Dodgy voltage detector

[tabbypurr]

I see the idiot's back

[tabbypurr]

You must be using yours differently to what I do

NT

[tabbypurr]

if you take a voltmeter and place the red wire on a live, and the black wir e on ground, or your finger, you can get a voltage reading even if no volta ge is present, due to induction. These things can't tell the difference be tween a bit of induced voltage and 240V with current carrying capabilities.

depends on the meter. That's not normally a problem

NT

[tabbypurr]

to detect voltage?

you could always read up on how they work and why they give false negatives and false positives.

NT

[tabbypurr]

You can and it's easy enough, but I no of no product that does it.

NT

[James Wilkinson Sword]

My (decent) multimeter will read about 80 volts if I touch one wire to live and the other to my finger, or even just leave the other wire in mid air. Presumably this is why ones sensing through walls have a problem. How could they possibly tell the difference between a far away live wire and induction in a nearby wire?

[John Rumm]

Not exactly "non contract" though is it!

[John Rumm]

Are you saying the indication you got from the live and dead circuits were the same, or was the "dead" reading "different" but not quite what you expected?

[Cursitor Doom]

I have a Megger that can do this. It beeps when in close proximity to a high voltage. Not sure about the possibility of false positives, though (inductive coupling) I must check it out one day when I can be arsed.

[tabbypurr]

No. Is that an issue?

NT

[leenowell]

Hi John

That's a good point. When the power was on it had a more or less continuous beep. When the power was off the beep was more of a series of individual beeps. The thing that spooked me more was the self test seemingly not working.

Thanks

Lee.

[Brian Gaff]

Oh where to start on this one. Most of these only have one connection, a bit like the old fashioned Neon Screwdriver. I'd really like one that makes a sound myself of course. However they seem to have no way to really detect the voltage that is there that can harm you as against an induced voltage that can't. They normally use you as if you were the other connection so really unless you can sink some current on the line you are measuring if anywhere along the run it picks up induced voltage from a live wire you get a false positive. In my mind all wires really should have small bleed resistors on them to earth but I guess over a whole city this would mean a significant power wastage. Brian

[Brian Gaff]

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

[Andy Burns]

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.

[John Rumm]

Yes! The whole purpose of the volt stick style detector is that it can make an assessment without needing any direct contact to the circuit, and so avoids all the risks associated with that.

Also keep in mind that a false positive reading is equally possible from a high impedance volt meter.

[John Rumm]

If you use them according to the instructions, you should not get false negatives.

[mechanic]

Great contribution.

[Robin]

And what's that got to do with the *principle* of capacitive voltage sensing?

Do you understand that any volt meter is capable of giving a false negative? With a good quality one used properly they will be very rare indeed but not impossible.

[Mike Clarke]

I don't know if they all use the same principle but some non-contact voltsticks work by detecting a potential gradient in the space near the conductor so don't rely on any leakage current through you to ground.

If a cable runs alongside other live cables then there could well be enough induced voltage in the 'dead' cable to show a false positive. This type of false indication can sometimes be cancelled by placing your hand on the outer surface of the cable insulation.

[Andy Burns]

I don't remember mine (Kewtech) warning about not detecting live SWA cables, presume the Flexishield type cable will always give a negative too?