Architectural Electromagnetic Shielding Handbook IEEE PRESS 445 Hoes Lane, PO Box 1331 Piscataway, NJ 08855-1331 1991
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Architectural Electromagnetic Shielding Handbook
IEEE PRESS 445 Hoes Lane, PO Box 1331 Piscataway, NJ 08855-1331 1991 Editorial Board Leonard Shaw, Editor in Chief William C. Guyker, Editor, Selected Reprint Series
1. E. Brittain S. H. Charap R. C. Dorf 1. 1. Farrell III L. 1. Greenstein 1. D. Irwin
W. K. Jenkins S. Luryi E. K. Miller 1. G. Nagle 1. D. Ryder A. C. Schell
M. Simaan M. I. Skolnik G. S. Smith Y. Sunahara R. Welchel 1. W. Woods
Dudley R. Kay, Executive Editor Carrie Briggs, Administrative Assistant Karen G. Miller, Production Editor
Architectural Electromagnetic Shielding Handbook A Design and Specification Guide
Leland H. Hemming Engineering Consultant
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I EEE PRESS
The Institute of Electrical and Electronics Engineers, Inc., New York
This book and other books may be purchased at a discount from the publisher when ordered in bulk quantities. Contact: IEEE Press Marketing Attn: Special Sales 445 Hoes Lane P.O. Box 1331 Piscataway, NJ 08855-1331 Fax: + 1 732 981 9334 For more information about IEEE Press products, visit the IEEE Online Catalog & Store: http://www.ieee.org/ieeestore.
© 1992 by the Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, 17th Floor, New York, NY 10016-5997 All rights reserved. No part of this book may be reproduced in any form, nor may it be stored in a retrieval system or transmitted in any form, without written permission from the publisher.
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ISBN 0-7803-6024-9 IEEE Order No. PP2824
The Library of Congress has catalogued the hard cover edition of this title as follows: Architectural electromagnetic shielding handbook: a design and specification guide / Leland H. Hemming, engineering consultant. p. em. Includes bibliographical references and index. ISBN 0-87942-287-4 I. Shielding (Electricity) 2. Magnetic Shielding. I. Hemming, Leland H. II. Institute of Electrical and Electronics Engineers. TK454.4.M33A77 1991 621.382-dc20
91-21458 CIP
Contents
Foreword
xiii
Preface
xvii
Chapter 1
Introduction
1
1.1 Scope
1
1.2
2
Chapter 2
Radio Frequency Shielding Definitions and Terminology
The Need for Radio Frequency Shielding
2.1
Introduction 2.2 The Electromagnetic Environment 2.2.1 2.2.2 2.2.3 2.2.4
7 7
Introduction, 7 Electromagnetic interference (EMf), 8 TEMPEST, 8 Electromagnetic pulse (EMP), 9
2.3 Facility versus Equipment Shielding 2.4
7
Shielded Anechoic Test Facilities
10 10
2.4.1 Introduction, 10 2.4.2 Shielding of anechoic facilities, 10
2.5 2.6
Chapter 3 3.1 3.2
Conclusions References
Shielding Theory Introduction Shielding Effectiveness
11 11
13 13 15
3.2.1 Introduction, 15
v
vi
Contents 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6
Reflection, 16 Absorption, 18 Internal reflection, factor B, 18 Total shielding effectiveness, 20 Performance degradation, 20
3.3 Typical Shielding Materials 3.4 Seams 3.4.1 3.4.2 3.4.3 3.4.4
Introduction, 26 Welded Seams, 26 Clamped seams, 26 Single-shield seams, 30
3.5 Conclusions 3.6 References
Chapter 4
25 26
33 34
Modular Shielded Enclosures
35
4.1 Introduction 4.2 Zinc/Galvanized Steel Modular Shielded Enclosures
35 36
4.2.1 4.2.2 4.2.3 4.2.4
Introduction, 36 System description, 36 Typical performance, 40 Procurement specifications, 40
4.3 Double-Isolated Shield System
40
4.3.1 Description, 40 4.3.2 Procurement specification, 43
4.4
Double-Isolated Screen Enclosure
43
4.4.1 Description, 43 4.4.2 Procurement Specification, 44
4.5 Single-Shield Modular Enclosure 4.5.1 4.5.2 4.5.3 4.5.4 4.5.5
44
Introduction, 44 Nonferrous NMR enclosures, 44 The Lindsay system, 46 Bolt-together wooden frame system, 50 Low-performance clamp-up shielding System, 50
4.6 Advantages/Disadvantages of Modular Enclosures 4.7 Critical Considerations
51 51
4.8 4.9
53
Chapter 5 5.1
Performance Specifications References
Welded Shielded Enclosures Introduction
52
55 55
5.1.1 Scope, 55 5.1.2 Defining the shielded volume, 55 5.1.3 Shield materials, 56
5.2 Welded Shield Design and Construction
57
5.2.1 General information, 57 5.2.2 Shield seam construction, 57 5.2.3 Corner seams, 64
5.3 Additional Shield Design Details 5.3.1 General discussion, 64 5.3.2 Shield wall supports, 65 5.3.3 Interior support columns and walls, 65
64
Contents
vii 5.3.4 5.3.5 5.3.6 5.3.7 5.3.8
Suspension of ceiling shield from roof joists, 66 Expansion joints, 66 Suspended acoustic ceiling, 70 Other equipment mounting, 70 Corrosion control, 70
5.4 Penetrations 5.5 Quality Control Recommendations 5.6 Shielding Effectiveness Performance 5.7 Sample Procurement Specifications 5.8 Conclusions and Recommendations 5.9 References
Chapter 6
Architectural Shielding
6.1 Introduction 6.2 Critical Considerations in Architectural Shielding 6.3 Aluminum Foil Shielding 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 6.3.6
79
81
Description, 81 Performance, 82 Material description, 82 Installation procedures, 82 Procurement Specification, 89
6.6 The Sandwich Seam Shielding System 6.6.1 6.6.2 6.6.3 6.6.4 6.6.5
75 75 76
Description, 79 Installation, 79 Performance, 80 Recommended Procurement Specifications, 81
6.5 Copper Alloy Shielded Enclosures 6.5.1 6.5.2 6.5.3 6.5.4 6.5.5
75
Introduction, 76 Shielding material, 77 Joining methods, 77 Installation procedures, 77 Shielded enclosure performance, 78 Procurement specification, 78
6.4 Copper Foil Shielded Enclosures 6.4.1 6.4.2 6.4.3 6.4.4
72 72 72 73 73 73
83
Introduction 83 Theory of operation, 86 Installation procedures, 87 Typical performance, 89 Procurement Specification, 89
6.7 Other Shielding Systems,
89
6.7. 1 Introduction, 89 6.7.2 Description of the INSTAR shielding system, 90 6.7.3 Copper Screen System, 91
6.8 Other Shielding Materials 6.8.1 6.8.2 6.8.3 6.8.4 6.8.5
91
Introduction, 91 Aluminum/Polyethylene material, 92 Nonwoven shielding materials, 92 Woven shielding materials, 93 Conductive copper paint, 93
6.9 Structural Considerations of Architectural Shielding
75
viii
Contents 6.9.1 Introduction, 94 6.9.2 Shielding of existing structure, 94 6.9.3 Decorative treatment of interior walls, 96
6.10 References
Chapter 7 7.1
Penetrations and Their Control Introduction
7.2 General Design Criteria for Penetration 7.3
Doors
97
99 99 99 100
7.3.1 Introduction, 100 7.3.2 The important features of an RF door, 101 7.3.3 The ReM or knife-edge door, 103
7.3.4 The compression door, 107 7.3.5 Moderate-performance RF doors, III 7.3.6 Vestibule and waveguide tunnel entrances, 111 7.3.7 Special-purpose shielded door systems 115
7.4
Heating and Air Conditioning
118
7.4.1 Introduction, 118 7.4.2 Description and theory of operation, 118 7.4.3 Performance, 120 7.4.4 Procurement Specifications, 121
7.5
Piping
121
7.5.1 Introduction, 121 7.5.2 7.5.3 7.5.4 7.5.5
7.6
Pipe penetrations for welded enclosures, 123 Piping for modular shielding, 124 Piping for architectural shielding, 124 Procurement specification, 125
Fiber Optics and Nonmetallic Hoses 126 7.6. 1 Design guides, 7.6.2 Procurement specification, 127
126
7.7 Shielded Windows 127 7.7.1 Description, 127 7.7.2 Shielding effectiveness of windows, 127 7.7.3 Window installation, 128
7.8
Fire Protection Systems
130
7.8.1 Introduction, 130
7.9 References
Chapter 8
Electrical Filters
8.1 Introduction 8.2 Fitter Theory
130
131 131 131
8.2.1 Introduction, 131 8.2.2 Filter configurations, 132
8.3
Filter Characteristics 8.3.1 Introduction, 133 8.3.2 Frequency characteristic, 134 8.3.3 Impedance levels, 134
133
ix
Contents 8.3.4 8.3.5 8.3.6 8.3.7 8.3.8 8.3.9
Voltage rating, 134 Current rating, 134 Insulation resistance, 134 Size and weight, 134 Temperature, 134 Reliability, 135
8.4 Filter Specifications
135
8.4.1 Introduction, 135 8.4.2 MIL-F-15733 requirements, 135 8.4.3 UL 1283 filter requirements, 136
8.5 Power Line Filters, 8.5.1 8.5.2 8.5.3 8.5.4
8.6 8.7 8.8 8.9
Chapter 9
Communication Filters Data Line Filters (Computers) Control Line Filters Reference
Enclosure Performance Specifications and Testing
9.1 Introduction 9.2 Performance Specification Review 9.2.1 9.2.2 9.2.3 9.2.4 9.2.5 9.2.6
137
Introduction, 137 Description of available configurations, 137 Duo-shield electromagnetic filters, 137 Procurement specification, 140
140 141 141 142
143 143 144
Introduction, 144 MIL-STD-285, 144 NSA 65-6, 145 NSA 73-2A, 145 IEEE 299, 146 Other specifications, 147
9.3 How to Select or Prepare a Performance Specification
147
9.3.1 Introduction, 147 9.3.2 Defining the shielding requirements, 148
9.4 Enclosure Performance Testing
9.5
Introduction, 149 Interpretation of text specifications, 149 Testing Considerations, 151 Accuracy of measurements, 154 Data presentation, 154 Common testing problems, 155 Recommended shielding effectiveness test specification, 156 Seam Leak Testing (SELDS) 9.5. I Introduction, 156 9.5.2 Principles of operation, 157 9.5.3 Recommended use, 157
149
9.4.1 9.4.2 9.4.3 9.4.4 9.4.5 9.4.6 9.4.7
9.6 Magnetic Particle Testing
156
158
9.6.1 Introduction, 158 9.6.2 Principles of operation, 158
9.7 Dye Penetrant Testing 9.8 References
159 159
Contents
x
Chapter 10
Grounding of Shielded Enclosures
10.1 10.2
Introduction Grounding Principles 10.2.1 10.2.2 10.2.3 10.2.4
161 161 161
Introduction, 161 Fault protection, 161 Enclosure isolat ion, 163 Grounding of signal references, 163
10.3 Selecting the Grounding System 10.4 The Earth Ground Test
164 165
10.4. 1 Introduction, 165 10.4.2 The direct method, 167 10.4.3 Fall-of-potential method, 167 10.5 References 167
Design Checklists
Chapter 11
11.1 Introduction 11.2 Checklist for Modular Shielding 11.2.1 11.2.2 11.2.3 11.2.4 11.2.5
11.3
177
Introduction, 177 Architectural checklist, 177 Electrical checklist, 178 Mechanical checklist, 178 Shielding checklist, 178
Appendix A A-I A-4
174
Introduction, 174 Architectural checklist, 174 Electrical checklist, 175 Mechanical checklist, 175 Shielding checklist, 176
11.4 Checklist for Architectural Shielding 11.4. 1 11.4.2 11.4.3 11.4.4 11.4.5
169 169
Introduction, 169 Architectural considerations, 169 Electrical considerations, 170 Mechanical considerations, 171 Shielding considerations, 171
Checklist for Welded Enclosures 11.3.1 11.3.2 11.3.3 11.3.4 11.3.5
169
Additional Definitions and Terminology Architects and Engineers Specifications
179 179 180
4.1 Galvanized modular enclosure procurement specifications, 180 4.2 Procurement specification for a version of the doubly isolated modular shielded enclosure system, 182 4.3 Procurement specifications for a copper screened enclosure, 185 4.4 Sample NMR RF shielding specification, 186 4.5 Sample procurement specification for the Lindsay singleshield modular enclosure system, 187 4.6 Sample procurement specification for a single-shield modular galvanized sheet metal shielding system, 188
A-5
Sample Specification for a Welded Enclosure
190
Contents
xi
A-6 Specifications for Architectural Shielding Systems 6.1 6.2 6.3 6.4
198
Specifications for aluminum foil shielding systems, 198 Procurement specifications for copper foil shielded enclosures, 199 Specification for copper alloy Shielded Enclosures. 20 I Procurement specifications for the sandwich seam shielding system, 202
A-7 Shielded Penetrations
203
7. 1 Procurement specifications for ReM or knife-edge door, 203 7.2 Procurement specifications for doubly isolated shielded door assembly, 204 7.3 Procurement specifications for moderate-performance shielded doors, 205 7.4 Procurement specifications for electromagnet latched RF shielded doors, 206 7.5 Procurement specification for doubly isolated shielded door assembly, 207 7.6 Procurement specification for shielded vents, 208
A-8 Sample Procurement Specifications for RF Filters
209
8.1 Specification for RF power line filters, 209
A-9 Sample Test Specification
215
Appendix B Appendix C Index
212
Selected Bibliography
217 219
Foreword
As microcircuit technology has evolved, integrated circuits (ICs) have continued to increase in complexity and capability. Individual ICs can now process enormous amounts of information in microseconds. By combining the capabilities of several ICs, electronic devices and machines playing vital roles in medicine, finance, manufacturing, and national defense have been realized. These roles cannot be jeopardized either by upset or damage from natural and manmade electrical signals. Unfortunately, the same technology that achieves high speed, high density processing capabilities tends to exhibit heightened sensitivities to the extraneous voltages and currents from electromagnetic fields, lightning, and switching transients. Consequently, there are many situations where electronic equipment must be electromagnetically isolated from its surroundings. For example, nuclear magnetic resonance imaging (NMRI) machines can be perturbed by stray magnetic fields, and many patient monitoring instruments are upset by electrical transients. Computers are often damaged by lightning transients or they experience data errors from strong radiated RF fields. Many other signal processors must be isolated from these effects as well as from others, and the electromagnetically shielded room is an essential ingredient of the protection. Defense and diplomatic information is often highly classified and must be protected against unauthorized interception. Communications and data processing centers handling classified information must conform to TEMPEST requirements; these requirements are often met by installing appropriate shields around the processing equipment. The principles behind electromagnetic shields have been known for over 50 years. Specially constructed modular rooms based on these principles have been commercially available for approximately 40 years. In the last 20 years, however, numerous applications have arisen where the traditional rectangular, double-walled, free-standing room will not suffice. Particularly in the last decade, the requirement to provide an adequately isolated volume has become an integral part of the design and construction
xiii
xiv
Foreword
of many buildings. The need for a facility-integrated shield means that architects and engineers must become familiar with the design, specification, construction, and testing of electromagnetically tight enclosures. The design, construction, and test of shielded rooms has traditionally been left to specialists in electromagnetic interference (EMI), electromagnetic pulse (EMP), and TEMPEST. As the need for protection has broadened to encompass all electromagnetic concerns and to protect facilities much more diverse than military installations and EMI testing rooms, other engineering specialties have begun to address electromagnetic shielding. The results have been mixed, ranging from extreme overdesign to installations that are said to be shielded, but the materials and construction of which violate many fundamental principles of shielding. Architectural Electromagnetic Shielding Handbook provides the responsible designer with an understandable exposition of the basics of the various aspects of realizing an effective shield: selecting a material appropriate for the application; joining the materials so that effective attenuation of electromagnetic fields is obtained; properly treating all necessary penetrations for power, communications, HVAC, and personnel access; proper grounding; and correct testing to verify that performance requirements are met. The modular room is fabricated at the factory and erected at the designated location by factory personnel or specially trained installers. The architectural shield, i.e., one that is tailored to fit the structural constraints and user needs of a specific building, however, must be constructed on site by normal building trades personnel who often are unskilled in the unique assembly techniques required for seams, doors, and penetrations, or who do not appreciate the special care required, as for example, when erecting interior finish walls of shielded rooms or installing air conditioning ducts. Therefore, detailed instructions must be given, special construction must be detailed, training must be conducted, and careful onsite inspection provided. Each of these needs is addressed in this handbook. As construction proceeds, carefully controlled tests of shield perfomance are required. Specialized test equipment not normally owned by construction companies must be used; the equipment must be operated by test personnel who are well versed in the nuances of radiated electromagnetic field measurements. Otherwise, test results can be meaningless, potentially leading to costly overruns or to an ineffectively shielded installation. The tests must be performed at the time the basic shield is complete and again upon completion of the shielded facility. An essential element of the architectural design package for a facility is the specification detailing construction and performance requirements. For a shielded facility, the specification must reflect the nature of the equipment or system to be protected. For example, an EMP shield around a large communications complex will possess different requirements from the shield required around an operating room in a hospital. The design specification must accommodate such differences. The specification must be technically realistic, easily interpreted by the construction contractor, and legally enforceable. This handbook addresses the shielding needs of a wide range of facilities. It is intended to assist architects and engineers in the design and specification of electromagnetically shielding rooms, whether encompassing only a small region inside the building or the entire structure. It provides guidance in the unique assembly and testing
Foreword
xv
techniques to achieve and verify the required electromagnetic isolation of the volume. Sample specifications are provided which may be tailored to fit a variety of construction situations. Hugh
w. Denny, ~E.
NARTE Certified EMC Engineer Director, Electromagnetic Environmental Effects Laboratory/GTR I
Preface
About ten years ago while writing procurement specifications for a number of shielded radio frequency anechoic test chambers I became aware of the lack of published information on architectural electromagnetic shielding systems. This book was written to fill that gap and I believe it is the first single-volume text covering all aspects of that subject. The purpose of the handbook is to provide the practicing architect/engineer with a single source of information on electromagnetic shielding. Included are chapters on the need for shielding, basic shielding theory, and complete descriptions of the three major types of commercial shielding. Extensive supporting information on penetrations, such as doors, vents, piping, and electromagnetic filters is provided for each type of shielding. Performance specifications and methods of testing necessary to prove performance are detailed. Finally, a set of design checklists is provided for the three most common forms of shielding so that the architect/engineer can be sure he has covered all aspects of the shielded enclosure installation. In Appendix A, suggested specifications are provided to aid the architect/engineer with formulating a complete shielded facility specification. These are arranged in the same order as the chapters that describe the various forms of shielding, supporting hardware, and services. This material was organized by the author who is solely responsible for all technical information included in the handbook. The presentation is made as factual as possible and treats all shielding products in a fair and unbiased manner. For specific products, services, and commercial shielding information, the reader is referred to Appendix B. This appendix describes an industry annual publication which lists most of the currently active shielding suppliers and installers in the United States as well as some overseas.
xvii
xviii
Preface
ACKNOWLEDGMENTS
The author wishes to thank his wife, Valda, 'for her unending support during the four years it took to complete this project. She was a great help in turning my long rambling sentences into organized, readable material. Jeff Eckert was very helpful, and I thank him for finding a publisher willing to put this handbook into print. Hugh Denny made a series of excellent recommendations, most of which I incorporated into the technical presentations and thus improved the professionalism of the book. I also wish to thank my many work associates who made helpful suggestions, especially Tom Witkowski for suggesting the arrangement of the appendices. Finally, my thanks to the staff of the IEEE Press who worked with me in a very professional manner throughout the final preparations and publication of this handbook. POWAY, CA
Leland H. Hemming
CHAPTER 1
Introduction
1.1 SCOPE The use of radio frequency shielded enclosures has become quite common in our increasingly electromagnetically crowded environment. Until recently, the design, manufacture, and installation of shielding was the province of a handful of specialty contractors. Two basic systems were developed by the shielding industry: the modular or clamp-up enclosure, and the welded structural steel enclosure, with the latter being built in place. Only a few attempts at other types of shielding have been made. The most notable was the aluminum foil system developed by the National Security Agency as described in their detailed specification NSA 73-2A. However, the need for shielding has become so widespread that a number of companies have developed a variety of single-skin shielded enclosures. The purpose of this handbook is to provide architectural and facility engineering personnel with the necessary information which will permit them to make informed decisions on the different types of shielding and how to properly specify them sufficiently so that the constructed shield performs its intended purpose. As will become clear from the text, the completed enclosure must be properly tested to ensure compliance with the desired level of performance. A second purpose is to point out the performance differences between the various shielding systems so that intelligent choices are possible. In the past, it has been common practice by the shield-buying community to request the highest performance specifications available, thus ensuring a safe decision, but in many instances, paying excessively for protection they did not need. This handbook gives the shielding buyer a survey of the systems available, so that he can select the best system for his particular application. The next two chapters are introductory in that they establish what shielding is and how it is used to control electromagnetic interference. The first part of the "Theory" section has been provided to help bridge the gap between the layman's knowledge of electricity and the deeper understanding necessary to appreciate how an electromagnetic
2
Chapter 1
Introduction
shield performs its function. It is not the purpose of this book to teach shielding design, but rather how to select from existing systems the one best suited for the job at hand. The next three chapters deal with shielded enclosures, first. those that are provided by the speciality shielding contractor, and then those that are built into the structure of a building. This is not an arbitrary selection, but is based on the current practices in the shielding industry. Currently, the most common shielding systems provided by the established shielding manufacturers are prefabricated modular or "clamped-up" rooms and welded enclosures which are built on site. These products have evolved with time, and are proven systems available from a number of large and small companies worldwide. A newer class of shielded enclosure is the one that is built into the structure, i.e., within the walls of a building. This later class has been grouped in Chapter 6 under the heading of "Architectural Shielding." Several variations of these shielding systems are described. These are usually installed by a general contractor either as described by the drawings or by subcontracting to a specialty contractor. The key to success in these installations is how well they are detailed in the building drawings. Since the shielding is built in, it must be done right the first time because retrofitting in the field is extremely costly and time consuming. If the information given in this handbook is carefully considered and implemented, it will successfully aid those who have the responsibility to implement shielding in their facilities or design/specify them for a client. The selection of the basic enclosure is very important to the overall performance of the facility, but a shield is only as good as the treatment of the penetrations through it. The next two chapters discuss how the designer selects methods of providing doors, vents, piping, power, communications, and security to a shielded enclosure. Just how badly an improperly installed penetration can degrade an enclosure is illustrated in the theory section. Another very important requirement is that of testing the facility while in the process of construction and finally on completion. The latter is absolutely essential. After all trades have completed their work, the shielded enclosure must be tested to the applicable shielding effectiveness specifications. Chapter 9 details the common specifications, how to use them effectively, and how to write a test plan that will ensure that the completed facility will perform its intended function. Drawing on the theory section, each type of test is described, along with the common problems encountered in field testing. Accomplishing an acceptable grounding system for a shielded enclosure is often a problem in the field. Signal security and electrical safety requirements do not always appear to be compatible. Using MIL-HB-419A as a guide, guidelines are given on how the various types of grounds can be designed to obtain a harmonious installation, one which is safe for personnel and also meets the needs of the security community. Also refer to MIL-HB-232 for grounding information. The last chapter provides a series of design checklists for the. three types of shielded enclosures, so the A&E can draw a complete enclosure specification together for the various forms of shielding. A number of appendices is provided for reference, such as a reference to an annual directory that details shielding manufacturers, installers, designers, and consultants. A large part of the material for this handbook was drawn from MIL-HB-419A, which is entitled "Grounding, Bonding, and Shielding for Electronic Equipments and
Section 1.2
Radio Frequency Shielding Definitions and Terminology
3
Facilities." This handbook is recommended reading for all who are concerned with large industrial/government facilities. It is also useful for designing or specifying a single room.
1.2 RADIO FREQUENCY SHIELDING DEFINITIONS AND TERMINOLOGY In any technical field of knowledge, a certain amount of special terms unique to that field must be understood in order to comprehend what is being presented. Therefore, this section is placed deliberately here in the first chapter so that a working vocabulary necessary to understanding the material presented can be easily acquired by those not familiar with shielding prior to the introduction of the technical concepts. Further definitions and supporting terminology are given in Appendix A-I.
Absorber. A material which absorbs electromagnetic energy by converting the wave energy into heat.
Absorption Loss. The attenuation of an electromagnetic wave as it passes through a shield. This loss is primarily due to induced currents and the associated heat loss.
Ambient Level. Those levels of radiated and conducted energy existing at a specified location and time when a test sample is deenergized. Atmospheric noise signals, both desired and undesired, from other sources and the internal noise level of the measuring instruments all contribute to the "ambient level." Antenna. A device employed as a means for radiating or receiving electromagnetic energy. Aperture. An opening in a shield through which electromagnetic energy passes. Attenuation. A general term used to denote a decrease in magnitude of power or field strength in transmission from one point to another caused by such factors as absorption, reflection, scattering, and dispersion. It may be expressed as a power ratio or by decibels. Bond. The electrical connection between two metallic surfaces established to provide a low-resistance path between them. Bonding. The process of establishing the required degree of electrical continuity between the conductive surfaces to be joined. Conductive Interference. Undesired signals that enter or leave an equipment along a conductive (wire or metallic) path. Coupling. Energy transfer between circuits, equipments, or systems. Coupling, Free-Space. Energy transfer via electromagnetic fields not in a conductor. Cutoff Frequency. The frequency below which electromagnetic energy will not propagate readily in a waveguide. dB. Decibel, a unit of voltage or power ratio. Defined as follows: dB = 10 log P'21P 1 for power or dB
= 20 log V2IVI
for voltage.
HdB" is commonly used to specify shielding effectiveness since very large differences in the input/output fields are generally required by the shielding specification.
4
Chapter I
Introduction
Specifications on the order of 60-100 dB are typical. This means that if one watt of power impinges on the shield, then only one millionth to one ten trillionth of a watt exits on the other side. Degradation. A decrease in the quality of a desired signal (i.e., decrease in the signalto-noise ratio or an increase in distortion), or an undesired change in the operational performance of equipment as the result of interference. Earth Electrode System. A network of electrically interconnected rods, plates, mats, or grids installed for the purpose of establishing a low-resistance contact with earth. The design objective for resistance to earth of this subsystem should not exceed 10 O. Electric Field. A vector field about a charged body. Its strength at any point is the force which would be exerted on a unit positive charge at that point. Electromagnetic Compatibility (EMC). The capability of equipment or systems to be operated in their intended operational environment at designed levels of efficiency without causing or receiving degradation owing to unintentional electromagnetic interference. Electromagnetic compatibility is the result of an engineering planning process applied during the life cycle of the equipment. The process involves careful considerations of frequency allocation, design, procurement, production, site selection, installation, operation, and maintenance. Electromagnetic Interference (EMI). Any conducted, radiated, or induced voltage which degrades, obstructs, or repeatedly interrupts the desired performance of electronic equipment. Electromagnetic Pulse (EMP). A large impulsive-type electromagnetic wave generated by nuclear or chemical explosions. Facility. A building or other structure, either fixed or transportable in nature, with its utilities, ground networks, and electrical supporting structures. Far Field. The region of the field of an antenna where the radiation field predominates, and where the angular field distribution is essentially independent of the distance from the antenna. A variety of guidelines is used; for some shielding calculations, 1/6th of a wavelength has been found useful. Fault. An unintentional short circuit or partial short circuit (usually of a power circuit) between energized conductors or between an energized conductor and ground. Field Strength. A general term that means the magnitude of the electric field vector (in volts per meter) or the magnitude of the magnetic field vector (in ampere-turns per meter). As used in the field of EMC/EMI, the term "field strength" shall be applied only to measurements made in the far field and shall be abbreviated as FS. For measurements made in the near field, the term electric field strength" (EFS) or "magnetic field strength" (MFS) shall be used, according to whether the resultant electric or magnetic field, respectively, is measured. Filter. A device for use on power or signal lines, specifically designed to pass only selected frequencies and to attenuate substantially all other frequencies. Ground. The electrical connection to earth through an earth electrode subsystem. This connection is extended throughout the facility via the facility ground system, consisting of the signal reference subsystem, the fault protection subsystem, and the lightning protection subsystem. These different systems are detailed in the following chapters. Magnetic Field. A vector field produced by a continuous flow of charge. Multipoint Ground. More than one path to ground. 44
Section 1.2
Radio Frequency Shielding Definitions and Terminology
5
National Electrical Code'", (NEC®). A standard governing the use of electrical wire, cable, and fixtures installed in buildings.
