Light switch wiring

[mrcharly-YHT]

Riddle me this oh wise people.
There is one switch on the wall two gang.

One grey cable coming to that switch from the fuse box.

The switch controls two lines of lights.

How can that work?

The only way I can see it working is if the the single cable is carrying three live wires. One from the source, and then one each for each of the lines of lights. Inside the fuse box. The live wires for the lights must be connected onto the cables. Is that right?

I can't actually open the fuse box. It is metal and rusted shut

[Diver300]

The only way I can see it working is if the the single cable is carrying three live wires. One from the source, and then one each for each of the lines of lights. Inside the fuse box. The live wires for the lights must be connected onto the cables. Is that right?

That is basically correct. Strictly speaking, that would be a live wire and two switched live wires. Triple and earth cable is available for lighting circuits.* Such cable is wider than the more normal twin and earth. Some cables say what cores are inside them, and on the grey mains cable it's often embossed into the surface.

*It used to be red, blue, yellow and uninsulated. It's now three shades of grey and uninsulated.

[mrcharly-YHT]

Thanks, that makes sense.
This doesn't look like four strand( three way plus earth). I suspect it is ordinary three strand cable. That fits with the rest of the botch in the building.

[Feanor]

A photo of the inside of the switch would help.

Does the cable go directly from the consumer unit to the switch?
Is it actually the same cable?
Or does it go to some junction box first?

It could be that it's regular 2-core from the consumer unit, to a loop-through junction box somewhere (where it will continue as 2-core to the next light), then with a 3-core drop to the switch which returns 2 switched lives to the JB, to feed the 2 lights.  That would be my suspicion.

Lighting circuits have historically been done in different ways:
Loop-through at the switches ( where the incoming 2-core L+N come to the switch, and loop out to the next light switch; then a 2-core cable with Switched-L and N then go up to the fitting);
Loop-through in junction boxes ( where the incoming 2-core L+N come to a JB, and loop out to the next light. Other 2-core cables run from the JB down to the switch, sending a L and returning a Sw. L, and then another carrying the Sw. L +N to the light fitting.  THere is no N at the switch.);
Loop-through at the ceiling rose ( where the incoming L+N are come into the ceiling rose, and loop out to the next light. A cable runs down to the switch carrying L and returning Sw. L. There is no N at the switch.  Basically, the same as the JB scenario, but the JB is built-in to the ceiling rose.).

[Kim]

Someone needs to obtain a Bill Wright style collection of unorthodox switch wiring photographs and curate them into an electrical puzzle book.  With answers at the back.

[mrcharly-YHT]

Switch box is rusted shut as well. I'll get round to replacing all of this. It's on a barn and I can see all of the cables so there is definitely only one cable going to the switch.

[Diver300]

Switch box is rusted shut as well. I'll get round to replacing all of this. It's on a barn and I can see all of the cables so there is definitely only one cable going to the switch.

If the cable is three core, and two sets of lights are controlled separately, that doesn't leave a wire for earth. The switch box is rusted shut, so that must be metal. There must be dampness to cause the rust. Three core cable has one uninsulated core, which should only be used for earth.

That all looks very dangerous. Moisture in the switch could easily make the case live.

With all those things wrong I would be surprised if there is correct earthing of the fuseboard, or effective RCD protection.

I'm glad to hear that you'll be replacing it all.

[mrcharly-YHT]

I've taken a second look at it this morning.

Can't see any way of getting a feed for another light from the old consumer unit.

We have a new unit in the barn (it is on its own feed from the house board) which has an unused MCB. I'll take the new light wiring from that one.

MrsC would like some more sockets, but I think they'll have to wait until I replace the original consumer unit.

[Diver300]

We have a new unit in the barn (it is on its own feed from the house board) which has an unused MCB. I'll take the new light wiring from that one.

MrsC would like some more sockets, but I think they'll have to wait until I replace the original consumer unit.

You could just replace the new consumer unit with a larger one, while keeping its circuit breakers etc, so you would only be paying for the housing. You would then run the barn on just one consumer unit.

Of course, I don't know what the layout is, and how large the barn is etc etc.

[mrcharly-YHT]

We have a new unit in the barn (it is on its own feed from the house board) which has an unused MCB. I'll take the new light wiring from that one.

MrsC would like some more sockets, but I think they'll have to wait until I replace the original consumer unit.

You could just replace the new consumer unit with a larger one, while keeping its circuit breakers etc, so you would only be paying for the housing. You would then run the barn on just one consumer unit.

Of course, I don't know what the layout is, and how large the barn is etc etc.

No, because there are two main supply cables to the barn.

Original - for lights and general sockets
New - currently only for kiln (which is 7kW)

[CarlF]

Switch box is rusted shut as well. I'll get round to replacing all of this. It's on a barn and I can see all of the cables so there is definitely only one cable going to the switch.

If the cable is three core, and two sets of lights are controlled separately, that doesn't leave a wire for earth. The switch box is rusted shut, so that must be metal. There must be dampness to cause the rust. Three core cable has one uninsulated core, which should only be used for earth.

That all looks very dangerous. Moisture in the switch could easily make the case live.

With all those things wrong I would be surprised if there is correct earthing of the fuseboard, or effective RCD protection.

I'm glad to hear that you'll be replacing it all.

I assume the light fittings are fed by cables coming from the old fuse box enclosure?

If it's three-core & earth:
Core 1: Permanent live, from the fuse box to the common terminal on the first switch, jumper wire from there to common on the second switch.
Core 2: Switched live 1, back from the L1 connection on the first switch to a terminal block inside the fuse box enclosure, feeding the cable to the first light fitting.
Core 3: Switched live 2, back from the L1 connection on the second switch to a terminal block inside the fusebox enclosure, feeding the cable to the second light fitting.
(which seems safe* as long as the earth is connected properly - which you can't assume! so proceed with extreme caution anyway...)

If it's twin & earth: same as above, but using the uninsulated conductor instead of one of the three cores above. 
(which as Diver300 says is not safe at all, especially given the switch and fusebox enclosures are metal)


*well, assuming there's also an RCD upstream of the old board somewhere, and there are no issues with earthing given this is an outbuilding, which introduces its own complexities...

[mrcharly-YHT]

You are all making me want to open up that switch enclosure.

I have some new switches on the way. Might replace the existing on when they get here (will have to drill out the screws I think).