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Guidelines on earthing/grounding/bonding in the oil and gas industry

This document is issued with a single user licence to the EI registered subscriber: Shell. It has been issued as part of the Corporate access Technical Partner membership of the Energy Institute. IMPORTANT: This document is subject to a licence agreement issued by the Energy Institute, London, UK. It may only be used in accordance with the licence terms and conditions. It must not be forwarded to, or stored, or accessed by, any unauthorised user. Enquiries: e:[email protected] t: +44 (0)207 467 7100

GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY First edition September 2016

Published by ENERGY INSTITUTE, LONDON The Energy Institute is a professional membership body incorporated by Royal Charter 2003 Registered charity number 1097899

This document is issued with a single user licence to the EI registered subscriber: Shell. It has been issued as part of the Corporate access Technical Partner membership of the Energy Institute. IMPORTANT: This document is subject to a licence agreement issued by the Energy Institute, London, UK. It may only be used in accordance with the licence terms and conditions. It must not be forwarded to, or stored, or accessed by, any unauthorised user. Enquiries: e:[email protected] t: +44 (0)207 467 7100

The Energy Institute (EI) is the chartered professional membership body for the energy industry, supporting over 23 000 individuals working in or studying energy and 250 energy companies worldwide. The EI provides learning and networking opportunities to support professional development, as well as professional recognition and technical and scientific knowledge resources on energy in all its forms and applications. The EI’s purpose is to develop and disseminate knowledge, skills and good practice towards a safe, secure and sustainable energy system. In fulfilling this mission, the EI addresses the depth and breadth of the energy sector, from fuels and fuels distribution to health and safety, sustainability and the environment. It also informs policy by providing a platform for debate and scientifically-sound information on energy issues. The EI is licensed by: −− the Engineering Council to award Chartered, Incorporated and Engineering Technician status; −− the Science Council to award Chartered Scientist status, and −− the Society for the Environment to award Chartered Environmentalist status. It also offers its own Chartered Energy Engineer, Chartered Petroleum Engineer and Chartered Energy Manager titles. A registered charity, the EI serves society with independence, professionalism and a wealth of expertise in all energy matters. This publication has been produced as a result of work carried out within the Technical Team of the EI, funded by the EI’s Technical Partners. The EI’s Technical Work Programme provides industry with cost-effective, value-adding knowledge on key current and future issues affecting those operating in the energy sector, both in the UK and internationally. For further information, please visit http://www.energyinst.org The EI gratefully acknowledges the financial contributions towards the scientific and technical programme from the following companies BP Exploration Operating Co Ltd RWE npower BP Oil UK Ltd Saudi Aramco Centrica Scottish Power Chevron SGS CLH Shell UK Oil Products Limited ConocoPhillips Ltd Shell U.K. Exploration and Production Ltd DCC Energy SSE DONG Energy Statkraft EDF Energy Statoil ENGIE Talisman Sinopec Energy (UK) Ltd ENI Tesoro E. ON UK Total E&P UK Limited ExxonMobil International Ltd Total UK Limited Kuwait Petroleum International Ltd Tullow Oil Maersk Oil North Sea UK Limited Valero Nexen Vattenfall Phillips 66 Vitol Qatar Petroleum World Fuel Services However, it should be noted that the above organisations have not all been directly involved in the development of this publication, nor do they necessarily endorse its content. Copyright © 2016 by the Energy Institute, London. The Energy Institute is a professional membership body incorporated by Royal Charter 2003. Registered charity number 1097899, England All rights reserved No part of this book may be reproduced by any means, or transmitted or translated into a machine language without the written permission of the publisher. ISBN 978 0 85293 924 6 Published by the Energy Institute The information contained in this publication is provided for general information purposes only. Whilst the Energy Institute and the contributors have applied reasonable care in developing this publication, no representations or warranties, express or implied, are made by the Energy Institute or any of the contributors concerning the applicability, suitability, accuracy or completeness of the information contained herein and the Energy Institute and the contributors accept no responsibility whatsoever for the use of this information. Neither the Energy Institute nor any of the contributors shall be liable in any way for any liability, loss, cost or damage incurred as a result of the receipt or use of the information contained herein. Hard copy and electronic access to EI and IP publications is available via our website, https://publishing.energyinst.org. Documents can be purchased online as downloadable pdfs or on an annual subscription for single users and companies. For more information, contact the EI Publications Team. e: [email protected]

This document is issued with a single user licence to the EI registered subscriber: Shell. It has been issued as part of the Corporate access Technical Partner membership of the Energy Institute. IMPORTANT: This document is subject to a licence agreement issued by the Energy Institute, London, UK. It may only be used in accordance with the licence terms and conditions. It must not be forwarded to, or stored, or accessed by, any unauthorised user. Enquiries: e:[email protected] t: +44 (0)207 467 7100

GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

CONTENTS

Page

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1

Introduction, scope and application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.3 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3 Functional performance requirements of earthing/grounding/bonding systems . . . 13 4 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.1 Electrical power earthing/grounding arrangements . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.1.1 TT systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.1.2 TN-C systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.1.3 TN-S systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.1.4 TN-C-S (PME) systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.1.5 IT (unearthed) systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.1.6 Ship’s systems earthing/grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.1.7 Summary of LV system supply features . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.2 AC substations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.3 HV/LV interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.4 Electrical equipment classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.5 Static electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 4.6 Lightning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 4.6.1 The likelihood of a strike, and risk management . . . . . . . . . . . . . . . . . . . 27 4.6.2 Earth/ground-terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.6.3 Physical damage and life hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.7 Circulating currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.8 Cathodic protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.9 Electromagnetic interactions between systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.9.1 Lightning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.9.2 Power lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.9.3 RF induction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.10 Earthing/grounding/bonding interconnections as a system . . . . . . . . . . . . . . . . . . . 34 4.11 Hazardous areas, Ex certified apparatus and ignition sources . . . . . . . . . . . . . . . . . 36 4.12 Earth/ground electrode resistance requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 4.13 Touch and step voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.14 Temporary installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 5 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 5.1 Electrical machines and power systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 5.2 Machine sets with non-electric drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.3 Ex I systems and apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.4 Above ground tanks and fixed storage units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 5.4.1 Note on cathodic protection (CP) of tanks . . . . . . . . . . . . . . . . . . . . . . . . 45 5.5 Cross-country pipelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3

This document is issued with a single user licence to the EI registered subscriber: Shell. It has been issued as part of the Corporate access Technical Partner membership of the Energy Institute. IMPORTANT: This document is subject to a licence agreement issued by the Energy Institute, London, UK. It may only be used in accordance with the licence terms and conditions. It must not be forwarded to, or stored, or accessed by, any unauthorised user. Enquiries: e:[email protected] t: +44 (0)207 467 7100

GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Contents continued Page 5.6

Tankers and fuel transfer/dispensing systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 5.6.1 Road tanker loading and unloading facilities . . . . . . . . . . . . . . . . . . . . . . 46 5.6.2 Bulk railcar loading and unloading facilities . . . . . . . . . . . . . . . . . . . . . . . 47 5.6.3 Sea tanker loading jetties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 5.6.4 Aircraft fuelling facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 5.6.5 Filling Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6 Detailed design/constructional requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 6.1 Field cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 6.2 Protective conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 6.2.1 CPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 6.2.2 Power supply system earthing/grounding conductors . . . . . . . . . . . . . . . 57 6.2.3 Bonding conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 6.2.4 Instrument and telecommunications systems, and intrinsically safe system cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 6.2.5 Lightning protection system earth/ground conductors and down conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 6.2.6 Static electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 6.3 Cable tray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 6.4 Earth/ground electrode design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 6.5 Above ground floating roof storage tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.6 Steel structures (onshore) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6.7 Vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6.8 Metallic stacks and towers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6.9 Non-metallic structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 6.10 Metallic guy ropes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 7 Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 7.1 Portable container filling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 7.2 Tank cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 7.3 Scaffolding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 7.4 Connecting/disconnecting conductive paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 7.5 Operations during lightning storms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 7.6 Radio silence during product transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 7.7 Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 7.8 Tank dipping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 8

Maintenance and inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 8.1 Permanently installed earthing/grounding/bonding connections . . . . . . . . . . . . . . . 70 8.2 Portable earthing/grounding equipment for power system maintenance . . . . . . . . . 70

9 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 9.1 Current tests (using clamp meters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 9.2 Earth/ground fault loop impedance testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 9.2.1 Example method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 9.2.2 Measurement of Rmain– see Figure 22 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 9.2.3 Measurement of R1 – see Figure 24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 9.2.4 Measurement of R1 + R2 – see Figure 25 . . . . . . . . . . . . . . . . . . . . . . . . . 75

4

This document is issued with a single user licence to the EI registered subscriber: Shell. It has been issued as part of the Corporate access Technical Partner membership of the Energy Institute. IMPORTANT: This document is subject to a licence agreement issued by the Energy Institute, London, UK. It may only be used in accordance with the licence terms and conditions. It must not be forwarded to, or stored, or accessed by, any unauthorised user. Enquiries: e:[email protected] t: +44 (0)207 467 7100

GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Contents continued Page

9.2.5 9.2.6 9.2.7 9.2.8 9.2.9 9.2.10 9.2.11

Determination of Rtotal – see Figure 23 . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Completion of test protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Alternate methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Existing installations (with MV transformers) where Ze is not known . . . . 76 Existing installations (without MV transformers) where Ze is not known . . 77 Modifications/additions to existing installations . . . . . . . . . . . . . . . . . . . . 77 Example method using a conductivity meter . . . . . . . . . . . . . . . . . . . . . . 83

Annexes Annex A Glossary of terms (adapted from various EI publications and British standards) and acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 A.1 Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 A.2 Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Annex B

Touch and step voltage limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Annex C Functional performance of earth/ground electrodes . . . . . . . . . . . . . . . . . . . . 90 C.1 Soil characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 C.2 Electrode geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 C.2.1 Annular current distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 C.2.2 Hemispherical current distribution . . . . . . . . . . . . . . . . . . . . . . 91 Annex D

Measurement of earth/ground electrode resistance . . . . . . . . . . . . . . . . . . . . 93

Annex E

Legal requirements in Britain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 E.1 Electricity safety, quality and continuity regulations . . . . . . . . . . . . . . . . . 95 E.2 Electricity at work regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 E.3 Summary of British legal requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Annex F

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

5

This document is issued with a single user licence to the EI registered subscriber: Shell. It has been issued as part of the Corporate access Technical Partner membership of the Energy Institute. IMPORTANT: This document is subject to a licence agreement issued by the Energy Institute, London, UK. It may only be used in accordance with the licence terms and conditions. It must not be forwarded to, or stored, or accessed by, any unauthorised user. Enquiries: e:[email protected] t: +44 (0)207 467 7100

GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

LIST OF FIGURES AND TABLES Page FIGURES Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 Figure 23 Figure 24 Figure 25 Figure D1 Figure D2

Earthing/grounding system conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Earthing principles onshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 PME supplies and diverted neutral current (DNC) . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Conversion of PME supply or public TN-S to local TT supply . . . . . . . . . . . . . . . . . . 17 Conversion of PME or public TN-S to local TN-S supply . . . . . . . . . . . . . . . . . . . . . . 18 Petersen coil-fault on blue phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Typical leakage current route to trigger RCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 HV/LV Interface – separate earth/ground electrodes . . . . . . . . . . . . . . . . . . . . . . . . 23 HV/LV Interface – combined earth/ground electrodes . . . . . . . . . . . . . . . . . . . . . . . 24 Typical earthing/grounding/bonding system for instruments . . . . . . . . . . . . . . . . . . 32 Surge protection of signal processing equipment . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Typical example of earthing/grounding system interconnections . . . . . . . . . . . . . . . 34 Example of the surface potential profile and resulting touch and step voltages . . . . 38 Earthing principles offshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Typical earthing/grounding arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Typical floating roof tank installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Typical arrangement of CP for shore systems, jetty and ship . . . . . . . . . . . . . . . . . . 48 Some typical filling station earth/ground/bond interconnections . . . . . . . . . . . . . . . 51 Earthing/grounding system design flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Earth/ground loop impedances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Layout of a TN-S system with the earth/ground fault loop resistances identified. . . . 78 Measurement of Rmain at the distribution board. . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Measurement of Rtotal at the field device. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Measurement of R1 at the distribution board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Measurement of R1+ R2 at the distribution board. . . . . . . . . . . . . . . . . . . . . . . . . . . 82 'Fall of potential' electrode resistance measurement . . . . . . . . . . . . . . . . . . . . . . . . 93 Wenner method of soil resistivity measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

TABLES Table 1 Table 2 Table B1 Table B2

Supply system features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Summary of earthing/grounding/bonding conductor sizes . . . . . . . . . . . . . . . . . . . 59 Permissible body currents depending on duration of exposure . . . . . . . . . . . . . . . . 88 Limiting values for severe conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

ACKNOWLEDGEMENTS This publication was prepared at the request of the EI’s Electrical Committee by Bernard Emery. It was subsequently reviewed and developed by members of the Electrical Committee. At the time of publication the Electrical Committee comprised: Jim Adams Neville Harris Terry Hedgeland Gary Holcroft Justin Mason Toni Needham Ian Neve Zaur Sadikhov Jonathan Slark John Stevens Chris Turney Steve Wilkinson

BP Valero Energy Limited Consultant Health and Safety Executive BP Exploration Energy Institute Total Lindsey Oil Refinery Shell Valero Energy Limited BPA F.E.S Phillips 66

The EI wishes to record its appreciation of the work carried out by the members of the Electrical Committee and to recognise the contribution made by those individuals, companies and organisations that provided comments during technical review of earlier drafts. Project coordination was undertaken by Toni Needham (EI).

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

FOREWORD Earthing/grounding and bonding are of major importance for the safety of personnel and the protection of material assets in the energy industry, wherever electrical energy is present. This applies not only where electricity is generated, distributed, stored or used, but also includes the natural phenomena of lightning and static electricity. It is a subject that is often misunderstood and considered to have 'grey areas'. As a 'safety critical' feature of installations in the energy industry, onshore and offshore, the effectiveness of earthing/grounding and bonding is a prime factor in the protection of personnel against electric shock, fire and burns due to the presence of electricity and the prevention of ignitive sparks in hazardous areas associated with potentially explosive atmospheres. This ranges from protection against static electricity to minimising the possible effects of lightning strikes. Within the industry there are many discrete activities or locations having specific earthing/grounding and/or bonding requirements relating to them. It can happen that earthing/grounding or bonding provided to satisfy one set of requirements may be incompatible with requirements satisfying other purposes, creating an unforeseen potential hazard. The provision of a connection allowing undesirable current to pass to earth/ground from an installation also provides a route for undesirable current from elsewhere to pass into the installation, with possibly serious consequences. The EI has an extensive portfolio of codes of practice and other guidance publications for a range of topics, many of which include provisions for earthing/grounding and bonding relevant to the topics concerned. This guidance publication brings together, from that portfolio, the essential requirements relating to earthing/grounding and bonding for installations in hazardous areas in the oil and gas industry. Whilst providing an overview to show a 'bigger picture', this publication does not replicate all the detailed requirements contained in individual publications in the portfolio, which should, in any event, otherwise be referred to. Within this guidance publication the terms 'earth' and 'ground' mean the same thing, as do 'earthing' and 'grounding'. This publication embodies relevant recommendations in the EN 60079 series; BS 7671 Requirements for electrical installations – IEE Wiring Regulations; relevant aspects of the UK statutory Electricity at Work Regulations and the Electrical Safety, Quality and Continuity Regulations; and gives cognizance to the relevant aspects of the recommendations from IEEE 80 Guide for Safety in AC Substation Grounding. The contents of this publication are provided for information only and while every reasonable care has been taken to ensure the accuracy of its contents, the EI cannot accept any responsibility for any action taken, or not taken, on the basis of this information. The EI shall not be liable to any person for any loss or damage which may arise from use of the information contained in any of its publications. The above disclaimer is not intended to restrict or exclude liability for death or personal injury caused by own negligence. Suggested revisions are invited and should be submitted to the Technical Department, Energy Institute, 61 New Cavendish Street, London W1G 7AR.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

1

INTRODUCTION, SCOPE AND APPLICATION

1.1 INTRODUCTION This code is aimed at providing an overview of electrical 'earthing', 'grounding' and 'bonding' to address the following phenomena: −− −− −− −− −− −− −−

electrical power system earth/ground faults; touch and step voltage hazards; lightning electrical and ignition hazards; ignition hazards associated with Ex apparatus; ignition hazards associated with the interruption of currents; ignition hazards associated with electrostatic discharges, and disturbance of signal transmission.

Earthing/grounding/bonding practices consist of interconnecting certain conductive parts of a system by engineered electrically conductive paths, primarily for the following personnel safety and asset protection purposes: −− −− −− −− −− −−

To provide a path for power system fault currents to flow back to the source of supply, and to mitigate arc flash hazards. The elimination of electric shock hazards (touch and step voltages). To provide a path to dissipate lightning currents into the general mass of the Earth. The elimination of ignition hazards, whether related to Ex certified apparatus or the prevention of the interruption of stray currents. The dissipation of electrostatic charges that could cause potentially incendive sparking. To ensure the integrity of signal return paths, and to minimise electrical interference with such signals.

Earthing/grounding/bonding systems are important for electrical safety, lightning safety, and the control of sources of potential ignition. They contribute to the operability of process control systems and to the integrity of active safety functions; hence these systems make a vital contribution to continuity of operation and to ongoing asset integrity. Note: A protective function, and the system that implements that function, may be regarded as 'safety critical' if a purpose of that function/system is to reduce the likelihood, or the consequences, of an accidental event which may result in major injuries to personnel; earthing/grounding/bonding systems will often meet that definition of safety criticality, and indeed may be subject to specific regulatory requirements (further information is given in the EI Guidelines for the management of safety critical elements). However, earthing/grounding/ bonding systems that are designed, maintained, tested and operated in accordance with this document and the underlying standards should normally meet both personnel safety and commercial objectives.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

The overall structure of this document includes discussions of −− −− −− −−

the underlying processes; the functional requirements and performance standards of earthing/grounding/ bonding systems; some specific applications, and detailed design and construction issues.

1.2 SCOPE This publication covers earthing, grounding and bonding practices in the upstream and downstream oil and gas industry, most notably in hazardous areas, but including adjacent areas classified as non-hazardous. The petroleum industry is distinctive because of the flammable nature of the product; this requires the control of sources of potential ignition of flammable product, e.g. the prevention of the interruption of stray currents that could result in an incendive spark. This publication does not cover earthing/grounding/bonding practices in other industrial sectors. References to current international, British and EI standards and guidance are provided.

1.3 APPLICATION This publication is intended for global application to oil and gas facilities such as upstream production installations, storage facilities, terminals, refineries, filling stations and product transfer, but not downstream gas facilities. It covers the design, operation, inspection, test and maintenance of both new and existing facilities, portable/temporary equipment, and to operational interactions with bulk fuel tankers and aircraft refuellers (but no other aspect of tankers or aircraft). This publication creates no general requirement to upgrade a legacy installation designed to obsolete standards, providing that it remains safe, operable and in compliance with legal requirements. However, if a significant modification is required, it should meet current standards where possible. Note: The legal requirements described in this publication are specific to Britain, and any reference to regulations in this publication refers only to British legislation; other jurisdictions may have different requirements. Metric units are used throughout.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

2 BACKGROUND This publication embodies and builds on relevant recommendations from the standards listed in Annex F (note, some of these standards do not apply offshore). It is consistent with the British Electricity at Work Regulations 1989 and the Electricity Safety, Quality and Continuity Regulations 2002, and provides some additional guidance. Note: There can be differences between standards, e.g. between those intended for offshore installations and those intended for ships such as floating production, storage and offloading installations (FPSOs). For example, the requirements for offshore installations in BS IEC 61892 differ from the requirements for ships (as applied to FPSOs) in BS IEC 60092-502, which itself is not in full concurrence with the system earthing/grounding requirements of the IMO International Convention for the Safety of Life at Sea (SOLAS). However, this publication makes no general attempt to identify inconsistencies between standards, or to resolve them. Where there is a conflict between standards, the chosen arrangement should be agreed with the insurer, classification society, regulator, and any other interested parties such as commercial partners. The characteristics of, and distinctions between, earthing, grounding and bonding are best understood by reference to their functions. However, any given earthing, grounding or bonding conductor may provide more than one of those functions. Earthing/grounding/ bonding connections may be achieved by specifically installed conductors, but in many cases there will be other incidental or fortuitous paths in parallel, e.g. cable armour, cable tray, metallic structures, stray capacitance, or the body of the Earth itself. The terminology in various publications is not necessarily consistent. The term 'earthing' often indicates a protective conductor intended to carry power system fault currents (often called 'earth faults' or 'ground faults') from exposed conductive parts (such as electrical apparatus housings) back to the source of supply, but this function may also be called 'grounding'. However, although in some cases fault current may flow through the body of the Earth back to the return side of the source of supply, the body of the Earth serves simply as a conductor in the flow path of the fault current, not as a 'zero' potential reference point. Alternatively, the term 'earthing' may refer to a connection from certain conductive part(s) to the general mass of the Earth (considered to be a zero potential reference point), sometimes called 'ground' but which may be water, e.g. the sea, but this function may also be called 'grounding'. The terms 'earth(ing)' and 'ground(ing)' are used interchangeably in the literature. Note: 'Exposed conductive parts’ are defined as parts of an electrical installation that are not normally live, but which may become live under fault conditions (e.g. the exposed metallic housing of electrical apparatus); 'extraneous conductive parts' are not part of an electrical installation, and so are not normally live, but may similarly become live under fault conditions (e.g. structural steelwork, metallic pipework or conductive cable tray). This and other publications make general use of such terms as protective earth, protective conductor, earth/ground conductor, etc. but the following terms have specific meanings (defined in Annex A) and are used in a consistent way in this publication: −−

Circuit protective conductors (CPCs): provide the primary route for currents due to a fault between live and conductive parts of the apparatus not intended to carry normal functional current (e.g. the housing of electrical apparatus) to flow back to the main earthing/grounding terminal, and hence to the return side of the supply (generally the supply transformer star point).

