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  1. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    Did a board change today and put in rcbos for mcbs in common areas of a block of flats. Meggered out ok before i started by the way!!

    Rcbo for outside lights didnt hold. Mcb was fine before. All others held fine. So neutral to earth fault right? Also disconnected neutral from rcbo no problems til neutral connected, with live connected all good. Connected neutral trip. Connected just neutral - trip. Earth in all the time.

    Disconnected that circuit's earth and functional earth to see if it would hold. No joy. Traced earth and it goes nowhere, cut off at contactor! However outside lights gave me a Zs so earth must be taken off another circuit.

    Thought about borrowed neutrals - but all other rcbo's held - didnt trip in a pair. Also meggered between neutrals came out ok. Even disconnected all other neutrals except outside lights. No joy.

    It's a main board in common areas that feeds that building's power and lights and outside lights plus feeds a few submains. Flats are on separate main incomer individual fuses. Could it be a borrowed neutral from a lighting circuit in one of the flats.

    Also, old board had charring round where the main neutral from 100a switch meets neutral bar. Is this probably unrelated and due to bad connection??

    What a horrible day. Going back tomorrow to fault find some more. But it's hard to find the ordering of the outside lights there are groups of them all over the place. I suppose get Zs on all of them and lowest is closest to the board.

    Wish me luck, but more importantly some highly experienced insight would be great!!
     
  2. Plonker 3
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    Plonker 3 Guest

    Have you IR between the neutral and all the earths and main bonds?
     
  3. Marvo
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    Marvo GMES....You absolute beauty. YOU ROCK DUDE!!!! Staff Member

    Location:
    South Africa
    Also have you IR tested the neutral of the tripping circuit to all the other neutrals whilst they're disconnected?
     
  4. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    Yes and yes!!
     
  5. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    Just for reference if u had a borrowed N then they would both trip right??
     
  6. Marvo
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    Marvo GMES....You absolute beauty. YOU ROCK DUDE!!!! Staff Member

    Location:
    South Africa
    Maybe but one might win the race and prevent the other one from tripping and probably both would trip only if both circuits had load on them. If one of the circuits has zero load current then maybe it wouldn't trip. I'm not very familiar with RCBO's, we only occasionally come across them where I am so hopefully someone will confirm this or give you better info.
     
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  7. baldsparkies
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    baldsparkies Electrician's Arms

    Get the offending circuit going again on an mcb. You know its not a live to earth fault otherwise the mcb would have gone. Then try and use an educated guess as to which is the first fitting in the circuit. disconnect the rest from there and get the rcbo back in. See if it holds with just the one fitting and then re connect downstream until by process of elimination you find which run of cable or fitting is giving you problems. Not very technical granted, but you will be home in time for tea.

    One other point if its a borrowed neutral once the outside lights are on an mcb the borrowed circuits rcbo will start to tripp. At least you will feel like your getting somewhere
     
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  8. Badged01
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    Badged01 DIY

    Location:
    Portugal
    Whats this the technical term for?
     
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  9. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    Put outsides on mcb til tomorrow. All other rcbos held. Thanks for advice. This confirms my suspicions not borrowed neutral. Will 'insulation resistance test' bit by bit (technical term ;-) )
     
  10. stantheman
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    stantheman Regular EF Member

    Hoovering!
     
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  11. thomas-1981
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    thomas-1981 Guest

    If everything is showing as clear in regards to the insulation resistance test, could the rcbo be faulty?
     
  12. baldsparkies
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    baldsparkies Electrician's Arms

    IR's might throw you. You have a lot of resistive loads in parallel plus feedbacks from parallel earths ect. Have you got any galv conduits in use, or gear trays going down by way of fixing screws into masonry that is damp (especially with recent weather)
    Use your test gear by all means but proof of the pudding is in the eating, and trying to find an IR fault thats only a fault when RCBO is connected can be time consuming and unless your really seeing methodically what your readings mean real world scenario, things will soon get frustrating.
    Process of elimination rules. Good luck, and happy hunting.
     
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  13. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    Thomas 1981. Tried 2 different rcbos!

    Bladsparkies- yes galv conduit in use. Are u saying parallel earth could be a problem from these?? You're damn right it's frustrating!!
     
