The substitute leakage test on the PT100 may not give a pass on larger class 1 equipment as the actual pass value for this is 3.5mA – the PT100 is pre-set to the lower value 0.75mA and you will need to determine if this is a pass or fail, note this can’t be altered on the instrument (see to ch15.5 of the 4th Code of Practice).
Substitute 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 (eg as with some washing machines) the substitute leakage method will measure both leakage currents. 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.
According to ch15.5 (of the 4th Code of Practice), the insulation test should be applied (which it is with a PT 100) and according to ch15.6 the protective conductor/touch current measurement is an alternative or complementary test to be carried out if the insulation test in ch15.5 can’t be performed or gives suspect results.
Note, many of our instruments that do a powered leakage test (eg PT250+ or Apollo):
Differential leakage (generally a class l test – the Protective Conductor Test) 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.
Direct leakage (generally a class ll test – Touch Current test) is a direct measurement of the leakage in the earth using a 2kOhm body model – for this to be performed correctly the earth probe needs to be used.