Near Field. The region of the field immediately surrounding an antenna where the inductive and capacitive fields predominate. In this region, the angular distribution of the field varies with distance from the antenna. Neutral. The ac power system conductor which is intentionally grounded on the supply side of the service disconnecting means. The neutral provides a current return path for ac power currents, whereas the ground (or green) should not, except during fault conditions. Penetration. The passage through a partition or wall of an equipment or enclosure by a wire, cable, pipe, or other conductive object. Plane Wave. An electromagnetic wave which predominates in the far-field region of an antenna, and with a wavefront which is essentially a flat plane. In free space, the characteristic impedance of a plane wave is 377 O. Radiation. The emission and propagation of electromagnetic energy through space. Radiation Resistance. The resistance which, if inserted in place of an antenna, would consume the same amount of power that is radiated by the antenna. Radio Frequency Interference (RFI). Synonymous with electromagnetic interference. RF-Tight. Offering a high degree of electromagnetic shielding effectiveness. Reflection Loss. The portion of the transition loss, expressed in decibels, that is due to the reflection of power at a barrier or shield. Reflection loss is determined by the magnitude of the wave impedance inside the barrier relative to the wave impedance in the propagation medium outside the barrier. Shield. A housing, screen, or cover which substantially reduces the coupling of electric and magnetic fields into or out of circuits or prevents the accidental contact of objects or persons with parts or components operating at hazardous voltage levels. Shielding Effectiveness. A measure of the reduction or attenuation in the electromagnetic field strength at a point in space caused by the insertion of a shield between the source and that point. Shielded Enclosure. An area (box, room, or building) specifically designed to attenuate electromagnetic radiation, or electromagnetic radiation and acoustical emanations, originating either inside or outside the area. Necessary openings in shielded enclosures, such as doors, air vents, and electrical feedthroughs, are specially designed to maintain this attenuat ion. Signal Reference Subsystem. This subsystem provides the reference points for all signal grounding to control static charges, noise, and interference. It may consist of any one or a combination of the lower frequency network, higher frequency network, or hybrid signal reference network. TEMPEST. A code word (not an acronym) which encompasses the government! industrial program for controlling the emissions from systems processing classified data. Individual equipment may be "TEMPESTED" or commercial equipment may be placed in shielded enclosures. Wave Impedance. The ratio of the electric field strength to the magnetic field strength at the point of observation. Wavelength. The ratio of C, the speed of light, to F, the frequency. Wavelength (ft) = 984/F (MHz).
CHAPTER 2
The Need for Radio Frequency Shielding
2.1 INTRODUCTION The need for shielding has evolved with radio communications from the very beginning, but since World War II, the need has become critical because of the tremendous growth of electronic devices in the home, office, and factory. Today, shielding against EMI is being practiced by government, military, private industry, medical facilities, and R&D laboratories. Shielding serves two basic functions: that of preventing interference and preventing electronic eavesdropping. The type of shielding required is a function of the purpose or use of the equipment within the shield. High-performance shielding is required where sensitive equipment must be protected from a nearby high-power radar. But only moderate shielding may be required to prevent eavesdropping on classified processing of data on a commercial computer. The exact degree of shielding required is a function of many variables, among which are the type of equipment involved, the distance between equipments, and the sensitivity of the information being processed. In order to gain some insight into the type of situations where shielding may be in order, the following sections outline the nature of electromagnetic interference, how it is generated, and how it reaches the equipment that may require shielding.
2.2 THE ELECTROMAGNETIC ENVIRONMENT 2.2.1 Introduction The electromagnetic designer must consider various interference sources and threats prior to specifying a certain shielding requirement for a given installation. Among these are the spectrum to be covered, typically 1 kHz-40 GHz, local sources such as licensed transmitters, government equipment such as radars, nearby local sources such as CB equipment, walkie-talkies, and electronic test equipment. Threats may include light7
Chapter 2
8 TABLE 2-1
The Need for Radio Frequency Shielding
SOURCES OF CONDUCTED INTERFERENCE Spectrum
Source Circuit Breaker Cam Contacts Command Programmer Signal lines Power lines Computer Logic Box Corona Fluorescent Lamps Heater Circuits (Contact Cycling) Latching Contactor Motor Armatures Mercury Arc Lamps Power Controller Power Supply Switching Circuit Power Transfer Controller Vacuum Cleaner
10-20 MHz
50
50 50
50
0.1-25 MHz 1-25 MHz kHz-20 MHz 0.1-10 MHz 0.1-3 MHz kHz-25 MHz kHz-25 MHz 2-4 MHz 0.1-1.0 MHz 2-15 kHz 0.5-25 MHz kHz-25 MHz 0.1-1.0 MHz
ning, electromagnetic pulse, and finally, sensitive eavesdropping receivers. Knowing the power levels and locations of interfering sources, a shielding effectiveness profile can be determined versus frequency and type of field for a given enclosure. In a like manner, knowing the sources of emissions from a data processing system and the possible location and sensitivity of a listening receiver, the amount of shielding effectiveness required for a given TEMPEST enclosure can be determined. In order to have some understanding of what is involved, consider the following.
2.2.2 Electromagnetic Interference (EMI) EMI (RFI) can occur via conducted or radiated interference. The former is transmitted by power, data, telephone lines, or just metallic paths connecting the source of the interference and the equipment being victimized. Table 2-1 illustrates the types of sources that commonly generate conducted interference. As illustrated, the conducted spectrum extends over a wide frequency range. Not shown is that many complex equipments can generate conducted interference well up into the gigahertz frequency range. Conducted interference is controlled by filtering all leads going into a shielded enclosure and ensuring that proper grounding and bonding are achieved. Radiated interference is any interference transferred through space by an electromagnetic field. The level of interference is a function of directivity of the energy as it leaves the source, the losses in propagating to the device, the degree of coupling into, and the susceptibility of the device to the characteristics of the energy. In addition to the sources listed in Table 2-1, radiated interference is caused by atmospheric disturbances, cosmic noise, solar radiation, and manmade sources such as automobiles, industrial, scientific, and medical equipment. Intentional transmitters from LF communications to millimeter radars and satellite communications also can interfere with other services. Finally, two man-created threats of electronic eavesdropping and EMP must be considered.
2.2.3 TEMPEST Electronic eavesdropping on classified and sensitive information both in the defense and general industry has, in recent years, become a real-world problem. As a result, the
Section 2.2
9
The Electromagnetic Environment dB
o -10
-20
-30 -40 -50
-60 -70
-80 -90 -100
10 1 10 2 103 10 4 10 5
10 6 10 7 108 10 9 10 10
Frequency (Hz)
Figure 2.1 EMP frequency versus energy spectrum.
government established a joint industry/government program for setting standards for controlling the emissions from equipment that processed classified information. This is generally known as the TEMPEST program. An elaborate set of information is available from the government on the control of emissions from equipment. A large number of individual computing devices is now available which are TEMPEST certified. But where a large amount of equipment is involved, it is customary to house them within shielded enclosures. As a result, the whole defense industry in recent years has been installing shielded computer centers which range from small room-size prefabricated enclosures to multistory shielded buildings. The level of shielding varies from a low of 30 dB to 120 dB or from foil-covered walls to 1/4 in welded steel facilities. The amount of shielding effectiveness is set primarily by the government agencies that are sponsoring the work being done by the defense contractor or other government agency.
2.2.4 Electromagnetic Pulse (EMP) When even a small nuclear device is exploded, it is possible for a large amount of electromagnetic energy to be released which can be very damaging to modern solidstate electronic equipment. A high-altitude nuclear explosion is considered to be the most likely, since one weapon exploded at a height of 500 km could effectively paralyze all communications or other unprotected digital equipment within the whole continental United States. Protection from HEMP is best provided by a properly designed shielded enclosure. The bulk of this energy, as shown in Fig. 2-1, is confined to the frequency range below 100 MHz, and lies at the lower end of the frequency spectrum. Thus, both magnetic and electric field coupling must be considered as threats to the equipment to be protected. The shielding system must include devices which will protect against large current and voltage surges, in addition to attenuation, to prevent radiated interfer-
10
Chapter 2
The Need for Radio Frequency Shielding
ence with the equipment within the enclosure. All leads must be especially designed to prevent magnetic coupled energy from reaching the equipment via the power lines or other conducted services. For a detailed description of the EMP problem, refer to [1, Sect. 10].
2.3 FACILITY VERSUS EQUIPMENT SHIELDING Because of the large number of electronic equipment now available, a large part of which uses digital circuitry, the opportunity for electromagnetic interference has increased to the point that government agencies of all the large industrial nations have found it necessary to set emission limits on most commercial electronic equipment. In the United States, the limits are set by the FCC. As a result, all commercial equipment must meet a given level of radiated emissions. But these levels are specified to prevent interference with neighboring equipment, not to prevent electronic eavesdropping. Therefore, the U.S. government has established another set of standards known as the TEMPEST requirements. These requirements are classified, and can only be made available to those working in the field and who require a need to know. But the requirements are very stringent, and as a result, add great expense to the cost of equipment which meet the requirements. A qualified products list exists which contains a large variety of office and data processing equipment. A brief review of the cost tradeoffs clearly indicate that if more than a few equipments are going to be used in a given office area, then a shielded enclosure should seriously be considered. First, the initial cost of TEMPEST equipment is high; next, any upgrades or changes to the equipment require recertification; and finally, the cost has to be repeated should the old equipment be replaced with the next generation. On the other hand, standard commercial equipment housed in a shielded enclosure will meet the TEMPEST requirements regardless of the equipment changes since the enclosure provides the protection for the equipment and is a one-time cost. The U.S. Air Force in [1] has determined that the shielding effectiveness of TEMPEST facilities need only be on the order of 50 dB within the continental United States.
2.4 SHIELDED ANECHOIC lEST FACILITIES 2.4.1 Introduction There exists a class of specialty facilities known as anechoic chambers which are commonly shielded. These facilities are used to test the electromagnetic properties of missiles, computers, televisions, microwave ovens, satellites, antennas, and aircraft.
2.4.2 Shielding of Anechoic Facilities The shielding of anechoic test facilities can take almost any of the forms of shielding discussed in the following chapters. The type of shielding required is a function of the nature of the test facility. If it is primarily used to establish a reflective backing for the anechoic material, then foil shielding is generally quite adequate. If it is to provide a TEMPEST environment for the test equipment of the facility, then a more elaborate shielding system may be required. If it is to test high-power equipment or the electromagnetic compatibility of an aircraft, then a welded enclosure probably is in order. The
Section 2.6
Reference
II
anechoic material is mounted to the interior surfaces of the chamber generally using adhesives. The design of the geometry of the chamber and the types and locations of the anechoic material are the purview of the design specialist, and it is suggested that if an anechoic design requirement is needed, then an appropriate specialist should be enlisted to perform the design. Generally, the layout of the various services, lights, ventilation, power, and fire protection require special consideration over and above the normal shielded enclosure requirements. 2.5 CONCLUSIONS Once the shielding effectiveness requirements have been defined either by edict or by analysis, the latter being preferred since the cost of shielding is directly a function of the amount and type of attenuation required, the following sections of this handbook can be utilized to design, specify, install, and test a cost-effective shielding system. 2.6 REFERENCE [1] USAF Handbookfor the Design and Construction of HEMP/TEMPEST Shielded Facilities, Dec. 1986.
CHAPTER 3
Shielding Theory
3.1 INTRODUCTION When a shield encloses an EM source, as shown in Fig. 3-1, the field strength outside the shield will be reduced. When the shield is used to enclose a sensitive (susceptible) assembly located near an external EM source, the field strength inside the enclosure is reduced substantially. Three types of electromagnetic fields exist, each of these act differently depending on the nature of the shielding material present. Magnetic shielding below 100 kHz is difficult to achieve and requires very thick ferrous shields or high permeability materials. Electric field shielding is relatively easy in that a thin metallic barrier will normally suffice. Plane wave or far-field shielding is mainly a function of maintaining an RF tight skin. All penetrations such as doors, vents, filters, and piping must be carefully designed and constructed to maintain the RF-tight requirements. The purpose of RF shielding is to confine or to prevent radiated energy from entering or exiting an enclosure. The mechanism of this radiated interference is by means of electromagnetic coupling. Two forms of coupling take place, near and far field, the difference being the distance between the circuits. The near field can be subdivided into inductive and capacitive or low- and high-impedance coupling, according to the nature of the electromagnetic field. In inductive coupling, a magnetic field linking the susceptible device is set up by the interfering source. In capacitive coupling, the electric field transfers energy between the two circuits. In the far field, radiation of energy byelectromagnetic waves is the principle coupling mechanism. When two or more wires or other conductors are located near each other, currents and voltages on one wire will be inductively and capacitively coupled to the other wires. The wire acting as the interference source for this near-field coupling may be any conductor such as a high-level signal line, an ac power line, a control line, or even a lightning downconductor. The currents or voltages induced into the other wires can fur-
13
14
Chapter 3
1.0
Shielding Theory
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Figure 3-1 Shield enclosing an em source.
ther be conductively coupled into yet other circuits. In the far field, coupling is achieved by radiation. The conductor does not have to be specifically designed to radiate energy; it may be any current-carrying conductor, i.e., a signal line, a power line, or even a ground lead. The equations defining these coupling modes are expressed as a sum of three terms. Each term is inversely proportional to a power of the distance r from the currentcarrying conductor. These power terms of l/r, l/r 2 and l/r 3 determine the nature of the field, as illustrated in Fig. 3-2. Close to the conductor (near field), the l/r and l/r components dominate, and the electromagnetic energy oscillates between the space surrounding the conductor and the conductor itself; zero average energy is propagated by the near-field terms. In the far field, the I/r term dominates, and the energy escapes from the conductor (antenna) into free space. This mechanism of radiation occurs as the frequency is raised and the current in the wire cannot reverse as fast as the field is built up, resulting in the field being released from the vicinity of the wire and propagating outward. On the receiving end, the reverse occurs, i.e., the incoming wave induces charges in the conductor (antenna), and a current is created. The strength of this field is a function of the distance from the radiating wire, the efficiency of the radiating wire as an antenna, and the amplitude and frequency of the signal on the radiating wire. The efficiency is a function of the wire's length in wavelengths. Wires on the order of a quarter wavelength make excellent antennas; those less than one hundredth wavelength are poor radiators.
Section 3.2
15
Shielding Effectiveness
Dipole
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Figure 3-2 Electromagnetic field about a conductor. (Courtesy of Carl T. A. [ohnk, John Wiley and Sons.)
3.2 SHIELDING EFFECTIVENESS 3.2.1 Introduction The attenuation provided by an RF shield results from three mechanisms, as illustrated in Fig. 3-3. 1. Incident energy is reflected by the surface of the shield because of the impedance
discontinuity of the air-metal boundary. This mechanism does not require a par-
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16
Chapter 3
Shielding Theory
ticular material thickness, but simply an impedance discontinuity. A special case exists when a gap in the shield has length, such as a honeycomb vent. 2. Energy that does cross the shield surface (i.e., not reflected) is attenuated (absorbed by turning into heat energy) in passing through the shield. 3. The energy that reaches the opposite face of the shield encounters another airmetal boundary, and thus some of it is reflected back into the shield. The first reflection results in a "single-reflection loss" R. Absorption through the shield is designated A, and subsequent reflections result in a "multiple-reflection correction term" B. B is significant only if A 1
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Chapter 3
Inside Shielded Room
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3.4.4 Single-Shield Seams Various means are used to seal the seams of single-shield systems. The clamped seam is used in some cases by the major suppliers of modular systems, but the most common is the field-installed system. With two exceptions, these are lower performance shielding systems. The exceptions are illustrated in Fig. 3-15. The sandwich seam is a patented concept [3] which uses the waveguide-beyond-cutoff principle to achieve the RF seal, whereas the other seam is flame-sprayed with zinc to achieve a metal-to-metal seal across the lap joint. The sandwich seam has been successfully used by the author in over a dozen large 26 gauge galvanized single-shield enclosures. In copper foil installations, continuous or spot-soldered seams are used. The latter should only be used in low-frequency, below 100 MHz installations since higher frequency energy will pass through the joints where the spacing exceeds 0.1 wavelength. In foil systems, it is absolutely essential that the material be lapped so the solder has a chance to properly seal the joint. A good method of sealing is to use copper tape that has been pretreated with a solder coating. A hot iron can then be used to seal the seams. Care must be exercised to prevent air bubbles from forming when using this method, and cold joints are often a problem. With the exception of the waveguide-beyond-cutoff seal, all seaming approaches rely on achieving a continuous metal-to-metal seal. The degree to which it is achieved determines the effectiveness of the construction method in accomplishing the overall
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32
Chapter 3
Shielding Theory
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _~ Cop per Foil
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Figure 3-16 Conductive adhesive shielding tape . shielding performance. At 10 GHz, the high end of most specifications, a very slight crack in a solder seam will cause a leak of 20 dB or, instead of measuring 100+ dB, the test will yield 80 dB . This is especially true around the seams at doors and other penetrations. For moderate performance installations «60 dB) another method of sealing seams in shielded enclosures is to use various forms of copper tape. Three forms of this tape currently exist. The most common is copper foil or tinned plated copper foil with a conductive adhesive . The conductivity of the adhesive is provided by filling the adhesive with metal particles. The most effective form uses silver-plated copper particles. This is illustrated in Fig. 3-16. Another form of the tape is the embossed tape, shown in Fig. 3-17. Here , conductivity is achieved by a metal-to-metal contact between the
Foil Cross Section
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Adhe sive
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Figure 3-17 Embossed pattern impressed onto the foil backing of shielding tape . (Courtesy of Electrical Specialties, Div. 3M Co.)
Section 3.5
33
Conclusions
Solder Plate
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Figure 3-18 Waveguide-beyond-cutoff shielding tape. patches of adhesive. A third form is illustrated in Fig. 3-18. This tape relies on the waveguide-beyond-cutoff principle. The waveguides are formed by the ridges in the tape; one form uses adhesive to hold the tape foil in intimate contact with the shielding surface, and the other has solder on the ridges. This concentrates the heat during soldering along the ridges and improves the metal-to-metal seal. For further details on this patented shielding tape, refer to [4]. Other methods of sealing seams are illustrated in Fig. 3-19.
3.5 CONCLUSIONS Shielded enclosures can be made of just about any type of metal foil, screen, metallic cloth, or metal sheet, the degree of shielding depending on the frequency and field type to be attenuated. The real key to an effective shielding system is how the sheets are seamed together and how penetrations are controlled through the shield. The next chapter .iiscusses commercial shielding systems and how they are designed and specified to achieve their stated performance.
Chapter 3
34
Shielding Theory
Weld Material
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Formation of Permanent Overlap Seam
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Note: Soldering or Welding Is Desirable for Maximum Protection
---1L CD
Figure 3-19 Other methods of sealing seams. 3.6 REFERENCES [I] D. R. 1. White, "Shielding design, methodology and procedures," Interference Control Technologies, Inc., 1986. [2] - - , "Electromagnetic shielding materials and performance," Interference Control Technologies, Inc., 1975. [3] "Shield system and sandwich seam for attenuation of electromagnetic energy," U.S. Patent 4 733 013, Mar. 22, 1988. Technology licensed to author by ALCOA. [4] "Radio frequency shielding tape," U.S. Patent 4 977 296, Dec. II, 1990. Inventor: author.
CHAPTER 4
Modular Shielded Enclosures
4.1 INTRODUCTION A shielded enclosure is a six-sided structure with all doors, vents, and other items which penetrate the shield treated to maintain shielding integrity. It may be a freestanding structure, such as the modular clamp-up structure, or a welded steel room built in place on the site. A more recent advent in shielding is the architectural shielding system, where metal foil or sheet metal is built into the walls, floor, and ceiling of existing or new construction. This chapter reviews the modular shielded systems commonly provided by the commercial shielded enclosure manufacturer. Both performance and procurement specifications are provided for the various types of modular enclosures. These range from the most popular form, a galvanized steel paneled room, to copper screen wire mounted on wood frames. Single- and double-shielded systems are available in modular form. The latter provide high levels of shielding effectiveness and are the most common. The single-shield systems provide moderate amounts of shielding effectiveness, and have become popular in recent years. The choice of which type of shielding system to use in a given application is a function of several factors. The first factor is performance. Only performance levels which are actually needed in terms of shielding effectiveness for each type of electromagnetic field and frequency range should be specified. The required shielding effectiveness is primarily based on the operational purpose of the facility. Hospital and secure communication facilities generally require shielding for different reasons. The operational purpose also determines the other physical factors required for the shielded enclosure such as appearance, HVAC, physical layout, and environmental requirements. All of these factors must be taken into account when selecting the type of shielding system for a given application. After all the requirements are clearly defined, the data given in this handbook can be used to select or specify a particular shielding system. The most common form of modular shielded enclosure is the clamp-up system using galvanized sheet metal bonded to both sides of a wood core and tied together by
3S
36
Chapter 4
Modular Shielded Enclosures
Emergency Lights Outlet Box Electrical Outlet for Door Emergency lights ~~--RFDoor
Platform
Ground Stud
Extractor Fan
Figure 4-1 Modular, clamp-up, demountable shielded enclosure.
a framework made of plated steel. This form of modular system will be discussed first. The remaining types of premanufactured or prefab structures will follow. The choice of which system to select is mainly based upon the need for the shielding. The galvanized panel system is the most common because it is readily available from a large number of suppliers and provides good performance over a broad frequency range. The remaining modular shielding systems described and specified provide a variety of capabilities, each of which depends upon the type of shielding material to be used and the method of their construction.
4.2 ZINc/GALVANIZED STEEL MODULAR SHIELDED ENCLOSURES 4.2.1 Introduction The majority of shielded enclosures sold by commercial shielding companies is of a modular, demountable, and clamp-up design. They consist of plywood or particle board laminated on both sides with galvanized or zinc-plated steel. They are assembled into enclosures using strapping systems which are known as hats and flats. This type of construction is illustrated in Fig. 4-1.
4.2.2 System Description The shielding panel generally consists of a 3/4 in. plywood or particle board panel sandwiched between two sheets of galvanized or zinc-plated steel. The sheet metal is most often 24 or 26 gauge. These are bonded together using adhesives developed to adhere metal to wood. The sheets are fabricated into panels 4 ft wide by 8, 10, and 12 ft in length. The strapping material or hat and flat assemblies are made a variety of ways. Each manufacturer uses the same basic method to achieve the bolted seam. The fabrication
Section 4.2
37
Zinc/Galvanized Steel Modular Shielded Enclosures Floor Seam-Typical
Inside Shielded Room
Wall Seam-Typical
Insi de Sh ield ed Room
Figure 4·2 Clamping unit using captured nuts . techniques vary between manufacturers; thus, the individual parts are not interchangeable. Figs. 4-2, 4-3, and 4-4 illustrate the various techniques used. All achieve the same end, i.e . , they provide a continuous metal-to-metal seal between the panels of a modular shielding system . Since the performance of a given enclosure is dependent on the degree of continuity between the panels and the absence of cracks or breaks between the panels, installation workmanship is critical. Because of this dependence on workmanship, it is difficult to determine if anyone joining system is inherently more reliable than another. To achieve an even clamping pressure, the strap should be on the order of 1/8 in. thick, a minimum of 21/2 in. wide and the screw spacing not more than 4 in. Clamping pressure is supplied by 1/4-20 bolts or screws torqued to a specified amount, thus assuring maximum metal-to-metal continuity and an RF-tight seam. Most clamping systems are designed to permit the room to be totally assembled from the inside and located within an inch or two of existing walls.
Counter Sunk Screws Used For Floor
2 in . Flat Channel
Both Sides
Hat Channel
Plyw ood o r Nov aply
Ceil ing or Floor Section
Figure 4-3 Clamping unit using self-tapping screws.
38
Chapter 4 Str ap
'/.- 20 Screw
Chan nel
Nut
Modular Shielded Enclosures
St rap
Shield ing Panel
Figure 4-4 Clamping unit using floating nuts. Floor panels are the same as the wall and ceiling panels and provide the same RF shielding properties. Floor panels will support a total floor loading of 1200 lb/ft 2 provided the parent building floor is rated to support that loading. The additional loading of the enclosure, which typically runs about 12 lb/ft 2, must be included. The corner assemblies vary, depending on the manufacturer, and the three most common designs are illustrated in Figs. 4-5 - 4-7. Again , very little commonality exists between manufacturers on framing systems; thus, if a shielding kit is ordered, the buyer must coordinate carefully with the vendor to ensure that adequate supplies of all parts are ordered; otherwise, expensive delays in the field can result. Water is the worst enemy of modular shielding systems. A vapor seal must be installed between the enclosure floor panels and the floor of the parent building. The "Users of the enclosure must be warned not to spill liquids on the floor, and care must be taken to prevent water spillage during sprinkler tests because even the smallest amount of water will cause contamination of the clamped seams. Waterproof adhesive used for laminating the steel sheets to structural core material in some shielded systems is not a complete answer to the issue of moisture damage. It is the core material itself that swells with moisture and causes the damage. All manufacturers of these types of
RC Channel
Corner Cap (Cast Bronze)
I
Encl osure
Figure 4·5 Corner unit using bronze cap .
Section 4.2
Zinc/Galvanized Steel Modular Shielded Enclosures
r
3 in. Overlap
39
Corner Clamps I~_ _
L
Inside Clamp
Welded Seam
Outside Clamp - -......... V4-20 Phillips Head Screw
Plywood
Inside Shield Floor Tile
Outside Shield
Masonite
Figure 4-6
Corner unit using welded strapping.
8
R
Figure 4-7 Corner unit formed from shielding panels.
Outside Clamp
40
Chapter 4
Modular Shielded Enclosures
shielded enclosures are aware of this limitation, but no solution has been offered other than installing a pan-type floor; i.e., the floor must either have all seams solder sealed or welded. The weight of plywood/particle board systems is about 12 lb/ft". This may be a problem on the upper floors of office buildings. The other systems described in this handbook should be considered where weight is a problem. Also, when the ceiling is to be hung off the parent building, then the effect of that loading must be evaluated. If the loading exceeds the rated load of the parent building, then the enclosure must be specified to be self-supporting, and will therefore require a steel support structure to carry the weight of the ceiling panels. If the climate where the system is to be installed has continuous high humidity, then the shielding panels cannot be made of particle board, which has a tendency to swell with moisture and destroy the shielding integrity. A very good grade of marine plywood should be used in this type of application. The U.S. Army [I] determined during a long-term stability test of several modular enclosures that their service life is limited without extensive maintenance, especially of the RF doors. Another concern is the floor panels. These tend to work loose under varying loads and the constant stresses caused by people moving about the enclosure. Modular enclosures should be recertified at least every three years.
