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How a simple job can go wrong quickly....

Former Community Member
Former Community Member
Not my work before I relate:


Existing 3-ph circuit breaker DB in a shop has a 30 mA 4 pole RCD belatedly fitted in a separate enclosure to provide blanket RCD protection. OK, not ideal.

Electrician asked to install extra 13 A socket-outlet in window during shop hours so padlocks off the circuit's circuit-breaker and proceeds, He lets the circuit neutral and cpc touch when fitting the socket-outlet and out trips the RCD as expected. Resets and shop keeper then announces that the card reader, till, air-con and some lights not working.


All that equipment now duff (technical term!).


For an interesting weekend quiz, what happened?


Without hindsight and the work being done during opening hours, what would you have done differently?


Regards


BOD






  • 0
    That's one of the perils of working on an installation when you cannot isolate the whole of it. I must admit that it took me a while to figure out why the RCD tripped whilst stripping out an apparently isolated circuit - of course it was when my cutters united the N and CPC, but that was in a TT installation. I am not sure whether or not it would happen in a TN-C-S installation - I don't quite see why, but I haven't tried.


    What to do differently? Detach the N.


    Cause? Reversed polarity. In the milliseconds during which the RCD was tripping, L and E were united and that killed the equipment.


    What to do differently? Check polarity before you start. That's very much with hindsight.


    I might add that 3-phase boards are all or nothing. You can have an RCD incomer, but there is no such thing as a split board. So unless you are happy for an earth fault to kill everything, you need to use RCBOs. The price of 3-phase ones (for sockets) is eye-watering. ?
  • 0
    Where would you detach the neutral the neutral?


    In the distribution board whilst all the other circuits are live?


    Are you sure you have the right one?


    I presume the equipment was damaged due to their circuits going up to 415 volts, but I’m still trying to figure out why.


    And I’m sure we have all tripped an RCD working on a circuit that has had the live isolated.


    Andy B.
  • 0
    Possible, but a reverse polarity supply on 3 phase is very hard not to notice, and while near the origin  shorting L1 to N does indeed over-volt L2-N and L3-N, you do not need to be many metres down the line for most of the volt drop to be in the final cct.

    It could be a 4 pole RCD and breaking neutral first.


    In TNC-s shorting CPC to N on a final circuit bypasses the neutral coil of the RCD - unless you bypass the phase coil too, I expect a trip.
  • 0

    mapj1:

    Possible, but a reverse polarity supply on 3 phase is very hard not to notice, and while near the origin  shorting L1 to N does indeed over-volt L2-N and L3-N, you do not need to be many metres down the line for most of the volt drop to be in the final cct.




    I was thinking of a reverse in polarity somewhere along the SP circuit. I have to admit that my offering is guesswork, but I expect that BOD will tell us tomorrow. ?

  • 0
    A bit of insulation tape on the end of the neutral conductor as you dismantle the old fitting is a good start, maybe even a piano type Wago connector clipped onto it, also fold the CPC back when you are pulling the cable back out of a plastic enclosure.


    Regards the problem, were there still appliances connected to the circuit that was being worked on?


    Andy B.
  • 0
    I am still pondering the speed at which this happened.


    The live was locked off, the neutral was connected and had a small voltage on it, as did all the neutral conductors downstream of the RCD, enough to push more than 30 mA down to earth when the electrician touched the neutral down to earth by allowing it to touch the CPC or an earthed enclosure bringing the neutral down to near true earth potential.


    The RCD tripped in milliseconds, was the unintentional connection to earth from the neutral still in place when the damage occurred, I assume it would not have been as it would have tied the neutral down to earth stopping the circuits going phase to phase.


    So I assume the unintentional connection to earth from the neutral was flicked in and out in less time than it took for the RCD to trip, not a lot of milliseconds at all.


    Andy Betteridge
  • 0
    From the sound of it, all of the final circuits are SP.


    Reverse the polarity of a circuit on L3. Connect what looks like N to E. That goes back to the N-E link so you now have N at L3 potential. The devices on the other circuits now have phase to phase (L1-L3, L2-L3) for those few milliseconds before the RCD trips.
  • 0

    Chris Pearson:

    From the sound of it, all of the final circuits are SP.


    Reverse the polarity of a circuit on L3. Connect what looks like N to E. That goes back to the N-E link so you now have N at L3 potential. The devices on the other circuits now have phase to phase (L1-L3, L2-L3) for those few milliseconds before the RCD trips.




    I might add that all the equipment on L3 will have survived.

  • 0

    perspicacious:

    Not my work before I relate:


    Existing 3-ph circuit breaker DB in a shop has a 30 mA 4 pole RCD 






    Mike has it. The neutral was disconnected before the phases when the R.C.D. tripped off and the loads were subjected to 400 Volts. Even though the loads would be connected like a star connected motor, the star point being  at the neutral bar,  the equipment could have been damaged by a transient over Voltage if the N. of the R.C.D. opened before the L1, L2 and L3 contacts.



    Z.



     




     

  • 0
    Here is an excellent video presentation showing how a broken neutral can cause a high Voltage on single phase loads derived from a three phase supply. It is Canadian so the Voltages are not U.K. standard Voltages. I did not understand how an A.C. Ammeter can be wired backwards or display in reverse though.


    Enjoy.....https://www.youtube.com/watch?v=d1imn9lbxOs


    Z.

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