  14. Guest55
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    Guest55 Guest

    that makes no sense , all earthing is "parallel" as it originates at the same point.

    if its outside lights tripping then look for water damage in a fitting or a faulty pir / photocell , i dont see it being a borrowed neutral at all.
     
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  15. baldsparkies
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    baldsparkies Electrician's Arms

    Yes galv is conductive you have another earth return, and plenty of places on route where the cables IR might be failing or a fitting gear tray. To get an accurate insulation resistance reading you will have to disconnect so much stuff. Your far better isolating parts of the circuit and trying the rcbo to see when the fault clears. That way you can narrow your search down to maybe a chaffed neutral touching the conduit or a faulty gear tray or loss of IP rating letting moisture in. Could even be a high resistance live to earth fault. Not degraded enough to throw the mcb but enough to take out a 30ma rcbo.
    Like I said real world, Proof of the pudding, vs all kinds of readings that your trying to rationalise your choice. I have my preference. Go find your gremlin.

    And Biff is right, its not the parallel earth path provided by the galv conduit that will cause you problems with the RCBO. BUT the galv will provide the cable with possible earth contact points on route ie chaffed cables. And that kind of scenario makes IR readings interesting.
     
    Last edited: Mar 11, 2014
  16. netblindpaul
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    netblindpaul Trusted Advisor

    Location:
    South Wales
    Measure the leakage currents?
     
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  17. guest119
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    guest119 Guest

    Is there a control circuit for the contactor?
    If so where does the neutral go and where is the circuit fed from?

    Boydy
     
  18. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    Boydy. Contactor is controlled by external PIR. Disconnected that and still fault. I'm there now. Lots of food for thought lads. Thank you. I'll get cracking! Ps def not L-E high resistance fault because fine with live connected. Introducing the neutral brings all the problems. I am doing what baldsparkies said and disconnecting neutrals bit by bit. I have a suspicion where roughly fault may be...
     
  19. dazza1981
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    dazza1981 Electrician's Arms

    Location:
    essex
    Good luck eddie let us know how you get on im intrigued!!
     
  20. Pete999
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    Pete999 Trusted Advisor

    Top Poster Of Month

    Location:
    Northampton
    I was meggad when I went to bed last night, sorry that was due to too much Grolsh
     
  21. Rampantchilli
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    Rampantchilli Regular EF Member

    Location:
    Dudley
    Surely when testing for borrowed neutrals you don't test between neutrals? Surely the live of one circuit against neutrals of all other circuits, neutral to neutral shows only interconnection between neutrals
     
  22. Richard Burns
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    Richard Burns Trusted Advisor

    Location:
    Cambridgeshire
    Business Name:
    Richard Burns
    Testing between neutrals would work so long as everything is on as there would be a connection through the faulty load and one of the OK loads to give the reading, but from line to another neutral would only be testing through the faulty load so gives a chance for diagnosis.
    However if the load is electronic and near infinite resistance it may not show up.

    This is my thought:
    Shared Neutral Test.jpg
    be interested in feedback.
     
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  23. dazza1981
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    dazza1981 Electrician's Arms

    Location:
    essex
    That makes perfect sense to me and the drawing makes it even easier to understand. Thanks mate
     
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  24. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    nice diagram. very useful post.
     
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  25. mnbs 4x4
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    mnbs 4x4 Active EF Member

    Location:
    Australia
    is it possible that the lighting neutrals are crossed at the rcbo's , are the neutrals easily traced back at the board to match the feeds to double check
     
  26. MDJ
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    MDJ Guest

    Water in a photo cell or a light more than likely lol
     
  27. R G
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    R G Guest

    I would try and trace with earth leakage clamp meter with an MCB in for now, and trace it through with just that 1 circuit live then liven up other circuits and see what happens with the readings , you could try with lamps in first and all switches off , then try with switches on one at a time , trace it that way ,
     
  28. londonlec
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    londonlec Regular EF Member

    Location:
    London
    Does the circuit definitely need an RCBO...
     
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  29. Plonker 3
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    Plonker 3 Guest

    Why have you asked this?
     
