1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Dual RCB CU, RCB trips but MCB is not tripping.

Discussion in 'Electrics UK' started by platforminc, 4 Jun 2018.

  1. platforminc

    platforminc

    Joined:
    26 Aug 2007
    Messages:
    379
    Thanks Received:
    4
    Location:
    London
    Country:
    United Kingdom
    Hi All,



    I hope I am using the right terms here, I have a situation where my consumer unit is 15 way, and I believe MCB are the breakers which trip when there is a fault with a specific circuit, and RCB is the main big one which controls either half of the CU or a group of MCB’s.

    Now, if my assumption above is correct, what I am experiencing at the moment is situation whereby one of the RCB’s trip, but no specific MCB trips, so I don’t know what circuit is faulty and causing the RCB to trip. Whenever this happens, half of the house has no electricity and once I pull the RCB back up, everything works ag again, then randomly in the space of 1-2 weeks the same thing happens again, there is no definitive pattern, something just makes the RCB trip.



    The RCB which trips is the one that is circled in the picture, and everything to the right of it stops working until I pull it back up again.




    CU.jpg
     
    • Thanks Thanks x 1
  2. Sponsored Links
  3. plugwash

    plugwash

    Joined:
    28 Mar 2004
    Messages:
    9,140
    Thanks Received:
    307
    Country:
    United Kingdom
    MCBs and RCDs detect different things. MCBs detect overcurrent. RCDs detect imbalance between the currents in the live and neutral conductors.

    For whatever reason it has become accepted practice to have only a couple of RCDs for the whole installation. This is cheaper than using a RCBO (integrated MCB/RCD) for every circuit but it can turn into a nightmare when you have an intermittant fault.

    Performing insulation resistance and earth leakage tests on all equipment in the house may find the problem, but then again it might not.

    Moving circuits between the two RCDs may be an option, but it may be inconclusive especially if the actual problem is not one device but a small number of slightly leaky devices adding together.

    Turning off a MCB may or may not stop the devices connected to it from tripping the RCD because MCBs only disconnect the live and RCDs can be tripped by a neutral to earth fault.

    A final option is to rebuild (or replace) the CU to have a RCBO for each circuit from the problem group. If the problem was a bunch of small leakages adding together this may well fix it, if not it will at least narrow the problem down to one circuit.
     
  4. Taylortwocities

    Taylortwocities

    Joined:
    28 Jul 2006
    Messages:
    19,441
    Thanks Received:
    1,959
    Location:
    Oxfordshire
    Country:
    United Kingdom
    What are the circuits that the RCD protects? It looks like one of those might be the garage? If so, I would start by looking carefully at things like outside electricity things like pond pumps, lights, etc that could be run from the garage itself.

    Especially if the tripping co-incides with wet weather. It only takes a tiny bit of moisture to trip an RCD.
    The long way is to by disconnect the garage circuit from the board (that means the live and the neutral)
    The short way is to find an electrician with an insulation resistance tester.
     
  5. EFLImpudence

    EFLImpudence

    Joined:
    7 Jul 2010
    Messages:
    33,119
    Thanks Received:
    3,605
    Location:
    Retired to:
    Country:
    Portugal
    Sledge hammer and nut springs to mind.

    Fourteen RCBOs will be quite expensive.

    I think the first course of action is to determine the cause.
    If it is accumulated leakage then this should be fairly apparent as there will presumably be substantial leakage most of the time.
    You need a meter to measure this.
    If not then it must be an intermittent fault - might not easy to find but has to be found.

    Unless you, platforminc, can find this by trial and error, then, either way, you need an electrician.
     
    • Like Like x 1
  6. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    43,802
    Thanks Received:
    2,770
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    I suspect that the original 'whatever reason' for it becomi ng 'accepted practice' (after a previous period when a single RCD was quite accepted) was that it became 'accepted practice' at a time when RCBOs for domestic CUs either didn't exist at all or were not readily available (and/or at at sensible prices).

    A good few years down the road, the differential cost of MCBs and RCBOs remains a major consideration. That might eventually change - but, in the meantime, the nightmares to which you refer are fairly rare events. It's also the case that RCBOs themselves are not totally without 'downsides' when it comes to fault-finding, since those currently used domestically do not give an indication of whether they have been tripped by over-current or residual current.

    Kind Regards, John
     
    • Thanks Thanks x 1
  7. platforminc

    platforminc

    Joined:
    26 Aug 2007
    Messages:
    379
    Thanks Received:
    4
    Location:
    London
    Country:
    United Kingdom
    Thanks everyone for those contribution, the circuits under the RCB are as follows.


    Garage ( 4 x sockets and 3 x lights) and ip 66 outside socket.
    Garden lights (3 x up and down light and 1 flood light)
    Front door lights (2 x up down lights)
    Drive over lights (5 x drive over lights and 2 x bulk head lights)
    Kitchen sockets
    Kitchen lights
    1 free


    I only have basic electrical knowledge and don't have any special tools. What I have done is isolated everything bar kitchen lights to see what's causing it. A friend did say it sounds like earth touching neutral somewhere.
     
