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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



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  • 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.

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  • 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.

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