Confusing odd situation!

[JohnW2]

I get annoyed at these regular statements.

So do I.

Such devices have their uses,

They do. The issues/problems/dangers relate to the people who use them, not the devices. Very many tools have their limits and/or limitations and if ignorant people try to use them without recognising those limit/limitations can create problems, sometimes dangers.

I carry neon and LED screwdrivers in most of my toolkits and in some of the work I do are an absolute godsend to my safety.

Similar here.

Kind Regards, John

 

[bernardgreen]

and if ignorant people try to use them without recognising those limit/limitations can create problems, sometimes dangers.

That is the crux of the matter, in the right hands and an appropriate situation they can be useful.

It is incidents where for example a neon screwdriver lights up when touching an earth bonded pipe and does not light up when touching a wire that is Live where the ignorant person is at high risk.

 

[Harry Bloomfield]

Have a read of this

I am not entirely convinced by that website, it seems to be suggesting that a capacitive earth path is essential, yet a volt stick will detect a live wire or surface, whether it is in your hand or not, whether there is an earthed surface adjacent to it or not.

 

[SUNRAY]

I am not entirely convinced by that website, it seems to be suggesting that a capacitive earth path is essential, yet a volt stick will detect a live wire or surface, whether it is in your hand or not, whether there is an earthed surface adjacent to it or not.

In order to detect a voltage there must be some sort of reference and a PD. If it were opssible to find/create a space where no electrical reference was present/available then how do you think a pen could detect a live conductor?

 

[SUNRAY]

That is the crux of the matter, in the right hands and an appropriate situation they can be useful.

It is incidents where for example a neon screwdriver lights up when touching an earth bonded pipe and does not light up when touching a wire that is Live where the ignorant person is at high risk.

Yes, and I've been there.

 

[Harry Bloomfield]

In order to detect a voltage there must be some sort of reference and a PD. If it were opssible to find/create a space where no electrical reference was present/available then how do you think a pen could detect a live conductor?

How do you think such a volt stick might light up, when there is no earth near it - by the varying at 50Hz field in the area surrounding a live part.

A 50Hz field is detectable for a considerable distance from where there are live conductors. I can walk 30 yards from my home, into my back garden, and with a portable scope, still detect the 50Hz field.

 

[JohnW2]

How do you think such a volt stick might light up, when there is no earth near it - by the varying at 50Hz field in the area surrounding a live part. ... A 50Hz field is detectable for a considerable distance from where there are live conductors. I can walk 30 yards from my home, into my back garden, and with a portable scope, still detect the 50Hz field.

True, but therein lies one of the problems. If the device is (like your scope) sensitive enough to pick up 50 Hz fields 'virtually anywhere', that obviously frustrates attempts to determine whether something specific is, or is not, 'live'. Choice/adjustment of sensitivity is therefore crucial, and will presumably vary considerably between situations.

Kind Regards, John

 

[SUNRAY]

How do you think such a volt stick might light up, when there is no earth near it - by the varying at 50Hz field in the area surrounding a live part.

A 50Hz field is detectable for a considerable distance from where there are live conductors. I can walk 30 yards from my home, into my back garden, and with a portable scope, still detect the 50Hz field.

Yes indeed, you are absolutely correct however the mere fact that you see that means the reference voltage being used by a voltstick in (let's call it) free space is derived from the same place as the wire being tested. The net PD is effectively zero.

EDIT: again I started writing this 75 minutes ago and been on the phone ever since. John said the same thing in a better way.

2nd EDIT: The other thing is the scope may be picking up the electromagnetic radiation which is different to a voltage field.

 

[Harry Bloomfield]

Choice/adjustment of sensitivity is therefore crucial, and will presumably vary considerably between situations.

Exactly, and you can reduce the sensitivity/ direct and localise the sensitivity, by shielding the tip with your fingers.

 

[Harry Bloomfield]

electromagnetic radiation

Exactly, that is what the volt-stick is sensitive to - the electromagnetic radiation field, which does not require an earth, rather than the voltage gradient.

 

[SUNRAY]

At the end of the day all of these sticks and screwdrivers mentioned are voltage detectors and the only way bpossible to do that is by detecting a current flow.

Imagine using a 2 prong volt meter, only connecting one prong to the live cable and leaving the other free. I think all of us will say it will show zero volts and after a moment might add; 'or nearly zero'.

