Beware The Borrowed Neutral

[Monkey1984]

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.

 

[NoSparks]

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.

 

[Des 56]

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

 

[snowiespark]

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...

 

[NoSparks]

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.

 

[sparky1502]

sorry but why would u broow the neutral wire for the lighting circuit

 

[NoSparks]

The concept was that you had a live wire from the downstairs lights in the switch in the hall. This would be connectd to the common for the landing light. Then a 2 core (often without cpc) connected to L1 & L2 (although they were not marked as such in those days) to the landing switch, then a single core to the landing light, before "borrowing" the neutral of the upstairs lights.

Why? probably to save on the cost of 3 core between switches, or maybe because it meant that the electricians did not have to carry 3 core cable, or maybe just because that's the way everybody did it! Also in those days everybody had a pendant and there was no concept of changing a light fitting, so I suppose the dangers were not even considered.

What concerns me is what practices we are using today that will be considered dangerous in 50 years time? Maybe putting wire in the wall at all. I can just imagine it " Those idiots back in 2010 used to put cable in the wall. Why didn't they use proper radio switches?"

 

[hotXbun]

OK .... so I gather it's probably just me but something I find hard to resolve ...

I don't know about anyone else but I haven't come across a customer who phones me and says, "Hi hotcrossbunny, please, I want one of those new fangled 17th Edition all singing all dancing consumer units that every plumber/carpenter/plasterer/tiler/decorator/roofer/apprentice insists I need .....................and then said customer rejects the idea of replacing the borrowed neutral!

As I understand it, some interfering busybody comes along (most prob a plumber ) and tells customer that all is not well .... "I'm sorry to tell you this, but your electrics are not up to date. You need an "update"! Said customer, being mighty impressed, pleads with plumber for guidance and along comes PartP to the rescue ........ No doubt he/she(!?) has a "P" emblazoned on the underpants worn 'Superman' style ..... "Hi! Part P man (don't ask which part) to the rescue! ... What's your problem?" ... Hmmm,

Excuse the digression, but why is it ok as long as everything is 50mm beneath the surface? What is it that the plumber knows and I don't?????...

 

[NoSparks]

Maybe the plumber on his training course was told about BS7671.

 

[Gravid]

"As I understand it, some interfering busybody comes along (most prob a plumber ) and tells customer that all is not well .... "I'm sorry to tell you this, but your electrics are not up to date. You need an "update"! Said customer, being mighty impressed, pleads with plumber for guidance and along comes PartP to the rescue ........ No doubt he/she(!?) has a "P" emblazoned on the underpants worn 'Superman' style ..... "Hi! Part P man (don't ask which part) to the rescue! ... What's your problem?" ... Hmmm,"

HaHa brilliant, wish I'd got part P underpants instead of a mouse mat.

Maybe they'll stop selling screws and nails longer than 1.5" to anybody who owns a house soon. Who knows why 50mm ? All i know is it says so on page 100 BRB reg 522.6.7, anything less than or not complying with........, needs an RCD.