4.2.3 Typical Performance The galvanized sheet metal modular enclosure, when properly installed, meets most of the current specifications for shielding. It has a moderate amount of magnetic field shielding effectiveness, as illustrated in Fig. 4-8, and generally exceeds 100 dB for electric and plane-wave fields. The actual measured performance of these enclosures is a function of the installation workmanship; therefore, careful selection of a qualified installer is very important. 140 130 120
Cii 110
~
90
~100 93 >101 >130 >130 115 105 110
NSA 65-6 (dB) 20 34 56 74 90 96 70 100 100 100 100 100 100
approach produces good EMI shielding properties for a wide frequency band. The material works well from the kilohertz frequency range up to the microwave frequencies. A single-shield enclosure exceeds the requirements of NSA 65-6 when good construction methods are implemented and properly designed doors and other penetrations are used.
6.5.2 Performance The performance of a Cu-49-Cu alloy 0.012 in. thick is listed in Table 6-1. It is compared to the requirements specified by NSA.65-6. 6.5.3 Material Description The Cu-49-Cu alloy comes in 24 in. wide rolls and in several thicknesses. A common thickness is 0.010 in. A variety of preformed materials is available, as illustrated in Fig. 6-6. These are a help in achieving a high-performance shielded enclosure. 6.5.4 Installation Procedures The installation details are shown in Fig. 6-7. The shielding material should be installed on the floor first. The foil is cut to the length of the floor plus a minimum of 6 in. The latter will be bent and run up the wall to form the lap joint illustrated in Fig. 6-7. The floor is soldered in place using the 2 in. wide solder strips. It is recommended that an 800-1000 W soldering iron with a built-in thermocouple and a temperature controller be used. The iron is set in the 350°C range for optimum soldering operations. A good grade of contact adhesive is recommended to hold the shielding foil in place. The floor should then be covered with some form of protective covering. Plywood held down with a thick construction grade mastic works well. It is important that the shielding foil has no penetrations or holes, but if such exist, they should be patched with the solder tape. When the floor is in place and covered with plywood, the ceiling foil should be installed, again bringing the foil down the wall on all sides by at least 3 in. After the seams are sealed with the 2 in. solder tape, the wall material is
Section 6.6
83
The Sandwich Seam Shielding System
Sn/Pb
::
Cu
Tinned Strip
~
2.0 i n . - . j
Sn/Pb
Two-Dimensional Contoured Corner Strip
Three-Dimensional Corner Closure
"U't-Shaped Structural Material
Figure 6-6 Preformed alloy shielding materials. (Reprinted from ITEM 1988 with permission. © 1988. R & B Enterprises, a Division of ROBAR Industries.) installed. Overlap the material from the floor and ceiling onto the wall, as illustrated in Fig. 6-8. Finally, the doors and other penetrations are installed. This particular material lends itself to building double-isolated shielded enclosures with high performance if the interior shield is properly isolated from the outer shield. One form of this could be drywall" mounted on furring strips so that the inner shield would be well isolated from the outer shield. It is important that the penetrations, such as doors and EMI filters, be of the proper type for double-isolated installations. See Chapters 7 and 8 for the appropriate types of supporting equipment. For other details, see [4].
6.5.5 Procurement Specification Appendix A-6.3 provides a suggested specification for the procurement of copper alloy shielded enclosures.
6.6 THE SANDWICH SEAM SHIELDING SYSTEM 6.6.1 Introduction A shielding system that uses ordinary construction skills and materials was patented in 1988 [5]. This system is known as the sandwich seam shielding system since the seams of sheet metal are sandwiched between two layers of sheet metal. This seam design is
Gypsum Wall or Plywood
Shielding Foil
Floor
Solder
Figure 6-7 Installation detail for laminated foil. (Reprinted from ITEM 1988 with permission . © 1988. R & B Enterprises, a Division of ROBAR Industries.)
Wall
.... co
::
~ 0
" , .; ',
... , . ' ~.
..
I
f
12 in .
:::.
2 in.
-i IFloor
Figure 6·8
84
Ceiling/waIVfloor overlap .
Section 6.6
85
The Sandwich Seam Shielding System
- - - - 6 in. j-
CIearance
I
- --
-
_
oe
I
,
I
0
I I
II' I ! I
II;
I
I : I
I
i I
I: !
l
I
---r-.
0 --
I II I, I I
0- -- --,
0
_J __
J
r------ -
I
I
I
I
I
!
I
I
I
I
I
I I I
I I I
1.25 in.
0
I
7'
,i )
l
l
l
Figure 6-9 Sandwic h seam.
illustrated in Fig. 6-9. Thi s technique of electromagnetic sealing is based on the waveguide-beyond -cutoff princ iple . Tha t is, the width of the seam and the screw pattern form gaps with length (waveguides) , thus cutting off the electromagnetic energy below a given frequency. This cutoff performance is illustrated in Fig. 6-10 and demonstrates the calculated versus measured perfo rmance of a commonly constructed version of the sandw ich seam . Over a dozen of these sys tems have been constructed to date by a variety of sheet meta l installers without any seam failures reported . The pr incipal use has been for TEMPEST facilities where NSA 65-6 performance was required up to 400 MHz . These were achieved with a single shield of 26 gauge galvanized sheet metal mounted over plywood. High-performance doors and EMI filters were used to ensure that full performance was achieved by the total enclo sure . The system is cost effective and lends itself to common construct ion method s. As is common with all sing le-sh ield system s, care must be taken to ensure that the shielding effect iveness is not compromised by improper pene trat ions. Sect ion 6.2 discusses the limitations of single-shield systems. The sandwich seam is compatible with any type of shielding material, and enclo sures can be constructed to meet a varie ty of shielding requ irement s. The most
86
Chapter 6
Architectural Shielding
150 140 0- - - - -0 Measured 130
- - - - Calculated
120!~...... 110 Cii'
.s
~,
6 in. Seam Width 1'--- ...... _
~
100 90
2.5 in. Screw Spacing ~,-
~ ~~
50 >50 >50 >50 >55 >60 >40 Flammable
>50 >50 >50 >50 >50 >55 >55 >50 Will burn, but with some difficulty
>50 >50 >50 >50 >50 >55 >60 >40 Flammable
This level of performance is consistent with the requirements of most TEMPEST applications. No magnetic field shielding effectiveness has been reported.
6.8.4 Woven Shielding Materials A large number of metallized cloth materials is also available for constructing low performance shielded enclosures. Tables 6-4 and 6-5 list a variety of these materials with their respective properties. The most common metal used in metallized fabrics is nickel. The shielding effectiveness data were acquired using coaxial or waveguide transmission test fixtures. No magnetic field data have been reported.
6.8.5 Conductive Copper Paint A number of conductive paints is available on the market, all of which claim to be an economical method of obtaining moderate shielding. As with the case of any shielding it is not the barrier material that sets the level of shielding actually achieved, rather it is the methods used to control the penetrations in conjunction with the shield. In the case of painted systems it is very important that all penetrations have some form of metal flashing around the penetration in which the pipe/venUfilter is mounted and then the paint is applied over the flashing so that a continuous film of material is present from the penetration to the painted shield surface. This is especially important around door frames and very important at the door threshold.
94
Chapter 6
TABLE 6-5
Architectural Shielding
PROPERTIES OF WOVEN MATERIALS, MONOFILAMENTS Property
Substrate Construction Metal Nominal Thread Diameter (JLm) Nominal Mesh Opening (JLm) Nominal Mesh Count/inch Nominal Open Area (%) Weights a. Substrate (g/rrr') b. Metal (g/rn'') c. Finished Material (oz/yd ') Width (inches) Thickness (urn) dc Resistance (rnfl/sq) Shielding Effectiveness Frequency (dB) 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz I GHz 10 GHz 40 GHz Flammability
Type of Cloth Polyester Plain Weave Nickel
Polyester Plain Weave Nickel
Polyester Twill Weave Nickel
103
41
42
307
41
21
61
305
420
58
27
9.5
90 20-30 3.24-3.54 40-59 225
35 30-40 1.92-2.21 40-59 62
45 40-50 2.51-2.80 40-59 73
120-160
60-100
40-80
>50 >50 >50 >50 >50 >50 >35 >20 Flammable
>55 >55 >55 >55 >55 >60 >65 >50 Flammable
>55 >55 >55 >55 >55 >70 >80 >80 (26 GHz) Flammable
One form of copper conductive paint is reported to have a shielding effectiveness in excess of 60 dB through 1000 MHz. It consists of a water-based resin system.with a copper filler. It is formulated to maintain its conductivity and has a surface resistivity of O. 10 fl/ft 2 for a 2-mil coating.
6.9 STRUCTURAL CONSIDERATIONS OF ARCHITECTURAL SHIELDING 6.9.1 Introduction Most architectural shielding employs foil installed onto an existing structure. Careful methods are required to protect the shielding from accidental or intentional penetration.
6.9.2 Shielding of Existing Structures Figure 6-17 illustrates various methods of mounting architectural shielding on existing exterior walls. The key to maintaining the shielding is to minimize the penetrations necessary to hold the shielding in place. For foil shielding, a good grade of adhesive is
Section 6.9
Structural Considerations of Architectural Shielding
•.:*-
-
11-'1--
-
- - Masonry Outer Wall
-
-
. --
-
-
95
Moisture Barrier
-
Dryw all Att ach ed to Out er W all
1-- - - Shield ing Foil (Coppe r/A luminu m) Steel Studd in g (Sup ported Top /Bott om Only ) Dryw all
+- - - - a..-- - - . --
-
-..-
-
-
Outer Mas onry W all Marin e Plywo od Sheet M etal Shield ing (Sandwi ch Seam )
- - Dryw all
t:1:::+---- RC Channel
Figure 6-17 Installing architectural shielding on exterior walls . best since no penetrations are required. Figure 6-18 shows methods of mounting the shielding material on interior walls. such as stud walls of various kinds . To ensure that the integrity of a single-shield system, especially a lightweight material such as foil. is maintained during use, it is necessary to use special techniques for installing interior walls and decorative treatments inside an architectural shielded enclo-
96
Chapter 6
.If-Jv - - - - - -
o
~----
Architectural Shielding
Exte rior Drywall Steel Stud
~---- ';' in . Plywood/Drywall
..-
o
Shielding -Foil/Drywall Sheet Metal / Plywood
o fr .;
RC Channel
1'- ";, ::f - -
·0
Drywa ll
~ .:
rJ :. /
Figure 6-18 Installing architectural shielding on interior walls. sure . For discussion purposes, a copper foil system is assumed , which has been installed over a drywall interior wall/ceiling, and Y4 in. plywood has been installed with an industrial grade mastic adhesive over the floor shielding. This is illustrated in Fig. 6-19.
6.9.3 Decorative Treatment of Interior Walls In an installation as described above , Y4 in. wood furring is adhered to the ceiling around the perimeter of the room and across the ceiling where interior partitions are to be installed . Metal studding track is then screwed down to the floor and ceiling using S/s in. drywall screws, and metal or wood studs are then installed . Electrical, alarm, and telephone wiring is then installed, and the interior walls are finished out in the usual manner. The important advantage of this design is that no nails or screws will penetrate the shielding. If penetrations must be made into the shielding, a metal-to-metal seal must be maintained since if a screw or a nail passes through the shield without the
Plywood Floor
Wood Furring IHeid by Construction Adhesive)
~~~:r
Seam Copper Folded
Shield ing Foil
.l IVt?J~ / Tile
/
Plywood
/
/ ; Me;"",, I / Barrier
f f ( :)i:j/ .:;: ).·;:·:":.·;.:..::..:.;:..:. .~...:.:.:.:".:;::: : .~. :..':.;.:"'::...... ~. : ·: :\·.·:;~:::·:c.~.~c~~ie·;: Figure 6-19 Example of a copper foil floor installation.
Section 6.10
97
References Ceil ing
Lead
Lead Steel Washer
Shield CAD Plated Drywall Screw
Concrete
Figure 6·20 Interior wall treatment in a sheet meta l shielded enclosure .
metal-to-metal seal, it becomes an " antenna," and will couple energy from the inside to the outside or vice versa . In galvanized sheet metal facilit ies, such as the sandwich seam system, the meth ods shown in Fig . 6-20 have been used success fully. Here, the " antenna effect" is killed by using the soft lead to provide a metal-to-metal seal wherever a screw is used to penetrate the shielding. In these installations. it is important that the screw be securely fastened, and that the metal under the screw be under compression . If the screw spins out, then a larger screw must be used or the hole patched and the fastener moved to another location .
6.10 REFERENCES [I) NSA 73-2A, "National Security Agency specification for aluminum foil shielded enclosures," Nov. 15, 1972. (2) S. C. Jewell , "Aluminum foil RF shielding systems." ITEM. pp. 64-71.1988 . [3] G . Trenkler and L. McBride, " Composite metal shields for electromagnetic interference," ITEM. pp. 242-246, 1987. (4) G . Trenkler and R. Delagi, "The application of clad metals for EMI room shielding," ITEM . pp. 222-226, 1988. [5] "Shield system and sandwich seam for attenuation of electromagnetic energy ," U.S . Patent 4 733 013 issued Mar. 22, 1988. (6) Tech . Rep. 87.6.4., Ark Electronics, Inc., " Shielding effectiveness measurements of electromagnetic shielded enclosure." [7] Test Rep. ERC Corp., "Shielded enclo sure performance test report for Rantron systems," Sept. 1987.
CHAPTER 7
Penetrations and Their Control
7.1 INTRODUCTION A six-sided metal room is not very practical. Many penetrations must be made to develop it into a useful working shielded enclosure. Doors, heating and air conditioning ducts, piping, fiber optics, electrical power, telephone lines, and other services must be provided. Each of these have special design requirements, and must be treated with as much care in the design phase of the project as the overall enclosure. Figure 7-1 illustrates a variety of penetrations. First, criteria will be discussed for effective RF-tight penetrations, and then a thorough treatment will be given on each type of penetration commonly encountered. Guidance will be provided for mounting the various penetrations in welded, modular, and architectural shielded enclosures.
7.2 GENERAL DESIGN CRITERIA FOR PENETRATION As discussed in the "theory" chapter, there are two basic methods of obtaining shielding effectiveness. The first is to have a continuous barrier of the right material and thickness. The second is to have any penetrations in the shield be small in cross section (in terms of wavelength) and have a gap/length ratio greater than 7. Degradations to a shield include breaks in the continuity of the shielding material, changes in the composition (homogeneity) of the material, corrosion to mating members, and lack of metalto-metal contact of mating materials. Each of these problems behaves differently, depending on the type of electromagnetic fields present and the frequency of operation. All of these concerns must be considered when designing or selecting a given penetration for a shielded enclosure. When making penetrations through a welded enclosure, a good practice is to provide the penetration with a sufficiently large flange around the device so that the heat of the welding does not distort the shop-made welds. Also, adequate support structure
99
Chapter 7
100
Penetrations and Their Control
Antenna Honeycomb Vent
Welded Seam
Shielded Lead-In
Shielded Lines
Gasketed Seam (Service Panels)
I ~
~
Fingerstock Seam (Doors) Filtered Lines
Insulated Water Pipe
Metallic Waveguide
Clamped Seam
Coaxial Line
Figure 7-1 Types of penetrations.
be provided around the location of the penetration to maintain the physical integrity of the system. Penetrations through modular shielding must be made so that a metal-to-metal seal is obtained on both sides of the shielding panel. For connector panels, vents, and other penetrations of large size, it has been found best to use frames made from the hat and flat material used in the panel framing. These should be formed into picture frames using welded corners. These frames must be flat and square. Single-shield enclosures require additional care since the usually thin materials, foil or sheet metal, will not physically support the penetrations. Therefore, it is essential that additional support structure be supplied for each type of penetration. Large flanges must be provided to ensure that a low-impedance junction is provided between the shield and the penetration. The following sections discuss doors, heating and air conditioning, piping, and fiber optics. Chapter 8 discusses electromagnetic filters. Blust
7.3 DOORS 7.3.1 Introduction The radio frequency shielded door is the most important of the penetrations in a shielded enclosure. It is generally the weakest link in the system and the most difficult
Section 7.3
Doors
101
to maintain due to its high usage. It is a precision device, and must be designed and manufactured to maintain its mechanical and electrical functions under the most demanding circumstances that may be encountered in the installation. High shielding effectiveness can only be maintained if the structure of the door and frame is strong enough to maintain the tight tolerances needed to achieve the RF seal around the perimeter of the opening. Various techniques and methods of construction are used in the manufacture of doors, depending on the shielding specifications, door use, size, and environment. Each of the most common designs is described with their advantages and disadvantages. One of the most difficult requirements for an RF door is that of handicap access. Various solutions to meeting this requirement are incorporated, but no one solution has been completely satisfactory in meeting the full NSA 65-6 performance requirements. Generally, the performance is limited to the requirements of MIL-STD-285 in that the magnetic field shielding effectiveness is lower and the plane-wave performance is limited up to 400 MHz. The various designs currently available will be illustrated. In high-traffic areas, such as shielded buildings or computer centers, it is necessary to provide a vestibule with double doors or a waveguide or sinuous tunnel entrance. These are described in Section 7.3.4.
7.3.2 The Important Features of an RF Door A metal-to-metal seal must be obtained around the perimeter of the door opening. This seal must be maintained under thousands of door operations with a minimum of service. Experience has shown that it is necessary that the RF seal consist of two rows of contacts when the plane-wave performance must exceed 60 dB. The three basic highperformance RF seal configurations are illustrated in Figs. 7-2 and 7-3. The first configuration is based on compressing two rows of RF gasket material between the door leaf and the door frame. It requires a great deal of clamping force to achieve the seal, and it is commonly achieved using a rack and pinion closure system at three points opposite the hinge side. The hinges are designed to provide the necessary closing force as the door is latched. The second form of RF seal is also illustrated, and it is called the ReM or recessed contact mechanism or knife-edge door seal. It is now the most common form used because it is the most reliable and the easiest to maintain. The third version of a door seal is illustrated in Fig. 7-3. The advantage of this version is that a low threshold is possible for certain types of installations. Another version of the compression door is illustrated in Fig. 7-4. Here, a metal-to-metal seal is obtained by air pressure holding the door leaf in contact with the door frame. This form of door is very useful for welded enclosures requiring very good low-frequency magnetic shielding effectiveness. Figure 7-5 illustrates door seals that are commonly used for low-performance (60 dB) doors. These doors are commonly found in high-use areas in vestibules (two-door systems) where the two-door combination can approach the performance of the single high-performance door. All of these configurations meet the following electromagnetic requirements. First, metal-to-metal contact is maintained around the perimeter of the door. Properly designed and constructed, the electrical contact is continuous and provides minimum distortion to the current paths in the wall of the shielded enclosure. This is necessary in order to maintain a good magnetic seal. No unsealed gaps are permitted through the
102
Chapter 7
Penetrations and Their Control
Fingerstock Gasket Under Cornpression
Door Leaf (a)
Recessed Contact Mechanism (RCM) Fingerstock RF Gasket
----~e ~---
Knife Edge on Door Leaf
(b)
Figure 7-2
Door seals: (a) compression; (b) ReM.
Section Through Door and Jamb
Figure 7-3 Wedge RF seal.
Section 7.3
Doors
103
Welded Door Frame
Metal-to-Metal Seal
~---
,..L..-.;::::"_~~~'----';~-""'-....;:=II~---.;:lIC..-_~-~-~~~"-"_""'--a,
Air Bladder
Sliding Door
Figure 7-4 Sliding door compression seal. Holder RF Gasket
Metal Clad Door ' - - - - - - - - - Metal Door Jamb - - - - - - - - '
___ Door
Fingerstock
........
.....-...:~
Figure 7-5
~~""""'-'-
....~-- Brass Threshold
Common low-performance door seals.
door or the frame, such as the bolts used to mount the hinges, where microwave energy could leak. Another important design consideration is how the door handle is designed. Since a shaft must penetrate the door leaf, it must be designed so that the shaft is grounded so that it does not become an antenna. It is also important that the door frame tie well into the surrounding shielding. This is especially important for single-shield systems. Special care must be taken for foil systems since any strain placed on the foil near a door opening could fracture the foil and degrade the shielding effectiveness. The most common RF doors are next described with their advantages and disadvantages. Procurement specifications for each of the types described are given in Appendix A-7. The most commonly specified door is the ReM design.
7.3.3 The ReM or Knife-Edge Door The simplest form of the ReM door is illustrated in Fig. 7-6. It consists of a shielded panel (particle board with 24 gauge steel laminated to both sides) framed with an ex-
104
Chapter 7
Penetrations and Their Control Hinge
Handle in Closed Position
Surface Latch (Two Point)
Door Elevation Door in RF Shielded and Closed Position
Gasket
, ,, \ , , ,\ I
Door Frame
\ I
Removable Contact Fingers
r-- L
'1-\' \ \
"
\ \\ \\
t \\
---
\\
f\
\ \\
~~~~
- - - - -
\\
, ..1 L_l
L---r----
--- ---- .... ---
t.------
Figure 7-6 Basic ReM door geometry.
truded phosphor bronze knife geometry and mated with a pocket in the door frame containing the beryllium fingerstock RF gasket. One of these extrusions is soldered to the perimeter of the door leaf, and the other to the inside of the door opening made in a shielding panel. The door assembly is mounted in the enclosure wall using the same hat and flat assemblies used for the remainder of the enclosure. Each manufacturer has a variation of the ReM design, with the method of attaching the fingerstock being the largest difference. Several of these are illustrated in Fig. 7-7. This is the only patentable feature since the ReM concept is now part of the public domain. Several companies have introduced the double-knife-edge concept illustrated in Fig. 7-8. The double knife edge is primarily useful for very high performance,
Doors
Section 7.3
105
Fingerstock
Sheet Metal ~-+-+---
Fingerstock Fingerstock
Brass Extrusion
Fingerstock 2-56 Screw Sheet Metal Bronze Extrusion Fingerstock
Figure 7-7 Other forms of ReM door seals. i.e., 120 dB requirements, and is generally very heavy, on the order of 700 lb. Most of these types of doors are used on welded facilities. The ReM door has several advantages, the best of which is that the RF seal is recessed and protected from wear and tear because the fingerstock is mounted perpendicular to the door leaf, the threshold height is minimized, and the wiping action of the knife against the fingerstock helps keep the contact resistance at a minimum. The disadvantage is that the construction tolerances are very tight, and the door must be realigned regularly or the knife edge soon destroys or cuts up the RF gaskets in the ReM pocket. Weekly maintenance is usually required to keep the doors up to full RF perfor-
106
Chapter 7
Penetrations and Their Control
---------------------, 0 0 ___ ___ _ _ _0 _ _ _ _---i 0
--------------------,
o
o
Figure 7-8 Double knife-edge door seal.
Door Leaf
Bottom Plate Finger sto ck
Brass Th resho ld
Figure 7-9 Low-threshold geometry for ReM door.
Section 7.3
107
Doors
mance. An obvious disadvantage is that of handicap access. A lip must exist at the floor level to accommodate the ReM pocket. The most common solution is a poor compromise, as shown in Fig. 7-9. This design is subject to dirt, high maintenance, and poor magnetic and microwave shielding effectiveness. Some suppliers have a ramp option, which automatically positions a short ramp up to the lip of the door as the door is opened, rather than compromise the performance of the door. Refer to Appendix A-7.1 for suggested procurement specifications for the RCMtype shielded door.
7.3.4 The Compression Door A variety of compression doors is commercially available. The most common variety is a door leaf which extends around the perimeter of the door opening by about 3 in. The door frame is designed to be very flat, and two rows of fingerstock are mounted to the face of the door leaf. The door leaf is mounted on three or four very heavy hinges, and the door is closed using a very heavy rack and pinion closing mechanism that clamps the door leaf against the door frame. Very high closing forces are required to achieve the necessary RF seal, especially at the microwave frequencies. The advantage of the door is its simplicity. Its disadvantages are the height of the doorsill, somewhat like a ship's hatch, the exposed fingerstock which catches on clothing and breaks off, and the weight of the door and frame which must be very strong to achieve the necessary closing forces. A better version of the compression door is illustrated in Fig. 7-10. Here, the fingerstock is recessed in a channel on the frame of the door, protecting it from accidental damage common to the standard compression door. The door leaf and frame must be precision made so that proper compression of the fingerstock is achieved. The third form of a door seal is a variation of the compression door, where the fingerstock is mounted on the edge of the door as illustrated in Fig. 7-11. This door must be thick to provide the mounting surface for the fingerstock, and it is subject to the same limitations as the standard compression door. The door frame is designed to mate with the door leaf, which must fit very closely to achieve the tight RF seal around the perimeter of the door leaf. Several versions of this door are made: a highperformance version for NSA65-6 operation, a very high-performance door (doubleisolated system; see the example of procurement specifications for details), and a moderate-performance type with a low threshold for hospitals. The door is made of a honeycomb construction, which makes it lightweight, and it uses a three-point closure
Door Frame
Door Leaf
Figure 7-10 Improved compression RF door seal.
108
Chapter 7
Penetrations and Their Control
/-:-'. --L. •• •
figure 7-11
Wedge-type RF door seal.
system which reliably pulls the door leaf into contact with the door frame . A version of this door is used in MRI installations which do not have the high-performance requirements of the other installations. This door geometry is illustrated in Fig. 7-12; note the low threshold , which is suitable for hospital gurneys. These doors also come in very attractive finishes suitable for the hospital environment. A fourth form of the compression door is the sliding pneumatic door, which is illustrated in Fig. 7-13. The RF seal is obtained either by the air pressure expanding the thickness of the door leaf, making contact with the door frame, or the door frame expanding, providing a metal-to-metal seal with the door leaf. This door configuration is only suitable for welded installations since the door and frame are extremely massive in order to attain the extreme forces required for this type of RF seal to function prop-
x
o
ac.
< .S
/ .~
Grounded Threshold
figure 7-12
Low-threshold medical RF door.
-. .:. '.
. '
Section 7.3
109
Doors
Track
Ground Steel Box Beam Shield
Track Metal-toMetal RF Seal
Air Bladder Shield
Track
Section A-A
Figure 7-13A Sliding compression RF door.
erly. The disadvantage of this type of construction is its cost, on the order of six times that of the ordinary ReM door. The advantage is that it is an extremely highperformance door system, capable of very high magnetic shielding, very low in frequency, and also of very good performance, well up into the millimeter frequencies. Two versions are available: the fully automatic door which operates by air pressure at the push of a button, or the manual door which must be slid manually (very difficult in most instances; the doors are extremely heavy) and then sealed by pushing a button. A back-up air cylinder is recommended in case a factory power failure shuts down the building air system. This is recommended for critical installations where the shielding must be maintained, even under emergency conditions. It does not meet the fire codes since they require a breakout hinged door. A swing-out version of the pneumatic door is
110
Chapter 7
Penetrations and Their Control
Electric Field
Plane Wave
110 100
90 80 70
60 Attenuation in Decibels
50 40
30
20
,/
I
/
I
Magnetic Field
4
10
a 1
kHz
10
100
1
10
100 400
MHz
1
10
GHz
Frequency Guaranteed Performance When Tested To NSA 65-6 Specifications.
