I'm in the process of trying to add a smart relays to my light switches across the house. In most cases I've managed fine and I understand how the circuit works, to me that is critical before I make any changes.

In one case, the bathroom, i'm struggling to understand what's going on. I've attached a picture of the existing setup below. The main question I have is why are there 2 connections out of common so I can understand what impact adding to this circuit will have.

For reference, the left hand side currently controls 3 downlights. The right hand side controls an extractor fan within the bathroom (which can stay powered on after the switch is turned off if the humidity is high). The extractor fan also has an isolating switch separate from this double gang switch, could this be something to do with it?

Right hand side is the standard neutral block. Colours are accurate for UK wiring.

Existing Wiring

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1 Answer 1


The wiring in the switch box isn't only for the switches. It is also carrying supply power onward to other points of use. That is the reason for the extra wires coming off COM.

This would be easier to understand if the 3 brown wires you drew terminated in a block just like the neutrals, and then 2 pigtails from the block went on to feed the two switches on their COM side.

Pretend it's like that, or get blocks and pigtails and make it actually like that.

  • Not sure I follow. Maybe my drawing is a little unclear, but the little circles are the terminals on the back of the switches where wires are connected. Top being COM, bottom are both L1.
    – Dan
    Nov 6, 2019 at 20:07
  • I understood that 100%. There's nothing magical about the wires going to COM; they're simply using that as a splice block to distribute always-on power to other points of use. In other words the wiring in this switch box isn't only about the switches. Nov 6, 2019 at 20:13
  • Ok that's sort of what I expected, I guess i'm just confused as to what else it would be powering at this stage. I assume i can largely ignore where these go as I know that L1 on each switch is going to be the switched live powering the devices?
    – Dan
    Nov 6, 2019 at 20:23
  • @Dan It never hurts to spend a little time to find out where the wires go. It might help you in the future to solve another problem and also help you to understand this crazy world of wiring.
    – JACK
    Nov 6, 2019 at 20:25
  • 1
    I assume there are other switches and outlets in your house? Do you imagine that all of them are a cable straight back to the service panel, of course not... they daisy chain off nearby sources, just like this. Yes, you don't need to care where they go, heck, you don't even need to care which one is supply and which are onward. As long as you are sure they are directlly connected to each other. Nov 6, 2019 at 20:25

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