PORTABLE APPLIANCE TESTING
All You Need To Know About PAT Testing, PAT Testers & Related Topics

EARTH CONTINUITY TEST

This test is carried out on all Class I appliances during PAT Testing. The purpose of the test is to check that there is a good connection between the Earth pin on the plug and the case of the appliance. A good connection is defined as having a resistance of less than 0.1 ohms (or 100 milli-ohms).

The Earth Continuity test is sometime referred to as the Earth Bond test or the Earth Resistance test. In effect they all measure that there is a good connection between the mains plug and the Earth point. The conventional way to carry out this test is to plug the appliance into a PAT Tester and clip the Test lead to a suitable earth point.

When the test is done, if the resistance measured is less than 0.1 ohms, then there is a good earth connection and the appliance is considered safe. When carrying out this test during PAT testing, there are a number of practical things to be taken into consideration.
 
Earth point on appliance

In order to carry out this test, it is important of course to clip the test lead to a suitable earth point. If this is not done, then there is no return path for the current during PAT testing. This will result in the appliance failing the test.

Finding a suitable earth point will come with experience. It is essential to get a good connection to the Earth point. Rusty metal or scaling on kettle elements may prevent a good low resistance connection. If this is the case then rotate the connector and use the teeth of the crocodile clip to scrape away the rust or the scaling to get a good connection. It is also acceptable to clip onto a screwdriver and use this to connect to an Earth point.

The Golden Rule is: When testing Earth Continuity, if you do get a fail initially, do not immediately fail the appliance. Make sure you have a good connection and try PAT testing again. If necessary try a different Earth point as not all metal parts might be connected to earth.
 
PAT Testing - Test Current
 

This test should be carried out using a test current of between 100 and 200 mA. During the test the power cable should be flexed to bring out any obvious faults. This is a very safe test and can be used on ALL Class I appliances including IT equipment.

 

Higher test currents up to 25 Amps can be used where there is a good connection to the earth on the appliance, such as on extension leads. Our recommendation is to use the low current test (between 100 to 200 mA) unless there is a compelling reason to use a much higher current.

 
PAT Testing - Test Limits

As mentioned above, to pass this test, the Earth Continuity resistance has to be less than 0.1 ohm when PAT testing is carried out. As this is from the pin on the plug to the Earth point, an allowance can be made for the Mains lead that is used on the appliance.

 

For example, a Class I lamp has a 5m mains lead (current capacity of 3A) attached to it. The total resistance of this lead is 0.195 ohms. To this one would add the 0.1 ohms for the appliance itself to give a limit of 0.295 ohms. When testing this lamp for earth Continuity, if the resistance was say 0.22 ohms, then one would pass the appliance. 

On Pass/Fail PAT Testers, there is an automatic allowance made for the mains lead. Typically the limit is set to around 0.25 ohms to allow for up to 5m of mains cable. On some Pass/Fail PAT Testers, allowance can also be made for longer cables, say when testing 10m or 20m extension leads.