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Tripping RCD in different property - builders say it's due to hairdryers...

Former Community Member
Former Community Member
HI everyone


Hope you've  had a good week. Had a call from a client at the end of today. For the last 6 weeks (not sure why he left it so long to call) the RCD for his tenants property has been tripping while the builders are working on the ground floor flat (GFF). The set up is a bit odd. Converted to flats probably in the 80's. Main head has 2 x 60A fuses and is TNCS. Each flat has its own switched isolator, then 16mm T and E to the respective CU's. The GFF still has a 60A rewardable fuse in the isolator. The first floor flat  (FFF) switched isolator (my clients) has been 'upgraded' to a 100mA time delay RCD and switch, presumably someone thought this would give protection to the buried supply cable. This is what is now tripping. Both boards now have RCBO's. Before the recent building work, the ground floor one didn't.


The builders apparently turn off the main switch of the GFF CU each night and at the weekend. When it's off, the 100mA RCD doesn't trip. Neither my client nor his tenants have witnessed the tripping, but have come home after work to find it's tripped OR that there has been a power loss (eg clocks need resetting) so the builder must be resetting it. Builder seems to think the issue is EITHER loss of power from the street (in which case he'd loose power as well, but he's not reported it) or that the tenants hairdryers are causing the 100mA RCD to trip but only after they've gone to work! Hmm. He's not said explicitly that he's rest the RCD but he must have (unless its a special self resetting one that I don't know about)


I just popped in to run some quick tests. Neither 16mm T and E supply cable is damaged. Ze is within limits, I loop tested 1 socket in the ground floor flat and it was fine so I assume polarity to the GFF CU is OK. All RCD's tripping within required times. I didn't have my megger with me, so couldn't ramp test.


Since the issues have only started since the builders started work and all was fine and dandy before, it MUST be something the builders have done that's now creating the fault?  I tested the installation about a year ago and there was a previous test cert from 2010.

I'm a bit stumped and would welcome any thoughts on where to go next. I did suggest getting UKPN out to check the supply, is it worth this?


many thanks

BB



  • 0
    I can't help thinking one circuit has been cross connected somewhere between the flats supplies either Line or Neutral - which is causing an imbalance whenever it's used.


    Does sound odd


    One way to determine any cross connection is to isolate the power to both and place a toner/oscillator on each circuit and see if there's any kind of cross connection?
  • 0
    Could be a N-E short somewhere. Increasing the load (e.g. hairdryers)  anywhere in either of the two flats will cause a larger volt drop along the shared N cable (and possibly the N cable to the RCD too),  which causes a larger voltage across the N-E short, which causes a larger leakage current, eventually tripping the RCD. Note that the N-E short load doesn't have to be downstream of the RCD or in the same flat.
  • Former Community Member
    0 Former Community Member
    Hello both!


    Thanks so much for taking the time to reply on a Friday night! Yes I did think about shared N either deliberate or 'caused' by a fault, but then surely the RCBO's in the FFF would have been tripping if the fault was prior to the new building work (and second RCBO CU) or both could be tripping now that there are 2 CU's with RCBO's. And since they are 30mA, why is the 100mA S type tripping?

    If the hairdryers were to blame, surely they would cause the trip at the time of drying not after the tenants have left the building?!?


    I'm not sure how keen the builder is to give access - I just managed to sneak in with my loop tester today as I sweet talked the decorators...


    Still scratching my head..

    BB


  • 0
    The first thing I would do is, with the S-type RCD open, do an insulation test between the E and the load-side N terminal of the RCD
  • Former Community Member
    0 Former Community Member
    I tested the load cable (disconnected from the RCD) between all 3 conductors and it was >500 MOhm. Is this what you meant?

  • 0
    It could be coincendental that a fridge or CH pump has gone faulty at the same time the builders started but I would suspect that the builders have managed to do something to the FFF wiring especially as it is a conversion. I agree the self actuating hair dryers are unlikely to be the problem. If the decorators are in I assume the flat is nearly finished and the builders are probably trying to defer any issues until they have left. 


