Test Leads - insulation resistance testing

Failure to check that the instrument test leads are not open-circuited can result in the maximum possible value that can be indicated on the test instrument being unwittingly recorded as the insulation resistance value for the circuit (or group of circuits) under test.

If the true value for the circuit(s) was below the minimum value permitted by BS7671, a risk of electric shock, and possibly even fire, could remain undetected, posing a danger to users of the installation and surrounding areas.

Solution

The insulation resistance measured in accordance with the method given in Regulation 612.3.2 is considered satisfactory if the insulation is not less than the appropriate value given in Table 61 of that regulation.

Checking test lead continuity can be achieved by shorting or connecting the test probes of clips together and measuring the resistance of the leads, using the low resistance setting or appropriate button on the test instrument. The resistance measured should be the resistance of the leads and connections, and therefore, will generally be less than 0.5 Ohms.

To ensure that the test leads are operating correctly, such continuity checks should be undertaken immediately prior to carrying out insulation resistance measurements and again after the recording of a series of insulation resistance measurements.

Regulation 612.3.2 (part of)

The insulation resistance measured with the test voltages indicated in Table 61 shall be considered satisfactory if the main switchboard and each distribution circuit tested separately , with all its final circuits connected but with current-using equipment disconnected, has an insulation resistance not less than the appropriate value given in Table 61.