Help please with understanding a borrowed neutral

baza · Feb 26, 2013

Hello everyone
Could someone explain why the following happens with a borrowed neutral?
Did a cu change today and installed a dual rcd board, Previously the up and downstairs lights were on the same mcb. I was half expecting the rcd/s to trip when I separated them, which they did. I dont need help in how to remedy this, but my question is when disconnecting the link at the back of the switch feeding the strappers linked to the other switch, I would have thought that I had removed the inbalance and the rcd would remain on.
Thanks in advance

lofty84 · Feb 27, 2013

thanks trev. HOw would they do that though as surely any new cable would have a cpc, L, and neautral unless they cut it of then it would be there for them to use.

or do you mean Mr Jones decides to put an outside light at the front of his house right by the front door so he can see where the hole is for his key as he is fed up with it taking 5 mins every night through the winter. Instead of logically taking his feed from the the hall light for example he has decided that he will take it from the outside socket which was installed to power his lownmower when he wants to cut his front lawn because it was a shorter route. therefore when I go in and switch the mcb of for down stairs lights if I wasnt to double check it was dead then I would get a shock because there was still power feeeding it from the outside socket which was on the down stairs ring?

farmerian · Feb 27, 2013

They a clue in the name borrowed neutral, it when a neutral has been connected to a different circult basically, a RCD will only pick this up

derek · Feb 27, 2013

http://www.electriciansforums.net/i...cal-installations/37498-borrowed-neutral.html

I think the above link has a decent explanation. In the example you have given if he took the Line from the lighting circuit and the neutral from the socket (or another lighting circuit on a different circuit breaker) as his return path then it would be a borrowed neutral and so require isolating two circuits to make safe.
The actual example you have given could mean that he is using 1.5mm cable on a 32A breaker- undersized cable/oversized breaker.
You should always - always - always use a voltage indicator (meeting GS38) before working on a circuit.

electric will · Feb 27, 2013

Imagine connecting up a new load but take the live (line) from 1 circuit then take the neutral to another circuit.
So power comes from one circuit but returns via another.
Im not the best at explaining things but that how I see borrowed neutrals.

rickm · Feb 27, 2013

Used to be a acceptable way of wiring before 3core and rcds. Pretty easy to rectify . It trips rcds cos its not a balanced load .ie current is being drawn from 1 side of the split load and returns on the other side of the board. The rcd thinks the current is leaked somewhere

derek · Feb 28, 2013

Although the borrowed neutral may have been common practice I don't think it has ever been acceptable (at least not for 65 years or so), a little research on the IET forum seems to suggest that even the 11th and 12th editions didn't allow it and the 13th edition 1955 definitely didn't allow a borrowed neutral, "The wiring of each final sub-circuit shall be electrically separate from that of every other final sub-circuit".

Flakey · Feb 28, 2013

Borrowed neutral will only become apparant when rcds are installed. Its like crossing neutrals in the board. For example say you have two lighting circuits one on one RCD the other on the other RCD but the neutral of one of the lighting circuits is incorrectly connected to the neutral bar of the other lighting circuit then the RCD wil trip. so its like connect the neutral of one circuit is connected to the neutral bar of another circuit just further down the Circuit!!

lofty84 · Feb 28, 2013

Cheers boys I thought it was something like that it doesn't sound right so not to sure why people would do it but I guess I you don't do it you wouldnt really know. Sometimes people say electricians do it because they are lazy but I'm trying to understand how this could be as if your taking a live and cpc from one circuit you migh as well take the Neauteal from there as well so it just sounds like more work to me. Good point about taking a neutral from a ring to feed a light I wouldn't actually do that but it was a silly example none the less. Cheers for the link

VoltzElectrical · Feb 28, 2013

I still can't understand the physical wiring of this borrowed neutral business. As has been stated, if you need a feed from somewhere, just take the neutral from the same source i.e. a ceiling rose. Seems like more work cobbling a feed from two sources.

I don't work in houses so have never personally come across it. I understand the science of why the RCD trips, but TBH I have given up trying to understand the rest.

(i'm going to read that other thread now to see if that helps lol.)

Flakey · Feb 28, 2013

A scenario i could make up is say: you have a one gang light switch that switches a pendent for example all you had in the switch was the switch line and permanant but no neutral. Now say you wanted wall lights that were switched from the same switch location you would need a 2 gang switch but there would be no neutral there may be a socket locally that may be "easier" to access near the switch so borrow a neutral from there. (DONT DO THIS) Example scenario purpose only!!!

derek · Feb 28, 2013

Perhaps the reason for so many borrowed neutrals in the UK is that many DIY 'experts' have been having a go at domestic electrics for many years before (and since) part P came in.

Scornio · Feb 28, 2013

It happens a lot when you have 'loop in' systems that sends the live and switch live to switch say by the front door... Then the average DIY er decides he wants an outside light switched from same location and thinks he'll get the feed from there.
probably wires the cable in as well before realising there is no neutral... Then the inevitable happens.... He either uses the cpc or borrows a neutral from nearest location.

VoltzElectrical · Feb 28, 2013

It all confirms WHY i don't now, or ever want to, work in houses!

Commercial/ Industrial is way easier and more rewarding. WHY does every new member on here want to become a flippering Domestic Installer? I'd clean toilets before doing that! (I have cleaned toilets by the way - straight out of university too I might add! Then i went on to factory work before doing teacher training, but that's another story!)