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Shed RCD tripping out power in house....

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  • 27-04-2011 9:34am
    #1
    jarvisob
    Registered Users Posts: 35 ✭


    Alright so i'v recently wired up a shed with two socket circuits and one lighting circuit. All 3 circuits have their own MCB. Also all 3 are fed from an RCD.

    The main SWA cable coming to the shed( 3 x 4sq) is coming from a junction box just outside the house and running into the board in the shed. In the board i have the live and neutral from the SWA going into a bottom of an RCD, with the live coming out of top of RCD and feeding bottom of all the MCBs. The neutral coming out of top of RCD is connected to the only neutral bar in the board.

    Right so when i have power switched on at the main board in the house i have power and the incoming (bottom) of the RCD. Even with all the MCBs down, whenever i switch up the RCD the power in the house is cut.

    What could the possible reasons for this be? If the swa cable was damaged then is would trip before i put up the RCD would it? Many thanks in advance....

    p.s The shed is not on it's own circuit from house...there was a junction box where a circuit feeding an existing smaller shed was split to provide power to an outside socket. This shed will have no load in future except a light so i connected up the live, neutral and earth from the new cable to the connectors in the junction box.


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    #2
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Its tripping the RCD in the house so? If the circuit out to the sheds is through the house RCD then there was not much point in putting another RCD in the second shed, except as an isolator maybe.

    But anyway, it could possibly be a neutral earth short in the second shed after the RCD, assuming you have the polarity into the shed board correct? If it was reversed you would have neutral through the MCBs and live to the neutral bar in which case any neutral earth fault would now be a Live-Earth fault.

    To test for neutral earth problem, first unplug anything if anything is plugged in and retry RCD. If its not that then remove all the circuit neutrals from the neutral bar in the problem board in shed and turn on the RCD then, and see if that works. If it stays on then switch it off each time and connect one neutral at a time and try again to find the problem one if that is the problem.

    If one does cause it to trip then check that circuit out, check socket boxes, flourescent light fitting etc.


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    #3
    jarvisob
    Registered Users Posts: 35 ✭jarvisob


    Cheers Robbie yeah it's tripping the RCD in the house...the one in the shed was used as an isolator really. Yeah it sound likely that it is a neutral-earth fault. I'm going there again in a few house so will try sort it then. Any further questions later and I may ask again. Much appreciated!


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    #4
    M cebee
    Closed Accounts Posts: 4,431 ✭✭✭M cebee


    could be a fault in shed

    mixup at the joint outside

    problem with existing circuit even-but prob not

    when you series the rcd's the ones upstream can trip first anyway


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    #5
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Either way, if you think of the shed RCD as simply a 2 pole isolator, and when its being switched on, its tripping the house one, so the likely problem is a circuit after the shed RCD. As the OP said he had all MCB`s off (in the shed i assume) then its likely a neutral earth problem in the shed, again assuming the polarity in is correct.

    It should be also causing the shed one to trip too as you switch it on? Unless there is just enough inbalance to add to the upstream house RCD slight imbalance that may be on that one already.


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    #6
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    M cebee wrote: »
    when you series the rcd's the ones upstream can trip first anyway

    They can, as all inbalances add up as you go upstream with series connected ones if each upstream one has its own circuits fed from it.


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    #7
    jarvisob
    Registered Users Posts: 35 ✭jarvisob


    Right I'v been able to rule a few things out but the whole thing is after getting more confusing! Will try explain everything as best I can.

    1) Using my meter I got continuity between the L and N at the bottom of the RCD ( and at top if switched on ) so thought "faulty RCD". Disconnected the wiring and tested for continuity again...none.
    2 ) Now tested for continuity between the incoming live and neutral and yes it was there.
    3 ) Went back and checked junction box to make sure polarity was correct. Everything looked ok so thought " Cable must of got damaged". Disconnected from connectors and now tested cable - no continuity!
    4) Now with my new cable disconnected I tested for continuity between the Brown and Blue connectors in the junction box and yes there is continuity between the two!

