Insulation testing - Measurement in medical environments

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Bill Earlie, of test instrument and precision measurement company Cropico, explains the implications of new guidance on electrical installations in medical environments

New Medical Electrical Installation Guidance Notes (MEIGaN) have recently been issued on the safety requirements of electrical systems installed in diagnostic imaging, patient treatment and radiotherapy rooms.

This guidance has been issued for new buildings, refurbished rooms and transportable diagnostic or treatment rooms in medical or health premises. The notes stipulate a range of measures to be taken in relation to the electrical mains supply and include new instructions regarding the earthing and equipotential bonding connection of permanently installed medical devices and associated equipment.

Developed for healthcare organisations and medical device suppliers, the new MEIGaN guidelines embody the basic principles of the BS7671 IEE Wiring Regulations, but refine this with specific requirements for medical environments.

Earthing & equipotential bonding 
Many electrically operated medical devices and equipment including sterilisation baths, heaters, treatment tables, drug cabinets and some lighting fittings can have exposed metalwork that could become live if a fault occurred. Anyone touching these surfaces could then receive a shock or even be killed depending on the current flowing through them to earth.

Equipotential bonding is therefore necessary as part of the safety measures associated with electrical installations to prevent significant touch voltages being generated within the patient environment.

MEIGaN requires earthing and equipotential bonding conductors shall not intentionally carry load or control circuits. In addition, it is stipulated that equipotential bonding conductor continuity between equipment and the associated mains supply isolator(s) shall not depend solely on the continuity of conduits, cable braiding, ducts or trunking and should be achieved with a dedicated copper earth cable connected with brass or copper fittings.
An earth reference bar (ERB) is required in medical a room that comprises one or more copper connection bars installed in an enclosure as part of the room's protective earth system. The ERB is designated as a reference or datum point for the purpose of defining and measuring resistance values.

All installed equipment needs to be earthed to the ERB if there are any conductive surfaces that are accessible to either patients or staff. This could include warning lights, injectors, water baths, contrast media warning equipment, viewing boxes and powered drug cabinets.
All non-powered equipment with metal surfaces must also be similarly bonded to the ERB. This includes protective screens, metal sinks and work surfaces, heating pipes and radiators, water pipes, drug cupboards, ceiling mounted hardware and other steel or wire cable trays, steel floor ducts and similar hardware. In all cases, such items must be returned to the ERB by means of a cable.

Each equipotential conductor connected to the ERB must be individually labelled and permanently identified. A typical ERB layout is as shown below (with black alligator clips of leads from test equipment).

Testing and verification
MEIGaN requires earthing and equipotential bonding connections are inspected and tested to verify compliance with the new guidance.
It advises resistance should be measured between each protective earth terminal, socket outlet or every accessible metal part and the ERB.  In terms of bonding resistance, the maximum acceptable resistance is 0.1Ω (or 100mΩ). Anything above this level would represent a failure of the earth continuity provided.

As a point of caution MEIGaN points out with the testing of socket outlets, care must be taken to ensure that the contact resistance of an inserted earth pin of a 13A plug should also be low. This is because there can be some variation in the resistance of plug pins, which could be manufacturer dependent. For this reason it is suggested the socket resistance is tested to the back of an inserted pin and not by probing the socket itself.

There has also been much debate about the current and voltage levels that should be used for this test. Traditionally the standard instrument used for equipotential bonding was a mains powered unit capable of supplying up to 35A at up to 25V for a 5 second test.
However, some modern electronic medical equipment could be susceptible to damage by large earth current pulses, especially on PCBs. In addition, the 35A, 5 sec test does not necessarily fuse frayed connections as easily as one might think.

Tests have shown a single strand of earth flex can withstand the test, so it is not in itself a reliable measure of ‘robustness'. Hence, testing at lower currents is not necessarily a weaker test that would miss potential failures.

The new MEIGaN guidelines appear to have taken these sort of considerations into account and now stipulate that a minimum test current of 1A can be used to test the equipotential bonding.

In doing so the new guidance means highly effective testing of the earthing and equipotential bonding of medical devices and fixed equipment in a treatment room or diagnostic suite can now be undertaken with lightweight, hand held digital microhmmeters.
One new example, the Cropico DO4002, has been specially developed for this application.
This instrument utilises forward and reverse current measurement with auto averaging to maintain highly accurate measurement of very low resistance. It also has the advantage of battery power which means equipotential bonding measurements can be taken without the risk of earth leakage currents from the main power supply interfering with resistance values.
In addition, the tester can be used with extended test leads of up to 20 metres without any loss of measurement accuracy, making room socket tests easily accessible and enabling individual plug tests can be undertaken fast and effectively.

Maintaining the integrity of electrical integrity in medical environments is critical to prevent patients being put at risk. The availability of new hand held test instrumentation enables electromedical service and installation engineers to meet this need by helping them undertake earthing and equipotential bonding testing quickly and effectively - without compromising the accuracy of test results.

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