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R.C.D. tripping and earthing concerns.

Zoomup
Evnin' All,

                  I have had a hard day and my brain is in meltdown so please excuse nonsense and silliness.


I installed a new socket in a house today. TT earthing presumably. New A.B.C. overhead supply. Nearest D.N.O. P.M.E. pole earth rod about 15 metres away from the house. When I tested the R.C.D. it worked on the test button, but the switch seemed a bit graunchy. It would not trip off on 30mA test, but did trip off on the 5X R.C.D. test at a socket. I replaced the R.C.D. The same thing happened. No trip on 30mA test setting, but it did trip off on the 5X setting.


An earth fault loop test at a socket showed a reading of 185 Ohms.


A  length of 6.0mm2 green and yellow cable was found in the front yard but the electrode is not inspectable as it is covered by block paving is old. It may have rotted away. With the main switch off the resistance from N. to the earth bar in the consumer unit is about 180 Ohms. 


The consumer unit is a 10 way unit with just one 30mA R.C.D.


I converted the earthing to P.M.E. and then nuisance tripping was seen. I intend to install an all R.C.B.O. board tomorrow.


Sometimes a fridge when turned on will trip the R.C.D., other times it is a freezer, then the kettle. But the appliances test o.k. with an insulation resistance test. Once or twice the tungsten lighting circuits tripped off the R.C.D.


I assume that the addition of individual leakage current is responsible for the R.C.D. tripping, but there is no logic to which individual appliances/circuits seem to cause it. Two suspected rings test o.k. for insulation resistance.


The house owner reported that the gas fitter received a shock when doing something under the AGA. The main gas bond was loose at its clamp. I have replaced the earth clamp.


Any thoughts please?


EDIT. P.S. I have a Voltage tester that will trip off a 30mA R.C.D. if connected to the outgoing R.C.D. L. and E. at the same time. When this was connected to the original R.C.D's LIVE INCOMING terminal and the E. bar the R.C.D. tripped off. Did this raise the Voltage on the C.P.C.s sufficiently to trip off the RC.D?


Z.


  • 0
    Current injected into the CPC, that does not come from the load side of the RCD (neutral or live) will not trip the RCD.

    There may be a partial path from CPC to the load side neutral of the RCD, in that case pulling up the neutral earth would destabilise the RCD.


    Not tripping at X1 is more of a concern, presuming the tester is  pushing current from outbound live to CPC - though again a partial  NE fault might put some of that 30mA not into terra-firma but back into the load-side neutral of the RCD.


    Be aware on TT, with a significant electrode resistance, an NE short is not the immediate trip on any light loads that it would be on a TNS or TNC-s supply , as if the rod is 180 ohms, even if the NE fault is dead short, load current shares in the ratio of 180 ohms to Rn, and that may mean you need a very large load current before false tripping occurs.  (Rn may be tenth of a ohm, so 30mA * 180/0.1  ~ 60A before tripping such a current may never be seen on the defective circuit.)


    I'd be wondering if drilling and driving a second rod was on the cards,  if the original is in doubt, and also looking for an N-E problem in an appliance or one cct.

    250V or 500V test L+N to E will help shake that out.

    It may not be N to E the CPC of the cct, it could be N to E as in some pipe or fence post in the ground  or something, so test not just to CPC but to electrode too.

  • 0
    Sounds like a N-PE fault somewhere to me (possibly relatively high resistance) - thus random trips when things are under load and the current the RCD tester injects isn't all seen as out of balance by the RCD (as some finds its way back though N and reduces the imbalance).


      - Andy.
  • 0

    Zoomup:

    No trip on 30mA test setting, but it did trip off on the 5X setting.




    That's a pass, innit?

  • 0
    Have you done a global insulation test between both lives connected together to earth with all the loads connected, then narrowed it down from there?


    Andy Betteridge
  • 0
    Thanks all for your help. I woke up in the middle of the night and thought that a N. to E. fault was the most likely situation causing the problem. It was not possible to disconnect individual final circuits yesterday due to the rat's nest of wiring in the consumer unit covering the screws of M.C.B.s and the neutral bar. and its awkward location. I was more interested in just keeping the fridge and freezer working. Thanks Mike and Andy, I am sure that there is the likelihood of a N. to E. fault of some Ohms somewhere. No apliance was found to be faulty, so I suspect the fixed wiring.


    I will install a new all R.C.B.O. consumer unit today as the existing one with just one R.C.D. and many M.C.B.s is likely to prove troublesome in the future, as it is now. Hopefully with the cables all labelled up with the old consumer unit gone I will be able to test each circuit thoroughly.

     I will report back for others' benefit.


    Thanks again,


    Z.
  • 0

    Sparkingchip:

    Have you done a global insulation test between both lives connected together to earth with all the loads connected, then narrowed it down from there?


    Andy Betteridge 




    I did not do a global test, but did test many appliances individually at 240 Volts. No appliance fault was found. Thinking about it now, the nuisance tripping is related to load current. The kettle tested o.k. but always tripped off the R.C.D. if the freezer and fridge were running. The fridge or freezer would trip the R.C.D. on startup, but on other occasions the R.C.D. stayed on if the loads were limited in number. Perhaps we have a rusty nail through a cable or a socket screw just piercing a N. in a damp metal backbox.


    Z.

  • Former Community Member
    0 Former Community Member

    Zoomup:

    Evnin' All,


    I converted the earthing to P.M.E. and then nuisance tripping was seen. I intend to install an all R.C.B.O. board tomorrow.

    Any thoughts please?


    Z.


     




     

    That was either brave or foolish of you I would have thought !!


    Is your conversion still in place ?


    OMS
  • 0
    Update. I installed a new consumer unit with 10 new R.C.B.O.s. I tested every circuit thoroughly. A neutral to C.P.C. fault was found on a lighting circuit. The lighting circuit is about 20 years old and has some old hot running recessed ceiling lights with tungsten R080 reflector lamps in them. Also on the same circuit there is an external P.I.R. lantern. Perhaps that is damp inside due to rain ingress, or perhaps heat has damaged a lighting cable above the ceiling. I will investigate that circuit tomorrow. It is disconnected now and all other circuits are fully operational along with all fridges and freezers etc. Also  the electric kettle does not cause any tripping off of an R.C.B.O. All is now o.k.


    Z.
  • 0

    OMS:




    Zoomup:

    Evnin' All,


    I converted the earthing to P.M.E. and then nuisance tripping was seen. I intend to install an all R.C.B.O. board tomorrow.

    Any thoughts please?


    Z.


     




     

    That was either brave or foolish of you I would have thought !!


    Is your conversion still in place ?


    OMS 

     




    It is a P.M.E. area with newish A.B.C. overheads. Other properties have been converted also.


    Z.

  • 0
    making it PME is not as simple as joining N to E on the customer side,  ignoring for now the legal aspect of doing so in the UK.

    The rest of the installation has to be PME ready, with suitable bonding to water and gas pipes, or insulating joints in the service pipes,  and larger CSA earthing in any places likely to suffer from diverted neutral.

    The line to the house has to be suitable, which usually means that all connections in the PEN are double crimped and no screw line taps.

    Strictly the only authority that can know this for certain is the DNO.

    I appreciate that the chance of a PEN fault during a quick diagnostic  test to joint the N and E busbars to see if a nuisance trip problem gets better or worse  is not the same risk as a supply side PEN fault as having them linked for years, hough  I'd very strongly recommend either getting the DNO to do it, or changing it back to TT.

    I'm sure you know this really, so this is just an observation and partly for the benefit of any other readers finding this post in the future who may be inspired.

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