  8. John D v2.0

    John D v2.0

    Joined:
    26 Aug 2016
    Messages:
    6,315
    Thanks Received:
    876
    Country:
    United Kingdom
    Regarding two RCDs providing adequate separation.
    Presumably this is for minimising inconvenience due to a single fault causing a protective device to trip. Since the minimal install would usual be two lighting circuits and a socket circuit, the only reason the lights aren't on the socket circuit is because of the differing over current requirements. Hence the minimum two rcds.
    So a dual RCD meets the regs so is usually most cost effective in the absence of other customer requirements. If the customer has equipment that must remain powered up and other high risk circuits, rcbos may be more appropriate
     
  9. Taylortwocities

    Taylortwocities

    Joined:
    28 Jul 2006
    Messages:
    19,441
    Thanks Received:
    1,959
    Location:
    Oxfordshire
    Country:
    United Kingdom
    I would suspect the garden lights or the drive-over lights. As i said, you would need to properly isolate each circuit*, one at a time and see if the tripping goes away.
    *To do this you need to remove the live and neutral connections inside the consumer unit. Just turning off the MCB only isolates the live side of the circuit and will not help in identifying the faulty circuit.

    This is a rather hit and miss method. Unless you have the proper test equipment, and/or are prepared to spend many weeks of trial and error, then get an electrician in to find it for you.
    Could be, but more likely to be water/damp in one of the outside light fittings or outdoor junction. I have seen many problems like this. For example, a bollard light fitting that was partof a garden illumination. Ants had built a nest right up inside the bollard, right over the terminal block inside. Every time the weather got wet, the nest got just a bit damper and the RCD tripped.
     
  10. Sponsored Links
  11. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    43,802
    Thanks Received:
    2,770
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    If I understand the OP correctly, the RCD trips are occurring once every 1-2 weeks and, when the do occur, the RCD can be reset immediately (and then remains reset for 1-2 weeks). If that is the case, I think that the OP probably should be warned that diagnosing the problem may not be a trivial task even for "an electrician with proper test equipment".

    Having said that, I have to agree with you that water/dampness in 'something outside' is by far the most likely culprit (although a kitchen appliance is far from impossible).

    Kind Regards, John
     
  12. aptsys

    aptsys

    Joined:
    16 Sep 2007
    Messages:
    1,784
    Thanks Received:
    134
    Location:
    Cambridgeshire
    Country:
    United Kingdom
    Indeed, I would actually suggest this can make fault finding more difficult depending on the configuration of the consumer unit.
     
  13. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    43,802
    Thanks Received:
    2,770
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Quite - that's obviously what I meant by a 'downside' (of RCBOs).

    I can recall at least one example - when a friend and myself spent ages hunting for a postulated intermittent L-E or N-E fault which was causing occasional RCBO trips, only to eventually discover that the cause was an intermittent L-N fault in a kitchen appliance.

    Kind Regards, John
     
  14. platforminc

    platforminc

    Joined:
    26 Aug 2007
    Messages:
    379
    Thanks Received:
    4
    Location:
    London
    Country:
    United Kingdom
    Hi all.

    In the absence of test equipment, after a heavy downpour can I open up the outside lights and check for water ingress. There are a lot of junction boxes which means a potential point of failure.

    It would just be nice to be able to isolate rather than opening up stuff willy nilly.

    I would also look at the option of disconnecting live and neutral from individual mcb. Funny thing is that during heavy rains, it works as normal, no trips. It's so random. Something triggers it which is a combination of some unknown factors.
     
  15. platforminc

    platforminc

    Joined:
    26 Aug 2007
    Messages:
    379
    Thanks Received:
    4
    Location:
    London
    Country:
    United Kingdom
    I am also thinking would it be best to call an electrician when it's raining to fault find.
     
  16. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    43,802
    Thanks Received:
    2,770
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    You could - but from what you go on to say it sounds as if the trips are not directly chronologically-related to heavy rain, so that approach might not succeed.
    That would certainly narrow things down and, as has been suggested, it would make sense to start with the 'outside' circuits (one at a time). However, if I'm understanding correctly that it can be 1-2 weeks between trips, you would probably have to do without some of those circuits (e.g. garden lights) continuously for several weeks before you narrowed the problem to one circuit - assuming you could. The 'worst-case' scenario would probably be that the combination of small leakages (e.g. due to water ingress) in two or more circuits was causing the problem, but with no one circuit having enough leakage to cause a trip. In that scenario, you might find that you could get rid of the problem by disconnecting two or more circuits (individually)
    As above, if that is the case, then examining things during, or soon after, heavy rain might not be fruitful. It's the RCD trips which are due to "a combination of unknown factors" than can be the greatest nightmare.
    As above, what you say suggests that might not necessarily be fruitful. I suppose what you want/need is to get an electrician there as soon as possible after a trip occurs.

    An electrician would also (hopefully) test the RCD. It's just possible (although unlikely) that it has become 'trigger happy' and is tripping when it shouldn't.

    Kind Regards, John
     
  17. ban-all-sheds

    ban-all-sheds

    Joined:
    27 Aug 2003
    Messages:
    69,784
    Thanks Received:
    2,857
    Location:
    London
    Country:
    United Kingdom
    Was that just a bit of careless wording, or what you really think you would do?

    If the latter it shows a lack of knowledge of what the inside of a CU is like - do be careful.
     
    • Like Like x 1
Sponsored Links
Loading...

Share This Page