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Household electrics

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10th May 2004 | 18:56

This weekend I learnt how NOT to replace a ceiling light fitting, and then how to do it properly.

The lighting circuit consists of a number of segments going from the consumer unit to each light fitting in turn. At a fitting there is a spur to the switch and a spur to the light (though the latter is removed with the fitting). So the circuit is something like

                          live  _______     |   \ switch
                          +---==\      \===-+  |
                          | +-==/______/===----+
      _________   live    | |      _______________
C ===\         \=========-+-)---==\               to next
U ===/_________/=========---)-+-==/_______________fitting
        cable    neutral    | |
                            | | <- neutral
           appears to be -> | |
          neutral but is     to
      either off or live    light

So you have three cables hanging from the ceiling, one of which is special (it goes to the switch) but which might not be easy to distinguish from the others.

Do not connect all the red wires together and all the black wires together. This means the light is on when the switch is off, and turning the switch on causes a short circuit which upsets the CU. *bang*

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Comments {6}


from: simonb
date: 10th May 2004 13:41 (UTC)

In this situation its required to mark the black wire from the switch with a piece of red (either electrical tape or red sleaving) or use a two core pieces of cable were both cores are insulated with red sleaving so that this doesn't happen.

There are also other lighting systems which use junction boxes which are much simpler - you only get the live-neutral-earth triple going to the ceiling rose with the switching logic dealt with in the junction box. Its not amazingly common tho as it costs more due to needing junction boxes - I prefer it tho as it is simpler to work with.

And you can get strange systems which are switched neutral, but they aren't as common in the UK.... mainly since its possible to electrocute yourself when changing a light bulb.

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Tony Finch

from: fanf
date: 10th May 2004 14:12 (UTC)

"required to mark"? in a 40-year-old council house?

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from: simonb
date: 10th May 2004 14:27 (UTC)

True; I was more talking about modern wiring standards :)

However I have encountered wiring of that sort of age which did do the right thing, however back then it wasn't a requirement.

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from: timeplease
date: 10th May 2004 17:17 (UTC)

I don't believe that switched neutral systems are legal for new installations.

Switching being dealt with in the junction box is much better as far as I am concerned; I installed all of the new electrics in the Coalheavers Arms using this system. The previous system that I stripped out had been done in single and earth wiring! Really hard to trace when you're trying to work out what wire goes where - connect a red wire to live and a random black wire ends up at live potential too...

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from: kaet
date: 10th May 2004 15:57 (UTC)

Surely the switch cable is of the exciting double red variety? Or there's something odd about your house, :).

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from: kaet
date: 10th May 2004 15:58 (UTC)

Ah, okay, read comments before posting....

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