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Borrowed Neutral


m4tty

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Hi,

Just found some info thought id share. May help some of the learners ... like it did me.

The most likely instance where you will find a borrowed neutral is the hall/landing light circuit with a two way switch which enables you to switch on the light from the bottom or top of the staircase.

Domestic lighting is usually split into an upstairs and downstairs circuit each with it's own breaker being a fuse or MCB. Now when the house was rewired back in the 80's a not very scrupulous electrician (but following what was very common bad practice) decided he could save himself a bit of time and cable when wiring the landing light. He used the live red wire from the downstairs light circuit to provide the supply to the downstairs light switch, which was wired in with the upstairs light switch in two way configuration. So the live supply to the landing light upstairs was from the downstairs light circuit.

But, he also needed a neutral supply to the upstairs landing light and what could be more easy than running an neutral to this light from the upstairs light circuit. He thinks this is no problem because all neutrals are connected together back at the fuse board anyway. Electrically the circuit will work.

Now, assume you have this arrangement and you decide to fit a new light fitting in the bedroom and pull the fuse on the upstairs lighting circuit (or switch off the MCB). You carry out a voltage test on the ceiling light that you are about to disconnect and happy that the circuit is dead you continue to disconnect the wires from the ceiling rose. Whilst wiring up your new light somebody in the house comes up the stairs and turns the staircase light on. Bang! Your dead.

The reason, because of the borrowed neutral. When the landing light which was on the downstairs circuit is switched on and you were working on the upstairs, this current flowed through the light bulb and into the upstairs neutral circuit. Whilst the neutrals on your upstairs fitting were connected together they were at neutral potential. When you took the neutrals out of the terminal block the one that went back to the landing light became live because it was connected through the light bulb to live supply. When you touched it the current that would have flowed through the neutral circuit back to the main neutral at the fuse board is now trying to flow through you to ground.

Electricians have been killed as a result of borrowed neutrals so be advised, the best solution is to switch off at the main switch.

Cheers

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I've come across borrowed neutrals a couple of times in larger installations where the neutral has been connected to another DB completely, once a colleague got a belt from the neutral block of an isolated DB & once when breaking the neutral link on a 3-phase db for IR testing I noticed sparks. In both cases putting a test lamp between the neutral & earth blocks showed 230v, highlights the reason for testing between neutral & earth when carrying out safe isolation!

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i still dont fully follow why it was easer to borrow the neutral.

i must be wrong but do they take the live from the downstairs switch and take this to the upstairs light and then take a neutral from the upstairs light circuit??

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i still dont fully follow why it was easer to borrow the neutral.

i must be wrong but do they take the live from the downstairs switch and take this to the upstairs light and then take a neutral from the upstairs light circuit??

Yes.

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You just put the hall light on the upstairs circuit.

Borrowed neutral was a widespread common practice in the 1950s , 1960s . All about saving money, so you will find in a small 3 bed semi built in 50s and 60s , a 4 way board, 1x 5a light circuit (up/down and unearthed) 1 x 30a ring main (whole house) 1 x 15a Immersion htr. 1 x 30a Cooker. Spare ways were unheard of.

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You just put the hall light on the upstairs circuit.

Borrowed neutral was a widespread common practice in the 1950s , 1960s . All about saving money, so you will find in a small 3 bed semi built in 50s and 60s , a 4 way board, 1x 5a light circuit (up/down and unearthed) 1 x 30a ring main (whole house) 1 x 15a Immersion htr. 1 x 30a Cooker. Spare ways were unheard of.

Deke, forgive my vetty dumbness but if there's only 1 lighting circuit the neutral isn't borrowed!

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I knew some one would say that , had to be you didn't it ? Its just bad sentence construction, TBH, borrowed neutral etc... FULL STOP !! Then related information about cheap installations .... as I clicked the button I saw " only one circuit etc" how could you borrow, but too late it was into the ether.

Obviously jobs with two circuits would have the borrowed neutral. Trust you!!

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Be aware that even if the hall light is energised, and the borrowed neutral is in use, your voltage testing will tell you its dead until neutral is disconnected (because there is no volt difference since neutral is still at neutral voltage). once you remove the neutral, it will then become live

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  • 2 weeks later...

