Insulation test alternatives compared

Jim Wallace of Seaward looks at the relative merits of different electrical safety tests used to assess the integrity of the insulation in electrical appliances.

As part of any routine electrical safety testing programme there is a requirement to assess the integrity of the insulation between live parts and parts accessible by the user.

Conventional insulation resistance measurement (insulation test) has been the traditional method of carrying out this requirement, with protective conductor current or touch current as a complementary test for use when an insulation test cannot be performed or gives questionable results.

The recent publication of the third edition of the IEE Code of Practice for in-service testing has introduced two new additional test methods as potential alternatives.

The new IEE code recognises that testing insulation resistance at 500V DC might be problematic in some circumstances and as an alternative suggests the use of a reduced voltage (250V) DC test, a protective conductor/touch current measurement or substitute/alternative leakage current test.

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

Insulation resistance 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.

With all measurements of insulation resistance, the appliance under test must be switched on before performing the test, otherwise the test voltage does not pass beyond the mains switch, in which case we are only testing the mains in cord.

However, as the insulation resistance test does not power up the appliance and though this is regarded as an advantage (reducing the time taken to test and eliminating the danger of moving hazardous parts) extra care should be taken that the equipment switch is still put into the 'on' position to make the test meaningful.

In addition, 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 particularly older IT equipment which does not comply with BS EN60950 may be damaged by 500V.

However, in practice, this may not be a significant issue as BSEN 60950 has been around a lot longer than most IT equipment currently in common use.

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

On the plus side, the insulation resistance test is relatively quick and easy to carry out, which is why it is probably the most widely used.

Reducing the test voltage to 250V, as now officially included in the IEE Code of Practice, overcomes the problem of surge protection and in reality was introduced as an option in some PAT testers some time ago at the request of end users testing multiway adapters or extension leads as these often have surge protection.

Surge protection devices are designed to suppress large voltages, typically those above 300V.

As a result, the 500V test will trigger the surge protection whereas a 250V test will not.

The alternative leakage measurement technique is similar to the insulation resistance method and involves the application of an AC test voltage between both the live and neutral conductors connected together and protective earth.

This is referred to as alternative or substitute leakage measurement.

However, the test voltage is now AC at a frequency of 50Hz (UK mains frequency) 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.

However, the substitute/alternative leakage test still has some limitations because any electronic switches present will not be on (as with insulation testing) and relays or other active circuitry that may effect measurements may not be activated.

The protective conductor current or touch current is a complementary test to the insulation test for use when an insulation test cannot be performed or gives questionable results.

In the interests of clarity, the protective conductor test was previously known as earth leakage or Class 1 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.

The measurement is made while the equipment is operating at nominal mains voltage and the test readings are compared with 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, for example, when testing a circular saw.

Equally, accessible conductive parts of the appliance should not be touched during a protective conductor/touch current test.

It is also good practice to perform an insulation test before any powered test, as the insulation test 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, because the equipment is in normal operating mode, equipment with rotating parts can pose a mechanical hazard.

In addition with the use of mains voltage there is a risk to the test operative if an (unknown) insulation fault is present.

Overall this form of testing may be more time consuming.

For example, if powered tests are being performed on a PC, the test duration can be several minutes taking into account the need for the PC to boot up and then shut down again - 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 these alternative methods of assessing the integrity of the insulation of electrical equipment, those responsible for testing need to carefully consider which test option best suits their own particular needs and circumstances.

They should then choose the right test instrument with the versatility to enable them to carry out the most appropriate type of test at any required time.

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