Dodgy voltage detector

[Harry Bloomfield]

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.

[Andy Burns]

Though I see the instructions for a newer model do

[John Rumm]

I have never tried that brand, and so can't really compare against my experience. I have a similar looking thing by Fluke that I find to be reliable.

Generally if you follow the instructions - i.e. test against known live and known dead before trusting a reading they should be ok.

[tabbypurr]

it creates a bigger risk by misinforming users on occasion that a live cable is dead.

not from a standard multimeter. Especially high impedance ones are a different fish of kettle.

NT

[tabbypurr]

The reality is it does happen. I've explained why.

[tabbypurr]

wrong again. As I said...

[Robin]

Does your faith require you to deny the existence of intermittent faults?

[tabbypurr]

No, but it does require me to plonk people that lack expertise yet keep arguing.

NT

[Robin]

Then why not tell me a volt meter which is incapable of giving a false negative and prove me wrong?

[tabbypurr]

try sci.electronics.basics

[John Rumm]

You have that back to front. High impedance meters are the norm. Any common digital multimeter will likely have an input impedance in the megohms range, and that is high enough to see capacitively coupled floating conductors as live.

This is why test gear makers specifically make kit for mains test applications that have low or adaptive impedance measurement options to eliminate these spurious readings.

Goo look at the specs for something like a Fluke 114, or 115

[John Rumm]

If you are using it right, it won't happen. i.e. If you have proven it lights on a known good circuit, and does not light on a known dead one, and nothing else has changed, then it will light for the live test circuit.

Actually you haven't that I can see, although you have made the assertion several times.

[John Rumm]

+1

And preferable to making physical connection to a circuit under test.

[Robin]

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 ;)

[Harry Bloomfield]

newshound brought next idea :

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.

[Harry Bloomfield]

Brian Gaff submitted this idea :

The neon drivers needed two, the tip to the cable, the metal top to you. They depended on you having reasonable path the ground, which wasn't always easy to achieve at the top of an insulated ladder.

A volt stick needs no ground, they work on the difference in field over the length of the stick.

[whisky-dave]

An old smilie[1] meter would be good, provided you understood how to use it.

[1] AVO

[Robin]

I can see how that avoids false positives but not how it guarantees no false negatives: I used an AVO in 1973-4 which had an intermittent "I see nothing" fault (until the workshop resoldered a couple of joints).

[tabbypurr]

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

[tabbypurr]

Nothing guarantees zero false negatives, equipment failures, operator errors & failure to follow safe procedures all occur. But voltage detectors that reference to the hand have the additional inbuilt screwup mode already described.

NT