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Dimmer switch with 2 red wires + earth?

Discussion in 'Electrics UK' started by SRJDIY, 2 May 2021.

  1. SRJDIY

    SRJDIY

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    I have a dimmer switch that recently stopped working, so I'm hoping to replace it with a new one.

    This is the only switch that controls the light in question, so based on my googling I expected to see one red or brown live wire and one black or blue switched live wire going into the switch. However, both wires going in are red.

    My understanding is that I need to attach the live wire to common and the switched live to L1 on the new switch, but I'm not sure how to do this as I'm not sure if both red wires are live or if only one is and the other's been mislabelled.

    Adding to the confusion, the label on the old switch says 'connect either way around', but one terminal has a red dot underneath it and the other doesn't, suggesting there is a difference.

    Does anyone know what's going on please? Is this something with a simple answer I just don't know, or is it time to call an electrician in?

    I've also attached a picture if it helps. The label says "Connect wires either way around, 1 way 250w, 200-250v, suppressed to BS800, Pats 1514161 & 2063573''.

    Thank you!
     

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  3. AndyPRK

    AndyPRK

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    It doesn’t matter which way round you connect
     
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  4. sparkwright

    sparkwright

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    Ideal time for a bit of learning.

    The switch interrupts the live wire, so one wire is permanent live, the other wire is switched live.

    The switches that you have seen with one red and one black (or one brown and one blue) are NOT live and neutral. They are live and switch live.

    So what should happen is the black wire gets sleeved with a short piece of red sleeving, to indicate it is a live wire and not a neutral wire.

    Your cable consists of two red wires, which is the 'best' cable to use, but is relatively rare.

    Electricians test to use red and black (or brown and blue as it is now) for convenience - also it is far easier to tell which wire is which - should the need arise.
     
  5. SRJDIY

    SRJDIY

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    Okay, so from what you're saying it sounds like my switched live wire is also red because it can be live depending on whether the switch is on or off. That makes sense.

    This is the problem - I can't tell which is which. If it doesn't matter which way around I connect them to common and L1 on the new switch, why is this the case please? And if it does matter, what are the consequences likely to be if I get it wrong?
     
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  7. sparkwright

    sparkwright

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    It's just a switch, so usually it doesn't make any difference.

    SOME manufacturers of dimmers will stipulate in their instructions which way round the wires are supposed to be. They will include a diagram, showing one wire from the LIVE supply, and one wire going to the LiVE of the lamp.
     
  8. flameport

    flameport

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    A single switch with only two wires just connects those wires together to turn the light on, and separates them for the light off.
    Makes absolutely no difference which wire is connected to which terminal. All that matters is connected together or not.

    For switches with more than one wire or multiple switches, it does matter, as connecting the wrong pairs of wires together will operate the wrong light(s).

    The only difference with a dimmer is that it alters the amount of current that can flow between the two wires, rather than connecting them directly together.
    Still doesn't matter which way they are connected.
     
  9. EFLImpudence

    EFLImpudence

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    Nor does it matter what colour they are.

    Even if they were different colours, you still wouldn't know which was which.
     
  10. SRJDIY

    SRJDIY

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    Excellent; thanks for the advice!
     
  11. DIYnot Local

    DIYnot Local

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