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PAUL M

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 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!
 
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 ?
 
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!!
 
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...........
 
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.
 
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
 
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:
 
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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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.
 
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.
 
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.

I wouldn't want to be going upto 20A on lighting. I've not seen above 10A in domestic yet. Maybe just been lucky.

Like you say its definitely safer then leaving them on 2 circuits on the same RCD. but installation cert should be noted and board should be labelled.

Its not ideal but it does solve a problem.
 
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.

Watch the cable size though. 1.0mm T&E if not bunched and not run through insulation etc can safely handle a bit more than 10A (but you can't get MCBs a bit more than 10A!) . That gives you upto 2300 Watts. Its supprising how quickly this figure can be reached with modern downlighters.

I don't have the OSG to hand to look up the limit for 1.5mm T&E, but on houses with this fault its more likely to be 1/044 or 3/026 and I don't know how comparable they are with 1.0mm and 1.5mm.
 
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99 times out of hundred the borrowed neutral problem exists only where the landing and hall light switches are grouped
It was unfortunately the most common way of wiring lighting in the 60s and 70s and was not given much importance at the time.because it did not impact on rcds because there were few of them
Today it becomes more of a problem
Getting a feed to the landing 2-way in the hall is usuallly the best method,but whatever the solution finding this problem and being surprised by its presence is the thing to avoid
When considering cons unit changes,always check for this situation
Its better to spend a few mninutes checking than getting into all sorts of trouble doing the change blind
Moaning about the borrowed neutral seems pointless because without botch ups mistakes faults and the rest,our work would be less fruitful
 
done 3 board changes this month sofar, all 3 had a borrowed N, so i fixed by putting in one MCB... none of the clients wanted any more work and were happy for it to be noted on cirt...
 
Des, While we all seem to agree with the analysis of the problem, the danger always existed, its just that it is more detectable with 2 RCDs in use. Furthermore it seems that amongst the community using this forum that we have a concensus on what is a very simple solution.
 
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