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Electrical Safety in Medical Equipment

© D. J. McMahon 141008

rev cewood 2017-10-23

Key Points Electrical Safety in Medical Equipment: - know the significance of NFPA-99 and the annual editions - define 'the patient care vicinity' (PCV) - know about the 'wet location' and LI vs GFCI - know the definitions AND SYMBOLS for applied parts B, BF, CF - recognize the symbols for 'defibrillation proof' - know the methods for electrical signal isolation & power isolation - be able to define leakage current; what contributes to it? - recognize the electrical safety standards and know which one is commonly used in the US - MEMORIZE the 2015 NFPA-99 standards; 3 values, including the units - do these values apply to non patient care devices? - know the easy fix for high leakage, often built-in to devices - when do we do electrical safety testing? - understand the problem of finding an actual chassis ground point - how do these standards apply to two-prong plug devices? - know about testing 'systems' - know about Line Isolation & LIMs - are extension cords and power strips allowed? what are the requirements?

Birth of the HTM Profession : (March 1971)

http://kami.camp9.org/Resources/Pictures/Ralph-naders-most-shocking-expose[1].pdf

NFPA 70®: National Electrical Code® (NEC®) Sets requirements for building design, construction, operation, and maintenance. Applies to all buildings. Adopted as code in 46 states, the VA system, and the Centers for Medicare and Medicaid Services (CMS).

National Electrical Code (NEC) Section 517: Health Care Facilities Discusses electrical service for hospitals and clinics > General electrical distribution > Power supply backups > Defines emergency power distribution > Specifies electrical concerns for specialty areas: - anesthetizing locations - isolated power systems and their monitors - pediatric areas - X-ray

NFPA 99 Sets criteria for health care services based on risk to the patients, staff, or visitors in health care facilities to minimize the hazards of fire, explosion, and electricity. Includes installation, inspection, testing, and maintenance of facilities, equipment, and appliances, including medical gas and vacuum systems.

The “Patient Care Vicinity” Per NFPA 99: A space within the location intended for patient care, extending 1.5 m (~ 6 feet) beyond the normal location of the bed, table, etc that supports the patient during examination or treatment, and extending vertically 2.5 m (~7.5 feet) above the floor.

- Clinical Engineering is responsible for all patient-impact equipment that is within the Patient Care Vicinity. Any equipment in that area must meet NRTL or NFPA-99 electrical safety standards. - Equipment in non-patient care areas (nurses stations, lounges, etc) does not need to meet those standards but may be checked by Facilities Engineering or Clinical Engineering, depending on hospital policy.

The “Patient Care Vicinity” (the ‘bubble’)

Patient Care Vicinity: a crowded place!

“Wet Location” “The area in a patient care area where a procedure is performed that is normally subject to wet conditions while patients are present, including standing fluids on the floor or drenching of the work area, either if which is intimate to the patient or staff.” - NFPA-99 (2005) 3.3.185 > typically applies to surgical suites and other areas > controversial topic lately – - architects tending away from Line Isolation in O.R.s - GFCI’s being used instead

“Applied Part” of medical equipment

as defined by the IEC standard 60601-1: Type B: Applied parts that are generally not conductive and can be immediately released from the patient. May be grounded. eg: Non-invasive BP monitors

[think: Body]

Type BF: Devices that have direct contact with the patient, or parts that have long term contact with the patient. eg: ECG monitors [think:

Body, Floating (ground)]

Type CF: Applied parts that have direct contact with the heart. eg: Invasive pressure monitors, defib paddles

[think: Cardiac Floating (ground)]

ONE of these ratings should be present on a medical device

see the little defib paddles?

If ‘Defib Proof’, ONE of these

IEC Definitions: Class I Equipment – Device is protected against shock by use of a grounding wire connected to the exposed conductive parts of the device. Class II Equipment – Device is protected by additional insulation in addition to the basic design insulation. Usually called “Double Insulated”.

Why aren’t these birds electrocuted ? …

… because they’re not grounded.

Electrical safety is all about current path to ground.

How to electrocute homo sapiens…

(through skin)

(under skin)

I = 120V / 100KW = 1.2 mA = 1200 µA

I = 120V / 25KW = 4.8 mA = 4800 µA

Ungrounded (2-wire) wiring

Grounded (3-wire) wiring

Chassis Ground (at power entry)

Electrical Isolation: transformer isolation for power

opto-isolator for electrical signal:

Leakage current:

or “distributed capacitance”

Leakage current is both capacitive, caused by intrinsic capacitance between conductors; and resistive, caused by imperfect insulation.

RF filters at power entries will add to capacitive leakage:

Where does the leakage current go?

Open ground ! Where does the leakage current go now?

Fatality by microshock scenario:

Electrical Safety Testing Terminology IEC (international):

NEMA / AAMI (US): -

-

-

L1

Neutral

-

-

-

L2

Ground

-

-

-

Earth

Line Voltage

-

-

-

Mains

Hot

-

Patient Contacts (leads, paddles) Applied Parts Enclosure Chassis Ground Wire

-

-

-

Protective Earth (PE)

Leakage Current in Ground Wire

Earth Leakage Current

Chassis leakage

-

-

Enclosure Leakage

Lead leakage -

-

-

Patient Leakage

Leakage between Patient Leads

Patient Auxiliary

Lead Isolation -

-

Mains on Applied Parts (MAP)

-

Earth Resistance

-

Ground Wire Resistance

Insulation Resistance between Hot and Neutral to Ground

Insulation Resistance

What are the electrical safety standards ? AAMI ES-1 (same as NFPA99) - most commonly used in the U.S. IEC 60601-1 (same as UL2601-1 in the U.S. CSA C22.2 in Canada) - most widely used, but written for manufacturers IEC 62353 - written for end-users; a bit looser requirements AN/NZS 3551 (modified 60601-1)

The NFPA-99 Electrical Safety Test: 1) ground integrity 2) chassis leakage 3) patient contact lead-to-ground leakages 4) patient contact lead-to-lead leakages 5) patient contact lead leakage while in contact with the line voltage (MAP)

Know your analyzer !!