  30. Des 56
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    Des 56 Trusted Advisor

    Location:
    Gliese 581C
    Originally Posted by londonlec [​IMG]
    Does the circuit definitely need an RCBO...
    Because he wants to know if the circuit needs an Rcbo sorry dillb, couldn't resist [​IMG]
     
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  31. londonlec
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    londonlec Regular EF Member

    Location:
    London
    Because the landlord/Local Authorities don't want callouts due to tripping RCDs.

    Obviously the fault still needs to be found. Have you IR tested the outgoing side of the contactor?
     
  32. Plonker 3
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    Plonker 3 Guest

    Why wouldn't they?

    They have a duty of care to ensure everything is safe, if it's tripping find fault repair then all is good.

    Had this yesterday, RCBO keeps tripping narrowed it down to faulty light, removed fitting all ok now. RCBO still on circuit and not tripping anymore.
     
  33. Plonker 3
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    Plonker 3 Guest

     
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  34. Matt85
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    Matt85 Active EF Member

    Location:
    Nottingham
    Without reading the tread in full I had this a few weeks ago. Turned out the photo cell was on a common neutral.
     
  35. mobile
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    mobile Guest

    rcbo

    Try swapping rcbo with a known good one from another circuit. I had a similar problem some time ago spent ages doing all checks and tests turned out to be duff rcbo very annoying had to go home and kick the dog!
     
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  36. Badged01
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    Badged01 DIY

    Location:
    Portugal
    Re: rcbo

    Bet she must love waiting for you to come home! Tea ready on the table, he's walking up the garden path, i'm heading down to the outside loo for a while .....
     
  37. guest119
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    guest119 Guest

    Re: rcbo


    No need to swap if it was ramped.

    Boydy
     
  38. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    Riiiiight. So I have made a bit of an error. When doing IR test I disconnected cpc neutral and live and did it that way. The cpc wasnt going anywhere, the lights bprrowed the cpc from various local points. Not awful but very rough I know! I had a fun day disconnecting sets of lights bit by bit. Turns out they are pretty much all letting in water. Going back to change 10 lights tomorrow and Monday. Eurgh...

    Cant believe i made that IR cock up. I must admit, i have a love hate relationship with IR tests. So many variables to give you low results and the wiring is slapdash in this place to say the least with so many variables I'm not sure where to start!!
     
  39. NickD
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    NickD Regular EF Member

    Location:
    Vulcan
    You're meant to IR test with CPC connected to the means of earthing, I believe?
     
  40. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    You're right there. I did learn that somewhere. But it'd been a long day and I wasnt with it.

    Now this is odd: All the lights are on when PIR is overrided. When i switch a particular submain (on its own rcbo) one light goes out. Now this HAS to be borrowed neutral surely because switched live is fed from one contactor?! Or perhaps switched live and neutral from different circuit is feb from contactor? I'm a bit baffled. Lights are on mcb til i sort probs out. But rcbo feeding submain does NOT trip which is why borrowed neutral looks v unlikely. Someone help me i feel like i'm losing the plot, not slept much since son was born, i swear having young kids turns your grey matter into cauliflower....
     
  41. guest119
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    guest119 Guest

    Do you mean submain to elsewhere or are you refering to contactor supply feed?

    Boydy
     
  42. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    Boydy - i may've mentioned it in my original post but there is an RCBO board that feeds several submains (i thought thats what they are called - they don't have their own DNO fuse if that's what you are asking )around the common parts of a large block of flats.

    I rewired that one light so it is on the correct switched live , neutral and earth now.

    The place is a total wiring mess!!!
     
  43. guest119
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    guest119 Guest

    So is the rcbo tripping now sorted?

    Boydy
     
  44. Eddiesparks
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    Eddiesparks Regular EF Member

    Location:
    Londinium
    Haha good question. Not sure. Still have to replace 6 lights then i'll check. They had a non-existent IP rating so had to be done anyway. I'll let u know monday. For once i have sat AND sun off and i plan to put work woes out of my mind for 60 hours!

    Ps whoever suggested clamp meter for earth leakage cheers. I'm gonna do that too.

    Its all wired in the same size for cpc and neutral so N to CPC short no problem right?! JOKE
     
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