Use of an A.C. Milli-Amp Clampmeter for Fault Finding Purposes

In my experience undertaking periodic inspection and testing, I have always been interested in non-invasive ways of finding out whether the electrical installation under scrutiny has excessive leakage due to deteriorating wiring insulation or connected equipment. If we look at a simple domestic installation, we can use an AC milli-amp clampmeter to measure the amount of leakage in the entire electrical installation with everything connected and working. This test can be seen as a non contact way of doing an insulation resistance test.

On a single-phase installation, the device can often be simply clipped around both the Line and Neutral tails and will pick up the difference in current flow between the two conductors ie, what is going in on the line and returning on the neutral - the difference being the earth leakage. On a healthy electrical installation the leakage will be usually in the order of 3 to 8mA and will be dependant on what is connected at the time of test. Clearly, this level of leakage would not normally trouble a 30mA RCD, whose tripping range is in the order of 17mA to 27mA, dependant on the make. However, where nuisance tripping is a problem then the milli-amp clampmeter comes into it's own in determining which circuit the high leakage is present on by turning each circuit off one by one until the leakage level detected drops.

The device can also be used on the main earth conductor or individual cpc's to see what leakage current is flowing. Another use for the milliamp clampmeter is to determine the earth leakage on peices of I.T. equipment, which is particularly useful when high protective conductor currents are anticipated at the design stage when considering Section 543.7 of the IET Wiring Regulations. Other loads like washing machines, cookers and fridges are also leaky by nature and can sometimes cause nuisance tripping problems in a domestic installation.