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Main bonding conductors: provide a connection between extraneous conductive parts, or the lightning down conductor, to the main earthing/grounding terminal of the electrical installation. Supplementary bonding conductors: make a local connection between exposed or extraneous conductive parts.

−−

−−

Figure 1 illustrates some possible layouts for the main and supplementary bonding conductors, CPC etc. 'Bonding' in the sense of main or supplementary conductors is employed to deal with power system fault currents, stray current or lightning currents, and is different from 'bonding' applied to prevent electrostatic charges from accumulating; see 4.5 for further guidance on electrostatic charges. Main earthing/grounding terminal

Earthing/grounding conductors

Extraneous conductive part

Exposed conductive part

Supplementary bonding conductor

Exposed conductive part

Exposed conductive part Lightning down conductor

TN

Supply system earth/ground electrode

or

Main bonding conductors

TT/IT

Metallic service pipework

Local earth/ground electrode

Circuit protective conductors

Lightning earth/ground electrode

Figure 1: Earthing/grounding system conductors

Figure 1: Earthing/grounding system conductors

CPCs, main bonding conductors and supplementary bonding conductors are not intended to carry electrical system operating currents. Note: Sometimes, adjacent systems require some degree of separation from each other, for example, where it is necessary to isolate cathodic protection systems so as to confine the cathodic protection current to the steelwork intended to be protected. In this case, the protected steel is at a finite potential with respect to the general mass of the Earth, and the anode is the earth/ground reference point. Low resistance paths provided for earthing/grounding and bonding purposes provide a fortuitous current path for stray currents; these can arise from a number of sources, some inherent in the situation (e.g. galvanic currents resulting from corrosion of buried metallic objects), some introduced deliberately (e.g. cathodic protection), and some due to faults (e.g. in electrical power supply circuits). Where a current path is interrupted or reconnected, a potentially incendive arc or spark may result. Therefore, the management controls of earthing/grounding/bonding arrangements are particularly demanding in the oil and gas industry, given the need to avoid the risk of ignition of potentially flammable atmospheres.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

3

FUNCTIONAL PERFORMANCE REQUIREMENTS OF EARTHING/GROUNDING/BONDING SYSTEMS The design and operation of functional systems such as earthing/grounding/bonding are best understood by reference to the defined function and the required performance of that function. The functions and performance requirements of earthing/grounding/bonding systems in the oil and gas industry are as follows: −−

−−

−−

−−

−−

−−



Electrical protection is designed to operate rapidly in the event of a power system fault between the live circuit conductor(s) and exposed or extraneous conductive parts (this is commonly called an 'earth/ground fault'); protection is designed to mitigate the risk of damage to apparatus or harm to personnel from arcs, under single fault conditions in otherwise normal operation. This protective function relies on a low impedance circuit protective conductor (CPC) in order to allow a sufficiently high current to flow so as to initiate the disconnection function within an acceptable time. Recommended minimum conductor sizes for the CPC and maximum isolation times to achieve satisfactory performance are given in BS 7671, and see 6.2. Accessible exposed conductive parts are earthed/grounded, and extraneous conductive parts are bonded, in order to reduce 'touch voltages' and 'step voltages' to a non-hazardous level (see Annex B for a discussion of tolerable touch and step voltages). Lightning strikes can cause direct damage or injury. Some degree of protection is provided if the earthing/grounding system impedance is low enough to limit the voltage rise of the lightning protection conductors. BS EN 62305-3 recommends low earth/ground resistance, if possible lower than 10 Ω. A spark can result if a current is interrupted or reconnected. BS EN 60079-11 indicates that for gas subdivisions IIA, IIB and IIC, in a circuit with no reactance, a voltage below 10 V appears not to be able to create an incendive spark, and for voltages below 20 V, a current below 100 mA cannot create an incendive spark. Earthing/grounding/bonding conductors are considered to have little reactance. For petrol filling stations earthed via a TN-C-S system it is considered that currents below 100 mA may not present an ignition hazard if interrupted or reconnected. However, currents (including stray or diverted neutral current) above 100 mA in CPCs or bonds in hazardous areas should be investigated in case the operational situation changes and the current rises to a potentially incendive level. Electrostatic charges can reach incendive levels in certain conditions, but are readily dissipated to a non-incendive level by bonding paths of less than 1 MΩ resistance (see 4.5). Reliable functionality of instrument and telecommunications signals cannot be assured in the presence of significant signal interference, so signal return paths (especially safety system signal paths) may require protection. Voltages and currents that can interfere with correct circuit operation can be determined only on a case-by-case basis. Note: e.g. that many instrument signals use 4 – 20 mA, so that levels of interference in the range of a few mA or less, are likely to cause a malfunction in instrumentation applications (this may be a relatively high level of signal interference, but is normally much less than a potentially incendive level).

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

4 PRINCIPLES 4.1

ELECTRICAL POWER EARTHING/GROUNDING ARRANGEMENTS For low voltage (LV) systems, the IEC 60364 series of standards distinguishes three classes of earthing/grounding arrangements for power supplies, using the two-letter codes TT, TN and IT. These apply equally to three-phase AC and single-phase supplies. The first letter indicates the connection between earth/ground and the power supply generator: −− −−

T: direct connection of a point (often the generator star point) with earth/ground (the letter T is derived from the Latin, terra). I: no point is connected to earth/ground (except through a high impedance or stray impedances).

The second letter indicates the connection between earth/ground and the electrical apparatus being supplied: −− −−

T: direct connection with earth/ground, independent of any earth/ground connection in the supply system. N: connection with earth/ground via the supply system.

There are three variants of TN systems: −− −− −−

TN-S: the protective earth/ground and neutral are separate conductors, joined together at or near the power source. TN-C: a single conductor functions as a combined neutral and protective earth/ ground; this conductor may be earthed/grounded at several points. TN-C-S: supplier’s part of the system uses a combined protective earth/ground and neutral conductor, but this is split at some point, usually on entry to the consumer’s facility, into separate protective earth/ground and neutral conductors connected to the electrical load apparatus; the protective earth/ground conductor is earthed/ grounded at several points, typically at each consumer’s site. This variant is also known as protective multiple earth (PME) or multi-grounded neutral (MGN).

There are advantages to both unearthed/ungrounded and earthed/grounded neutral systems, and both systems are widely used. Protection through earthing/grounding will only be effective if the earth/ground path impedance is sufficiently low so that there is no danger from any voltage developed due to any prospective fault current flow. Earthing/grounding of the neutral via an impedance designed to limit fault current may be undertaken provided there will be an effective earth/ground fault protection arrangement, the neutral is insulated from earth/ground to the requirements of phase voltage and in all other respects is regarded as a potentially live conductor. For high voltage (HV) applications, generator neutrals will generally be earthed/grounded via neutral earthing/grounding resistors so as to limit the earth/ground fault current and to prevent overvoltages that may otherwise occur on electrical systems. The neutral earthing/ grounding resistor should be rated for the short-term fault current, but should also be rated for the continuous circulating current where generators with earthed/grounded neutrals are intended to run in parallel with the neutrals interconnected (especially with generators of different rating or winding pitch).

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

HV distribution systems are usually earthed/grounded at the transformer in order to control phase to earth/ground voltages and avoid overstressing the insulation by keeping neutral voltages as close to earth/ground potential as possible. The LV neutral is usually earthed/ grounded at the switchboard. Figure 2 shows typical arrangements. DC supplies are widely used in well drilling and well control power supply applications, switchgear tripping supplies and in power supplies to control and safety systems. These applications usually use unearthed/ungrounded supplies, as they generally require the associated improved availability of supply. 11KV/415V Trans.

415/240V Switchboard

Plant

R N Cable armour glands bonded internally to earthing/grounding bar/terminal

Earthed/grounded steel structure forming part of the general earthing/grounding system on the plant

Ground level

Substation power earthing/grounding system

Interconnections

Lightning and static earthing/grounding system

Figure 2: Earthing principles onshore

Figure 2: Earthing principles onshore 4.1.1 TT systems TT systems require electrically independent earth/ground electrodes at the supply and at the consumer’s installation. This may be a costly option, and the earth/ground loop impedance will be high, so protection against earth/ground faults is typically provided by residual current device (RCD.) 4.1.2 TN-C systems TN-C systems are the least safe because a failure of the combined neutral and earth/ground conductor results in the neutral side of the load apparatus rising towards line potential. This problem can be reduced if the system has multiple earth/ground connections in addition to the low impedance earth/ground at the source (see 4.1.4 for a discussion of the effects of a broken combined neutral and earth (CNE) conductor in this situation, and on the effects of diverted neutral currents).

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4.1.3 TN-S systems TN-S systems can be costly because of the requirement for a separate protective conductor from source to load. Also note that failure of this conductor is not revealed, and if the protective earth (PE) conductor is open circuit, the potential of exposed conductive surfaces may become hazardous in the event of an earth/ground fault in the apparatus. 4.1.4 TN-C-S (PME) systems The TN-C-S system is often known as the PME system, but is sometimes known as the MGN system. The protective earth and neutral (PEN) conductor, also referred to as the CNE conductor, should have earth/ground connections at a number of points in order to mitigate the risk of a broken neutral conductor. In PME systems, the protective earth/ground (PE) conductor within a given facility is connected to the incoming CNE conductor near the boundary of the site; all exposed conductive parts (e.g. electrical apparatus housings) and extraneous conductive parts (e.g. steelwork) in that facility then are bonded to the sitespecific PE conductor, therefore providing a connection to the supply system combined neutral/earth (CNE) conductor. The PE is also connected to service pipework (e.g. conductive utility pipework such as water mains) to provide de facto earth/ground connections at every facility supplied by the PME supply; however, the resistance and the integrity of these earth/ground connections are unknown. In simple terms, for a self-contained facility, PME systems should not create a shock hazard in the event of a failure of the CNE conductor (when the CNE conductor may be at a significant potential with respect to the general mass of the Earth in spite of the local 'earth/ground' connection provided by connections to service/utility pipework), because comprehensive main and supplementary bonds should eliminate potentially hazardous touch voltages; this situation is likely to be revealed by unacceptable voltage variations. Similarly, the failure of the PE conductor or a main bonding conductor should not create a shock hazard if the CNE conductor remains connected to the protective earthing/grounding system and exposed conductive parts remain insulated from live circuit conductors. However, this PE or bonding conductor failure is not revealed, so a subsequent earth/ground fault may create a shock hazard. Therefore periodic tests should be conducted on PE and main bonding conductors to confirm the integrity of the protective function, but PME systems generally will tolerate any single fault condition. However, if it is possible to touch a conductive surface at CNE potential and 'true' earth/ground, e.g. by standing on a (wet/conductive) surface in contact with the general mass of the Earth, there is indeed a potential electric shock hazard under single fault conditions, e.g. failure of the CNE conductor. Therefore PME systems cannot be recommended for oil and gas industry applications, even in areas classified as non-hazardous. Furthermore, even in fault-free PME systems, some proportion, called the 'diverted neutral current' (DNC), of the load return/neutral current will flow through any conductor that is connected to the CNE conductor and is in effective contact with the general mass of the Earth - this could, for example, be any extraneous conductive part, e.g. metallic cable tray (see Figure 3). The accidental making or breaking of joints in such conductors, or their deliberate making or breaking, e.g. in maintenance work, could result in a potentially incendive spark. Therefore these systems should not be used in classified hazardous areas, except where a risk assessment determines that such use cannot be hazardous, e.g. if the DNC (including the effects of DNCs from other consumers on the same supply) can be shown to be below potentially incendive levels. Note: It is possible to convert PME or public TN-S supplies to local TT or local TN-S, and that this eliminates the DNC problem (see Figures 4 and 5).

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Supply

Consumer’s site

L

L

Load

CNE

N DNC

Supply earth / ground electrode

E

Other consumers

Bond to local (earthed/grounded) metalwork DNC to other consumers’ local earth/ground bonds

DNC from other consumers’ local earth/ground bonds

DNC to source

Figure 3: PME supplies & diverted neutral current (DNC)

Figure 3: PME supplies and diverted neutral current (DNC)

Supply

Consumer

L

L

RCD (CNE)

N

N

E Supply earth/ground electrode

Supply earth/ground facility NOT USED for consumer’s installation

Consumer’s earth/ground electrode

Figure 4: Conversion of PME supply or public TN-S to local TT supply

Figure 4: Conversion of PME supply or public TN-S to local TT supply

17

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Supply

Consumer

Isolating transformer

L

L

Circuit protection

(CNE)

N

N

E

E

Supply earth/ground electrode

Consumer’s earth/ground electrode

Figure 5: Conversion of PME or public TN-S to local TN-S supply 4.1.5 IT (unearthed) systems The IT system is generally used for high reliability of electrical supply, especially in safety critical applications, e.g. in marine stability applications, drill rigs and DC supplies to control and safety systems. In these applications, the loss of equipment availability owing to one earth/ ground fault might have adverse safety implications or adverse effects on the operability or survivability of the installation. IT systems might also be used to reduce the possibility of earth/ground fault currents flowing in hazardous areas, e.g. cargo tanks. IT systems are best suited for small electrical systems (e.g. a ship’s electrical system in relatively dry, below-deck conditions), and are less suited to systems situated in a damp environment (e.g. the electrical systems associated with process plant on the deck of an FPSO). An appropriate risk assessment needs to balance the above risks together with the possibility of low arcing faults going undetected. Where a mixture of earthed/grounded and unearthed/ ungrounded electrical distribution systems is to be implemented on the same installation, an appropriate risk assessment should be undertaken to ensure that the systems are compatible and to identify any essential control measures. There is a balance of risk between, on the one hand, the control of high energy electrical faults, and on the other hand, major accident risks, such as the ignition of flammable atmospheres or loss of availability of safety critical systems. Where unearthed/ungrounded systems are installed, it should be recognised that the power system phase conductors could constitute an electric shock hazard or potential ignition hazard as a result of contact between them and an extraneous conductive part, notwithstanding the apparent lack of a current route to earth/ground. This arises because of stray capacitive coupling between the power system phase conductors and the local mass of adjacent conductive material, usually at or near earth/ground potential. It is this capacitance that is the basis for Petersen coils; these coils connect the star point of the IT supply to the local mass of conductive material via a relatively high inductive impedance (so preserving the IT property, i.e. that a single earth/ground fault is tolerated). The coil and

18

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stray capacitances are tuned to the power frequency so that in the event of an earth/ground fault, much of the fault current flows back to the star point through the Petersen coil, rather than through the fault itself. The result is that any arc at the site of the fault may be less intense and extinguish more easily. Figure 6 illustrates the operation of Petersen coils; note when the Petersen coil is activated, an alarm should be given.

VR VY

Star point

VP

I IR VB

IR Stray capacitances from red and yellow phases to earth / ground

Fault on blue phase

I

VR

Earth / ground

IY

VB

IY Star point

VY

VP

Figure 6: Petersen coil – fault on blue phase IT system integrity relies upon there being no power system earth/ground contact; if an earth/ground fault occurs then it becomes earthed/grounded via the fault impedance, but without the earth/ground fault detection or the neutral earthing/grounding resistor that play an important role in electrical protection arrangements in earthed/grounded systems. Therefore, any fault is potentially at full load current, i.e. there is a risk of arc flash faults. By contrast, incomers on a conventional earthed/grounded system would typically have earth/ ground fault protection with settings (say at 10 to 15 % full load current) designed to clear the fault before damage could occur. The occurrence of a second earth/ground fault on IT systems may not cause the overcurrent protection to operate if the fault impedance restricts the fault current below that required to initiate the protection. When other earth/ground

19

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faults occur, a situation can develop that results in an IT electrical distribution system having multiple earth/ground faults present. There is a risk of low-arcing faults, greatly in excess of normal load currents, and the upstream overcurrent protection may not be able to clear the fault(s) sufficiently quickly to prevent damage or danger. The risks of such faults occurring on an IT system should be assessed and, where necessary, appropriate action should be taken to reduce the risks. When using IT systems, the designers need to pay particular attention to the need for earth/ ground fault monitoring and ease of tracking and rectifying the first earth/ground fault as soon as practicable. Designers should therefore structure the system so that switchboards are not too large, and should consider the use of isolation transformers for the supplies to electronic and electrical equipment that might have low impedance networks (i.e. filter circuits) which, inherently in their design, will cause leakage to earth/ground. Insulation monitoring arrangements play a crucial role in assisting with the tracing of earth/ ground faults. The three lamp system and the basic insulation level monitor system provide only limited assistance to maintenance technicians. More sophisticated systems are now available that facilitate the rapid tracing of earth/ground faults. As well as reducing the time that a fault remains present, the use of these systems minimises the requirement for supply disconnection and technician intervention on energised electrical equipment. Residual current devices (RCDs) can have a role in protecting against electrical shock, and can assist with the tracing of earth/ground faults when used on IT electrical distribution systems. The IT electrical distribution system stray impedance to earth/ground will often be at a value low enough to allow an RCD to function when an earth/ground fault occurs. This is particularly so with electrical distribution systems that incorporate harmonic line filters. Where continuity of supply is required, it will be appropriate to detect the fault by means of a residual current monitor (RCM), but not to trip. For protection against electrical shock, it is recommended that 30 mA rated RCDs (detect and trip) be incorporated in IT electrical supplies to portable electrical tools and equipment, unless other risk reduction measures have been taken, e.g. the use of a reduced voltage. The use of 30 mA rated RCDs on unearthed/ungrounded systems should also be considered in non-industrial areas such as accommodation on offshore installations, because if two earth/ ground faults occur (on separate phase conductors), such protection could provide better protection against electric shock. Also, where the distribution system stray impedance to earth/ground upstream of the RCD is low (e.g. where there are harmonic line filters), the RCD may provide protection if a person comes into contact with a live conductor. An RCD that is sensitive to an imbalance between live and neutral circuit currents is more likely to disconnect a circuit that has developed an earth/ground fault than a fuse or circuit breaker. Incorporation of RCDs into final circuits that are susceptible to earth/ground faults, such as deck lighting, can therefore assist in the tracing and clearing of faults. Figure 7 illustrates typical imbalanced current routes that can be detected by RCDs.

20

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Figure 7: Typical leakage current route to trigger RCD For unearthed/ungrounded systems on offshore installations, further guidance is given in IEC 61892-1 Mobile and fixed offshore units. Electrical installations. General requirements and conditions, and in BS IEC 61892-2 Mobile and fixed offshore units. Electrical installations. System design. The requirements for unearthed/ungrounded systems on floating production storage and offloading installations (FPSOs), which are different, are given in BS IEC 60092-502. 4.1.6 Ship’s systems earthing/grounding BS IEC 60092-502 and IEC 61892 discuss four classes of neutral earthing/grounding arrangements; any of these is permitted on ships: −− −− −− −−

three-phase, three-wire high impedance earthed/grounded neutral; three-phase, three-wire low impedance earthed/grounded neutral; three-phase, three-wire directly earthed/grounded neutral, and three-phase, three-wire insulated neutral.

On systems with an insulated neutral, the fault could last a relatively long time, so the system should be rated this condition. For system with a high resistance earthed/grounded neutral, the system should incorporate a trip, so the system requires only a short-term rating. In either case, the insulation of the neutral should be designed for the phase-to-phase voltage. 4.1.7 Summary of LV system supply features The TN-S system has the disadvantage that a broken PE conductor is unrevealed, and the user installation is not earthed/grounded. Then if an earth/ground fault occurs, the exposed conductive surfaces may be at a hazardous potential. The TN-C three-phase system is similarly susceptible to a broken CNE conductor, depending on the imbalance between the three phases; in TN-C single phase systems, the potential of the neutral conductor can rise to the line potential if the CNE conductor fails. The TN-C-S (PME) system has a significant earth/ ground loop impedance under fault conditions (higher than the impedance of a normal CNE conductor), and this may lead to unacceptable load voltage variations. Some other features of the various supply types are shown in Table 1.

21

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Table 1: Supply system features

PE conductor cost

4.2

TN-S

TN-C

TN-C-S

TT

IT

Highest

Least

High

Low

Low

Earth loop impedance

Low

Low

Low

High

Highest

Risk of broken neutral

High

Highest

High

No

No

Safety

Safest

Least safe

Safe

Safe

Less safe

Needs site electrode

No

No

No

Yes

Yes

EM interference on the earth/ground system

Low

High

Low

Least

Least

Advantages

Safest

Cost

Cost and safety

Safe and reliable

Availability and cost

AC SUBSTATIONS An AC substation requires a low resistance earth/ground electrode, usually implemented by a buried earth/ground plate, mesh or grid which covers the whole footprint of the substation; this provides a route to the general mass of the Earth and hence back to the source of supply (see 6.2 for a discussion of conductor sizing, and 6.4 and Annex C for a discussion of earth/ ground electrode design). During earth/ground fault conditions in a substation, the current flow via this earthing/grounding system will result in a rise in the system potential relative to the general mass of the Earth, i.e. a ground potential rise (GPR) or earth potential rise (EPR). The sub-station earthing/grounding system should be designed to have a low enough resistance to limit the GPR such that touch and step voltage limits are not exceeded (see 4.13 for a discussion of touch and step voltages). This GPR will be fairly uniform across the area covered by the earthing/grounding system so that the gradient and step voltages are small, but where a grid is used, the GPR will undulate across the site between conductors of the grid, creating local gradients that cause higher step voltages. Around the periphery of the earth/ground electrode system, the gradient will be higher, generating high step voltages; earth/ground potential grading rods may also be included around the electrode periphery to control these step voltages. For a discussion of touch and step voltage limits, see Annex B.

