Borrowed Neutral test - sense check.

Hi, I don't wish to open the 'borrowed neutrals' can of worms again but I'd just like a more experienced testers view of how to test for them.

I've not done my 2391 but I'm hoping my logic will prevail.

To test for a borrowed neutral would it work to:

Disconnect N on lighting circuit 1 and switch off CB while leaving circuit 2 live and all connected.

Check which lights all work on circuit 2 then reverse the test to ascertain the fittings on circuit 1.

re-connect both circuits as they were originally, make circuit 1 live while leave 2 off and visa versa and see if any of the lights suddenly do work on the 'wrong' circuits?

I'm assuming that if the landing/hall lamp neutral were borrowed it wouldn't work on either of the first 2 tests but would at the end?

I'm assuming a barrage of corrections and questioning of parentage but I'm asking because I need correcting/confirming.

Thanks

Dave 

thats one way of doing it but be aware the disconnected neutral at the board may be live

Hi Dave ... this is supposed to be "The  UK's  Friendliest Forum"      so I sincerly hope that  your parentage is not called into question ....welcome to the Forum...ask your questions , we'll try to answer them . 

 The safe way I suppose , is to switch off both ,  pull the Neuts off the bar ,  ohmeter or buzzer  across them , switch lights on/off .  

If you wanted to automate it, stick the 2 circuits on different RCD's and keep switching switches until an RCD trips.

(Not recommending this, terms and conditions apply etc)

That would involve having more than 1 rcd in the ccu to begin with. 

The point of the testing is with a view to rcbo'ing the board.

DRW said:

That would involve having more than 1 rcd in the ccu to begin with.

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no it wouldnt. only 1 circuit would need to be on the RCD, the other could be on the non RCD side

yes I see, my apologies, removing the neutrals feels a bit simpler and less faff. I'm glad my thinking was at least accurate even if it's not the standard way, if there is such a thing.

DRW said:

I'm assuming a barrage of corrections and questioning of parentage but I'm asking because I need correcting/confirming.

Thanks

Dave 

Click to expand...

your mother was a hamster and your father smelt of elderberries :^O

easiest way is at the CU, disconnect the suspect neutral and test for a voltage with circuit you suspect neutral is borrowed from still live.

Borrowed neutrals, hate the things, years ago we'd done a board change on a school and we got a call back saying the rcd kept tripping. we went back and I decided to ask a few questions before doing anything, well this teacher commented that it seemed to trip every time she went into her store cupboard, this gave me an idea. I powered everything up and got my mate to operate the switch on the "suspect fitting and sure enough one of the rcd's tripped, I tested the neutral bar while rcd tripped out and it was live, indicating that the offending fitting was getting it's live from the other side of the board. I then proceeded to remove each neutral in turn and checking to see if it was live, I eventually found one that was and dropped it onto the other neutral rail, the light came on, happy days. I was putting the other cables back and dropped a screw, I picked it up and was holding it between my fingers trying to get it into the neutral bar, suddenly I got a hell of a belt, I dropped the screw and stood there cursing. A couple of minutes later another teacher appeared, "excuse me, when you've finished can you have a look at a light in my store cupboard?". It turned out there were several lights that we didn't even know about so we hadn't found them .

Ok I should have fully isolated the board, that was my fault, but it also revealed that the previous contractor had not been keeping records of work they'd carried out at the school, this is often the case on public buildings. I had one where there was no power to a full floor of a nursing home, it turned out to be an rcd that had tripped, the rcd had been fitted in the feed cable to the board for that floor but because there was no space to fit it adjacent to the board it had been fitted in a store cupboard on the floor below.