Electrium consumer unit and RCBO trip why?

Discussion in 'Electricians' Talk' started by Joseph William Smith, May 24, 2023.

  1. Thank you for that, I do understand the 2 way circuit etc, but there are 2 circuits one up one down where does the borrowing come in? if this is as you say (can both be combined onto one RCBO) to get round this, problem is newly decorated property and already tenanted doesnt help) Whats the proper solution? Thanks for your reply.
  2. Thanks for reply Bob, that was my first thoughts because if fitting is removed it doesn't trip, I'm not sure whether we could change that device now tbh, certainly not until I know 100% But 2 circuits 1 up and 1 down in this board as it was previously?
  3. adgjl

    adgjl Screwfix Select

    Not really relevant. You need to be registered with a scheme or involve the local council building control to replace / install a consumer unit in a domestic property, especially a rented property. I am guessing you don’t have insurance to cover this work either.

    In terms of testing, it should have been tested before energizing, as per the current edition of BS7671. The “bang test” where you assume all is good if it doesn’t go bang was obsolete many years ago, and for good reason. This is a simple fault, and should not take more than a few minutes to identify, although it may take longer to fix.

    As has already been said, you need to go back to doing the “dead tests” you should have already done - continuity, polarity, insulation resistance. These need to be done on individual circuits, as well as testing between circuits to prove they are really separate.
    Ind spark likes this.
  4. You mean the old board?
  5. Ind spark

    Ind spark Screwfix Select

    No the new one sorry.
  6. Bob Rathbone

    Bob Rathbone Screwfix Select

    Now change the fitting for a standard 60 watt incandescent lamp and see what happens. That will confirm if its the luminaire or the wiring. Borrowed neutrals were specifically prohibited in the 15th edition, and ever since by the requirement to start and end P & N of a circuit in corresponding ways in the CCU or DB, so use CB no 1 use neutral No 1 (and if you are vigilant, use earth No 1).
    As for being a member of the partially qualified electricians club (Part P and domestic installer), that's going to upset a few on here :), I find myself in a situation where I trained apprentices and taught qualified electricians 16Th Edition, and this scheme has the gall to tell me I can't install a two bit domestic circuit, but I can still install a supply from a 1600 KVA transformer to the busbar chamber. The world is crazy, run by idiots who have the political power but do not understand the industry or those who operate within it. As for Part P, all it has achieved is flooding the market with 'Electricians' who have been on a Part 'P' course, who think they can do the job. The fees charged for enrolment into the scheme are added on to the customers bill, making it too expensive, so they still go an get the guy from down the pub to do the job. Rant over, back to sanity.
    Last edited: May 26, 2023
    Ind spark and arrow like this.
  7. Thats next check Bob, i,ll let you know, most are telling me it will trip, but i haven't tried this yet. Decoration etc will pose a problem if I have to run a separate cable, also tenant in there, doesn't help at all??
  8. MGW

    MGW Screwfix Select

    We see on here so many times where we don't get the whole story, going back to your original post, you seem to say disconnect the one lamp at ceiling rose and all is OK.

    So if it were my job, the question is overload or earth leakage, so for a couple of quid you can get an MCB, so you can rules out earth leakage if it trips, and put the clamp on around the feed cables Testing for live.jpg and see what leakage if any there is, I assume you have already tried with the insulation tester VC60B.jpg and shown no DC leakage, so there is a possibility that the leakage is only there with AC due to some capacitive or inductive linking. You could not have fitted the consumer unit without the test equipment so I am assuming you have it, however it seems common not to test for AC leakage as there is no provision on the test certificate to enter readings for AC leakage, and I know I tend to use the installation certificate as a check list for what needs testing.

    If you did not fit or test the consumer unit, you will still one hopes have the test results, however I note when Part P first came in it said the LABC did not have to pass on their results to the owner, only the completion certificate is required, but still seems odd it would not be passed on.

    If you don't have test equipment then the temp fitting of a tungsten lamp as already suggested seems good. I have only once gone down the inform the LABC I am doing the work, and I know they were keen to view my test equipment, however they did not seem to want to see the calibration certificates, lucky, as I did not have any.