4.3

HV/LV INTERFACES HV and LV systems associated with a ground mounted HV/LV substation or transformer may either share a common earth/ground electrode or have a separate electrode for each system. For a system where HV metalwork (transformers, associated switchgear and cable armour) and LV neutral-earthing/grounding share a common electrode, the earth/ground resistance should not exceed 1Ω. Where this cannot be achieved, separate electrodes should be provided for the HV and LV systems, with a minimum separation distance of three metres. Depending on earth/ground resistance, it may be necessary to increase the separation distance to avoid electrode area overlap. Where LV equipment is located within metalwork connected to the HV earth/ground electrode, care must be taken to adequately insulate the LV neutral-earthing/grounding bars and LV cable armour from the HV metalwork.

22

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Where HV and LV systems have their own earth/ground electrodes whose spheres of influence do not overlap, there is no interaction between the two earthing/grounding systems, but a touch voltage can exist between the two earthing/grounding systems, as shown in Figure 8. Where HV and LV systems share a common earth/ground electrode, the temporary overvoltage of the HV earth/ground electrode (which occurs on a HV system during a fault condition) is transferred into the (common) LV system earth/ground and there is no such touch voltage, as shown in Figure 9. Where the separate HV and LV electrodes have spheres of influence that overlap, part of this earth/ground potential rise is transferred into the LV system. HV

LV

Touch voltage V = IR I

Return to source

R

V = IR

V=O

Figure 8: HV/LV interface – separate earth/ground electrodes

23

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HV

LV

Touch voltage = O I

R

V = IR

V = IR

Return to source

Figure 9: HV/LV interface – combined earth/ground electrodes An interconnected earthing/grounding system (i.e. global earthing/grounding system) should be used where practicable because the fault current returns directly to the supply star point, and the global earthing/grounding arrangement limits touch voltages between exposed conductive surfaces of HV apparatus and exposed conductive surfaces of LV apparatus (touch voltages between HV apparatus exposed conductive surfaces and extraneous conductive surfaces may exist; whichever configuration is chosen). In a global earthing/grounding system, the earth/ground fault current divides between the various local earthing/grounding systems, and this results in a reduction in the earth/ground potential rise at the local earthing/ grounding system where the fault current originates. This may make for a more economical design than individual local earthing/grounding systems. Note: The current sharing process may result in other local earthing/grounding systems being overloaded, depending on their current rating and earth/ground resistance. Where the global earthing/grounding concept is applied, the entire system should be designed as such; inservice system performance can be verified by either calculation or by test. For further information on HV/LV interfaces and on the protection of LV installations against overvoltages due to faults in their HV supply, see BS 7671 (Chapter 44, Section 442), and BS 7430; for HV installations including substations, see BS EN 61936-1 and BS EN 50522 Fig. NA 7 (none of these standards applies on ships or offshore installations). 4.4

ELECTRICAL EQUIPMENT CLASSIFICATION LV electrical equipment is classified in IEC 60364 according to its insulation characteristics and any protective earthing/grounding arrangements, and this classification determines the practical uses to which the equipment may be put. 24

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Class I equipment has live parts surrounded by basic insulation (which may be air), and exposed conductive parts have a means of connection to a circuit protective conductor (CPC) that is part of the fixed wiring of the installation; if the equipment develops an internal fault between a live part and exposed conductive part(s), a path is available for the fault current to flow back to the source and operate the circuit protection, which should be designed to disconnect the power supply within the defined disconnection time. Much fixed equipment is Class I. Class II equipment has live parts surrounded by basic insulation and supplementary insulation (or has reinforced insulation that meets the requirements for double insulation), but has no provision for any exposed conductive part to be earthed/grounded; this arrangement is commonly called 'double insulation' or 'all insulation'. Class II protection is often used for smaller items of equipment whose casings lend themselves to construction from non-conductive material. Class II equipment remains safe with failure of one layer of insulation (or degradation of the reinforced insulation), and is usually used with circuit protection by RCD. Class II equipment has the advantage of simple installation, and cannot have the possible adverse effects of diverted neutral current, and can have no touch voltage under single fault conditions. Class III equipment operates at separated (or 'safety') extra-low voltage (SELV) and relies on protection against electric shock by using an ELV supply of no greater than 50 V ac or 120 V dc. Exposed conductive parts may be earthed/grounded, or may be unearthed/ungrounded (e.g. for electromagnetic compatibility purposes). Portable equipment often relies on the SELV concept, and inherently cannot give rise to a hazardous touch voltage. ELV limits are defined in IEC 61201.

4.5

STATIC ELECTRICITY Whenever two dissimilar materials come into contact, charge separation occurs at the interface. If one (or both) of the materials is a very poor conductor, recombination of these charges is impeded and a charge accumulates – this is static electricity. Unless relaxed (i.e. conducted away), charge may build up on, for example, isolated conductive surfaces, on any particulates in a flow stream, or on the internal surfaces of a pipe wall. The conditions that cause static charges to be separated are present in many working situations, including processes that rub surfaces together or agitate fluids. Examples include: sampling from live hydrocarbon lines into sample containers, product loading, steam cleaning, grit blasting, water jetting, pouring of fuels, decanting of chemicals, and product flow through a nozzle (see 7.1 and 7.2 for further comments). Charge separation is substantial in a two-phase flow due to agitation of the mixture and the increased surface area of the phases, and downstream of fine filters due to extensive agitation of the fluid. Note: Where the fluid has significant conductivity (typically greater than 50 pS/m), charge will relax quickly, rather than accumulating; conductivity may be a natural property of the fluid, or may result from anti-static additives; static relaxation tanks may be used to allow sufficient time for the relaxation process to occur. Equally, personnel and their clothing and boots can become charged, and this charge could persist unless relaxed, e.g. by contact with secondary structures such as conductive flooring.

25

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The energy dissipated by the discharge of static electricity generated in typical oil and gas industry applications is unlikely to cause any direct damage, or to harm personnel, but greatly exceeds the minimum ignition energy of some common oil or gas products (see the EI Model Code of Safe Practice Part 1: The selection, installation, inspection, and maintenance of electrical and non electrical apparatus in hazardous areas). Therefore, electrostatic charge may create an incendive spark when brought into close proximity with a conductor at a different potential. Note: An aircraft in flight will typically acquire an electrostatic charge. However, most of the parts of the aircraft will be bonded (either explicitly, or incidentally, in some cases via paths of high resistance), and the occupants of the aircraft will be in contact with the aircraft body. The aircraft itself will be unaffected by the electrostatic charge, as this does not cause a potential difference between aircraft parts, and similarly the occupants will be unaffected by (and indeed unaware of) the charge. When the aircraft lands, the electrostatic charge will generally be dissipated into the general mass of the Earth via (semi) conductive tyres and the runway/apron/helideck, with no observable effects on the aircraft or its occupants (but the aircraft may need a bonding conductor during refuelling, to avoid ignition of fuel vapours, see 5.6.4). The generation of static charge may be a continuous process; the charging current depends on numerous factors, but practical experience in the oil and gas industry indicates that the current will be 10-4 A or less, and tests indicate that in normal oil and gas applications, at least 300 V are required to initiate an incendive discharge from an electrostatic source (see the EI Model Code of Safe Practice Part 1: The selection, installation, inspection, and maintenance of electrical and non electrical apparatus in hazardous areas). Therefore, it is recommended that an electrostatic potential of 100 V should not be exceeded, but static electricity is easily relaxed via conductive or dissipative bonds (sometimes called semi-conductive bonds) with a resistance not normally exceeding 100V/10-4 A, i.e.1 MΩ (bonding assists in the relaxation of static charges i.e. bonding prevents the build-up of a dangerous electrostatic voltage, but does not prevent charge separation and the creation of static electricity). Therefore, where conductors are required to be effectively isolated for other reasons, they may nonetheless be bonded via high resistance paths in order to relax static electricity. Note: Vehicle and aircraft tyres can provide a suitable dissipative path to the general mass of the Earth (though tarmac may have too high a resistance to allow this process, see the EI Model Code of Safe Practice Part 1: The selection, installation, inspection, and maintenance of electrical and non electrical apparatus in hazardous areas). Earthing/grounding/bonding conductors provided for electrical or lightning protection have very low resistance values in relation to the requirements for static electricity. Consequently, where such conductors are provided, there will also be protection against electrostatic energy. A mixture of conductive and non-conductive components in one installation should be avoided, as this may result in isolated conductive sections that may acquire a potentially incendive electrostatic charge; such an arrangement can also create difficulties in the periodic verification of the earthing/grounding/bonding functionality. For example, insertion of a section of non-conductive pipe into a conductive pipe run breaks the continuity along that run; it is then necessary to ensure that the conductive parts at both ends are reliably connected to earth/ground, or to install a bonding conductor to bridge the non-conductive pipe section. Alternatively, the use of semi-conductive/dissipative pipe should ensure that a continuous conductive path is maintained throughout the length of the pipe run (provided that electrical contact is maintained at interfaces).

26

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Where an open-ended pipe discharges into a vessel or where a line incorporates an electrically discontinuous flexible coupling to a vessel, bonding between the end of the pipe and the vessel is required in order to prevent the accumulation of potentially incendive electrostatic charges during product flow. Further advice on the control of hazards arising from static electricity is given in EI Model code of safe practice Part 21 Guidelines for the control of hazards arising from static electricity.

4.6 LIGHTNING Lightning is the discharge of static electricity generated within clouds by the charge separation mechanism described in 4.5, but on a very large scale. It results from collisions between water droplets, ice/water slush or ice crystals, driven by air circulation or wind within the cloud. Negative charge tends to accumulate on the base of the cloud, whereas the top of the cloud becomes positively charged. Ionised 'leaders' reach down from the cloud, and leaders reach up from ground level or from tall structures. Eventually, a massive discharge occurs via a conductive plasma that ‘attaches’ to some point on the surface of the Earth or to an earthed/grounded structure. Following the initial fast strike, a lower current may flow for a longer period. The lightning current flows into the general mass of the Earth, where it need be considered no further (unlike the situation with electrically powered systems, where currents always return to the source of supply, typically the transformer star point). Most cloud-to-ground lightning is a discharge from the negatively charged base of the cloud. Positive lightning arises from the positively charged top of the cloud, and may travel a considerable distance away from the cloud before forming a large bolt of lightning which can be more severe than negative lightning, by about a factor of 10; only about 10 % of strokes are positive. Lightning strikes have currents and voltages that might be capable of damaging unprotected structures, process apparatus or electrical/electronic apparatus, as well causing ignition of flammable inventories. However, the structures of large oil and gas installations are generally made of conducting materials such as steel, and are generally in contact with the soil or the sea. Larger items of process plant are generally made of steel and are in contact with the basic structure of the facility, and machinery is generally in metallic casings, once again in contact with the structure. The sizes and thicknesses of these structures and casings may exceed the BS EN 62305-3 requirements for thickness and cross-sectional area (e.g. 4 mm for steel sheets, 25 mm2 for copper conductors not prone to mechanical damage), and so offer adequate protection; bonding to the site earth-termination electrode is required. The energy of a strike is sufficient to cause injury to personnel; a stroke near a facility, or a stroke on a service connected to a facility (e.g. a pipeline, power cable, signal cable, or other conductive structure), can also cause injury. Because soil is not a perfect conductor, large voltage gradients can appear adjacent to the immediate site of the strike, and cause hazardous 'step voltages' between the feet of personnel (see 4.13). 4.6.1 The likelihood of a strike, and risk management Maps of the world showing lightning flash density ('isokinauric maps') are available and give the average number of lightning flashes per square kilometre per year. Data are available from many sources, for example, the National Lightning Safety Institute (891, N. Hoover Avenue, Louisville, CO, USA) and the Council on Large Electrical Systems (CIGRE, 21 rue d’Artois, FR-75008, Paris, France). The number of flashes in some parts of the world may exceed

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10 per square kilometre per year, and in others is less than 0,1. For small facilities (say 10 m x 10 m) in benign locations with an annual probability of a strike of e.g. 0,1 per square km, the annual probability of a strike is 10-5, the threshold of risk to human life or permanent injuries suggested in BS EN 62305-2 as requiring consideration. However, in general the likelihood of a hazardous strike and resulting losses should be assessed. The methodology defined in BS EN 62305 is to identify the lightning protection zone (LPZ) which requires protection by a lightning protection system (LPS). BS EN 62305-1 defines four lightning protection levels (LPL), I, II, III and IV, I being the most onerous, and defines the specific lightning parameters which the LPS should be designed to withstand (i.e. in terms of cross section area of bonding conductors, thickness of shielding, etc.). Note : BS EN 62035-2 indicates that at least class II should be adopted for facilities with an explosion risk, but the need to define a performance standard for the lightning protection function is self-evident for oil and gas facilities, as these contain a significant inventory of flammable fluids which could be ignited by a strike. The parameters of individual lightning strikes vary greatly, but their statistical distribution is given in BS EN 62305-1; for LPL II there is a 98 % probability that the parameters of individual lightning strikes will be within the values given (so only 2 % of strikes will be more severe than the values given, see BS EN 62305-1 Table 5), and LPL II systems give a good degree of lightning risk reduction. The mechanical effects of lightning are related to the peak value of the current (for LPL II, 150 kA), the thermal effects are related to the specific energy, (for LPL II 5,6 MJ/Ω where resistive coupling is involved) and to the charge (for LPL II, 75 coulombs) in cases when arcs develop. Overvoltages and sparking caused by inductive coupling are related to the average steepness of the lightning current front (for LP II, 20 kA/µs). The functions of the external LPS are (a) to intercept the lightning flash via an air-termination system (not addressed in this publication), (b) to provide a conductive path via a downconductor system (not addressed in this publication) therefore bypassing the protected facility and ideally routing the lightning current away from hazardous areas, and (c) to disperse it into the general mass of the Earth via an earth/ground-termination system, usually a system of earth/ground electrode(s), though the underground metallic structures of the facility such as foundation steelwork may form an effective earth/ground-termination. Surge protection systems may be required to intercept surges resulting from resistive and inductive coupling, transmitted by lines connected to the structure, or magnetic fields directly coupling with apparatus, and to provide protection against the failure of internal systems, see 4.9.1.1. 4.6.2 Earth/ground-terminations The lightning earth/ground termination, and related conductors and bonding bar(s) generally used to interconnect extraneous conductive parts, etc, should be of sufficient strength and current carrying capacity to tolerate the peak current and heating effect. BS EN 62305-3 gives recommended minimum conductor sizes (which are independent of LPL). Earth/ground-termination conductors should also have low inductance so as to reduce overvoltages caused by the steep edges of the lightning current pulses. The inductance (L) of a linear conductor in free space is approximately (see EB Rosa): L = 2.l.[ln(2l/r)-1]x10-7 H

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Where: l = length r = radius = 5 µH for a conductor of radius 10 mm and length 5 m So on this basis, for the maximum di/dt of 20 kA/µs defined for LPL II, the voltage generated is 100 kV for a 5 m conductor; however, di/dt can be up to 10 times higher for positive lightning, and the voltage generated correspondingly higher. Stray capacitance reduces these overvoltages, and in practice an inductance up to 100 µH may not be a problem (see Cul/LT-0373 Lightning simulation testing to determine the required characteristics for roof bonding cables). Nonetheless, earth/ground-termination conductors should be kept as short as possible, and bends should be avoided as they increase inductance. The resistance of copper earth/ground-termination conductors is of order 0,005 Ω/m, so for the peak current defined for LP II (150kA), the voltage generated is around 3,75 kV for a 5 m conductor. This voltage is dwarfed by the inductive component, but since side flashes may occur at potential differences of 5 kV or more, structures adjacent to earth/groundtermination conductors should be bonded to the earth/ground-termination conductor at intervals of no more than 5 m. To disperse the lightning current into the general mass of the Earth, so minimising potentially hazardous overvoltages, the earth/ground electrode system should have a low resistance. BS EN 62305-3 recommends as low a resistance as practicable and where possible a maximum resistance of 10 Ω (measured at low frequency). On this basis, the LPL II peak current of 150 kA gives a voltage of 1 500 kV, so the earth/ground electrode resistance is a significant contributor to overvoltages. 4.6.3 Physical damage and life hazard For loss of human life or permanent injuries, BS EN 62305-2 gives a representative value for tolerable risk of 10-5 per year (a suitably small share of the 10-3 widely applied as the maximum tolerable risk in workplaces). The risks in a particular situation should be compared with the tolerable risk, and where the risk is higher than the tolerable risk, an LPS should always be provided. Health and safety legislation and practice in Britain have a second requirement, i.e. 'grossly disproportionate' expenditure to reduce the residual risk to a level which is 'as low as is reasonably practicable' (ALARP.) This final level of residual risk may be lower than the limit of tolerability (10-5), if cost-effective risk reduction measures are available. For economic loss, a straightforward cost-benefit analysis may be adopted, as discussed in BS EN 62305-2; in general, this requires the identification of all types of loss and the evaluation of the risk associated with each. The cost-effectiveness of each possible protective measure is evaluated by comparison of its costs with the economic value of losses averted by that protection measure (see BS EN 62305-2 Annex D).

4.7

CIRCULATING CURRENTS Circulating currents may occur in any loop, for example, induced by nearby power transmission conductors, RF radiation, CP currents, galvanic currents, etc. See 4.1 for comments on circulating currents between generators.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Where practicable, loops should be avoided and radial layouts are preferred, i.e. a single conductive earthing/grounding/bonding path should be used, although there are special cases (e.g. Zener barrier systems) where two or more conductors are required to provide a sufficiently low impedance path, or a path of high integrity that will withstand a conductor failure; in these cases, the conductors may need to run side-by-side in order to have the lowest practicable mutual inductance.

4.8

CATHODIC PROTECTION Steel structures in contact with the sea, and underground steel tanks and structures, are often protected from corrosion by a combination of external protective coatings and cathodic protection (CP). There are two types of CP, (a) a sacrificial anode system, and (b) an impressed current system. The anodes of either type are inherently in direct/intimate contact with the general mass of the Earth and form the ‘reference potential point’ for bonding purposes; the steel cathode is protected against corrosion by the current which flows from the anode. Any structure residing in the same electrolyte, and in electrical contact with the structure intended to be protected, may divert part of the cathodic protection current by acting as an alternative conductive path. This current detracts from the effectiveness of the CP scheme, and may create an incendive spark if interrupted or reconnected. Therefore structures such as connected pipework or cable armour that could divert the CP current should be electrically isolated from the protected structure, e.g. by means of isolation bushes. A method of testing to confirm the continued effectiveness of the isolation(s) should be incorporated. Isolations allow potential differences to appear between conductive parts, which should therefore be placed far enough apart that they cannot be touched simultaneously (to create a 'touch voltage' hazard, or a potentially incendive spark). Where electrical power is supplied to systems on a CP protected structure, the supply system and structure should not share earth/ ground connections; for example, the power supply system on a CP protected jetty should be a local TT (with RCD protection) or a local TN-S system, as shown in Figures 4 and 5. The design of CP for a new facility should include isolating joints or insulating gaskets between flanges to ensure that new steel pipework and storage tanks are electrically isolated from the site earth/ground bar, but will not isolate these structures from any static earth/ ground electrode required. All sections of buried steelwork and pipework to be protected should be made electrically continuous by the use of a separate insulated and sheathed bonding conductor. On retrofitted CP systems it should be noted that it may not be possible to isolate every unintended earth/ground connection. For example, CP current may be diverted by the armouring/screening of instrument cables, and this could also interfere with the instrument function. Where possible, instrument cable connections to the protected steelwork should be electrically isolated from that steelwork, and should have the cable armouring/screening earthed/grounded at one point only. Isolated flanges and the pipework on each side of the flange should be protected with an insulating material (e.g. for a distance of 1 metre from the flange) by wrapping with a cold applied PVC tape. The aim is to avoid unintentional potentially incendive discharge between earth/ground and the cathodically protected structure in the event of accidental contact, e.g. tools short-circuiting the flange. The wrap will also prevent ingress of moisture and debris which could result in a reduction in the electrical insulation properties of the insulated flange.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Where equipment such as a submersible pump is installed in a CP protected tank, an isolating joint/insulating gasket is normally installed between the pump head and the tank flange to provide isolation of the electrically earthed/grounded metalwork of the pump. For further guidance on CP, see 4.10 and BS EN 13636.

4.9

ELECTROMAGNETIC INTERACTIONS BETWEEN SYSTEMS Any current flow will induce currents in adjacent conductive materials, and capacitive coupling also causes energy transfer from one conductor to another. Therefore, electrical and electronic systems can be susceptible to interference from lightning currents and radio frequency transmitters. Interference can also arise from adjacent power systems, so segregation of HV, LV and signal circuits may be required.

