How to identify the circuit cables in a light fitting?

[Milleniumaire]

Hi, my neighbour has just asked me if I might be able to help him fit a new light fitting in his bedroom as he can't get it to work. I have a basic understanding so went round with my testing screwdriver, but gave up on the grounds that I didn't fully understand what was happening, so didn't want to risk incorrectly wiring the new fitting.

It is now bugging me!

When I looked at the wires in the ceiling, there where 3 x 3 core (twin and earth cables). He had connected one of these to a terminal block, which plugs into the new light fitting. The other two had been connected live to live and neutral to neutral with the earth cables unconnected. Clearly wrong, so no wonder his light didn't work.

First thing I did was disconnect the terminal block and test each red core. Only one of them was live, so that must be the supply. I'll also point out that the en-suite light was working, but the next bedroom wasn't, so makes sense that two of the 3 core cables is the circuit and the other must run to the switch.

On examining the switch I could see a three core cable, with red and black connected to the switch terminals.

Back at the ceiling, the live supply had been painted partially white and one of the other red cores had also been painted white, so I'm thinking this would be the live supply to the light switch and therefore connected them together. If I'm correct, the black core in this cable must be the switched live. To prove this I left all the black cores unconnected and turned on the power. To my surprise the black core that I thought was going to the light switch was permanently live, regardless of the switch position. At this point I decided I didn't understand and walked away.

Why would the neutral core be permanently live?

Here's my attempt at a diagram and what I think should be the correct wiring:

Cable 1 (ring circuit - I'm fairly sure about this)
1A - red (painted white) - currently permanently live
1B - black

Cable 2 (ring circuit - not 100% sure)
2A - red - should be permanently live
2B - black

Cable 3 (to switch - not 100% sure)
3A - red (painted white) - should be permanently live
3B - black - will be switched live

Cable 4 (light fitting)
4A - red - will be switched live
4B - black - neutral

So, when I connected 1A to 3A, with NO other cores connected, 3B was permanently live despite the position of the wall switch. I don't understand why.

I think the wires should be connected as follows (but I'm not prepared to do this until I understand the reason for 3B being permanently live or if I am correct below):

1A to 3A and 2A - Live supply to the rest of the ring circuit and to the switch

3B to 4A - Switched live (black core from light switch) to light fitting live

1B to 2B to 4B - Both ring circuit neutrals and light fitting neutral connected together.

Although I know which is the live supply on the ring circuit, I don't know which of the other two cables goes to the light switch. I'm assuming it's the one with the red core painted white. Is there any way I can be sure? Trial and error doesn't seem appropriate when dealing with electrics!
Also, understanding why one of the black cores is permanently live would help

Thanks in advance.

 

[John D v2.0]

Your first problem is a neon screwdriver. That isn't a reliable way to detect live wires. You need a multimeter or other 2 pole voltage tester and it and its leads should be marked CAT III or CAT IV.

 

[EFLImpudence]

Firstly, onlily?, you cannot rely on testing screwdrivers so the black probably wasn't live.

 

[John D v2.0]

permanently live, regardless of the switch position. At this point I decided I didn't understand and walked away.

Why would the neutral core be permanently live?

That was probably the feed to the later lights and one of them was probably on, therefore pulling the disconnected neutral to live voltage.
Even if it wasn't, see the above post about testing.

 

[John D v2.0]

If you use your multimeter you can do dead testing, so turn off the main switch and test from the faceplate screws of the switch to the earths in the ceiling, then you'll know which is which. Then check that red to black on that responds to switching. Now you know the supply and switch, so you can connect as you intended.

Edit sorry for replying 3 times, your post was long so hard to compose a reply on my phone

 

[Milleniumaire]

Thanks for the prompt responses. Although I have a multimeter, I've only ever used it for checking continuity when installing alarm systems, so feel a little unsure about using it on electric cables.

I suppose I could turn off the lighting circuit and then use the continuity at the switch on the red and black cores. If I connect together one of the red and black cores a the ceiling and leave the others disconnected, I will get a continuity reading from the cable that runs to the switch, then I can connect up as I described above?

 

[John D v2.0]

Thanks for the prompt responses. Although I have a multimeter, I've only ever used it for checking continuity when installing alarm systems, so feel a little unsure about using it on electric cables.

I suppose I could turn off the lighting circuit and then use the continuity at the switch on the red and black cores. If I connect together one of the red and black cores a the ceiling and leave the others disconnected, I will get a continuity reading from the cable that runs to the switch, then I can connect up as I described above?

Yes that would also work. Just be sure to turn off the power first (and test that there's 0v between all wires and earth), then it's just like using it to check cables on an alarm system.

 

[winston1]

There are no ring circuits on lighting. It will be a radial circuit.
My guess is (shouldn't guess so please confirm with your multimeter) cable 1 is the supply, cable 2 is the switch and cable 3 is the onward feed to the next light.

So all reds together in a terminal block. 1B, 3B, and 4B together. 2B to 4A (put a red or brown sleeve on 2B). Don't forget to also join the earths in a terminal block and to the new light if it needs it.