ARW wrote:
On 03/12/2017 18:42, Mr Pounder Esquire wrote:
wrote:
On Sunday, 3 December 2017 17:41:40 UTC, Robin wrote:
On 03/12/2017 17:28, tabbypurr wrote:

They use an unsound principle and cannot be relied on. Don't buy

I'd be interested to learn more about that.

they produce false positives and false negatives, relying on one
is a hazard


I'm well aware of the possibility of a false negative with them (as
with any device I've ever used). But I don't see why that makes
the *principle* of capacitive voltage sensing unsound.

it makes the way that principle is always applied unsound.

As for false positives, I take it you mean the way they will detect
voltages which are totally safe for human contact. On the whole I
prefer to see that not so much a false positive as an indication of
the need to check further.

it isn't, it's just false positives. Come back when you understand
them.

Years ago I was given one of those gadgets. It was dead posh and
detected voltage in the walls where I knew there were wires.
Then it tried to tell me that there was voltage in the lounge door!
I recently bought a volt stick to detect voltage should I have a
broken wire in the vac cleaner etc. That is about they are good for.



I find that they are pretty useless on some flexes.

Still, they have their uses.

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.

On Wednesday, 6 December 2017 01:16:24 UTC, John Rumm wrote:
On 05/12/2017 23:45, tabbypurr wrote:
On Tuesday, 5 December 2017 18:44:08 UTC, John Rumm wrote:
On 05/12/2017 16:31, tabbypurr wrote:


The physics/electronics behind it is quite basic.

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! ;-)

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.

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

Capacitive 'non contact' volt detectors measure the voltage between their 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 case the detector works fine.

But if the human is off the floor and say leaning on a mains wire, or a wall 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 current 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 surroundings capacitance.


NT

Terry Casey wrote:

I recall getting a belt off a loudspeaker cable. It
was in an old working man's club

Quite likely to be so called "100 volt line" speaker wiring ...

each speaker has a transformer to tap-off its signal.

On 06/12/2017 11:11, Terry Casey wrote:
In article ,
says...

Correct -if a wire with no connections at either end, is adjacent to
another live cable, the first can pickup from the second, causing the
stick to light up. If there is some sort of load remaining on the first
cable, then that will not happen because it is effectively grounded via
the load.

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.

Sometimes "extension loudspeaker" sockets in valve radios had a high
impedance output - i.e. connected to the primary of the output
transformer, so one end would be at HT potential to earth - 250V
typically. Moving coil extension speakers would have a transformer,
perhaps offering high or low impedance with different sockets. I assume
this was for the benefit of horn speakers, which may have been high
impedance.

--
Max Demian

On Wednesday, 6 December 2017 18:29:11 UTC, tabby wrote:
On Wednesday, 6 December 2017 01:16:24 UTC, John Rumm wrote:
On 05/12/2017 23:45, tabbypurr wrote:
On Tuesday, 5 December 2017 18:44:08 UTC, John Rumm wrote:
On 05/12/2017 16:31, tabbypurr wrote:


The physics/electronics behind it is quite basic.

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! ;-)

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.

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

Capacitive 'non contact' volt detectors measure the voltage between their 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 case the detector works fine.

But if the human is off the floor and say leaning on a mains wire, or a wall 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 current 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 surroundings capacitance.


NT

(If I wrap the live wire round the handle of the stick, then it just indicates 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

On 05/12/2017 23:28, John Rumm wrote:

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

http://wiki.diyfaq.org.uk/index.php/...ck_experiments



Typos

"resister"

'operators' should be 'operator'.

'matt' should be 'mat'

"Now one suggestion was that it might be possible to invert the test
result, so its possible..." should be 'it's'.

Interesting stuff!

Bill

In article ,
says...

Terry Casey wrote:

I recall getting a belt off a loudspeaker cable. It
was in an old working man's club

Quite likely to be so called "100 volt line" speaker wiring ...

each speaker has a transformer to tap-off its signal.


It wouldn't matter as, in this case, the speaker wiring was
disconnected from the amplifier so completely floating.

We were probably disconnecting the amplifier for repair or
possiblyy reconnecting it after repair but it is now a very,
very long time ago ...

--

Terry

---
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http://www.avg.com

On 07/12/2017 05:20, Bill Wright wrote:
On 05/12/2017 23:28, John Rumm wrote:

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

http://wiki.diyfaq.org.uk/index.php/...ck_experiments



Typos

Yup sorry, was lashed together in a hurry!

ok fixed em...

Interesting stuff!

Bill



--
Cheers,

John.

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On 07/12/2017 03:28, wrote:

(If I wrap the live wire round the handle of the stick, then it
just indicates 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.

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.


--
Cheers,

John.

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On 07/12/2017 16:19, John Rumm wrote:
On 07/12/2017 03:28, wrote:

(If I wrap the live wire round the handle of the stick, then it
just indicates 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.

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.

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"



--
Robin
reply-to address is (intended to be) valid

On 07/12/2017 18:47, Robin wrote:
On 07/12/2017 16:19, John Rumm wrote:
On 07/12/2017 03:28, wrote:

(If I wrap the live wire round the handle of the stick, then it
just indicates 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.

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.

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"

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


--
Cheers,

John.

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On Thu, 7 Dec 2017 18:47:12 +0000
Robin wrote:

a gold leaf electroscope[1]
....
[1] probably too old-fashioned a device for youngsters like you and
Tim with your "modern maths"

We did "new maths" and old science. Lots of mahogany and brass in
the physics labs. :-)

On 08/12/2017 21:27, Rob Morley wrote:
On Thu, 7 Dec 2017 18:47:12 +0000
Robin wrote:

a gold leaf electroscope[1]
...
[1] probably too old-fashioned a device for youngsters like you and
Tim with your "modern maths"

We did "new maths" and old science. Lots of mahogany and brass in
the physics labs. :-)

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

--
Max Demian

On Thursday, 7 December 2017 16:19:17 UTC, John Rumm wrote:
On 07/12/2017 03:28, tabbypurr wrote:

(If I wrap the live wire round the handle of the stick, then it
just indicates 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.

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.

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

On 09/12/2017 02:19, wrote:
On Thursday, 7 December 2017 16:19:17 UTC, John Rumm wrote:
On 07/12/2017 03:28, tabbypurr wrote:

(If I wrap the live wire round the handle of the stick, then
it just indicates 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.

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.

You're not going to get the thing at live potential by sitting one
end on a wire.

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

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.

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.

--
Cheers,

John.

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