You ask: I was doing a no-trip three-wire loop test on a lighting installation recently and the RCD kept tripping. I thought my test set was faulty, so I borrowed one from a colleague – which was a different make. This time the RCD didn’t trip, but the results were obviously wrong. What’s going on?
We answer: These problems usually occur when testing lighting circuits that include dimmable LEDs, irrespective of what make or type of test set you use. Different test sets and different test methods may give different results, but all the results are suspect. You might think that the solution would be to remove or disconnect all of the LED lamps before carrying out the test but this doesn’t help. The results with the lamps disconnected are different, but still can’t be relied on. In the Megger application laboratory, we’ve investigated what’s going on and we’ve found that it’s the “intelligent dimmers” used with the LED lamps – not the lamps themselves – that cause the problem. The solution is to consider removing electronic dimmers from the circuit before testing, or establish ZS by calculation using R1+R2. Remember that the NICEIC and other regulatory bodies consider calculation of ZS more suitable than live measurement for lighting circuits. Also remember that, even in non-LED applications, dimmers or any other loads that remain in circuit may significantly affect loop test results.