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Screwing in bulb temporarily kills power in cables

Discussion in 'Electrics UK' started by Scramble, 15 Oct 2017.

  1. Scramble

    Scramble

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    I have two outside lights on the same circuit and they weren't working, so I went to change the bulbs. One bulb had blown, but one was fine -- I tested it inside, and it still works. But it wasn't working with the outside lights, and neither did a new bulb. But using my current detector pen I could verify that the wires were live. I tried screwing the bulb in again, and the weird thing was that doing this made the wires go dead.

    I did this over and over, with both light sockets, and the same thing happened every time. The circuit is live, but screwing in a working bulb kills the power (but not by tripping any switch on the fusebox). Unscrew it a bit and the wires are live again. I turned off the power at the fusebox and checked all the wires and connections, everything was fine, no loose connections, no wrong wiring, no damaged wires. One of the sockets had some carbon on a contact from where one of the bulbs had blown, so I cleaned that off, but that didn't help.

    Might this be some safety feature with the socket (which looks simple enough)? Or is there likely to be a fault somewhere in the circuit? Do I need to get an electrician in?
     
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  3. stillp

    stillp

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    There's your problem. They often give false results. Get a proper two-terminal voltage tester, or a multimeter.
     
    Last edited: 15 Oct 2017
  4. mwatsonxx

    mwatsonxx

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    Seems you have a volt drop problem, cables too long and inadequate csa to provide the required volts. Smaller wattage lamps might work.
     
  5. stillp

    stillp

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    Your "current detector pen" is picking up stray voltage capacitively coupled into the cables from adjacent wiring. Inserting a lamp shorts out that stray voltage. Get a proper two-terminal voltage tester, or a multimeter, or an electrician, because this apparent strange behaviour is not why your lights don't work.
     
  6. JohnW2

    JohnW2

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    Are you serious?

    As stillp has said, what the OP is observing is very probably a consequence of the inadequate/unreliable 'tester' he is using.

    Kind Regards, John
     
  7. Scramble

    Scramble

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    >Seems you have a volt drop problem, cables too long and inadequate csa to provide the required volts. Smaller wattage lamps might work.

    It won't be that, these lights have been working fine for years (originally installed by a proper electrician). I should have said that these are lights that are on the back wall of the house, not ones way down the back of the garden.

    I'll investigate again tomorrow, maybe it's the tester.
     
  8. Taylortwocities

    Taylortwocities

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    If you have a bad connection somewhere, then when there is no current flowing you may see voltage. But under load the bad connection can break down which means no volts and the lamp doesnt light.

    To find this problem, follow stillp's advice:,
     
  9. JohnW2

    JohnW2

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    Another thought has occurred to me ....

    As has been said, your 'detector pen' may be fooling you into thinking that the lights are 'live' when they aren't. What circuit (at the fusebox) are these lights connected to - a 'sockets' circuit or a 'lighting' one? If the former, it is very possible that there is a 'fused connection unit' (FCU - a box with a fuse, and possibly a switch) somewhere in the feed to the lights. If an FCU is present, if the blowing of the bulb resulted in carbon deposits in the socket, it's quite possible that the fuse in the FCU will have blown - so maybe you should 'hunt for an FCU'!

    Even if the lights are supplied from a 'lighting' circuit, it is by no means impossible that the person who installed it will ('unnecessarily') have included an FCU - so the above comments still apply.

    An FCU will look something like ... [​IMG] or (with a switch) ... [​IMG]

    Kind Regards, John
     
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  11. Astra99

    Astra99

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    Surely with outside lights it is common practice to place a double-pole isolator (switch) or FCU in the circuit so that any nuisance tripping of the RCD caused by water ingress can be prevented?????????
     
  12. ban-all-sheds

    ban-all-sheds

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    It is good practice.

    Not the same thing.
     
  13. EFLImpudence

    EFLImpudence

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    I doubt anyone has ever undertaken a survey.
     
  14. JohnW2

    JohnW2

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    Indeed. When I said 'unnecessarily' as regards the possibility that there could well be an FCU even if the light was fed off a lighting circuit, I perhaps should have added that, even though an FCU would not be required in that situation, a DP switch would be desirable.

    However, in context, all that mattered was the possibility that there is an 'overlooked fuse' which may have blown as a result of a bulb/lamp dying.

    Kind Regards, John
     
  15. ban-all-sheds

    ban-all-sheds

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    An FCU is often the cheapest way to provide a DP switch.
     
  16. winston1

    winston1

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    Or a plug and socket. Or even cheaper twist the ends together and cover with tape.

    What I'm saying is the cheapest is not usually the best or correct way.
     
  17. ban-all-sheds

    ban-all-sheds

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    There is not a single thing sub-optimal or incorrect about using an FCU to provide DP switching.
     
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