Borrowed Neutral/Mains humming?

[DNF2357]

Moved into a refurbed flat a couple of years ago and there were a few suspect things about the electrical work which I’ve not got round to rectifying yet.

I swapped out all the lighting myself over time (funny how all of the installed lamps were incandescent) and noticed that the Bathroom LEDs continued to glow faintly when off, even with the breaker off. Also picked up a lower voltage coming off the neutral on another light I changed, despite the lighting circuit being off.

Seems like a classic case of a borrowed neutral. The bathroom wall had been rebuilt after they moved the HW Cylinder and they’d put it a 2 gang switch for the hall/bathroom lights.

I didn’t manage to trace out the cabling fully to get to the bottom of it due to sudden health issues but it’s been at the back of my mind since.

I’ve got a split board with 2 RCBOs off the main isolator; with lighting on one and power on the other as well as a couple of other circuits for the HW and appliances etc.

There are also two installed breakers marked as Spare but it turns out they’ve both just been disconnected and left in connector blocks behind the panel cover. I didn’t really question the blanked off socket in each room when moving in as I knew there was previously an Economy 7 setup and I assumed they’d just not bothered to connect up that circuit when they left the old E7 supply in a Henly block.

Since there’s only 1 lighting circuit, I figured the crossed neutral might be with the power somewhere but again, I’ve not been able to bell everything out to see.

Reason for the post is that I’ve noticed a high pitched humming noise coming from the bathroom periodically over the past month or so and it’s been driving me mad. It stopped today when I unplugged the charging dock I got for Christmas (one of those ones with several USB outputs at different wattages and QI wireless charger).

Is this purely circumstantial or could that be clue as to the location of the offending neutral?

 

[DPG]

Borrowed neutral would more likely trip the RCD. Your issue with the LEDs sounds more like capacitive pick up. Have a search on here for threads about snubbers.

 

[Lucien Nunes]

No reason to think there's any connection, it sounds like a co-incidence . Although if you do find that the noise in the bathroom comes and goes with the connection of the dock, find out exactly what device is emitting the noise.

the Bathroom LEDs continued to glow faintly when off, even with the breaker off. Also picked up a lower voltage coming off the neutral on another light I changed, despite the lighting circuit being off.

These effects do not suggest a borrowed neutral to me. If the line conductor of a circuit is isolated at the MCB, the only way the LEDs can glow is through leakage, either capacitive or resistive, from another line conductor. Which neutral is in use is immaterial as far as the inward leakage is concerned. What voltage did you measure on this neutral, and relative to what? Did you measure similarly on another circuit at the time?

 

[loz2754]

Wireless chargers are notorious for giving off high frequency whining/warbling noises.

 

[DNF2357]

No reason to think there's any connection, it sounds like a co-incidence . Although if you do find that the noise in the bathroom comes and goes with the connection of the dock, find out exactly what device is emitting the noise.



These effects do not suggest a borrowed neutral to me. If the line conductor of a circuit is isolated at the MCB, the only way the LEDs can glow is through leakage, either capacitive or resistive, from another line conductor. Which neutral is in use is immaterial as far as the inward leakage is concerned. What voltage did you measure on this neutral, and relative to what? Did you measure similarly on another circuit at the time?

Thanks to you and @DPG, I’d not thought about this before but it’s interesting. Will have to have a deeper read to get my head around it fully.

If I recall correctly I’ve picked up around 50v N-E at a couple of lighting points with the lighting RCD off (and also I think with the RCBO for the lighting circuit off) but would disappear once I switched the other RCBO and RCDs off.

Will have to replicate it to be more exact as it’s been a while. Didn’t think to try looking for a ghost voltage on any other circuits but can look into that.

Am I wrong in thinking that previous posts on this have said that the voltage is unlikely to be inducted? So I need to be checking the junction boxes and anything else the cowboys touched where there might be a physical connection between line conductors?

Any chance the two incoming supplies might have anything to do with it at all? The E7 supply still has a leg from the meter into a Henley block next to the DB and both supplies run through a bit of boxing above the front door, which I’d imagine contains a few surprises.

 

[DNF2357]

Wireless chargers are notorious for giving off high frequency whining/warbling noises.

It’s definitely not from the charger itself, which is on my bedside table. I haven’t narrowed down what it’s coming from, besides being loudest in the bathroom.

All I know is that it was preventing me having a nap earlier and disappeared as soon as I unplugged the charger

 

[Lucien Nunes]

I’ve picked up around 50v N-E at a couple of lighting points

This is abnormal; it does not indicate borrowed neutral but rather that either the N or CPC is high-resistance or open-circuit. If the lights on this circuit work correctly, the neutral probably isn't at fault in which case it suggests the earthing is open-circuit. What you are measuring would then be 50V on the CPC, relative to the neutral which is near true earth/neutral potential. The voltage found on one circuit could be picked up as capacitive leakage from another circuit if the CPCs meet at a light switch, hence the variation when other circuits' MCBs are switched on/off. I would investigate this as a matter of priority by doing CPC continuity tests (R2, or R1+R2), where necessary separating the above mentioned points where CPCs of different circuits meet.

If the CPC is open-circuit it can increase the tendency for the LEDs to glow when off, since the layout of T+E causes the central CPC to intercept capacitive leakage from the permanent line before it reaches the switched line in a switch cable. A floating CPC could be boosting the total leakage into the switched line. Hold off troubleshooting the glow until you have confirmed good R2 and loop-impedance readings at all points of interest.

 

[DNF2357]

This is abnormal; it does not indicate borrowed neutral but rather that either the N or CPC is high-resistance or open-circuit. If the lights on this circuit work correctly, the neutral probably isn't at fault in which case it suggests the earthing is open-circuit. What you are measuring would then be 50V on the CPC, relative to the neutral which is near true earth/neutral potential. The voltage found on one circuit could be picked up as capacitive leakage from another circuit if the CPCs meet at a light switch, hence the variation when other circuits' MCBs are switched on/off. I would investigate this as a matter of priority by doing CPC continuity tests (R2, or R1+R2), where necessary separating the above mentioned points where CPCs of different circuits meet.

If the CPC is open-circuit it can increase the tendency for the LEDs to glow when off, since the layout of T+E causes the central CPC to intercept capacitive leakage from the permanent line before it reaches the switched line in a switch cable. A floating CPC could be boosting the total leakage into the switched line. Hold off troubleshooting the glow until you have confirmed good R2 and loop-impedance readings at all points of interest.

The new hall/bathroom switch seems to go straight up into a rose above the bathroom ceiling and into the lighting circuit/back to the DB from there so I guess that will be the place to start looking.

It looks like the rest of the lighting circuit is as originally installed (early 90s) and run in conduit above the plastered ceiling which wasn’t touched in the refurb so I’ll be hoping the problem doesn’t stem from anything up there.

Time to break out the Kewtech and see what I get.

 

[Markyd]

Lamps glowing when switched off....cheap lamps