Identifying shared neutrals

[LearningSpark]

A question to all you domestic electricians.
When carrying out a pre works survey for a consumer unit change, apart from going in the loft and tracing cables is there a simple check for shared neutrals on the lighting circuits prior to quoting?. Fortunately I haven't been caught out yet but I guess my day will come!.

 

[ricicle]

The most common occurence of borrowed neutrals is between the upstairs and downstairs lighting circuits via the landing light.
From my experience it tends to be on older installs where the line is linked between commons at the hall switch and a twin runs up to the landing switch as strappers. Then a single runs to the landing light. But the neutrals are from the upstairs lighting circuit.
You could check by inspection, but a definate way of doing it is by (carefully) using an ammeter in the consumer unit. By comparing current on Lines and Neutrals you can know for sure.

 

[Kaku]

Switch off all lights

Check the continuity between the lives and neutral to verify this
You can only do this if there are no loads permanently between l-n ie transformers
(Isolate at the consumer unit)
Switch on the Hall and the Landing lights,no others,

Remove the lives for up and down lights
Continuity test between the lives at fuses or mcbs
If you have continuity
Remove any one lamp
If continuity is lost its likely the neutral is shared

Read more: //www.diynot.com/forums/electrics/identifying-shared-neutrals.301544/#ixzz1f93xxda4

 

[333rocky333]

 

[RF Lighting]

That doesn't help at all.

The most common borrowed neutral will be for the landing two way.

Both the upstairs and downstairs circuit wioll have their own line and neutral at the CU, but the landing light will have it's live fed from downstairs, and it's neutral fed from upstairs.

 

[333rocky333]

yeah your right

 

[Kaku]

Switch off all lights

Check the continuity between the lives and neutral to verify this
You can only do this if there are no loads permanently between l-n ietransformers
(Isolate at the consumer unit)
Switch on the Hall and the Landing lights,no others,

Remove the lives for up and down lights
Continuity test between the lives at fuses or mcbs
If you have continuity
Remove any one lamp
If continuity is lost its likely the neutral is shared

I have edited my post to indicate testing between lives
The post now makes sense,pardon the clanger

 

[ban-all-sheds]

I have edited my post to indicate testing between lives
The post now makes sense,pardon the clanger

That's just what you haven't done - you quoted it.

You should go back and edit the original one.

 

[Electrifying]

Switch off all lights

Check the continuity between the lives and neutral to verify this
You can only do this if there are no loads permanently between l-n ie transformers
(Isolate at the consumer unit)
Switch on the Hall and the Landing lights,no others,

Remove the lives for up and down lights
Continuity test between the lives at fuses or mcbs
If you have continuity
Remove any one lamp
If continuity is lost its likely the neutral is shared

Am I missing something - you'ld have continuity doing this whether the neutral was borrowed or not...............wouldn't you??

 

[JohnW2]

Switch off all lights
Check the continuity between the lives and neutral to verify this
You can only do this if there are no loads permanently between l-n ie transformers
(Isolate at the consumer unit)
Switch on the Hall and the Landing lights,no others,
Remove the lives for up and down lights
Continuity test between the lives at fuses or mcbs
If you have continuity
Remove any one lamp
If continuity is lost its likely the neutral is shared

Am I missing something - you'ld have continuity doing this whether the neutral was borrowed or not...............wouldn't you??

No, I don't think you're missing anything. However (thinking rapidly on my feet, therefore not too carefully thought through!), if one disconnected the two neutrals from the neutral bar in the CU, one could then (with lamps in place and switches 'on') simply directly test for continuity (lamp resistance) between the L's and the 'wrong' N's, couldn't one?

Kind Regards, John.

 

[ericmark]

Since lights are most likely problem.
Switch off one lighting MCB/fuse and try each switch and see what works. Including with two way all four options.
Switch off the other lighting MCB/fuse and put first one back on. Re-do switch tests.
Compare results and see if any lights have worked on two MCB/fuses.
Although we call it shared neutral really it's a duplicated live.
So we all know most likely is stairs lighting. It does not take long to switch off/draw one MCB/fuse at a time and see if the stairs lights work on two MCB/fuses.

Where one gets caught out is when one tried to separate circuits which were originally all on one MCB/fuse. I have seen myself where an electrician has stated that the two floors should be separated and most old houses did not have separate supplies for the two floors. Including my house. And there is no real point in separating the supplies. In fact it can be dangerous. My stairs are lit with an emergency lamp and due to open plan of house this gives enough light to leave the house and auto switches on with a power failure. If I split the lighting I could be unaware of a failure and either have flat batteries on emergency lamp or no light.

 

[ricicle]

Since lights are most likely problem.
Switch off one lighting MCB/fuse and try each switch and see what works. Including with two way all four options.
Switch off the other lighting MCB/fuse and put first one back on. Re-do switch tests.
Compare results and see if any lights have worked on two MCB/fuses.
Although we call it shared neutral really it's a duplicated live.
So we all know most likely is stairs lighting. It does not take long to switch off/draw one MCB/fuse at a time and see if the stairs lights work on two MCB/fuses.

Eric, I'm afraid in the context of a borrowed neutral what you have posted has no relevance at all. Yes the landing light has borrowed the neutral from the upstairs light or conversly "borrowed" a line from the downstairs circuit but the light is still only fed from one protective device so will not show up as being on two.

 

[Electrifying]

However (thinking rapidly on my feet, therefore not too carefully thought through!), if one disconnected the two neutrals from the neutral bar in the CU, one could then (with lamps in place and switches 'on') simply directly test for continuity (lamp resistance) between the L's and the 'wrong' N's, couldn't one?

Kind Regards, John.

This is the correct procedure, John, but you have to keep in mind - it will only show up the borrowed neutral if one of the lamps that you've left in is the one with the 'borrowed' neutral......so you would use the landing light as one of them (powered off the downstairs light circuit), as this is the most likely, and then choose an upstairs fitting as the other.

Unless, of course, you're leaving all lamps in and all switches on - which will work as well.
Otherwise, what you suggest is correct......obviously, you only want continuity between individual pairs of 'L' & 'N'.


A quick inspection of the hall two-gang switch can also give you a clue - if the 'lInes' of the two switches have a small link between them, then the chances are the neutral is borrowed upstairs.
If there's no link, with a 3-core running off one of the switches, it should be OK.

Not fool proof though.

 

[JohnW2]

... if one disconnected the two neutrals from the neutral bar in the CU, one could then (with lamps in place and switches 'on') simply directly test for continuity (lamp resistance) between the L's and the 'wrong' N's, couldn't one?

This is the correct procedure, John, but you have to keep in mind - it will only show up the borrowed neutral if one of the lamps that you've left in is the one with the 'borrowed' neutral......so you would use the landing light as one of them (powered off the downstairs light circuit), as this is the mosty likely, and then choose an upstairs fitting as the other. Otherwise, what you suggest is correct.

I'm not sure where you got the idea that I was only going to 'leave in' some of the lamps. If, per the OP, it was a 'general search' for any borrowed (or shared) neutrals (e.g. before installing multiple RCDs), I think I would do exactly as I said (perhaps, uncharacteristically, in slightly less detail than I could have done!) - leave all the lamps in, with all the light switches 'on', then diconnected all (2 or more) lighting circuits (L & N) at the CU and check for continuity (lamp resistance) between each L and each of the corresponding 'wrong' Ns.

Kind Regards, John.

 

[Electrifying]

I'm not sure where you got the idea that I was only going to 'leave in' some of the lamps. Kind Regards, John.

Yeah, I'm not sure where I got that idea from, either.