This test is carried out on Class 1 appliances and looks to see if there is any leakage current flowing from the Live Wire to the Earth wire of an appliance. This current is a measure of any significant breakdown of insulation.
This test is done at full mains voltage ie at 240V. The appliance is plugged into the tester and while 240V mains is being applied, the tester measures the current flowing through the Earth wire and presents this reading.
As the earth wire is known as the Protective Conductor, this test is sometimes referred to as the "Protective Conductor current" test.
Great care needs to be taken during this test as full mains voltage is present and the appliance will start working.
This test is carried out on Class 2 appliances. In every aspects it is identical to the Earth Leakage test except one. Since Class 2 appliances do not have an earth connection, the test lead is connected to any user touchable metal part when this test is done. For this reason, it is called the Touch Current test as it measures the current that would flow through a person if they were touching the appliance.
Sub Earth Leakage (or Substitution Earth Leakage)
The Earth Leakage and the Touch Leakage tests require full mains voltage. For this reason, battery powered PAT testers do not provide this test. However some battery powered testers provide a much lower test using 40V AC. The resulting leakage current is multiplied by 6 to give the current that would be flowing if 240V mains were to be used. This test is used for Class 1 and Class 2 appliances.
One of the disadvantages of the Sub Earth Leakage test is that it does not power the appliance. Faults that may only be obvious when full mains is applied, will not be shown up by this test.
This blog post explains the different terminology used when describing Earth Leakage tests on appliances.