PAT250 Portable appliance testing (PAT) instrument

Earth continuity is applicable for Class I equipment and extension leads. The purpose of the test is to ensure that the earth terminal has a low resistance connection to the conductive metal casing of the appliance by electrical bonding. Two test methods are available, and the technique used should depend on the circumstances. 

Take care when measuring earth bond on appliances where the construction includes exposed metalwork having casual contact to earth. This metalwork is primarily protected by double or reinforced insulation. The earth connection is only classed as ‘fortuitous.’ So the resistance value is unlikely to be as low as truly bonded metalwork, and a test performed on this unbonded metalwork may give misleading results. Examples of this construction may include a steam iron’s soleplate or a kettle’s metal chassis. A high current bond test using up to 26 A may also damage casual-contact components. You should perform a low-current test first to identify potential problems. 

To pass the earth continuity test, the measured value should not exceed those shown: 

If the resistance R of the protective conductor cannot easily be measured, the table below provides nominal cable resistances per metre length for various types of flexible cable. 

You should identify the cable and measure its length. You can calculate the protective conductor resistance using the app PAT toolbox or by using the table and a calculator. The Megger PAT400 testers have built-in compensation for resistance in long supply cables. 

This test is an alternative to an insulation test. We recommend its use when an insulation test could damage the asset or when the insulation test results are suspect. For this test, you energise the asset at its normal operating voltage, and any current flowing to earth is measured. For Class I, this is to the earth pin of the plug; for Class II, this is to any accessible conductive surface using a remote probe.  

Note: a low voltage insulation test should be performed before any energised test, such as the touch current test, to identify any potential danger from poor insulation and shorted connections. This test is only available on the more sophisticated portable appliance testers, such as the Megger PAT. 

It is essential to ensure that the asset is safe to run during the test. For example, if testing a kettle, it should be part-filled with water so as not to damage the element during this test. 

This test is useful where it may be deemed dangerous to run the asset during a protective conductor or touch current measurement. This test uses 40 V AC, which will not operate the asset during the test. This test can be battery powered, removing the need for a mains supply during testing. 

In all cases, the current is measured within five seconds after the application of the test voltage; the values must not exceed those given below. 

A functional test is carried out during the “load” test or the mains-powered protective conductor or touch current measurement. This test will determine the following: 

This can be a good indicator of future problems and potential failures in an appliance. Problems, like worn bearings on a drill, would probably result in increased current drawn from the supply and, therefore, an increase in the VA reading.