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Replacing a Light Switch

Discussion in 'Electrics UK' started by Mshepp, 22 Feb 2016.

  1. Mshepp


    9 Jul 2015
    Thanks Received:
    United Kingdom

    I am trying to replace a lightswitch but have run into one small problem which isn't helped by the fact the lighting setup is a bit weird so hopefully I'm able to explain the setup correctly.

    In the lounge there is a two way switch which operates either of the lights in the lounge. One of the switches must be on in order for the other lounge light to turn on, and this also has to be on in order for the hallway light to turn on.

    The plug socket I'm replacing has the following connections, L1, L2 and Common where as the new one only has 2 x L1, 2 x L2 and 2 x L3. Will this replacement socket work because I'm struggling to figure out what should be plugged in where. The cables that are available to plug in are 1 x blue, 1 x red and 1 x black with a short red cable that was bridging the "common" connection on the old socket.

    Hopefully I have explained this correctly, I'm finding it very confusing
  2. PBC_1966


    1 Sep 2015
    Thanks Received:
    Redding, California
    United States
    Some newer switches have adopted this much less clear designation system. The usual "translation" is that Common/L1/L2 are L1/L2/L3 respectively.

    It does sound as though the existing 2-gang switch was wired incorrectly at some time. Are the three conductors you have listed (red/black/blue) all that are present in the box?
    Last edited: 22 Feb 2016
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  4. Taylortwocities


    28 Jul 2006
    Thanks Received:
    United Kingdom
    Take a photo of the old switch for reference, before you disconnect anything!
  5. ban-all-sheds


    27 Aug 2003
    Thanks Received:
    United Kingdom
    I'm afraid you aren't the only one.

    Do you mean two-way, i.e. there are two switches, each one turning the same light(s) on and off, or do you meant two-gang, i.e. the switch has two rockers, each one controlling different lights?

    As an aside, it is neither a plug nor a socket, it is, as you originally said, a light switch.

    So the old switch is a one-gang, and the new is two-gang?

    Colours aren't much help, as electricity doesn't know what colour the insulation is, and the switches and conductors will do what they do based on how they are connected up, not on what colour they are.

    Bridging it to where?

    As long as you make absolutely sure that you keep an accurate record of which conductors go where, and which sets of 3 terminals (C/L1/L2 and L1/L2/L3) go together ( they may well not be laid out in the same arrangement on the old & new switches) you can transpose C to L1, L1 to L2 and L2 to L3.


    That is weird, and wrong - it's been fiddled with by someone who doesn't know what they are doing, and really you should not continue that tradition.

    You now have to choose between Plan A and Plan B.

    PLAN A:
    PLAN B:
    • Get an electrician.
    There are some irresponsible people here who will tell you that there is a Plan C, which is to start trying different things without really knowing what's going on, hoping to get it working by luck, or by blindly following instructions to put-this-wire-in-that-hole without any idea as to why. Please don't listen to them - you must know what, and truly understand what, you are doing.

    Electrical-installation-by-guesswork is a foolish idea.

    There is no Plan C for anybody sensible.
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