20 Volt oddity at recent rewire

[M-B-Electrical-Services]

Hi all,

I have an unusual issue with a recent rewire I have just completed which I hope you guys can assist with.
For background, this is a complete strip out and rewire of a domestic property (bungalow) with an unusual but appropriate reference method.
For this rewire, due to the fact that it is a bungalow with easy loft access and zero insulation installed, I opted for reference method E (perforated tray) with a mix of Klik roses and Klik lighting distribution units used for the lighting which has worked extremely well in this case as the homeowner wanted to maximise loft space which we have certainly achieved here.
Anyway, the issue I am having is on one of the lighting circuits where I am getting 20v between N-CPC - N-SwL - Swl - CPC when the switch is in the open position.

All voltages between L - N, L - CPC, N - CPC are correct when the switch is in the closed position.
To me, this indicates either a loose connection or high resistive joint but I have checked everything and all appears in good order.
I have turned all circuits off except the offending circuit with no effect.
Due to the reference method used, I can see all conductors and can trace them for damage easily and all wiring within the walls is contained within oval trunking with no pictures hung within the prescribed zones.

Have I missed something obvious here? I have my NICEIC assessment coming up soon and wold like to use this property for assessment purposes but until this unusual fault is found, I obviously cannot do that.

Mike

 

[timhoward]

Are your IR tests all clear?
Is this actually causing any issues with any normal test results or causing LED lamps to stay on, flicker or flash?
If you have lots of cable in parallel on cable tray could this simply be induced voltage?

 

[M-B-Electrical-Services]

Are your IR tests all clear?
Is this actually causing any issues with any normal test results or causing LED lamps to stay on, flicker or flash?
If you have lots of cable in parallel on cable tray could this simply be induced voltage?

All IR testing comes back ok and, because I can easily remove loads, I can happily test between live conductors too which further confirms the solidity of the results.
There are no anomalies with the lighting at all (all LED) and this does not seem to be affecting the standard lighting.

Agree on induced voltage due to the tray work but when I open all other circuits, the 20v remains on this circuit.

The DNO have recently removed a looped connection to this property, could that be a contributing factor?

I have never seen this before in over 20 years so I'm at a loss really.

 

[M-B-Electrical-Services]

Could there be an ever so slightly loose connection at one of the greg roses that I've missed maybe?? Loose CPC maybe?

 

[M-B-Electrical-Services]

The slight twisting motion of installing the greg rose may play a part here but I am quite certain on my connections.

I'm going back on Thursday so I think I will have to go back to basics and start from scratch by splitting the circuit down bit by bit.

 

[timhoward]

I'm going back on Thursday so I think I will have to go back to basics and start from scratch by splitting the circuit down bit by bit.

Sounds like a plan. Though I'm not (yet) convinced there is actually a problem, induced voltage from 2 way lighting strappers is fairly common, also from bathroom fan 3 core. I don't usually even measure this!

The DNO have recently removed a looped connection to this property, could that be a contributing factor?

Earthing type? It doesn't sound like a DNO issue if you are not seeing it on other circuits.

 

[M-B-Electrical-Services]

Sounds like a plan. Though I'm not (yet) convinced there is actually a problem, induced voltage from 2 way lighting strappers is fairly common, also from bathroom fan 3 core. I don't usually even measure this!

Earthing type? It doesn't sound like a DNO issue if you are not seeing it on other circuits.

I'm relieved really that you don't seem to think of it as something to be concerned about!

I hate little issues like this as it can plant a seed of doubt in your mind regardless of your level of experience!
I delayed messaging on here as I assumed I would be ridiculed for not knowing what the issue was so I'm happy that's not the case here!!

I appreciate your input and will post back with my findings once I have re-tested the wiring.

Mike

 

[davesparks]

What are you measuring the voltage with?

Its most likely capacitive coupling is the reason for the voltage reading, try using a meter with a lower input impedance or connecting a resistance across it and see what you get.

 

[M-B-Electrical-Services]

What are you measuring the voltage with?

Its most likely capacitive coupling is the reason for the voltage reading, try using a meter with a lower input impedance or connecting a resistance across it and see what you get.

Hi Dave.

Can you explain to me what capacitive coupling is? Is it an electromagnetic effect?

I'm using my Megger MFT on its voltage setting. I only saw it by chance whilst double checking the switch wiring was correctly terminated.
I then freaked out a bit as it is a brand new system.

 

[Avo Mk8]

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

 

[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