20 Volt oddity at recent rewire

[M-B-Electrical-Services]

https://uk.prysmiangroup.com/sites/default/files/atoms/files/Stray%20Voltages%20in%20Multicore%20Cables%20%282%29.pdf

Thanks for this very helpful information!

 

[M-B-Electrical-Services]

Ok so this gets more and more confusing.

I am back on site and have identified the offending lighting circuit which is now showing 49v between switched line and CPC when switched off.

I have split the circuit in half so only 2 Greg roses are powered. All loads removed. The hallway light (removed) is fed via 3 switches consisting of 2 x 2 ways and an intermediate.
I have confirmed r1 + r2 with a good result and carried out a 500V IR test ensuring to switch each switch after each test again with beyond 999 results.

All other circuits are in the off position so only that circuit is powered.

I am bewildered by this and am at my wit's end.

 

[M-B-Electrical-Services]

Could it possibly be a faulty switch?

 

[newfutile]

I don’t think it’s a problem at all , as stated already it’s most likely caused by capacitance or inductance . You have already proved its not resistive .
its not a test listed in GN3 or BS7671 .
if it passes all other relevant test its fine.
try loading the circuit down with a lamp and lamp holder and i think the voltage will disappear .
meters generally measure volts in a high impedance mode (in the megohms) once any real current is drawn from the circuit the voltage should drop a great deal

 

[DPG]

Agree with the above, probably a coupled/induced voltage.

What results did you get when you tried measuring the same voltage, but with a low impedance meter, or with a very small load connected?

 

[M-B-Electrical-Services]

I don’t think it’s a problem at all , as stated already it’s most likely caused by capacitance or inductance . You have already proved its not resistive .
its not a test listed in GN3 or BS7671 .
if it passes all other relevant test its fine.
try loading the circuit down with a lamp and lamp holder and i think the voltage will disappear .
meters generally measure volts in a high impedance mode (in the megohms) once any real current is drawn from the circuit the voltage should drop a great deal

I have come to the same conclusion after exhausting all other avenues.
I have my NICEIC inspection coming up in the next 2 weeks and will be using this property as one of my examples which is why I have gone a bit overboard on it.
All I have really achieved is plenty of wasted time and lots of frustration.
Luckily, the client is a friend so they weren't too put out by my being there.

I'll take this one on the chin and chalk it up to experience (or lack of in this case) and know that in future, unless there is clearly a broken conductor, I will be familiar with this scenario and know how to deal with it.

Thanks for the input!

 

[M-B-Electrical-Services]

Agree with the above, probably a coupled/induced voltage.

What results did you get when you tried measuring the same voltage, but with a low impedance meter, or with a very small load connected?

There was no difference whatsoever in this case bud.

 

[DPG]

There was no difference whatsoever in this case bud.

That's more interesting then. I'm surprised at that.

 

[M-B-Electrical-Services]

That's more interesting then. I'm surprised at that.

Something you may find interesting is that, sporadically, my meter would jump to a DC voltage and show a negative result of around minus 35v just for the slightest of seconds then go back to it's original 49v.

At this point my head fell off!

Although I have 25 years in the game, this one has truly stumped me and, although I now know the cause, it still has given me much to think about!!

 

[M-B-Electrical-Services]

My MFT has just been calibrated before anyone asks