Borrowed Neutral

[Kevin Holohan]

I was investigating a crackling light switch in the garage yesterday. The switch also provides power to an outside PIR security light which the previous owner of the house had provisioned. Removing the switch showed there was just live/switched live and earth for the garage light, so the individual (not me!) who connected the outside light used the earth as the neutral. I guess the logic was that neutral and earth are connected back together at some point (either in the meter cupboard or at the local substation) but I don't think this arrangement is entirely acceptable. I have disconnected the outside security light and I will run a cable from the garage ceiling light to allow a more suitable connection to the lighting circuit for the security light. A couple of years ago I did replace my old CU with one using RCBOs. One lighting circuit (this one!) would not allow the RCBO to latch so I replaced it with a regular MCB. I wonder if borrowing the neutral was the reason for the RCBO not wanting to latch on?

[Andy Burns]

That's not the usual definition of borrowing a neutral (which is using a neutral from one circuit as part of another circuit)

But using earth as though it was neutral will absolutely trip an RCD/RCBO unless it's really tiny current such as a neon indicator.

[Kevin Holohan]

Should have called it stealing a neutral!

[Harry Bloomfield, Esq.]

Kevin Holohan submitted this idea :

It is not acceptable. If you were to loose your main earth connection, then many metal cased appliances would then become live, with the obvious risks.

The above would be an obvious cause of the RCD tripping, though possibly not the only cause.

A 'borrowed nuetral' is not one where the earth is used as the nuetral. It describes the case where the nuetral from another entirely separate circuit is used. That would also cause an RCD to trip.

[John Rumm]

Have you checked at the other end of that cable to see what the earth wire is connected to there?

Obviously using an actual earth as a neutral is unacceptable for many reasons, however it might be that the earth wire was repurposed as an actual neutral by connecting it as such at *both* ends.

(if you do that then it must be over marked at both ends - and you can only do it on a wire that is part of a cable - not a single).

I am guessing from the RCD comment that was probably not what happened!

Yup that would do it if there was any load on the connected back to earth circumventing the RCD.

[charles]

very likely Different currents in L&N.

[ARW]

If it was a true borrowed neutral, on an all RCBO setup, I would expect two RCBOs to trip.

But it looks like the OP has found his problem

[Kevin Holohan]

Not looked yet but it will be clear when I access the wiring for the garage light to see what is connected to where.

Thanks for the feedback from you and others - it's much appreciated.

[Brian Gaff (Sofa]

The design of the original installation does sound very peculiar to me. There can be loads of reasons why your so called earth and neutral are not the same potential or lack of potential when the live follows a completely different path that its asking for somebody to get a nasty shock. Brian