2-gang to 1-gang switching

Discussion in 'Electricians' Talk' started by thestraycat, May 17, 2023.

  1. thestraycat

    thestraycat Member

    Hi guys,

    I know this is likely an 'electrical first week question', but just wanted to theoretically talk it out.

    - I have 7 downlights in my kitchen, they operate in 2 zones. All is currently working fine.
    - 4 are controlled by gang-1 on the switch
    - 3 are controlled by gang-2 on the switch

    My current no-neutral dimmable smart switch has the lives from both circuits going to L

    and then the switch live from circuit 1 going to L1
    and then the switch live from circuit 1 going to L2

    My new switch is a smart, dimmable, 1-gang.

    Could i theoretically combine both L1+L2 together to make a single switch live and use that on my new 1-gang switch to control both zones?

    Picture for reference from the old switch wiring:

    upload_2023-5-17_14-22-48.png


    Picture of back of new switch:

    upload_2023-5-17_14-23-52.png
     
  2. thestraycat

    thestraycat Member

    Ammendment:

    "and then the switch live from circuit 1 going to L2"

    should be:

    "and then the switch live from circuit 2 going to L2"
     
  3. FlyByNight

    FlyByNight Screwfix Select

    What lamps are fitted ? What power are they?
     
  4. MGW

    MGW Screwfix Select

    Step one is to work out what circuit each light is supplied from. To combine line feeds can be very dangerous, it can result in borrowed neutrals, and overloading, there may be no problem, but one can't assume.

    I have in my hall a 4 gang 2 way switch plate, this has three supplies at least, one for upstairs lights, one for entrance level lights, possibly one for lower level lights, and also extra low voltage for landing lights, I found wrong lines had been used when I went all RCBO rather than fuses, and I am well aware I have multi circuits to same switch.

    I have removed a few smart switches and gone to smart bulbs instead, mainly due to compatibility issues, lights not switching off, and a shimmer when on, I know £8.48 per bulb is off putting when you have seven of them, and also power cut can cause lights to come on. However with smart bulbs you can also adjust the warmth and even colour of the bulbs.
     
  5. thestraycat

    thestraycat Member

    @MGW - Thanks for the info. There were no neutrals in the switches... can the neturals still be borrowed?

    To be honest, i run all my smart automation in code and issue my commands directly to the light switches, gives me everything other than colour temp (which in my case is a hardwired set-and-forget switch on the downlights. I generally use smart bulbs in lamps where the colour and temp is more commonly tweaked. But for main lights i feel switches are the best to go. (If you want you can even decouple the light switch relay to allow the light switch to accept a smart bulb with brands like Aqara for example.)
     
  6. MGW

    MGW Screwfix Select

    Yes, in real terms it is a borrowed line, but it's called a borrowed neutral. The problem is you can think some thing is isolated, then when some thing is turned on, it makes the neutral line through the device connected, which is why it is such a no no.

    The other problem is joining the two lines, it means one could have two MCB's feeding all the lights, which means twice the power to what was designed is required to trip the pair of MCB's.
     
  7. thestraycat

    thestraycat Member

    This work has been done on a ground floor extension with no 2nd floor above it. In the worst place, if the line that was 'borrowed' was also on the ground floor and all ground floor lights have been tested prior by an electrician to be corresponding to the single ground floor lights RCBO. Would that mitigate the risk? (worst case)

    Again would this be mitigated somewhat by knowing all ground floor lights are going to a single ground floor RCBO?

    Sorry not an electrician just trying to understand :)
     
  8. MGW

    MGW Screwfix Select

    Since using RCBO there is really no problem, as if there is a borrowed neutral it would trip.
     
  9. thestraycat

    thestraycat Member

    @MGW Phew good to know!

    Hmmm got me thinking though.

    My downstairs WC (small box room toilet) has a a '24w LED suface mount downlight' installed. And we've always had issues with it.

    It is the only light on the downstairs circuit that will trip the ground floor RCBO 1 in 20 or so times its turned on...

    Due to the WC having an old pull switch when we moved in we replaced it with one of these RF recievers with a 24w inverter so we could use a wireless light switch and relocate the switch someone a bit more convenient. Any idea why it might be tripping? At first i asked the sparky to replace the extractor fan thinking a surge of inrush was causing it, but it's still doing it even after the new extractor went in. Last week, i killed the power and moved all the connectors over to wago's and made sure nothing was shorting, and it's still doing it.

    Either the wiring or the light i'm assuming? Or an incompatibility with the RF reciever and the LED inverter??

    https://ilumos.com/products/i-lumos...d-remote-controls?_pos=1&_sid=d78d39bb1&_ss=r
    upload_2023-5-19_18-7-49.png [​IMG]



    [​IMG][​IMG]
     
    Last edited: May 19, 2023
  10. MGW

    MGW Screwfix Select

    One can draw around 6 watt max without tripping an RCD/RCBO so with a LED smaller than 6 watt one could have a borrowed neutral situation. But very few lamps are less than 6 watt.

    I would use my meter Testing voltage.jpg and measure the leakage, I have done it for whole house, and got around 26 mA, which split between 14 RCBO's is not real problem, we tend to use a meter like this VC60B.jpg to test insulation resistance, since using 500 volt, it tends to show any faults, but it measures using DC, and our supplies are AC, so any inductive or capacitive linking will not show up, one needs the clamp-on to show that.

    However the most likely is a neutral to earth fault, which I would look for first, with no load earth and neutral are at same potential so even if connected together no current will flow between the two, but as we increase the load, the neutral voltage raises towards the line, so then it can trip, so with the isolator off, I would be checking the neutral to earth resistance.
     

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