4.9.1 Lightning Electrical and electronic systems can be susceptible to the secondary effects of a lightning strike, e.g. introduced into the electrical/electronic system internals via functional connections such as signal conductors, or via cable screens or armour; this can occur even though such systems are enclosed in a conductive housing that offers some protection against a direct strike. Surge arrestors may be used to limit the secondary effects of a lightning strike, see 4.9.1.1. It is generally recommended that any pipe joints that have insulating gaskets in them for the purpose of cathodic protection isolation should have those joints equipped with isolating spark gaps (ISGs), suitably certified for the area classification in which they are installed, to allow current to bypass the insulated joint at the time of a strike and to spread between the various structures, see 4.9.1.1. It is better for minimising effects to the instrumentation that the current can disperse over a wider area than just the one structure which is struck, and ISGs permit this by making parallel current paths available. Buried cables may be provided with a protective earthing/grounding conductor laid in the trench; this may give some lightning protection and risk reduction, especially for instrument/ telecommunications cables in lightning-prone areas. This arrangement may also provide some protection against electromagnetic interference. For further information, see the API/EI Research report. Verification of lightning protection requirements for above ground hydrocarbon storage tanks, the BS EN 62305 Protection against lightning series of standards and BS EN 62561-3. 4.9.1.1 Surge protection Surge protection is installed to provide protection against the presence of high voltages in the event of a lightning strike or similar high voltage surge. There are two general types of surge protector. Each provides a low impedance path for normal operational currents; in the event of a surge, some clamp the outgoing voltage to a safe maximum and conduct excess current to their earth/ground connection, and others provide a very low resistance path to their earth/ground connection, rendering their output voltage low. All surge arrestors divert the surge current to the earth/ground system, which therefore requires a low resistance. Surge arresters reduce the likelihood of damage to internal systems such as control systems and instrumented safety systems; they limit the 'series mode' voltage to the signal interface (i.e. the signal voltage itself), and limit the 'common mode' voltage between the two input

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

terminals of the electronic equipment and earth/ground. Therefore, the signal itself may be corrupted for the duration of the surge (a few µs in the case of lightning), but the interface equipment is protected against damage. The power supply to the signal interface equipment may also need protection against surges, in order to prevent surges on the input power supply from coupling into the signal interface apparatus (corrupting the signal, and possibly damaging the electronics). Figures 10 and 11 illustrate the interconnections in this situation. Note: Where the impedance of the earth/ground conductor serving the surge protector is significant, the common mode voltage at the protected equipment can be reduced by a 'surge link', as shown in Figure 11. A related (but converse) protective function is provided by isolating spark gaps (ISGs); these are two-terminal devices, and normally have a high impedance. In the event of a surge, they have a low impedance and provide a conductive path for surges, e.g. to bypass an isolated joint. Hazardous area

Screen (isolated)

Non-hazardous area Field junction box

Marshalling cabinet

Instrument internals

Instrument housing

Surge arrester Armour earthed / grounded via cable gland

TERMINAL BLOCKS

Armour earthed / grounded

Zenner barriers

Signal processing

I.S. System Earth

Clean earth / ground Plant bond - structures + conductors + soil Supply Two parallel conductors preferred to facilitate testing – see 5.3

Earth / ground electrode Figure 10: Typical earthing / grounding / bonding system for instruments

Figure 10: Typical earthing/grounding/bonding system for instruments

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Protected electronics

Signal SPD Series mode voltage Common mode voltage

Surge current

Surge link if SPD close to protected equipment

L Power supply

N

E

L

N

Power SPD

E L N

Other protected equipment

Surge current

Mains earth / ground terminal

Figure 11: Surge protection of signal processing equipment 4.9.2 Power lines Electrical and electronic systems can be susceptible to induced voltages from adjacent power lines, but the screening of signal cables and earthing/grounding of any cable armour are usually sufficient to deal with this problem. Where a pipeline is near a power transmission system there are two mechanisms that can give rise to a voltage between the pipeline and earth/ground: −− −−

The power transmission line suffers an earth/ground fault causing a short term but significant voltage to be impressed on the pipeline. The electrical capacitive and inductive coupling between the power transmission line and the pipeline cause a voltage to be present continuously.

Note: Because cross country metallic pipelines may be inherently earthed/grounded, it is difficult to prevent some earth/ground fault or other currents passing along a pipeline. Power line effects should be controlled by judicious pipeline earth/ground application together with provision of material that may increase earth/ground impedance at points where a pipeline may be touched, e.g. at test points and where the pipeline comes above ground. The continuous voltage produced on the pipeline due to electromagnetic coupling between the pipeline and an adjacent power transmission line should be mitigated by the pipeline earthing/grounding arrangements to ensure that at no point does the pipeline voltage to earth/ground exceed 15 V. Where the effect is short term, (generally less than 0,3 seconds), the allowable voltages could be higher than may be allowed for a normal standing voltage. See Annex B for guidance on touch and step voltage limits.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

4.9.3 RF induction Where a conductor is close to high power transmitters, it is possible for the conductor to act as an aerial and, due to the electro-magnetic field, have voltages induced in it. Therefore for earthing/grounding/bonding circuits, radial layouts should be used in preference to ring circuits, in order to minimise induced RF current, though some loops cannot be avoided. PD CLC/TR 50427 describes procedures for establishing whether potential hazards may exist; a simple procedure for undertaking an initial assessment is given. PD CLC/TR 50427 includes methods that may be used to establish the field intensity from fixed and mobile transmitters, including those on board vehicles, ships and aircraft. Both pulsed and continuous transmission are considered. Additional guidance is given in EI Recommendations for radio communication equipment and its installation in petroleum road tankers, and further information on field intensity may be found in OCIMF International safety guide for oil tankers and terminals (ISGOTT).

4.10

EARTHING/GROUNDING/BONDING INTERCONNECTIONS AS A SYSTEM There are several considerations to be taken into account in designing an entire earthing/ grounding/bonding system for an oil and gas facility; the system should therefore be considered in a holistic way. For example, some earthing/grounding/bonding connections require segregation where they perform different functions or where they may interfere with each other; Figure 12 illustrates some typical interconnections and isolations. The following notes discuss some potential interactions/interference which should be avoided where possible. Supply system

Consumer’s site

Surge arrestor L

Substation

N

L Switch board

Electrical plant

N E

Nonelectrical plant

CP Supply

Induction Earthing / grounding bar

E

Isolation Test socket

Gravel Grid

External pipeline

Earth / ground electrode system CP anode

Earth/ground electrodes

Anti-static electrode

Figure 12: Typical example of earthing/grounding system interconnections Figure 12: Typical example of earthing / grounding system interconnections

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Significant voltage drops can occur on the earth/ground-termination conductor of lightning protection systems, so earthing/grounding connections from other systems that share the same earth/ground electrode should be made as low down as possible so as to avoid coupling voltages into those other systems. However, the earth/ground electrode voltage itself can rise substantially when a lightning strike occurs, so it is inevitable that some lightning induced voltage will be coupled into other systems, and surge protection may be needed. Where electrical power is supplied from overhead power lines, a surge may occur in the power system at the facility in the event of a lightning strike on the power lines, even at a significant distance. Interactions between the incoming supply and on-site systems should therefore be minimised by the use of surge protection at the power system entry to the site (this protective function diverts the surge to a local earthing/grounding connection), see 4.9.1.1. Where a pipeline incorporates isolation joints (e.g. to isolate CP protected steelwork), isolating spark gaps (ISGs) may be required to provide a path to earth/ground for lightning currents. Steelwork having cathodic protection should be electrically isolated from adjacent earthed/ grounded conductive systems so that the cathodic protection current is not diverted away from the steelwork requiring protection. Also, such protected steelwork may be at a significant potential with respect to other earthed/grounded conductive surfaces, and so present an electric shock hazard (touch voltage), or an ignition hazard if the gap is bridged by a conductor such as a tool. Where earthed/grounded/bonded systems require isolation(e.g. to avoid CP interactions), the isolation should nonetheless be bridged by a resistor to allow for the dissipation of static electricity (see 4.5). If an impressed current CP system is used to protect steelwork, the impressed current anodes should not be relied upon to act as anti-static earth/ground electrodes because they are connected with the steelwork only via the CP supply, and are not in direct electrical contact. If the steelwork and associated pipework require anti-static earthing/grounding, this should be achieved by a connection (via a resistor) to the general site earthing/grounding system, or by installing a dedicated earth/ground electrode connected directly to the steelwork; however, this electrode will draw current from the CP system, which will need to have sufficient spare capacity to accommodate the increased current demand. In the case of a sacrificial anode connected directly to, e.g. a tank, a separate anti-static earth/ground electrode should still be provided, since there will be no means of testing the effectiveness of a single anode. However, where at least two sacrificial anodes serve as anti-static electrodes, one may be disconnected for testing without affecting the tank static earthing/grounding. Where sacrificial anodes are used as static earth/ground electrodes, they should be identified as such. The electrode material should be manufactured from galvanised steel or zinc so as not to cause galvanic corrosion. Instrumentation and telecommunications systems earthing/grounding arrangements should be connected to the site primary earth/ground electrode at a single point so as to avoid interaction from power system operational currents and fault currents. This earthing/ grounding point is commonly called a 'clean earth/ground' and returns instrument and telecommunications system current to the power system star point (see Figure 10). There is little merit in a separate earth/ground electrode for the 'clean' earth/ground, especially for Ex i systems, as the resistance between the main plant earth/ground electrode and a separate 'clean' earth/ground electrode may not be able to achieve a sufficiently low resistance (e.g. the 1Ω required by BS EN 60079-14 for systems with TN-S supplies). Screened or unscreened

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cables may be specified depending on the installation and on the possibility of interference from other types of circuit. The preferred arrangement for the earthing/grounding of instrument cable conducting screens, which are to be earthed/grounded at the non-hazardous area, is shown in Figure 10. More than one earthing/grounding connection is permitted on a circuit provided it is galvanically separated into subcircuits, each of which has only one earthing/grounding point.

4.11

HAZARDOUS AREAS, EX CERTIFIED APPARATUS AND IGNITION SOURCES Electrical and mechanical equipment employed in areas classified as hazardous (e.g. as described in the EI Model code of safe practice: Part 15 Area classification code for installations handling flammable fluids) should be Ex certified in accordance with the zone classification. However, note that there may be potential sources of ignition other than electrical or mechanical equipment, e.g. the discharge of static electricity. A particular concern arises in the interior of atmospheric tanks and associated pipework, e.g. the crude oil storage tanks on offshore installations (such as FPSOs), petrol storage tanks, hydrocarbon slops tanks, etc. Unless an inert gas blanket is employed, air may be drawn in during emptying operations or may enter by normal 'breathing' or diffusion. Therefore, the interior of atmospheric tanks containing hydrocarbon products may have a potentially flammable atmosphere and require a formal hazardous area classification, and may indeed be a zone 0. Attention should therefore be paid to internal fixtures in such tanks, e.g. fill pipes, to ensure that they cannot become electrostatically charged by, e.g. agitation or splashing of the product; such items should be bonded to the tank so as to safely dissipate static electricity. Similarly, vents or take-offs from atmospheric tanks could give rise to a potentially flammable atmosphere in the vicinity. If such vents require lightning protection, it should be provided in a suitable area away from vents so as to reduce any ignition risk at the vent. Note: Pressure systems for hydrocarbon products are closed systems containing little or no oxygen; gas/vapour concentration is generally well above the upper flammable limit, and the interiors of such vessels are not classified as hazardous areas. The hazardous areas and non-hazardous areas of tankers (road, rail, sea) can overlap the hazardous and non-hazardous areas of fixed installation (e.g. when the tanker is positioned incorrectly). Therefore, consideration should be given to the possibility of the uncertified apparatus of tankers, or any other potential source of ignition, entering the hazardous areas of the fixed installation, and vice versa. For further information on the requirements for earthing/grounding and bonding of hazardous area apparatus, see 5.3 and BS EN 60079-14.

4.12

EARTH/GROUND ELECTRODE RESISTANCE REQUIREMENTS The required resistance of a given earthing/grounding system depends on its defined function and associated performance requirements. For a lightning earth/ground electrode system, the EN 62305 series of standards requires the resistance to be as low as practicable, with a maximum of 10 Ω, and for neutral earthing/ grounding it should be as low as practicable.

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For substations, the earth/ground electrode system should be designed to accommodate the maximum fault current and in so doing to keep touch and step voltages within safe limits. BS 7430 provides extensive information on the design of earth/ground electrode systems for electrical installations, and requires that the resistance be low enough (to ensure that fault current is high enough) to operate safety mechanisms to isolate the supply in the fault situation. However, some electrical 'earthing/grounding' systems do not rely on significant current flow into the general mass of the Earth, as fault current is returned to the source of supply by the circuit protective conductor. For such cases, the resistance of the earth/ground electrode is less critical. Most electronic 'earthing/grounding' systems have a very low current flow to the power supply return or into the general mass of the Earth (though they may provide a nominally 'zero' volt reference, e.g. to protect low signal voltages against pick-up); for these systems earth/ground electrode resistance is not critical. Also, dissipation of static electricity into the general mass of the Earth can be achieved with a very high resistance earth/ground electrode system. An exact calculation of earth/ground electrode resistance is not practicable because soil generally has non-uniform resistivity that is likely to vary over time as the water content of the soil varies. See Annex C for examples of calculations of electrode resistance and resulting step voltages for two simplified electrode geometries, and for further information. Similarly, touch voltages may be a function of earth/ground electrode resistance. Annex B gives information on what step and touch voltages may be regarded as acceptable to avoid danger to people.

4.13

TOUCH AND STEP VOLTAGES A 'touch voltage' is the potential difference between (a) both of a person’s outstretched hands, considered to be 2 metres, or (b) a person’s foot/feet and an outstretched hand. Potentially hazardous touch voltages can arise in a number of ways. For example, where an 'earth/ground' fault results in a current flowing through exposed conductive parts into the earth/ground electrode system, a 'ground potential rise' will occur, shown as V1 in Figure 13, and a touch voltage may exist between: −− −−

Any exposed conductive part and an accessible extraneous conductive part (e.g. a steel flooring). Figure 13 illustrates examples of such touch voltages (V2 and V3). Or, Any accessible pair of exposed conductive parts (e.g. apparatus housings) giving a 'metal-to-metal' touch voltage.

A form of touch voltage called a 'transferred voltage' arises when an extraneous conductive part such as a wire fence, cable armour, etc. is at a significant potential with respect to the nominal zero potential of the remote general mass of the Earth. This potential is transferred away from an electrical installation via the conductive structure to a person whose feet are nearer to the nominally zero potential of the remote general mass of the Earth, resulting in a potential difference between hand(s) and foot/feet, i.e. a touch voltage. Alternatively, a nominally zero voltage may be transferred into an installation to the hand(s) of a person whose feet are at a significant GPR, also creating a transferred touch voltage. Figure 13 illustrates how these touch voltages (V5) are created. The step voltage is the difference in surface potential between a person’s outstretched feet, considered to be 1 metre. Step voltages arise directly from the surface voltage gradient caused by an electrical system fault current or by lightning. V4 in Figure 13 illustrates GPR

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gradient. The anticipated GPR and its gradient can be calculated from a knowledge of the maximum fault current and the characteristics of the earth/ground electrode system. See 6.4 and Annex C for details of how earth/ground electrode resistance can be predicted, measured and minimised. Simplified examples of calculated gradients are given in Annex C. For guidance on acceptable touch and step voltages limits, see Annex B. V1 = Earth/ground potential rise V2 = Touch voltage V3 = Touch voltage V4 = Step voltage V5 = Prospective touch voltage/ transferred potential

V3 V1

V2

V4

Surface potential V5

Conductive structure

Main earth / ground electrode system Grading electrodes Fault current return to source

Figure 13: Example of the surface potential profile and resulting touch and step voltages

4.14

TEMPORARY INSTALLATIONS Temporary installations should be designed, maintained, tested and operated to the general principles described in this publication and in the underlying standards, but the details can be determined only on a case-by-case basis.

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5 APPLICATIONS 5.1

ELECTRICAL MACHINES AND POWER SYSTEMS Earthing/grounding/bonding conductors for electrical systems are provided to deal with fault currents, lightning, static electricity, and touch and step voltages, and may consist of one or a parallel combination of: −− −− −− −− −−

dedicated earthing/grounding/bonding conductor; cable armouring or metallic sheath; rigid screwed conductive conduit, cable tray, etc; earth/ground core within a multi-core cable, and structural steelwork.

Figure 14 shows a typical offshore layout of an electrical supply system earthing/grounding, and the typical earthing/grounding of electrically driven machines. In this case, the all-welded steel structure of the installation is regarded as 'earth/ground'. All tanks, vessels, pipes, ducts, cable trays, access ways etc. are bolted or welded to the structural steelwork. This arrangement also provides a lightning earth/ground-termination, and ensures that any static potential build-up will be avoided by continuous drain to 'earth/ground'. The impedance of a large all-welded structure used as a protective conductor cannot be readily calculated, but it can be assumed that a very large structure such as an offshore installation has negligible impedance for the purpose of fault current calculations. 11KV/415V Trans.

Cable phase conductors

415/240V Switchboard

R N

Y

Circuit protective device (fuse or c/b) Cables boxes bonded internally to earthing/grounding bar

Cable armour Routes for fault current

B F

Bonding connections

Earthing/grounding bar

Platform steel work Bonding connection required if apparatus is insulated from steel structure

Electrical apparatus in permanent electrical contact with the structure via bolts, welds and pipework

Figure 14: Earthing principles offshore This concept may apply to other permanent all-welded steel structures in an onshore process plant. However, where there is no local connection to the main steelwork, the cable armour may form part of the return path/CPC for fault currents, as shown in Figure 14.

Figure 14: Earthing principles offshore

When high voltage motors are in permanent direct electrical contact with steelwork forming (part of) the common earthing/grounding system, no additional bonding conductors are required. Metallic enclosures of local control stations and other associated electrical devices

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local to the motor should be bonded to the motor enclosure or to the earthing/grounding system to which the motor is connected. This may occur inherently through the mounting bolts and earthed/grounded steelwork. Enclosures of high-voltage motors etc. not in direct electrical contact with earthed/grounded steelwork should be connected to the local static and lightning earthing/grounding system, or to local earth/ground electrodes. A common earth/ground electrode system may be used for several motors in the same area. Low-voltage equipment enclosures in permanent direct metallic contact with earthed/ grounded plant steelwork require no further connection to the earthing/grounding system. Exposed conductive parts of electrical equipment and extraneous conductive parts of nonelectrical apparatus are normally effectively bonded to the steel structure by holding down bolts and bedplate welds etc. Enclosures not in direct metallic contact with earthed/grounded plant steelwork or pipework should be bonded to the static and lightning earthing/grounding systems or to the adjacent earthed/grounded steelwork by means of a bonding conductor. An example of this latter arrangement is shown in Figure 15. If the plant/apparatus is remote from the sub-station the earthing/grounding systems need not be interconnected; in this case earth/ground fault current will return via the cable armour or earthing/grounding core. Account should be taken of parallel earth/ground paths via the armour of other cables. EQUIPMENT EARTHING/GROUNDING Diagram show that earthing/grounding may be provided by:

EARTH/GROUND STUD 3-PHASE MOTOR

R

Y

B

[1] Dedicated conductor [2] Cable armour and glands [3] Auxiliary earth/ground bond The designer may use any one or a combination of these methods to ensure that earthing meets National Standards. There is no preferred method.

E [1]

[3]

[2]

LOCAL PLANT EARTH/GROUND

R

Y

B

E

MOTOR STARTER

Figure 15: Typical earthing/grounding arrangements

40

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Note: The hull of a ship (e.g. floating production, storage and offloading vessel (FPSO)) should not generally be used as a return path for neutral currents or as a circuit protective conductor; BS IEC 60092-502 allows the use of the hull as an 'earth/ground' only for limited and locally earthed/grounded systems, intrinsically safe systems and impressed current CP systems. 5.2

MACHINE SETS WITH NON-ELECTRIC DRIVES Earthing/grounding/bonding conductors for non-electrical machine sets in hazardous or nonhazardous areas are provided to deal with lightning and static electricity, and for general supplementary bonding purposes. When driving and driven machines are in direct metallic contact with an earthed/grounded steel structure (e.g. offshore), no additional earthing/ grounding is required. When driving and driven machines are bolted to a common metallic bedplate on a concrete or other poorly conducting foundation, one connection should be taken from the bedplate to the earthing/grounding system. When driving and driven machines are on separate bedplates mounted on separate concrete plinths or other poorly conducting material, the bedplates should be bonded together and a connection taken to the general earthing/grounding system. Earthing/grounding/bonding connections should be a copper conductor of appropriate size (see 6.2).

5.3

EX I SYSTEMS AND APPARATUS Intrinsically safe systems utilising zener barriers may be fully floating (isolated) or bonded, at one point only, to the reference potential associated with the hazardous area; this safety 'earth/ground' connection should have low impedance, and is of fundamental importance, so a special 'clean' earthing/grounding terminal for the intrinsic safety earth/ground point should be provided. Zener barriers are connected to this point, which should be isolated from the equipment cabinet metalwork and from other earthing/grounding connections except the dedicated insulated protective conductor connecting back to the power system earthing/grounding terminal. This is because under fault conditions, transient voltage differences between the barrier busbar and the supply return (typically the neutral star point) are transferred into the hazardous area and could reach incendive levels if the resistance of the safety earthing/grounding conductor is too high. Note: More than one earthing/grounding connection is permitted on each circuit provided that the circuit is divided into sub-circuits, each of which has only one earthing/grounding connection. Mechanical protection of the conductors may be required. The total conductor c.s.a. should not be less than that recommended in 6.2, but this may be increased to maintain the earth/ground impedance below 1,0 Ω. Two earthing/grounding conductors in parallel are preferable to a single conductor as this enables each conductor to be tested without loss of system earth/ground continuity (see Figure 10). Galvanic isolators (as opposed to Zener barriers) do not have such stringent earthing/ grounding requirements, because they isolate the barrier busbar from connections into the hazardous area. Another advantage of galvanic isolators over zener barriers is that they allow the use of field devices that cannot be isolated from earthed/grounded conductive parts (e.g. devices which make an inherent contact with steelwork or the process fluid). In ship’s systems, there is an historical preference for galvanic isolators so that there can be no currents from the Ex i system transferred into the ship’s hull (and no currents can be transferred from the ship’s hull into the Ex i system). For further information on the earthing/grounding provisions for intrinsically safe systems, see BS EN 60079-14. 41

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5.4

ABOVE-GROUND TANKS AND FIXED STORAGE UNITS In practice, an above-ground tank, whether bunded or not, may be inherently well earthed/ grounded by contact with the soil, unless coated or landed on an insulator (e.g. concrete or insulating membrane). This inherent earthing/grounding may be supplemented by earth/ ground rods. There will also be incidental earthing/grounding through the many pipes, cables, handrails, and any ladder(s) that connect to each tank. This is not to say that the tanks can be left without specific earthing/grounding conductors; for personnel safety reasons, for electrical earth/ground faults, and for safety of both the electrical supply system and the instrumentation, tanks and fixed storage units should be provided with specific earthing/grounding/bonding conductors. The earthing/grounding systems of adjacent tanks in a tank farm should also be connected to each other, and to the instrumentation/control centre, to help limit lightning transients in the instrumentation circuits. An additional issue arises from potentially incendive static electricity; for example, fast flow through pumps, filters, etc, causes a charge separation process leaving a net charge in the liquid. This charge relaxes over a period of time; during the charge relaxation process, charge flows to the tank skin, and hence to earth/ground. The shunts and other metallic items make a sufficiently low resistance path as regards the tank floating roof, but tank internals, e.g. mixers, gauge floats and sling arms, should also be bonded to tank shell at one or more locations (depending on size of the internal object); bonding can preferably be achieved by direct bolting. A major issue arises with storage tanks, because there may be a potentially flammable atmosphere around the rim of a floating roof tank (FRT), or within a fixed roof tank. There is therefore a risk of ignition in the event of a lightning strike, but earthing/grounding may make a useful contribution to reducing this risk. Potential ignition hazards exist particularly at shunt/shell interfaces for open top floating roof tanks; most of the following discussion is relevant only to floating roof tanks, but ignition hazards may also occur in fixed roof tanks, especially those with geodesic roofs (which tend to be of thinner construction, and therefore are more readily punctured by a lightning strike). The nature of the hydrocarbon, its temperature and any ventilation, determine whether the vapour could be within the flammable range. In order to understand how lightning might affect a facility, and therefore be able to protect against it, it is useful to understand where the lightning current will flow. The fast, high energy component of the lightning current will not follow a single bond path, but will generally spread out between as many paths as possible. Consideration of a generic floating roof design demonstrates what this means (see Figure 16). A lightning strike anywhere in the vicinity of the tank roof will drive a current across the surface of the roof, and this current will flow on and off the roof via the shunts or other contact points. As described in section 2.1 and Figures 5a and 5b of Cul/LT-0234 Review of lightning phenomena and the interaction with above ground storage tanks, current flow paths from a strike to the top of the shell, or the roof, involve the whole tank; there is a distributed path to earth/ground all over the outside of the tank, and down to ground level. Therefore, even with, for example, only four earth/ground electrodes, the tank is earthed/ grounded well enough to conduct the strike into the general mass of the Earth. Since the earthing/grounding points are so remote from areas of flammable vapour/air mixes (such as the rim seal or roof), effects at ground level have little or no bearing on lightning induced fires. For tanks with a conducting floating roof with multiple bonds to the shell, very little lightning current will flow inside the shell.