Electromagnetic Door Frame Contact Area Trim Plate
Door Leaf
Door Frame Steel Tubing Stiffener
V2 in Square Tube Spacer
3/4
Door Frame V4 in Thick Steel Plate
in Shielded Panel
Thin Gauge Steel (One or Two Sides as Applicable)
Figure 7-138 Electromagnet compression RF door.
now available, suitable for fire exits. Sample specifications are given in Appendix A-7 for both versions. A form of the compression door is those that are required to be compatible with the double-isolated modular shielding systems described in Chapter 5. A procurement specification for this type of door is described in Appendix A-7, paragraph 7.3.
Section 7.3
Doors
III
Another form of compression door is the permanent magnet or electromagnetoperated shielded door. Various forms of the permanent magnet type have been available for several years; generally they are only useful for moderate shielding effectiveness installations. A high performance electromagnet-operated door [I] is now available that meets the full NSA 65-6 performance requirements; that door is specified in paragraph 7.4 of Appendix A-7. The advantage of this door is that it does not have any fingerstock that requires servicing and the door is rated for 500,000 operations with minimum service, making it useful for high use installations. The door operates with fingertip control and comes standard with push bar exit operation. A slightly lower performance version that meets Title 24 requirements for handicapped operation is also available. The geometry and performance of this door is given in Fig. 7-13B.
7.3.5 Moderate-Performance RF Doors For moderate-performance shielded enclosures, such as those designed to meet NSA 73-2A or its equivalent, doors are available that have been adapted from a good grade of metal industrial door. Figure 7-14 illustrates how these doors are made to meet the required shielding effectiveness. Performance on the order of 60 dB is advertised. It is
very important that galvanized plated steel be used for both the door frame and the door leaf. With care, these can be painted after installation. To achieve the stated performance, the doors must be very carefully connected into the surrounding shielded wall, especially at the floor; otherwise, the desired performance will not be achieved. A metal-to-metal seal between the door frame and the door leaf is essential. The frame must also have a metal-to-metal seal from the frame to the surrounding walls and floor. A sample procurement specification for moderate-performance shielded doors is given in Appendix A7, paragraph A7.5.
7.3.6 Vestibule and Waveguide Tunnel Entrances 7.3.6.1 Introduction. In TEMPESTED facilities where high traffic is expected, a means must be provided to overcome the fact that the doors must be opened extensively, with the risk of compromising the shielding effectiveness of the facility. This is accomplished in two ways. The simplest is to provide a vestibule with two doors. The distance between the doors should be such that it will hold a number of people, and the doors must be interlocked so that only one door is open at a time. Because of the high traffic use, it is desirable that these doors operate easily and not be susceptible to damage. Criteria are given and doors are described that have been used in these types of installations. Another form of high-traffic entrance is the waveguide-beyond-cutoff or sinuous entrance. These are especially useful when an entire building is a shielded enclosure, as it saves on the number of doors and their subsequent maintenance. 7.3.6.2 Double-Door Vestibule. The principal TEMPEST threat due to an opened door is in the plane-wave region; therefore, the vestibule system must provide adequate protection from 50 MHz up in frequency. Because the common shielded door is subject to very high maintenance with use, i.e., the fingerstock of all standard commercial doors is subject to damage in high-use areas, they do not perform well in highuse applications. In addition, where the people count is high, wheelchair access is generally essential; thus, a low threshold door is required. Fortunately, most of the high-use areas are for TEMPEST, and in the continental United States, the shielding
112
Chapter 7
~-----
3 ft
Penetrations and Their Control
------41-..
A
A
7ft
Metal Door
8
8 RF Gasket
Section A-A Ball Bearing Hinges
k:;
".-_______
4110.-/
F GaSket Brass Threshold
~
Section B-B
Figure 7-14 Moderate-performance shielded door.
effectiveness requirements are usually specified to meet NSA 73-2A, which calls for a moderate level of shielding. A conservative level of 60 dB-l GHz is acceptable, which means that the doors discussed in Section 4.3.3.4 are usable. The double-door arrangement is illustrated in Fig. 7-15. The doors are provided with electric locks which are
Section 7.3
113
Doors
! RF Door SW Outside Shield
~
I I I
x· -
Fire Door With Controlled Access
Safety Mat (Required)
"~::-::=======:::::J Shield Control Mat (Option to Door Switch)
RF Door SW Notes: 1. Doors are electrically interlocked, with only one open at a time. 12 ft
Safety Mat (Required)
Shield RF Door Switch Inside Shield
Figure 7-15 Vestibule entrance.
interlocked so that one of the doors must be in the closed position before the other is opened. It is recommended that the distance between the doors be a minimum of 6 ft for low-traffic enclosures and 12 ft for high-traffic facilities. The minimum width should be on the order of 4 flo For high-performance, high-use facilities, it is recommended that the sliding air-operated door be used in the vestibule system if the fire code will permit its use. If not, it is common to add a fire door at another location in the facility to meet the code requirements, thus maintaining the high level of performance without serious degradation due to high usage normally experienced by the conventional RCMtype RF door.
7.3.6.3 Waveguide Tunnel Entryway. Inspections of actual electromagnetic shielded facilities reveal that entryways are the most commonly abused elements of shielding. Because of heavy traffic, normally closed doors are found broken or blocked in the open position and interlocks are subverted. Waveguide entryways constitute an
114
Chapter 7
Penetrations and Their Control
Notes: 1. Labyrinth Footprint Approximately 32 x 42 h. 2. Mean Path Through Labyrinth A to 8 = 58 ft.
10 Gauge Welded Steel Shield Plane \
10 in'.
Microwave Absorbers
20 h, 6 in.
t
B
-~-
: - 11 ft,10in. 8tt,Oin.1
....
I
20 tt
I
l=ijilllllllll"iIiIii:'~::I~j::j:'::j::::::'iIIi"'ii"'ii"iir
10 Gauge Welded Steel Shield Plane
Figure 7-16 Waveguide-beyond-cutoff entrance. (Reprinted from ITEM 1989 with permission. © 1989. R & B Enterprises, a Division of ROBAR Industries.)
alternate and possibly better approach, implementing a passively failsafe penetration protection method to the maximum extent possible. They permit the use of lower quality (60 dB nominal) and faster acting doors which can be more easily maintained. A waveguide entryway is achieved when the entryway shield walls are configured to form a hollow tube. The height and width of the waveguide should be as small as possible, and the length should be made as long as possible. The height should be the minimum value which will permit personnel to comfortably stand within the guide, about 86 in. The width must allow side-by-side passage, but should be about half the height. The length along the shortest path must be at least five times the height.
A new form of waveguide entry is the sinuous path entrance, which is a waveguide entry that is lined with absorbing material and has no doors. This form of entry is
Section 7.3
Doors
us
illustrated in Fig. 7-16. The design of these entrances is still in the formulative stages, and the designer is referred to the current literature such as that given in [2].
7.3.7 Special-Purpose Shielded Door Systems A variety of special-purpose RF shielded doors has been developed and is available on a special-order basis. These include automated doors, semi-automatic doors, doors with permanent magnets to seal the doors, doors with electromagnet RF seals, and special large doors for a variety of electromagnetic test chambers. The semi-automatic doors are equipped with an automatic unlatching device which aids operation in high-traffic areas. This door is illustrated in Fig. 7-17. The automatic version of this door is outfitted with operating and safety mats, as illustrated in Fig. 7-18. A fire rating of shielded doors is very difficul t to obtain. While a few suppliers can provide 45 min doors, only a small number of configurations have been tested and rated by Factory Mutual. A number of special-purpose RF doors use permanent magnets to hold the door closed with metal-to-metal seals. No known systems are fully qualified to NSA 65-6 performance requirements. A door using electromagnets is available, and it is being used in vestibule applications where semi-automatic operation is required. Large doors are usually sealed with pneumatic seals, as illustrated in Fig. 7-19. A double row of fingerstock is mounted around the perimeter of the door leaf, and it is compressed against the door frame by means of air pressure when the door is in the closed position.
7.4 HEATING AND AIR CONDITIONING 7.4.1 Introduction One of the major penetrations that must be considered is how to handle the need for conditioned air within a shielded enclosure. Small rooms generally are tied to the existing building air for supply and return, or they are just vented to the room in which it is placed. The penetration used for ductwork has several names; among them are waveguide vent, honeycomb vent, or shielded vent. The first name is related to how the device behaves electromagnetically, the second to its appearance, and the final to the fact that it maintains the integrity of the shield while permitting the flow of air in/out of the enclosure. In larger enclosures, conditioned air may be by ductwork, but it is most often accomplished using heat pumps or air handlers which are controlled by an overall heating/cooling system from the main building. The general piping guidelines are used for these types of penetrations.
7.4.2 Description and Theory of Operation Honeycomb vents are structures which appear as shown in Fig. 7-20. Equation (3-9) expresses the shielding effectiveness of these types of penetrations. With an inch of thickness over 100 dB attenuation is achieved up to 30 GHz with a 3/16 in. cell size. The most common form of construction is honeycomb made from steel or brass and then hot solder plated. Panels up to 2 x 3 ft are available. The most important
Door Closer
-HH-+t-t-----.~
Door Door Frame Emergency Release
Unlatch and Stop Buttons
latch Unit
Adjustable Hinges
Figure 7-17 Semi-automatic ReM RF door.
Overhead Opener Emergency Release
Door
Door Frame--_--.u..-
Adjustable Hinges
latch Unit
Key Switch (Hold Open) Opening Mat
Opening Mat
Figure 7-18
Automatic ReM RF door.
116
Section 7.4
Heating and Air Conditioning
117
aspect of these vents is how they are mounted in the wall or ceiling of the enclosure. In modular enclosures, it is common to solder the honeycomb directly into the galvanized steel shielding panel. These panels must be preengineered so that they get placed in the right location of the shielded enclosure. In welded enclosures, the honeycomb is factory mounted into a frame, which is then welded in the field in a rough-cut opening in the shield. The frame must be designed so that field welding will not damage the vent material. For single-shield systems such as in architectural shielding, the vents are mounted in clamp-up frames, which are then bolted into place at the job site. Some preparation of the rough opening must be done to ensure that a proper RF seal is achieved around the perimeter of the vent frame. Also, provision must be made to support the weight of the vent since the single-shield system usually is not capable of providing support. Three different forms of vents are available, depending on the type of shielding to be installed. The most common is steel honeycomb with 3/16 or 1/8 in. cells, 1 in. thick. This is either soldered into a shielding panel or mounted in a frame. For 60 dB requirements, the frame can be screwed into the shielded wall using gasketing, but for higher performance enclosures, a clamping arrangement such as the hat and flat is recommended. For welded enclosures, the frame should be of steel and designed to be field welded into place in a rough opening cut out of the enclosure; a large overlap is recommended. The vent frame must be plated prior to installation. The second most common vent material is made of brass. These are recommended for NMR installations where nonmagnetic materials are usually used. They are also recommended in applications where the humidity is high; experience has shown that the steel types will rust out in the presence of high levels of moisture. These vents provide excellent electric and plane-wave shielding, but the magnetic shielding effectiveness is lower than the steel versions. For very high-performance installations (120 dB), a composite version of the brass and steel is recommended. This consists of a sandwich of the two types of honeycomb mounted in the same frame. The brass version should be located on the exterior side of the frame. In the doubly isolated enclosure, two frames are used, isolated from each other. Provisions shall be made to attach ductwork to the vent, and this normally takes the form of a flange located around the perimeter of the clear opening. The size of the vents is usually selected to be 15% larger than the size of the ducts to reduce the blockage provided by the honeycomb cells. For more precise sizing, the pressure drop data given in Table 7-1 and Fig. 7-21 should be considered in the design of these devices. In those installations requiring electrical isolation, a dielectric break in the form of an insulated sleeve is provided by the mechanical contractor.
7.4.3. Performance The vent performance shall be the same as that of the shielded enclosure. If lowfrequency magnetic field requirements are necessary, then the vent must be fabricated from steel. The typical performance of these devices is shown in Table 7-2.
7.4.4 Procurement Specification A sample procurement specification is given In Appendix A-7, paragraph 7.6.
Chapter 7
118
Penetrations and Their Control
Brass
Air Bladder
Fingerstock Gasket
Door Leaf
---...,tt---::J.....
Air Bladder
----' f.-l.25in.-.j '----Section A-A
Figure 7-19 Large door RF seals .
7.5 PIPING 7.5.1 Introduction Pipe penetrations in shielded enclosures must be designed so that a metal-to-metal seal is achieved at the point of penetration. The size of the piping and how it is terminated inside the enclosure is critical to maintaining the overall shielding effectiveness of the enclosure. The three different types of shielded enclosures, welded, modular, and architectural, require different forms of the penetrations. Electromagnetic energy will pass through a pipe that is large in terms of wavelength , and will provide leakage paths into a shielded enclosure unless the proper design procedures are not invoked . If the piping is continuous and completely sealed, such as a gas or water pipe inside a shielded enclosure, then the piping need only be metalto-metal sealed at the wall entrance to the shielded enclosure, regardless of pipe size . In
119
Piping
Section 7.5
Duct Work
~
__
by Others Canvas Connector and Collar When Required
l~
Vent Size
Equal to Scheduled Duct Size
CLR Opening (See Penetration Schedule for Size)
Duct by Mech. 1in. Thick Tin-Plated Brass Honeycomb Air Vent with Va in. Cells, Size Same as CLR Opening. Solder to Collar
11 Gauge Hot-Rolled Steel Bent Collar Reference
Symmetrical ~ of Duct
_
No Plating on this Flange (Either Side) RF
_----a...~_ _--+"\.)
Va Inside Shielding RF Shield Outside Shielding
va Note: Collar corners to be mitered & cont. RF Welded.
See
L:R
= 0.08
11 Gauge Hot-Rolled Bent Collar Where Required for Duct-Work Dielectric Flex Connection 6 in. Minimum Length by Mechanic
RF Air Vent Installation Detail
Figure 7-20 (a), (b) Honeycomb shielded vent. very high-performance enclosures, it may require that all the pipe joints be welded or solder sealed rather than using plumbers' dope or teflon tape in the pipe joints since the normal method of sealing the pipe does not provide a high-performance RF seal. Where the piping is not terminated in a sealed system within the shielded enclosure, then the size of the pipe must be limited to a maximum size, as illustrated in Fig. 7-22. If the pipe size cannot be reduced, then a honeycomb union must be placed in
120
Chapter 7 TABLE 7-1
Penetrations and Their Control
HONEYCOMB VENT STATIC PRESSURE DROP
Inches of water: F/min: (Multiply by area to obtain CFM.)
0.015
0.025
400
600
0.042 800
0.065
0.30
1000
2000
.06 .05 Q.
0OS .04 ~
~
Q)C'O
:;~
(/)'t-
(/)0
~ ~
.03
Q...r:.
.~
g
--
(0(J)
.02 .01 O-+---+----+----t--+---+---+----+---.1
200
300
400
500
600
700
800
900
1000
Velocity FPM
Figure 7-21 Static pressure versus air flow. series with the line, as illustrated in Fig. 7-23. This special fixture will act like a physical filter in the line, and it must be periodically cleaned out. The best method to achieve this is to provide an auxiliary fixture connected to an air line. Then the filter can be periodically cleaned out with air pressure just by cracking the valve mounted adjacent to the filtered pipe. This is especially important on air conditioning units with drip pans or stand-alone NC units for computers. For piping carrying heated or cooled liquids, special insulated penetrations such as is shown in Fig. 7-24 must be used.
7.5.2 Pipe Penetrations for Welded Enclosures The welded penetration takes the form illustrated in Fig. 7-25. For a copper pipe, it is first brazed into a steel plate, and then welded into the wall of the enclosure. For steel
TABLE 7-2 PERFORMANCE OF SHIELDED VENTS 3/16 IN. CELLS, I IN. THICK Steel Honeycomb: Magnetic Fields I kHz 20 kHz 100 dB 25 dB Brass Honeycomb: Magnetic Fields I kHz 20 kHz 20 dB 70 dB
100 kHz 120 dB
Electric Fields 10 MHz 120 dB
100 MHz 120dB
Plane-Wave Fields I GHz 10 GHz 120 dB 120 dB
Electric Fields 10 MHz 120 dB 120dB
100 MHz 120dB
Plane-Wave Fields 1 GHz 10 GHz 120 dB 120 dB
100 kHz
Section 7.5
121
Piping
piping, it is shop welded into a plate, and then the plate is field welded. The two-step setup procedure is highly recommended since it is very important that the weld around the pipe be of the very highest quality and be done under controlled conditions. The field weld also must be very carefully done. A 2 in. overlay between the penetration plate and the shield is recommended since this will provide a waveguide-beyond-cutoff effect, thus enhancing the shielding effectiveness of the enclosure. The pipe should have connections on either side of the wall, and the outside connection may require a dielectric decoupler installed so as to isolate the enclosure from the rest of the building.
7.5.3 Piping for Modular Shielding The piping for modular shielding should be arranged so that the shielding panel is compressed between flanges mounted on either side of the enclosure wall, as shown in Fig. 7-26. This arrangement provides the best overall shielding performance since it seals at both sides of the panel.
7.5.4 Piping for Architectural Shielding The same type of fixture can be used with single-shield systems if sheet metal is used as the barrier. The only additional requirement is that the penetration be supported independently of the shield. For foil installations, the penetration should be mounted in a fairly large plate and the plate mounted in the wall of the enclosure so that the foil is
160
t
150
~
= 5 * 1.0.
'~
140 130
,'
~ 110 w
~en 100 ~
>
.~
\
80
, , I
\ ~ 0.5"
70
~ 60
:.c Qi 50
:c en
\
\'
90
Q)
ffi
"'" i\.
\ \\ \ \ \~ \ \0.25" \ \ ~\ \
iii 120
Q)
~ ~~~
~0.75"
40 1"
30 20 10
o 10 kHz
\
100 kHz
1 MHz
10 MHz
100 MHz
1 GHz
Frequency
Figure 7-22 Waveguide cutoff versus pipe size.
10 GHz
122
Chapter 7
Penetrations and Their Control
RF Gasket
11 in min I
Thread
1.5 in
l-
-r 0.5 in
T
Weld or Braze Shield
RF Gasket
Honeycomb Core (Brass) 1116 in Cell Size > 10 GHz Operation
Air Fitting
RF Gasket
Thread Weld or Braze
d
T r--1in~ ~--Shield
Piping> 0.5 in 1.0. for 100 dB SE Through 10 GHz
Figure 7·23 Honeycomb pipe union.
not stressed. If copper foil is used, then the plate should be made of brass and then solder sealed to the foil on the wall. If other foils are used, then it should be RF taped into place with a high-performance shielding tape. These techniques are illustrated in Fig. 7-27.
7.5.5 Procurement Specification The only requirement is that the piping be RF sealed around the perimeter of the pipe where it passes through the shielded wall. If used as a waveguide, then the length must
Section 7.5
Piping
123
Shield
Insu lation
1
---t
T
..--~
D = 3d
d
Figure 7-24 Insulated pipe penetration. be seven times the inside diameter of the pipe to achieve waveguide-beyond-cutoff performance . The diameter of the pipe is selected to cutoff above the operating frequency of the shielded enclosure.
-
Installati on by Mechanical Inside Shield ing
-t-I
Installation by Shielding Install er
8 Diameters or 12 in. Wh ichev er is Greater
--t-I _
Installati on by Mechan ical Outsid e Shi eld ing
2 in Ma x
6 in M in
Schedule 40 PVC Pipe Sectio n
RF Shielding
- - - - - ----1 Typical Wet Pipe & Drain Penetr at ion
Figure 7-25A
Welded pipe penetration
124
Chapter 7
Penetrations and Their Control
Inside Shielding
Outside Shielding
Installation by Shielding
Installed by Mechanical
24 in Min
6 in Min Fire Protection 4:JPiPing by Mecha1nical RF08 M22
3/16 in Mild Steel Backing Plate
I
PolypropyleneLined Steel Pipe Spool by Mechanical 150 Ib Raised Face Steel Flange Pipe, Furnished by Mechanical
RF Shielding - - - - - - . ...
Fire Protection Penetration
Figure 7-258
Welded pipe penetration.
7.6 FIBER OPTICS AND NONMETALLIC HOSES 7.6.1 Design Guides Fiber optics and other forms of nonmetallic conductors can be taken through the wall of a shielded room by means of a short length of pipe called a waveguide. The fiber optic cable must be all dielectric. No wire or shielding is permitted. The length of the pipe must be a minimum of seven times the diameter. The diameter must meet the requirements of Fig. 7-25. Multiple fibers or small plastic tubes can be run through a fitting illustrated in Fig. 7-28. In some installations, it may be desirable to bring water service in by rubber hose. Waveguides can be used for this application, but if the water contains a high mineral content, then the water may act like a conductor, picking up and carrying RF currents. Then the hose will behave like an antenna or wire stuck through the wall of the shielded enclosure. Distilled water is preferred since it is a nonconductor. Care must be taken to ensure that nonconductive rubber is used in the hose. Experience has shown that a large number of commercial hose designs use carbon black as a reinforcing agent, and the rubber becomes conductive and therefore acts like an antenna if run through a waveguide in a shielded enclosure. 7.6.2 Procurement Specification The only requirement is that a good RF seal be obtained around the outside of the pipe as it passes through the enclosure wall. The best seal is welded or brazed.
Section 7.7
125
Shielded Windows
RF Panel Copper Tub ing
Contin uously Sw eat Solder Both Sid es Fixed Flang e Brass Hex NUl Solder (Sw eat )
13 /16 in .
I~
-\
RF Panel Flanged Brass Hex Nuts
----tnT 1\\\\\\\\\ \
I
I
11 \\11\ 1 \\
I 111 I
Wavegu ide W it h Cont inuou s N.P.T. Tapered Ends Thr eaded Type
Figure 7-26 Modular pipe penetration.
7.7 SHIELDED WINDOWS 7.7.1 Description Some installations require the use of a shielded window, such as hospital operating rooms and NMR examination areas . Shielded windows are expensive , and should only be used where absolutely required . Generally, they are limited to low-frequency or lowperformance applications. Two forms of shielded windows are commercially available . The most common form is a sandwich made of copper screen and glass or plastic . The copper screen must be well attached (low conduct ive path) to the window frame , which in turn must be bonded to the surrounding shield. Two layers of coppe r screening are best , with different threads per inch . The second form of shielded window relies on the conductivity of vapor-deposited metallic film to provide a means of reflecting the elec tromagnetic energy. Both of these techniques are discussed .
Chapter 7
126
Penetrations and Their Control
Foil
Solder Outside Shield
Brass Plate
Inside Shield
Dielectric Union Where Required
Metal Union
Nut
Metal Clamp
l/e Steel Washer RF Gasket
Metal Support Bracket Plywood
1/16in. Lead Seal
Screws as Required Clearance Hole in Metal Bracket
Solder
Figure 7-27 Foil pipe penetrations.
7.7.2 Shielding Effectiveness of Windows Copper screening provides a higher degree of shielding effectiveness than does the continuous film systems for the same degree of optical transmission at the lower frequencies, but it rolls off in performance in the plane-wave region. This is shown by comparing Fig. 7-29 and 7-30. The data shown are for a single screen. It is common to install two screens separated by an air space. The disadvantage of the mesh is that it forms a moire pattern, which can be objectionable to the viewer. As indicated in the performance curves, high levels of shielding effectiveness are not possible for windows. Therefore, they should not be used unless they are necessary, such as for patient viewing in MRI examination rooms where the shielding performance is generally needed at 100 MHz and below.
Section 7.8
Fire Protection Systems
i
127
0.375 in. 1.0.
----------- T Shield
Figure 7-28 Multiple-waveguide penetration for fiber optics.
7.7.3 Window Installation It is essential that the conductive mesh or film be terminated completely around the perimeter of the window frame with a metal-to-metal seal. The frame, in turn, must be terminated into the shielded wall with a metal-to-metal seal. Soldering or a good gasket seal are required; just clamping the frame to the shield is not adequate unless the clamping system is similar to that of the hat and flat design.
7.8 FIRE PROTECTION SYSTEMS 7.8.1 Introduction Two forms of fire protection are commonly used in shielded enclosures. The most common is the sprinkler system, but it is also the most risky in terms of accidental damage. Water is an enemy of shielding, as described elsewhere. Only welded rooms are immune to the effects of water since these enclosures behave as welded tanks. The other form of fire protection is a halon gas system. These are normally used where computer systems are installed since, again, water damage is a greater risk than fire. But halon is a CFC-based chemical, and it will soon be unavailable because of the ozone risk. As of this writing, no known safe substitute has been found. In lieu of halon, which is the best fire protection for shielded enclosures, it is recommended that a dry pipe system be used, triggered by the use of smoke detectors. This minimizes the risk of accidental discharge, ruining both the equipment within the shield, and potentially damaging the shielding system as well.
128
Chapter 7
Penetrations and Their Control
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Section 7.8
129
Fire Protection Systems
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Figure 10-3 Single-point signal ground system for tempest enclosures.
For small shielded enclosures a single-point grounding system is practical, and it is provided by attaching a single, very large conductor from a grounding stud attached near the electromagnetic filters, and then routed by the shortest path to a separate ground located within the perimeter of the building shell. This ground wire shall have less than 1 n of resistance between the enclosure and the ground connection. The ground connection must have less than 10 n of resistance under all-year conditions. Controlled multiple-point grounding is recommended for very large enclosures so that dangerous voltage potentials do not develop due to the large capacity effect of floating a large steel box above ground. This technique is illustrated in Fig. 10-4.
10.3 SELECTING THE GROUNDING SYSTEM It is recommended that the design information given in MIL-HDBK-419A be used in the design of the grounding system for shielded enclosures.
165
Section 10.4
Shielded Enclosure
Grounding Stud
< 1 11 Resistance , ......1 - - - - -
to Ground
Chemical -=-~ Ground
Nonisolated Shielded Enclosure
Figure 10-4 Multiple-signal grounds used to ensure low-resistance signal path to ground on nonisolated shielded enclosure.
Where a single-point ground system is dictated for security reasons, the configuration shown in Fig. 10-3 is recommended. Note that all conduits have dielectric breaks, that the filters are mounted to the exterior of the shielded room, and that an extra wire is run along with the signal ground to an electrode installed near a chemical ground connection which is provided for the single-point signal ground. Note also that the green wire is connected via a stud in the interior barrier of the RFI filter cabinet. This configuration meets the following criteria. a. The National Electrical Code for fault protection. b. The requirement for a single-point ground. c. Provides a method of checking the ground which is to be monitored and logged on a quarterly basis. d. The chemical ground unit ensures that the ground will measure less than 10 {l, which is the standard requirement. The cables from the enclosure ground connection and the earth ground are selected to have less than 1 n of resistance. e. The shielded enclosure is isolated from its surroundings by building the shield on top of a dielectric substrate, and all connections to the shield are isolated using dielectric breaks in all piping and HVAC systems. All electrical leads are filtered.
10.4 THE EARTH GROUND lEST 10.4.1 Introduction There are two basic earth ground test methods, shown in Figs. 10-5 and 10-6. The first method is the direct or two-terminal test; the second is the fall-of-potential method or three-terminal test.
166
Chapter 10
Grounding of Shielded Enclosures
Building
Ground Stake Water Pipe
Figure 10-5 Two-terminal earth ground test.
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Section 10.5
167
References
10.4.2 The Direct Method When using a four-terminal instrument, P and C terminals connect to the earth electrode under test; P and C terminals connect to an all-metallic water-pipe system. With a three-terminal instrument, connect X to the earth electrode, and P and C to the pipe system. If the water system is extensive, its resistance should only be a fraction of an ohm. The instrument reading can be taken as the resistance of the electrode under test. The direct method is the simplest way to make an earth-resistance test. With this method, the resistance of two electrodes in series is measured-the driven rod and the water system. But there are three important limitations.
