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Screwing in bulb temporarily kills power in cables

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

  1. JohnD

    JohnD

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    I just carried out a survey.

    100% of the two houses I inspected did not have DP switching of external lights.
     
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  3. JohnW2

    JohnW2

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    Nothing more 'sub-optimal' than, say, having two RCDs in series - but we do usually advise people against such a practice (unless it is a deliberate act to provide RCD redundancy).

    Kind Regards, John
     
  4. ban-all-sheds

    ban-all-sheds

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    What's inadvisable about using an FCU?
     
  5. Scramble

    Scramble

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    Solved now. I thought that if the pen is giving me faulty readings, and the circuit isn't live, then possibly it's the switch that's gone. It's a double dimmer switch, the other dimmer controls another two outside lights (maybe the false live reading was coming from this setup).

    I don't have a multimeter at the moment, and had to be at home yesterday so I coulnd't go out to buy one, but I remembered that I have a spare double dimmer in the loft, so I changed that one for the existing one, and everything works now.

    Thanks for the help.
     
  6. JohnW2

    JohnW2

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    I'm sure that many (most?) don't.

    I'm actually far from convinced that there really is a need, since I would think it pretty (very?) unlikely that water ingress would ever result in a low enough impedance N-E 'fault' to cause and RCD to operate (which is the main reason people advocate DP switches).

    In terms of my personal experience, I've certainly experienced RCD trips due to water ingress into external lights etc., but it has always been possible to reset the RCD after opening the light's SP switch.

    Kind Regards, John
     
  7. JohnW2

    JohnW2

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    The same thing as having a second 'unnecessary' RCD - the 'inconvenience' associated with having two protective devices doing the same thing, so that one, the other or both may operate in the event of a fault.

    Kind Regards, John
     
  8. JohnW2

    JohnW2

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    I'm glad that the problem is solved. Dying bulbs certain can kill dimmers.

    I must say that I can't recall having previously heard of outside lights on dimmers!

    Kind Regards, John
     
  9. bernardgreen

    bernardgreen

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    I have found the opposite. The problem is that the leakage Neutral to Earth through the water in the lamp is a very small percentage of the total Neutral current and thus the RCD trips when a heavy load on another circuit puts a large current through the Neutral. Time for this again..


    I dim my outside lamps by connecting two lamps in series
     
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  11. ban-all-sheds

    ban-all-sheds

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    I think that a 13A fuse in an accessible location operating as well as or instead of a device in the CU when an outside light has failed anyway is so unlikely to be "inconvenient" that objections are nugatory.
     
  12. JohnW2

    JohnW2

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    I can't really disagree with that, but many people 'advise against', say, having a (dedicated) RCD/RCBO at both ends of a cable supplying an outbuilding.

    Kind Regards, John
     
  13. Taylortwocities

    Taylortwocities

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    Glad it is working. but random substitution of parts isn't the way to fix electrical problems.

    Before anything else happens, why don't you follow the advice given at the weekend
    Then you'll be prepared for the next weekend of fun.
     
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  14. JohnW2

    JohnW2

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    I obviously can't argue with your experience but, as I have said, I have never experienced an N-E 'leak' due to water ingress causing an RCD to operate.
    Your famous diagram assumes a 'short' (of negligible impedance) between N and E, but everything is (quantitatively) very different when the fault path (e.g. through water) has a relatively high impedance ...

    ... an L-E fault path (through water) of about 7.67kΩ would be just enough to trip a 30mA RCD with a 230V supply. It therefore follows that a fault of the same impedance between N and E would only result in an RCD trip if the N-E potential difference were 230V - which is obviously an impossible situation, no matter how high the 'total neutral current'. For an RCD to trip as the result of a N-E fault (of non-negligible impedance) at any credible N-E potential difference would require the impedance of the fault to be dramatically less than 7.67kΩ - an impedance which I do not think one would hardly ever see as a result of water ingress alone.
    If you tried that with new-fangled 'lamps' (CFLs, LEDs etc.), I think that the results (if any!) would be rather unpredictable.

    Kind Regards, John
     
  15. aesmith

    aesmith

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  16. JohnW2

    JohnW2

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    Indeed - that's what the last page or so of discussion has been about. I generally do it and advise it but, as above, am far from convinced that it's actually necessary.

    Kind Regards, John
     
  17. Astra99

    Astra99

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    The discussion about FCUs, double-pole isolation, single pole isolation or even no isolation at all, is all well and good until you remember Sozdt's Law, something with which we are all VERY familiar!
     
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