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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






Parents
  • 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. ?
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  • 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. ?
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