    When the builders have left can you do a continuity test between the L,N & E on each flat? The fact they turn the GFF off every night would indicated to me they know there is a problem. Ask that they leave it on to see if the fault occurs then.


    I suspect it could get messy and a strongly worded letter possibly from a solicitor may be needed to get some action.
  • 0
    Are the 2 flats on the same phase ?


    IF they are then earth leakage and neutral currents will be in phase.


    In any case something causes the outbound L current and the returning N current not to be equal at the 100mA RCD, and it is not clear quite what.


    IF the flat that trips all works OK, and no missing circuits, until the other flat is switched  on, my money would first be on a small load between the live of the second flat, and the neutral of the flat  with the RCD. If it was the other way round, when the flat was switched off something  in the flat with the RCD would not work.

    So what may be shared ?

    Fire alarm?

    lights - perhaps via lights in the  common area, or wall lights from a light switch with no local neutral picking up a lighting neutral from the wrong place.


    But both flats have RCBOs on all circuits and do not trip, so we are looking at not a lot of leakage, or a fault that is already there is being augmented by a new one pushing a marginal situation over the edge..

    If it in not the obvious cross wiring, then it become a Pandora's box of possible horrors.


    If it is near the knuckle, then neutral faults to true earth (plumbing /building steels etc) may not quite trip until there is enough load -  it does not have to be N to CPC, it could be N to a screw in a damp wall and need a bit of N-E voltage drop to drive current through it.

    A 100mA RCD needs at least 50mA of imbalance to fire, so you need to find either 5 k ohms L-E or a rather indeterminate but lower handful of ohms N-E depending rather on the NE voltage offset which in turn will depend on total load downstream of the NE bond. (and that is why I asked about phases - same phase makes this more credible)

    Good luck, this might well be a sod to find, and probably really obvious once found.


    A meggar, a wander lead and perhaps a garden fork or screw driver in the flower bed and a lot of readings between all sorts of combinations of things (starting with L+N of upstairs to L+N of downstairs ) may be needed to find exactly what.

  • 0

    The builders apparently turn off the main switch of the GFF CU each night and at the weekend. When it's off, the 100mA RCD doesn't trip. Neither my client nor his tenants have witnessed the tripping, but have come home after work to find it's tripped OR that there has been a power loss (eg clocks need resetting) so the builder must be resetting it. Builder seems to think the issue is EITHER loss of power from the street (in which case he'd loose power as well, but he's not reported it) or that the tenants hairdryers are causing the 100mA RCD to trip but only after they've gone to work! Hmm. He's not said explicitly that he's rest the RCD but he must have (unless its a special self resetting one that I don't know about)



    So it only trips when the builder is there? and sometimes it's reset. Could it be just that the builder (or one of his lads) doesn't really understand what does what and has just been switching off random things until the desired effect (GFF dead) is achieved - sometimes resetting the things they switched off and didn't do what they hoped, sometimes not. Then too embarrassed to admit exactly what happened? Or some of the builder's equipment is causing so much RFI that it's upsetting nearby electronics - including the RCD.


      - Andy.
  • 0
    "Then too embarrassed to admit exactly what happened? "

    This sentence is a good one to remember with all fault finding excersizes too.

    Take onboard what the customer says by all means but never take it as 100% gospel
  • 0
    Definitely going to second the 'N-E fault on the consumer's side'


    A fault between N&E won't always pass enough current to trip an RCD, especially a 100mA time delay type.


    An increase in load on a system sharing the same PEN-splitting point, or in most cases, apparently 'separate' TN-S supplies, which happen to actually be PME'd elsewhere along the main cable run, will necessarily increase the N-E voltage. Given a fault of low, but measurable impedance between N&E, and you get this exact scenario.  Even so far as RCD's tripping with ZERO load on the 'load' side (see David Savery's "curious fault #1" video for what I believe to be an example of this.)*


    The RCD, especially type AC ones, which may be non electronic, don't CARE if the line conductor is actually live, even. If they spot 30mA on the neutral with zero on the line, they're gonna trip.



    *https://youtu.be/IMD5VwroawU

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