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Big Bang Prevented by R.C.D.

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I should be put out to pasture.

I installed a new double socket today and a new metal box for it. The noggin was already in the wall for a single metal box and socket. It was big enough for the new double socket box as well. So I installed the new socket and tested it. All was good.

I went away to do another job and get a t.v. coax lead and returned later to test the t.v.

The t.v. would not come on when it was plugged into the original single socket mentioned above.

The consumer unit R.C.D. had tripped off. I took both sockets off and initially saw no problems with the wiring. Then I saw a small place on the L. conductor where the insulation had been damaged. It had been in contact with the box fixing screw and the conductor had migrated through the insulation and touched the screw.

 There had been a L. to E. short, but no big bang or black marks occurred. The R.C.D. had tripped off and saved the day.

I suppose that as it is a hot day the P.V.C. insulation had been soft and sufficient pressure had been present to push the copper wire onto the screw head, which by the way was not sharp on its head at all.

I have not done that for many years and could kick myself. (The other incident involved some 6.0mm2 cooker cable in a metal box and a sharp box edge.

Doh!

 

Z.

 

 

 

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    It may be embarrassing, but does illustrate how the near-instant trip RCD limits the energy at the point of fault by opening very promptly indeed. 

    I squared t, with a very small t. 

    As you say, if it was down to an MCB, or indeed the old style hot wire fuse, you'd be looking at a blackened back box and a length of  wire to cut back and re-terminate.

    I had a similar ‘aha’ moment with a tumble drier whose element would sag (of course only when warm) and touch the case. The RCD went so fast that there was no sign of this contact, and once unplugged and metered, no fault found. Actually it took quite a bit of tracking down to see what exactly was happening. 

    I did ponder that in the pre RCD days the element would probably have been fused to the case, making the fault much easier to find and also  much  harder to fix. In the case of the tumble a bending of the brackets to pull the slinky element out straight again. 

    Mike.

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  • 0

    It may be embarrassing, but does illustrate how the near-instant trip RCD limits the energy at the point of fault by opening very promptly indeed. 

    I squared t, with a very small t. 

    As you say, if it was down to an MCB, or indeed the old style hot wire fuse, you'd be looking at a blackened back box and a length of  wire to cut back and re-terminate.

    I had a similar ‘aha’ moment with a tumble drier whose element would sag (of course only when warm) and touch the case. The RCD went so fast that there was no sign of this contact, and once unplugged and metered, no fault found. Actually it took quite a bit of tracking down to see what exactly was happening. 

    I did ponder that in the pre RCD days the element would probably have been fused to the case, making the fault much easier to find and also  much  harder to fix. In the case of the tumble a bending of the brackets to pull the slinky element out straight again. 

    Mike.

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