Alternatives for insulation testing

18 August, 2008

It is a routine part of any electrical safety testing programme that the integrity of the insulation between live parts and parts accessible to the user is assessed.

The publication of the Third Edition of the IEE Code of Practice for portable appliance testing has introduced other tests as potential alternatives to the traditional insulation test.

All tests have their relative merits and their place in periodic testing, provided the different limitations of each test method are understood.

Insulation resistance

This is normally checked by applying 500V DC between both live conductors (phase and neutral), connected together and protective earth when testing a Class I appliance, or a test probe applied to accessible metal parts when testing a Class II appliance. The resistance is measured and compared with the minimum acceptable value to assess pass or fail conditions.

The appliance under test must be switched on before performing the test, otherwise the test voltage does not pass beyond the mains switch, meaning only the mains cord will be tested.

However, since this test does not power up the appliance, extra care should be taken that the equipment switch is still put into the ‘on’ position to make the test meaningful.

Also, appliances fitted with electronic mains switches or RCD plugs cannot be tested in this manner as it is not possible to close the mains switch (as they require mains to be present).

In some cases sensitive electronic devices and older IT equipment which does not comply with BS EN60950 may be damaged by 500V. In practice this may not be a significant issue as BS EN60950 has been around a lot longer than most IT equipment currently in common use.

Finally, some electrical equipment components connected to the live/neutral conductors for EMC filtering or surge protection, can influence the measurement, indicating an erroneous failure of the test.

Reducing the test voltage to 250V, as now included in the IEE Code of Practice, overcomes the problem of surge protection. Surge protection devices are designed to suppress large voltages, typically those above 300V, and will be activated by 500V but not by using 250V.

Alternative leakage

This involves the application of an AC test voltage between both the live and neutral conductors connected together and protective earth. This is also referred to as substitute leakage measurement.

However, the test voltage is now AC at a frequency of 50Hz, which means the leakage paths will be similar to those present when the equipment is in operation. This avoids the problems associated with EMC filtering or surge protection affecting DC insulation tests. This test has its limitations because any electronic switches present will not be on and active circuitry that may effect measurements may not be activated.

Protective conductor/touch current test

This is complementary to the insulation test, for use when an insulation test cannot be performed or gives questionable results.

The protective conductor test was previously known as the earth leakage or Class I leakage test and the touch current test was often referred to as the touch or Class II leakage test.

However, the principles are the same and the protective conductor/touch current is measured from the internal live parts to earth for Class I equipment or the internal live parts to accessible metal surfaces of Class II equipment.

Measurement is made while the equipment is operating at nominal mains voltage. Test readings are compared to the maximum acceptable current value to assess pass or failure.

As this measurement involves powering the equipment under test, care should be taken to avoid mechanical hazards. Also, accessible conductive parts should not be touched during a protective conductor/touch current test.

It is good practice to perform an insulation test before any powered test, as the former may detect a potentially dangerous loss of insulation. If the insulation test gives a very low or zero reading, the reason should be investigated before any further tests are performed.

One of the benefits of the protective conductor/ touch current test is that the equipment is in normal operational mode and therefore gives the most accurate assessment of the insulation. As the test voltage is the nominal supply voltage, and no elevated voltages are involved, there is no risk of damage to the equipment.

However, this test may be more time consuming. For example, if powered tests are being performed on a PC, the test duration can be several minutes as the PC will need to boot up and shut down. It is not possible to simply stop the test once measurement has been made.

With the IEE Code of Practice now giving PAT contractors the opportunity to use alternative tests to assess the insulation of electrical equipment, consideration should be given to choosing the right test instrument with the versatility to carry out the most appropriate type of test at any required time.