Borrowed neutral

[Dambo]

Just a quick one on this guys:

(1) Why would there be any need for borrowing a neutral when using 2C+E / 3C+E? Or does the issue arise when not using 2C+E / 3C+E?

(2) Before carrying out any electrical work what would cause an electrician to even suspect that something might be wired this way i.e. when would you know when to check?

 

[ProDave]

Borrowed neutrals was common on the old octupus wiring system where everything (every light switch, every light fitting) all goes back to one huge junction box.

The problem is there is usually one junction box per floor. So what happens is the upstairs landing light gets it's neutral from the upstairs junction box, but gets it's live via the switches from the downstairs junction box.

With modern wiring methods, either loop in the light fitting, or loop in the switch it shouldn't happen.

In the old days, it didn't matter and was pure laziness as all neutrals went back to the same block, long before RCD's and split CU's were envisaged.

So a big clue is any property wired on the old octopus system is very very likely to have a borrowed neutral somewhere.

 

[steptoe]

or, on the common loop in/out system where it was simply a 2c&E strapper run to the landing switch, then a single red&E to landing light, with a single black N from upstairs circuit,

very common in 15th and I would say early part of 16th.

 

[ProDave]

very common in 15th and I would say early part of 16th.

Borrowed neutrals were outlawed in 16th right from the start. I still remember the lecturer drumming it into us.

 

[PBC_1966]

And even though they might have been not exactly uncommon in practice, they were not permitted by the Wiring Regs. for long before that.

14th edition, regulation A.25, and 13th edition, regulation 112(B):

The wiring of each final sub-circuit shall be electrically separate from that of every other final sub-circuit.

 

[sparkytim]

So "Borrowed Nuetral" really just means the Live for a particular light or lights is from one circuit and its nuetral is from another.

Whats the problem?

Well if you pull the fuse to disconnect a lighting circuit then disconnect the looped thru nuetral wires it is possible to get a shock of the Nuetral if it is serving another light on another circuit.

As others have said we often come across this on CU Changes when the upstairs landing light has its nuetral from the Upstairs lighting circuit and the live comes from the Downstairs circuit as it is fed off the downstairs switch then via strappers to the upstairs switch then to the landing light.

(This is really a borrowed LIVE, but we still call it borrowed Nuetral)

With a twin RCD CU we have upsairs lights on one RCD and downstairs on the other, so with Live from 1 RCD and N from the other it won't work it just trips the RCDs.

What do we do?

One way is to rewirethe landing 2way switching to ensure the live comes from upstairs circuit to the down stairs light switch but often difficult.

Many of us have found the easier solution is to take a DOWN STAIRS lighting Nuetral up to the Landing light to save disturbing the switches.

 

[ADS]

Before carrying out any electrical work what would cause an electrician to even suspect that something might be wired this way i.e. when would you know when to check?

If the property that you are working on has two lighting circuits (one up, one down), then there is a possibilty of a borrowed neutral.

It wouldn't rear it's ugly head until you fit a 17th board and split the two lighting circuits - one on each RCD. Unless, of course, you go looking for it.

If you have a house like mine was - i.e. one lighting circuit for whole house - then you haven't got a borrowed neutral, even though the neutral is connected upstairs, as it's the same circuit.

The problem will arise though, if you try to split the single lighting circuit into two, in order to comply with the 17th

Hope that's helpful.

 

[Dambo]

Thanks guys