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Cul/LT-0234 Review of lightning phenomena and the interaction with above ground storage tanks, describes current paths for the fast current component (the first return stroke and restrikes). For these components, skin effect and the inductance of the tank dominate current flow routes, and result in current flow through the shunts. For external (not submerged) shunts, arcing at the shunts caused by the fast component of lightning is unavoidable and is not significantly attenuated by roof bonding cables. Fast component arcing is primarily a hazard near its location at the shunt/shell interface. Where the shell is either corroded or contaminated with non-conducting material, e.g. wax etc. (or where there are no shunts), there will be sufficient inductive voltage to cause break-down of any gaps, in the form of voltage sparks (arcs) and possibly thermal sparks (Figure 16 (a)).

If we look at a generic floating roof design we can see what this means (Figure 1). Shell Shunt

Secondary seal Roof bonding cable

Primary seal fabric Metallic shoe

Floating roof

Weight (or spring) to hold shoe against the shell

(a) Fast current arcing

(b) Typical floating roof with bonding cable

Figure 1 Typical floating roof tank installation of primary seal, secondary seal, and shunt. The shunt is inten to help provide electrical continuity to the shell, but in practice any other fortuitously touchin Figure 16: Typical floatingfrom roof the tankroof installation nearly-touching contacts will also carry current. Sparking at shunts is more severe for the long duration component of the lightning current.

In the event Thermal of a strike the or to current willbecause spreadthe out divide sparkstofor thisroof, situation arethe morerim, of a the problem than arcs, arcand is localised between the atavailable even if itofhas to flow across poor such as the shunt. paths, However, showers thermal sparks spray out andcontacts. tend to fall Interfaces downwards into the seal region and into any gaps between the seal and the shell, or on to or behind the shunts and metallic shoes are inevitably going to generate arcing, because it is not possible to secondary seal, and so they can spread their effect over a larger volume, especially below the provide a good bond there. source of the sparks, therefore increasing the likelihood of flammable vapour ignition. This particular arcing threat may be reduced if low resistance roof-bonding cable(s) can be fitted

Arcs formed(Figure around theThe metallic between the primary and secondary sealallowed will occur in a 16(b)). voltageshoe at which the arc extinguishes determines the maximum resistance of the roof bonding cable system. Normally, arcing is extinguished if the resistive vapour space that may be flammable. A way to lower this threat is to provide isolation in the drop down the cable(s) is 200 A) the arc could be maintained as low as should be designed to hold off tens kV,reliably and so flashover distance of at (1least 6 V. The 14 V extinction will of occur if aa narrow spacer keeps a small mm)3”/ gap75mm open between the shunt and the shell. Since the average value of long duration 'continuing should be maintained.

2.1

current' is considered to be 400 A (see Cul/LT-0235 Review of tank base earthing and test current recommendations), then this would require the bonding cable system to have a combined resistance of 300 mm gives improved protection, but submersion at 100 mm to 300 mm would require bonding cables Arcs formed around the metallic shoe between the primary and secondary seal will occur in a vapour space that may be flammable. A way to lower this threat is to provide isolation in the pusher plate or pantograph arrangement so that this cannot be a current path. This isolation should be designed to hold off tens of kV, and so a flashover distance of at least 75 mm should be maintained. It may be that the shunts are not the only current path, so that some other fortuitous contacts may exist near or above the fluid line (shoe, pusher plates, etc). In these cases, any arcing is much more likely to occur outside the fluid, and would present a hazard; shunts or roof bonding cables will not provide any significant protection, as some current will always flow through the primary seal. Ladders may provide a conductive path, and so help to protect the structure from the effects of lightning, as they will present a fairly low inductance path; however, due to the nature of the contacts (either sliding with wheels, or hinges), the resistance of these interfaces is likely be high, or at best ill-defined, with the possibility of arcing. It is better not to assume credit for such a path. When a rolling ladder is installed, a flexible copper bonding conductor should be applied across the ladder hinges, between ladder and tank top, and between ladder and floating roof.

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The recommendation in BS EN 62305 is that the resistance to earth/ground of each structure should be as low as possible, and less than 10 Ω is recommended. In practice, because of contact with the soil and the multiple interconnections between tanks through earthed/ grounded armouring on cables, instrumentation earths/grounds and pipework, the earth/ ground resistance of a tank will be very low, usually much less than 1 Ω, without any recourse to special earthing/grounding measures. As a precaution, however, each tank should be equipped with earth/ground rods distributed equally around the circumference, to ensure that adequate earthing/grounding is achieved, e.g. if an insulating membrane is incorporated under the base. As indicated above, the earthing/grounding systems of adjacent tanks should be bonded together; periodic verification of earth/ground resistance is recommended (see Annex D). 5.4.1 Note on cathodic protection (CP) of tanks All tanks and steel structures should be earthed/grounded for electrical and lightning safety considerations. Where impressed current cathodic protection is used to minimise corrosion, pipe connections might require insulated joints to confine the cathodic protection current to the tank or structure that requires that protection. Lightning could, however, affect the cathodic protection system because some fraction of the lightning current might cause transient damage to the cathodic system power supply and monitoring circuits, which might therefore require surge protection (see 4.9.1.1 for guidance on surge protection). Electrical apparatus associated with the tank should also be isolated from the tank, and should have its own CPC to allow fault currents to flow back to the source.

5.5

CROSS-COUNTRY PIPELINES Underground cross-country metallic pipelines may be inherently earthed/grounded by virtue of their construction (unless covered with an insulating layer provided for thermal insulation, or to confine cathodic protection currents to the pipe material). Similarly, above-ground, cross-country metallic pipelines may be inherently earthed/grounded via metal supports that have earth/ground contact. However, cross-country pipelines should not be relied on as an earth/ground electrode for other systems or indeed as a protective conductor. Sometimes it is necessary to earth/ground buried pipelines that have impressed current cathodic protection, e.g. to alleviate the effects of local overhead power lines. In such cases, the pipeline should be earthed/grounded by polarisation cells, or, alternatively, by the use of earthing/ground rod materials of a suitable galvanic potential. Unless adequately connected to earth/ground elsewhere, other metallic utility and process pipelines above ground should be bonded to a common earthing/grounding conductor by means of earthing/grounding bosses or pipe clamps and connected to the earthing/ grounding system at all points where they enter or leave a hazardous area. Where pipelines enter a structure that incorporates a lightning protection system, a risk assessment should be carried out to evaluate the bonding requirements for lightning protection. See 7.4 for guidance on the operational practice of breaking joints in pipelines.

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5.6

TANKERS AND FUEL TRANSFER/DISPENSING SYSTEMS As with any other oil and gas installation, tanker loading/unloading and fuel transfer/ dispensing facilities may require CPCs to deal with earth/ground faults, earthing/grounding to mitigate touch and step voltage hazards, and lightning earthing/grounding arrangements. Potential ignition hazards also arise from sparking in hazardous areas as a result of electrical apparatus, cathodic protection systems, radio frequency transmission and static electricity. The following general recommendations should be followed for all tanker and fuel transfer systems: −− −− −− −−

Electrical apparatus should be earthed/grounded/bonded to provide a return path (i.e. a CPC) for fault currents and to eliminate hazardous touch and step voltages. Radio silence may be required during loading/unloading operations; for further guidance, see 7.6. Tank dipping may result in the generation of static electricity and a potential ignition hazard – see 7.8 for guidance on tank dipping operations. Impressed current CP systems can result in current flow in conductive parts, and energy may be stored for a significant time after the CP supply has been switched off; making or breaking connections may result in a potentially incendive spark – see 7.4 for guidance on operations on cathodically protected systems.

Further general guidance is available in the EI Model Code of Safe Practice Part 1: The selection, installation, inspection, and maintenance of electrical and non electrical apparatus in hazardous areas, but note that sources of potential ignition within tankers, aircraft etc. are not considered in this publication. 5.6.1 Road tanker loading and unloading facilities The general recommendations of 5.6 should be followed, but note the following points specific to road tankers: −−

−− −−

−−

−−

Each loading gantry should preferably have a connection to earth/ground at each end. Connections may be direct to independent earth/ground electrodes or to the general earthing/grounding system of adjacent plant. Alternatively, where there is the prospect of creating a ‘stray’ current loop within a gantry, a single earthing/ grounding point separate from other parts of the electrical installation may be considered appropriate. The resistance of individual connections to the general mass of the earth/ground should be less than 10 Ω. Product pipelines should be bonded together and bonded to the loading gantries either by pipe clamps or pipe flange bolts. Loading hoses should be electrically continuous from the product pipeline to the loading nozzle or flange. A bond may be required across each swivel joint in metallic loading arms. Loading bays should be provided with a number of flexible copper connections bolted to an earthing/grounding boss in the loading gantry at one end, and have a robust earthing/grounding clamp for bonding tankers to earth/ground during loading and unloading. Earthing/grounding devices and interlock arrangements shall be provided so tanker loading or discharge is possible only when effective earthing/grounding has been achieved (also note that proprietary interlock systems are available that recognize that a tanker is present).

46

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−−

For autogas/LPG/CNG loading bays, vehicle earthing/grounding arrangements should ensure that earthing/grounding connections are completed in areas classified as nonhazardous via an earthing/grounding switch, or by means of a suitable Ex certified switch.

5.6.2 Bulk railcar loading and unloading facilities The general recommendations of 5.6 should be followed, but note the following points specific to bulk railcars: −− −−

−−

−−

−− −−

Product pipelines, standpipes, loading hoses, loading arms and gantry structures should be electrically continuous and be earthed/grounded. The railway lines in the loading/unloading area should be isolated from the remainder of the rail system by means of insulated joints, in order to isolate potentially incendive track-signalling voltages or traction system currents from the loading/unloading area. A number of sections of rail may need to be insulated to prevent the railcar units short-circuiting the insulated joints. The tracks in the loading/unloading area should be bonded to the earthing/grounding system and gantry. Separate railcar tank earthing/grounding is not a requirement due to effective earthing/grounding via the car wheels being in contact with the earthed/grounded track. Stray current flows may arise in extreme cases from local power system earth/ground faults, power system neutral current flow or from cathodic protection systems. In such cases, placing an insulating flange in the delivery pipe should be considered as a control measure. Where top loading, the filling nozzle should be bonded to the railcar before loading (see 7.4). On electrified rail systems, live contact rails or overhead conductors should terminate outside the loading compound.

5.6.3 Sea tanker loading jetties The general recommendations of 5.6 should be followed, but note the following points specific to sea tanker loading jetties: −−

−−

−−

−−

All pipelines, loading gantries, masts and guy wires, floodlighting towers, cranes, structural and plant steelwork should be connected to an earthing/grounding system having a resistance to the general mass of the Earth at any point of not greater than 10 Ω. The path to the earth/ground-termination system for lightning protection should be as short and direct as possible. Where revolving cranes are mounted on rails, earthing/grounding the rails at more than one point should provide adequate protection. For jetties using hose loading gantries, the jib handling rig hook should be insulated. Where damage to bearings is a concern as a result of earth/ground currents, specialist advice should be sought. To avoid the possibility of potentially incendive sparking occurring when an oil jetty loading or unloading arm is connected to, or disconnected from, a ship, an isolating joint should be inserted in each of the lines as recommended in the OCIMF ISGOTT. Loading arms on jetties should have an insulated flange inserted in the 'outboard' end to prevent flows of stray current and hence potential sparking when connection is made between the ship and the jetty. The section of loading arm that is 'downstream'

47

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of the flange should be connected to the ship, and the 'upstream' section connected to the jetty earthing/grounding system. Insulating flanges should be inserted in each pipeline at the shore end. The jetty structure and earthing/grounding system should also be isolated from the shore earthing/grounding system. The intention of sea tanker isolations is to create three separate systems, isolated from each other; the shore system, the jetty system and the ship system, see Figure 17. Where a shore electrical supply is used to power systems on the jetty (or the ship, see 6.6.3.1), the incoming supply should be a local TT (with RCD protection) or local TN-S, so that the power system protective earthing/grounding does not bridge the isolations. Isolations serve two purposes; they prevent the flow of stray electrical system currents between the ship, the jetty and the pipeline, and they prevent the diversion of cathodic protection currents. When positioning isolating joints on different types of loading line, the following points should be considered: −−

Wholly flexible loading line – the isolating joint or flange should be fitted to the jetty manifold immediately prior to the hose. Partly flexible loading line, i.e. where the line is partly flexible and partly metal boom – the joint should be fitted between the flexible hose and the metal boom. All metal loading line – an isolating joint should be inserted at a convenient point close to the ship connection point.

−− −−

L

Cold ironing supply

N Isolation joint Hose

Pipeline

CP

CP

Supply

Jetty

Isolation joint

CP

Isolated mooring system

Shore earth / ground electrode Pipeline

CP anode

Impressed current CP anodes

Jetty isolation

Figure 17: Typical arrangement of CP for shore systems, jetty and ship Figure 17: Typical arrangement of CP for shore systems, jetty and ship

The insulation resistance across any isolation joint should not be less than 1 kΩ measured on site at 20 V (10 kΩ at 1 kV in the factory). All joints should be cleaned and tested periodically (based on site experience of failure rates), to maintain this value in accordance with the requirements of the OCIMF Design and construction specification for marine loading arms. Loading hoses should be tested in accordance with the procedures given in BS EN ISO 8031 Rubber and plastics hoses and hose assemblies. Determination of electrical resistance and conductivity. Breasting dolphins, fenders and quays having metallic parts connected to the jetty earthing/ grounding system should be protected from direct electrical contact with ships’ hulls (e.g.

48

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by wooden linings). Mooring dolphins should be insulated from the jetty earthing/grounding system or located in a non-hazardous area but insulated from the shore earthing/grounding system. Steel jetties that have impressed current cathodic protection cables entering the jetty area from the shore should have their metallic sheaths and/or armours bonded to the jetty earthing/grounding system but isolated from the shore earthing/grounding system (the steel wire armour should be cut back and the correct gland/termination used). Alternatively, the gland could be connected into an insulating plate. Further advice on earthing/grounding and bonding systems on tanker loading jetties can be found in the OCIMF ISGOTT and BS EN 1474 or equivalent. 5.6.3.1 Shore conections (cold ironing) The operational practice of supplying electrical power requirements from a shore connection to a ship whilst it is docked is often known as 'cold ironing'. The purpose is to reduce emissions produced by the ship’s engines by using the shore supply, which may have lower emissions regulated under the jurisdiction of the port nation. The use of shore supplies may also create an opportunity to carry out maintenance work. In rare cases, supplies may be from the power generation plant of another ship. Although commonly practised, there does not seem to be any generally accepted standard for supply voltage and frequency, or for the types of cable or connector; power requirements may range from a few hundred kW to 10 or more MW. Each operation is handled on a caseby-case basis. However, it is usually necessary to isolate the incoming shore supply from the ship’s earth/ground systems, so the supply interface should include an isolation transformer. Further advice on shore connections (cold ironing) is available in BS IEC 60092-201. 5.6.4 Aircraft fuelling facilities The general recommendations of 5.6 should be followed, but note the following points specific to aircraft fuelling: Depending on the scale of operation at an airport, the usual fuelling methods used are mobile fuellers or a hydrant system. Where mobile fuellers are employed, the airport depot facilities allow for the fuel required by the aircraft to be pumped from the storage tank into fuellers that then proceed to the aircraft where the fuel is delivered. Fuellers may be loaded directly from a hydrant pit valve if they are equipped to do so, but the fueller should never be electrically bonded to the hydrant pit valve internals because the pit is likely to contain a potentially flammable atmosphere. In the case of hydrant fuelling, the facilities provide for the fuel to be pumped directly from the airport fuel depot storage tanks to the aircraft parking apron by means of pipelines, and then transferred from the hydrant pit(s) installed there into the aircraft via hydrant servicer(s). Stray currents may exist in any of these systems, created by stray DC., induced by power frequency AC or by RF, or introduced by earth/ground loops from other services such as ground power connected to the aircraft. During refuelling, it is possible that the aircraft may already be earthed/grounded by other electrical connections, and if the fuelling connection were to provide a second low resistance path, stray currents could flow around the loop, with the possibility of potentially incendive sparking. This situation is different from, e.g. road tanker loading, and any earth/ground path connected to the aircraft should have a resistance of at least 103 Ω to limit the current to below a potentially incendive level.

49

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The mobile fueller and the aircraft may be at different electrostatic voltages unless each has a leakage path to earth via its tyres; equally, an aircraft may be at a different electrostatic potential to a hydrant. Concrete aprons generally provide a suitable path via fueller and aircraft tyres, but tarmac may offer too high a resistance. During the fuelling operation, fuel transfers are made through flexible hoses, and static electricity is generated during this operation. In view of the charging effect of filters, a residence time of one second for fuels with a conductivity of not less than 50 pS/m should be allowed where possible between the outlet of the filter and the aircraft tank. When handling fuels with a conductivity of less than 50 pS/m, the residence time should be increased to 30 seconds. Strict attention should be paid to the control of static electricity charging during fuel movement, a condition that is exacerbated by the use of the essential filtration equipment. For further comments on limiting flow velocity in order to control electrostatic charge generation, refer to 4.5. Attention is also drawn to the differences in maximum flow velocities included in the API and EI publications. The limit appropriate to the location should be applied. Control measures to reduce the ignition risk due to static electricity include: −−

−−

−−

−−

Hoses should be electrically dissipative/semi-conductive, e.g. selected from the types detailed in BS EN ISO 1825 Rubber hoses and hose assemblies for aircraft fuelling and defuelling. Aircraft bonding to the fuelling system should utilise a bonding cable that has one end permanently attached to the fuelling vehicle or hydrant dispenser. This should be connected to the aircraft prior to dispensing any fuel and remain in place until the operation has been completed. Bonding between the aircraft fuel orifice and the metallic end of the fuelling hose for under-wing fuelling is achieved by the metal-to-metal contact between hose and coupling adapter. For over-wing operations it is important to bond the metallic hose nozzle to the aircraft using a specific bonding cable, prior to opening the fuel filler cap. The bond should remain until filling has been completed and the cap replaced. Bonding precautions are required for other filling operations, e.g. from drums, to ensure continuity between any drum, pump and aircraft, see 7.4. Where personnel use a ladder to access the filler on the aircraft, this ladder should be designed to offer a resistance of less than 108 Ω through its tyres, and should have conductive steps so that personnel do not retain any electrostatic charge. In addition, personal protective equipment, including gloves and boots worn by personnel, should be static dissipating.

If earthing/grounding to the general mass of the Earth is required by local regulations, only a single earthing/grounding point should be used so as to avoid stray current loops. For further guidance see EI Model code of safe practice Part 21: Guidelines for the control of hazards arising from static electricity. For offshore helicopter refuelling, a reel of single core flexible cable, of suitable size, should be installed in an area classified as non-hazardous beside the helideck, for bonding helicopters to the offshore installation during refuelling. The reel should incorporate a slip ring to pick up the connection to earth/ground. The Civil Aviation Authority publication CAP 437 provides further information on helicopter refuelling. For further information on civilian aircraft fuelling operations see API/EI RP 1540 Design, construction, operation and maintenance of aviation fuelling facilities.

50

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5.6.5 Filling stations The general recommendations of 5.6 should be followed, but note the following points specific to filling stations. Above ground tanks for diesel, gasoil and kerosene should be protected against lightning, and the lightning protection system should be earthed/grounded locally to the tank, i.e. this earth/ground should be segregated from the electrical earthing/grounding for the site. For advice on above ground tanks for petrol, see 8.4.4 of the APEA/EI Guidance for the design, construction, modification, maintenance and decommissioning of filling stations. If underground storage tanks (USTs) and associated pipework incorporate metal parts that are not inherently connected to earth/ground, they require protection against the accumulation of static electricity, and the metal parts should be electrically bonded together and connected to a dedicated earth/ground electrode. They should not be connected to the filling station electrical earthing/grounding arrangements. If a sacrificial anode CP system is installed to protect the storage tank(s), the sacrificial anodes (provided that there are at least two to allow for testing) will act as static earth/ground electrodes and a separate static earth/ground electrode may be omitted, providing that the resistance to the general mass of the Earth of a sacrificial anode when first installed is less than 10 kΩ.

