Borrowed Neutral Testing

[brummydave]

I did a search for 'borrowed neutral' and it got no results on here. I was a little staggered considering it was such a common practice that now falls foul of splitting lighting circuits onto separate RCDs so thought i'd post on the subject!

So I was wondering what methods anyone uses for testing/solving this issue?

I'm in the process of replacing an MCB only board with an RCBO board and have had some weird occurences on the lighting circuits. There are two, one up, one down. The landing & hall lights are connected to the downstairs circuit with two way switching on just the landing light. When both circuits were re-energised all the lights operated as expected, except for the landing light. That tripped both the up and down RCBOs. After being reset (and the landing light off) then followed a series of random tripping that had no logic to it! My current solution is to put both circuits on one RCBO until I've time to solve the problem....

As well as all the usual tests which came up fine, I tested continuity at the board between the two lighting neutrals and found there to be none so that went part way to me expecting the neutral was not borrowed.

There's a fair number of single neutral cables dotted around the light fittings but as i've no access under boards I've not been able to trace every route. Think i'll have to get my wander lead out

Any tips or tricks please?

Thanks

D.

 

[brummydave]

I have just 'googled' this and of course found a lot of hits. Including this! http://www.talk.elec...needed-pleasel/

So will get reading. This post http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=43994 seems to have some great ideas.

I appreciate any input anyone can write here tho.

I should add that the wiring is in good nick, was all replaced when the Crabtree SB6000 was fitted (I think about 20 years ago?), there's no records, charts or anything useful like that.

 

[brummydave]

Best bit of info so far:

"borrowing is not interconnection..

borrowing is using a live from one circuit and a neutral from another..

there is no connection between the neutrals of the first and second circuit."

​

Oops.

:Blushing Not that everyone agreed with it, but it makes sense to me.​

Testing wise, I've found loads!

:Applaud​

Think I have made a redundant post really, but will leave it here in case it's useful for others.

​

 

[paul b b]

Dave... You need to test continuity from the N in the cu to the N in the switch..so wander lead ( or a roll of 1mm) is needed.

 

[Steve3948]

I suspect the problem is with the landing and hall light, hall light on ground floor circuit and landing on 1st floor circuit,

switching altered so both come on together.

 

[SPECIAL LOCATION]

I did a search for 'borrowed neutral' and it got no results on here. I was a little staggered considering it was such a common practice that now falls foul of splitting lighting circuits onto separate RCDs so thought i'd post on the subject!

There are loads of threads on this subject in the bowels of the forum.....

maybe shared, or borrowed, or RCD tripping with lights etc.. etc...

The problem you mention has nothing to do with falling foul of needing lights on two RCD's...

it is the far more dangerous practice of having two protective devices energising a single piece of cable..

which falls foul of...

314.4 page 46 or 314-01-04 'old money'

e.g. one section of neutral cable that if disconnected at the CU can become live by TWO protective devices..

Consider:-

The risk of electric shock when DIY BOB is changing a light in the bedroom thinking all the upstairs lights are off but power from the downstairs circuit via the landing light and its floating neutral, now live in the wires at the bedroom where it happens to be interconnected..

as he grabs hold of the live cable up his stepladder!! {or vice-versa with a downstairs arrangement}

So it shouldn't have been acceptable for many years now!!!!

Please do not trivialise this poor workmanship implying it is only need fixing because it contravenes dual RCD CU's..

It is easily identified before changing a CU with a few quick test readings done at the CU

PS..

don't assume shared neutrals only occur on lighting circuits...

Some dodgy conservatory fitters have used downstairs lights to downstairs sockets neutrals to work the lights!!!!

 

[SPECIAL LOCATION]

I should add that the wiring is in good nick, was all replaced when the Crabtree SB6000 was fitted (I think about 20 years ago?), there's no records, charts or anything useful like that.

So it was wired in contravention of 314-01-04 back then by the sounds of it.!!!

"The wiring of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent indirect energising of a final circuit that needs to be isolated"

thats from 20years ago!

To resolve the issue you will need to install an additional neutral wire from the CU to the point where the old circuit interconnects with the other circuit...

{probably somewhere adjacent to the landing light, so cable from CU to loft}

As this is the reason why the neutral was shared in the first place...

singles running the lives upstairs via switches..

but no neutral of the same circuit available at the load {light}..

:C

 

[sparkytim]

As has been discussed many times the usual problem is the landing light which has two way switching with the downstairs switch live fed of the downstairs lighting circuit and the nuetral from the upstairs circuit.

As specs has suggested I have found the easiest solution is to remove the Landing light from the upstairs circuit and take a nuetral from the downstairs lighting circuit up to it.

 

[Steve3948]

If its a 2 gang switch downstairs, it may be configured incorrectly.

 

[brummydave]

Thanks all, very much for the reply. As I was having a fit of insomnia I didn't do my due diligence of scouring properly for threads. I did put 'borrowed neutral' in the TEF search bar but should've relied on trawling the bowels of TEF, and Google instead.

Anyhoo, in the light of day all seems much better and thank you for your replies. I definitely don't want to trivialise any aspect of wiring!

Dave... You need to test continuity from the N in the cu to the N in the switch..so wander lead ( or a roll of 1mm) is needed.

Do you mean from the CU to the Luminaires? As there's not a single N in any switch on this job.

I suspect the problem is with the landing and hall light, hall light on ground floor circuit and landing on 1st floor circuit,

switching altered so both come on together.

Thanks, it is the most common problem area and will be the first I look at but I won't stop there. Especially as there's so many single neutrals going off all over the place!

