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Discuss Finding borrowed neutrals in the UK Electrical Forum area at ElectriciansForums.net

Raptor0014

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Evening all.

I did an EICR recently and then changed a consumer unit.

As part of the EICR process I take off around 25% of all accessories to check terminations etc.

Property was well maintained and EICR was satisfactory with a few C3s. Board was a split load single RCD board. Light and smoke detector circuits being the ones not RCD protected. Owner requested a CU upgrade as I was also installing a EV charge point and they wanted everything in one board.

So I replaced it with a full DP RCBO surge protected board.

After the install and doing all the tests etc I did a final go around to check everything worked and nothing was causing any issues.

Turned the landing light on at the bottom of the stairs and both down and up light RCBOs tripped.

When I removed the switches at the bottom and top of the stairs I noticed a single core red conductor in with the rest of the conductors.

So realised then that the previous installers had borrowed a neutral somewhere, probably in the loft which is fully insulated, boarded and had a load of storage items in there.

At the moment I’ve put the lights on one RCBO. There is still some of the lights on another RCBO as they had an extension built around 10yrs ago. So kitchen, utility and garage are on one RCBO and rest of lights on another. I’ve also labelled the board to say there’s a borrowed neutral on the offending circuit as I need to go back and rewire the circuit properly.

I was just wondering if there was something I was missing in my testing that would give me an idea that a neutral is borrowed? I’ve asked a couple of sparks I know and even my yearly assessor for the CPS. But no one seems to have any sure fire way to tell.

I’ve made a mental note to always take hallway and landing light switches off now as part of the 25%.

But was just wondering if the hive mind of the forum and experience of the members could tell me a way to pick it up so I don’t get a repeat occurrence in the future. 🤷🏼‍♂️
 
check for earth leakage with a clamp meter

Strictly speaking I think we should say 'check for differential current' as it will be to the other circuit's neutral, not earth. It can be shown up more clearly by switching off a circuit's MCB and checking for any current in its neutral.
 
On a related note, Irish regs call for an “erroneous connections test”, which was new to me until I helped on a job over there. I’ve always thought was a good idea since.
But I believe that is mainly for inter connected line conductors, and you turn off a breaker and test the bus bar to the outgoing supposedly disconnected live.
I’ve had a few inter-connected rings over the years, or both ends in different breakers which this has thrown up.
It saved me a bit of head scratching recently when the immersion and one end of the ring were mixed up at the board.
 
Last edited:
I’ve requested access to the Arms but as yet not been granted access. Will take a look once i get access.
In the meantime, here's what it said:
Initial verification of a new circuit? Or addition to an existing circuit/periodic?

For the latter:

All lamps in, all lights, fans etc switched on, everything. Isolate board. Disconnect L+N of the circuit to be tested and join together. Test first at 250V to E, then to L of every other circuit, then to N of every other circuit. Then if all results reasonable, repeat at 500V. Finally if any additions/alterations have been made, a continuity test between L and N (not an IR test!).

This should prove IR to earth, separation of circuits, and the continuity test will show if there is a direct L to N short. 250V first for the IR, because if there is a borrowed neutral and you test at 500V, then you'll put 500V across the luminaire.

It has been a long time since I've done IV on a completely new lighting circuit, but I would probably treat it the same way. I know we're expected to do a full 500V L-N IR test, but I see little value in it, particularly when afterwards you do a load of more work reconnecting luminaires, dimmers and so on. Any of this work could accidentally introduce a fault which would go untested.
After L+N to E, I personally do continuity first to other circuits (mainly out of habit) as in my mind I liked to eliminate low resistance faults before sending 250v between circuits, but thinking about it there's no partic reason for this caution!
 
An experienced domestic electrician should be specifically on the look out for this kind of thing, if not actively expecting it, when splitting lighting circuits onto different RCDs or connecting them to RCDs for the first time.
If you haven't warned the customer that there may be extra chargeable work after fitting RCDs, then you'll be doing some free work.
 
An experienced domestic electrician should be specifically on the look out for this kind of thing, if not actively expecting it, when splitting lighting circuits onto different RCDs or connecting them to RCDs for the first time.
If you haven't warned the customer that there may be extra chargeable work after fitting RCDs, then you'll be doing some free work.
We 're all continuously becoming more experienced. The op has just added this experienced to his collection, and so might others after reading about it.
 

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