L N E

Surge arrester

Canopy

L L N RCD N E Cathodic protection

Site earth / ground electrode

Dispenser

Bond

Earth / ground terminal

Conductive hose

Concrete apron

LNE

ELN

Insulating joint

Impressed current CP anode

or

Sacrificial CP anodes

Figure 18: Some typical filling station earth/ground/bond interconnections Where a tank and pipework system incorporate non-conductive components with isolated metal parts, protection against static electricity may be required. Detailed provisions for this form of protection are given in paragraph 14.4.7 of the APEA/EI Guidance for the design, construction, modification, maintenance and decommissioning of filling stations.

Figure 18: Some typical filling station earth / ground / bond interconnections

Protection against touch voltages for personnel or users of the filling station should be provided by earthed/grounded bonds and automatic disconnection of supply, or by the use of equipment of Class II construction (double insulated). The automatic disconnection of supply to forecourt circuits should not exceed 100 mS in the event of an earth/ground fault. Where a vehicle filling station installation is part of a TT system, or where earthing/grounding arrangements are such that disconnection times cannot be achieved with overcurrent

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protective devices, RCDs will be required to facilitate automatic disconnection of supply. It is inappropriate to provide a single 'front end' RCD at the main switch position as the sole means of earth/ground fault disconnection for the installation. Circuits will usually be RCD protected individually in order to satisfy site operational requirements. In any event, an RCD incorporated in a circuit serving a hazardous area should be independent of RCDs protecting non-hazardous area circuits. If RCDs are connected in series, discrimination of operation of a load side device should be ensured by incorporating a supply side device having a suitable time-delay. For each RCD the leakage current of the associated circuit should not exceed 25 % of the rated residual current of the RCD, or it might trip spuriously. Guidance on the use of PME systems at filling stations is given in APEA/EI Design, construction, modification, maintenance and decommissioning of filling stations, and see 4.1.4. Where a legacy site has a PME supply taken from a public utility, it is considered that the risk of a broken neutral near to the filling station is minimal, and that normally there should be no need to achieve a TT or TN-S arrangement within the premises. In such cases, an annual inspection and test regime should be implemented to ensure the continued effectiveness of the PE and main bonding conductors (for reasons discussed in 4.1.4), and to monitor the level of the DNC (which may pass into hazardous areas via metalwork or protective conductors) to ensure that it does not approach potentially incendive levels (as discussed in Section 3). However, the design should conform to local national legislation and this may require that under certain circumstances, isolation transformers are required. Where additional equipment, such as that for autogas/LPG/LNG, is to be installed on an existing filling station where the earthing/ grounding system is connected to a PME terminal, a risk assessment should be carried out to determine any possible adverse effects on the autogas/LPG/CNG installation – where the metalwork of the autogas/LPG/CNG installation is cathodically protected and employs isolating joints, the effects of diverted neutral currents should be obviated. On a new filling station, or one under major refurbishment, a PME supply should not normally be used. The AC supply system should use a site-specific earth/ground electrode with no connection to the PME earthing/grounding terminal provided by the electricity distributor, to create a local TT system as shown in Figure 4, or a TN-S supply as shown in Figure 5. Extra-low voltage data and other systems not intrinsically safe should use SELV circuits supplied via a BS EN 61558 safety isolating transformer or equivalent safety source. Care should be taken to ensure that the earthing/grounding arrangements for data cable screening do not introduce potentially dangerous levels of ignition energy into a hazardous area. Generally, screening should be earthed at one point only, in the non-hazardous area. For autogas/LNG/CNG installations, the general guidance on earthing/grounding/bonding in section 14.8 of the APEA/EI Guidance for the design, construction, modification, maintenance and decommissioning of filling stations should be applied. Provision should be made for the electrical connection of autogas/LPG/CNG road tankers to the metalwork of the autogas/ LPG/CNG system. The bonding point, which should not be located in an access chamber or other hazardous area location, should be suitable for making a bond to the tanker prior to the commencement of, and until completion of, the final transfer operation. Where the autogas/LPG/CNG tank and pipe metalwork are cathodically protected, care should be taken to ensure that use of the tanker bonding point does not bridge the cathodic protection arrangements. For autogas/LPG/LNG applications, a conspicuous, durable and legible notice should be fitted adjacent to the terminal or other provision for earth/ground bonding of road tankers during fuel transfer, bearing the words: 'TANKER EARTH BONDING POINT'.

52

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At all filling stations, an all-insulated test socket for measuring earth/ground loop impedance and prospective fault current should be installed at the origin of the installation in conjunction with the main earth/ground terminal test link – a suitably labelled insulated protective conductor that is segregated from the earthing/grounding arrangements within the installation should connect the earth/ground terminal of the test socket to the earthing/grounding conductor side of the main earth/ground terminal test link. The test socket and its related all-insulated protective device incorporating isolation and overcurrent protection should comply with section 14.4.5 of APEA/EI Guidance for the design, construction, modification, maintenance and decommissioning of filling stations. The means of isolation should be capable of being locked and should be provided with a label identifying its purpose. A filling station should not be installed below overhead conductors (electricity, telephone lines, etc.), except where protected overhead by an earthed/grounded metallic canopy as detailed in the APEA/EI Guidance for the design, construction, modification, maintenance and decommissioning of filling stations.

53

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6

DETAILED DESIGN/CONSTRUCTIONAL REQUIREMENTS

6.1

FIELD CABLES Power cables are usually armoured in order to provide mechanical protection, and the armour should be earthed/grounded at each end because the fault currents then tend to flow in the armour, and the current flowing in adjacent steelwork, cable tray, etc, may be lessened, reducing the likelihood of potentially incendive sparking. Unarmoured cables can also be used where mechanical protection is not required (unarmoured cables are lighter, cheaper and easier to install), but bonding of the cable tray is required to reduce the risk of potentially incendive sparking at intermittent contacts (see 6.3). However, for Zone 0 and Zone 1 hazardous areas, cables should preferably be armoured unless otherwise mechanically protected. Where a number of installations have separate earthing/grounding arrangements, any protective conductors common to these installations should either be capable of carrying the maximum fault current likely to flow through them, or be earthed/grounded within one installation only and insulated from the earthing/grounding arrangements of any other installation. Conductors and armour of unused power cables should be earthed/grounded in a nonhazardous area, and well insulated or securely earthed/grounded at the field end. Decommissioned underground cables may be cut off short at each end, and abandoned. Proprietary cable abandonment kits may be used to terminate and seal unused power cables, and to provide protection against accidental re-energisation of the cable. Note: The armour of single core armoured cable may carry a circulating current if earthed/ grounded at both ends, and should be rated accordingly. For instrument/telecommunication cable, the major operational issue is signal interference; the advice of the vendor of instrument or telecommunications devices should be sought as to the susceptibility of the device. The main cause of interference is magnetic coupling between signal conductors and power conductors; note that signal paths consisting of twisted pairs are much less prone to interference than untwisted pairs. Screened or armoured cables are less prone to magnetic (and capacitive) coupling. The screen should be earthed/grounded at one point only so as to avoid circulating currents; this earthing/grounding point should be in the non-hazardous area. Also, this one point should be chosen so that signal return paths do not share a common path with heavy current returns (see Figure 10). Armour on instrument/telecommunications cables may be earthed/grounded at multiple points if a screen is also present, but see 4.8 for comments on isolations in CP systems. It is not necessary to have a separate earth/ground electrode for sensitive circuits – in every case, instrument/telecommunications currents are returned to the supply star point, and this is achieved economically and reliably by returns to the local plant earthing/grounding bar. Especially in the case of a remote neutral star point, there could be a large voltage between local plant earth/ground and the power supply neutral star point, and the local plant earthing/ grounding bar/terminal should be used for the signal earth/ground, so that no power system return currents flow through the signal return conductor. Unused instrument cable cores should be earthed/grounded in the non-hazardous area, and the field ends should be terminated in unused terminal blocks in the field junction box (rendering the unused cores safe, and conveniently available for future use).

54

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Cables for use with impressed current anode systems should generally be armoured, unless installed in ducts. Non-armoured cable should be used only with sacrificial anode systems operating at a current output of less than 100 mA, to avoid the possibility of potentially incendive sparks in the event of a fault. Where armoured cable is used, the armour is for mechanical protection only and is not required to be electrically earthed/grounded; in this case, the armour should be cut back and isolated properly at each end, with the correct cable gland. However, if an earth/ground connection is provided, care should be taken to ensure that the armour and its terminations are earthed/grounded at one point only. Where extra-low voltage (ELV) circuits are contained in the same trunking, duct or multicore cable as higher voltage circuits, the latter should be provided with an earthed/grounded metallic screen or sheath with a current carrying capacity equivalent to that of the higher voltage cores. Alternatively, the conductors of an ELV system should be insulated individually or collectively for the highest voltage present on higher voltage conductors in the same enclosure. The higher voltage grade insulation should be applied throughout the ELV system. High voltage cable glands or terminations should incorporate a lug for bonding the cable armour to earth/ground or to the equipment enclosure. If a low-voltage cable enters a metallic enclosure having threaded entries, a bonding conductor between the gland and enclosure is not required, provided there is no electrical discontinuity at the enclosure. If non-metallic enclosures are used, means shall be provided to preserve the electrical continuity of the armouring and/or metallic sheaths of cables (this is normally achieved with an earth/ground continuity plate located inside the enclosure). Earth/ground tag washers are required where cable glands enter thin-walled enclosures; they should be on the outside of the enclosure against the shoulder of the gland so as to provide low resistance electrical continuity, and should be sized for the fault current. Steel wire armoured cables terminated with glands approved for Zone 1 or Zone 2, as appropriate, in hazardous areas may be employed subject to the installation of earth/ground tag washers at the cable glands in non-hazardous areas – a lugged cable connection being provided between the earth/ground tag washer and the enclosure earthing/grounding bar or terminal. 6.2

PROTECTIVE CONDUCTORS Clearly, every earthing/grounding/bonding conductor should have sufficient strength and current-carrying capability to discharge electrical energy to earth/ground, or to the supply return point, as appropriate. The conductor and any insulating material should be capable of withstanding the thermal and mechanical effects of the maximum fault current, taking into account the peak value of the prospective short-circuit current (especially during the first half cycle), and in the case of circuit protective conductors, the operating time of the circuit protection. These requirements apply to all earthing/grounding/bonding conductors, e.g. lightning earthing/grounding conductors, circuit protective conductors (CPCs), main bonding conductors and supplementary bonding conductors. The conductive paths of the system shown in Figure 1, may consist of: −− −− −− −− −− −− −−

steel wire, tape or braid armouring of cables; a separate protective conductor incorporated in an armoured cable; a separate earthing/grounding/bonding core in any other cable; a separate, bare or insulated conductor of copper or steel; screwed metal conduit or conductive cable tray; the metal sheath of mineral insulated cables terminated in earth-tail pots; steel structure. 55

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Sole reliance on cable armouring and/or metallic sheathing should only be made if it is adequately fault-rated as a protective conductor for the case of faults within electrical equipment. In the case of faults within the cable, it may not be possible to have armour that is fully fault-rated, particularly if the armour is steel braid type. However, the advantages of steel braid armour over steel wire armour on cables used offshore are considered to outweigh this possible disadvantage. Bonding conductors do not replace the need for CPCs connecting exposed conductive parts. Welding or solidly bolting together the appropriate parts may be adequate to provide bonding. However, if it is credible that corrosion may adversely reduce the conductivity of the connection, a bonding conductor should also be applied. Earthing/grounding/bonding conductor cables should have an appropriate degree of corrosion resistance, and, typically, copper should be used. Mechanical protection should generally be provided for protective conductors where potential damage is credible. Joints in earthing/grounding/bonding conductors should be avoided where possible and should not be used in Zone 0 hazardous areas; terminations and joints in stranded conductors should engage all of the strands, typically by means of compression/swaging/crimping. Guidance for the minimum value for the cross-sectional area (c.s.a.) of earthing/grounding/ bonding conductors will be found in the IEC and British standards; for U.S. applications, values will be found in the NFPA guidance. These IEC, British Standard and NFPA values are, in general, somewhat different, but this publication can give no further guidance on the value appropriate to a given application. Irrespective of electrical requirements, a certain minimum value may also be specified to ensure adequate mechanical strength of the conductor. The IEC, British Standard and NFPA values for conductor c.s.a are summarised below and in Table 2. 6.2.1 CPCs The conductor c.s.a. of a CPC for electrical apparatus should normally comply with the minimum requirements determined by methods given in the underlying standards (all sizes quoted in this document refer to copper conductors). If the CPC consists of an extensive mesh of interconnected copper earthing/grounding conductors, its earth/ground impedance may be ignored for fault current calculation. However, for short cable circuits, earth/ground impedance may be of comparable magnitude to that of the cable sheath and should be considered. The c.s.a. of any CPC should take into account the prospective fault current that could pass through the CPC, and the operating time at maximum prospective fault current of the protective device together with its current limiting capabilities. The c.s.a. of the CPC is usually determined without taking credit for any earth/ground paths through metal cable sheaths, armouring or other fortuitous earth/ground paths. For CPCs, the c.s.a. required by BS 7430 and BS 7671 is given by: S = (I√t)/k where: S = CPC cross-sectional area, in mm2 I = fault current, in A which can flow through a fault of negligible impedance and the associated protective device, with account being taken of the current-limiting effect of circuit impedances and the protective device t = operating time in s of the disconnecting device corresponding to the fault current I

56

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k = maximum acceptable fault current density for a 1 s interval, in A/mm2, taking into account the resistivity, temperature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures (both conductor material and insulating material, if any). Note: I2t (here expressed as I√t) is proportional to the energy dissipated (i.e. heating effect) in the conductor, and k takes account inter alia the maximum acceptable temperature rise of the conductor, to derive an approximate value for the conductor size thay will survive the fault event undamaged. In practice, the conductor c.s.a. S is rounded up to the next standard size. BS 7430 gives a methodology for establishing the factor k: k=K√(ln((T2+β)/(T1+β)) where: T1 = initial temperature, T2 = final temperature. For copper, K=226 A/mm2 and β=254 ºC; for steel K=78 A/mm2 and β=202 ºC; for aluminium K=148 A/mm2 and β=228 ºC. Therefore, for example, for copper conductors at an initial temperature of 30 ºC and for a final temperature of 200 ºC (a typical maximum for electrical apparatus CPCs), k = 159. BS 7671 gives values for k for insulated copper conductors; for example, for an initial temperature of 70 °C and a final temperature of 160 °C, k=115. BS 7671 also gives values for k for other configurations, e.g. protective conductor incorporated in a cable, and for various conditions. Where it is appropriate to use a simplified methodology with no requirement to establish values for I or t, or to derive the factor k, BS 7671 indicates that for a phase conductor up to 16 mm2 the protective conductor should have the same c.s.a. as the phase conductor; for a phase conductor greater than 16 mm2 and up to 35 mm2 the protective conductor should be 16 mm2; for phase conductors over 35 mm2, the protective conductor should be half the size of the phase conductor. 6.2.2 Power supply system earthing/grounding conductors For power supply system earthing/grounding conductors (see Figure 1), e.g. in the vicinity of solidly earthed/grounded substations, final conductor temperatures of up to 700 ºC for copper and steel, and 300 ºC for aluminium may be acceptable, and these lead to higher values for k calculated as shown in 6.2.1, and hence to smaller conductors than would be the case for electrical apparatus CPCs (where the final temperature is lower). However, the method for calculating protective conductor c.s.a. remains the same. Note: Where a corrosion protected conductor is installed in the soil with no mechanical protection, a conductor c.s.a greater than 16 mm2 is recommended by BS 7671; if there is no corrosion protection, a c.s.a greater than 25 mm2 for copper or 50 mm2 for steel is recommended. IEC 62305-3 recommends a minimum of 50 mm2 for conductors buried directly in the earth/ground. 6.2.3 Bonding conductors A main bonding conductor is a protective conductor providing a connection from an extraneous conductive part such as a metallic pipe, structural steelwork or other metalwork not forming part of an electrical enclosure, to the main earthing/grounding bar/terminal of

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an electrical installation. Except where PME conditions apply, the c.s.a. of a main bonding conductor should not be less than half of the c.s.a. of the earthing/grounding conductor for the installation, with a minimum c.s.a. of 6 mm2. The c.s.a. need not exceed 25 mm2 for copper conductors. Other conductor materials must have a c.s.a. affording equivalent conductance . See BS 7671 for further details. According to BS 7671, supplementary bonding conductors connecting two exposed conductive parts should have a c.s.a. not less than the smaller CPC, but not less than 4 mm2 (in order to achieve adequate mechanical strength). A supplementary bonding conductor connecting an exposed conductive part and an extraneous conductive part should have a c.s.a not less than half the protective conductor connected to the exposed conductive part. A supplementary bonding conductor connecting two extraneous conductive parts should have a c.s.a. not less than 4 mm2 (2,5 mm2 if mechanically protected), but not less than half the protective conductor if one of the extraneous conductive parts is also connected to an exposed conductive part. 6.2.4 Instrument and telecommunications systems, and intrinsically safe system cables Instrument and telecommunications earthing/grounding cables (including functional return paths) are sized so that the signals perform with adequate integrity; vendor advice may be needed to establish the required conductor c.s.a. It is generally the case that a c.s.a. of 4 mm2 is adequate for conductors without mechanical protection and a c.s.a. of 2,5 mm2 is generally adequate where mechanical protection is provided. For intrinsically safe zener barrier systems, BS EN 60079-14 specifies a permanently installed earthing/grounding conductor (i.e. with no plug/socket connections) with a minimum c.s.a of 4 mm2, or two 1,5 mm2 conductors. These sizes may require increasing to maintain the earth/ground impedance below 1,0 Ω. Mechanical protection of the conductors may be required to ensure integrity of the conductive path. 6.2.5 Lightning protection system earth/ground conductors and down conductors For lightning earth/ground conductors and lightning protection system down-conductors, BS EN 62305-3 recommends a minimum c.s.a. for large extraneous conductive structures of 50 mm2, but this may be reduced to 25 mm2 where adequate support is provided and mechanical strength of the conductor is not an issue. For connections between different bonding bars, or connections between the bonding bar and the earth/ground-termination, the minimum conductor size is 16 mm2. For internal system cables that are screened or in conduit, it may be sufficient to bond only the screen/conduit, otherwise surge protection devices might be needed (see 4.9.1.1). Conductors designed according to fault current requirements are normally adequate for carrying short time surges caused by lightning, but additional bonding connections should be made at welded bosses local to equipment to avoid long cable runs. Further advice may be found in IEC 62305-3 (for US applications, NFPA 780 provides appropriate advice). 6.2.6 Static electricity Protection against static electricity is provided inherently by all low resistance (i.e. less than 1 MΩ) common earthing/grounding/bonding systems to which all structures and items of process equipment are connected, either directly, indirectly, or fortuitously (see 4.5). Static and lightning earthing/grounding systems should be connected to power earthing/grounding system at two points for facilities located in proximity to process area substations. Where it is required to isolate conductors but to retain a high resistance connection to dissipate static, a resistor of 100 kΩ to 1 MΩ rated at 2 W should be used (even though the power dissipated is

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well below 2 W, because short electrical transients can damage resistors with a lower power rating; also a 2 W rated resistor is mechanically robust).

Table 2: Summary of earthing/grounding/bonding conductor sizes Protective conductor function

BS/IEC installations

CPC conductor

BS 7430 and BS 7671. S ≥ I(√ t)/k (see 6.2.1). or Phase conductor ≤16 mm2, same as phase conductor. Phase conductor >16 mm2, 50 % of phase conductor with a minimum of 16 mm2.

Earthing/grounding conductor

BS 7430 and BS 7671. 16 mm2 with corrosion protection for copper and steel. 25 mm2 for copper without corrosion protection. Strip connections > 3 mm thick. IEC 62305 – 3. 50 mm² with an earth/ground electrode rod diameter ≥ 15 mm.

Main and supplementary bonds

IEC 60079-14. ≥ 6 mm2 for main bonding conductor. ≥ 4 mm2 for supplementary bonding conductors. Mechanical strength equivalent to 4 mm2 copper. IEC 61892-6. ≥ 6 mm2 generally. ≥ 35 mm2 for bonding conductors exposed to lightning, e.g. vessels not seam welded to structure, HVAC ducting. IEC 62305-3. ≥ 16 mm² connection from earthing/ grounding bar/terminal to another earthing/ grounding bar/terminal ≥ 6 mm² equipment to earthing/grounding bar/terminal

Intrinsically safe system earthing/grounding conductors

IEC 60079-14. 2 x 1,5mm2 or 1 x 4 mm2.

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Table 2: Summary of earthing/grounding/bonding conductor sizes (continued) Instrument and telecommunications functional earthing/grounding returns

Apparatus vendor advice needed. Minimum of 4 mm2 for conductors with no mechanical protection, or 2,5 mm2 with such protection would generally be adequate.

Lightning protection earthing/grounding conductors and down-conductors

IEC 62305-3. 50 mm² copper/stee.l

6.3

CABLE TRAY In any application, conductive cable tray (and similar conduit, trunking, etc.) should be earthed/grounded throughout its length; in classified hazardous areas, other types of cable tray should have anti-static properties. Where unarmoured cables are used, the bulk of any fault current will tend to flow in the cable tray if it is conductive throughout, as this will offer the path of lowest inductance, especially with non-magnetic cable tray; also it is essential to ensure that there are no electrical discontinuities, e.g. at joints, in the length of the cable tray, as discontinuities may allow potentially incendive sparking. Bolted joints between sections of tray are usually electrically sound, and bolted connections to earthed/grounded steelwork usually provide an adequate conductive path. However, supplementary bonds/straps may be required over mechanical slip joints to increase the reliability of the conductive path; metallic covers over cable tray may similarly require bonds to the tray, especially in hazardous areas, to prevent potentially incendive sparking at joints.

