The Megger Team continue their monthly Made2Measure by answering your questions about RCD Testers.
I’ve been having problems testing RCDs on circuits powered from 110V site transformers. My RCD tester simply doesn’t seem to work properly in these situations. What do you recommend?
The most likely reason you’re having problems is that the secondaries on these site transformers have an earthed centre tap, so you’re actually working with a 55-0-55 V supply. Your RCD tester is, therefore, seeing just 55 V, and there are many types that don’t work properly with such a low voltage. It looks like you’re going to need a new tester but, before you decide which to buy, be sure to check the data sheet carefully! Whilst you’re buying a new instrument, another thing you might want to watch for is its frequency range for testing RCDs. The mains supply is, of course, accurately controlled at 50 Hz but, if you ever have to test RCDs on a system powered by a generator, you may well find that the frequency is a long way from this. If you’re looking for a dedicated RCD tester, then Megger’s RCDT320 would be a good choice. If you prefer a multifunction tester, then one of the models in Megger’s MFT1700 range will meet your needs. These instruments will test RCDs reliably at voltages down to 48V and at frequencies anywhere between 46-65 Hz.