Yes, this was common practice in the 60's and even well into the 70's. One other tell tale sign is lighting circuits wired in singles, with a t&e (usually 2 reds) used for the strappers.

I put both 'circuits' onto one mcb creating one circuit when I'm doing a board change.

What methods do others use to find this condition?

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why does it trip the rcd when a borrowed neutral of 2 circuits are on each of the 2 rcd's

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why does it trip the rcd when a borrowed neutral of 2 circuits are on each of the 2 rcd's

RCD detects difference between L&N so if power 'going in' the live but 'going out' the neutral of a different circuit the RCD trips.

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RCD detects difference between L&N so if power 'going in' the live but 'going out' the neutral of a different circuit the RCD trips.

also, both RCD's should trip, since one has too much through live phase line, and the other has too much through neutral

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I've read and re-read some of the posts in this thread and I'm still confused...

Don't suppose there's someone out there who could put an example of a "borrowed neutral" in a diagram to help me understand it a bit more.

I think I have that situation with the circuits at my mum's house which became apparent when doing some work. I turned off both up & down circuits to be on the same side.

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Nat,

It's actually really simple. Lazy spark has lights on the stairs (usually) and takes 1 live feed from downstairs but feels lazy and uses the neutral from the upstairs lighting circuit on the light. Why? Save a bit of cable? This was sort of ok and worked before we had RCD's, but they don't like the imbalance.

Can have any example you chose - DIY outside lights etc

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Nat,

It's actually really simple. Lazy spark has lights on the stairs (usually) and takes 1 live feed from downstairs but feels lazy and uses the neutral from the upstairs lighting circuit on the light. Why? Save a bit of cable? This was sort of ok and worked before we had RCD's, but they don't like the imbalance.

Can have any example you chose - DIY outside lights etc

so, it's usually done between up and down circuits, but can be done between any in theory? Also, does the exchange usualy occur at the switch?

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If you've got a copy of the book "Snags and solutions" (the second book, not the earthing and bonding one) then there is a cracking explanation in there with some lovely pictures... I got a copy from the library so haven't got a copy that I can scan at the moment (will see if I can loan it again and do so later).

The problem that I've found is that if you put the two circuits on separate sides of the board, then because the RCDs will have slightly different reaction times, then there is no knowing which will trip first, and it seems to vary too.

I've worked round it by ignoring discrimination (I know, but it's easier than chasing out and adding a new neutral) and moving both MCBs onto one side of the board. Simple, but effective!

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I've worked round it by ignoring discrimination (I know, but it's easier than chasing out and adding a new neutral) and moving both MCBs onto one side of the board. Simple, but effective!

:o

if u do that then u still have a borrowed neutral?

the dangers still excist?

if u turn off 1 mcb then is it not posible to get a belt?

is it not ok to put both circuits in the same mcb (10 amp) if voltage drop is ok???

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Is this an example of a borrowed neutral...

There's a downstairs light on - when I switch off the breaker labelled "down lights", it doesn't go off. I put the breaker back on and switch off "up lights", it still doesn't go off! It only goes off when I switch off BOTH up and down light breakers.

Could someone kindly confirm if this is the case, and if not, why it's like this?

Cheers

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is it not ok to put both circuits in the same mcb (10 amp) if voltage drop is ok???

It's either that or mend it! IMO much safer than just moving it over to same side of board and then some other spark comes along and kills themself!

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It's either that or mend it! IMO much safer than just moving it over to same side of board and then some other spark comes along and kills themself!

i dont want to kill my self with electricity, unless its after a long custody battle with x wife, a toster and a bath,. but wait would the rcd save me, dame foiled by my own demise:_|

:D:D:D

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The best way to describe this method of wiring is to compare it to the way you would wire 2 way switching in conduit, in singles.

Wiring in 3c&e is slightly different to conduit method.

I googled 2 way switching and found this diagram, not the best in the world, but it shows 2 way swithing wiring, conduit style.

http://2wayswitch.com/wp-content/uploads/2008/01/2-way-switch-wiring-diagram.jpg

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