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Leakage testing

PostPosted: Thu Feb 23, 2012 12:17 pm
by MikePope
I use a Primetest 250. I had a situation with a washing machine where the substitute leakage test gave 4mA, ie: a fail. So I repeated the test as a differential test by using a mains supply into the tester. This gave 1.8mA, which is OK. As it was a newish machine with electronic controls, inc On/Off, I presumed it is the RFI filtering on the incoming mains causing the leakage, thus the machine is OK. Is my presumption a safe one and is my process of only using the differential test if the substitute test fails a safe one?

Re: Leakage testing

PostPosted: Tue Feb 28, 2012 11:30 pm
by Jim Wallace
Hi Mike

Substiute leakage is measured between both line and neutral and the protective earth conductor. It is similar to an insulation resistance measurement except that a test voltage of 40V/50Hz is used rather than 500V DC. The measured current is then automatically scaled by the tester to show the leakage that would present at mains voltage (i.e. scaled by a factor of 6). If the appliance under test has filter components between both line-earth and neutral earth (as appears to be the case with the washing machine) the substitute leakage method will measure both leakage current. The IET Code of Practise includes the substitute leakage method but does not have limit values that take into account the additional leakage path between neutral-earth.

Differential leakage is measured at the normal supply voltage with a normal supply connection i.e. it is a true indication of the leakage under normal operating conditions and is therefore a much more robust measurement. If the differential leakage measurement value is less than the limits specified by the IET CoP then the appliance is OK.

Hope this is clear......