RCD, no trip

Discussion in 'Electricians' Talk' started by seneca2, Jun 4, 2009.

  1. seneca2

    seneca2 New Member

    Leitzz, although the Ra was around 150 ohms, a loop test with all bonding in place gave a reading of 120 ohms. As the rod, gas and water are all bonded together I still think a n-e fault would cause the rcd to trip. All will hopefully be revealed next week when I do a full test when I change the c/unit,
    thanks for replies everyone.
     
  2. leitzz

    leitzz New Member

    If your loop test was showing 120 Ohms then that suggests to me that the incomimg services are plastic (bonded yes but not really acting as a low impedance "earth") - so your neutral earth fault is effectively bypassing the RCD when you are testing - did you try putting the earth lead on the incoming neutral that should tell you for certain

    Leitzz
     
  3. wally2

    wally2 New Member

    While some of you chaps are much better than me on calculations & electrical understanding there might be some truth in what your saying regarding n/e fault but it is not my view. Might have read it wrong but indications (correct me if im wrong) seem to be if its a TT install & the bonding is of a lower reading than an earth stake then the RCD might not trip. It is often found that bonding is better than earth stake but it does not stop the RCD from working. Why would we install RCDs on such systems if they were unlikely to work.
     
  4. Simannjo

    Simannjo New Member

    Let me see if I,ve got this right...
    The RCD tester takes some line current and dumps it to earth ansd ... trip.
    In Senecas case the E/N fault is allowing the dumped current to go back through the RCD Neutral as the Earth Path is too difficult... No trip.
    If no E/N fault then the dunped current would be forced to earth upto 1667 ohms...
     
  5. seneca2

    seneca2 New Member

    I still can't see this being a n-e short. I'm sure that most of us have had the situation where we're working on a circuit with the breaker switched off and whilst working we touch the n & e together, tripping the rcd!
    Now some are saying if the n & e are shorted the rcd won't trip, doesn't make sense does it? I can't wait to get back there next week to do a full test!
     
  6. Simannjo

    Simannjo New Member

    When RCD trips 'cause E/N are touched together it's usually when consumer unit is still live and only single pole MCB is isolating circuit being worked on. Therefore Neutral is a live conductor and carrying some current that goes down the parallel earth path when shorted.
    Put me down for an E/N fault.
     
  7. seneca2

    seneca2 New Member

    Well the c/unit is live in this case, just to re-cap, rcd in tails feeding 3036 c/unit. Rcd won't trip unless main switch turned off.
     
  8. Simannjo

    Simannjo New Member

    With the main switch off you have disconnected the faulty neutral and are testing with a good neutral.
     
  9. Simannjo

    Simannjo New Member

    To prove... Turn off main switch and link in and out main switch neutrals.
     
  10. wally2

    wally2 New Member

    If the N/E are shorted on the outgoing the RCD may not trip as no current is flowing, as soon as there is current flowing in the circuit it is then likely to trip because of the n/e fault if that makes sense. Take the following problem of say a householder rings up & says what ever he plugs in a socket to use, the rcd trips. There is no deffinate appliance at fault but there is likely to be a N/E fault on the system
     
  11. clarktspark

    clarktspark New Member

    did you get a resolve to this? i have very similar issue again on a tt sysytem as none of the rcd's on site will trip, checked every thing shared neutrals etc eventually took the  test kit home to test my house, the megger at one point was showing odd voltages(151vac) after the rcd DIDNT trip on test, turn it ff and back on then displys almost a flosting 230ish vac but flashing..... tt location sub 200ohms on rod old mem 200 series board 100ma front end 30ma  socket protection, second feed to outbuildings 63a from 100ma front end rcd feeding second mem 200 series with a min switch for basic circuits and 30ma for sockets and pool quipment(all off and pool drained and coverd recession blamed!!) no RCD will trip in this building on test but on of the 30ma trips for a hobby until i'm there!! perhaps two faults earth neutral on the front end or even supply, and a fault on the cct protected by the 30ma that trips when i'm not looking. I've some hours here now and pulling hair out.... any ides????
     
  12. J.P.

    J.P. New Member

    Fish and chips?
     
  13. unphased

    unphased Screwfix Select

    Hi clark. I had a situation on a job a couple of years back when I changed a fuseboard where there were two faults. One of the faults turned out to be a DIY wire joint in a lamp flex where the plonker had joined neutral on to earthand that was causing the RCD to trip when the plug was inserted so that was easily sorted. However, with that particular flex unplugged the RCD would not trip uinder test nor when the test button was pressed. This was quite perplexing and took ages to resolve. I found (eventually!) that when the shower circuit was disconnected the RCD would work and trip under test no problem. So the second fault was in the shower wiring. I rewired the shower circuit and it was during the rewire that I found the fault on the cable. It had been routed over a joist in the loft where there was a hidden screw that had penetrated the wire shorting neutral and cpc together on the supply side to the shower switch. So, when the shower was off and no load on cable there was a neutral and earth short created by the screw. My explanation for the non-tripping is that in effect under no load conditions no imbalance could be detected by the RCD because the neutral cpc was shorted permanently creating a single conductor and balancing the circuit around two conductors ie phase and neutral/earth.. Had the shower been switched on then the massive increase in current would have instantly tripped the RCD as it would easily have detected the earth neutral connection but in a no load situation it was somehow interfering with the testing. So, if your installtion has a similar condition where a n/e fault is present under no load condition and when nthat circuit is energized the RCD trips that is what I would be looking for. Hope this helps and good luck.
     
  14. clarktspark

    clarktspark New Member

    Unphased, thanks for the advice/experience, looks like its heading towards a n-e fault as that looks to be the theme for this type of fault. gonna go back with the knowledge the test kit is working(thats a horrid feeling when you start to doubt your own kit) and a fresh bag hot bag of fish and chips
    I'll post my findings if good or my new career choice if bad(JP)
     
  15. J.P.

    J.P. New Member

  16. Lokkars Daisy

    Lokkars Daisy New Member

    On yer bike UP ,, we all know your game ,, you have read through all the posts and couldn't bear to be left out and so you concocted your  own scenario using available data but changing a few spurious detals

    ***

    cheers



     
  17. clarktspark

    clarktspark New Member

    Update.......
    Looks like a supply issue, voltage all over the shop 170 to 265vac p+E  250 to 260's p+p 80 odd  n+E, supply company out within an hour. Not bad for rural wiltshire on a monday night!!!
    Will investigate further when the supply correct
     
  18. palavaman

    palavaman Well-Known Member

    senecal2, i got news 4u.    your ohms reading is too high?     get a bucket of water and pour it where you stuck your earth rod.  when that sips away, pour another bucket of water to it.  afta a while do your tests again.  BINGO, you'll get a betta ohms reading.  SIMPLES.

    BTW, your client 4got to tell you that there is a lamp in the loft which is actually connected via a plug (in ring main).  now, being dat you know nofink of it, you havent pulled dis plug out.  so, you can be there all day and youll continue to av problems.
    HTH,s.  
    Just so you know, senessa, dat £8,000 I paid for my 5 day electrical course, i failed it.  now dey want anoda £2,000 for a re-sit???  i aint paying it.  i'll just call it a bad investment.
     
  19. seneca

    seneca Screwfix Select

    I can't remember much about the job now, it was 2 years ago!  From what I remember I did get the rod down to about 120 ohms and then after fitting the new board and  doing the bonding etc all was ok.
     

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