Dodgy voltage detector

[leenowell]

Hi all

Just bought one of these

=287869

Reviews seemed good only needed it to confirm circuit is off before I cut i nto it. It seems to detect voltage but when I switched the circuit off it s till seems to think there is voltage although the beep / led is not continu ous. I know the socket is off as I had something plugged into it which then went off when I switched the circuit off.

The instructions say that it self tests when you press the power button and indicates ok with a flashing LED. Mine gives a solid green.

Just took it back to Screwfix and tried 3 others and non seem to do the sel f test thing. Brought a new one home to test on the off circuit and same pr oblems.

It seems unlikely that all 4 and dead (maybe a dodgy batch) but now thinkin g maybe I am missing something?

Thanks

Lee.

[Bob Minchin]

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

[newshound]

I'd go with a bad batch; what did Screwfix say? You should post feedback on their web site. Never had one with self test (could be a bad instruction leaflet)? The false positive sounds like it might result from inductive coupling with a parallel circuit. You sometimes see this with neon screwdrivers and DVMs, although I've never seen it with a "volt stick".

What I like about volt sticks is that they can often be used to distinguish non-invasively between a plug fuse failure, a line switch failure, or load device failure.

[leenowell]

One thing I forgot to mention is that the first one was from a different Sc rewfix branch. doesn't preclude the bad batch theory though. The only rea son I wanted this is to quadruple check I have the correct wire before brea king in to it. The wiring for first floor is a little odd as most sockets are fed from below but for some reason this one seems to be fed from above although it is on the correct circuit. Hence I want to be extra sure I hav e the correct wire before cutting.

[alan_m]

The false positive sounds like it might result

Surely more likely with a contactLESS device as is this volts stick

[Robin]

I'd be interested to learn more about that.

[tabbypurr]

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

NT

[leenowell]

If this type is so unreliable what is the safest way to determine if a cable is safe to cut?

[Robin]

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.

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.

[tabbypurr]

voltmeter

NT

[Mike Hunt]

What if there's no neutral, either missing (fault), or unavailable (switch drop)? Possibly no earth either, in the case of a switch drop wired in singles.

[tabbypurr]

it makes the way that principle is always applied unsound.

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

NT

[charles]

use an extension lead

[mechanic]

In this case it seems a question of what is positive and what is negative - the presence of mains voltage as a result of the test could be taken either way.

[Mr Pounder Esquire]

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.

[leenowell]

A volt meter will only be useful once you have cut into the cable. In my situation I want to prove that a cable is part of a particular circuit and is off

[James Wilkinson Sword]

One of those things worked out which Christmas tree bulb had failed. Apart from that they're utterly useless.

[James Wilkinson Sword]

Detecting voltage without actually touching the wire doesn't work. Even if you take a voltmeter and place the red wire on a live, and the black wire on ground, or your finger, you can get a voltage reading even if no voltage is present, due to induction. These things can't tell the difference between a bit of induced voltage and 240V with current carrying capabilities.

[Chris Green]

I don't believe you can do that certainly without having a physical connection to the wire involved. I'd certainly not want to rely on something that detects the (weak) radiated field from the wire.

[Robin]

I'd be more impressed by your confidence if you'd bothered to state what you are taking to be the purpose of a volt stick. (Note I never said I approved the use of a volt stick to prove a conductor "dead". I do continue to fail to see why you claim they are "always unsound in principle" to identify a live conductor - eg when there are several conductors in a conduit.)