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.
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.
Last edited: