Mysterious RCD fault, won't trip!

Discussion in 'Electricians' Talk' started by 14th edition, Nov 26, 2018.

  1. 14th edition

    14th edition Well-Known Member

    Went to customers house following A N Other who had extended socket circuit for a conservatory, They said to customer that RCD wasn't tripping properly, interestingly the put NA against trip values on test cert for sockets that could be used outside... but thats another issue.
    So I repeated tests and couldn't get the RCD to trip on any setting, ZS at test socket was 0.70. Its a TT system but I was getting a ZE of 0.55 so probably parallel earths on water and or oil pipe. Overhead supply and not converted to TNCS. Changed the CU RCD for a new one still not tripping. There was also a much older RCD unit in series with the RCD on the CU, took that out and put tails into 100a DP as it could fail in some way, made no difference to RCD CU test. Oddly I found another mains RCD in the cupboard identical to the original in the CU, so it looks like someone tried changing it already,,,?
    Tried testing the RCD on the load side mains connection with and without MCBs on, still fails. Test button works fine. So to rule it out I changed for another new RCD, makes no difference. Tried disconnecting one neutral at a time and retesting but wouldn't trip either at mains or sockets.
    I haven't done any IR tests as I was running out of light and time but I recall somewhere that a N-E fault can cause this even though normally it should trip.
    Any ideas please?!
  2. KIAB

    KIAB Well-Known Member

    A TT arangement, so if there a higher resistance to Earth than there is to Neutral, current would return via the netrual, as it will have less resistance,& therefore the RCD will not detect this,so won't trip out.
  3. HappyHacker

    HappyHacker Active Member

    Most manufacturers recommend that you disconnect the load before testing as it can impact on the tested trip times and some RCDs need the output cables connected, as they are designed to be part of the trip circuit for reasons I forget, and the tests to be done away from the RCD. There is one make who's name I can't remember that usually has this problem. A search on the web may produce some more answers unless anyone here has a better memory than me , not difficult these days :-(
  4. unphased

    unphased Screwfix Select

    There is a circuit somewhere that has the neutral and earth connected, like a screw through the cable. I had exactly the same issues on a job a few years ago and that was multiple faults but one was a screw through the shower circuit cable shorting earth and neutral. Why it prevented RCD tripping instead of making it trip is still a mystery.
  5. 14th edition

    14th edition Well-Known Member

    I have tried 2 new RCD's, there was another cu [run from MCB in main board] in the kitchen extension which has its own RCD. This red tests fine and as it relies on main earth from main cu I dont think it can be the TT earth....
  6. Sparkielev

    Sparkielev Well-Known Member

    Had this issue with BG boards one of the reasons i wont fit them
    barbaricduck likes this.
  7. retiredsparks

    retiredsparks Well-Known Member

    Wiring NE fault will be found on full IR tests of all ccts from the MCB's..... all N's disconnected from board.
    ZE should be purely the earth bonding in place, otherwise you have not confirmed the integrity of the main earth.
    This might give you a heads up if its very low..(PME ?)
  8. Bazza

    Bazza Well-Known Member

    Aye. Some people forget that earth and neutral are connected, somewhere…
  9. 14th edition

    14th edition Well-Known Member

    Ta, will go back and check for wiring faults and true Ze
  10. 14th edition

    14th edition Well-Known Member

    Went back here. Fault was a skinned neutral on a ring main, close to CU. Why that didn't trip the RCD is beyond me but RCD worked normally afterwards. Although it would sometimes not reset after tripping on test, had to switch off all the mob's first....
    unphased likes this.

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