hella dodgy wiring!

[robbM]

Hi All,
first time user and total DIY novice here, I hope someone can help.
I've recently moved in to an old flat where all the wiring for the lights is well dodgy! Just lots of wires coming from the ceiling in to the bare metal bones of the light fitting. My problem is that the light for the hallway is not connected at all and there are 3 cables hanging loose. I've bought a new pendant set for it but my problem is that I don't know which of these cables is the switch cable.
Does anyone know how I can find out easily?
Many thanks,
Robb

 

[slippyr4]

you need a test meter. you can get these for a few small quids in a DIY shed.

with the power turned off, and meter on the smallest resistance mode, test each cable. one cable should read infinite when the lightswitch is off, and very low resistance when it's on. that is your switch cable.

 

[fido]

If you don't have a multimeter it is worth getting one as they can be bought quite cheaply. An electricians test screwdriver is also handy. It complicates matters if there are two switches that operate the same light but if it has one way switching it's not too bad. Typically the 3 wires will be :
Unswitched mains input
Unswitched mains output to next room or light
Switch

If you turn the power to the house off you can identify the switch wire using the lowest resistance (ohms) range on the multimeter. Hold the meter probes on the live and neutral ends of each cable in turn whilst getting an assistant to switch the light switch on and off. The one that gives a change in reading ie short circuit to open circuit according to switch position is the one you want. Finding the unswitched mains wire is more dangerous as you need the power on and the meter set at a suitable AC volts range. You again touch the meter probes on the pairs of wire ends. Find the wire with 240 volts on it and you now have enough information to wire up the new fitting. The red mains wire goes to the red switch wire, the black mains wire goes to the bulb and the black switch wire provides the other bulb connection. Note this is for one way switching only.

 

[ban-all-sheds]

If you don't have a multimeter it is worth getting one as they can be bought quite cheaply.

And as with any other tool, the policy should be to buy the best you can afford not the cheapest you can find, particularly when it comes to tools on which your life can depend.

IIRC, the last discussion on the topic was here: http://www.diynot.com/forums/viewtopic.php?t=6649

An electricians test screwdriver is also handy.

Only if it is a decent screwdriver. Once upon a time they were, but now they are made out of tinsel-wrapped mazak. As an item of test equipment it makes a good bit of landfill.

It complicates matters if there are two switches that operate the same light

Not at the ceiling rose it doesn't - there is still just the one switch cable.

but if it has one way switching it's not too bad. Typically the 3 wires will be :
Unswitched mains input
Unswitched mains output to next room or light
Switch
.
.
.
Note this is for one way switching only.

So how do you think it would be different at the ceiling rose with two-way switching?

 

[fido]

I have seen 2 way lighting done with a run of 3 core & earth from each switch to the rose, which was used as a junction box.

 

[dwain dibley]

look at the diagrams at the begining of the forum http://www.diynot.com/forums/viewtopic.php?t=7553
they should help

 

[ban-all-sheds]

I have seen 2 way lighting done with a run of 3 core & earth from each switch to the rose, which was used as a junction box.

But he's only got 3 cables at his rose.

OK - I guess it could be the last rose on the circuit, and have 2 switch cables, but he didn't mention that any of them were 3C+E....