Dodgy voltage detector

[Bob Eager]

I have two!

[Cursitor Doom]

I believe the surest way is just to stick your finger in there. That method never lets me down. But I would not suggest other people do that; I have a higher skin resistance than most folk. 240V for me starts as a slight tingle and builds up slowly. Takes about 5 seconds before it becomes intolerable, presumably due to skin pores releasing moisture that is then trapped and so increases conductivity.

[John Rumm]

Its also interesting to note that many of the makers instruction sets allude to the possibility of non detection due to a lack of reliable ground reference (which they suggest is provided by the user). However experiments demonstrate that the technology is somewhat more sophisticated than that anyway since they function reliably even when nowhere near an operator (or any other significant source of free electrons (i.e. earth)).

If you look at the sequence of photos of the Fluke LVD2 in operation:

It is able to detect the presence of a live wire even while sat on an insulating bench and not obviously coupled to any ground reference.

I tried a similar experiment with my Fluke 1AC-II VoltAlert and got the same result

[bin]

That reminds me of a teacher I had in about 1964. I took an old wind-up generator (from a telephone) to school and had horseshoes of kids holding hands with the ones on the ends holding wires - much hilarity when I wound the handle. When I tried it on the teachers he showed no effect so explained about skin resistance and demonstrated his ability to a class of 10-11 year old boys by putting wires into a mains socket and touching them both. There was no "don't try this at home" warning. A stupid thing to do, but we were suitably impressed.

[Robin]

similarly - but is *much* less sensitive than the one in the Wiki photos (or I'd wager your Fluke).

I suppose one explanation is that you have discovered homeopathic coupling, where he volt stick remembers the capacitance from when it was last held. Could be an opportunity to ensure your name liveth for evermore in references such as "that's a Rumm effect".

[tabbypurr]

of course they are. Now hold them with some sort of hand coupled to a live conductor and they'll give inverted readings. The physics/electronics behind it is quite basic.

NT

[ARW]

I find that they are pretty useless on some flexes.

Still, they have their uses.

[ARW]

Take a 230V fluorescent task light in one hand (must check to see if its double insulated), a Fluke volts stick in the other and the Fluke says everything is live when it gets near to an earth!

[John Rumm]

Again never seen any evidence of that.

Could you suggest a test scenario that you think will fool it?

Which by rights would suggest you could make some accurate predictions based on the theory. So far it seems my Fluke has you outwitted at every turn! ;-)

[John Rumm]

The nearest equivalent I have been able to reproduce is to get one to light on proximity to an earth or neutral when the operator is coupled to live. However mine still reliably detects live as well in that circumstance. If I can couple the butt end of the stick well enough to live (i.e. wrapping an individual live wire round its handle), then it just lights up and says "live" even if sat isolation on an insulating surface. So far I have only been able to get a false positive, but not a false negative...

[Robin]

Adam mentioned his (I think) Fluke doesn't cope well with all flex; and my cheap LAP is definitely capable of giving a flase negative on some stuff unless I place it at the right angle.

I think false negatives were easier before they built in a "low battery" signal :(

[John Rumm]

My Fluke does a double flash of its light every two secs to show its "working" - so should be harder to get caught by either it being off or having a flat battery.

Here are the results of some experiments I did earlier tonight:

[tabbypurr]

I don't know why you're being silly. The cause of the problem is simple, well known, and I described it upthread. Nothing you've done has even begun to contradict what I said.

NT

[tabbypurr]

the circuit is completed via the stray capacitance of the human.

I would first ask what the values of C were for live to human and human to ground.

NT

[Cursitor Doom]

Yup. And 10 years later the teacher *would* say, "don't try this at home." Fast forward to 2017 and just recounting the story with the caveat would probably end that teacher's career. We live in excessively safety- conscious times. A 'good' war would soon sort that out for a few generations.

[John Rumm]

You can eliminate the human since the detector works in isolation - even if the human walks to the other side of the room.

[John Rumm]

Must have missed it then. What did you say upthread?

[Roger Hayter]

I share your distaste for the excessive safety consciousness, and the restriction of children's 'right to roam'. But the fact is that illness and even accident kill very few children now, and a risk that would be lost in the noise 80 years ago is a doubling of mortality now. Thus safety culture saves much heartache and misery.

We may not like it, but it is probably a worthwhile thing.

[Terry Casey]

Not long after I started work - so the best part of 60 years ago - I recall getting a belt off a loudspeaker cable. It was in an old working man's club (it was demolished soon after and replaced by a modern building) so I've no idea how old it was.

The walls and extremely high ceilings were all clad in wood and, as far as we could ascertain, the wiring ran up the wall behind the cladding and across the ceiling of a large hall then across the ceiling of an adjacent large room and down the wall to where the remote speaker was sited.

We actually measured close to the full mains voltage on this cable with an AVO 8! I can only assume that the mains wiring consisted of physically separate conductors and that the speaker wiring was run alongside for its considerable length.

I'm not sure now how we proved that this was purely induced and not dangerous and fortunately, the amplifier to which it was connected was earthed - not always the case with such ancient wiring, of course - otherwise using the microphone could have been quite 'entertaining'!

[Mr Pounder Esquire]

Light switch fell apart on my office, bloody sparks using cheap crap. It was only 9 years old. House had been rewired. Yeah, breaker in the meter cupboard labeled "Lights". I know what tricks sparks get up to and did not trust the label. Tested volt stick on the exposed wires to the wall switch and beep, turned breaker off and no beep. Without that volt stick I would have turned all the power off.