In Earth Leakage Testing for Class I appliances, the amount of current which leaks through the insulation (No insulation is perfect; some current, however small, will leak through) is measured.
In Touch Current Testing for class II, because there is no earth, two different methods of testing can be used. One is a probe test where the Earth Bond Lead is used as a probe to measure any current leaking onto the enclosure or casing.
The second method is called a ‘Differential’ test; the tester measures how much current it is supplying to the appliance; it measures how much current is being returned and calculates the difference. The pass/fail limits are on page 77 table 15.3 of the IEE Code of Practice 3rd edition.
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