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Fault or no fault?

Hi,When replacing an ordinary plastic 13a double socket in my kitchen and even after I activated the trip switch to cut off the power to whole of the downstairs power circuit which includes the kitchen,(I left all the other trip switches on)I noticed that when the neutral wire within the socket touched the earth wire,the whole power to the whole of the house tripped.I found this very worrying but wondered if there is an explanation for it?

Thanks very much for all the replies.Wow!! thats a really first class service.I can rest easy now!!!

3 Answers from MyBuilder Electricians

Best Answer

There is no permanent fault present. With the live supply isolated at the MCB for the sockets, the Residual Current Device (RCD) is still monitoring the the neutral and earth conductors of the isolated circuit.

Unlike an MCB which can only protect against overload and Live to Neutral and Live to Earth short circuits, an RCD can detect the flow of very small ammounts of current and so will trip when the Neutral and Earth conductors are shorted together. As worrying as it may have appeared, your RCD has worked as it should have and isolated the supply due to the fault.

Although its a technical document this may be of use if you want to delve deeper into the theory and mechanics of how RCD's work.

Hope this helps and remember to push the test button on your RCD once every three months to make sure its still working correctly.


Answered 28th Nov 2012


The answer to your question is straight forward once you undestand how an RCD actually functions.

An RCD monitors the current that runs through the live conductor and the neutral conductor to earth. So although you isolated the socket circuit via the circuit breaker which means you break the flow of current through the live, the RCD is still monitoring current flow through the neutral conductor.

Now to answer the question: When your neutral conductor touched the earth it caused an immediate imbalance in your RCD and the RCD trips as the nuetral is not isolated from the mains when you switch off single pole domestic circuit breakers.

I hope this was clear enough for you without being over technical.



Answered 28th Nov 2012

This is normal you tripped the RCD which covers several circuits you will find the switch that tripped has a test button on it. RCD(residual current device)detects imbalance between live and neutral when you touched earth and neutral you formed an alternate pathway for neutral creating an imbalance. I have tried to say this in laymans terms to make it understandable but if I have failed rest assured what you have described is supposed to happen.


Answered 28th Nov 2012

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