1. The water-pipe system must be extensive enough to have negligible resistance. 2. The water-pipe system must be metallic throughout, without any insulating couplings or flanges. 3. The earth electrode under test must be far enough away from the water-pipe system to be outside its sphere of influence. In some locations, the earth electrode may be so close that the two cannot be separated; under these circumstances, met, the water-pipe system can be used as the ground. As possible future changes in the water-pipe system, an earth installed.
to the water-pipe system if conditions 1 and 2 are a precaution against any electrode should also be
10.4.3 Fall-of-Potential Method This three-terminal test is the method shown in Fig. 10-6a. With a four-terminal tester, P and C terminals on the instrument are jumpered and connected to the earth electrode under test. With a three-terminal instrument, connect X to the earth electrode. The driven reference rod C should be placed as far from the earth electrode as practical; this distance may be limited by the length of extension wire available or the geography of the surroundings. Potential-reference rod P is then driven in at a number of points roughly on a straight line between the earth electrode and C. Resistance readings are logged for each of the points. A curve of resistance versus distance, like Fig. 10-6b, is then drawn. Correct earth resistance is read from the curve for the distance that is about 62% of the total distance from the earth electrode to C. In other words, if the total distance is D, the 62% distance is 0.62D; for example, if D is 120 ft, the distance value for earth resistance is 0.62 x 120 or 74ft.
10.5 REFERENCES [1] MIL-HDBK-419A. [2] C. S. Snow "Grounding of RF shielded enclosures," ITEM 1982. [3] H. W. Denny "Grounding for the control of EMf," Interference Control Technolo-
gies, 1986.
CHAPTER 11
Design Checklists
11.1 INTRODUCTION This chapter provides a set of checklists as a guide for developing a complete design for each of the three basic types of shielding: modular, welded, and architectural. Part of the material in the checklists refers to normal architectural design considerations required in any facility. Some overlap exists between the three examples, but all three should be studied prior to finalizing a given specification.
11.2 CHECKLIST FOR MODULAR SHIELDING 11.2.1 Introduction For the purposes of this example, the galvanized panel modular enclosure has been selected. The checklist is divided into four sections normally found in architectural specifications. These are architectural considerations, electrical considerations, mechanical considerations, and shielding considerations. The shielding specifications are normally found in Section/Division 13 of a project's specifications, although Section 17 may be used for TEMPEST shielding. Cross references should be made in the early sections of the shielding specifications to support requirements in the electrical, mechanical, HVAC, door, and hardware sections as necessary to ensure clarity of the complete enclosure requirements. The format of the checklist is an item number, followed by a dash, and then the recommended item to be considered.
11.2.2 Architectural Considerations ITEM NO.
ITEM TO BE CONSIDERED:
1__ Floors: The floor slabs under a shielded enclosure should be recessed if a flush threshold is desired at a door opening. The depth of the recess depends upon the floor finishes. The slab must be smooth and level to I/S in.110 f1.
169
170
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Design Checklists
2__ Interior Partitions: May not be required if formed by the RF shielded construction. If standard partitions are to be used, then care must be taken in their design and installation so that the shielding is not compromised.
3__ Wall Finishes: Care must be exercised in finishing exterior or interior surfaces of shielded enclosures. The best method is to provide furring channels for the application of standard wallboard-finished interiors or to provide steel studding to mount the electrical power and interior finishes. 4__ Ceilings: A conventional suspended ceiling may be used. Special hardware is required to interface with the ceiling of the shielded enclosure.
5__ Door and Frames: All doors, frames, and hardware for doors in shielded rooms must be part of the complete RF shielded construction. 6__ Enclosure Isolation: If required, the shielded enclosure shall be isolated from the building or earth ground by constructing the shield on a dielectric insulator such as a heavy plastic film or sheet, and ensuring that the shield does not come in contact with the building steel. See Chapter 10 for a discussion on the need for enclosure insulation.
11.2.3 Electrical Considerations ITEM NO.
ITEM TO BE CONSIDERED:
1__ Filters: All electrical power, alarms, HVAC control lines, intercoms, telephone lines, or any other service requiring wiring are to be filtered utilizing electromagnetic filters, each selected for the particular type of service involved. A full line of these filters is available from a number of sources. Under the electrical section of the specifications, it should be stated that all power for the shielded enclosure is to be fed through filters installed on the shield. For TEMPEST enclosures, all filters are to be installed in a cabinet, with provisions for an internal ground stud for the safety ground. Each leg of the power, including the neutral, shall be fed through filters, individually or in a common cabinet. Conduits feeding the filters may require dielectric fittings. 2__ Drawings: The electrical drawings should indicate the requirements of filters by symbol or note. They shall indicate in the details a typical hook up of the filters from the main feeders and distribution of the electrical conduits within the shielded rooms. In enclosures where ground detection and isolation transformers are used, these components must be mounted within the enclosure. 3__ Lighting Fixtures: For EMI enclosures or for enclosures to be used for EMI testing, incandescent lights are used. Fluorescent-type lighting should only be used in TEMPEST or communication facilities. These light fixtures should be specified under the electrical section of the specification. 4__ Grounding: Each shielded enclosure shall be provided with a ground stud adjacent to the filters on the outside of the enclosure. A size 2/0 AWG minimum grounding wire shall be provided from the grounding stud to the building or earth ground. This shall be done prior to power hook up to the filters. 5__ Power Distribution: For the most economical power distribution within the shielded enclosure, only the main feeder should be brought in through
Section 11.2
Checklist for Modular Shielding
171
power line filters to a distribution panel inside the shielded enclosure. All internal circuits for conventional distribution should originate from this panel. Only one power penetration should be made. 6__ Explosive Installations: Light tubes with exterior lamp fixtures should be considered to prevent RF or electrical voltages from igniting explosives. In most cases, standard explosion-proof light fixtures are acceptable.
11.2.4 Mechanical Considerations ITEM NO.
ITEM TO BE CONSIDERED:
1__ Penetrations: The shielded enclosure shall have special penetrations to receive all required mechanical inputs such as water, gas, air, or waste. From the outside, the mechanical line may require dielectric connectors, and then be attached to the special fitting in the wall of the enclosure's wall. The attachment shall provide a metal-to-metal seal both inside and outside the shielded panel. Piping distribution within the enclosure can be performed in a conventional manner.
2__ Drawings: Mechanical drawings shall indicate on the floor plan which rooms are to be shielded. A typical detail should be added, showing the method of connecting building ductwork to the waveguide vents in the shield. Details should also be provided specifying how pipes penetrate through the shielded walls using the details provided in this handbook. 3__ Doors: The door schedule should indicate the type of doors to be used in the shielded enclosure. These should be selected from Chapter 7 and described in the specifications. 11.2.5 Shielding Considerations ITEM NO.
ITEM TO BE CONSIDERED:
1__ Scope of Work: Ensure that all work necessary to accomplish the shielding task is properly delineated. The scope should include furnishing all labor, material, equipment, plant tools, scaffolding, and all incidental and related items to provide, fabricate, deliver, and test all radio frequency shielded rooms as shown on the drawings and specifications. Only experienced installers should be used in the installation of radio frequency shielding. 2__ Applicable Specifications: The A-E should select from the following the appropriate specifications for the installation being specified. a) MIL-STD-285-Method of Attenuation Measurements for Electromagnetic Shielding Enclosure for Electronic Test Purposes. b) NSA 65-6-RF Shielded Enclosure for Communications Equipment: General Specifications. c) MIL-STD-220A-Method of Insertion Loss Measurements for Radio Frequency Filters. d) UL 1283-Standard for Safety-Electromagnetic Interference Filters. e) ASTM E90-83-Recommended Practice for Laboratory Measurements of Airborne Sound Transmission Loss of Building Partitions. f) ASTM E413-73-Standard Classification for Determination of Sound Transmission Class.
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g) Federal Specification SS-A-118B-Flame Resistance Test. h) ASTM E84-81 A-Test for Surface Burning Characteristics of Building Materials. 3__ Functional Performance: The function of RF construction is to permit an interference-free environment with a dedicated point ground and/or to retain RF signals inside the shielded environment. 4__ Shielding Effectiveness: The shielding effectiveness of the enclosure, as a minimum, shall be as follows. Magnetic Field; 56 dB at 14 kHz, increasing to 100 dB at 200 kHz. Electric Field: 100 dB from 200 kHz to 50 MHz. Plane Wave: 100 dB from 50 MHz to 10 GHz. (NOTE: The above specifications are only applicable to the standard clamp-up or modular shielded enclosure.) 5__ If applicable, panels and doors shall have sound transmission class (STC) 30 when tested per ASTM E90-83. 6__ If applicable, panels shall have a flame spread rating of Class A, UBC class one when tested in accordance with ASTM method E84-81. 7__ Work Included: The shielding installer as a minimum shall install within the building in sizes detailed, with all accessories and facilities required, fully tested to meet the performance requirements specified, and shall include the following. a) All floor, wall, and ceiling-shielded panels and RF framing members, including all necessary supports, fasteners, etc. b) RF door unit including frames and hardware. c) Waveguide-type air vents for supply and return air. d) RF filter for all electrical, control, and communications services. e) All necessary waveguide-type shielded fittings for all pipes penetrating the shield, including any required dielectric decouplers. f) All hangers as may be required for the suspension of the RF enclosure ceiling from an overhead structure. g) Provision to receive conventional finishes, both inside and outside the shielded environment. h) Coordination of other building and mechanical trades, including equipment suppliers. i) Performance test and guarantee. 8__ Work Not Included: The work listed below is not normally included, but should be supervised by the RF shielding contractor. a) Electric wiring and wiring materials such as lighting, outlets, and receptacles both inside and outside the shielded environment. b) Any duct work to and from the waveguide-type air vents. c) Any piping to and from RF penetrations. d) Field painting or any other finishes. e) Any construction work in connection with preparing surfaces to receive RF shielded environment and/or conventional construction enveloping the shielded rooms.
Section 11.2
Checklist for Modular Shielding
173
9__ Shielding Panels: The wall, floor, and ceiling panels shall be rigid laminated panels, faced with heat-treated, annealed steel, treated to resist corrosion without degrading the electrical continuity or RF attenuation. Panels shall also be of such design that the core material is water resistant. The sheet steel shall be 24 gauge minimum. 10__ The framing system shall consist of Vs in. thick zinc-plated steel, a minimum of 3 in. wide. The shapes of the framing members shall provide a clamping action of panel edges with uniform and constant pressure contact against the shielding panels. Provisions shall be made for fasteners of a maximum distance of 4 in.; blind inserts or "weld nuts" are the preferred method of attaching the cadmium-plated V4-20 screws which should have a minimum tensile strength of 135 000 lbf/in. 2 . 11__ Corners: The corners shall consist of prewelded framing members or preformed shielded panels. 12__ Mechanical Performance: The deflection of the walls under a static load of 75 lb applied normal to the wall surface shall cause a deflection not to exceed V250 of the unsupported span. The RF ceiling shall be supported from the parent room overhead construction by means of adjustable-type hangers, unless specified otherwise. The deflection of the panels, including finishes, lights, and diffusers, shall not exceed 1/270 of the span. 13__ Doors: The door unit shall be factory assembled, consisting of a removable door leaf, door frame, threshold, hardware, and electrical contacting strips. The door and frame perimeters shall be rugged and provide for the recessed contact mechanism consisting of a fingerstock RF gasket held in a groove, making contact with a knife-edge contact on the door leaf. The gasket is to be mechanically mounted. 14__ Door Hardware: The door hardware shall consist of the following. Two radial and thrust-bearing hinges, with provisions to adjust the door in the hung position. The locking device shall consist of cam-actuated type latch. It shall be operable from both sides of the door. It shall have permanently lubricated bearings at all points of pivot or rotation. Contact with the strike shall be by a cam roller bearing. With the door leaf at rest with the fingerstock in light contact with the frame, the mechanism shall, on rotation of the lever handle, draw the door into its final closing RF-tight position with an operating pressure of not more than 20 lb. 15__ Ventilation: Waveguide-type air vents shall be of such design as to provide air passage for cooling and ventilation and still maintain the specified shielding effectiveness. Flexible dielectric connectors shall be supplied on enclosures requiring isolation. 16__ Mechanical Penetrations: Provide waveguide-type pipe penetrations for all pipes or tubes entering the RF shield. Insulated-type penetrations shall be used where condensation may occur on the piping. 17__ Electrical Service: All incoming electrical power shall be provided with UL listed radio frequency filters. The filters shall be provided on each electrical conductor. The filters shall be designed to attenuate RF energy on the incom-
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Design Checklists
ing power by 100 dB from 14 kHz to 10 GHz when tested in accordance with MILSTD-220A. 18__ Ground Provisions: A single-point signal ground shall be provided by means of a brass stud located as near as possible to the power line filters. 19__ Installation: Only experienced installers should be used in the installation of RF shielding. 20__ Floor Installation: The floor panels shall be laid on 6 mil dielectric film placed on the structural floor of the parent room. Over this film, additional I/s dielectric filler material shall be furnished to provide uniform support of the panels. 21__ Cleaning: All exposed surfaces shall be thoroughly cleaned of all dirt, finger marks, and foreign matter resulting from handling or installation, and all areas shall be left free of defects. The edges of all shielding panels shall be thoroughly cleaned prior to the assembly of the shielding. 22__ Preliminary Performance Test: A preliminary test shall be performed after the enclosure is installed and before any other finishes or building construction are erected within the shielded enclosure. As a minimum, the test should be conducted at 100 kHz magnetic and 1 GHz plane wave. 23__ Acceptance Test: Upon completion of the facility and just prior to occupancy, a full test shall be conducted to the applicable performance specification such as MIL-STD-285 or NSA 65-6. A test plan shall be submitted by the installer and approved by the purchaser. The final acceptance test shall be conducted by an independent testing contractor, and it shall be witnessed by a representative of the purchaser.
11.3 CHECKLIST FOR WELDED ENCLOSURES 11.3.1 Introduction The welded enclosure can attain the highest performance of all the methods of achieving shielding. The welded seam, if properly done, is the optimum method of achieving a seam in a shielded enclosure. Assuming that the welding is done properly, which only a good quality assurance program can ensure, the performance of a welded enclosure is determined by the penetrations used to provide the various services. Each type of penetration must be designed for the overall performance requirement; this is especially true of the doors. Shielding suppliers advertise 120 dB welded enclosures, but they put 100 dB filters on the power lines; thus, care must be exercised in specifying this level of performance since, in reality, it may not be practical. This section provides a checklist of items that should be considered when specifying welded enclosures.
11.3.2 Architectural Checklist ITEM NO.
ITEM OF CONCERN:
1__ Shielded Volume: The cost of field welding is very high; thus, the A-E must carefully consider the total volume, floor space, and types of work areas to be included within the shielded volume.
Section 11.3
Checklist for Welded Enclosures
175
2__ Submittals: Define all deliverables such as shielding specialist credentials, welder certificates, material certifications, test reports, and shop drawings. 3__ Shop Drawings: State that shop drawings of the shielded enclosure with all details, materials, and erection data shall be submitted to the purchaser for approval. All pertinent details shall be supplied. 4 Welds: Full penetration butt welds or lap welds shall be used, as illustrated in Chapter 5. Where backing material is required, it is recommended that it shall overlap by at least I in. on both sides of the weld location. 5__ Welder Qualification: Ensure that welders shall be qualified in the specific procedures as detailed in MIL-STD-248B. Require that welders supply qualification certificates to the builder. 6__ Entries: The method of entry into the shielded enclosure needs to be selected carefully. High-traffic facilities may require the use of vestibules or waveguide entrances. Normal traffic or low-use facilities should consider using single doors, with their performance level consistent with the overall requirements of the enclosure. A safe guide is to require the doors to be specified to have a minimum of 10 dB performance above that of the basic enclosure. 7__ Door Design: Mechanical design of an RF shield door is extremely important for maintaining the electromagnetic performance over the life of the facility. The mechanical strength of the hinging mechanisms and the resistance to warpage must be demonstrated. See Chapter 7 for suggested methods of evaluation. Sliding doors with air bladders and steel-to-steel RF seals provide the best lowfrequency magnetic shielding.
11.3.3 Electrical Checklist 1__ For HEMP facilities, electrical surge arresters (ESA) shall be installed at the filter terminals which connect to the exterior wiring. 2__ The conduit from the filter/ESA assembly to the RF shield shall be a rigid steel conduit, circumferentially welded at all joints and at the penetrations into the RF filter cabinet and RF shield.
11.3.4 Mechanical Checklist 1__ Materials: Ensure that the necessary specifications for sheet stock, welding rods, and other steel elements used to fabricate the RF shield comply with ASTM, AWS standards, and other requirements as needed. Require the installing contractor to certify to the purchaser that the material meets the requirements.
2__ Penetrations: All penetrations must be designed to maintain the integrity of the shield; a note should be made to the drawings to the effect that all known penetrations are identified on the drawings, and no additional penetrations should be made without the approval of the purchaser. 3__ Special Door Hardware: When security locks, such as card keys, are installed in shielded doors, the installation should be conducted by the door supplier. This applies even if the locks are customer supplied.
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Design Checklists
4__ Door Alarms: Emergency exit/equipment access RF doors should be alarmed to indicate an open condition. Vestibule doors should be alarmed to indicate when both inner and outer doors are open simultaneously. Electrical interlocks on vestibule entrances are recommended. 5__ Ventilation Penetrations. Honeycomb vent structures must be used for these services. The frame of the vent structure must be welded into the RF shield with continuous seam welds. 6__ Piping Penetrations: For 100 dB performance up through 10 GHz, all piping should be circumferentially welded to the steel liner at the penetration. The penetration stub shall have an unbroken length at least five times the inside diameter of the pipe, and it shall form a waveguide beyond cutoff with a minimum cutoff frequency of 15 GHz (inside diameter less than 0.39 in.). Dielectric linings are not permitted in the penetrating pipe stub. If an adequate fluid flow cannot be achieved with a 0.39 in. diameter pipe, a honeycomb waveguide insert in the pipe should be used for penetration protection.
11.3.5 Shielding Checklist 1__ Scope: The scope of the type of enclosure should be carefully defined, especially if the facility is for HEMP protection. 2__ Shielding Specialist: Welded enclosures require an on-site specialist who oversees the RF welding and ensures that proper techniques are employed. 3__ Applicable Specifications: Ensure that the proper specifications have been referenced, especially those that require special testing or welder qualifications. Shield Requirements: Define the shielding performance requirements; this should be based upon a thorough evaluation of the facility needs, not on just choosing a blanket specification. The performance requirements may be stated in a tabular or curve format, but it must be stated that the performance throughout the entire frequency spectrum must meet the minimum requirements. The recommended useful life of a welded shielded enclosure shall be 30 years, and the shielding effectiveness shall satisfy the shielding performance requirements for a minimum of three years, when maintained using the procedures supplied by the installation contractor. The shielding effectiveness requirements shall apply to the finished structure, with all electrical and mechanical penetrations installed and operating. 4. _ _
5__ Welding: The quality of the welding sets the performance achievable by the shielded enclosure. The steel sheets must be assembled into an RFtight shield by continuous welding of all seams, joints, and corners. The metal electrode inert gas (MIG) process has been found to be the best. The surfaces of the metals must be prepared by removing rust, scale, and other foreign materials, and completed welds must be free of slag, gas pockets, wormholes, cracks, or incomplete fusion. 6__ Require that an in-process testing program be provided, and that all welds shall be 100% tested using the methods described in Chapter 9.
Section 11.4
Checklist for Architectural Shielding
177
7__ Preliminary Shielding Effectiveness Testing: An empty shell test should be conducted prior to installing the final finishes. This should include doors and other mechanical penetrations; a temporary filter installation is acceptable. As a minimum, a magnetic field test at 100 kHz and a 1 GHz plane-wave test should be conducted using the procedures of MIL-STD-285 or NSA 65-6. Final Acceptance Testing: After completion of the RF shield 8__ and all finishes, the entire shielded enclosure should be tested in accordance with the full performance requirements. The electromagnetic filters should be installed and under load. An independent testing service shall perform all final acceptance testing using a contractor-prepared, customer-approved test plan. 9__ Door Frame Welds: Welds between the door frame and the RF shield are primary shield welds, and they should be inspected in accordance with the provisions discussed in Chapter 9.
11.4 CHECKLIST FOR ARCHITECTURAL SHIELDING 11.4.1 Introduction Architectural shielding must be detailed very carefully since it is designed into the structure of the facility. It is especially important that every penetration be defined and detailed since the overall shielding integrity is directly related to the soundness of the penetrations. Copper foil shielding is assumed for this example. A performance of 80 dB is assumed through 1 GHz.
11.4.2 Architectural Checklist 1__ Layout: The floorplan of the space to be shielded should be laid out with consideration being given to minimizing the number of penetrations and RF doors required. If the space is to be divided into a number of rooms, plan on using nonshielded interior walls, and only shield the outer envelope. In high-traffic facilities, vestibules using a pair of RF doors are recommended. 2__ Floor: A plywood floor should be first installed, and then the shielding foil applied. All seams are sealed with solder-coated shielding tape. The foil is then covered with another layer of plywood, and glued down with a good grade of construction adhesive. Interior Partitions: Ensure that the design of the interior par3_ _ titions does not compromise the floor and ceiling shielding. 4__ Wall Finishes: The wall finishes should be mounted on furring strips which have been glued to the face of the shielding. Nails and screws are not compatible with foil shielding since it is difficult to get a long-lasting metal-to-metal seal at each fastener location. 5__ Ceilings: Acoustical ceiling tile can be used, but the hangers mounted in the shielded ceiling must be arranged so that the dead weight of the suspended ceiling does not pull out the ceiling fasteners.
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6__ Doors and Frames: The door performance must be at least 10 dB better than the enclosure requirements. The door frames must be solder sealed around the perimeter of the frame to ensure an RF-tight installation. This is particularly true at the door threshold.
7__
dc Isolation: If the enclosure must be isolated from the parent building, then special care must be exercised to ensure that all fasteners do not reach building steel.
11.4.3 Electrical Checklist 1__ All RF filters should be mounted in filter cabinets, and a single penetration should be passed through the shield and clamped on both sides of the foil shield. The penetration should be solder sealed on the inside.
2__
Foil is not a good surface to mount mechanical devices. It is recommended that an interior stud wall be installed around the perimeter of the shielded space. Conduit, receptacles, and light switches can then be mounted without disturbing the shield.
11.4.4 Mechanical Checklist 1__ All penetrations through the foil shield must be mechanically supported, and a good solder seal must be made between the flange of the penetration and the foil shielding. It is recommended that penetrations be grouped using a common metal plate for the penetration. 2__ The junctions between the foil shielding and the door frames are especially critical. Care must be taken to see that a good solder seam is made between the door frame and the shielding foil. This is especially true at the door threshold.
11.4.5 Shielding Checklist 1__ Scope of Work: Ensure that all work necessary to accomplish the shielding task is properly defined; this includes the shield, all supporting structures, finishes, mechanical items, electrical items, and testing. 2__ Performance specifications need to be carefully drawn and specified. Foil shielding exceeds NSA 73-2A, but will not meet NSA 65-6. 3__ Preliminary performance testing must be conducted prior to adding final finishes to the shielded enclosure. All doors must be installed, and a power line filter also must be installed. It is suggested that shielding effectiveness testing be conducted at I GHz as a minimum.
APPENDIX A
A-I ADDITIONAL DEFINITIONS AND TERMINOLOGY BLACK Designation. A designation applied to electronic components, equipment, and systems which handle only unclassified signals, and to areas in which no classified signals occur. Radiation Resistance. The resistance which, if inserted in place of an antenna, would consume the same amount of power that is radiated by the antenna. REDIBLACK Concept. The concept that electrical and electronic circuits, components, equipment, and systems, which handle classified plain language (not encrypted) information in electric signal form (RED), be separated from those which handle encrypted or unclassified information (BLACK). Under this concept, RED and BLACK terminology is used to clarify specific criteria relating to and to differentiate between such circuits, components, equipment, and systems and the areas in which they are contained. RED Designation. A designation applied to: 1) all communication electronics (CE) within the terminal or switching facility carrying classified plain language, 2) all (CE) between the encrypted side of the on-line crypto equipment used and individual subscriber sets or terminal equipment, 3) equipment and sets originating or terminating classified plain-language processing equipment, and 4) areas containing these wire lines, equipment, and their interconnecting and auxiliary equipment.
A-2 No appendix material.
A-3 No appendix material.
179
180
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A-4 ARCHITECTS AND ENGINEERS SPECIFICATIONS The following is a selection of architectural and engineering specifications for a variety of modular RF shielded enclosures. 4.1 GALVANIZED MODULAR ENCLOSURE PROCUREMENT SPECIFICATIONS The following detailed specification is suggested as a model for specifying the enclosure portion of a galvanized modular enclosure specification suitable for insertion in Section 1300 of an overall architectural specification. Doors, filters, and other penetrations used in enclosures are discussed in their respective chapters.
1.0 General The radio frequency shielded, solid wall enclosure described and specified herein shall be designed and installed for the containment and/or exclusion of radio frequency energy and shall be manufactured by a qualified supplier. 2.0 Applicable Specifications MIL-E-18639A: MIL-E-8881: MIL-E-4957A: MIL-STD-285: MIL-F-15733: MIL-STD-220A: NSA 65-6:
Enclosures, Electromagnetic Shielding, Knockdown Design. Enclosure, Electromagnetic Shielding, Demountable, Prefabricated. Enclosure, Electromagnetic Shielding, Prefabricated, Demountable. Method of Attenuation Measurements for Electromagnetic Shielded Enclosures for Electronic Test Purposes. Filter, Radio Frequency Interference, General Specification for. Method of Filter Insertion Loss Measurement. National Security Agency Specification for RF Shielded Enclosures for Communications Equipment; General Specification.
3.0 Materials All materials used in the enclosure, including all ancillary equipment, shall be new, undamaged, installed, and used in such a manner that the normal operation does not affect the specified shielding effectiveness. Shielding Panels: Shielding panels shall be low carbon electrical steel, zinc clad, per specification QQ-S-775. All shielding panels shall have a base of % in. CCX exterior grade plywood or equal, unless otherwise specified. Plywood and particle board ratings shall be in accordance with requirements of the U.S. Plywood and U.S. Particle Board Associations. Framing Members: The electrical shielding panels shall be connected to each other using suitable clamps to provide a continuous electrical bond from panel to panel. The clamps shall be made of heavy duty, I/S in. steel. Suitable prefabricated plated steel clamps shall be provided in matched sets to meet the structural and electrical requirements for the walls, ceiling, and floors. Walls, floor, and ceiling shall use suitable
Section 4.1
Galvanized Modular Enclosure Procurement Specifications
18t
"hats" and "flats," and all corner sections shall be of a matching HW" and HU" configuration. The structural framing members shall be electrically and mechanically secured to each other with a 1/4 -20 plated machine screw installed at 4 in. maximum intervals along all framing members. The screw shall mate up with a threaded insert or nut attached to the hat section. All clamping action machine screws shall be set at a torque of 70-80 in. Ib to assure electrical bonding and structural firmness. All structural steel clamps shall be zinc plated per QQ-Z-235, Type II for maximum electrical continuity. All three-way corners shall have suitable assemblies to ensure electrical bonding, mechanical, and structural strength.