NFPA99 Chassis Leakage (touch) Current • 1999 edition • 2005 edition • 2012 edition

• 2015 edition 10.2.6* Touch Current – Portable Equipment. The touch current for cordconnected equipment shall not exceed 500 A with normal polarity and the ground wire disconnected (if a ground wire is provided).

still valid: .5

1999 rev; PRE-2012! standard has changed!

Electrical Safety Testing Values (per NFPA-99, 2012 edition) Resistance of Ground wire of power cord: 500 milliOhms (500m or .5 ) Leakage current at the chassis: NC (normal condition) 100 microAmps (100 A) SFC (Single Fault Condition, e.g. open ground) 500 microAmps (500 A) Leakage current for any one patient contact lead to ground: 10 A with ground closed, 50 A with ground open. Leakage current for each patient contact lead to any other lead: 10 A with ground closed, 50 A with ground open. Leakage current for patient leads exposed to line voltage: 50 A

Standard circuit described in NFPA 99 for measurement of leakage current:

“Applied Part” = any component of a device that is directly applied to the patient. (Electrodes, paddles, sensors, etc)

Test device measuring chassis leakage current:

note: no switch to open neutral leg; grounding switch shown in SFC (single fault condition)

Test device measuring patient lead leakage current:

(DUT)

Test device measuring isolation current with MAP (Mains on Applied Parts):

(DUT)

What about non-patient care devices? For Non-Patient Care devices, within the PCV: Leakage current at the chassis: 500 microAmps (0.5 mA) e.g. a mobile computer terminal

For Non-Patient Care devices, not in the PCV: Leakage current at the chassis: 3500 microAmps (3.5 mA) e.g. a vacuum cleaner

the PCV

Non-Medical Grade equipment esp Medical computers, TV’s, etc… - can have high leakage current due to capacitance in line filters - corrective action for these if used in patient care areas: - connect a redundant ground wire, or - go thru an isolation transformer in the power input

What about plastic-enclosed, (sometimes battery-driven) medical devices? Many medical devices have no electrically conductive surfaces to access as the “enclosure” for the purposes of connecting a test device to check leakage or ground integrity. NFPA-99 has a specific way to establish a temporary reference point using a 10x10 cm sheet of foil, but it is not very practical. You will often need to find any possible metallic point that is continuous with the device’s ground. In some cases, the device simply can’t be checked for leakage.

e.g. an irrigation pump used in surgery:

When to we do Electrical Safety Testing? 1) on any incoming inspection of new equipment, including rented, borrowed, or donated equipment 2) after any repair or modification 3) as part of scheduled Performance Verifications (depending on department policy) - may also be useful as part of troubleshooting

Typical Electrical Safety Analyzers

In the NSCC Lab:

Rigel SafeTest 50 Fluke ESA612 Dale 544D+

The resistance measurement problem:

One solution: Kelvin (4-wire) resistance leads

Kelvin Lead set on an electrical safety tester:

Chassis Ground Lug (or POAG: Point of Actual Ground)

Grounding wire

POAG connector

Where do we find chassis ground?

What about testing multiple medical devices (or ‘systems’) ?

“System” in the context of medical equipment inventory, refers to a number of medical devices commonly used together, which share a common power strip. Generally, the electrical safety check of a system can be done at the common power cord that feeds it. - be certain that the total current draw of the system, when at maximum, does not exceed ~75% of the power strip’s rating.

Example: typical medical video tower

What about “Fixed Equipment”? i.e. equipment that is permanently installed to the building structure. e.g.: imaging systems, treadmills, large centrifuges. ‘Point-to-Point’ testing for leakage current and resistance is done with ES analyzers on equipment that is fixed. Acceptable leakage limits are higher.

Standard power distribution grounding:

Isolated power branch (“Line Isolation”):

Isolated Power System Generally used in Operating Rooms

Line Isolation Monitor (LIM)

LIM’s alternately look at each side of the line to check for excessive leakage relative to ground:

Circuit Breaker Panel

Line isolation transformer

Line isolation monitor (LIM)

Typical setting and reading for a LIM

Extension Cords: Generally allowed for use in the patient care vicinity, IF: > They use wire that exceeds current capacity of the power outlet being extended. (usually 14 gauge or 12 gauge, + ground) > Designed with medical-grade components.

‘Multioutlet Extension Cords’ ‘Multiple Outlet Strips’ ‘Power Strips’ ‘Relocatable Power Taps’ Standardized in UL #1363. Allowed by NFPA 99, IF: - they are made of medical grade components - they are of adequate ampacity for the need - they are inventoried and maintained

Not Hospital Grade

Hospital Grade

Relocatable Power Taps are now acceptable IF: ------

Are permanently attached to equipment. The total current drawn is