Discuss RCD tripping, problems identifying fault in the Electrical Forum area at ElectriciansForums.net

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leep82

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I received a phone call from a friend the other night telling me his socket werent working and whenever he tried to reset the MCB the RCD would trip immediately. As i couldn't get to him that night i asked him to unplug anything in the affected sockets and then try to reset the MCB, hoping that it may have been a faulty appliance. It wasnt!

I managed to get round to him the next day and repeated the procedure myself for piece of mind only to get the same results. I asked my friend if he had been doing any work around the house just before he noticed the MCB had tripped, which he told me he hadnt, although he had attempted to change a halogen lamp in his kitchen.

The board is a split load type with 100ma time delay RCD protecting two lighting circuits and a 30ma RCD protecting all other circuits. I started to test the circuit and although when doing my insulation resistance test L-N was low. I didnt think it low enough to create a short ( think it was over 10Mohms ), L-E and N-E were clear. I connected the circuit back into the board and just thought id check once more to see if the MCB would reset, and it did. Now i hadnt done any other work other than disconnecting and then reconnecting the circuit from the board prior to my tests. I checked to see if either of the cables were damaged entering the board, wondering if i had inadvertantly disturbed them, but they were fine.

Really struggling to explain this one so any thoughts welcome
 
The only thing you can do is ask your friend to keep a record of when it trips - that said L-N issues tripping a MCB should be easier to find!
 
From your description it is not entirely clear what tripped first.

Did the MCB for the sockets trip - suggesting a L to N fault somewhere?
Or actually did only the 30mA RCD trip, and the MCB was subsequently opened in order to be able to reset the RCD?
 
Can you test the RCDs too?
For what it's worth - I have replaced an mcb which liked tripping without due cause (that I ever found). It's rare though.
 
If it was the MCB tripping that would suggest a L-N fault rather than an earth fault. It's possible that the substantial fault current will have blown the fault clear causing damage if continued attempts were made to reset the mcb.
 
Did you look inside all of the sockets as I've had it where a damp wall had corroded the back box and socket but wasn't tripping all the time just intermittently.
 
Cannot fathom what was tripping,one of the two RCD's on closing the MCB,or the MCB itself...

It might just be a sign,like was prevalent in the old testament...
 

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