Borrowed Neutral? Advice Needed Please!l

[cr0ft]

Hi all. I've put a few posts up about my first job, changing a consumer unit for my dad. When I took out the old unit I noticed that 2 of the 3 lighting circuits in his house were wired into the same MCB. At first I thought it was a ring mains to be honest until I noticed a 6 amp mcb there.

I've worked out why the previous electrician did this (I think). I separated all 3 lighting circuits on the new dual RCD board I put in today. The problem is that whenever the upstairs hall light is switched on (either from the downstairs or upstairs switch) it trips the RCD, but the one on the other side of the board, not the one the relevant light is wired into.

I'm assuming the problem is a borrowed neutral. When I remove the bulb from the upstairs light and flick either lightswitch nothing trips. This makes sense to me as there is no longer a return path down the neutral wire from the pendant in question.

So I think I've worked out what it is, but how do I go about confirming it and fixing it? Is it as simple as getting up in the loft and tracing the neutral wires and then sorting them out properly?

Everything else is working fine to be honest. The whole job was a bit of a pig to be honest, having to crimp lots of wires, extend the meter tails etc. An interesting first job!

Many thanks in advance for all help.

 

[NozSpark]

You can normally tell if you have a borrowed neutral by having a look behind your hall switch,,, the live will be linked across the switches if it's borrowed,, then you could check the landing switch for only having a single conductor going up into the loft.

Another way (dead testing) is to turn on the landing light (with a lamp in) and then carry out a resistance test between the downstairs line and the upstairs neutral conductors at the CU. You will get a resistance equal to the lamps filament and the L+N conductors.

---------- Post Auto-Merged at 18:22 ---------- Previous post was made at 18:18 ----------

If they are borrowed whack them in the same MCB!

 

[cr0ft]

Hi. Definitely no cross-linked live in the downstairs switch. I will check the landing switch on Monday. It wouldn't surprise me if it's been wired off another circuit to be honest. Their house has several lights just wired off the ring mains too.

All a bit of a mess but seems safe enough, other than the borrowed neutral milarky.

I'm assuming that if I connect them in on the MCBs on the same RCD then I won't have the tripping issue but I will have an issue where the neutral could possibly be live even when the circuit is isolated?

 

[NozSpark]

Do not put these circuits on seperate MCBs for exactly the reason you said

 

[Electroglow]

Reeeeeee wire

 

[Manator]

A word of caution here, if you are not experianced " enough", this is a bad job to show your assessor, he will ask more questions of you to make sure you fully understand everything you did, and why.

 

[cr0ft]

Hi and thanks. I am happy with what the problem is to be honest, I'm going back on Monday to check the issue out but I am sure it is a borrowed neutral. Starting to think it is a bad job to show the assessor though to be fair! If I can't easily fix the wiring issue I will just end up putting them on the same MCB I think. That is how they were before and as long as the MCB is rated low enough to cover each cable individually then there shouldn't be any issues at the assessment as any safety issues have been fixed by effectively making it one circuit, right?

Definitely needs a re-wire though but there is no way the folks are going to pay for that one so I just have to note any obvious issues/defects down on the certificate. There's nothing else I can do if the customer doesn't want to pay is there?

 

[Electroglow]

Does ur dad have a garage? If so change that cu bit easyer??

Also have u tried to put one side of the lighting on one side of the rcd and the other on the same side but in separate mcbs.. ? Explain why u have done it to the assessor and tell him u gave the client (dad) a notice of recommended remedial work?

 

[Zeespark]

The lighting circuits may only need a partial rewire. Due to the age of the installation, your assessor shouldn't really pull you up on it I dont think, but do write down that you have a suspected shared neutral in the box marked 'comments on current installation' or some such I think it is.

AndyGuinness

 

[SPECIAL LOCATION]

The lighting circuits may only need a partial rewire. Due to the age of the installation, your assessor shouldn't really pull you up on it I dont think, but do write down that you have a suspected shared neutral in the box marked 'comments on current installation' or some such I think it is.AndyGuinness

You are correct in you assumptions of what an assessor should be checking.. :Salute

i.e. ONLY your work..

NOT the existing installation that you have not worked on!

BUT...

there wont be a shared neutral to comment on because you will not be leaving the circuits so arranged!

Does ur dad have a garage? If so change that cu bit easyer?? Also have u tried to put one side of the lighting on one side of the rcd and the other on the same side but in separate mcbs.. ? Explain why u have done it to the assessor and tell him u gave the client (dad) a notice of recommended remedial work?

:red card NEVER have two MCB's feeding the same section of cable.. (in this case a neutral)

It is an absolute NO NO.. 314.4 page 46 BGB!

It can risk electrocuting someone who disconnects one circuit thinking that they can change a light fitting...

only to find the neutral is live from another circuit!!!

EITHER..

1/ find the incorrect wiring and install new cable as appropriate to rectify the problem..

OR

2/ Keep them all supplied from one MCB.

:C

---------- Post Auto-Merged at 00:54 ---------- Previous post was made at 00:47 ----------

Hi and thanks. I am happy with what the problem is to be honest, I'm going back on Monday to check the issue out but I am sure it is a borrowed neutral. Starting to think it is a bad job to show the assessor though to be fair! If I can't easily fix the wiring issue I will just end up putting them on the same MCB I think. That is how they were before and as long as the MCB is rated low enough to cover each cable individually then there shouldn't be any issues at the assessment as any safety issues have been fixed by effectively making it one circuit, right?Definitely needs a re-wire though but there is no way the folks are going to pay for that one so I just have to note any obvious issues/defects down on the certificate. There's nothing else I can do if the customer doesn't want to pay is there?

Correct....

And your assessor, as well as checking you can interpret the regs correctly, should be well aware of real world limitations of budget constraints.....

i.e.

We must all do the best we can within budget and the clients specification....

IF your new work is going to be leaving the installation in a serious danger risking injury to people property or livestock...

Then DONT DO THE ALTERATION....

But providing your work is safe & in accordance with regs....

you can only advise a customer as to any other areas needing attention.

 

[Electroglow]

No to test if it a crossed N . Wen both cct are on opposite sides of the rcd then the N will be crossed so is u put them on the same side thn this would test if the N was crossed?

 

[NozSpark]

No to test if it a crossed N . Wen both cct are on opposite sides of the rcd then the N will be crossed so is u put them on the same side thn this would test if the N was crossed?

Sorry, but I'm not too sure what you're asking here. But have a look at my post above #2

 

[cr0ft]

Quick update. Put both radial circuits onto the same MCB today and the problem is gone. Well, the borrowed neutral is still there but at least it's safe and working now. I guess I'll just note it on the electricial installation certificate. Thanks to all for the advice.