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Discuss Beware The Borrowed Neutral in the Lighting Forum area at ElectriciansForums.net

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PAUL M

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i had a phone call off my mate today he had done cu change ,dual rcd as per 17th,he said alls fine until i turn on the landing light (2 way switch) and both the rcds keep tripping,turns out he had put up lights and down lights on different rcds as recomended,turns out when the council wired the house 10 years ago(now private)they had borrowed the neutral for the landing light,so he put both circuits on the one rcd and it worked no problem.he has now told the customer she needs a neutral running from down light circuit to landing light and she has agreed to the work.these new boards are gonna cause some headaches.lol:)
 
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BEN5637

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  • #2
Had the exact same problem mate where landing switch was tripping out both rcds after a board change. did the same put them on the same rcd and made a comment on the installation cert, advised cutomer but they declined a quote. def one to watch out for!
 
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TONY JONES

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  • #3
i had a phone call off my mate today he had done cu change ,dual rcd as per 17th,he said alls fine until i turn on the landing light (2 way switch) and both the rcds keep tripping,turns out he had put up lights and down lights on different rcds as recomended,turns out when the council wired the house 10 years ago(now private)they had borrowed the neutral for the landing light,so he put both circuits on the one rcd and it worked no problem.he has now told the customer she needs a neutral running from down light circuit to landing light and she has agreed to the work.these new boards are gonna cause some headaches.lol:)
had similar problem myself so looks like we have to allow in price to change consumer unit an additional cost for fault finding ?
 
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PAUL M

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  • #4
it didnt happen to me as i said it was a good mate of mine but for a short while it did have him scratching his head:D
 
E

EWebster

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  • #5
it didnt happen to me as i said it was a good mate of mine but for a short while it did have him scratching his head:D
Done 5 consumer units around my village in the last couple of weeks (60's housing) and 3 had borrowed neutrals. Must have been the done thing in the 60's/70's!!
 
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Graeme Harrold

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  • #6
Thats where taking a little time to test before jumping in and changing the CU. Gives you the bargining power before rather than after the change...........
 
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ezzzekiel

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  • #7
Thats where taking a little time to test before jumping in and changing the CU. Gives you the bargining power before rather than after the change...........


if a jobs worth doing.......
 
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EWebster

  • Thread Starter Thread Starter
  • #8
Thats where taking a little time to test before jumping in and changing the CU. Gives you the bargining power before rather than after the change...........
Absolutely. I've learned my lesson now, and haul apart the light fittings first. So far it's almost always the live from the common bridged between downstairs and upstairs hall lighting switches.
 
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Deleted member 9648

  • Thread Starter Thread Starter
  • #9
Thats where taking a little time to test before jumping in and changing the CU. Gives you the bargining power before rather than after the change...........
As I'ts likely to be an insulation test carried out before changing a CU, a borrowed neutral wouldnt show up.And with the usual jungle of out of order neutrals in the N-bar I doubt whether faults like this can reasonably be expected to be found prior to the work being carried out.
When quoting for a CU change we always add the get out clause that any faults not apparent before work starts may need to be rectified at additional cost.
Alan
 
E

EWebster

  • Thread Starter Thread Starter
  • #10
As I'ts likely to be an insulation test carried out before changing a CU, a borrowed neutral wouldnt show up.And with the usual jungle of out of order neutrals in the N-bar I doubt whether faults like this can reasonably be expected to be found prior to the work being carried out.
When quoting for a CU change we always add the get out clause that any faults not apparent before work starts may need to be rectified at additional cost.
Alan
Yup, I've got that on my quotes as well. It's amazing how many people would rather you bodged the job than chase their walls to run a new line :eek:
 

acvc

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Arms
A while ago I had the situation on a testing job where the upstairs and downstairs lighting circuits had been put into the same mcb.

I took out the L N and E of the upstairs circuit, left the downstairs in. Connected upstairs R1 and R2 for a continuity test. Switched the c/u back on, with only the downstairs circuit with its rcd left on. (I realise the c/u should have been dead at this stage:eek:).

The point is that when doing the R1+R2 at an upstairs ight fitting, it tripped the RCD. I put this down to a neutral being borrowed from the downstairs light circuit. Was this the right assumption to make?
 
So what is the solution? Most people will simply say its been like that for 50yrs why should I change now.

Placing the two circuits on the same RCD will hide the problem, but retain hidden dangers.

Suppose circuit 1 borrows the netral of circuit 2 . Maintenance work starts on circuit 2, let us suppose replacement of a rose with a modern light fitting and circuit 2 is isolated by locking off the relevant MCB. The wires are drawn out of the rose ready for re-wire, one nutral is attached to the light which is wired from circuit 1, the other nutral is connected to the consumer unit nutral. The light in circuit 1 is switched on.

Now obviousley the light does not work. The nutral from the the light becomes live (because it has no return path), while the nutral connected to the CU is still solid. thus we have a live and nutral side by side, both marked nutral, and both waiting for the maintainer to touch them in the belief that the circuit has been isolated!

In my opinion the only safe remady (apart from a re-wire) is to make them a single circuit , both on the same MCB (and of course therefore the same RCB and nutral bar).
 
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M

Monkey1984

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  • #14
So what is the solution? Most people will simply say its been like that for 50yrs why should I change now.

Placing the two circuits on the same RCD will hide the problem, but retain hidden dangers.

Suppose circuit 1 borrows the netral of circuit 2 . Maintenance work starts on circuit 2, let us suppose replacement of a rose with a modern light fitting and circuit 2 is isolated by locking off the relevant MCB. The wires are drawn out of the rose ready for re-wire, one nutral is attached to the light which is wired from circuit 1, the other nutral is connected to the consumer unit nutral. The light in circuit 1 is switched on.

Now obviousley the light does not work. The nutral from the the light becomes live (because it has no return path), while the nutral connected to the CU is still solid. thus we have a live and nutral side by side, both marked nutral, and both waiting for the maintainer to touch them in the belief that the circuit has been isolated!

In my opinion the only safe remady (apart from a re-wire) is to make them a single circuit , both on the same MCB (and of course therefore the same RCB and nutral bar).
The way I have been told to do it is to put both lighting circuits into 1MCB and note as a departure on certificate and to also label the board to show this has been done.

As you said otherwise your running a risk of an inadvertantly live circuit. being fed from the other side of the board.
 
E

EWebster

  • Thread Starter Thread Starter
  • #15
The way I have been told to do it is to put both lighting circuits into 1MCB and note as a departure on certificate and to also label the board to show this has been done.

As you said otherwise your running a risk of an inadvertantly live circuit. being fed from the other side of the board.
That's interesting. I guess it depends on how many lights you have on the two circuits as well, although it is permissible to run lighting MCBs up to 20A. Not sure I would want to though.

And saying that, it is definitely safer running them off the same MCB, versus two MCBs on the same RCD.
 
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