4.0 Construction The shielded enclosure shall be of the prefabricated, modular type, and shall be fully capable of being assembled and disassembled without the use of special tools. The shielded enclosure shall be capable of being assembled/disassembled from the inside. The completed enclosure, when specified, shall be capable of being electrically isolated from the parent building floor and walls. The enclosure shall be rigid and plumb with good installation procedures, and shall be such that the maximum sag of the ceiling is less than V240 of the ceiling span. The enclosures will be subjected to various live loads, repetitious usage of the shielded door(s), and continuous use of the electric power, light, and other accessories. The design of the shielding system must tolerate these requirements. The shielded room floor shall be supplied with suitable hardboard subflooring to provide leveling when necessary, and to provide for electrical isolation from the parent room floor. When specified (single-point grounding), the subflooring shall be insulated and moisture protected by a suitable layer of heavy-duty plastic film. The shielded room floor, when suitably supported by the parent room floor, shall be capable of supporting a total floor load of 1200 lb/ft". The completed shield room shall be provided with a suitable floor of asphalt tile in a neutral color. The tile loading shall be suitable for a loading of 300 Ib/ft 2 and concentrated loads of 1000 Ib on suitable casters. Vinyl tile shall be supplied when specified. Each shielded room shall be supplied with an instruction handbook and a complete set of assembly and disassembly drawings.
5.0 Doors (Select from the door specifications in Chapter 7 the type and size that meet the intended use.)
6.0 Accessories Vents. (See Chapter 7 for waveguide air vent specifications.) Grounding. Each shielded enclosure shall be supplied with a permanently installed grounding stud of solid brass or bronze of not less than V2 in. diameter. The grounding stud shall extend a suitable distance both inside and outside the shielded enclosure for installation of ground leads. The ground stud shall be provided with its own washers and locking nuts.
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Appendix A
Penetrations. All necessary penetrations for specified services shall be supplied and installed, and with suitable caps where necessary, on a permanent base and in such a manner that the total shielding effectiveness shall not be degraded. Where single or controlled grounding is specified, then dielectric decouplers shall be inserted in all pipe penetrations. The shielding installer shall provide or supervise all penetrations in the shielded enclosure. Access Panels. When specified, suitable coaxial access panels shall be supplied as part of the shielded enclosure. All access panels shall be of suitable plated steel or brass so that electrical connectors can be soldered to the panels. When specified, the panels shall be removable, supplied with their own RF gaskets, and shall meet the same shielding effectiveness of the shielded enclosure. Filters. (Filters should be selected from Chapter 8.)
7.0 Shielding Effectiveness The shielding effectiveness (attenuation) shall be equal to or greater than (choose from Chapter 9, typically NSA 65-6) and be conducted on the completed installation. An independent testing service shall conduct the test. All penetrations shall be tested. Doors shall be tested at a minimum of ten points around the perimeter. Depending on the size of the enclosure, tests should be conducted at 15 ft intervals around the perimeter of the entire enclosure. Power line filters shall be in place and under load.
8.0 Quality Assurance An independent testing service shall conduct shielding effectiveness tests in accordance with the applicable testing specifications (see Chapter 9 for details) and certify that the enclosure was so tested. If the interior surfaces of the enclosure are to be architecturally treated, then a preliminary test of the enclosure shall be conducted prior to their installation. After all work is completed, the final test shall be conducted.
4.2. PROCUREMENT SPECIFICATION FOR A VERSION OF THE DOUBLY ISOLATED MODULAR SHIELDED ENCLOSURE SYSTEM A suggested procurement specification is as follows.
1.0 General The shielded enclosure shall be constructed with prefabricated, modular panels consisting of interior and exterior 3 oz copper and 24 gauge galvanized steel which shall be electrically isolated to provide maximum attenuation of radio frequency signals. Panels shall be capable of being assembled, disassembled, moved, and reassembled without suffering a degradation of shield effectiveness. Panel-to-panel seams shall provide solid, continuous, low-resistance electrical contact between respective shields. To the extent consistent with the required dimensions and configuration of the enclosure, all like components shall be interchangeable. The enclosure shall be self-supporting up to a width of 12 ft. Enclosures exceeding 12 ft in width shall be provided with a ceiling support system.
Section 4.2.
Procurement Specification
183
Field soldering and/or welding shall not be required for assembly or maintenance of the shielding integrity of the enclosure. All accessories and components necessary to maintain attenuation and other requirements shall be as specified herein. All components shall maintain isolation between shields. There will be no penetrations of shields, except as specified herein.
2.0 Applicable Documents MIL-E-18639A: MIL-E-888I : MIL-E-4957A: MIL-STD-285: MIL-F-15733: MIL-STD-220A: NSA 65-6:
Enclosures, Electromagnetic Shielding, Knockdown Design. Enclosure, Electromagnetic Shielding, Demountable, Prefabricated. Enclosure, Electromagnetic Shielding, Prefabricated, Demountable. Method of Attenuation Measurements for Electromagnetic Shielding Enclosures for Electronic Test Purposes. Filters, Radio Interference. Method of Filter Insertion Loss Measurement. National Security Agency Specification for RF Shielded Enclosures for Communications Equipment: General Specification.
3.0 Materials Shielding materials shall be 3 oz solid copper FED UU-P-147B Type V182, C 1, A, B, C, or equivalent, and 24 gauge steel zinc galvanized FED QQ-S-775-D, Type I, Class D. All frames shall be 13/4 in. thick D-select kiln-dried pine or better in accordance with FED SPEC MM-L-751-H. Plywood used for the floor shall be a minimum of Type A-D fir, Type II waterresistant in accordance with FED SPEC NN-P-530 and covered with vinyl tile ComNBS L-T-00345. Pressure clamp angle bars shall be cold-rolled angles MI020 (Merchant Quality) in accordance with FED SPEC QQ-S-630 and zinc dichromate plated in accordance with FED SPEC QQ-Z-325. Fasteners shall be steel and shall be plated with the same basic metal as that of the parts connected. When fasteners make contact with steel, they shall be zinc coated or cadmium plated. Solder shall be in accordance with FED SPEC QQ-S-571-D Type RA 50 SN and FED SPEC QQ-S-571-D Type RA 60 SN. Brass fittings shall be in accordance with FED SPEC QQ-B-626.
4.0 Construction Panels shall be of wood frame construction, with shielding materials covering one side of the frame, but electrically isolated from each other. Frames shall be constructed with plate-forming techniques using wood plates and adhesive to prevent "antenna action" caused by metal fasteners. Frames shall have cross braces, glued solid at one end and left floating at the other, so that panels can "work" as ambient conditions change without degrading shielding integrity and effectiveness. Panels shall be joined, and their support shall be augmented by specially designed pressure clamps fastened to the panel, and by cover shield seams to provide continuous, constant, and uniform shield contact pressure to prevent electromagnetic energy leaks at seams.
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Appendix A
Wall panels shall be 40 x 96 in., except as required to meet specified enclosure sizes. Wall panels will have horizontal braces spaced nominally 32 in. on center, and shall have a V4 in. thick wood grained protective paneling on interior surfaces. Service panels will be provided with solid sections as required to furnish proper mounting surfaces for power filters, telephone filters, RF connector assemblies, and waveguide feedthroughs. Service panels shall have 32 in. high wood grained wainscoat kick panels on interior and exterior surfaces. Ceiling panel frames shall have cross members spaced nominally 30 in. on center. Floor Panels: Cross braces spaced 12 in. on center. Floor panels shall have a 2 3/4 in. plywood overlay capable of supporting a uniform load of 1200 Ib/ft • All panels shall be set in a straight, true line, with level and even surfaces, such that all panel seams will be in alignment and provide electromagnetically tight seams. Exposed surface areas shall be thoroughly cleaned of all foreign matter resulting from handling. All contacting surfaces shall be free from defects which may inhibit good contact between panels. The manufacturer shall provide assembly instructions for the shielded enclosure.
5.0 Doors (See Chapter 7 for a discussion on double-isolated door specifications.)
6.0 Accessories Filters: The filters are to be "duo-shield" filters as described in Chapter 8. Waveguide Air Vents: A pair of vents shall be used: one on the interior shield, and the other on the exterior shield. They should be of brass material as described in Chapter 7.' Feedthrough Connectors: Special double-shield connectors shall be used. Grounding: A standard V2 in. brass grounding stud shall be used, connecting the two shields together, thus providing a single grounding point.
7.0 Shielding Effectiveness The shielding effectiveness of the enclosure, when properly assembled, shall be as follows. Magnetic Field
Electric Field
14 kHz 75 dB
120 dB
14 kHz
Microwave
Plane Wave 450 MHz 120 dB
I GHz
120 dB
10GHz
110 dB
8.0 Quality Assurance Experience: The shielding room installer must have at least three years' experience in the RF shielding industry. A written guarantee of workmanship, materials, and attenuation for a period of at least three years, except for working parts such as the door, shall be provided by the manufacturer. All working parts, doors, EMI filters, etc., shall be warranteed for a period of one year after acceptance of the installation.
Section 4.3
Procurement Specifications for a copper screened enclosure
185
Testing: After assembly, the enclosure shall be tested for shielding effectiveness per the specifications using MIL-STD-285 test methods at the frequencies specified in NSA 65-6. An independent experienced test service shall be used, under a separate contract. 4.3 PROCUREMENT SPECIFICATIONS FOR A COPPER SCREENED ENCLOSURE The following specification is suggested for use in specifying copper screen enclosures.
1.0 General The radio frequency shielded enclosure described and specified herein shall be designed and installed for the containment and/or exclusion of radio frequency energy, and shall be manufactured by a qualified supplier.
2.0 Applicable Specifications MIL-STD-285.
3.0 Materials The copper screen shall be 22 x 22 x 0.015 in. copper wire mesh mounted on wood frames. All wood framing shall be in accordance with FED SPEC MM-L-751-H. See Section 4.3.2 for remainder of material specifications.
4.0 Construction Same method as described in Section 4.3.2. A "kick" panel shall be installed on the lower portion of the enclosure, inside and out, for physical protection of the screening. A minimum height of 24 in. is recommended.
5.0 Doors (A suitable door from Chapter 7 should be selected.)
6.0 Accessories Filters, vents, feedthrough connectors, and grounding studs should be selected from Chapter 7.
7.0 Shielding Effectiveness The shielding effectiveness of the enclosure, when properly assembled, shall be as follows. Magnetic Field
14 kHz 68 dB
Electric Field
14 kHz 100 dB
Plane Wave 450 MHz 100 dB
Microwave
1 GHz 90 dB
10 GHz 50 dB
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Appendix A
8.0 Quality Assurance Same requirements as given in Section 4.2.
4.4 SAMPLE NMR RF SHIELDING SPECIFICATION The following detailed specification is suggested as a model for specifying the enclosure portion of a copper single-shield NMR facility suitable for insertion in Section 13000 of an overall architectural specification. Doors, filters, and other penetrations used in enclosures are discussed in their respective chapters.
1.0 General The radio frequency shielded, single-shield enclosure described and specified herein shall be designed and installed for the containment and/or exclusion of radio frequency energy and shall be manufactured by a qualified supplier.
2.0 Applicable Specifications MIL-STD-285.
3.0 Material Shielding materials shall be 3 oz solid copper FED UU-P-147B Type V182, Cl, A, B, C, or equivalent, such as 3 oz copper bonded to sisalkraft paper (see Chapter 6 for material specifications).
4.0 Construction Wall Panels: The wall panels shall be constructed from solid wood frames with 3 oz copper exterior surfaces. Interior surface to be treated conventionally. Framing materials shall be in accordance with FED SPEC MM-L-751-H. Ceiling Panels: The ceiling panels shall be similar to the wall panels. Include wood frame backing at the light fixture locations. Wood mounting pads shall be provided for the light fixtures. Include structural supports as needed to maintain a span deflection of less than L/240. Floor Panels: The floor shall be of one-piece pan construction. This is achieved by solder sealing the seams between the copper sheets. A covering shall be provided to protect the copper shield and support the floor covering.
5.0 Doors (Select from the door specifications in Chapter 7 the type and size that meet the intended use.) 6.0 Accessories
Each shielded enclosure shall be supplied with a permanently installed grounding stud of solid brass or bronze of not less than 1/2 in. diameter. The grounding stud shall extend
Section 4.5
Sample Procurement Specification
187
a minimum of 1.0 in. inside and outside the shielded enclosure for installation of ground leads. The ground stud shall be provided with its own washers and locking nuts. Air vents are to be 3/16 in. brass cell size, 1 in. thick, as described in Chapter 7.
7.0 Shielding Effectiveness The shielding effectiveness shall be equal to or greater than the following: Electric Field
Magnetic Field I kHz 20 dB
10 kHz 40 dB
I MHz 80 dB
10 kHz 100 dB
10 MHz 100 dB
Plane Wave 100 MHz 100 dB
8.0 Quality Assurance An independent testing service shall conduct the shielding effectiveness tests in accordance with the applicable testing specifications (see Chapter 8 for details) and certify that the enclosure was so tested.
4.5 SAMPLE PROCUREMENT SPECIFICATION FOR THE LINDSAY SINGLE-SHIELD MODULAR ENCLOSURE SYSTEM 1.0 General The radio frequency shielded, all-metal single-shield enclosure described and specified herein shall be supplied and installed for the containment and/or exclusion of radio frequency energy.
2.0 Applicable Specifications NSA 65-6.
3.0 Materials The flanged gavannealed RF panels shall be die drawn to ensure a proper fit with the framing system. The panel tensioner and channels shall be cold formed on roller dies, precision die cut, die punched, and die notched for a proper fit with the RF panels. Tensioner screws and spline nuts shall be used to assemble the RF panels and framing members.
4.0 Construction The panel sheet flanges fit into the frame channels and are gripped by the panel tensioners. When tensioners are tightened with the tensioner lock screws, the panels are pulled into uniform tension in all directions, affecting a solid and continuous metalto- metal contact.
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Appendix A
5.0 Doors (Doors shall be selected from Chapter 7 and shall be consistent with the performance capability of the shielding system.)
6.0 Accessories Electromagnetic filters shall be consistent with the enclosure performance and shall be selected from Chapter 8. Venting shall be accomplished using the waveguide-beyond-cutoff honeycomb structures described in Chapter 7. A V2 in. diameter brass grounding stud shall be located near the EMI filters. All pipe penetrations should be compatible with the single-shield concept and have a supporting structure on one side of the shield, as described in Chapter 7.
7.0 Shielding Effectiveness The shielding effectiveness shall be in accordance with Fig. I of NSA 65-6.
8.0 Quality Assurance Requirements The shielded room installer shall have a minimum of three years' experience in the RF shielding industry. A written guarantee of workmanship, materials, and attenuation for a period of at least three years, except for working parts such as the door, shall be provided by the manufacturer. All working parts, doors, EMI filters, etc., shall be warranteed for a period of one year after acceptance of the installation. After assembly and prior to final finishes, the enclosure shall be tested for shielding effectiveness at a minimum at 100 kHz magnetic and I GHz plane wave. Upon completion of all work, an independent experienced test service shall test and certify the enclosure to the test and performance requirements of NSA 65-6. 4.6 SAMPLE PROCUREMENT SPECIFICATION FOR A SINGLE-SHIELD MODULAR GALVANIZED SHEET METAt SHIELDING SYSTEM
1.0 General
The shielded enclosure shall be constructed with prefabricated, modular panels consisting of 24 gauge galvanized steel exterior shield. 2.0 Applicable Specifications MIL-E-18639A:
MIL-STD-285: MIL-F-15733: MIL-STD-220A: NSA 73-2A:
Enclosures, Electromagnetic Shielding, Knockdown Design. Method of Attenuation Measurements for Electromagnetic Shielding Enclosures for Electronic Test Purposes. Filters, Radio Frequency Interference, General Specification for. Method of Filter Insertion Loss Measurement. National Security Agency Specification for Aluminum Foil Shielded Enclosures.
Section 4.6
Sample Procurement Specification
189
3.0 Materials Shielding materials shall be 24 gauge steel zinc galvanized in accordance with Federal Specification QQ-S-775-D, Type I, Class D. All frames shall be 13/ 4 in. thick D-select kiln-dried pine or better in accordance with FED SPEC MM-L-751-H. Plywood used for floor shall be a minimum of Type A-D fir, Type II water resistant in accordance with FED SPEC NN-P-530 and covered with vinyl tile. Fasteners shall be steel and be zinc or cadmium plated. Brass fittings shall be in accordance with FED SPEC QQ-B-626.
4.0 Construction The shielding panels shall be of wood frame construction with shielding materials covering the exterior side of the frame. Frames shall be constructed with plate-forming techniques using wood plates and glue to prevent "antenna action" caused by metal fasteners. Frames shall have cross braces, glued solid at one end and left floating at the other so panels can "work" as ambient conditions change without degrading the shielding integrity and effectiveness. Panels shall be joined together by an inside through bolt with a tee nut V4- 20 fastener every 4 in. on center along the seams to provide continuous, constant, and uniform shield contact pressure. Service panels will be provided with solid sections as required to furnish proper mounting surfaces for power filters, telephone filters, RF connector, and waveguide assemblies. Floor panels shall have cross bracing 12 in. on center with a 3/4 in. plywood overlay capable of supporting a uniform load of 1200 Ib/ft2 •
5.0 Doors (Doors shall be selected from Chapter 7, based upon the shielding performance required and the type of activity to take place within the shielded enclosure.)
6.0 Accessories Filters shall be of the conventional type, the level of performance consistent with the shielding system. See Chapter 8 for details. Waveguide air vents shall be used to supply intake and exhaust air. See Chapter 7 for details. Grounding shall be accomplished with a 1/2 in. diameter threaded brass grounding stud located near the filter panel. Pipe penetrations shall be of the clamping type and supported on one side of the wall. The diameters and lengths shall be consistent with the recommendations given in Chapter 7. All panels shall be set in a straight, true line, with level and even surfaces such that all panel seams will be in alignment and provide electromagnetically tight seams. All surfaces shall be thoroughly cleaned of all foreign matter resulting from handling. All contacting surfaces shall be free of defects which may inhibit good contact between panels.
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Appendix A
7.0 Shielding Effectiveness The shielding effectiveness shall be in accordance with the following. Magnetic Field
14 kHz
200 kHz
60 dB
100 dB
Electric Field
10 kHz 100 dB
30 MHz 100 dB
Plane Wave
100 MHZ 100 dB
1 GHz 70 dB
Microwave
IOGHz 60 dB
8.0 Quality Assurance The installer shall submit shop drawings showing the layout of the enclosure and its integral accessories for the customer's approval. The installer shall have a minimum of three years' experience in the installation of RFI-EMI shielded enclosures. The installer shall provide independent test data which demonstrate that the shielded enclosure meets the requirements specified. The testing shall be in accordance with MIL-STD-285 (modified).
A-S SAMPLE SPECIFICATION FOR A WELDED ENCLOSURE 1.0 General 1.1 Purpose. This section specifies the design, construction, and quality assurance testing requirements relative to the electromagnetic performance of a welded radio frequency shielded enclosure. Other criteria related to the submittals and components specified herein appear in other sections of this document, as follows. • • • • •
Section: Section: Section: Section: Section:
General Requirements for Contractor Submittals Metals Doors and Windows General Mechanical Requirements General Electrical Requirements
1.2 Scope. This section defines the following. • • • • • • •
The welded steel shield. The shielded doors. All electrical and mechanical penetrations of the shield. All filter/surge arrester assemblies, including their RFI enclosures. All conduit runs between filter/surge arrester assemblies and the welded shield. All RFI welded pull boxes and junction boxes. All RFI shielded conduit runs.
1.3 Shielding Specialist. All work under this section shall be provided by a "shielding specialist" or under the supervision of a "shielding quality assurance specialist." A "shielding specialist" shall have successfully completed at least five (5) similar shielding projects within the past ten (10) years. A "shielding quality assurance
A-S
Sample Specification for a Welded Enclosure
191
specialist" shall have performed the quality assurance program for at least five (5) successfully completed similar shielding projects within the past ten (10) years. The government reserves the right to approve the specialist, based upon information and references provided as required under paragraph 3.1.
2.0 Applicable Documents The publications listed below form a part of this specification to the extent given. In the event of a conflict between a referenced document and this specification, the specification shall take precedence.
2.1 Military Standards MIL-STD-220A: MIL-STD-248B: MIL-STD-285:
Method of Insertion Loss Measurement. Qualification Test for Welders. Attenuation Measurements for Enclosures, Electromagnetic Shielding, for Electronic Test Purposes, Measurement of.
2.2 Military Specifications 2.3 Others. NSA 65-6.
3.0 Submittals The contractor shall submit data identified in paragraphs 3. 1-3.7.
3.1 Shielding or Shielding Quality Assurance Specialist's Credentials. The contractor shall submit identification and credentials of shielding specialist or shielding quality assurance specialist, establishing evidence of the experience required by paragraph 1.3. The contracting agency, contract number, and contracting officer shall be identified on those projects submitted in fulfillment of the experience requirement. 3.2 Materials Certification. Certificates attesting that the materials used in RF shield fabrication meet the specified requirements of paragraph 4.2 shall be submitted to the Contracting Officer. If requested, material samples shall also be delivered. 3.3 Shop Drawings. Shop drawings and as-built drawings shall be submitted for approval to the Contracting Officer as required by paragraph 4.3. All deviations from the project drawings shall be explicitly identified. Specifications and manufacturers' literature for commercial doors, honeycomb panels, filters, surge arresters, RF gaskets, and other commercial products used in the shielding penetration protection subsystem shall be attached to the shop drawings. See paragraph 3.7 regarding concurrent submission of shielding/penetration protection subsystem maintenance procedures.
3.4 Welder Qualification Plan and Qualification Certificates. Detailed procedures used for qualification of welders, as required by paragraph 4.5, and qualification certificates of personnel approved to perform RF shield welds shall be submitted to the Contracting Officer. 3.5 Quality Assurance Plan. The contractor's plan for in-progress testing of welds, complete (empty) shield performance testing, tests of shield doors, waveguides and honeycomb waveguide panels, filters, surge arresters, RF enclosures, and conduits,
192
Appendix A
special case tests identified in paragraph 7.8, and final acceptance testing shall be submitted for Contracting Officer approval before the start of shield fabrication. The plan shall establish the general framework of the quality assurance program and shall contain detailed test procedures as appendices. Detailed procedures shall identify system configuration for testing, instrumentation to be used, data requirements, test point locations, and measurement and calibration procedures.
3.6 Test Reports. Certified test reports, including copies of all original data, shall be submitted to the Contracting Officer. Any deviations from the test procedures submitted under paragraph 3.5 shall be discussed. Success or failure of the component to satisfy the criteria shall be clearly stated, and proposed resolutions of unacceptable performance shall be presented. It is emphasized that test data, where required, shall be for the shielding and penetration. 3.7 Maintenance Procedures. Procedures to preserve the performance of the shielding and penetration protection subsystem and maintain the contractors's warranty in effect shall be submitted to the Contracting Officer, along with shop drawings required under paragraph 3.3. Revisions of the maintenance plan, if required by as-built conditions, shall be submitted prior to performance of the final shield acceptance test. 4.0 RF Shield 4.1 Requirements. The RF facility shield shall be a fully welded structure which provides shielding effectiveness greater than or equal to minimum requirements of (state requirements). The RF shield shall be constructed for a useful lifetime of at least thirty (30) years when maintained in accordance with procedures supplied by the contractor. The shielding effectiveness requirements apply to the finished structure, with all electrical and mechanical penetrations installed and operating. Finish requirements for the RF shield appear in Section* of these specifications. 4.2 Materials. Sheet stock, welding rods, and other steel elements used to fabricate the RF shield shall comply with ASTM, AWS standards, and other requirements as specified. The contractor shall certify to the Contracting Officer that materials meet these requirements. If requested by the Contracting Officer, samples of these materials shall be submitted.
4.3 Shop Drawings. Shop drawings of the shielded enclosure with all details, materials, and erection data shall be submitted for approval to the Contracting Officer. Drawings shall indicate the materials, arrangements, thicknesses, sizes of parts, construction, fastenings, clearances, assembly and erection details, welding procedures, and necessary interfaces to the work of other trades. Drawings shall have been approved by a professional structural engineer and bear his seal. NOTE: Approval by a structural engineer may be unnecessary if the A-E drawings are very detailed and no deviations are made. The contractor is permitted to make minor deviations from the drawings to improve performance or producibility or to decrease cost. Such deviations shall be explicitly identified in the shop drawing and are subject to approval by the Contracting Officer. *Fill in as required.
A-5
Sample Specification for a Welded Enclosure
193
Such approvals, however, do not relieve the contractor of his obligation to conform to all performance specifications. Upon completion of the project, shop drawings shall be updated to show "asbuilt" configurations, and they shall be resubmitted.
4.4 Welding. The steel sheets shall be assembled into an RF-tight shield by continuous welding of all seams, joints, and corners. The metal electrode inert gas (MIG) process or other government-approved method shall be used. The surface shall be prepared by removing rust, scale, and other foreign materials, and completed welds shall be free of slag, gas pockets, wormholes, cracks, or incomplete fusion. Full penetration butt welds or lap welds shall be used, as shown in the drawings. Where backing material is required, it shall overlap by at least 1 in. on both sides of the weld location.
4.5 Welder Qualification.
Welders performing welding on the RF shield shall
be qualified in the specific procedures to be used in accordance with MIL-STD-248B,
Qualification Test for Welders. Qualification certificates for the welders shall be submitted to the Contracting Officer. Any welder producing an excessive number of unsatisfactory welds shall be retested and recertified or released.
4.6 In-Process Testing. The contractor's quality control plan shall include testing of 100% of all RF shield welds as described in paragraph 7.2. The plan shall address the procedures by which the contractor ensures that all welds are tested and that all defective welds are repaired and retested. 4.7 Complete (Empty) Shield Performance Testing. The contractor shall conduct measurements of the shielding effectiveness provided by the RF shield immediately after the shield has been completed, but before the concrete wear slab, interior finishes, and duct work have been installed. The testing shall be performed in accordance with procedures described in paragraph 7.3. NOTE: This test requirement imposes some constraints on the entire construction sequence. It is strongly recommended, however, for two reasons: 1) access for measurements is unrestricted at this time, and 2) deficiencies can be identified and repaired while the shield is still completely exposed. The government may wish to employ an independent test laboratory to perform the complete shield and final acceptance tests.
4.8 Final Acceptance Testing. After completion of the RF shield, installation of all penetrations, and installation and check out of all systems within the shield, the contractor shall demonstrate compliance with the shielding effectiveness requirements of (state requirements) using the procedures given in NSA 65-6 as described in paragraph 7.9. The warranty period shall commence on the successful completion of this final shield acceptance test. 5.0 Penetrations 5.1 General. All penetrations of the RF shielded enclosure and penetrations of the waveguide entryway (or vestibule) shield shall be protected in a manner which preserves the integrity of the shield.
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Appendix A
All known penetrations are identified on the drawings in the shield penetration schedule. No additional shielding penetrations shall be made without the approval of the Contracting Officer. 5.2 Main Personnel Entryway. The main personnel access into the shielded facility shall be via a shielded vestibule as detailed in the project drawings. The shield shall be constructed in accordance with Section 4 of these specifications. All penetrations of the vestibule shield shall be protected as specified elsewhere. NOTE: Alternative personnel access methods are discussed in Chapter 7. Since the entry doors are the most used and abused component of a shielding system, careful consideration should be given to their selection. 5.3 RF Shield Doors. All RF shielded doors shall provide at least 10 dB in excess of the enclosure shielding effectiveness requirements. Compliance with this requirement shall be demonstrated as required by paragraph 7.4. Emergency exit/equipment access RF doors shall be alarmed to indicate an open condition. Welds between the door frame and the RF shield are primary shield welds and shall be inspected as required in paragraph 7.2. Final acceptance testing of shielded doors shall be included in the acceptance test of the RF shield. Mechanical and hardware requirements on RF shield doors appear in Section* of these specifications. 5.4 Ventilation Penetrations. All ventilation penetrations shall be protected with honeycomb waveguide-beyond-cutoff panels with cutoff frequencies no less than 34 GHz. The frame of the honeycomb panel shall be welded into the RF shield with continuous seam welds. Maximum allowable pressure drops are as follows. Inches of water: 0.015 FtJrnin: 400 (Multiply by area to obtain CFM.)
