You asked: I’ve been having a few problems recently with nuisance tripping of RCDs and RCBOs. In most cases, the devices themselves are fine, so there really is leakage to earth, but finding out where it’s occurring is a real pain – and it’s time consuming. Any suggestions for making life a bit easier?
Megger replied: Sounds like you could do with an earth leakage clamp meter. This is a useful handheld device that you simply clamp round a conductor to measure the current flowing in it. You don’t have any direct connection to the circuit under test, which makes it safe, fast and easy to use. Earth leakage clamp meters, like the Megger DCM300E, can measure really small currents down to a hundredth of a milliamp so, if you’ve got nuisance tripping problems, you could clamp the meter round the protective conductor of the suspect equipment or section of the installation and you can instantly read off the actual earth leakage current but this will be the leakage current in the earth conductor and does not take in to account the parallel paths. Put the line and neutral in the jaws, and you can measure the imbalance between the two and hence the total leakage to earth of the system.
Using this technique you can find the problem circuit and either isolate it or go on to discover if there is a particular problem appliance. By making yourself a test lead that allows the protective earth to by-pass the clamp jaws, as shown in the illustration below, you will then pick up any imbalance between the live and neutral for each appliance on the problem circuit. Simple yet really powerful.
And, when you’ve got a DCM300E, you’ll find it useful in many other applications as well, since it also has high current ranges up to 300 A. This means that it’s a very convenient tool for making rapid and accurate current measurements in AC circuits of all kinds.
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