Switched Neutral Fault Finding

DyFamCont · Jan 19, 2024

Evening all,

I had a quick Friday afternoon domestic job that has turned into a PITA and I'd appreciate your help.

While adding some new lights to a circuit, I found that the switch where I was taking the additional line from was actually switching the neutral. After much swearing I checked the polarity at the ceiling rose and got some odd results from the ceiling rose as per the table below. The part that's confusing me is why N-E is at 230v with the switch open.

Being a domestic job, I can't follow the cables properly without wrecking the place but have done continuity to find out what each goes to. The camera in the ceiling revealed nothing of significance. I'm assuming there's a junction box somewhere in the middle where someone has messed it up. The rest of the lights in the circuit have the correct polarity.

Any help would be greatly appreciated because I said I'd go back tomorrow.

[ElectriciansForums.net] Switched Neutral Fault Finding

westward10 · Jan 19, 2024

Your live readings at the rose are they at the loop in, assuming there is one.

DyFamCont · Jan 19, 2024

westward10 said:
Your live readings at the rose are they at the loop in, assuming there is one.

Thanks for your interest.

There isn't a loop at the ceiling or switch, so I assume its in a box in the ceiling. I can't access it though, so just assuming

plugsandsparks · Jan 19, 2024

Have you tried it without the lamp in the holder ?

Eric12 · Jan 19, 2024

sure im not going mad but them readings are as they would be if the neutral is switched, so looks like you need to extend the feed so the line an be switched, and put a blank plate on the existing switch !

Eric12 · Jan 19, 2024

DyFamCont said:
The part that's confusing me is why N-E is at 230v with the switch open.

As the switch is breaking the neutral (causing open circuit) then the neutral will show potential to earth. The neutral is a live (potential to earth) conductor isnt it if its not going anywhere

OLDBOY · Jan 19, 2024

As above, remove the lamp and the N- E potential at the ceiling rose should disappear with the switch open.

Eric12 · Jan 19, 2024

OLDBOY said:
As above, remove the lamp and the N- E potential at the ceiling rose should disappear with the switch open.

Disappear at the switch aswell

Eric12 · Jan 19, 2024

the diagram looks unorthodox, is this wired in singles ? If not then how is just one conductor feeding the ceiling light rose ??

nicebutdim · Jan 19, 2024

Eric12 said:
the diagram looks unorthodox, is this wired in singles ? If not then how is just one conductor feeding the ceiling light rose ??

OP is working on the assumption that a junction box is hiding somewhere, presumably in which switched line and neutral are reversed.

DyFamCont · Jan 20, 2024

Thanks very much for your help chaps.

It turned out to be reversed polarity and some dodgy DIY stuff. I'll go back another day to sort it.

Really appreciate the quick response though!