Another bulb holder wiring dillemma !

Android 07 · 6 Apr 2021

Hi chaps

Following on from my thread where you guys generously helped me connect a lamp holder to two cables sticking from out of the roof
( https://www.diynot.com/diy/threads/stuck-trying-to-replace-a-bulb-holder.559600/#post-4814084 )
, I have bought a property where there are three cables coming from the roof above one of the lamp holders in the living room.

So....

Living room :

--Two cables > Bulb A -- Switch ...................... -- Three cables > Bulb B --- Switch

I used the same wiring technique I learnt for Bulb A and it worked perfectly. I got stumped at Bulb B. So, I twisted like-for-like wire for two of the cables so that I ended up with two cables and their wires. But, when I switched on the light, the breaker tripped and now none of these two bulbs light up. Additionally, there are two lights in the corridor next to the living room and none of these now light up either!

The wiring is the old school Red, Black and earth.

How can I get past this ?

ericmark · 6 Apr 2021

The wiki page linked here should help, however in real life this is more likely.

Note the black on switch cable is line, it should be over sleeved red, but this is rare, if that black is put with other blacks which are neutral it will be a direct short, so one has to be careful to identify it and ensure they don't get mixed up.

Taylortwocities · 6 Apr 2021

@Android 07 you are going to need an electric tester, even a cheap multimeter will do. Do you have one?

briwire · 6 Apr 2021

This is a very common mistake to make. In simple terms, one of the black wires is actually a live when the switch is turned on. You need to identify which of the three cables is the one that goes to the switch. As Android 07 says, you need a tester for this. With the power off, tester on resistance, leads on red and black and get someone to turn the switch on and off.
The regs say this black wire should be taped to indicate it is not actually a neutral.

Taylortwocities · 6 Apr 2021

briwire said:
As Android 07 says, you need a tester for this.

No he didn’t! @Android 07 is the OP!

Android 07 · 6 Apr 2021

Taylortwocities said:
@Android 07 you are going to need an electric tester, even a cheap multimeter will do. Do you have one?

I'll get one today from Screwfix / Toolstation

Android 07 · 6 Apr 2021

Despite disconnecting the cables , why are the other lights not lighting up ? Have I fried their switches?

Aragorn84 · 6 Apr 2021

because the supply loops around each light fitting in turn, and you've disconnected the circuit in the middle.

Android 07 · 6 Apr 2021

All fixed now. Thank you. I have builders working in the house doing something else. They noticed it and fixed it while I wasn't around.

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Another bulb holder wiring dillemma !

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