0.025
0.042
600
800
0.065 1000
0.30 2000
5.5 Piping Penetrations. All piping penetrations of the RF shield shall be circumferentially welded to the steel liner at the penetration. The penetration stub shall have an unbroken length at least five times the inside diameter of the pipe, and it shall form a waveguide-beyond-cutoff with a minimum cutoff frequency of 18 GHz. 5.6 Waveguide Penetrations. Waveguide penetrations for dielectric fibers or hoses shall be implemented in the same manner as piping penetrations. No conductors (wires, fiber cable strength members, conductive rubber, etc.) shall pass through the waveguide opening. 5.7 Electrical Penetrations. All electrical wiring penetrations of the shield shall be protected with filter/electrical surge arrester (ESA) assemblies. The enclosure for the filter assembly shall be a two-compartment RF enclosure, as shown in the drawings, designed and tested to demonstrate compliance with the shield*Fill in as required.
A-5
Sample Specification for a Welded Enclosure
195
ing effectiveness requirements of this specification. Welded construction shall be used, and bolted and RFI gasket shielded access covers shall be provided into each compartment which has a shielding requirement. Access covers into compartments without shielding requirements shall be designed for easy removal. The electromagnetic filters shall provide an insertion loss greater than or equal to the requirements (state requirements). The contractor shall select and install all filters and test as required by paragraph 7.6.2. The government reserves the right to perform additional tests on filter assemblies as specified in paragraph 7.6.4.
(NOTE: There are two kinds of power filters, generally known as "X" and "W" series, on the market. The "X' " series filters are designed to achieve rated insertion loss under load when tested in accordance with MIL-STD-220A using extended range buffer networks. "W" series device data sheets will not contain the phrase "tested using extended range buffer networks," and may not satisfy the stated performance under full load at frequencies below 100 kHz. "X" series filters can also be differentiated from "W" devices by the fact that they are usually two-three times greater in weight.) Consideration should be made of the requirements of UL1283 on electromagnetic filters; see Chapter 8 for details. Electrical surge arresters shall be installed on the "dirty side" of each filter on HEMP facilities. The static breakdown voltage of surge arresters shall be between 150 and 200% of the maximum operating voltage on the associated circuit. Dynamic breakdown voltage shall not be less than 5000 V at a transient voltage ramp rate of 1 kVIns. Extreme duty discharge current shall be at least 65 kA in circuits with a lightning threat, 20 kA in other circuits, and 10 kA in control and communication circuits with less than 28 V. The conduit from the filter assembly to the RF shield shall be a rigid steel conduit, circumferentially welded at all joints and at the penetrations into the RF enclosure and RF shield. These circumferential welds are primary shield welds, and they shall be inspected as required by paragraph 7.2. 6.0 Special Cases This section is used to allow for special cases such as equipment located outside the shield, but which must be attached to the shield and still maintain the enclosure's shielding effectiveness. Appropriate performance specifications and quality assurance provisions must be provided for each special case.
7.0 Quality Assurance
7.1 General. All testing required by this section of the specifications must be documented with test procedures and test reports as required by paragraphs 3.5 and 3.6. It is emphasized that certifications of specification compliance alone do not satisfy these requirements. The contractor shall notify the Contracting Officer at least two weeks prior to the performance of these tests. The government reserves the right to witness all required tests.
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Appendix A
NOTE: It is strongly recommended that qualified government representatives witness in-factory, as well as on-site, tests.
7.2 In-Process Weld Testing. All seam, joint, and corner RF shield welds, door frame and honeycomb panel frame welds to the primary shield, piping and conduit welds, and other welds in the shielding and penetration protection subsystem shall be tested for quality using one of the following methods. • • • •
Radiographic nondestruct ive test. Magnetic particle nondestructive test. Two-component dye penetrate test. SELDS nondestructive test.
All tests shall be in accordance with a contractor-prepared, government-approved test plan. The contractor shall maintain appropriate records to ensure that all welds are checked and to record results. All unsatisfactory welds shall be repaired and retested. Records shall also provide identification of welders producing an excessive number of unsatisfactory welds.
7.3 Complete (Shell) Shield Testing. Upon completion of the shield, but before starting interior work, the contractor shall perform a preliminary shielding effectiveness test of the shield. As a minimum, this test shall consist of SELDS or "sniffer" testing, H-field loop antenna measurements, and plane-wave sweep tests. 7.3. / . The sniffer test conducted at approximately 100 kHz shall be conducted by driving the entire shield in sections not larger than 500 ft2 • Sections shall overlay by at least 20%, and each section shall be driven from two perpendicular directions. All welds shall be probed for leaks. 7.3.2. H-field loop antenna measurements shall be taken in accordance with NSA 65-6 or MIL-STD-285 procedures to supplement the sniffer test. Testing shall be conducted at all penetrations. Door frames shall be tested at a minimum of ten (10) points around each frame. 7.3.3. A complete plane-wave sweep test of the shield shall be performed at 1 GHz using the procedures defined in MIL-STD-285 or NSA 65-6. 7.4 In-Factory Shielding Effectiveness Tests of RF Doors. All RF shielded doors with 120 dB shielding effectiveness requirements shall be tested by MIL-STD-285 procedures or in accordance with a contractor-prepared, government-approved test plan to demonstrate compliance with requirements of paragraph 5.3. These tests shall be completed after all mechanical tests have been completed.
7.5. In-Factory Tests of Honeycomb Waveguide Panels. At least one sample of the honeycomb panel of each type of construction to be installed in the facility shall be tested prior to installation to demonstrate compliance with shielding effectiveness requirements. Testing shall be performed in accordance with MIL-STD-285 or procedures in a contractor-prepared, government-approved test plan.
A-S
Sample Specification for a Welded Enclosure
197
7.6 Filter/Surge Arrester Assembly Testing 7.6.1. All RF filter enclosures shall be tested as described in paragraph 7.7. 7.6.2. One of each type of filter to be installed shall be tested in factory in accordance with the requirements of MIL-F-15733G and UL1823. 7.6.3. One of each type of surge arrester to be installed shall be tested in factory, and all surge arresters shall be tested after installation to demonstrate satisfactory operation under dynamic excitation conditions. Minimum acceptable drive levels are as follows. Peak Current (/): 500 A. Rise time to 90% J (1): 1.2 us (max). Fall time to 30% I (1): 6 x t or Ills, whichever is greater. Testing shall be performed in accordance with a contractor-prepared, governmentapproved test plan.
7.7 Final Acceptance Testing. Upon completion of the construction of the shielding and penetration protection subsystem and all interior and exterior work with the potential to affect its performance, the contractor shall perform final acceptance testing to ensure compliance with the shielding effectiveness performance requirements (stated elsewhere) of this specification. Testing shall be performed in accordance with MIL-STD-285 or NSA 65-6 using the frequencies specified in NSA 65-6. The test points shall be determined as follows. • The entire primary shield surface shall be divided into test areas approximately 8 x 8 ft, and a test point shall be chosen in the center of each area. The test areas shall be shown in the detailed procedures for the final acceptance test. • Each penetration through the shield shall be a test point. • The RF doors and access panels shall be tested around the perimeter of the opening. RF doors shall have a minimum of ten (10) points. • Transmitting Antenna Deployment: Both horizontal and vertical polarization shall be tested in the plane-wave region. Test equipment used in this final acceptance test procedure shall have a dynamic range at least 6 dB in excess of the shielding effectiveness requirement at the corresponding frequency.
8.0 Remedial Action If any material, equipment, or installation in the shielding and penetration protection subsystem fails to meet the specified performance requirements, the contractor shall replace the defective components, repair the defective installation, and take any other actions required to ensure acceptable performance at no additional cost to the government. Furthermore, any contractor-initiated changes to the facility shielding and penetration protection after final acceptance shall be retested without additional cost to the government.
198
Appendix A
A-6 SPECIFICATIONS FOR ARCHITECTURAL SHIELDING SYSTEMS 6.1 SPECIFICATIONS FOR ALUMINUM FOIL SHIELDING SYSTEMS 1.0 General This specification defines the requirements for a shielded enclosure that is constructed on site and uses a heavy gauge aluminum foil for the barrier material. The enclosure is designed to meet the requirements of NSA 73-2A.
2.0 Applicable Specifications 2.1 NSA 73-2A: National Security Agency Specification for Foil RF Shielded Enclosures.
3.0 Materials The following materials are to be used in the construction of the shielded enclosure. The aluminum foil shall be a minimum of 0.0059 in. thick and meet the requirements of FED SPEC QQ-A-1876. The width shall be a minimum of 36 in. The shielding tape shall be a minimum of 2 in. wide, be embossed, and meet the following typical requirements. Thickness: 0.0042 in. Tensile Strength: 25 lb/in. Adhesion to Steel: 35 oz/in. Electrical Resistance Through Tape: 0.00 1 O/sq Fungus: Inert
4.0 Installation The foil is installed by applying a good grade of contact adhesive to the surface of the space to be shielded and to the back of the shielding foil. The layout should be made and the adhesi ve applied so that a 6 in. overlap occurs at all seams and the seam is metal-to-metaJ (no adhesive between layers). The shielding tape is used to seal the joints.
5.0 Doors The doors for foil installations should be of the modified industrial door design as discussed in Chapter 7. The door frames must overlap the aluminum shielding material completely around the perimeter of the door frame and then be taped.
6.0 Accessories The various accessories must be mounted in plates and then taped into position on the enclosure walls. Support for piping, waveguides, and filters must be provided external to the shield. A variety of penetrations for foil shielded enclosures is discussed in Chapter 7. .
Procurement Specification for Copper Foil Shielded Enclosures
Section 6.2
199
7.0 Shielding Effectiveness The enclosure shall meet the shielding effectiveness requirements of NSA 73-2A.
8.0 Quality Assurance The completed enclosure should have a preliminary shielding effectiveness test at 1 GHz prior to completing the interior treatments. Upon the completion of the entire facility, a full NSA 73-2A performance test shall be conducted. Special care should be taken to see that the doors are thoroughly tested.
6.2 PROCUREMENT SPECIFICATION FOR COPPER FOIL SHIELDED ENCLOSURES 1.0 General The shielded enclosure shall be constructed with 3 oz copper foil on a paper backing with all seams solder seamed. The material will be applied to the existing paint-ready drywall-constructed walls and ceiling and to the plywood-covered floor.
2.0 Applicable Documents MIL-STD-285: Attenuation Measurements for Enclosures-Test Method. NSA 73-2A: National Security Agency Specifications for Foil RF Shielded Enclosures.
3.0 Materials The shielding material shall be a plastic lamination of 3 oz copper foil to high-strength extensible kraft, tridirectionally reinforced with fiberglass scrim. The material shall conform to the following specifications.
1. Beach Puncture-Exceed 135 units as defined in ASTM D-781 . 2. Tensile Strength-Exceed 200 lb/in. machine direction and 135 lb/in. crossmachine direction as measured by ASTM-828.
3. Bond Strength-The bond strength shall exceed 2 Ib/in. on an Instron or Scott-type tensile tester. 4. Sheet Dimensions-60 in. wide x 120 ft long. The solder-seal copper seaming tape shall consist of a layer of 2 oz electrodeposited copper bonded to a layer of 0.00075 in. solder. The application of heat from an industrial iron on the copper side of the tape causes the solder to melt, thus forming a permanent, electrically continuous bond between it and the two adjacent sheets of copper foil. Specifications. The copper seaming tape shall meet the following requirements.
1. 2. 3. 4.
Copper Thickness-0.OO28 in. Solder Thickness-O.OOO75 in. of 60/40 solder Total Tape Thickness-0.0035 in. Tensile Strength-70 lb/in. 5. Adhesion Strength-23 lb/in,
200
Appendix A
4.0 Installation The foil is installed by applying a good grade of contact adhesive to the surfaces of the space to be shielded and to the back of the shielding foil. The material is supplied in rolls measuring 5 x 120 ft. Sheets are cut to conform to the surface of the space to be enclosed. Corner units are made by forming 24 in. wide material into 12 x 12 in. 900 bends. These can run the entire length of a wall. Three-way corners are preformed. All of the material is applied to the room surfaces with a waterbased vinyl wall covering adhesive. The copper sheets are to be butted at the joints, with no overlap. All seams are sealed with the solder-coated copper conductive tape 2 in. wide. The tape must be applied to a clean surface. Surfaces to be taped shall be cleaned with a solvent such as methylene chloride, MEK, or toluene. (Be sure to take proper precautions when handling these chemicals.) After cleaning, the surface is to be lightly sanded to ensure a good bond between the tape and copper sheets. After surface preparation, a noncorrosive flux is applied to the bond line. The tape should then be soldered in place utilizing an industrial iron at a minimum temperature of 4500 F. The iron is held to the tape until the solder is melted. The edges of the tape must be smooth and in complete contact with the copper sheets. Any ridges in the edges of the tape must be soldered utilizing a 50/50 solder.
5.0 Doors A knife-edge door or similar high-performance shielded door should be selected from Chapter 7. Care must be taken to see that the door frame is connected to the shielding foil with a continuous metal-to-metal seal around the entire perimeter of the door.
6.0 Accessories Filters, grounding studs, pipe penetrations, and vents should be selected from those described in Chapter 7.
7.0 Shielding Effectiveness The shielding effectiveness of the shielded enclosure shall meet the following requirements. Magnetic Field
10kHz 30 dB
200 kHz 50 dB
Electric Field
Plane Wave
10 kHz-50 MHz 100 dB
100 MHz-l GHz 100 dB
Microwave
100Hz 80 dB
8.0 Quality Assurance The manufacturer/installer shall submit shop drawings showing the layout of the enclosure and its integral accessories for approval. An experienced independent test laboratory shall conduct the shielding effectiveness testing in accordance with the provisions of MIL-STD-285 (modified). Preliminary and final acceptance testing shall be performed.
Section 6.3
Specification for Copper Alloy Shielded Enclosures
201
6.3 SPECIFICATION FOR COPPER ALLOY SHIELDED ENCLOSURES 1.0 General This specification describes a single-shield copper alloy shielded enclosure that meets the requirements of NSA 65-6. Standard RF doors and penetrations are used to provide services to the enclosure.
2.0 Applicable Specifications NSA 65-6 National Security Agency Specification for RF Shielded Enclosures for Communications Equipment: General Specification.
3.0 Materials The copper clad alloy shall consist of a layer of copper on either side of a minimum foil thickness of 0.008 in. of a permalloy material commonly designated as alloy 49. The overall thickness shall be a minimum of 0.010 in. The material shall be provided in rolls 2 ft wide x 50 ft long or 100 Ib, whichever applies. Preformed two-way and three-way corners shall be provided with a minimum leg length of 2 in. A 2 in. wide solder-plated tape shall be used. The solder shall melt when a 450 0 F soldering iron is applied to the seaming tape.
4.0 Installation The material is supplied in rolls. The preformed corners are installed using an industrial-grade contact adhesive. The sheet material is cut to butt to the corner materials and edge-to-edge along the walls and across the floor and ceiling. It is recommended that the floor be installed first and then covered with plywood. Once the sheets are adhered to the wall and ceiling, then the seams are sealed with the solder-plated shielding tape. Any ridges in the tape must be sealed with solder.
5.0 Doors High-performance doors consistent with NSA 65-6 performance shall be selected from those described in Chapter 7.
6.0 Accessories High-performance accessories shall be used in the construction of this shielding system; these are described in Chapter 7.
7.0 Shielding Effectiveness The shielding effectiveness shall be in accordance with the performance requirements of NSA 65-6.
8.0 Quality Assurance A preliminary SE test shall be conducted prior to completing the interior finishes. All doors, filters, and other penetrations shall be in place during the inspection.
202
Appendix A
The enclosure's final inspection shall be conducted by an experienced independent test laboratory in accordance with the requirements of NSA 65-6.
6.4 PROCUREMENT SPECIFICATIONS FOR THE SANDWICH SEAM SHIELDING SYSTEM 1.0 General The shielded enclosure shall be constructed using the sandwich seam construction method. The enclosure shall meet the shielding effectiveness requirements of NSA 65-6 through 400 MHz.
2.0 Applicable Specifications NSA 65-6 National Security Agency Specification for RF Shielded Enclosures for Communications Equipment: General Specifications.
3.0 Materials The galvanized sheet metal shall be in accordance with ASTM A-52?, G90. The seam cover strips are to be 6 in. wide, the backing strip made from 26 gauge and the cover strip from 24 gauge. The cover strip is prepunched with two rows of 3/16 in. diameter holes 2.5 in. apart, staggered 1.25 in., at 2 in. on either side of the center line of the strip. The 3/4 in. plywood shall be an AID grade in accordance with the U.S. Plywood Association standards. The screws shall be hexhead drywall type, 5/8 in. long. They shall be cadmium plated. A good grade of steel is recommended to minimize the risk of twisting the heads off during installation.
4.0 Installation The space to be shielded is enclosed using conventional metal or wooden studs. The latter are recommended if the shield is to be electrically isolated from the parent building. The entire interior surface is covered with the 3,4 in. plywood. The seam pattern is laid out on the plywood, and seams shall be located 12 in. from all corners, floor, ceiling, and walls. The remainder of the seams are located on 48.25 in. centers across all six surfaces of the space. The length of the panels is a function of the material used and the convenience in handling. The corner material is installed using 90 0 bends with 12 in. legs. The three-way corners are shop fabricated and solder sealed. The backing strips are installed using small nails or staples. Next, the shielding panels are installed. These can be held in place by a few drywall screws if care is taken to ensure that a metal-to-metal seal is obtained. The cover strips are then instaJled along with the shims at the junctions of the cover strips. Torque guns are recommended, using a preset torque consistent with the type of screws and materials used. It is essential that the screws be pulled up as tightly as possible so that a compression seal is obtained at each screw.
5.0 Doors Standard high-performance shield doors using the knife-edge design are recommended for use with the sandwich seam shielding system.
Section 7.1
Procurement Specifications for ReM or Knife-Edge Shielded Door
203
6.0 Accessories High-performance penetrations should be used with this form of shielding.
8.0 Quality Assurance As construction progresses, all screws must be visually inspected to ensure that they are properly installed. The heads must be pulled up tightly against the metal, spun out screws must be replaced, and those that have lost their heads shall be patched over with a lead cover and the screw relocated nearby. Multiple screws can be used if necessary to hold the sheet metal flat should a wrinkle occur during the installation of the sheet. Make sure that shims are installed in all joints of the cover strips. The shims shall be the width of the seams and extend at least 3 in. on either side of the gap. Upon completion of the enclosure, the shielding effectiveness of the installation shall be measured in accordance with NSA 65-6 procedures. It is recommended that a preliminary inspection be performed prior to the installation of interior treatments, especially at penetration points such as piping, vents, and doors.
A-7 SHIELDED PENETRATIONS 7.1 PROCUREMENT SPECIFICATIONS FOR RCM OR KNIFE-EDGE SHIELDED DOOR 1.0 General The RF shielded door shall be of the recessed contact mechanism (ReM) type, and shall be supplied complete with a metal frame, mounting hardware, latching mechanism, and knife-edge RF seal with beryllium copper fingerstock.
2.0 Applicable Specifications NSA 65-6: National Security Agency Specification. 3.0 Materials Fingerstock: The shielded door shall be supplied with a double row of RF fingerstock gaskets installed around the periphery of the frame in a recessed pocket. The fingerstock shall be secured in the frame without the use of special tools and without the application of solders. Latching Mechanism: The shielded door shall be supplied with a suitable two- or three-point latching mechanism that provides proper force to engage the RF seal. The latching system shall be of the rack and pinion design, and supplied with a suitable safety handle so that during operations, the handle will not mechanically interfere with the door frame when the shielded door is opened or closed. The latching mechanism shall operate with a maximum 20 lb pull or push from the inside or outside of the door, and be capable of a minimum of 10 000 cycles without physical degradation. Hinges: All doors shall be provided with a minimum of three hinges of the bearing type with a vertical adjustment. The door leaf shall be capable of being lifted off the hinges. The door hinges must be capable of 10 000 operations without physical degradation.
204
Appendix A
Door Panel: The door shall be a minimum of ¥4 in. thick, laminated with 24 gauge galvanized steel. A suitable knife-edge treatment shall be provided on the perimeter of the door leaf. Door Frame: The door frame shall be of one-piece construction, with a minimum clear opening of 36 x 84 in. unless specified otherwise. A suitable pocket housing the fingerstock shall be provided in the frame. Mounting Hardware: A set of mounting hardware shall be provided for clamping the door frame into the rough opening of the shielded enclosure. This shall be of the hat and flat geometry unless specified otherwise.
4.0 Installation The door shall be installed by an experienced installer who has a minimum of three years' experience in RF shielding using the installation hardware supplied by the manufacturer of the door.
5.0 Quality Assurance The RF door performance shall be a minimum of 10 dB higher than the enclosure requirement. This shall be demonstrated in a factory test prior to shipment. A minimum of ten test points shall be taken around the perimeter of the door at 100 kHz H field, 1 and 10 GHz plane wave using the test procedures specified in NSA 65-6.
7.2 PROCUREMENT SPECIFICATION FOR DOUBLY ISOLATED SHIELDED DOOR ASSEMBLY
1.0 Scope The door shall be side hinged, swinging, with electrically isolated solid metal shields on wood frames. The door shall be hung from three chrome-plated Delrin bearing hinges, each to be fastened with nine # 12 x 2 in. long wood screws to framing to provide sag-free mounting. The door opening shall be 36 x 84 in. (unless otherwise specified) and equipped with an inside 28 in. panic-type bar handle.
2.0 Applicable Specifications NSA 65-6.
3.0 Materials Jambs: The jambs shall consist of dressed, kiln-dried, 13,4 in. thick wood frames covered with 24 gauge galvanized steel on the interior and exterior, and shall be electrically isolated from each other. Fingerstock: Two rows of heavy-duty alloy 510 Grade A spring temper bronze contact finger strips, in accordance with ASTM 8103, shall be mounted around the periphery of the door, with their respective planes mutually perpendicular to provide leak-free contact with interior and exterior enclosure shields and maintaining electrical isolation between interior and exterior shields.
Section 7.3
Procurement Specifications for Moderate-Performance Shielded Doors
205
Latching System: The latching system shall be accomplished by a three-point cam pressure contact system, operable from the interior and exterior of the room. The latch shall be provided with a gravity-spring interlock to prevent the latching mechanism contact from striking the jamb as the door is being opened or closed. Interior and exterior handles shall be electrically isolated from each other.
4.0 Installation The door shall come preassembled, and shall be installed by an experienced installer, one who has a minimum of three years' experience in RF shielding.
5.0 Quality Assurance Reliability Test: The door design shall be capable of mechanically enduring 2500 operating cycles without loss of shielding performance. The door design shall also be tested for a horizontal position loading of 40 Ibf/ft2 for a period of 10 min, and a 90 0 open position sag test of 120 Ib placed within 5 in. of the outer edge of the door for a period of 10 min, without any significant mechanical or RF shielding performance degradation. Shielding Effectiveness: The door shall be factory tested to meet the requirements of NSA 65-6, with a minimum of 120 dB electric and plane-wave shielding effectiveness.
7.3 PROCUREMENT SPECIFICATIONS FOR MODERATE-PERFORMANCE SHIELDED DOORS
1.0 General The shielded door shall consist of an electrogalvanized door and frame delivered as one assembly.
2.0 Applicable Specifications The publications listed below form a part of this specification to the extent referenced.
2.1 Military Standards MIL-STD-285. 2.2 ASTM Publications A591-77(R83):
Steel Sheet, Cold-Rolled, Electrolytic Zinc-Coated.
2.3 ANSI/SDI Publications 100-85: A151.1-80:
Recommended Specifications-Standard Steel Doors and Frames. Test Procedure and Acceptance Criteria for Physical Endurance for Steel Doors and Hardware Reinforcing.
2.4 National Security Agency NSA 73-2A:
Specifications for Foil RF Shielded Enclosures.
206
Appendix A
3.0 Materials Custom Hollow Metal Doors: Provide custom hollow metal doors where nonstandard steel doors are indicated. Door size(s), design, materials, construction, and finish shall be as specified. Steel sheet shall be a minimum of 16 gauge. Door Construction: Doors, frames, and gasket retainers shall be formed from electrogalvanized steel sheets conforming to ASTM A-591. Work shall be assembled using all welded construction conforming to pertinent requirements of AWS D1-1. Frame corners shall be mitered and welded. Both doors and frames shall be supplied with all necessary internal hardware reinforcements. Frames shall be supplied with adjustable anchors for built-in expansion shields for bolt-in installation. When bolt-in construction is specified, frame shall be provided with prepunched and dimpled mounting holes. Door RF Gasketing System: Each door frame shall be equipped at the head and jambs with EMI gaskets. The gasket shall be installed in a retainer that allows the gasket to be adjusted to attain maximum shielding effectiveness. The bottom of the door shall be equipped with a Monel knitted wire EMI gasket in a retainer that permits adjustment. A continuous strip of shielding tape (tin-copper) shall be bonded to the inside perimeter face of door at the sides and top where EMI gaskets make contact. The RFI door bottom gasket shall seal against a bronze low-profile threshold.
4.0 Installation The door shall be installed by an experienced installer who has a minimum of three years' experience in RF shielding using the installation hardware supplied by the manufacturer of the door. The door and frame shall be finished in accordance with ANSI/SOl 100. The final finish shall be two coats of low-gloss enamel in accordance with the color plan for the installation. 5.0 Quality Assurance
The RF door performance shall be a minimum of 10 dB higher than the enclosure requirement. This shall be demonstrated in a factory test prior to shipment. A minimum of ten test points shall be taken around the perimeter of the door using the procedures outlined in NSA 73-2A. 7.4 PROCUREMENT SPECIFICATIONS FOR
ELECTROMAGNET LATCHED RF SHIELDED DOORS 1.0 General The RF shielded door shall be of the electromagnet latched type and shall be factory assembled and individually tested for compliance to the contract requirements. Door units shall be complete consisting of door leaf, door frame, finish hardware, electrical contact strips, and electronic control circuitry. Doors shall be self-closing and selflatching without human or mechanical assistance, and shall be rated for a minimum of 500,000 cycles without loss of attenuation and without need for major adjustments.