    So it's nothing in the shed and nothing to do with the new cable I'v ran. Something to do with this junction box and how it is wired back to the house. Don't know who wired this so no one to verify anything.

    Few questions though...how can there be continuity between the brown and blue without constantly tripping out RCD in main house board? Could it be there are both live and earth is somehow used as a neutral?

    V confusing.... only get a chance to spend both two hours or so at a time there so had to stop and leave it again til tomorrow.

    The junction box at the moment is holding connectors with are used to tap of the cable going to old shed and feed an outdoor socket . So just to summarise each connector in juction box has 3 cables in each - 1)the feed from house, 2)the feed onto shed and 3)the feed to outdoor socket.

    Where to start?


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    #8
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    I assume you have the RCD in the house off. But you will get continuity through any appliances plugged into the house sockets.

    Continuity between the neutral and earth on the load side of the shed RCD is what you should be first looking for first, with the shed RCD off.


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    #9
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    jarvisob wrote: »
    Few questions though...how can there be continuity between the brown and blue without constantly tripping out RCD in main house board? Could it be there are both live and earth is somehow used as a neutral?

    RCDs trip when some current going out on the live is missing on the neutral, the difference has to be 30ma or more (0.03 amps)

    This is to protect people from electrocution from Live to Earth, as any current going to the earth will now be missing from the neutral conductor through the RCD, and the imballance trips it.

    As i said in the previous post, its likely the continuity from live to neutral is through appliances still plugged into the house sockets. Continuity live to neutral wont be tripping the RCD.

    If the house one only trips when you put on the shed RCD, then the problem is in the shed most likely, and as you said you had all MCBs off, its probably a neutral to earth fault.

    But it could be live to earth in the shed, but if you unplug anything in the shed, switch the lights off in it, switch off the shed MCBs, and if the RCD in the shed still trips the house breaker, then its likely a neutral to earth fault in the shed.

    Do a continuity test between neutral and earth and live to earth in the shed on the load side of the RCD with the shed RCD off.


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    #10
    jarvisob
    Registered Users Posts: 35 ✭jarvisob


    robbie7730 wrote: »
    RCDs trip when some current going out on the live is missing on the neutral, the difference has to be 30ma or more (0.03 amps)

    This is to protect people from electrocution from Live to Earth, as any current going to the earth will now be missing from the neutral conductor through the RCD, and the imballance trips it.

    As i said in the previous post, its likely the continuity from live to neutral is through appliances still plugged into the house sockets. Continuity live to neutral wont be tripping the RCD.

    If the house one only trips when you put on the shed RCD, then the problem is in the shed most likely, and as you said you had all MCBs off, its probably a neutral to earth fault.

    But it could be live to earth in the shed, but if you unplug anything in the shed, switch the lights off in it, switch off the shed MCBs, and if the RCD in the shed still trips the house breaker, then its likely a neutral to earth fault in the shed.

    Do a continuity test between neutral and earth and live to earth in the shed on the load side of the RCD with the shed RCD off.

    Ah ha... Thanks a million. Makes more sense now...first time doing any electrical work in bout two years since qualifying and brain is VERY rusty with fault finding. Thought when house RCD was down that there should be no continuity between L and N out side.

    Right will check for N-Earth faults and L-Earth faults tomorrow when there.... Cheers Robbie!


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    #11
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    jarvisob wrote: »
    Ah ha... Thanks a million. Makes more sense now...first time doing any electrical work in bout two years since qualifying and brain is VERY rusty with fault finding. Thought when house RCD was down that there should be no continuity between L and N out side.

    If you unplugged everything in the house, or switched off all MCBs controlled by the house RCD, and the same with the shed one, then there should be no continuity then between live and neutral. But you dont need to worry about that anyway. Thats all normal.
    Right will check for N-Earth faults and L-Earth faults tomorrow when there.... Cheers Robbie!

    Just switch off shed MCBs, and the light and anything else in the shed. Then see if switching on the shed RCD still trips the house RCD. If it does then its likely neutral earth fault. If it now doesnt trip it, then switch on each mcb in shed and the light etc and see.

    If the shed one does trip the house one, the shed one should also be tripping as you put it on.


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