Electrical query - can anyone explain?

[quelquod]

Lighting circuits in our house have to be switched off with two of the MCB's at the distribution unit, or they are still live. I have no idea if that's normal. But it's how ours is. The distribution unit was fitted new 3 years ago in a 28 year old house.

You’ve said that your ‘lighting circuits’ need to be turned off using 2 MCBs. It’s common for houses to have separate upstairs and downstairs lighting circuits of course. If you mean that you need to turn off 2 MCBs and that that then turns off ALL of the lighting circuits it sounds as though yours have somehow been shorted together. Not the sort of thing a competent electrician would do and it’s not wholly safe [*] as it would allow a higher fault current than the wiring is likely rated for. I think I’d get an electrician to check and correct it soonish. I might put the price of a pint on that assuming you have a landing light with 2 way switching someone has somehow miswired the live at both ends.

[*]Lighting circuit faults are usually shorts which would trip both MCBs.

[DrFfybes]

Thanks. For example, for the downstairs lights to go out, I need to switch two MCBs. They are both marked lighting circuit by the electrician who installed the consumer unit.

This sounds very wrong to me. So to turn off any one single downstairs light, any light, you have to turn off two MCBs at the same time? Or do you mean to extinguish ALL of the downstairs lights (as opposed to just some of them), two MCBs must be turned off?

That's how I read it, although it could be like our house - it is wired "North End" and "South End" so the lighting breaker marked "upstairs" is North end - when the board was replaced the sparky copied the labels(!). Interestingly the sockets are wired the same, so in one bedroom the 2 sockets at opposite ends of the same room are on 2 different breakers.

Paul

[Spot5]

.....
I am now completely bewildered as to why cutting a 'dead' wire should/could trip the circuit.

Any thoughts?

Are the wires in the armoured cable colour coded?

If the live and neutral are reversed, and the spur switch is only single pole, you would be been cutting a live cable....


s

[modellingman]

Lighting circuits in our house have to be switched off with two of the MCB's at the distribution unit, or they are still live. I have no idea if that's normal. But it's how ours is. The distribution unit was fitted new 3 years ago in a 28 year old house.

You’ve said that your ‘lighting circuits’ need to be turned off using 2 MCBs. It’s common for houses to have separate upstairs and downstairs lighting circuits of course. If you mean that you need to turn off 2 MCBs and that that then turns off ALL of the lighting circuits it sounds as though yours have somehow been shorted together. Not the sort of thing a competent electrician would do and it’s not wholly safe [*] as it would allow a higher fault current than the wiring is likely rated for. I think I’d get an electrician to check and correct it soonish. I might put the price of a pint on that assuming you have a landing light with 2 way switching someone has somehow miswired the live at both ends.

[*]Lighting circuit faults are usually shorts which would trip both MCBs.

The obvious way for Bulldog to test the soundness of his installation would be to (1) switch off one of his two lighting MCBs (say MCB1) with the other (say MCB2) left on and note which of his house lights work and then (2) to switch MCB1 on and MCB2 off and again note which lights work. If the installation is correctly wired then he should have two mutually exclusive sets of house lights - those which work when MCB1 in on and those which work when MCB2 is on. If the sets are not mutually exclusive then there is a wiring fault. Even if there's no fault it would still be a useful exercise, particularly if the lists of lights controlled by each MCB are recorded and left by the consumer unit, since it would inform Bulldog (and any future owner) what is on each of the two lighting circuits.

modellingman

[BullDog]

You’ve said that your ‘lighting circuits’ need to be turned off using 2 MCBs. It’s common for houses to have separate upstairs and downstairs lighting circuits of course. If you mean that you need to turn off 2 MCBs and that that then turns off ALL of the lighting circuits it sounds as though yours have somehow been shorted together. Not the sort of thing a competent electrician would do and it’s not wholly safe [*] as it would allow a higher fault current than the wiring is likely rated for. I think I’d get an electrician to check and correct it soonish. I might put the price of a pint on that assuming you have a landing light with 2 way switching someone has somehow miswired the live at both ends.

[*]Lighting circuit faults are usually shorts which would trip both MCBs.

The obvious way for Bulldog to test the soundness of his installation would be to (1) switch off one of his two lighting MCBs (say MCB1) with the other (say MCB2) left on and note which of his house lights work and then (2) to switch MCB1 on and MCB2 off and again note which lights work. If the installation is correctly wired then he should have two mutually exclusive sets of house lights - those which work when MCB1 in on and those which work when MCB2 is on. If the sets are not mutually exclusive then there is a wiring fault. Even if there's no fault it would still be a useful exercise, particularly if the lists of lights controlled by each MCB are recorded and left by the consumer unit, since it would inform Bulldog (and any future owner) what is on each of the two lighting circuits.

modellingman

Thanks. Next time I see the electrician who did the installation I'll ask him to check it again. I've known him for years. He'll check it out for me.