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Can somebody explain this lighting circuit?

Discussion in 'Electrics UK' started by Alex evans, 22 Jul 2021 at 1:36 AM.

  1. Alex evans

    Alex evans

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    So.. today i changed a couple of lights in my kitchen.

    The first light i changed was simple. Live to live neutral to neutral ect. So when putting in my new light fixture, i just did the same, obviously.

    However.. when i came to the 2nd light there was more going on.

    Have a look at this photo i took of it.

    I took a photo and marked with tape the black wire going into the live connection. I copied the set up EXACTLY, and of course it works fine.

    But was wondering WHY is it like this?

    Can somebody educate me?

    The lights are switched on by 2 switches (on a single pannel)
     

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  3. plugwash

    plugwash

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    A switch works by interrupting the live feed to a light.

    If you were wiring with single wires, you would take the permanent live to the switches, the neutrals to the lights and then take a switched live wire from each switch to the corresponding light.

    But while single wires in conduit or double insulated single wires are legitimate installation methods, most of the time we don't use single wires, we use cables. There are a few common arrangements for wiring lights with cables:

    1. The loop at light method, "circuit cables" carrying permanent live and neutral are routed to the lights, then a "switch cable" takes permanent live down to the switch and brings switched live back.
    2. the "loop at switch" method, "circuit cables" carrying permanent live and neutral are routed to the switches, then a "cable to lighting point", takes switched live and neutral to the light.
    3. the "junction box" method, "circuit cables" carrying permanent live and neutral are routed to junction boxes, then a "switch cable" takes permanent live down to the switch and brings switched live back and finally a "cable to lighting point", takes switched live and neutral to the light.

    In addition to the common arrangements there are a whole plethora of variants. I doubt anyone could usefully enumerate them all.

    When a black (or blue on more modern installs) conductor is used as a switched live it should be marked with red (or brown on more modern installs) sleeving, but in practice this often was not done.

    In your case I would guess the cable at the back is bringing in permanent live and neutral. The cable on the front right is taking the permanent live down to the switch and bringing the switch live back.

    The cable on the left which has it's red wire cut off is probably taking the neutral to another light fitting. I suspect that the live for that other fitting is tapped off at the switch.
     
  4. Alex evans

    Alex evans

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    Thanks plugwash.

    So taking a picture and recreating it on the new fitting WAS. A good idear.

    And yeah, it did occur to me to mark that black wire going to the neutral.. which i did also.

    Who knew, theres more then one way to skin a cat... Or wire a cat.. or something.
     
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