The insulation resistance test ensures that the insulation that surrounds an appliances’ live parts has a sufficiently high resistance level.
The test immediately preceding the insulation test should be the earth bond test. If the appliance does not pass the earth bond test do not proceed to the insulation test.
The insulation test is carried out at 500volts d.c for a period of 5 seconds. All PAT testers that carry out this test do so by shorting together internally the live and neutral pins within the appliance when carrying out the test.
This means that there is no potential difference between the live parts. Where an appliance has a suppressor fitted or a surge protection device then the test may indicate a fail. In these cases the insulation test may be substituted for the earth leakage test or substitute earth leakage test.
Both Class I and Class II appliances are subject to insulation testing. It’s particularly important you know the different between how each class is tested.
Many people are concerned that the insulation test damages the internal circuitry of IT equipment. With electronic equipment test voltages should be dropped to 250 volts.
Testing a Class I Appliance
A Class 1 appliance should have the power switch in the on position, if this is not possible pressing the test button a 2nd time would carry out the test. Plug the appliance into the tester and select the Class 1 test. The insulation test pass mark will be 1MΩ. This means that the Insulation reading must be over 1MΩ to Pass the test. The PAT tester will measure the resistance between the live parts (Live and Neutral) connected together and earth.
Testing Class II Appliances
If you decide you need to test a Class II appliance with the insulation resistance test, then there’s a slightly different procedure to follow.
The appliance switch should still be in the On position whilst carrying out the test.
The main different between the Class I and II test is that for Class II appliances you must connect a test probe (lead) to any metal point on the appliance in question. If you had a drill, you’d hold it onto the ‘chuck’ or screws within the bodywork. If someone had damaged the internal live parts or let’s say gone through the live cable within the appliance then this test would pick up that the screw had gone through the live cable.
It is important for most appliances that you complete this test multiple times on all the different metal sections in order to assess that the correct level of insulation is present throughout the entire appliance.
For this test the insulation resistance should be greater than 2MΩ between the live + neutral and the metallic part of the appliance.
Testing Appliances and Extension Leads with Surge Protection
Surge protection devices are more commonly found on extension leads fitted to IT equipment.
It is common to use a standard voltage of 500V when the testing is carried out. Appliances that are surge protected are different because they have circuitry built into the appliance to prevent a “surge” of electricity from entering and damaging the appliance.
This circuit is designed to monitor the incoming voltage into the appliance and if it rises above 250 volts the circuit will “divert” the extra voltage away safely down the earth cable of the appliance.
When we carry out a PAT test at 500v d.c. the voltage used often triggers the surge protection circuit. For this reason most appliance testers are fitted with a 250 volt insulation test option which will not trigger the surge protection device.
Not all PAT testers have this capability, so it is also acceptable to use a touch current test instead.
Written by Barry Atkins
