Strange Loft Light Switch Configuration

[sandydog]

Hello

Can anyone help me wire a light switch in my loft? We've been without light in there for about 6 years! It's our bedroom.

The thing is, there are three cables.

Cable 1 has 2 x red wires, and 1 earth Cable 2 has 1 red, 1 black and 1 earth Cable 3 has 1 red, 1 black and 1 earth

All three earths are joined together, as are cable 2 and 3's black wires.

The switch was working fine until we had to pull the loft walls down, and unfortunately we didn't make a note of what came from where. I can't even remember if it's a two way switch working the landing light. (The landing switch is a two way, one working the loft light).

Please, can anyone help me??

[sjones]

Any idea which of these wires goes to the light fitting? Get a electrical screwdriver (one that lights up on mains) and carefully check which of the reds are live. I suspect cable 1 is the switch cable and cable 2 or 3 will be power and the other the light feed.. If thats the case then cable 1 would connect to the two red wires together when switched. However I could be wrong so perhaps better to get a qualified electrician in.

[Dave Plowman (News)]

Where are these cables? Are they all inside the two *gang* (not way - although it will probably be a two gang two way) switch?

Or are they the cables at the light fitting?

[sandydog]

The cables were all inside the light switch. They are now sticking out of the wall, waiting for me to fit a new one. I've fitted a new light, and didn't find anything unusual about fitting it. (it was a while ago!)

[John Rumm]

OK, deal with the two red wires first - chances are they are the live in from the ceiling rose, and the live return to the pendant. If you have a multimeter you could check this. With the power on[1], one would be live and the other not. If you measure the voltage between them, you would expect to see 240V with the bulb in, and less with it out.

The other wires will be used for the two way switching. However it is now a bit more complicated since you only need three wires between the switches for this to work (you probably have four!) (so it is also more comon to use a bit of three core and earth for this wire instead of two separate cables). Have a look at the connected switch downstairs and see which wires go to the "com" terminal, that ought to at least tell you the colour of the one to take to com on the missing switch. You may find they have for example take both blacks to the com downstairs and used one of the lives from each for the L1 and L2 connections.

Report back on what you find in the downstairs switch and we can give you a bit more information.

[1] You need a multimeter for these tests, and you need to be confident working with mains voltage. If in doubt seek help!

[sandydog]

Thanks everyone for your super quick replies.

John, I've had a look at the switch downstairs (which also has 3 cables running into it.) This switch has a connector in it with 6 holes (sorry about the terminology!) There are 2 commons. Both of these have a red, but the bottom one has a loop of red (independant of cables) going from the common to the 1 way switch hole in the centre, which has a blue wire going into it. By the way, one of the cables has a yellow wire - no blacks in sight.

Unfortunately I don't own a multimeter, but I don't mind working with electricity. (turn the mains off, and when I turn it back on, switch the light on with a broomstick! ha ha)

Thanks for your help!

[John Rumm]

OK - that is a two gang, two way switch - i.e. two separate two switches built into one face plate. One no doubt for the light in the room, and the other for the light upstairs.

One of the cables sounds like a three core and earth (that will be the one with red, yellow, and blue wires). Normally you would just run this to the other switch - but something different has been done here.

Have a look at the diagrams here:

might be worht looking at the connections at the light fitting upstairs in the loft to see if that is where the three core and earth has been taken.

Might be worth picking up a cheap £5 jobbie from a local Maplin, they often come in useful for lots of things.