Q: I’ve just taken my City & Guilds 18th Edition exam and I noticed that the section on unwanted tripping of RCDs (531.3.2) says that I’ll sometimes need to measure earth leakage currents. How do I do this?
A: The regulations say you should select RCDs to limit the risk of unwanted tripping. To ensure this, the total leakage current (which is the leakage current in the protective conductor plus the earth leakage current) should not exceed 30% of the RCD rated residual current. The best way to be sure is to measure the leakage current with an earth-leakage clamp meter, such as the Megger DCM305E.
All you have to do is clamp the meter around the line and neutral conductor of each circuit, and it will measure any current imbalance – which is the leakage current – and show it directly in milliamps. Alternatively, you can clamp the meter around the incoming tails of the consumer unit/panel, then switch off all the MCBs. If you switch the MCBs back on one at a time, you’ll be able to see the leakage current associated with each circuit. The Megger DCM305E is a very good clamp meter to choose, because it can also measure AC currents up to 100A, so you’ll find many more uses for it in addition to earth leakage testing.
For more information visit: https://uk.megger.com