6.4

EARTH/GROUND ELECTRODE DESIGN Target values for earth/ground electrode resistance are discussed in 4.12. The resistance of an earth/ground electrode depends on its geometry and the resistivity of the soil. However, accurate prediction of the resistance of an earth/ground electrode to the general mass of the Earth is not possible, and manufacturers of earth/ground electrodes give empirical values of electrode resistance for their products, typically based on system geometry and soil resistivity. See Annex C for further guidance. Earth/ground electrode systems for substations may require an iterative design process where the basic data (earth/ground fault current, fault duration) are used to derive an initial design that meets the functional requirements, e.g. for electrical resistance, mechanical strength, thermal strength, resistance to corrosion, etc. Then this design is assessed to see if it meets other requirements such as touch voltage limits. If those limits are breached, the design is improved and reassessed against the limits for touch voltage etc. This iterative process continues until all requirements are met. A flowchart of this process is given in Figure 19.

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Basic data Earth/ground fault current, fault duration

Initial design to meet functional requirements Yes

Global earthing/grounding system

Design complete

No Determination of soil characteristics

Determination of current discharged into soil

Determination impedence of earth/ ground

Determination of earth/ground potential rise

Yes EPR

touch voltage limit? No

Determination of actual touch voltages

Actual touch voltages

No

limit

Yes Check

Allowable body current Step voltages

Yes

Requirements are fulfilled

Design complete

No

Improvement of design

Figure 19: Earthing/grounding system design flow chart

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From a lightning perspective, BS EN 62305-3 distinguishes two types of earth/ground electrode, as follows: −−

−−

Type A has horizontal or vertical electrodes installed outside the structure to be protected and connected to each down-conductor or foundation earth/ground electrode, but not forming a loop. Type A is satisfactory for installations containing electrical systems, but is not recommended for installations containing electronic systems. Type B has a ring conductor external to the structure to be protected, in contact with the soil over at least 80 % of its length, or a foundation earth/ground electrode, and forming a closed loop; such electrodes may also be meshed. Type B is satisfactory for installations containing electrical systems, and is recommended for installations containing electronic systems.

Earth/ground electrodes may consist of rod(s) driven vertically at least 2,5 m into the soil (where installed at an angle, the specified vertical depth should be achieved). A number of rods may be connected in parallel to obtain the required resistance; the distance between rods should be greater than their depth in order to reduce proximity effects. It may be more economical to install a number of such rods rather than to attempt to obtain the required resistance from a single, deeply driven rod, and the use of more than one rod may make it easier to measure electrode resistance. Multiple earth/ground rods are often situated around the periphery of the site for potential grading. If rods cannot be driven to a suitable depth their installation should be preceded by drilling. Care should be taken to ensure the satisfactory consolidation of the backfill soil. Resistance can be reduced by the use of a suitable soil conditioning agent such as bentonite, though this will require more frequent periodic testing, and replacement of the bentonite when it is exhausted. Earth/ground electrodes may also consist of buried earth/ground plates, a wire mesh or a grid of earthing/grounding strips, and cover an extended horizontal area. Foundation metalwork (e.g. welded steel reinforcing bars in concrete, or structural steel members) may provide a satisfactory earth/ground electrode. The use of service pipes such as water mains as a primary earth/ground electrode should be avoided, and where used on legacy sites, an alternative should be sought. The materials of the electrode should be selected to meet the requirements for corrosion resistance (high conductivity soils are likely to be more corrosive); BS 7430 gives recommendations for material section for earth/ground electrodes. Copper earth/ground electrodes may accelerate the corrosion of adjacent buried steel structures; austenitic stainless steel electrodes may be more suitable in this situation, but electrodes should not be allowed to corrode as this will reduce their effectiveness. Coated copper conductors may compromise performance by increasing the resistance of the system. Earth/ground rods should be terminated in an inspection pit. Earth/ground-termination conductors can be sized according to the methodology given in the IEEE Guide for safety in AC substation grounding section 11, based on the following criteria; they should (a) have sufficient conductance to prevent a hazardous voltage drop during fault conditions, (b) withstand corrosion and mechanical abuse during the design life of the facility, and (c) have sufficiently low resistance to limit the temperature rise under fault conditions. Buried earthing/grounding conductors leading to the electrode should be protected by a suitable covering to prevent corrosion (the covering should be coloured green/ yellow). Copper should be used where the c.s.a. is 16 mm2 or less. A test joint should be fitted low down in the lightning down-conductor, near to its connection to the earth/ground electrode system, to allow disconnection of the electrode for tests on its performance.

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6.5

ABOVE GROUND FLOATING ROOF STORAGE TANKS Heavy metallic structures of welded construction down to ground level are inherently in contact with the general mass of the Earth unless coated or landed on concrete or an insulating membrane, but separate earthing/grounding conductors are required to connect earthing/grounding bosses at ground level to the main earthing/grounding system. Tanks up to 30 m diameter should be provided with two, and tanks over 30 m diameter should be provided with three or more, equally spaced earthing/grounding bosses positioned near the base of the tank. Earth/ground electrodes common to a group of tanks should be installed so that each tank has as a minimum two paths to earth/ground, to ensure that during testing of one electrode, the tank will remain earthed/grounded by a system with an earth/ground resistance of appropriate value. Shunts provide a route to earth/ground for lightning currents, and take the form of a spring contact strip fastened to the top of the weather shield or secondary seal, to provide a conductive path to the shell. They are normally spaced about 3 m apart, and are often made from stainless steel (typically 50 x 0,6 x 400 mm), pressing against the tank shell. On floating roof tanks, multiple shunt connections should be provided between the floating roof and the tubbing shoe at adequate intervals around the roof periphery or one per pantograph (if these are fitted). If high winds prevail, shunt strips may be replaced with cables bolted into position. Shunts should be fitted above the sealing arrangement. Spacing of shunt connections should avoid the risk of discharge from the roof to the tank wall directly across the gap, rather than via a shunt, due to formation of re-entrant loops. The risk is increased when the path length of the loop exceeds eight times the width of the open side. In this case, the maximum loop length is half the peripheral distance of the closed path between adjacent shunts. Although hanger linkages on the pantograph offer an earthing/grounding path from the floating roof to the shell, they can be a source of arcs during a lightning strike if placed too close together (the accepted minimum separation on an empirical basis is 1 m). The arcs occur from sharp points or joints, but may be avoided by installing short insulated jumpers around each pinned hanger joint and covering sharp points of hangers with insulating material. Where floating roof tanks have a roof bonding conductor this should be highly flexible, typified by ‘welding cable’. The cable should have an outer neoprene sheath with insulation extending right up to the compression type termination lugs, so that when lying on the roof it does not make contact with other items and initiate sparking. For floating roof tanks without rolling ladders, earthing/grounding of the roof should be provided by flexible earthing/grounding cables laid along the roof drain – one earthing/grounding cable should be installed for each roof drain. Roof connections for double and single roof tanks should be a similar material specification to the tank itself. Roof connections should be sited close to roof drains with suitable facilities to securely fix the earthing/grounding cable to the roof. Connections should be protected against corrosion. A detensioner should be installed if the earthing/grounding cable exits the roof connection. The earthing/grounding cable should be fixed to the roof drain using suitable clips (e.g. stainless steel cable ties) with due allowance for movement of all swivels. When earthing/grounding cables have been installed, tests should be undertaken to ensure a low resistance between the roof and wall. Note: Experience shows that these cables can become entangled and break with the movement of the roof. Also, the tangling of the cable produces re-entrant loops.

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6.6

STEEL STRUCTURES (ONSHORE) Steel structures in contact with the ground form part of the common earthing/grounding system. An earthing/grounding lug or boss should be welded to main columns at regular intervals and at convenient heights above ground level. Where a steel structure forms a common earthing/grounding system, or part thereof, each earthing/grounding lug or boss should be directly connected to an adjacent earth/ground electrode. If steel structures do not form part of the common earthing/grounding system and/or direct earthing/grounding system for lightning, each lug or boss should be connected to the common earthing/ grounding system.

6.7 VESSELS When pressure vessels are mounted directly on, and in metallic contact with, an earthed/ grounded steel structure, further bonding may not be necessary; however, additional earthing/grounding may be required for lightning protection or to reduce overall earth/ ground impedance, and direct earthing/grounding for lightning should be applied when necessary. When the vessel mounting is insulated from earthed/grounded steelwork by materials of poor conductivity such as wood, concrete, rubber etc, two earthing/grounding connections should be taken from the vessel to the common earthing/grounding system. If the vessel is remote from the plant and connection to the common earthing/grounding system is impractical, two connections should be taken from the vessel to separate earth/ground electrodes and the resistance to earth/ground of each electrode should be less than 10 Ω. Looping earthing/grounding conductors between vessels is permitted provided a connection is taken from each end of the 'looped' system to the general earthing/grounding system or earth/ground electrodes, though a radial arrangement is preferred. Where a vessel is lined with a non-conductive lining (e.g. glass), the use of a tantalum plug to dissipate static electricity should be considered, especially for vessels where the fluid is agitated (as this increases charge separation). If a vessel has insulation and an outer metal cladding or wire reinforcement, the metal cladding or reinforcement should be electrically continuous and bonded to the vessel; for vessels containing hydrocarbons, two bonding connections should be used, but for non-flammable fluids (e.g. steam, air, water), a single connection may be satisfactory.

6.8

METALLIC STACKS AND TOWERS No lightning air-terminals or down-conductors are required when metallic stacks and/or towers are of welded, bolted or riveted construction. Two earthing/grounding lugs or bosses located near the bottom and on opposite sides of the equipment should be provided and independently connected either to the general earthing/grounding system or to two earth electrodes located near the base of the equipment. Armouring of cables entering metallic stacks should be bonded to the stack at the point of entry. (This guidance is not applicable to flare stacks.)

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6.9

NON-METALLIC STRUCTURES Non-metallic structures less than 9 m in height do not require lightning protection or earthing/ grounding. When greater than 18 m in height, non-metallic structures should be provided with lightning protection. Lightning protection for non-metallic structures between 9 m and 18 m in height should be determined taking into account the heights of adjacent structures, proximity of flammable materials, consequences of damage, etc. In classified hazardous areas, steelwork such as stairways, cable racks, handrails, etc, mounted on or attached to non-metallic structures should be bonded directly to the general earthing/ grounding system. However, when steelwork cannot be bonded as an earthed/grounded extraneous conductive part, an alternative to bonding should be carried out to ensure that: −− −−

6.10

Steelwork is sufficiently isolated from other potential lightning down-conductors to ensure that no side flashing will occur. No static build up above minimum ignition energy of hydrocarbons present is possible.

METALLIC GUY ROPES Metallic guy ropes used for supporting metallic stacks or other structures should be bonded at their upper ends to the stack or structure; in the case of non-metallic stacks or structures, guy ropes should be bonded to the lightning protection air-termination. In every case, the lower end of the guy ropes should be directly earthed/grounded.

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7 OPERATIONS 7.1

PORTABLE CONTAINER FILLING In addition to earthing/grounding items discussed in relation to road, rail and marine tanker loading (see 6.6), the following should be electrically continuous and bonded to the common earthing/grounding system or to an earthing/grounding system installed specifically for the filling installation: −− −− −−

weighing machine platforms and bases; conveyor tracks, and other ancillary equipment.

Filling hoses should be electrically continuous. A separate flexible earthing/grounding lead with a robust clip or clamp should be provided for connection to the drum during filling. This arrangement may not be necessary for filling pressure containers because they do not normally contain a potentially flammable mixture. An earthing/grounding unit, interlocked with the product delivery pumps, should be installed. 7.2

TANK CLEANING For electrical safety, electrical equipment should be securely and effectively earthed/grounded and bonded to the tank shell, as described in 6.4. Potentially incendive static electricity is a significant hazard in tank cleaning operations because some quantity of flammable residue is likely to remain. Accumulation of electrostatic charges on equipment, such as storage tank shells and internals, should be prevented by ensuring that they are made of electrically conductive materials and are properly bonded and earthed/grounded. Among other items requiring bonding, using a proper connecting terminal, are: −− −− −− −− −− −− −− −− −− −−

air blowers, eductors and ejectors; compressors; pressure hoses, jetting nozzles and couplings used for water or steam jetting; hoses, nozzles and couplings used for gas-freeing by inert gas displacement; vacuum trucks, air pumps, hoses, nozzles and couplings; air lines, etc. used in grit or shot blasting; ventilation lines; temporary lighting systems; portable tools, and scaffolding.

The system of bonding to the storage tank should not be disconnected during tank cleaning operations. Where steaming out is used, all fittings and nozzles should be properly bonded to the storage tank. It should not be assumed that satisfactory bonding already exists for typical storage tank fittings (e.g. owing to the presence of potentially insulating gaskets).

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It should be ensured, when circulating using temporary lines and nozzles, that the discharge point is kept submerged in the liquid at all times: a jet breaking the surface may generate static electricity. All temporary piping should be electrically bonded and its electrical continuity should be tested. To ensure an adequate bond to the storage tank shell, and to minimise testing required on site, the following provisions should be made: −− −−

−− −−

A designated bond connection point should be nominated for each storage tank. Bonding conductors of adequate mechanical strength should be used to bond equipment to the designated bond point. Where adequate bonding is inherent in the equipment, e.g. metallic braided hoses, a separate bond may not be necessary. Bonding cable connections should be mechanically tight and should provide a robust electrical connection. All connections should be visually checked and tested for electrical continuity before tank cleaning operations start, using an intrinsically safe instrument.

For further guidance, see the EI Model code of safe practice Part 16:Tank cleaning safety code.

7.3 SCAFFOLDING Scaffolding operations may increase the risk of electric shock – during installation, scaffolding might impinge on or puncture adjacent live electrical equipment, e.g. lighting fixtures or electric cabling, and cause the scaffolding to become live. Where lighting is fitted to scaffolding, or portable electrical equipment is to be used, the metal parts of the scaffold should be bonded and earthed/grounded so as to prevent potentially hazardous touch voltages; metal-tometal fixing is usually adequate to provide electrical bonds between scaffold poles and fixing connections. The use of low voltage electrical equipment and supplies, wherever possible, is recommended, and the low voltage lead should be the longer, with a short mains voltage lead connected to the nearest convenient supply. RCD protection may also reduce the risk of electric shock. All scaffolding structures that are at risk from lightning strikes should be properly earthed/ grounded, particularly those on the roofs of high buildings or process plant. Butting on to the building surface is not considered adequate to provide an effective earthing/grounding connection or to ensure that the lightning will not pass through any person’s body in contact with the metal framework. Where scaffolding is installed on a structure with a lightning protection system (LPS), the scaffolding should also be connected to the air-termination and earth/ground-termination of the LPS. However, where the scaffolding is connected to the LPS, it should not be connected to the main electrical earthing/grounding system (so as to avoid circulating currents between the electrical earthing/grounding system and the lightning earthing/grounding system). Static electricity may be generated by work activities carried out on scaffolded structures, though this may be dissipated by contact with conductive parts of the scaffolding; however, in hazardous areas (especially in confined areas such as tanks), ignition risks should be considered and appropriate earthing/grounding arrangements should be made. Further information is available in NASC Guidance SG3:08 Earthing of scaffolding structures and HSE GS6 Avoiding danger from overhead power lines.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

7.4

CONNECTING/DISCONNECTING CONDUCTIVE PATHS As noted in section 3, the making or breaking of contacts in conductors carrying current can result in a potentially incendive spark, so a structured approach to maintenance operations is required. In order to maintain earthing/grounding/bonding continuity, so as to avoid the risk of potentially incendive sparks from any current, a temporary bonding bridge should be fitted across any joint or section before it is disconnected or cut, and should remain in place until the break is reconnected, unless the area has been declared gas-free under a permit-to-work system. The final connection of the bonding conductor should be made in a non-hazardous area. This may involve, e.g. connecting a separate conductor to each side of a flange in an access chamber, then joining the other ends of the conductors together to complete the bond in a non-hazardous area outside the access chamber. When subsequently removing the bond, the join in the non-hazardous area outside the access chamber would be disconnected before detaching the conductors at the flange/pipe ends in the hazardous area. Alternatively, an Ex certified switch could be used. Note, in particular, that a site supplied from a PME supply is likely to experience diverted neutral currents that originate off-site and cannot be controlled; in this situation it is important to provide bonds over joints or sections being disconnected. Where transmission pipelines run parallel to overhead high voltage power lines, there may be induced voltages and currents such that a low resistance bond needs to be provided across any joint before it is broken. Particular attention should be paid to earthing/grounding across flexible connections. Disconnection of an earthed/grounded cable whilst a CP system is energised could cause a potentially incendive spark due to the CP current. Work on CP metalwork or any earthed/ grounded apparatus where CP is present should be carried out only when the CP power supply has been isolated (see 4.8) and stored energy has dissipated, see section 8. For tanker operations, care should be taken both during installation, and during subsequent loading operations, to ensure that any isolating joints are not short-circuited by structural steelwork or loads carried by cranes.

7.5

OPERATIONS DURING LIGHTNING STORMS Caution should be exercised if there is active lightning in the vicinity, or a likelihood of lightning. Operations such as fuel transfer should be suspended during lightning activity within the immediate vicinity of the facility.

7.6

RADIO SILENCE DURING PRODUCT TRANSFER RF radiation can cause current flows in adjacent conductive structures, and incidental interruption of those currents has the potential to be incendive. Therefore, radio silence may be required during fuel transfer or tanker loading/unloading operations, see guidance PD CLC/TR 50427 Assessment of inadvertent ignition of flammable atmospheres by radiofrequency radiation.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

7.7 WELDING Electric welding operations can introduce substantial stray currents into the installation structure, and if interrupted or reconnected, these currents may produce a potentially incendive spark. Where welding is to be conducted on a live plant, the risk of such sparking should be assessed, and controls put in place via the permit-to-work system.

7.8

TANK DIPPING Metallic or conductive objects should not be lowered into a tank during loading, or for a five-minute period after loading has finished, to allow for static electrical charge relaxation. However, this restriction is not applicable where dipsticks are used in dip tubes that have their bottom ends submerged in the product. See EI DVD Controlling the risk from static electricity, 2013.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

8

MAINTENANCE AND INSPECTION

8.1

PERMANENTLY INSTALLED EARTHING/GROUNDING/BONDING CONNECTIONS Earthing/grounding/bonding conductors should be designed and constructed so that they require minimal maintenance, though the layout of the system should allow for visual inspection. Periodic general inspections for corrosion, mechanical damage, etc. are part of the normal surveillance of the overall facility, and earthing/grounding/bonding conductors should be included in this activity. In the case of inspection of open floating roof tanks and fixed roof tanks with internal floating covers, checks should especially include: −− −−

all earthing/grounding shunts and bonding cables, and roof drains (also to be inspected at ground level).

Since the possibility of incendive ignition exists with any CP system, it should be temporarily switched off/de-energised, allowing sufficient time for stored energy to dissipate, before carrying out any type of work on the CP system. It is recommended that the system is isolated for a period of one to two hours before any disconnection/separation takes place. This is to allow sufficient depolarisation of the CP protected steelwork so that the stored energy from the CP system will dissipate. It is stressed that even with electrical systems de-energised, diverted neutral currents may still be present in earthed/grounded conductive parts such as pipework at a site with a PME supply. See 8.4 for guidance on the avoidance of potentially incendive sparking caused by the connection or disconnection of conductive paths.

8.2

PORTABLE EARTHING/GROUNDING EQUIPMENT FOR POWER SYSTEM MAINTENANCE Portable earthing/grounding equipment is used to connect a nominally de-energised circuit to earth/ground during maintenance work, and supplements any permanently installed earthing/grounding switches. This equipment: a) prevents a dangerous voltage rise at the worksite arising from unanticipated fault currents, and b) conducts to earth/ground any induced current or stored energy in the circuit. All phases should be earthed/grounded in multiphase applications. Clamps are employed to make connections to the circuit conductor(s) and a suitable earthing/grounding point. Clamps should have a high clamping force to make a reliable contact – special clamps are available that penetrate non-conductive scale, corrosion products or coatings. The circuit conductor(s) should first be proved dead, then the earth/ground end of the connection should be made, and finally the clamp(s) applied to the circuit conductor(s) by means of an insulated pole or other suitable device, so as to avoid any shock hazard. Note: It should be ensured that all temporary earthing/grounding connections are removed before re-energisation of the equipment concerned.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

This equipment needs to be sized for the maximum fault current and for the operating time of the circuit protection; it should safely deal with accidental re-energisation of the circuit, but the conductors may be smaller than normal circuit protective conductors as it is not necessary for this equipment to remain undamaged by such an accidental re-energisation. It also needs to withstand the large electromagnetic forces that act on conductors during the fault condition, so cables need to be configured with sufficient slack to avoid overstress. For outdoor applications, where an accidental re-energisation fault condition occurs, a substantial temperature rise in the cable is acceptable (the cables should be discarded in the event that they are subjected to such a circuit fault condition); for indoor applications, cables should be sized such that the temperature rise is limited, such that no potentially harmful gases are evolved from the insulating material. Where a neutral-earthing/grounding resistor or reactor is present, the potential fault current is reduced, and a smaller cable size may be used unless the resistor or reactor is bypassed as part of the maintenance work. Further advice is available in IEC 61230 Live working. Portable equipment for earthing or earthing and short-circuiting.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

9 TESTING Initial tests and periodic tests to confirm the performance and integrity of the earthing/ grounding/bonding system are recommended, particularly following major changes that might affect the fundamental requirements for the system or its performance. Test intervals should be tailored to the individual situation, and would depend on site experience of failure rates; a rolling programme based on samples of conductors (in a similar manner to sampled inspection of ex-apparatus) may be satisfactory. Trending analysis should be applied to determine whether the results are acceptable, and to identify any long-term deterioration/ ageing of the system. Note: Making a system electrically 'dead' for the purposes of testing (or maintenance) may not make the earthing/grounding system 'dead' – voltages can be impressed from e.g. adjacent substations via LV neutrals, cable sheaths, etc. Further advice on testing is available in BS 7430 (which does not apply offshore). 9.1

CURRENT TESTS (USING CLAMP METERS) Clamp meters are also known as current clamps or tongs, and are essentially current transformers with two jaws that clamp on to a conductor; these meters can be used to measure current flow in a conductor on a live plant without disconnection of the earthing/ grounding conductor. (Clamp meter testing in hazardous areas requires a suitably Ex certified instrument used within its certification conditions.) Most earthing/grounding conductors carry at least some current during plant operation, and if a finite current is measured, this indicates that the conductor is (at least partially) operable. However, any earthing/grounding/bonding conductor in a hazardous area carrying more than 100 mA should be investigated. Substantial earth/ground/bond currents within a non-hazardous area should also be investigated. Where a conductor carries no current at all, this could well indicate that the conductor is open circuit, and this too should be investigated.

