With filters or without? How can the impact of mains filters on the results from portable appliance test routines be overcome?
As electrical appliances become more sophisticated, tighter controls to limit electromagnetic interaction have been imposed by the regulatory authorities. These regulations can lead to extra circuitry being added, which can affect the outcome of electrical safety tests undertaken on the products.

Under EMC Directive 89/338 manufacturers must ensure that their products remain immune from and do not create unacceptable electromagnetic interference. To ensure immunity against voltage spikes on the power line IEC 61 000-4-4 should be followed.

If a product is susceptible to these fast transient spikes, one solution is to introduce an input mains filter. This is essentially a low-pass filter to high frequencies, connected electrically between the live power line and earth. Any fast transient spikes are shunted to earth and a clean ac waveform is presented to the power supply.

These mains filters can present problems when carrying out subsequent periodic electrical safety tests – the insulation test is most affected. This test is normally carried out by applying 500 Vdc between earth and the live and neutral, which are connected together internally in the portable appliance tester (PAT).

Unfortunately, when the voltage reaches a threshold of about 300 V the mains filter will behave as a low resistance, typically about 0·5 MV. This will cause a test failure if the pass limit has been set for 2 MV – a typical Class one minimum insulation requirement for most standards.

The failure is not caused by the product's insulation, but by the mains filter resistance being below the pass limit. This would lead to appliances being condemned as unsafe, where in fact this is not the case.

Clearly, all electrical safety failures registered by PAT testers should be treated as genuine unless proved otherwise. But how can it be determined if the failure is caused by a mains filter or is a genuine insulation problem?

One remedy introduced by PAT manufacturers recognises that the problem occurs when the 300 V threshold is exceeded. This has been resolved by the introduction of a lower insulation test voltage of 250 Vdc. In this case, the mains filter appears as an open circuit to this voltage and the insulation behind the filter can now be measured. Most modern testers are equipped with this voltage.

Another approach recommended by the IEE in its Code of Practice for inspection and testing of electrical equipment is to regard any mains filters as an intrinsic and necessary component within an appliance if deemed to be the case by the manufacturer.

Here, the insulation value as measured through the power cord can be treated as an in-service value. The IEE recommends in-service data as the preferred pass values when carrying out periodic electrical safety tests on equipment that is in use.

These values take into account mains filters and are typically 0·5 MV for Class one and 1 MV for Class two. Most testers developed since the publication of the IEE document allow these in-service values as pass limits.

The effect of mains filters on portable appliance test routines can be a troublesome factor and the best solution is usually obtained in consultation with the individual appliance manufacturers.