# Dodgy voltage detector

On Wednesday, 6 December 2017 01:16:24 UTC, John Rumm wrote:

ry

, well known, and I described it upthread. Nothing you've done has even beg un to contradict what I said.

Capacitive 'non contact' volt detectors measure the voltage between their d etecting tip and the field around them. When a human holds the thing, the h uman's stray capacitance keeps them not far from 0v, usually. In which case the detector works fine.
But if the human is off the floor and say leaning on a mains wire, or a wal l with a wire under the surface, human can be at higher voltage. If human v oltage goes high enough, they are near live voltage, albeit at minimal curr ent via stray capacitance. In this situation the capacitive meter will give inverted readings, live gets reported as ground and ground as live.
Wrapping yourself with a live wire raises your voltage, but to what extent we don't know, as we don't know human to wire capacitance or human to surro undings capacitance.
NT
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On Wednesday, 6 December 2017 18:29:11 UTC, tabby wrote:

ns

very

le, well known, and I described it upthread. Nothing you've done has even b egun to contradict what I said.

detecting tip and the field around them. When a human holds the thing, the human's stray capacitance keeps them not far from 0v, usually. In which ca se the detector works fine.

all with a wire under the surface, human can be at higher voltage. If human voltage goes high enough, they are near live voltage, albeit at minimal cu rrent via stray capacitance. In this situation the capacitive meter will gi ve inverted readings, live gets reported as ground and ground as live.

t we don't know, as we don't know human to wire capacitance or human to sur roundings capacitance.

cates live all the time)
Do that then unwrap it bit by bit until it just stops indicating live. Then when you approach it with a neutral wire it should indicate live. Approach it with a live wire and see what it thinks too.
NT
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On 07/12/2017 03:28, snipped-for-privacy@gmail.com wrote:

ok just tried that...
If I lay a live wire on the bench, then place the handle end on top of it (half a turn if you like), then it indicates live when in free space.
Now if I place a hand close enough to the handle end, it will then stop indicating live. You can adjust the hand spacing so that you are right on the threshold of detection. If at that point I then offer it a different live wire to its probe end, then it still indicates live when in proximity. It does seem remarkably hard to fool.
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Cheers,

John.
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On 07/12/2017 16:19, John Rumm wrote:

Your experiments remind me of - and replicate - the behaviour of a gold leaf electroscope[1] and the way a pre-charged leaf would fall if an opposite charged rod approached but then rise again as it moved even closer.
But I'd caution the OP not to expect such sensitivity with a cheap LAP. Mine performs best when held. And when held gives a pretty good indication of the presence or absence of the potential (sic) touch voltage. And that's what I want in most circs. Eg if I am about to open metal conduit I don't actually care if it is live relative to earth; I want to know if it is live relative to _me_.
[1] probably too old-fashioned a device for youngsters like you and Tim with your "modern maths" :)
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Robin
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On 07/12/2017 18:47, Robin wrote:

ISTR our physics lab at school was the proud owner of one... Did some experiments with it including demonstration of the photo electric effect ;-)
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Cheers,

John.
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On Thu, 7 Dec 2017 18:47:12 +0000

...

We did "new maths" and old science. Lots of mahogany and brass in the physics labs. :-)
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On 08/12/2017 21:27, Rob Morley wrote:

I assume there were Leclanché and Daniel cells on a high shelf.
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Max Demian

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On Thursday, 7 December 2017 16:19:17 UTC, John Rumm wrote:

You're not going to get the thing at live potential by sitting one end on a wire. It always has capacitances to anything & everything around it, the live wire is closer but also far less area than everything else.
I don't know where mine is so can't experiment, but I know exactly how it works.
NT
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On 09/12/2017 02:19, snipped-for-privacy@gmail.com wrote:

It gets it "live enough" to cause it to indicate live.

The upshot with mine[1] is that if you get the handle end into a field strength near enough to that of a live wire, then it indicates live. If the handle field strength is less than "live", then when the tip encounters live it will also indicate live.
I think that represents the most favourable behaviour since if used correctly[2] it won't tell you something is safe when its not, and will always tell you something is dangerous when it is. The only area for error seem to be the possibility of either not being able to get a reading at all (because it permanently indicates live), or getting false positive from a neutral / earth (i.e. telling you something is dangerous when its not)
(One thing I have not tested, is what happens when you want to detect a live at the tip that is at a lower potential than that of the handle (mine covers 90V - 1000V, so you could for example want to test 110V site wiring while coupled to a 240V field). My guess would be based on the artefact observed from the experiments of it seeing N & E as live when its in an elevated field, is it would probably indicate live in this case as well)
[1] Keep in mind these observations are all based on one particular model of one makers range - others may be different.
[2] i.e. Test on known live before and after your actual test - preferably in the same location.
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Cheers,

John.
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Robin expressed precisely :

Volt sticks can vary in quality, Fluke sells one of the better ones. If there are multiple live wires around it is possible your test wire might show as being live - the simple solution is to shield the stick from all but the wire you are interested in with your fingers. The validity then needs to be crossed checked with something which actually draws some current from the source. A test lamp works well for this.
I have never known a Fluke show a circuit as being dead, when it is live, but the stick should always be tested before use. One test is to rub the tip of the stick up and down your arm, whereupon it should light up. They are absolutely great for non-invasive tests, but only if you are practised in using one.
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On 04/12/2017 11:25, Harry Bloomfield wrote:

+1
And preferable to making physical connection to a circuit under test.
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Cheers,

John.
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On 04/12/2017 15:12, John Rumm wrote:

Yes - and I had in mind in particular the way a volt stick then makes it easy to check you've got the right one by turning on and off the circuit you want.

I don't know who makes the ones I've seen used to check 11kV overhead lines are dead but think they are probably considered to be pretty reliable too ;)
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Robin
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On Monday, 4 December 2017 15:40:18 UTC, Robin wrote:

n
f
ht

ll

ht

If you understand the limitations and follow correct procedure they are. Th at's very easy when trained & probing 11kV lines. It can sometimes fail to happen when Joe public uses them to check household wiring.
NT
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Well, in my view, any voltage detector needs two connections Maybe a probe on a piece of wire as well as the pen. Simply design a circuit which has a small load in it, say a resistor and connect one end, say the probe to neutral the other, say the pen to the possible live. You could work in one of two ways, detect the voltage under load and or the current flow though the load. An old Electrician had a wonderful thing made up with a little pigmy low wattage bulb in a translucent box and a couple of probes. If the light lit it was um, live. Only way to be sure he told me. Brian
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Brian Gaff wrote:

The whole point of a non-contact mains detector is that it has zero connections.
Like any tool they're fallible, they can give false negatives (e.g. for live SWA cable) and false positives (e.g. for a USB cable) but they're better than using your tongue.
Some do have audible output.
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On Mon, 04 Dec 2017 10:13:13 +0000, Andy Burns wrote:

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.
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On 04/12/2017 23:19, Cursitor Doom wrote:

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.
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On Tue, 05 Dec 2017 13:49:00 +0000, bin wrote:

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.
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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.
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Roger Hayter

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Odd that it didn’t with the last world war.
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