Odd voltages

[gazdkw82]

Sorted a shorting fault at a pendant earlier this evening.

Whilst i was there they asked if I could look at the wall lights that have never worked since they moved in 2 years ago.

2 wall lights. Getting 244v line to earth, 0v neutral to cpc but 74v line to neutral.

The 74v line to neutral is my guess as to why any lamp will not work in the fittings.

I could really do too much investigating because it was late but it got me thinking.

What's usually the cause of this type a reading?

A Lost/loose/floating neutral?

 

[Mini-ohm]

It was across SL & N they were a low energy lamp if i remember rightly! in the evening time they still lit a very dim glow but that makes sense now! It was one of them i just closed the circuit up and tried to forget about it haha
Forums are worthwhile
Thanks for the reply

 

[Mini-ohm]

It was across SL & N they were a low energy lamp if i remember rightly! in the evening time they still lit a very dim glow but that makes sense now! It was one of them i just closed the circuit up and tried to forget about it haha
Forums are worthwhile
Thanks for the reply

I dont think mine was induced voltage, im pretty sure of that as although a good sized builiding cables werent particuly grouped together, my understaying of induced voltages is mainly from singles in trunking (i may be wrong on this) this was T&E installation

 

[Mini-ohm]

Attached wrong piece on that meant to attach gazdkw82 piece regarding the induced voltage

 

[gazdkw82]

I dont think mine was induced voltage, im pretty sure of that as although a good sized builiding cables werent particuly grouped together, my understaying of induced voltages is mainly from singles in trunking (i may be wrong on this) this was T&E installation

My situation was almost identical. It was on 2 way lighting for hallway and landing. There was an intermediate switch also.

Wired in t+e. Still was getting 50 odd volts when the circuit was turned off. It was making the led 2d lamps flash. Snubbee fixed it

 

[Lucien Nunes]

a snubber which basically eats up the voltage.

It would be more appropriate to say it eats up the current. The source of the voltage is the mains in series with a high impedance (often the inter-core capacitance of a length of cable) which approximates to a current source. Like earth leakage, if you pull it right down to earth / neutral potential via a lower impedance, whether that is a snubber, an actual lighting load or even a direct connection to earth, a certain current will still flow, equal to V/Xc of the cable capacitance.

We always talk of the earth leakage current because the voltage (from the CPC to the MET) is negligible and the sources approximate to current sources. But in these 'ghost voltage' situations people often say ''I'm reading V volts on my meter' because they were testing for voltage at the time, but it would be better to divide that V by the input resistance of the meter and say 'I'm reading I microamps on my meter'.

 

[gazdkw82]

It would be more appropriate to say it eats up the current. The source of the voltage is the mains in series with a high impedance (often the inter-core capacitance of a length of cable) which approximates to a current source. Like earth leakage, if you pull it right down to earth / neutral potential via a lower impedance, whether that is a snubber, an actual lighting load or even a direct connection to earth, a certain current will still flow, equal to V/Xc of the cable capacitance.

We always talk of the earth leakage current because the voltage (from the CPC to the MET) is negligible and the sources approximate to current sources. But in these 'ghost voltage' situations people often say ''I'm reading V volts on my meter' because they were testing for voltage at the time, but it would be better to divide that V by the input resistance of the meter and say 'I'm reading I microamps on my meter'.

Seriously, where do you gain this level of knowledge?

 

[Mini-ohm]

Seriously, where do you gain this level of knowledge?

I did think exactly the same, my brains in bits

 

[gazdkw82]

I did think exactly the same, my brains in bits

It's not common electrician knowledge. This sort of thing is another level.

Electronics background? Electrical engineering background?

 

[Lucien Nunes]

Well yes I'm an electrical & electronic engineer, but most of the science of what is going on here is probably already within your field of knowledge. You sometimes have to take a step back and apply theory that you know in new ways and new situations. For example, you'Il be quite familiar with Ohm's law but you might not have tried applying it to the input of your multimeter. You probably know the formula for the reactance of a capacitor, but you might not have tried using it to calculate the leakage current of a cable from its stray capacitance.

If you like I'll walk through what I saw in my mind's eye when you posted the original question.

 

[gazdkw82]

If you like I'll walk through what I saw in my mind's eye when you posted the original question.

Yes that would be great