9.2

EARTH/GROUND FAULT LOOP IMPEDANCE TESTING The purposes of periodic tests of earth/ground loop impedance are: −− −−

to verify that earth/ground impedance is low enough so that fault currents are high enough to initiate circuit protection within the prescribed time, and to detect changes in circuit impedance from previous tests, or to detect fluctuations in impedance during a given test; these conditions might indicate a deterioration in conductors, joints or terminations (e.g. caused by corrosion).

Tests using a substantial current (e.g. 30 A to 200 A) are normally conducted on all electrical systems during the commissioning phase, e.g. the Ex initial inspection phase, before potentially flammable atmospheres are present. These tests are effective at detecting bad joints, etc. Tests should also be carried out at this stage using a light current continuity testing instrument, in order to provide a baseline for future post-commissioning periodic tests during the life cycle of the installation; these tests may be carried out using Ex certified instruments whilst hydrocarbons may be present. Further advice is available in the EI Model Code of Safe Practice Part 1: The selection, installation, inspection, and maintenance of electrical and non electrical apparatus in hazardous areas. 72

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Current injection (heavy current) testing can be performed in the operational phase subject to risk assessment (heavy current testing may cause localised heating effects or sparking, and so should be carried out only under gas-free conditions), see 9.2.11. Alternatively, earth loop impedance testing should be carried out during plant shutdowns when the area has been classified as being gas-free. However, care needs to be exercised where stray test currents might migrate back to the source of supply via a hazardous area that still contains hydrocarbons, i.e. is not classified as being gas-free. Post-commissioning periodic testing may be performed during a 'detailed' Ex inspection of the field device, e.g. Ex motor or light fitting etc. as part of a programmed or random sample, as described in BS EN 60079-17. The low current conductivity (resistance) test should be compared to the initial baseline readings and a judgement made as to whether the results are satisfactory. Where readings are found to be significantly different to initial readings, then this should be investigated and further tests performed on similar electrical equipment in the same location to determine the scale of any degradation in the earthing/grounding/bonding of electrical equipment. The following notes apply to all circuits (other than IS circuits, data circuits, telephone circuits, intruder alarm circuits, etc. but including process control circuits) operating at 50 volts single phase to 1 000 volts 3 phase AC and 120 volts to 1 500 volts DC. The following definitions are generally used in the literature (see Figure 20). Note: Tests carried out with the supply isolated, neglect the reflected impedance on the primary side of the transformer.

R1 L

Distribution board

Ze

Zs

Load

N

E R2 Figure 20: Earth/ground loop impedances

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Ze the external earth/ground fault loop impedance from phase to earth/ground at the point of supply (e.g. distribution board) Zs the total earth/ground fault loop impedance of the supply and final circuit cables, including R1 and R2 R1

the resistance of the final circuit phase conductor

R2 the resistance of the final circuit protective conductor (CPC) (the phase and CPC conductors are considered to have minimal inductance) Figures 21 to 25 use the following additional definitions: Rconductor the resistance of the phase conductor of a distribution circuit or a final circuit from the test point to the field device. (same as R1) RCPC the resistance of the CPC of a distribution circuit or a final circuit from the test point to the field device. (same as R2) Rmain the resistance of the circuit (phase plus earth/ground path) from the main incoming supply point to the test point at the source of supply for the final circuit (similar to Ze) Rtotal the total resistance of the circuit (phase plus earth/ground path) from the main incoming supply point to the field device (similar to Zs) 9.2.1 Example method This example method uses a low-resistance ohmmeter as a low current conductivity (resistance) test instrument to verify the life cycle integrity of earth/ground fault loop impedance tests. The test instrument should comply with BS EN 61557-4. It is likely that a standard multi meter will not comply. Note: The capacitance and inductance of the circuit under test may invalidate the certification requirements of an Ex 'i' test instrument. Electrical equipment associated with the test will need to be appropriately risk assessed to ensure that the test does not introduce any hazards. The equipment will require electrical isolation and a permit-to-work. Dependent on the physical layout of the site, testing may take place from a non-hazardous area into the hazardous area or may take place within the hazardous area. The use of a lowresistance ohmmeter in the hazardous area at any field device, e.g. Ex distribution board, Ex motor or Ex light fitting etc, will require a gas-free certificate to be issued and constant gas testing to take place during the test periods. The following typical sequence assumes that the test is being undertaken on a final circuit serving equipment in the hazardous area. It also assumes that the test equipment is located in a non-hazardous area. A typical TN-S system is shown in Figure 21. This diagram shows the various resistances that require determination.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

9.2.2 Measurement of Rmain– see Figure 22 1.

2.

Isolate the supply at the main incoming source of supply to the site. This means isolating the primary side of the transformer. Prove dead using an appropriate tester at the point where the test will be undertaken. At the source of supply for the final circuit (switchboard/MCC/distribution board/MCB board), isolate the incomer or switch off/isolate all outgoing circuits. If practicable disconnect the distribution circuit CPC. Measure the resistance between the distribution circuit phase and the distribution circuit CPC including the distribution circuit phase conductor, the transformer LV winding and the distribution circuit CPC, to obtain Rmain. (While this resistance is not actually impedance it is sufficiently accurate to be considered the equivalent of Ze at the point where the test is being undertaken).

9.2.3 Measurement of R1 – see Figure 24 1.

2.

3. 4.

At the field device, end e.g. Ex motor (or Ex light fitting), confirm area is gas-free. if appropriate, allow sufficient time for charges within equipment to dissipate (equipment may have an associated label), remove terminal cover and prove equipment is dead using an appropriate tester. For a single phase circuit, disconnect phase and neutral conductors and connect together into a separate independent insulated connector. For a two or three-phase circuit disconnect at the field device any two-phase conductors and connect together into a separate independent insulated connector. Replace the terminal cover on the field device so as to retain the integrity of the enclosure protection, e.g. EExd. At the source of supply for the final circuit (switchboard/MCC/distribution board/ MCB board), disconnect or separate the supply cables to the field device from all other circuits then connect the low-resistance ohmmeter and test between the two phase conductors (for three phase) or phase to neutral conductors (for single phase) as appropriate. Divide the reading by two to get Rconductor (R1). This is on the basis that the phase conductor and the neutral conductor have the same length and crosssectional area.

9.2.4 Measurement of R1 + R2 – see Figure 25 1.

2. 3.

At the field device end remove the terminal cover. For a single phase circuit, disconnect at the field device the phase and CPC and connect together into a separate independent insulated connector. For a two or three-phase circuit, disconnect at the field device any of the phase conductors and the CPC and connect together into a separate independent insulated connector. The CPC may be a separate conductor, cable armour or cable containment system. Where the CPC is cable armour or a cable containment system, the connection may be achieved by simply connecting the phase conductor to the earth/ground terminal of the field device. It is not necessary to disconnect cable glands etc. Disconnect, where practical, all bonding from the field device. It is not necessary to demount equipment from its supporting structure. Replace the terminal cover so as to retain the integrity of the enclosure protection, e.g. EExd.

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4.

At the source of supply for the final circuit (switchboard/MCC/distribution board/ MCB board), disconnect or separate the supply and CPC cables to the field device from all other circuits then connect the low-resistance ohmmeter and test between the phase conductor and the CPC (earth/ground) connection. Subtract the R1 reading to get RCPC (R).

9.2.5 Determination of Rtotal – see Figure 23 1.

The total loop resistance Rtotal = Rmain + Rconductor + RCPC (similar to Zs = Ze + R1 + R2). This can be calculated from the results obtained earlier or alternatively can be measured as shown in Figure 23. For the example shown in Figure 23 the test equipment is located in the hazardous area.

9.2.6 Completion of test protocol 1.

2.

Return all conductors and bonding at field device and source of supply for the final circuit to the correct terminals. Replace terminal covers to retain the integrity of the enclosure explosion protection, e.g. EExd and IP ingress rating. Re-energise the supply.

This test result Rtotal should be compared with the initial or datum resistance test to determine whether there has been any significant changes. The essential aspect is to verify that the time requirement for automatic disconnection of supply is maintained. 9.2.7 Alternate methodology In practical terms, for all but the smallest installations the above procedure could be disruptive to operations as the test can only occur with the supply isolated on the primary (MV) side of the transformer. An alternative arrangement would be to establish, using the low-resistance ohmmeter, at regular plant shut-down(s), the resistance Rmain at each main and sub main switchboard; at each main and sub main MCC; and at each main and sub main distribution board. Over a period of time, this would build up a record for the entire site. This pre-measured R(main) value could then be added to the resistance values R1 and R2 obtained at a later date (as per the above low-resistance ohmmeter method) for the final circuit. The overall test result Rtotal should then be compared with the initial or datum low current conductivity (resistance) test to determine whether there have been any significant changes. The essential aspect is to verify that the time requirement for automatic disconnection of supply is maintained. 9.2.8 Existing installations (with MV transformers) where Ze is not known For those installations, with MV transformers, which have been in existence for some time, the value of Ze at the main incoming source of supply or at any point within the distribution system may not be known. In these circumstances, the value of Ze should be determined as a 'one off' exercise only. The value of Ze should be determined at each incoming supply point to the site by means of a conventional high current earth/ground fault loop impedance test instrument. This test will require a prior risk assessment. This test would also require disconnection of the plant earthing/grounding network, including utility and main bonding from the supply earthing/grounding network.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

It may be prudent to ensure that a suitably labelled test socket for earth/ground fault loop impedance measurement purposes only is installed at the main incoming supply point. 9.2.9 Existing installations (without MV transformers) where Ze is not known For those installations that are supplied at LV, without a transformer dedicated to the site, the value of Rmain cannot be physically measured as described above as it is not possible to isolate the supply company network. In these circumstances, the value of Ze should be determined as a 'one off' exercise only. The value of Ze should be determined at each incoming supply point to the site by means of a conventional high current earth/ground fault loop test instrument. This test will require a prior risk assessment. This test would also require disconnection of the plant earthing/grounding network, including utility and main bonding from the supply earthing/grounding network. It may be prudent to ensure that a suitably labelled test socket for earth/ground fault loop impedance measurement purposes only is installed at the main incoming supply point. 9.2.10 Modifications/additions to existing installations Where modifications/additions are being undertaken on an existing site then the value of Rmain at the distribution point (switchboard/MCC/distribution board/MCB board) feeding the final circuit to the modified/additional installation will already be known from the tests described above. The total circuit resistance can then be determined by separately measuring R1 and R2 for the modified/additional installation and adding this to the already known value of Rmain.

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L1 Rtotal

Supply transformer

R main Distribution circuit

L2 L3 N E Rtotal

Rmain

Incoming fusible link/isolator Incoming neutral link/isolator Distribution board

Rc RCPCconductor

Field cable

Final circuit

Load (field device)

Figure 21: Layout of a TN-S system with the earth/ground fault loop resistances identified

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L1 Supply transformer

Rmain

L2 L3 N E Isolate supply

0,01

Distribution board Test point

Test instrument

Load (field device)

Figure 22: Measurement of Rmain at the distribution board

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L1 Supply transformer

Rtotal

L2 L3 N E Incomer must be closed Distribution board

CPCs from other circuits may remain connected to earthing/grounding bar/terminal during test

Test instrument t 0,01

Test point

Load (field device)

Figure 23: Measurement of Rtotal at the field device

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L1 Supply transformer

L2 L3 N E Isolate supply

Test instrument

Distribution board

0,01

Test point

R1+R1

Load (field device)

Figure 24: Measurement of R1 at the distribution board

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L1 Supply transformer

L2 L3 N E Isolate supply Test instrument t 0,01

Test point

Distribution board

R1+R2

Load (field device)

Figure 25: Measurement of R1+ R2 at the distribution board

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9.2.11 Example method using a conductivity meter Associated electrical equipment will need to be appropriately risk assessed, electrically isolated, and a permit-to-work issued to carry out the following work/testing: The use of a high current conductivity tester at the field device, e.g. Ex motor, Ex light fitting, etc, will require a gas-free certificate to be issued and constant gas testing to take place during test periods. −− −− −− −− −− −−

At the source of supply, carry out a conductivity test from the supply switchboard to main incoming earthing/grounding bar/terminal to get Rmain. At the field device end, e.g. Ex motor, Ex light fitting, etc, remove any two-phase conductors and connect together into an insulated connector. Replace the field device Ex enclosure covers to retain Ex integrity. At the source of supply, e.g. motor starter, connect the high current conductor and test between the two-phase conductors: divide reading by two to get Rconductor. At the field device end, connect the high current conductor between the field device outer enclosure and the local earthing point to get Rremote. The total high current conductivity test Rtotal = Rmain + Rconductor + Rremote.

Rtotal should be compared with the initial or datum high current conductivity test to determine whether there have been any significant changes. Provided that the results compare favourably, a meter certified for use in intrinsically safe circuits may be used for future purposes when intrinsically safe testing is carried out.

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ANNEX A GLOSSARY OF TERMS (ADAPTED FROM VARIOUS EI PUBLICATIONS AND BRITISH STANDARDS) AND ACRONYMS A.1 TERMS bond(ing)

An interconnection between conductive parts (generally adjacent exposed or extraneous parts) to prevent electric shock hazards from contact (i.e. touch voltages), or to dissipate static electricity (though not necessarily into the general mass of the Earth).

circuit protective conductor

A conductor that provides the primary route for currents due to a fault between live and other conductive parts of the apparatus not intended to carry normal functional current to flow back to the source of supply.

class I equipment

Equipment with basic insulation as provision for basic protection and protective bonding as provision for fault protection.

class II equipment

Equipment with basic insulation for basic protection and supplementary insulation as provision for fault protection, or in which basic and fault protection are provided by reinforced insulation.

class III equipment

Equipment relying on a limitation of voltage to ELV values as provision for basic protection and with no provision for fault protection. Class III equipment shall be designed for a maximum voltage not exceeding 50V AC or 120V DC.

conductive hose

Hose made of material having a high electrical resistance but with an embedded reinforcing or enshrouding wire, braid or armour incorporated during manufacture and connected to metallic fittings at both ends of the hose.

down conductors

Conductors which connect lightning protection air terminations or elevated process plant and steelwork to the lightning protection earth/ground terminations.

earth/ground

The conductive mass of the Earth, whose electrical potential at any point is conventionally taken as zero.

earthing/grounding conductor

The connection between the main earthing/grounding bar/ terminal and the earth/ground electrode.

earth/ground electrode

A conductor or group of conductors in intimate contact with, and providing an electrical connection to, earth/ ground.

earth/ground electrode resistance

The resistance of an earth/ground electrode to the general mass of the Earth.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

earth/ground fault loop impedance

An impedance of the earth/ground fault current loop (phase to earth/ground loop) starting and ending at the point of the earth/ground fault.

earth/ground potential rise

The voltage between an earthing/grounding system and a reference earth/ground.

exposed conductive part

The conductive part of apparatus which can be touched, which is not a live part, but which may become live under fault conditions when basic insulation fails.

extraneous conductive part

The conductive part of an installation liable to introduce a potential, (generally earth/ground potential) and which does not form a part of the electrical installation.

foundation earth/ ground electrode

A conductive structure embedded in concrete which is in contact with the Earth via a large surface.

global earthing/ grounding system

An equivalent earthing/grounding system created by the interconnection of local earthing/grounding systems that ensures that there are no hazardous touch voltages

hazardous area

A three-dimensional space in which a flammable atmosphere is, or may be expected to be, present in quantities such that special precautions are required for the construction, installation and use of electrical apparatus.

high resistance earthed/ grounded neutral

A system where the neutral is earthed/grounded through a resistance with a numerical value that is equal to, or somewhat less than, one-third of the capacitive reactance between phases and earth/ground.

HV/high voltage

A voltage exceeding 1 000 V AC.

low resistance earthed/ grounded neutral

A system where the neutral is earthed/grounded through a resistance that limits the current to a minimum value of 20 % and a maximum value of 100 % of the rated current through the largest generator.

LV/low voltage

A voltage less than 1 000V AC.

main bonding conductor

A conductor that connects an extraneous conductive part or parts (e.g. the exposed metallic structural steelwork or service pipes of electrical apparatus) or any lightning protection system to the main earthing/grounding bar/ terminal of the electrical installation.

main earthing/ grounding bar (or terminal)

A bar or terminal provided for the connection of protective conductors, including main bonding conductors, to the means of earthing/grounding.

minimum ignition energy (MIE)

The minimum ignition energy that can ignite a mixture of specified flammable material with air or oxygen, measured by a standard procedure.

non-conducting hose

A hose of high electrical resistance, having insufficient conductivity to disperse static electricity.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

non-hazardous area

An area in which flammable atmospheres are not expected to be present so that special precautions for the construction, installation and use of electrical apparatus are not required.

potential grading earth/ ground

An electrode laid at shallow depth around the periphery of equipment for controlling touch and step voltages.

potentially flammable atmosphere

This is atmosphere that could become flammable (the danger is a potential one, not present all the time).

protective conductor

A conductor used to provide some measure of protection against electric shock, intended for connecting together any of the following: −− exposed conductive parts; −− extraneous conductive paths; −− he main earthing/grounding bar/terminal; −− earth/ground electrodes, and −− the earthing/grounding point of the source of supply (usually the transformer star point).

step voltage

The voltage between the feet of personnel standing on a surface, over a distance of 1 m.

structural (earth/ ground) electrode

A metal part that is in direct contact with the Earth or water, whose original purpose is not earthing/grounding, but which fulfils all of the requirements of an earth/ground electrode without impairment of its original purpose.

supplementary bonding conductor

A conductor connecting exposed or extraneous metallic parts, which could be touched simultaneously, at substantially the same potential, normally earth/ground potential, in order to prevent a risk of electric shock.

(effective) touch voltage

The voltage between conductive parts when touched simultaneously by a person.

prospective touch voltage

The voltage between simultaneously accessible conductive parts when these parts are not being touched by a person.

touch current

The electric current passing through a human body or through an animal body when it touches one or more accessible parts of an installation or equipment.

A.2. ACRONYMS AC

alternating current

CNE

combined neatral and earth

CNG

compressed natural gas

CP

cathodic protection

CPC

circuit protective conductor

CSA

cross-sectional area

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

DC

direct current

DNC

diverted neutral current

ELV

extra low voltage

EPR

earth potential rise

FPSO

floating production, storage and offloading (vessel)

FRT

floating roof tank

GPR

ground potential rise

HV

high voltage

IS

intrinsically safe

ISGs

isolating spark gaps

LNG

liquefied natural gas

LPG

liquid petroleum gas

LPL

lightning protection levels

LPS

lightning protection system

LPZ

lightning protection zone

LV

low voltage

OCIMF

Oil Companies International Marine Forum

PME

Protective multiple earthing

RCD

residual current devices

RCM

residual current monitor

RF

radio frequency

SELV

safety/separated extra-low voltage

SPD

surge protection device

USTs

underground storage tanks

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

ANNEX B TOUCH AND STEP VOLTAGE LIMITS A current flow through the human body can cause serious harm, including potentially fatal ventricular fibrillation. The likelihood of harm depends on the duration of exposure, the current, etc. The route the current takes through the body is significant, for example, there are differences between routes through the left hand and through the right hand; tolerable step voltages (where the current flow is through the feet) are generally much higher than tolerable touch voltages, so where touch voltage limits are used, step voltage limits should not be exceeded. Also, any calculation of tolerable limits will be based on assumptions of typical body resistance, etc. and is of an indicative nature only. There are several sources of information on safe limits; not all give identical guidance, and a limit appropriate to the particular situation and country of use should be used. BS EN 50522 (which does not apply on ships or offshore) outlines a method of calculating limits (based on DD IEC/TS 60479-1 Effects of current on human beings and livestock. General aspects). Assumptions of a current path from one hand to feet, 50 % probability of body impedance being less than assumed value (this depends on the touch voltage itself), 5 % probability of ventricular fibrillation and no additional contact resistances, yield the current and voltage limits give in Table B1. However, for GB applications, BS EN 50522 Annex NA recommends a 5 % probability of body impedance, and yields somewhat different, lower limits.

Table B1: Permissible body currents depending on duration of exposure Exposure duration seconds

Body current mA

Permissible touch voltage V

0,05

900

715

0,10

750

654

0,20

600

537

0,50

200

220

1,00

80

117

2,00

60

96

5,00

51

86

10,0

50

85

The International Telecommunications Union (Reference: ITU) recommends limits for acceptable short-term touch voltages, also based on DD IEC/TS 60479-1, but with a different set of assumptions. For 'severe conditions', the results are given in Table B2.

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GUIDELINES ON EARTHING/GROUNDING/BONDING IN THE OIL AND GAS INDUSTRY

Table B2: Limiting values for severe conditions Exposure duration t seconds

Admissible limit. General V

Admissible limit when current through chest or hip need not be considered. V

t