2.0 SPECIFICATIONS NSA 65-6.
Section 7.5
Procurement Specification for Doubly Isolated Shielded Door Assembly
207
3.0 MATERIAL The door frame shall be welded cold-rolled steel, a minimum of 12 gauge and shall be plated after fabrication. The periphery of the door opening shall be free of protrusions made by latches, locks, or other devices. The inner perimeter of the frame shall be continuously bonded with an electromagnetic coil situated in an RF-shielded pocket. Electrical contact between the door frame and door leaf shall be accomplished by magnetic attraction of the door leaf to the contact strips of the coil pocket. No solder, fingerstock gasket, or other fragile components shall be employed in the construction of the frame. The door leaf shall be fabricated using a solid piece of steel, a minimum of 18gauge thickness, bonded to a supporting structure, to a finished leaf thickness of approximately 11/ 4 in. The door leaf shall be sized to overlap the electrical contact strips when the door is in the closed position. The door leaf shall be constructed in a manner which allows mounting of the opening devices, hinges, and other items without through-bolting or other penetrations which might limit the shielding effectiveness of the door leaf. The hinges shall be heavy-duty load-bearing allowing the door leaf to engage the electrical contact strips in a flush manner, with minimum rubbing or chafing of the contact surfaces. A minimum of three hinges shall be provided for 3 x 7 ft/doors or otherwise specified in the contract. The door shall be provided with an exterior opening device utilizing a pull-type handle mechanism designed to comply with all requirements of the uniform building code. The exterior opening device shall incorporate in its design an electrical switch which shall cause the electromagnet to de-energize whenever the opening device is activated. A set-up mounting hardware shall be provided for installing the door within the shielded enclosure, if for a welded enclosure then a suitable sheet metal interface shall be provided compatible with the installation. 4.0 INSTALLATION
The door shall be installed by an experienced installer who has a minimum of three years experience in RF shielding using the installation hardware supplied by the manufacturer.
5.0 Quality Assurance The RF door performance shall be a minimum of 10 dB higher than the enclosure requirement. This shall be demonstrated in a factory test prior to shipment. A minimum of 10 test points around the perimeter of the door shall be tested at 100kHz magnetic, I GHz and 10 GHz plane wave using the procedures specified in NSA 65-6.
7.5 PROCUREMENT SPECIFICATION FOR DOUBLY ISOLATED SHIELDED DOOR ASSEMBLY 1.0 Scope The door shall be side hinged, swinging with electrically isolated solid metal shields on wood frames. Door shall be hung from three chrome-plated Delrin bearing hinges and
208
Appendix A
each to be fastened with nine # 12 x 2 in. long wood screws to framing to provide sag-free mounting. Door opening shall be 36 x 84 in. (unless specified otherwise) and equipped with an inside 28-in. panic-type bar handle.
2.0 Applicable Specifications NSA 65-6
3.0 Materials Jambs: The jambs shall consist of dressed, kiln dried, 1~ inch thick wood frames covered with 24-gauge galvanized steel on interior and exterior and be electrically isolated from each other. Fingerstock: Two rows of heavy duty alloy 510 grade A spring temper bronze contact finger strips, in accordance with ASTM B103, shall be mounted around the periphery of the door with their respective planes mutually perpendicular to provide leak-free contact with interior and exterior enclosure shields and maintaining electrical isolation between interior and exterior shields. Latching System: The latching system shall be accomplished by a three-point cam pressure contact system, operable from the interior and exterior of room. The latch shall be provided with gravity-spring interlock to prevent latching mechanism contact from striking the jamb as the door is being opened or closed. Interior and exterior handles shall be electrically isolated from each other. 4.0 Installation The door shall come preassembled and be installed by an experienced installer, one who has a minimum of three years experience in RF shielding.
5.0 Quality Assurance Reliability Test: Door design shall be capable of mechanically enduring 25,000 operating cycles without loss of shielding performance. The door design shall also be tested for a horizontal position loading of 40 Ibf/ft2 for a period of 10 min, and a 90° open position sag test of 120 Ib placed within 5 in. of the outer edge of the door for a period of 10 min, without any significant mechanical or RF shielding performance degradation. Shielding Effectiveness: The door shall be factory tested to meet the requirements of NSA 65-6 with a minimum of 120 dB electric and plane-wave shielding effectiveness.
7.6 PROCUREMENT SPECIFICATION FOR SHIELDED VENTS 1.0 Scope
Air conditioning and heating ducts shall be installed through the walls and ceiling of the shielded enclosure with specially constructed vents using steel or brass honeycomb material a minimum of 1 in. thick.
Section 8.1
Specification for RF Power Line Filters
209
2.0 Applicable Specifications NSA 65-6.
3.0 Materials Frames: The frames shall be a minimum of 2.5 in. wide, made of zinc-plated steel, I/S in. thick. The hole pattern for the clamping hardware shall have a maximum spacing of 2 in. Heavy-duty V4-20 hardware shall be used to assemble the clamping structure. Honeycomb: The honeycomb structure shall consist of 1010 steel ribbon formed into hexagonal cells and then solder plated. The material shall be a minimum of 1 in. thick. Mounting: The honeycomb shall be mounted in a frame so that a good metalto-metal seal is obtained around the perimeter of the honeycomb structure. This may take the form of aframe soldered to the honeycomb or the clamping structure shall be designed to provide a good seal.
4.0 Installation The honeycomb vent shall be installed by an RF shielding contractor with a minimum of three years' experience.
5.0 Quality Assurance All vents installed in the shielded enclosure shall be individually tested in accordance with the applicable testing requirements. The vent shall be visually inspected to ensure that proper installation provisions have been provided for attachment of the HVAC ductwork.
A-8 SAMPLE PROCUREMENT SPECIFICATIONS FOR RF FILTERS 8.1 SPECIFICATION FOR RF POWER LINE FILTERS
1.0 Scope The filters specified in this section shall be supplied by the shielded enclosure manufacturer and mounted under the supervision of the shielded enclosure installer.
2.0 Applicable Specifications The publications listed below form a part of this specification to the extent referenced. The publications are referred to in the text by the basic designation only.
2.1 Military Specifications (MIL-SPEC) MIL-F-15733G:
Filter, Radio Interference, General Specification for.
210
Appendix A
2.2 Military Standard (MIL-STD) MIL-STD-202F: MIL-STD-220A:
Test Methods for Electronic and Electrical Component Parts. Method of Insertion-Loss Measurements.
2.3 National Fire Protection Association (NFPA) Publication 70-19XX:
National Electrical Code.
2.4 Underwriters' Laboratories, Inc. (UL) Publications 486A-1980: 1283-1984:
Wire Connectors and Soldering Lugs for Use with Copper Conductors. Electromagnetic Interference Filters.
3.0 Materials Radio Frequency Filters: Provide the filter units specified on the facility drawings. The units shall be designed to reduce conducted RF energy in the electrical power line according to MIL-F-15733 and UL 1283 for facility-type power line filters. Insertion loss between the load side of the filter and the power supply side shall be not less than 100 dB from 14 kHz to 10 GHz. Filter Units: Each filter unit (insert) shall be capable of being mounted individually such that a replacement is readily possible, and it shall include one filter for each phase conductor of the power line and neutral conductor. Enclosure: Filter units shall be provided in an RF modified NEMA Type 1 enclosure made of steel of not less than 14 gauge with welded seams. The enclosure shall be hot tin dipped after fabrication and welding. Internal configuration: The load terminal compartment shall be separated from the power input compartment by a solid steel barrier plate of the same gauge as the filter unit enclosure, extending across the entire width of the enclosure. The power input compartment shall house the individual power line filters and the power input terminal of the filters. Individual filter mounting: The load terminal end of the individual filter cases shall be attached to the RF barrier between the' two compartments. An RF-tight seal shall be provided between the filter case and the barrier. The load terminals of the filters shall project into the load terminal compartment. The case of each filter shall be attached to the enclosure to prevent stress being applied to the RF seal between the filter case and the RF barrier plate. Neutral connection: When the neutral conductor is not filtered, it shall be routed through the enclosure and connected to a stud welded to each side of the RF barrier plate so that the neutral is electrically connected to the filter unit enclosure. Conduit connections to enclosures: The load terminal and power input compartments shall have no knockouts, and each compartment shall have one threaded conduct hub. The hubs shall be seam welded in place, and shall be sized and located as required for the conduits per the purchase order. Access openings and cover plates: Access shall be from the front of the enclosure. The access opening for the load terminal compartment shall provide clear access to
Section 8.1
Specification for RF Power Line Filters
211
the filter load terminals and the standoff insulator terminals or insulated terminal blocks specified herein. The power input compartment opening shall provide clear access to the filter power input terminals and the standoff insulator terminals or insulated terminal blocks specified herein. It shall also allow easy removal of the individual filters from the enclosure. Provide two access cover plates. One plate shall cover the access opening to the load terminal compartment only, and when secured in place, shall provide an RF-tight seal with the compartment it covers. The second access cover plate may abut or overlap the cover plate for the load terminal compartment and shall cover the power input compartment. An RF gasket shall be provided for the load terminal compartment. The cover plates shall be secured with bolts having a maximum spacing of 3 in. Access cover plates shall be made of steel not less than 14 gauge, and the finish shall be the same as specified for the enclosure. Plates shall be attached so they may be easily removed and replaced. RF attenuation requirements for load terminal compartment: The load terminal compartment shall provide an attenuation of not less than 100 dB to radiated RF energy from 14 kHz to 10 GHz, with the individual power line filters mounted and the access cover plate attached. Filter Connections: Individual filters within a unit shall be equipped with insulated terminals, and shall incorporate suitably sized flexible leads from the insulated filter terminal to standoff insulator terminals or insulated terminal blocks. The standoff insulator terminals or insulated terminal blocks shall be mounted in the terminal compartments. Solderless lugs shall be provided for connecting the phase and neutral wires to the filter units. The lugs shall be of the hex head bolt or screw type and shall conform to UL 486A. Live parts shall be spaced in accordance with NFPA 70. Filter leads shall be copper. Individual Filters
Filter construction: Individual filters shall be sealed in a steel case. After the filter is filled with an impregnating or encapsulating compound, the seams shall be welded. When a solid potting compound is used to fill the filter, the filters may be mechanically secured and sealed with solder. Hermetically sealed impregnated capacitors shall be used, or the complete filter assembly shall be vacuum impregnated. Individual filter cases shall be fabricated from not less than 16 gauge steel and finished with a corrosion-resistant plating. Impregnating or potting compound: The filter shall be filled with an impregnating or potting compound meeting the requirements of MIL-F-15733 and having a flash point for operating range B as defined in Table VIII of MIL-F-15733. Overload requirements: Provide as specified in MIL-F-15733. Current rating: Provide filters to ratings specified in the shielded enclosure requirements. Passband: The passband shall be suitable for use with the 60 Hz power source. Total harmonics generated by the insertion of a power line filter shall not increase the line voltage distortion more than 2.5% with a unity power factor load. Voltage rating: As required for the circuits specified. de (resistive) voltage drop through the filter shall not exceed 0.5 V when the filter is operating at rated cur-
212
Appendix A
rent. The 60 Hz ac voltage within the resistive load variations from + 10 to 100% rated load shall vary not more than +1- 1% of the rated line voltage at a unity power factor. Drainage of stored charge: provide filters with bleeder resistors to drain the stored charge from the capacitors when power is shut off. Drainage of stored charge shall be in accordance with NFPA 70. Temperature rise: Temperature rise shall not exceed 25° C when operating at full rated load in a free-space environment equivalent to that specified in MIL-F-15733 with an ambient temperature of 65° C. When filters are mounted in an enclosure as specified herein, the temperature rise of the hottest filter shall not exceed 40° C at full load when operating in an ambient of 65° C. All components of the filter shall be suitable for continuous full-load operation at a temperature of 125° C without derating. Dielectric withstand voltage: Provide filters which, as a minimum, conform to the value of dielectric withstand voltage of UL 1283. Marking of Filter Units: Provide manufacturer's nameplate of each filter unit, stating its rated current, rated voltage, operating frequency, number of phases for which it is designed, manufacturer's name, total filter unit weight, and model number. The nameplate shall be mounted on the filter unit to be visible after installation without removing the cover plates or disturbing the interior parts or wiring. Each individual filter case shall be marked with the rated current, rated voltage, manufacturer's name, type of impregnating or potting compound, operating frequency, and model number. In addition, individual filter cases and the filter enclosures shall be durably marked by the manufacturer with the following: "WARNING: Before working on filters, terminals must be temporarily grounded to ensure discharge of capacitors." Nameplates and warning labels shall be attached with epoxy, rivets, or sheet metal screws.
A-9 SAMPLE TEST SPECIFICATION 1.0 Scope The shielded enclosure shall be tested in accordance with NSA 65-6, except as clarified herein.
2.0 Applicable Specifications NSA 65-6.
3.0 Test Equipment The external attenuator shall be capable of covering the range of the tests plus 10 dB. The dynamic range of the instrumentation shall exceed the test requirement by 6 dB.
4.0 Preliminary Tests A seam leak test (SELS) of the enclosure shall be conducted on every seam in the shield, and the signal level on the receiver shall not exceed a predetermined level based
Section 8.1
Specification for RF Power Line filters
213
upon the final shield effectiveness requirements. For example, a welded enclosure should not exhibit any discernible reading on the most sensitive scale. A 100 dB clamp up should not have any reading exceeding midrange on the most sensitive scale, etc. A preliminary test of the enclosure shall be performed after the entire shield has been installed, including all power line filters, air vents, doors, and other penetrations of the shield. The testing shall be conducted prior to the installation of any interior walls and finishes. The tests shall be conducted at 150 KHz magnetic and I GHz plane wave as a minimum. Any deficiencies in the shield will be brought to the attention of the general contractor at this time. Corrective measures shall be taken and the area retested to verify compliance. The following test points are where the measurements shall be performed for both the preliminary and final acceptance tests. All pipe penetrations, filter penetrations, and waveguide vents must be tested. The shielded doors shall be tested at a minimum of six points around the perimeter of the frame. The walls shall be tested at 10ft intervals at seam locations for the magnetic and electric fields and 20 ft for the plane wave.
5.0 Final Acceptance Testing The final acceptance test will be performed upon the completion of all construction work within and on the shield. The same test points as measured during the preliminary test shall be repeated. All specified test frequencies and field types shall be tested. These measurements will verify and document the shielded enclosure's compliance with NSA 65-6. The final acceptance test shall be conducted by an independent testing agency equipped and skilled at performing NSA 65-6 and MIL-STD-285 shield effectiveness testing. 6.0 Final Report
A final report shall be prepared and submitted within ten days of completing the test. All test data sheets shall be included as an appendix to the report. The data shall be summarized in a table showing the worst case data point at each frequency for each field type. A statement certifying that the test was conducted in accordance with NSA 65-6 shall be included.
APPENDIXB
A BRIEF INTRODUCTION TO INTERFERENCE TECHNOLOGY ENGINEERS' MASTER (ITEM), A SOURCE DIRECTORY FOR RF SHIELDING ENCLOSURE MANUFACTURERS AND INSTALLERS, PUBLISHED ANNUALLY BY R&B ENTERPRISES, DIVISION OF ROBAR INDUSTRIES, INC. 20 CLIPPER ROAD, WEST CONSHOHOCKEN, PA 19428-2721, (215) 825-1960.
This publication appears annually as a directory and design guide for the control of EMI and other electromagnetic effects. A series of design articles is provided, divided into approximately ten sections. A products and services section is also provided, as well as a directory of suppliers. Most of the prominent shielding manufacturers and installers advertise in this annual publication.
215
APPENDIXC
Selected Bibliography
For those readers who need additional details on a given subject, the following references may be of assistance. R. Aronson, "RFI/EMC shielded construction systems, methods and evaluation," ITEM, 1980. S. Austin, "Shielding options for penetrations," ITEM, 1987. H. Bloks, "NEMP/EMI shielding," EMC Technol., vol. 5, no. 6, Nov.-Dec. 1988. 1. J. Crenca, "Basic design parameters of acoustically treated shielded enclosures," ITEM 1990. G. P Condon, "Shielded enclosures leak detection: A simplified method," ITEM, 1989. W. E. Curran, "New techniques in shielding," ITEM, 1984. - - , "Shielding for HEMP/TEMPEST requirements," ITEM, 1988. H. W. Denny, "Grounding for the control of EMI," Don White Consultants, 1983. - - , "Grounding in the design of buildings and facilities for safety protection," EMC Technol., vol. 2, no. 1, Jan.-Mar. 1983. M. L. Eaton, "Shielding: An introduction," ITEM, 1989. M. Farsi, "EMIJRFI shielding: Theory and technique," ITEM, 1988. 1. Fuller, "Maintenance-free shielded personnel access device," EMC Technol., vol. 7, no. 8, Nov.-Dec. 1988. 1. A. Graham, "A new approach to architectual shielding," ITEM, 1990. - - , "RF shielding: Cost comparisons and tradeoffs," ITEM, 1987. V. W. Groh, "Shielded enclosure leak detection testing," EMC Technol., vol. 7, no. 5, July-Aug. 1988. F. L. Helene, 4' Architectural shielding: Introduction and applications," ITEM, 1990. - - , "Architectural shielding," a short course, R&B Enterprises, 1989.
217
218
Appendix C
"Satisfying NSA 65-6 shielding requirements without shielded doors," ITEM, 1989.
L. H. Hemming, "Anechoic materials for conducting EMC tests in shielded rooms," ITEM, 1982. S. C. Jewell, "Aluminum foil RF shielding systems," ITEM, 1988. B. Keiser, Principles of Electromagnetic Compatibility. Dedham, MA: Artech House, 1987. M. 1. Lahita, "RF shielding concepts and testing: An introduction," ITEM, 1990. 1. Lindenberger, "Facility systems selection for secure shielded environments," ITEM, 1987. E. A. Lindgren, "How meaningful are comparative tests on R.F. enclosures"," ITEM, 1980. 1. M. Magnusson, "EMI/RFI shielding, a designer's guide to foil tapes," ITEM,
1982. N. 1. Quesnel, "Architectural shielding keeps pace with changing times," EMC Technol., Jan.-Feb. 1990. B. D. Salati and C. 1. Chapman, "Maintenance of aged modular shielded enclosures" ITEM, 1988. B. D. Salati, "Maintenance of shielded enclosures: Maintaining the configuration of a single point grounding system," ITEM, 1989. 1. G. Sketue, "Aperture shielding effectiveness," ITEM, 1988. C. S. Snow, "Grounding of RF shielded enclosures," ITEM, 1982. - - , "In progress testing of shielded welded systems," ITEM, 1984. - - , "RF-shielding tape," ITEM, 1987. G. Trenkler and R. Delagi, "The application of clad metals for EMI room shielding," ITEM, 1988. 1. R. Wamsley, "Facility grounding and bonding: How does it work?," EMC Technol., vol. 8, no. 6, Sept. -Oct. 1989. D. Weber, "Large shielded anechoic facilities," ITEM, 1980. 1. Weinstein, "An enclosure system that will provide both RF and acoustical attenuation," ITEM, 1987. D. R. J. White, "Shield design, methodology and procedures," Don White Consultants, 1986. - - , "Electrical filters, synthesis, design and applications," Don White Consultants, 1980. - .- , "Electromagnetic shielding materials and performance," Don White Consultants, 1975. - - , "Electromagnetic interference and compatibility, vol. 3," Don White Consultants, 1973. D. M. Whiteside, "RF shielding projects: the facilities and procurement manager's viewpoint," ITEM, 1987. R. Willich, "Considerations for the integrated design and construction of secure shielded facilities," ITEM, 1984.
INDEX
A Absorber. definition, 3 Absorber lined tunnel entrance, 113 Absorption, 16, 18 loss, definition, 3 Acoustic ceiling, 70 Aluminum foil shielding, 76 Ambient level, definition, 3 Anechoic chamber, 10 Antenna: definition, 3 configurations, lSI effect, 14, 76, 97, 150, 155 Aperture, definition, 3 ASTM E84-8IA, 172 ASTM E90-83, 171 ASTM E413-73, 171 Attenuation, definition, 3 Automatic RF doors, 115
B Basic ReM door geometry, 104 Bolt together shielding systern, 47 Black designation, definition, 179 Bond, definition, 3
Bonding, definition, 3 Butt weld, 58~ 60. 61
c Ceiling hangers, 71 Ceiling span, 173 Chemical grounds, 165 Classified data processing, 9 Common testing problems, 155 Communication filters. 140 Compression seal, 107 Conducted emission, 8 Conducted interference: definition, 3, 131 descri bed, 8 Conductive coating. copper paint, 93 Conductivity, 18 Control line filter, 141 Copper: alloy shielding, 81 performance, 81 foil shielding, 79 screen shielding, 79 Corner seams, welded, 64 Corrosion, control, 70 Counterpoise, see Ground; systems Coupling:
capacinve, 13 definition, 3 far field, 13 free-space, definition, 3 inductive, 13 near field, 13 radiated, 14 Current rating, 134, 135, 137 Cut-off frequency: definition, 3, 17 equation, 17
D Data-line filters, 141 Definitions, 3-5, 179 Degradation: definition, 4 examples, 21 Doors, 100 Double-isolated shield systern, 41 Double knife-edge door, 104 Duo-shield filters, 137 Dye penetrant testing, 159
E Earth electrode system, definition,4
219
220 Earth electrode system test, 165 Earth ground: direct method, 165 two terminal method, 165 three terminal method, 165 See also Earth electrode system test Earth resistivity measurement, 166 Electric field: coupling, 14 definition, 4 measurements, 150 strength, 163 Electromagnet RF door, III Electromagnetic compatibility (EMC), definition, 4 Electromagnetic interference (EMI), 8 definition, 4 Electromagnetic pulse (EMP), 9 definition, 4 description, 9 Electronic surge arresters (ESA), 175 EMI, see Electromagnetic in-
terference, definition Emission, 9 EM~ energy versus frequency spectrum, 9 Enclosure isolation, 163 Enclosure stability, 40 Equipment shielding, 10 Expansion joints, welded, 66
F Facility: definition, 4 shielding, 10 Factor B, 16, 18 FaJl-of-potential method, 167 Far field: definition, 4 region, 4 Fault: definition, 4 protection, 161 Federal specifications, 179-213 Fiber optics, 124 Field strength, definition, 4 Filter:
Index
definition, 4 characteristics, 133 communications, 139 configurations, 132 control line, 141 current rating, 134 data, 140 frequencycharacteristics, 134 models, 137 power line, 137 specifications, 135 telephone, 139 theory, 131 transient suppression, 175 voltage rating, 134 Fingerstock gasket, 104 Fire protection: pipe penetration, 120 systems, 127 Flame spraying, 30 Floor loading, 38 Fluorescent lamp, 8 Foil, pressure-sensitive, 32 Foil, waveguide beyond cutoff. 32 Fraunhofer region, see Far field Frequency characteristic, see Filter
G Galvanized steel enclosure, 36 Gasket, 101 Green wire, 162 Ground: definition, 4 facility, 163 chemical, 165 multipoint, 164 plane, 161 resistance measurement, 165 rod, 164 single point, 165 systems, 164 vertical rod, Grounding: design guidelines, 164 principles, 161
H Handicap access, 101
Hats and flats, 36 HEMP, 9, 175 Honeycomb, 18 vent, 116 Hoses, 124
I IEEE 299, 146 Inductive coupling, see Coupling Inhomogeneous interface, 16 Insertion loss, see Filter INSTAR shielding system, 90 Insulated pipe design, 122 Interior, columns, 65 partitions, 96 wall treatment, 96 Internal reflections, 18 Installation, shield, 51
K Knife-edge door, 103
L Lap welding, 60 Leakage: effects, 20 sources, 20 Lightening effects, 13 Lindsay shielding system, 46 Low frequency magnetic shielding effectiveness, 73 Low performance door, III
M Magnetic field: coupling, 13 definition, 4 measurements, 151 Magnetic particle testing, 158 Magnetic permeability, 3 Magnetic shielding of NMI systems, 45 Metal-oxide varistor (MOV), 197 Metal-to-metal seals, 10I Metalized fabrics, 92, 93
221
Index Military specifications: MIL-E-4957A, 180 MIL-E-8881, 180 MIL-E-18639A, 180 MIL-F-15733, 135 MIL-HDBK-232, 2 MIL-HDBK-419A,2
Military standards: MIL-STD-220A, 171 MIL-STD-248B, 175 MIL-STD-285,52
test requirements, 144 Modular pipe penetration, 121-22 Modular shielding, 35 advantages of, 47 assembly requirements, 51 critical considerations of. 51 disadvantages of, 47 effects of water on, 51 moisture effects, 38 Moisture problems, 38 MRI enclosures, 44 Multiple refections, 16 Multipoint ground, definition,4
N National Electrical Code® (NEC®): definition, 5 referenced, 165 Nearfield, definition, 5 Neutral, definition, 5 NMR enclosures, 44-46 NSA 65-6, 145
specifications, 147 test, 148 Pipe design, 118 Piping: architectural enclosure, 121 modular enclosure, 121 welded enclosure, 120 Plane wave: definition, 5 measurements, 155 Pneumatic RF door, 108 Power line filters, 137 Purpose of handbook, I
R Rack and pinion, 101 Radiation, definition, 5 Radiation resistance, definition,5 Radiated emission, see Radiated interference Radiated interference, 8, 13 Radio frequency interference (RFI), definition, 5 Recessed contact mechanism (RCM), 103 Reflection, 16 ReM seal, 103 Red/black concept, definition, 179 Red designation, definition, 179 Reflection loss, definition, 5 RF doors, large, 115 RF door seal requirements, 100 RF-tight, definition, 5
NSA 73-2A, 1, 145-46
s
p
Safety ground, 161 Sample specifications, 179-213 Sandwich seam shielding systern, 83 typical performance, 86 Screen room, 43, 91 Seam leak testing (SELDS), 156 Seams: clamped, 25 taped, 30 sandwich seam, 30 single-shield, 30
Paint: conductive, 93 metal filled, 71, 93 Pan weld, 57, 58 Penetration: control of, 99 definition, 5 design criteria, 99 types, 100 Perfomance: degradation, 20
welded, 25 SE measurements, accuracy of, 154 Shield, definition, 5 Shielded enclosure, 35 definition, 5 most common form, 35 performance, 35, 147 Shielded windows, 126 Shielding: barrier, 13 door, 100 electric, 13 honeycomb, 116 magnetic, 13, 18 microwave, 150 nonwoven material, 92 panels, 36 performance specifications, 144 plane wave, 20 specialist, 176 theory, 13 volume, 55 woven material, 93 Shielding effectiveness (SE): absorption, 18 aluminum foil, 22 copper foil, 21 definition, 5 galvanized iron, 23 internal reflections, 18 reflection, 16 welded enclosures, 72 Shield material for welded enclosures, 56 Shield seam, see Seams Shielding requirements, defining, 55, 148 Signal grounds, 163 Signal reference subsystem, definition, 5 Single point ground, 165 Single-shield modular enclosure, 44 Slab leveling, 169 Sliding RF door, 108 "'Sniffer" testing, 156 Sources of conducted interference, 8 Specifications, military, see Military specifications
222 Spring finger, see Fingcrstock gasket Standards. military, see Military standards Static pressure drop. 120 Surge suppresser. 175
T Tape: conductive. 32 embossed, 32 waveguide cut-off, 33 TEMPEST: definition, 5 description, 8 Test antenna, 153 Test equipment. 151 Testing requirements: MIL-STD-285, 144
Index NSA 65-6, 145 NSA 73-2A, 145 Three-terminal ground test, 167 Total shielding effectiveness, 20 Two terminal ground test, 166 Typical modular enclosure performance. 40 Typical shielding materials, 24
u UL 1283, 136
v Vent SE performance, 117 Vestibule: design, 111 double-door, III Voltage rating. see Filter
Voltage standing wave ratio (VSWR),24
w Wall-floor corner joint, 63 Waveguide beyond cut-off, 17 Waveguide tunnel entrance, 113 Waveguide vent, 117 Wave impedance, definition, 5 Wavelength, definition, 5 Welded: pipe design, 124 seams, 25, 56, 58 shield design, 57 Welding, 55
z Zinc/galvanized modular enclosure, 36