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John Backes, Product Manager at Rigel Medical, part of the Seaward Group, considers the merits of 25A and 200mA test currents for testing protective earthing conductors in electromedical devices.
Debate in the medical industry on the most appropriate test current for checking the integrity of the protective earthing conductor of medical devices has been around for many years.
Historically, some have traditionally favoured a higher test current of 25A or 10A originating from the IEC 60601-1 requirements on the premise that it will best detect any damaged conductors present. In addition, when analogue instruments were widely used for low resistance measurement, it was often necessary to use high test currents to produce sufficient voltage drop across the sample to generate the necessary needle deflection.
With modern electronics and digital technology, this is no longer necessary and more recently, given the growth in hand held test instruments, others have come to favour a lower test current of 1A or less as a means of eliminating any risk of damage to the equipment under test.
In reality, the different test currents both have their merits. Various International Standards and Code of Practices for In-service Testing and Inspection of Medical Electrical Equipment recommend a variety of test currents from 25A down to 200mA. However, for routine testing and testing after repair of non-medical appliances and testing of fixed installations, the majority of European standards now specify a test current of 200mA.
Protective Earthing Conductors
Protective earthing conductors are designed to prevent electric shock by allowing the passage of electric current under fault conditions. In Class I electrical equipment the protective earthing conductor resistance needs to be of sufficiently low value to prevent the voltage on external metal parts rising to a level where the shock potential presents a hazard to life.
A variety of national and international standards define a maximum acceptable level of resistance of a protective earthing conductor and the precautions associated with medical equipment are significantly greater than those associated with industrial commercial and electrical products.
These standards not only lay down the maximum resistance values but also define the test current, the open circuit voltage and the duration of that test. Depending on the time at which the tests have been conducted, different criteria will apply at the design stage, the conformance testing stage, manufacturing and in service testing.
With any item of electromedical equipment it is likely that the protective earthing conductor will comprise various lengths of flexible cable linking the equipment to the point of electrical supply. It is also possible that various types of switching mechanism may exist including relays and electrical switches.
Any measurement of a protective earthing conductor will therefore encounter both bulk and contact forms of electrical resistance. Both these types of resistance can have implications on the use of different test methods with varying currents, voltages and time durations.
Figure 1 shows the different types of resistance making up the total measured resistance.
Bulk resistance is the material along the conductors' path. This will tend to be constant although it will be affected by temperature and in certain cases by physical pressure.
Contact resistance, however, is a variable resistance that occurs at the interface between two conducting<
