Strange switch wiring, explanation?

[monkey73]

Hi

I'm looking at installing some more zwave network controlled switches which i've done many of in the past, but i've come to the latest light that I want to enable and come across some wiring that i'm not entirely sure how it works. I'll try any explain the best I can.

The switch I am looking at is a 2 gang switch. 1 switch is part of a 2 way switching set up for a lighting circuit that I'm not interested in right now. The other switch controls the lighting circuit i want to enable. This switch has the normal wires I would expect, old colours blue and red for live and switched live. That wouldn't be a problem if that was it, except there is another short "bridging" wire that connects from L1 on the other switch, to COM on the switch I am interested in. I can't work out what this does or what its purpose is given that these are 2 separate circuits. Obviously the 2 lights are working and being controlled as i would expect now, so it must be a valid wire, but if someone could explain why it's there, that would be great.

Thanks

 

[ban-all-sheds]

Scary to think that you think you know enough about how lighting circuits are wired and how switches work to be gaily going around fiddling with them.

Just why is it not blindingly obvious why there would be a link from L1 (or L2) of a 2-way switch to COM of another?

I can't work out what this does or what its purpose is given that these are 2 separate circuits.

They aren't.

if someone could explain why it's there, that would be great.

Look at the diagram above.

If you wanted to swap the bottom switch for a 2-gang, from where would you get the permanent live for the new gang?

 

[monkey73]

Scary to think that you think you know enough about how lighting circuits are wired and how switches work to be gaily going around fiddling with them.

Except I don't, hence why I'm asking and haven't just gaily gone round rewiring things I don't know about.

If you wanted to swap the bottom switch for a 2-gang, from where would you get the permanent live for the new gang?

Why would the permanent live not come down one wire of the 2 core cable that is connected to that switch, like all the other rooms in the house?

 

[bernardgreen]

The other switch ( the one you are not altering ) is a two way circuit. It is getting its Live from the same wire as supplies the other switch.

 

[monkey73]

The other switch ( the one you are not altering ) is a two way circuit. It is getting its Live from the same wire as supplies the other switch.

AH! That makes so much more sense now. I was looking at it the other way round in my mind. See it's easy when you get a straightforward answer!

A follow up question then. The 2 way switch in the picture above (the top one) has live and switched live cable going back to the light. In my switch I have the 3 core joining the 2 switches and then a single wire also coming out. I presume then that single wire is the switched live going back up the light, and that is not a 2 core able because instead of getting it's permanent live from the supply cable, it's getting it from it's adjacent switch instead, as you answered. Right?

Edit: and if that is right, can you tell me why it's not getting it's live from the supply? I'm just curious