    In the past I have tested items for earth leakage with the PAT tester, mine did have a meter showing the leakage, but it was rather old, I have not a clue how it tests it, but clearly a standard multi-meter is useless.
  9. The Happy Builder

    The Happy Builder Screwfix Select

    If the landing light is tripping the RCBO:

    1. Carry out safe isolation of the entire installation and remove the consumer unit cover, then remove the circuit from the RCBO.
    2. Insulation test the lighting circuit with the live and neutral joined together to earth with the CPC connected along with all earthing and bonding in place and the lamps in place, operating the two way switching controlling the landing light and any other two way switching to ensure all the live conductors are tested to check for an earth fault.
    3. Insulation test the lighting circuit with the live and neutral joined together to the neutral bar in the consumer unit, operating the two way switching controlling the landing light and any other two way switching to ensure all the live conductors are tested with the lamps in, to check for a borrowed neutral.
    4. Reevaluate your choice of RCBO, presumably it a 6 amp Type A RCD and Type B MCB, does it need to be a 6 amp Type A RCD and a Type C MCB or possibly a 10 amp Type A RCD and a Type B MCB or even a 10 amp Type A RCD and a Type C MCB? Zs permitting of course, to accommodate any inrush current.
    5. Reassemble the circuit with the correct circuit protection having completed the dead tests then complete the live tests and all should be well.
    Joseph William Smith likes this.
  10. The Happy Builder

    The Happy Builder Screwfix Select

    There is no such thing as a time delayed 30 mA RCD or RCBO that will comply with the requirements of BS7671 for providing 30 mA RCD protection for a domestic lighting circuit and for cables concealed in a wall.

    So that is not an option.
  11. The Happy Builder

    The Happy Builder Screwfix Select

    If you do find a borrowed neutral then there's two options, rewire the landing light to remove the borrowed neutral or combine the two circuits into one at the consumer unit and supply them from the same RCBO.
    Joseph William Smith likes this.
  12. The Happy Builder

    The Happy Builder Screwfix Select

    There is another possibility, the light fitting may be miswired with the neutral actually connected to the circuit protective conductor.
    Joseph William Smith likes this.
  13. adgjl

    adgjl Screwfix Select

    After 32 posts, I am not sure we have discovered whether the rcbo is tripping on an earth fault or overcurrent yet.
  14. The Happy Builder

    The Happy Builder Screwfix Select

    A privately rented home requires a satisfactory Energy Performance Certificate, the landing light is LED and it would be a fair assumption that most or all the others are as well to get the required EPC rating, the current drawn by all the lights in the house is probably only around one amp.
    Joseph William Smith likes this.
  15. adgjl

    adgjl Screwfix Select

    Indeed, but we don’t know whether turning the switch on for the fitting is giving an earth fault or shorting phase to neutral.
  16. UPDATE ON THIS, Thank you first to everybody in replying, Im very grateful to all on here in trying to help, Just to confirm that I've been advised by many its a shared neutral giving us this problem, Yes the hallway and landing are on a 2 gang with landing feed looped off hallway, I've also been told to double up to get round it, which I don't want to do, having first suspected the led fitting, putting a separate rose/lampholder the problem was still evident in tripping, doubling up (as advised) gets rid of it but I dont want to do that tbh, All the wiring is newish previous test cert I saw was only 5 months ago, it was my brothers agent that told him he had not acted on a previous ADVISORY which had duly run its course, this was the MCU not being up to standard, it had MCBs, no problems were evident other than the mcu (plastic is not permitted in land lorded properties i assume) Yet mine at home is entirely plastic?? So where's the logic in that? as is next doors in fact all this estate was built to that spec. Landlord regs are different but lives are same imo??
    Again thanks for all advice, having established what was suspected by most on here Im looking for the EASIEST solution given the nearest point to the 2 gang up/down switches is the Consumer unit. Just to reiterate nothing is tripping until the landing light is turned on, nothing else trips anywhere all other circuits are healthy. Doubling up gets rid, but Id like what was there before if possible thats separate devices up/down?? Could a separate feed be directed to the landing 2 way therefore splitting and making the 2 switches separate (2 separate switches instead of 1 2gang) Thereby the landing is independent of the downstairs hall feed?
  17. They are I've checked!
  19. Ind spark

    Ind spark Screwfix Select

    You'll need to take a neutral from the downstairs circuit to the landing light and remove the neutral which probably comes from a bedroom light, it wont be easy.
  20. Ind spark

    Ind spark Screwfix Select

    If you have a seperate smoke detector circuit you could take a neutral from that and double that with the downstairs lights.

    Usually theres a smoke on the landing so could be simple.

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