live wire on lighting circuit when fuse at consumer unit off

[cookiemonster1]

Hi I seem to have a low votage live wire on the first floor lighting circuit when the fuse is switched off at the consumer unit. I realised this when it tripped the unit and the electrics went off. Upon testing I found that I needed to switch the ground floor lighting fuse off to clear the current. Does anyone know what might be the problem? Have the downstairs wiring and upstairs wiring been linked somehow?

 

[securespark]

Is it a landing light?

 

[cookiemonster1]

I will have a look to see if the landing light is in this junction box, thanks.

 

[Space cat]

Have the downstairs wiring and upstairs wiring been linked somehow?

If this is an old(ish) house, you might have the infamous borrowed neutral problem. The old fashioned way of wiring two-way lights was to connect live feed to COM at one end, run some two core cable between switches to link the L1 and L2 terminals and then take switched live from COM at the other end. It saved cable but it had a down side because live feed and neutral return ended up on different floors.

What this means in practice is that a junction box in the first floor lighting circuit might be getting live power through the switches from the ground floor circuit. (Although it's called a borrowed neutral, it could equally well be called a borrowed live.)

 

[ban-all-sheds]

Is it a landing light?

Doesn't have to be - anything upstream of the place where he breaks into the upstairs circuit would end up supplied via the borrowed neutral.

cookiemonster - take a look at the connection that runs between the hall and landing 2-way switches. If it's just twin & earth linking L1 & L2 like the top sketch here:

then your problem is almost certainly a borrowed neutral via the landing light.

 

[securespark]

Does not have to be, but that is the most common kind of borrowed neutral, the live coming from the downstairs circuit and the neutral from the upstairs.

 

[Space cat]

Doesn't have to be - anything upstream of the place where he breaks into the upstairs circuit would end up supplied via the borrowed neutral.

That's what happened to me about 25 years ago. (Was it really that long?) I was working on the downstairs lights at the time. When I loosened a neutral terminal in a junction box, the thing started to spark and the upstairs landing light went out - leaving me in the dark!

It took a bit of working out. I went on to discover that the downstairs hall light was powered from the upstairs circuit. A case of I'll lend you mine if you lend me yours.

 

[RF Lighting]

Hi I seem to have a low votage live wire on the first floor lighting circuit when the fuse is switched off at the consumer unit. I realised this when it tripped the unit and the electrics went off. Upon testing I found that I needed to switch the ground floor lighting fuse off to clear the current. Does anyone know what might be the problem? Have the downstairs wiring and upstairs wiring been linked somehow?

What do you mean by a 'low voltage wire'?

What are you using to test for power?

 

[ban-all-sheds]

What do you mean by a 'low voltage wire'?

50V - 1000V AC between conductors or 50 - 600V AC between conductors and earth?

 

[RF Lighting]

Yeah really helpful

 

[cookiemonster1]

thanks for all the info I need to do some checking with the junction boxes containing the landing light.

 

[ban-all-sheds]

Look at the switches first.

 

[ban-all-sheds]

Yeah really helpful

 

[ban-all-sheds]

Does not have to be, but that is the most common kind of borrowed neutral, the live coming from the downstairs circuit and the neutral from the upstairs.

Absolutely - it's guaranteed to happen if the lighting circuit is split into up/down and that's how the 2-way switching for the landing light had been done.

What I meant was it didn't have to be the landing light that he was working on.