The oldies among us may remember analogue meters of say 500Ω/V and a 250V range will require ~1.9mA to measure mains voltage, under that condition the meter is unlikely to deflect off the stop. even if set to the 1V range.

However some modern electronic meters may be 100MΩ impedance (I imagine more now) and only require ~2μA (apologies if my mental arithmetic have let me down) to read mains voltage and is far more likely to give a reading of a volt or so with just one probe connected.
I don't think I've ever done this before but a cheap Lidl autoranging multimeter of unknown input impedance set to AC V on a wooden table as far away as the test lead will allow from anything reads 2V. Just holding the meter in my hand I can make that reach 8v and adding the second lead and touching it is 100V however sitting a wooden chair and lifting my feet off the floor (concrete floor, parquet blocks, underlay, carpet and slippers) to the height of the seat the 8 & 100V drop to 3 & 40V. Touching probe on the painted metal radiator 160V.

All of those readings are dependant on current flow.
All voltages approximate and varying.

I WOULD NOT REPEAT THE TOUCH TESTS WITH AN OLD ANALOGUE METER OR A VERY CHEAP DIGITAL METER DUE TO THE ELECTRIC SHOCK RISK AND STRONGLY ADVISE OTHERS NOT TO

Moving on slightly there is also electrostatic voltage which is detectable using the principle of equal charges repel which does not rely on a current flow.

 

[JohnW2]

Exactly, and you can reduce the sensitivity/ direct and localise the sensitivity, by shielding the tip with your fingers.

Yes, but it's all a bit hit and miss, so it's difficult to be totally confident.

Kind Regards, John

 

[JohnW2]

At the end of the day all of these sticks and screwdrivers mentioned are voltage detectors and the only way bpossible to do that is by detecting a current flow.

That's true of the neon screwdrivers/similar.

With the more sophisticated 'filed sensing' devices it is only true of currents within the device - there is no current flow to anywhere 'outside the device in the manner that there is, via capacitance, with a neon screwdriver.

As a generalisation, it is not inevitable that current has to flow in order to detect voltage. A DVM or suchlike could theoretically have a 'near infinite' input impedance - so any current can be near-zero. As for 'actual zero current', you probably remember the "gold leaf electroscopes" we used at school etc.

Kind Regards, John

 

[Harry Bloomfield]

That's true of the neon screwdrivers/similar.

With the more sophisticated 'filed sensing' devices it is only true of currents within the device - there is no current flow to anywhere 'outside the device in the manner that there is, via capacitance, with a neon screwdriver.

As a generalisation, it is not inevitable that current has to flow in order to detect voltage. A DVM or suchlike could theoretically have a 'near infinite' input impedance - so any current can be near-zero. As for 'actual zero current', you probably remember the "gold leaf electroscopes" we used at school etc.

That is exactly right!

 

[SUNRAY]

That's true of the neon screwdrivers/similar.

With the more sophisticated 'filed sensing' devices it is only true of currents within the device - there is no current flow to anywhere 'outside the device in the manner that there is, via capacitance, with a neon screwdriver.

As a generalisation, it is not inevitable that current has to flow in order to detect voltage. A DVM or suchlike could theoretically have a 'near infinite' input impedance - so any current can be near-zero. As for 'actual zero current', you probably remember the "gold leaf electroscopes" we used at school etc.

Kind Regards, John

I've not used a voltstick myself but seen a few used and read the instructions for one (when I considered getting one) and that gave instructions to hold just the end to 'provide the earth' that term was used several times. seeing different people using the same voltstick I've seen differing responses, ie one found a cable to be live and the other wasn't so sure. That was not too surprising as people have different conductivity, wear different clothing and footwear etc, additionally I've not seen one remain lit when not being held (definitely not disputing due to minimal experience) therefore my very limited sight of such devices being used seems to depend very much on the reference being supplied by the operator. If that is incorrect then so be it and I'll welcome information on this.

Near infinite is an impossible state to reach so every DVM is current dependant/affected.

At school our electroscope was a man made fibre but yes it is where I was going in my previous but for the life of me the term would not come into my head. However the electroscope requires resetting regularly IIRC, although if it is using such a method to detect I'd imagine some automation is built in.

I've just done a quick google and 3 different hits describe voltsticks as capacitively coupled to the item under test and also to earth. That to me sounds like current detection rather than electrostatic.