Tracing supply to an Outside Light

[NottinghamRob]

Hi
I've recently moved into a new property & have the following issue.
There's an outside light at the back & I can't find the on/off switch for it. There's power to it, as the light comes on if the sensor is tripped, but I want to change it to a normal floodlight & only have it on when I switch it on.
Is there a way that I can try to trace if / where the switch is? I've take the light off the wall, but can't see where it goes from that side. It's at the same height as the cavity between the kitchen ceiling and the 1st floor room above it, so I could get access through the bedroom floor, but I don't really want to start pulling up carpets and floorboards. So, I'm hoping there's some form of gadget that I could buy to try and trace it? Or any other kind of awesome trick.
Thanks
Rob

 

[MJPD29]

Very helpful reply. I've had numerous electricians tell me that they either can't / won't get involved. I'll ask it another way. Do electricians have a way of tracing this source without ripping up floorboards / carpets etc.

Start with an educated guess. Where is the closest power?

 

[stewartrossinv]

There's no magic device that can trace the cabling involved for you. A professional electrician might be able to determine the most likely route that has been taken, but that might still be difficult as it all depends who fitted the external light and how competent they were. And as you've mentioned, so far you've not found an electrician who will help you with this.

If your property has a modern consumer unit with miniature circuit breakers (MCBs) you should be able to isolate which circuit the external light is on by switching off the MCBs one by one whilst your external light is on until you find the one that when off also switches the external light off. This if you are lucky may be one of the lighting circuits (on 6A MCBs), or it might be that the power for the external light has been taken as a spur off one of your sockets on your ring final circuits (on a 32A MCB typically) via a fused connection unit (FCU).

If your light is indeed powered by a spur from a mains socket you may find that the FCU is switched, so that you can at least turn the power off independently on that circuit before you take any action with the light itself. Be aware that the switch on an FCU may well be single pole, in which case it only interrupts the live connection - the neutral would remain connected.

I do agree that getting an electrician in is by far the best way forward. An electrician can verify that there is no missing earth connection for instance, and that the live and neutral have not been reversed and so on. If you can't engage an electrician then at least you can do some basic investigation at your consumer unit without having to poke around the wiring in ways that could be dangerous.

 

[Paul L]

There are ways and means of doing this but unfortunately none of them involve any magic solution. I would guess that it is a straight live and neutral from a light fitting somewhere near and doesn't have a switch so it's going to involve taking things apart and testing cables to find it.
There are some different scenarios too where it could come from a socket circuit potentially so unfortunately no real shortcut sorry

 

[telectrix]

There's no magic device that can trace the cabling involved for you. A professional electrician might be able to determine the most likely route that has been taken, but that might still be difficult as it all depends who fitted the external light and how competent they were. And as you've mentioned, so far you've not found an electrician who will help you with this.

If your property has a modern consumer unit with miniature circuit breakers (MCBs) you should be able to isolate which circuit the external light is on by switching off the MCBs one by one whilst your external light is on until you find the one that when off also switches the external light off. This if you are lucky may be one of the lighting circuits (on 6A MCBs), or it might be that the power for the external light has been taken as a spur off one of your sockets on your ring final circuits (on a 32A MCB typically) via a fused connection unit (FCU).

If your light is indeed powered by a spur from a mains socket you may find that the FCU is switched, so that you can at least turn the power off independently on that circuit before you take any action with the light itself. Be aware that the switch on an FCU only interrupts the live connection - the neutral remains connected.

I do agree that getting an electrician in is by far the best way forward. An electrician can verify that there is no missing earth connection for instance, and that the live and neutral have not been reversed and so on. If you can't engage an electrician then at least you can do some basic investigation at your consumer unit without having to poke around the wiring in ways that could be dangerous.

gave you an agree ther for a useful, helpful post, but most FCUs these days are double pole. depends how old it is ( if one is even fitted).

 

[stewartrossinv]

Thanks for the correction @telectrix - I was concerned that if the FCU (if fitted) was single-pole (unlikely now as you've said) then for instance accidentally touching the CPC to the neutral will trigger whatever RCB is protecting the ring and other circuits off straight away should the OP take the cables off whilst the FCU is apparently switched off. Anyway, glad to stand corrected on this!

 

[telectrix]

Thanks for the correction @telectrix - I was concerned that if the FCU (if fitted) was single-pole (unlikely now as you've said) then for instance accidentally touching the CPC to the neutral will trigger whatever RCB is protecting the ring and other circuits off straight away should the OP take the cables off whilst the FCU is apparently switched off. Anyway, glad to stand corrected on this!

if the FCU is old, it probably is single pole. i have found that shorting N to E is a good way of tracing the RCD supplying the circuit anyway. safer than the bang test.

 

[Ziggie]

Not recommending it but couldn’t you power off all circuits and use a tone / probe type instrument to send a signal and find the circuit?

Obviously , check circuit isn’t live before doing it.

I have heard of people using a 9volt battery and tasting the other end but definitely not recommending as it doesn’t taste very nice ?? My dad had me doing itfrom the age of 4ish.

 

[westward10]

I suspect the OP is gone due to earlier posts now removed.

 

[Lister1987]

I've got similar here, outside light (not working - either duff bulb or circuit issue), feed goes from light, into cavity then to God knows where.

First point of action is to ascertain supply (clamp meter on cable and if voltage present then get flicking MCBs), once isolated (and verified dead with Safe Isolation), try and figure routing using a mix of stud detector (while circuit was live to get an idea), getting up floorboards and trying the boroscope to see if I can get a visual.

My gut is saying fed from below so either off upper ring or lower lighting.

Got as far as using the stud finder to get an idea then the room got filled with junk (that junk room, we've all got it in some guise right?) so never got back to it. One day.... ?

ETA: safe isolation note