RCD tripping while circuit is off

N

NotSoFast

Hello forum. Couldn't find the virtual pub so I thought I'd dive right in.

I have a problem with the RCD tripping when the circuit is off. I discovered it earlier today while replacing a single socket with a double. Circuit was confirmed off with a plug in socket tester. Pulled the wires together to pull through the old back box and 'pop' the RCD tripped :eek: . I was able to repeat this on demand.

I had a similar problem a short while back which I put down to a faulty tester (an old Steinal). Putting it across live and earth or live and neutral tripped the RCD on both rings. However doing the same with my multimeter had no effect. It just read 230 something volts. At the time I presumed the tester had had it :(

I've had no issues with the electrics since moving in last year and the plug in neon socket tester suggests all sockets are correctly wired.

The house is about 12 years old. CU is the original Wylex with 2 rings, immersion heater and outside lights protected by a 80A / 30mA RCD.

What is the likely cause of the RCD tripping? Is it something I can trace and fix myself (I have a circuit tester, a multimeter and a fair amount of common sense)? If not, what should I be asking an electrician to ensure they have the skills and tools to fix?
 
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The RCD trips because in each instance, you are creating a fault between N and E. There is nothing wrong with it.
 
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A neutral-earth short will trip an RCD

The MCB only disconnects the live, so the RCD will trip even if the MCB is off.

You should really switch off the RCD before working on things, so that both live and neutral are isolated.
 
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N

NotSoFast

Well, you learn something new every day. Good to know this is expected behaviour. Came as a surprise as it never did that in my old house (8 or 9 y.o. CU of similar design).
 
T

tonyelectric

Yep its common enough alright, and quite normal, I would think your old house may have had a 100mA trip (not so sensitive) or the RCD was not working. Anyhow, there is no problem so all is well, Happy New Year
 

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