There are loads of threads on this subject in the bowels of the forum.....

maybe shared, or borrowed, or RCD tripping with lights etc.. etc...

The problem you mention has nothing to do with falling foul of needing lights on two RCD's...

it is the far more dangerous practice of having two protective devices energising a single piece of cable..

which falls foul of...

314.4 page 46 or 314-01-04 'old money'

e.g. one section of neutral cable that if disconnected at the CU can become live by TWO protective devices..

Consider:-

The risk of electric shock when DIY BOB is changing a light in the bedroom thinking all the upstairs lights are off but power from the downstairs circuit via the landing light and its floating neutral, now live in the wires at the bedroom where it happens to be interconnected..

as he grabs hold of the live cable up his stepladder!! {or vice-versa with a downstairs arrangement}

So it shouldn't have been acceptable for many years now!!!!

Please do not trivialise this poor workmanship implying it is only need fixing because it contravenes dual RCD CU's..

It is easily identified before changing a CU with a few quick test readings done at the CU

PS.. don't assume shared neutrals only occur on lighting circuits...

Some dodgy conservatory fitters have used downstairs lights to downstairs sockets neutrals to work the lights!!!!

It did occur to me that the previous owners were in danger but they'll never know as they moved out without ever having their rewire looked at professionally again. (Just added to, butchered, etc etc!)

Thanks for the tips, what are the quick test readings you're on about?

circuits disconnected, Bulbs in, switches on, and test between line of one and neutral of the other?

Someone suggested some IR tests but I'd be worried about dimmers and the like.

I have also found shared neutrals on socket circuits but thankfully it's rare, and they were on jobs that the whole lot was being ripped out.

So it was wired in contravention of 314-01-04 back then by the sounds of it.!!!

thats from 20years ago!

To resolve the issue you will need to install an additional neutral wire from the CU to the point where the old circuit interconnects with the other circuit...

{probably somewhere adjacent to the landing light, so cable from CU to loft}

As this is the reason why the neutral was shared in the first place...

singles running the lives upstairs via switches..

but no neutral of the same circuit available at the load {light}..

:C

Yes although the rewire was done to a professional standard, it seems this contravention slipped by. Unless it's come about due to fiddling/DIY/less than professional additional work. Of that I've found plenty of evidence! Grrr.

As has been discussed many times the usual problem is the landing light which has two way switching with the downstairs switch live fed of the downstairs lighting circuit and the nuetral from the upstairs circuit.

As specs has suggested I have found the easiest solution is to remove the Landing light from the upstairs circuit and take a nuetral from the downstairs lighting circuit up to it.

Thanks Tim, yes that's my plan. It just so happens I'm running a new shower cable (10mm to replace the existing 4mm!) from CU in garage to loft so a lighting neutral can be put in at the same time.

I must mention that when doing an install from scratch I avoid this scenario completely by fitting a minimum of three lighting circuits. For example: Downstairs rooms, Upstairs rooms and one for 'interconnecting areas', ie porch, hall, landing, loft etc. That way if a room goes dark, the adjoining area is still illuminated. And all have their own line and neutral which stay away from the others!

Cheers me ducks, time to get back in the sunshine.

 

[paul b b]

Dave...I've found the easiest way of finding it is use the wander lead from the N of the downstairs lights...the problem is normally on the hall/landing...so go there first...switches and fittings..

Last time i had a borrowed N on a cu change customer did not want it sorting so i put the lights on 1 10A mcb. Thus making it one circuit and eradicating the problem....Ofcouse, explain how it will be inconvenient to the customer but that's their choice!

 

[Nicky Tesla]

if the lights trip off when they are on seperate rcds then there is a borrowed neutral simples :|

 

[SLIPSHOD & SLAPDASH]

Be aware ! I have found this problem a few times when there is are 2, 2 gang switch's (1 up & 1 down) for the hall light and landing light, its nothing to do with the neutral. The sparky fitting the switches had crossed the strappers between upstairs and downstairs so depending on the positions of the 4 switches, 2 circuits are interconnected but still work correctly.

 

[ProDave]

if the lights trip off when they are on seperate rcds then there is a borrowed neutral simples :|

That's very true.but I think the point of the thread is how to test for it BEFORE you do a CU change, so you can warn the customer of the problem and factor in the time and price to put it right.

Just finding out when you power up a new CU makes you look unprofessional, and leaves you panicking at the end of the day to put it right and then trying to justify the extra time to the customer.

 

[steptoe]

easy test to find a borrowed neutral?

NEON anyone...?

I cant think of anything quicker or simpler.

 

[steptoe]

who else?

have had a few late nights at work this week, only just seen this thread now,

 

[Barx]

My first check is to inspect the wiring in the landing light, if there is more than one neutral conductor in there, chances are it's 'borrowed'. If only one then investigate further. A good test is to isolate up lights and remove the neutral from the cu. Power up the downstairs and see if landing works. It should work if not borrowed.

 

[Nicky Tesla]

just wait for someone to bring it back

 

[Steve3948]

You could always put a tone generator on the neutral disconnected and see where it goes.

 

[Evans Electric]

That's so true Nicky

I once diagnosed a faulty (supposedly motor) around 12 years ago at a place I shall not mention - I said to the maintenance sparks - it's not working because its lost a phase - he duly looked over each shoulder around the room and then asked me where i thought it had gone :slap

And in the words of (who was it !) - I kid you not

He was being serious....

Anyway ........ where HAD it gone ??? :C