Odd request regarding phantom voltage!

[HappyHippyDad]

Afternoon all...

I would like to experience phantom voltage!! Is there any way I can reproduce it so that I can test it?

I know this might sound a little silly, and I realise my new Drummond test lamp would simply not light up if it is phantom voltage, but I would just like to see it for myself on something that I know for certain is phantom voltage.

I also want to test the digital and analog volt meter on it so I can see what they record.

Again... I know this sounds daft but I just want to see it for myself.

Cheers guys.

 

[HappyHippyDad]

Not sure, haven't tried it, but if you took a nice length of three core and earth and wired it up to a load leaving the one middle core unconnected then you should get a "phantom voltage" on the unconnected core.

Well I went with your suggestion Richard, I used approx 30m's of 3 core and earth and popped a plug on the end. Live brown, neutral grey and black was the test conductor (left unconnected) to see if any induced voltage showed up.

Here are the results : (all results are voltage from black - N)

1. Fluke Multimeter (Thanks again Paul :smile5 shows 91.7V.



2. Fluke T5 shows 73V



3. Metrel shows 68V



4. Analog shows 37V



5. Drummond shows <50V (no light)



It was all quite interesting! Quite a range of results from all the testers. It was good to witness first hand phantom voltage and to see the Drummond in action.

I was surprised to see any voltage on the analog meter though, it showed between 35-50V!

 

[DPG]

Well I went with your suggestion Richard, I used approx 30m's of 3 core and earth and popped a plug on the end. Live brown, neutral grey and black was the test conductor (left unconnected) to see if any induced voltage showed up.

Here are the results : (all results are voltage from black - N)

1. Fluke Multimeter (Thanks again Paul :smile5 shows 91.7V.

View attachment 29152

2. Fluke T5 shows 73V

View attachment 29153

3. Metrel shows 68V

View attachment 29154

4. Analog shows 37V

View attachment 29155

5. Drummond shows <50V (no light)

View attachment 29156

It was all quite interesting! Quite a range of results from all the testers. It was good to witness first hand phantom voltage and to see the Drummond in action.

I was surprised to see any voltage on the analog meter though, it showed between 35-50V!

The input impedance of analogue multimeters can vary considerably. Older ones (AVO 8, etc.) are relatively low impedance and less susceptible to picking up stray voltage, but modern types (used to be called FET input types when this was a novel thing!) can have very high input impedances and be nearly as bad as digital types for stray voltages. Daz

 

[SPARTYKUS]

FET - field effect transistor...?

 

[DPG]

Yes indeed

 

[Lucien Nunes]

The next exercise is to account for those readings and compute the inter-core capacitances per metre of cable. If you know the input impedance of each meter, you can use the change in measured voltage from one to another to determine the effective source impedance of the phantom voltage, and what the open-circuit voltage on the black core would be in the absence of any meter. Don't forget that the impedance driving the test core of the cable is mostly capacitive and the meter input mostly resistive so their voltages are around 90° out of phase, hence the squares of their voltages add, not the voltages themselves.

Then look at the construction of the cable and estimate how that is made up from the various mutual capacitances and resistances between each core and another, and from that characterise the cable. You could then take use that data to predict the stray voltage on a different core with a different layout of energised cores, and compare with the actual result.

You can have hours of fascinating fun with half a roll of T+E!

 

[DPG]

The next exercise is to account for those readings and compute the inter-core capacitances per metre of cable. If you know the input impedance of each meter, you can use the change in measured voltage from one to another to determine the effective source impedance of the phantom voltage, and what the open-circuit voltage on the black core would be in the absence of any meter. Don't forget that the impedance driving the test core of the cable is mostly capacitive and the meter input mostly resistive so their voltages are around 90° out of phase, hence the squares of their voltages add, not the voltages themselves.

Then look at the construction of the cable and estimate how that is made up from the various mutual capacitances and resistances between each core and another, and from that characterise the cable. You could then take use that data to predict the stray voltage on a different core with a different layout of energised cores, and compare with the actual result.

You can have hours of fascinating fun with half a roll of T+E!

Blimey Lucien, is the telly broke? Daz

 

[Lucien Nunes]

I haven't got a telly, at least not one that gets all the modern programmes! And all I can get on the PC screen this evening is invoices and quotes...

 

[PEG]

...It is one of our wonderful electrical devices,i could easily do without... and the £145 a year i would save could go towards my next meter :yes:

 

[HappyHippyDad]

The next exercise is to account for those readings and compute the inter-core capacitances per metre of cable. If you know the input impedance of each meter, you can use the change in measured voltage from one to another to determine the effective source impedance of the phantom voltage, and what the open-circuit voltage on the black core would be in the absence of any meter. Don't forget that the impedance driving the test core of the cable is mostly capacitive and the meter input mostly resistive so their voltages are around 90° out of phase, hence the squares of their voltages add, not the voltages themselves.

Then look at the construction of the cable and estimate how that is made up from the various mutual capacitances and resistances between each core and another, and from that characterise the cable. You could then take use that data to predict the stray voltage on a different core with a different layout of energised cores, and compare with the actual result.

You can have hours of fascinating fun with half a roll of T+E!

I was just about to do all of that Lucien

 

[Lucien Nunes]

It was a completely serious suggestion. Only a basic understanding of AC circuit theory is needed to solve most of the mysteries that show up on the forum including ones about phantom voltage, all the calcs can be done accurately enough on a smoke packet. If more sparks turned their minds to solving problems in odd moments and applying equations they learned in college to real life situations, we'd have less dopey questions about why 100k IR doesn't trip an RCD. I'm not tarring you with the same brush BTW. You've gone the extra mile to investigate which most wouldn't.

 

[shanky887614]

It was a completely serious suggestion. Only a basic understanding of AC circuit theory is needed to solve most of the mysteries that show up on the forum including ones about phantom voltage, all the calcs can be done accurately enough on a smoke packet. If more sparks turned their minds to solving problems in odd moments and applying equations they learned in college to real life situations, we'd have less dopey questions about why 100k IR doesn't trip an RCD. I'm not tarring you with the same brush BTW. You've gone the extra mile to investigate which most wouldn't.

ill be honest but people dont really cover resistors much at college anymore.

i would have liked to have done electronics at college but couldn't

 

[markc123]

FET - field effect transistor...?

no no no

Fried Electrical Trainee