Understanding lighting switches

[Bromley86]

I've only ever seen switch wiring before that uses the same cable i.e. live in (red) and live out (black with red sheath). The stuff in our mid-80s house is different.

There are exceptions, but generally where there is one light there are two cables. The reds are both used in the switch and the blacks tidied into a choc box. Where there are two lights, there are three cables, again with only the reds used in the switch and the blacks in a choc box.

Would I be right in thinking that those black wires are likely not connected to anything at the other end? Or might they be neutrals?

Given that nice pattern of light fittings+1 = cables in switch, does anyone know why our living room, which has 2 wall lights, has four cables in it (same setup as the others though - one red in, three out, all the blacks in a choc box)? There are no other switches for that room or, as far as I can tell, any other electrical sockets/lights to account for the extra wire.

[auctions]

It sounds like you have an incoming supply (L+N), and a wire going on to the light (swL + N) - the wires are not looping at the ceiling rose.

The wall lights have been treated as 1 light, with a wire continuing from the first one to the second, on the basis that you're unlikely to want to switch them on and off independently.

A

[ARWadsworth]

Are you sure that it is one incoming live and 3 outgoing lives? It could be the other way around.

If it is as you said, then someone has removed a wall or ceiling light but left the cable connected in case it is needed again.

Adam

[Dave Plowman (News)]

I'm not quite clear about your description, but 1mm TW&E with two reds (now browns) is available for use as switch drops. Saves having to sleeve the black. Very few amateurs use it due to the cost of buying an extra reel that may not get used up.

[John Rumm]

That is "loop in" wiring - the power links from one ceiling rose to the next, and a separate drop wire is taken to the switch to provide a switched live at the rose.

Sounds like power is being take to the switch positions rather than the roses in your house. It is also likely since you only seem to have power in and a switched feed out that there is a separate junction box burried in the fabric of the house for each light that is picking off the switch power feed from a backbone of some description (either a radial or possibly a central star connection).

Almost certainly neutrals.

When you say "has two wall lights" - I presume these are the only lights in the room? My guess would be that even if that is currently the case, it was wired to allow for a central lighting position as well. Hence there is probably a wire dangling in the ceiling void where a light used to be or potentially could be.

[Bromley86]

Sounds like a good product. The reds in my cabling are from separate cables though. The switch itself only has red wires in it and, for example, if there are three red wires then there are three white twin core and earth cables.

For what it's worth (given my lack of knowledge) I think Auctions has the right explanation. If I understand him correctly, the wiring uses the switch itself in place of the junction boxes/ceiling roses. So what I took to be one cable in and two out to the lights is actually one cable out to one of the lights (which are then serialed together) and two cables maintaining the lighting circuit even when the switch is off.

[Bromley86]

Thanks! I think that you're right and the majority of the switches are being treated as ceiling roses.

That's what I was hoping for, as that means I'll be able to take a line off the switch in the cupboard to feed the alarm. It's got to be better than the current situation - it's fed from another room's socket circuit with the wire tucked between the skirt and the carpet gripper around half of that room!

Is there any way to confirm that a wire is a neutral? I have (but have never used) those basic test screwdrivers - I've just never summoned the courage to jam it into anything live before :) . I'll have to read up on how they work.

Thanks again (and to ARW). I wouldn't have thought of that. So the extra two cables in this switch are there to feed other switches (i.e. this particular switch is being treated as a junction box rather than a basic rose).

I'll test it all next week to see what wont turn on when I pull various wires, but at least I've got a better idea now.

[Bromley86]

Actually I'm beginning to think that the switch for the living room is that junction box. It has two "spare" cables. There has never been a ceiling light in the living room as it's got the original ceining with fake beams and no filled hole mark. That's not to say that there might not be a cable in the void - the beams were added after the house was built but before the first owner moved in.

Thanks also for the neutrals confirmation.

If I wire a lamp (my only test equipment :) ) between the pre-switch live and the neutrals and they turn out not be neutrals, is there likely to be any effect other than either nothing or the fuseboard tripping?

[Lurch]

On 27 Apr 2007 12:46:50 -0700, " [e-mail]" mused:

Yes, several ways. You need a decent multimeter of some sort.

Throw it in the bin, they're useless and unreliable.

[John Rumm]

I would expect the place to have been wired with the anticipation there would be a central light. The buyer may have asked the developer not to fit it, but there is still a good chance that the wire is in place.

If that were the case you would have four wires in total - power in, switched power out to three light positions (two wall, one ceiling).

The the only other thing they could be is live or not connected. In either of those cases the bulb won't light - nothing should trip.