For this test the test set will connect together the live and neutral pins of the appliance mains plug, and apply 500volts(dc) between this connection and the earth pin of the appliance. Hence the Earth Bonding or Earth Screen test should be carried out and passed first. Class 2 Appliances with no earth connection within the mains plug it is usual to connect a specially provided probe to the casing of the appliance whilst carrying out this test. The usually accepted pass levels are for Class 1 equipment not less than 2Mohms, and for Class 2 Equipment not less than 7Mohms.

Note: Class 1 heating appliances with a rating above 3KW can be considered as OK with insulation readings down to 0.3 MOhms. Some operators now regard these as ideal figures to be regarded as suitable for new equipment, and accept slightly lower figures for “in service” appliances. In practice much higher figures are usually encountered.

I.T. Equipment not manufactured to BSEN 60950 does not carry a requirement for this standard 500volt insulation test due to the possibility of damage. In this case if you have later generation PAT Tester it may provide a 250 volt DC insulation test you can use instead. This will protect internal spike arresters and other suppressors used in the power supplies of some of this type of equipment.

Some other equipment containing suppressors may also fail the standard 500volt insulation test, although they are perfectly sound. Filtered extension leads for use with computers for example fall into this category. However these days manufacturers usually supply this information by labelling.

If equipment falls into this category and you have a modern PAT Tester then you can use the “touch current” test instead. This will use the normal supply voltage to the appliance and the “touch current” must not exceed:

Portable/Handheld Class 1 <0.75mA
Class 1 Heaters 0.75mA/kWatt up a maximum of 5mA.
Other Class 1 <3.5mA
Class 2 <0.25mA