Non protected MCB tripping RCCB !! How?

Discussion in 'Electrics UK' started by sidewinder, 7 Jul 2009.

  1. sidewinder

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    OK here goes.
    I have a split consumer unit that is wired normally. One of the MCBs on the non protected side (I have checked that the neutral is on the non protected neutral block) trips the RCCB when ever the MCB is reset. The MCB does not trip itself, it just trips the RCCB! So I am unable to have the MCB (remember its on the unprotected side!) and the RCCB set at the same time.

    The MCB is question is on the non protected side so how can it be tripping the RCCB?

    Do I have an earth leak on that MCB?

    Any other help would be much appreciated.

    Thanks.
     
  2. rebuke

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    Have you checked all the neutrals (you mention the one for the circuit protected by the MCB in question, but not the others), as if any neutral is on the wrong side, then you could see strange issues.

    A picture of the inside of the CU might be useful - it may allow people to spot some obvious mistake or whatever...
     
  3. dannyboi2003

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    Whats on the circuit that wont reset, lights, sockets....?

    My money would be on a shared neutral taken from one of the RCD protected circuits somewhere in the house.

    Has the CU just been fitted or have you had any other electrical work done recently?
     
  4. sidewinder

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    I have checked all the other neutrals and they are correctly wired.
    This circuit seems to run some sockets but it is not a ring.

    In desperation I have contaced the previous owners of this house and managed to get some history. Here is the story. The MCB that trips the RCCB was on the protected side and worked fine except that Occationally the RCCB tripped. In an attempt to do some fault finding the previous owner moved one MCB at a time to the unprotected side and left it for a while to see if the RCCB tripped. I asked how occational it was and he said once or twice a month. This MCB was the second MCB out of 4 that he had moved (putting the first one back to the protected side when the RCCB tripped again).

    I have just moved the MCB in question to the protected side and everything seems to work fine. I guess the RCCB will probably trip once or twice a month as it did for the previous owner. Atleast I can now test the circuits.

    I am still confused why a non protected MCB would trip the RCCB! Is this circuit a good one to start with from a testing point of view. Can I infer that there is a fault in this circuit due to its non protected side history or am I missing the point somewhere?
     
  5. ericmark

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    If you look at first diagram I show an earth neutral fault.
    Now on second diagram I show same fault with an extra load going to a item which is not faulty
    This shows how a device on the non protected side can cause a neutral earth fault on protected side to exceed the level where the RCD will trip.

    I have used a program called Multisim which simulates electronic designs but also shows what happens with simpler circuits I have given each wire a resistance under an ohm and loads with ohm resistance.

    So you are looking for a earth neutral fault on the protected side. I would look at heating applances first as many use mineral insulated cables which tend to absorb mosture and many only have switches on the line so although switched off can still give you an earth neutral fault.

    Switch the isolator off on cooker and imersion heater for example.
     
  6. sidewinder

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    Ahh I see, I think!
    Great info. I think I need some specialist sparky test gear really.

    Thanks for the help.
    Cheers
     
  7. ColJack

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    if the breaker tripped the RCD straight away when not on the RCD side, but didn't once returned to the RCD protected side then it sugests that it has a shared neutral with a circuit on the RCD side or that you have a crossed neutral, but that ought to be obvious and more than 1 circuit would have caused the RCD to trip ( the one that it's mixed up with would get no current back on the neutral )

    pull all the neutrals out ( and take this opportunity to put cable numbers on them if they're not already on ) and test for something close to continuity between them.. if you find a pair that have contunuity ( or close enough ) that aren't two ends of a ring, then it's time to do a little digging, or more accurately a little bit of taking socket / switch / light fittings off..
     
  8. sidewinder

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    Thanks ColJack that gives me a good way forward and I fully understand that. Might be a few days though cos have to go away to work till Monday. I will update as soon as I get a chance next week.
    Cheers.
     
  9. securespark

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    eric- if you have a moment, would you be so good as to redraw your diagrams in the context of a split-load board?

    I can't quite follow your diagrams! :oops:
     
  10. ColJack

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    too many resistors to follow.. hurt my head the first time I traced it out too... ;) :LOL:
     

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