borrowed neutral question

[jaysean1]

Just changed a db at my house, I am unsure if i have a borrowed neutral between the 1st floor and gf lgts.

Am I right in saying if I Insulation test between the neutrals and lives of the two ccts (neutral to neutral, live to live ), this will prove it either way.

I have done the trick with the voltstick(switched a lgt on either cct and pulled the neutral out of the bar, and checked if the neutral becomes live)

(I know about part p, my dad is registered)

 

[ericmark]

The borrowed neutral is not easy to find as the item using borrowed neutral has to be switched on.

So for example stair lighting with borrowed neutral will only show in one of the 4 combinations of the two switches and only if bulbs are in place.

A volt stick is same as neon screw driver it is not a test instrument it is handy for quick tests but not to be relied on.

 

[RedHerring]

Excuse my ignorance, I'm not a sparky but how can a borrowed neutral be created by changing a distribution board? (which I assume is a db) .
Don't all the lighting circuit neutrals go into the same bar? Are they single-pole breakers? It could be out of place in the bar, but it's still in the same bar?

 

[ColJack]

it can't be caused by changing the CU, but it might cause problems when the new CU is dual RCD or RCBO..
they don't like borrowed neutrals one bit, where as older fuses and MCB boards don't really care..

the new regs mean that most CU changes mean you have to split the upstairs and downstairs onto seperate RCD's so that a fault on one light won't plunge the entire house into darkness...

stair lighting with borrowed neutral will only show in one of the 4 combinations of the two switches

well i don't know about you, but my 2 way switching makes the lights come on in 2 positions out of 4 of the switches...
both up or both down = off, either one oposite to the other = on..

 

[ericmark]

Excuse my ignorance, I'm not a sparky but how can a borrowed neutral be created by changing a distribution board? (which I assume is a db) .
Don't all the lighting circuit neutrals go into the same bar? Are they single-pole breakers? It could be out of place in the bar, but it's still in the same bar?

It is caused when the up-stairs and down-stairs lighting circuits are split which is often done when changing consumer units.

Years ago we used just two wires between the two switches on two way circuits and the third connection (Line) was "borrowed" from another switch normally using a two gang plate. This however could cause mains hum on radios and TV's etc. So it was stopped and now three cores are used. However when changing boards in old houses to comply with light must be available even if a single breaker trips it is common to split the lights into two feeds. This has been done for years but with RCD's it now trips out the RCD.

Easy cure is supply all lights from one MCB and to fit emergency light fittings. This is what I did in my house.

 

[jaysean1]

ive put the gf lgts into a rcbo they have not tripped so i think neutrals are seperate !!

 

[RedHerring]

It is caused when the up-stairs and down-stairs lighting circuits are split which is often done when changing consumer units.

Years ago we used just two wires between the two switches on two way circuits and the third connection (Line) was "borrowed" from another switch normally using a two gang plate. This however could cause mains hum on radios and TV's etc. So it was stopped and now three cores are used. However when changing boards in old houses to comply with light must be available even if a single breaker trips it is common to split the lights into two feeds. This has been done for years but with RCD's it now trips out the RCD.

Easy cure is supply all lights from one MCB and to fit emergency light fittings. This is what I did in my house.

eric, thanks for taking the time to explain. I used to be a sparky, over 35 years ago (was it 14th edition then?) and not uptodate on current regs.
The assumption I made is that the op was using one single-pole MCB for upstairs and one for downstairs. (Incorrectly, now) Thus my question how can a borrowed neutral be created because both neutrals would go into the same busbar. I assume an RCBO is a double-pole breaker, in order to measure the residual current.
Have no fear, I'm not updating my knowledge to start practising again.

 

[ban-all-sheds]

Imagine the following:

At the hallway 2-gang switch you link both COMs, and run twin & earth from the landing light side L1 & L2 to L1 & L2 in the switch on the landing.

From that you run a single, or one core of T/E from COM to the switched live of the landing light rose, through which runs the loop from which the light gets its neutral.

Come the day you separate the upstairs and downstairs lighting circuits you end up with the landing light getting its live from the downstairs circuit but its neutral from the upstairs one.

1) If up & down are on different RCDs, or on RCBOs, there will be a trip, because there will be an imbalance with a value of lamp watts ÷ voltage.

2) If not on RCBOs or separate RCDs, all will be well until someone turns off the upstairs MCB to do some work, cuts or disconnects the upstairs loop upstream of the borrowed neutral, and then finds to their chagrin that one end of the cut cable is live courtesy of the path through the light from the downstairs circuit.

I take your point about all the neutrals being commoned at the CU - (maybe the problem should be described as a borrowed live?), but they won't be commoned if RCBOs or different RCDs are used, and the "oops I thought I'd turned it off" doesn't happen until it's been disconnected from the neutral bar.

 

[RedHerring]

Blimey BAS, that was one hell of a response. Thank you.
I misunderstood the op's original post. I thought he said "I think I may have created a borrowed neutral". He didn't, he said "I may have a borrowed neutral".
My original question was how can you create a borrowed neutral by changing the CU.
Well, you can't, can you? Because both neutrals went into the same busbar, if you're using a MCB.
All becomes clear when I realised I'd made the mistake and realised the op was talking about taking the neutral out of the bar in the first DB, then putting the RCBO in the replacement CU.
And as you and others point out, if using RCD or RCBO they will trip.

 

[bongos]

With reference to the OP, what is the best way to test for borrowed neutrals - although this post seems to have some good ideas...

 

[jaysean1]

I thought I had a borrowed neutral because i disconnected the ground floor lgts neutral and gf floor lgts when still on. But when i took the neutral out of the upstairs, the gf lgts stopped working.

So the next day I traced the cct, belled it out and 'meggered' gf neutral with ff neutral same with the lives, which proved I did not have a borrowed neutral and I put the ground floor lgts on its own rcbo in my mem db.

I think as I put the gf lgts on a rcbo which has not tripped i dont think I have a borrowed neutral.