Light circuits - seemed to be wired together

[scottmeister71]

I noticed when I switched off the upstairs light circuit (MCB) the garage light was still on (consumer unit is in garage). So I switched this back on and switched the downstairs light circuit off and the garage light was still on. On further investigation the downstairs curcuit will only switch off by switching both MCB's off (the same applies for upstairs). Basically the only way I can isolate either circuit is to switch both MCB's off

Because of this I looked in the consumer unit and noticed that the downstairs lighting MCB has two live cables going into it and the upstairs has one.
Can someone tell me why this would be and is it dangerous.

Hope you can help.

 

[andy]

dodgy wiring... could have shared live and neutral or summit. its extremely dangerous and should be resolved ASAP. this is because:

1: the MCB has in effect been doubled.
2: someone working on the circuit could switch of 1 MCB and not the other

 

[scottmeister71]

Thanks for that.
How would I go about identifying what the third cable is supplying, i.e. as far as I'm aware there are only two light circuits in the house yet there are two wires in one MCB and one in the other?

 

[plugwash]

if your lucky someone has wired one of the cuircuits as a ring (uncommon but not unheared of) and put the wrong pair of wires in a breaker

do you own a multimeter?

 

[andy]

Thanks for that.
How would I go about identifying what the third cable is supplying, i.e. as far as I'm aware there are only two light circuits in the house yet there are two wires in one MCB and one in the other?

move 1 of the 2 wire to the other MCB. try it. if thats worked, good. if not try the other wire. if that doesnt work then youll have to go further in. see which light has an odd number of cables (i.e a single live/neutral leaving or summit)

or they could be wired as a ring, with 1 cable to each mcb

 

[scottmeister71]

Plugwash - I do own a multimeter but I've only just bought it and not looked into how to use it yet. I am a keen DIY'er and would like to learn more about electrics but I'm not daft enough to just do something without checking.

 

[andy]

Plugwash - I do own a multimeter but I've only just bought it and not looked into how to use it yet. I am a keen DIY'er and would like to learn more about electrics but I'm not daft enough to just do something without checking.

before using the meter on live circuits, get used to it with a battery first. you can use the multimeter so see if it is on a ring or not. to do this remove all lives from terminals (power off, obviously) and check to see if there is a very low resiatance between any of them. if there is you probably have a ring

 

[mapj1]

Is the weirdness confined to the garage light, or do both the MCBs have to be off to kill any light?

If the latter you really only have 1 big lighting circuit fed by 2 breakers and all 3 wires would be much better off under 1 breaker until resolved.



Easiest test of "whats it feeding"is to remove it, insulate it, switch back on and see what no longer works. (then note it down before repeating test with the other wires in turn.)

 

[Spark123]

Is it possible there is a 2 gang light switch common to both circuits with a strapper across the supplies for both circuits? If there is it will more than likely be on a staircase. Just a thought.

 

[scottmeister71]

mapj1 - I will try that, thanks. It isn't only confined to the garage, that is just where I happened to notice it first. Both MCB's have to be off to kill any light.

Spark123 - Funny you should ask that question but there is a two way, two gang light switch controlling two seperate light fittings at the top and bottom of stairs. The switch at the bottom of the stairs doesn't work though. I tried switching the cables but there were varying results from the light being on all the time, to the light working as it should but then the two gand switch not working. What is clear is the 3 core and earth from the single switch at the bottom of the stairs doesn't go to the switch with the two gangs. This has two two core flex with the two blue cables twisted together and connected to the one terminal

Sorry if this is totally confusing but without pictures it's not the easiestthing to explain.

 

[Spark123]

To prove this, switch off the electric, make a note of where all the wires go now and remove the light switch. Separate all wires and connect into seperate pieces of terminal block. Ensuring that no-one can touch any of the live terminals switch one circuit breaker back on and see which lighting circuits power up (except stairs up and down). Switch the breaker off and the other one on and see which lighting circuits power up this time. If this separates the circuits again, this is where the problem lies.