This test is carried out on Class I and Class II appliances during PAT Testing. The
purpose of this test is to ensure that there is adequate insulation between the Live
parts of the appliances and the user touchable metal parts. Adequate insulation is
defined as greater than 1 M ohm for Class I appliances and 2 M ohm for Class II appliances.
PAT Testing a Class I appliance
Plug the appliance into a Pat Tester. Make sure that the appliance is switched on.
When the test is done if there is more than 1 M ohm resistance between the Live +
Neutral and the Earth pins of the plug, then there is adequate insulation and the
appliance is considered safe.
PAT Testing a Class II appliance
Plug the appliance into a PAT Tester. Make sure that the appliance is switched on.
Connect the test probe to any metallic part on the appliance. For example on a drill,
this would be the chuck.
When the test is done, if there is more than 2 M ohm between the Live + Neutral and
any metallic part then there is adequate insulation and the appliance is considered
Note that it is very important to connect the test probe to all metallic part on
the Class II appliance in turn. This may require multiple tests. If this is not done
then any problems with the insulation will not be detected during PAT testing.
When carrying out this test there are a number of practical things to take into account.
PAT Testing - Test Voltage
This is normally carried out using a voltage of 500V. However surge protected appliances
have components that present a low resistance at this test voltage. When carrying
out the Insulation Resistance test during PAT Testing, if one gets a failure on surge
protected appliances, it is normal to repeat the test at a lower 250V. For example
the Kewtech tester has this facility.
If the PAT tester does not have a 250V test option, then is is adequate to make a
note about this and pass the surge protected appliance.
The BattPAT PAT tester does the insulation Resistance Test at a voltage of 350V and
will PASS surge protected appliances.
PAT Testing - Heating and Cooking Appliances
In some appliances using mineral insulated heating elements, absorbed moisture may
result in a failure when this test is carried out during PAT Testing. In this case,
switch on the appliance for a period to drive off excess moisture and then retest.