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rcd controlling circuit

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Jan. 11th, 2012 | 10:27 am

This is for the long tail, because Google failed me when we needed it last night.

Our electrical sockets tripped last night. The fuse box trip switches were all still on apart from a special one labelled "RCD controlling circuit". RCD is Residual Current Device. Turns out that one of our appliances (an electric radiator) had developed an earth-neutral fault, where the earth and neutral wires get connected internally. The RCD detects this and trips out. An electrician located the fault and advised us to dispose of the radiator. Problem solved.


This is not our fusebox but it's very similar.

We could have solved this ourselves, however, had we known how to search for RCD faults. We had carefully turned off all the sockets at the wall and were confused when the RCD kept tripping. It wouldn't trip immediately but would take a few minutes. Even more strangely, when we turned various appliances on, sometimes that would trip the RCD immediately. This caused us to think that e.g. our TV was the problem. Other times it tripped when we had turned several appliances on (ones we had decided were OK), as if we were overloading the circuit. But the RCD doesn't detect overloading, that's what the normal trip switches are for.

Googling didn't help us diagnose these symptoms and so I've written this:

How to search for RCD faults in your home:
  • Unplug everything from the sockets (turning them off at the wall isn't enough!). Turn everything else off.
  • Reset the RCD. If it still trips then there's a problem in your wiring and you need an electrician.
  • Plug things in one by one, and wait for the RCD to trip again. Eventually you should be able to find which one is causing the problem. It's tricky, because the problem may not show up immediately, and the appliances interact. But elimination should get you there eventually. Probably the only reliable method is to turn on everything except one thing, and see if the RCD stays on.

Disclaimers: I'm not an electrician, so to be safe you should definitely call one. We are in the UK, so this advice might not apply where you live.

Our electrician was very friendly. We were in the cellar and our 4-year old daughter had come down to watch. The electrician was talking to himself while he tested the circuits in the fuse box, saying "it must be one of these monkeys". And of course my daughter immediately informed him that there weren't any monkeys in there. He conceded the point and suggested that she was a monkey. She said that actually she was a turtle, and it was Mummy that was a monkey. (This is a nickname thing.)

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