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RCD tripping - mcb's off (split-load unit)

Discussion in 'Electrics UK' started by Stattman, 8 Mar 2014.

  1. Stattman

    Stattman

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    Hi Forum,

    I'm in a bit of a quandary..

    I have a split-load consumer unit, 5 MCBs on left-bank with RCD and 5 MCBs on right-bank with RCD.

    I'm having some building work done and during the week, one of the MCBs on the right-bank tripped off. I thought it would be ok, as the work is affecting the old out-house, the remaining MCBs on the right-bank keeping me going.
    No issues at all on the left-bank.

    However today the RCD tripped off on the right-bank, which means a bigger problem now.

    I've unplugged everything I can think of in the house and have switched off all MCBs on the right-bank but the RCD refuses to engage. I was planning to bring each circuit back up one-by-one...

    The left-bank continues to operate fine...

    I'm struggling as it's over the weekend and was wondering two things:

    1) any thoughts on the above scenario points which I could check on?
    2) are there any basic testing kits / devices recommended that I could get from B&Q to help in diagnosis? I have seen something like an earth tester...

    The builders will need to take a look early next week, but any advice is appreciated...

    Regards
    Mark.
     
  2. flameport

    flameport

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    There is a fault between neutral and earth on one or more of the circuits. Probably somewhere in the area where building work is being done, although could be anywhere, or more than one fault etc.

    Not likely.

    The only realistic option without test equipment would be to switch off all of the MCBs which are supplied from the affected RCD, disconnect the neutral for one of those circuits, put that wire into a separate insulated terminal block, then see if the RCD will switch on.
    If not, replace neutral, and repeat with the next circuit and so on.
    Once found, switch off, remove the line wire for the affected circuit from the MCB and put that into a separate insulated terminal block, so that both line and neutral for that circuit are not connected to anything.

    However this does involve the CU being opened with some live parts exposed, therefore if not absolutely sure what you are doing, do not attempt this.

    Once resolved, get a new CU with all RCBOs instead, which will ensure problems like this never happen.
     
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  4. securespark

    securespark

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    Yup.

    It is possible that the circuit that tripped is the guilty one.

    There may be fault involving water which tripped the MCB. So you left it off, but now the whole bank has gone down as the RCD has sensed a fault current > its value between the neutrals on that side and earth.

    If by your comments you mean that the outhouse is supplied by the MCB that tripped originally, then this could mean a fault with a junction box suffering ingress, a damaged (underground?) cable or something you have forgotten (or are unaware) is attached to the circuit. This could include garden sockets, garden or security lighting, pond pumps, greenhouse supplies, CH boiler, water or space heaters, kettles, etc....
     
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  5. Stattman

    Stattman

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    Hi!

    Many thanks for your feedback!

    Just to let you know: there was an outside plug which had been partially disconnected by the builders and covered with a plastic bag.
    So the only reason why I can think it tripped over the weekend is perhaps outside condensation.

    Taking precautions: I rewired the unit correctly, and brought the socket in doors. (Kitchen door has to stay open). The RCD now re-engages...

    It really does demonstrate how delicate the circuits can be and thankfully I have had my electrics regularly maintained. Now it's back to the builders to make good as they continue.

    Thank you again!

    Regards
    Mark
     
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  7. securespark

    securespark

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    Good stuff!
     
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