Neon screwdrivers

[Pete Verdon]

How are they supposed to work?

I tagged one of these onto the end of a TLC order just to get an ok electrician's-sized screwdriver. I don't intend to use the alleged "testing" facility, but I'm curious how it's supposed to work. It looks like it's meant to be touched to a live terminal and have the current flow up the shaft, through a capsule which I assume is the bulb, and then go via a brass end-cap *into my hand*. The last part of that I'm not particularly keen on.

I assume that, while they're not well-regarded by most here, neon screwdrivers don't actually electrocute their owners during intended use. Is it just that the current flow is very small, and almost all the voltage is across the neon? Can anyone with a better grasp of electrical theory explain?

Cheers,

Pete

[geoff]

In message , Pete Verdon writes

well, not if they are OK

else ...

Chuck it and spend a tenner on a meter

[Owain]

This is why they are Not Allowed to be used in workplaces.

IF they are working correctly. That's a big if.

Basically yes.

They are too likely to fail dangerous and hurt somebody, and too likely to give a false positive or negative, that they are completely unsuited to the tast and not fit for purpose. A proper electrical tester is much better.

Always test the tester against a known voltage, then test the unknown wire, then test the tester again, to be certain you aren't getting a false reading.

Owain

[John Rumm]

You touch the tip on something live or not, and they may light up or not.

Yup, is about right - your provide the neutral path.

No, that is usually when they do try to electrocute you. You stick the driver on a live wire, it neglects to light, so you go ahead and touch the wire!

[terry]

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Do they usually have high value resistor inside to limit current to a few microamps???? Or maybe something less than a milliamp? e.g. 230 RMS volts/230 k.ohms =3D 1 milliamp. and 115/230 k.ohms =3D 0.5 ma. Peak voltage of 115v about 160 volts more than enough to fire most neons.

PS. Took a small pocket version half way across the globe and to Malta and back over Christmas, mixed in woth pens and pencils in brief case; didn't even realise it was there. Only to have it confiscated on a short day trip between two Middle Eastern countries! Length of the blade or something. Strange thing was that on another flight we were provided with metal fork and knife, the blade length of which exceeded the confiscated pocket screwdriver. So thinking of making one that 'looks' like a ball pint or 0.5mm drafting pencil! Any suggestions?

[John Rumm]

yup - you won't even feel a tingle.

Was chatting to a friend a while back who had just got back from a business trip in the middle east. He ended up coming back on a commercial jet where about 95% of the passengers were returning US soldiers. A Sargent stood up at the front of the plane, and explained to them that because (of what he thought were ridiculous) FAA rules, they now had to pass round a plastic bucket into which they must deposit any pen knives, screwdrivers or other sharp objects etc. that could potentially be used as a weapon...

However since they were military personnel, there were allowed to keep their Ammo, M16s, 9mm side arms, bayonets, and any grenades or claymore mines they had about their persons!

[matthew.larkin]

I've read this before - any links to this actually happening? Surely B&Q wouldn't be selling them for =A31.99 if there was anything more than a very remote possibility of someone electrocuting themselves through "proper" use of the tool. Failures to the extent that are likely to harm must be incredibly rare.

Of course, that completely ignores the false negative potential for them, and that is the real problem with them. Though again they aren't sold with warning flags all over them. Meters can be just as dangerous for ignoramuses like me - when I first bought a meter I would always check the result that I saw with a neon screwdriver as a few times when I first used the meter I used the wrong settings and got a false negative, and the screwdriver was useful to help me out with that.

I'm more confident now that I use the right settings on the meter (its not that hard to put it on AC > 200v really is it???), but I still keep the screwdriver to hand just in case.

Matt

[Pete Verdon]

I already have a meter. It's somewhat less effective than the screwdriver at doing up terminal screws.

(You snipped the bit where I said I bought it as a screwdriver, and didn't intend to use the neon part.)

Pete

[Pete Verdon]

Thanks. That's what I was asking.

I went to some length to make clear that that wasn't what I was asking :-)

Pete

[The Medway Handyman]

I've followed that advice since you first mentioned it & feel a lot safer, so thanks for that.

I have a non contact 'voltstick' which is used as you describe. Always wondered though, why does it light up if waved rapidly from side to side? No idea how they work.

[Tournifreak]

I've never seen or heard of one that has failed dangerously (at least not where it's not obvious - snapped in two).

But isn't this the same process one should always follow when using an electrical test screwdriver? So what's the difference? Much easier to get a false reading from a meter anyway - dodgy leads, low battery, wrong setting etc.

Jon.

[robgraham]

The reason they don't light is that the user is ignorant of the why they light up and is standing on a rubber mat wearing rubber soled boots. If I've got any doubt I usually find some way of earthing my other hand to give a definite earth path.

Personally I'd rather use a neon light stick than a meter - leads fall out of meters, they unhook themselves, batteries fail, you require two hands and I'm in more danger trying to find somewhere to clip the lead to earth, and keep it there, than anything else. It's a case of know your tools, be aware of their limitations and use them properly - applies to everything from circular saws to screwdrivers.

I've never heard of one failing - has anyone? or is this nanny state protectionism ? I've been working / playing with electricity since my Dad gave me an electric train (12vac) when I was 5 and that was over

60 years ago. I've had the odd shock over the years - show me someone working with electrics regularly who hasn't, but if I'm at all concerned about a wire, I will just brush my finger across it quickly to see if it is live.

Rob

[Peter Twydell]

In message , Pete Verdon writes

They do try to electrocute their users if they're the wrong ones. I once had a 24V version for car use and a 400V (?) for domestic. Guess which one I used to test a mains wire?

[John Rumm]

I have never really considered failure of the tool to be a major issue (i.e. it conducting more than it should).

Yup, I have been bitten by that - once many years ago testing the end of a live cable to see if I had pulled the right fuse from the CU. Touched it on the live conductor - no light. Fortunately a little voice somewhere seemed to doubt - so I shorted live to earth with the tip of the driver just to make absolutely sure. Flash pop, and the driver removed itself from the gene pool with a somewhat molten end where the screwdrivery bit used to be!

[tinnews]

I nearly always use some electrical tool or lamp or similar to test if a circuit is live. Much of the time I will have been using (for example) my SDS drill so to check if the circuit is off I check that the drill doesn't run when plugged in to the circuit I'm working on.

As I have nearly always just used the drill I know that it *does* run when there's electricity there.

The alternative I often use is a mains inspection lamp which I leave permanently plugged in to the circuit and switched on, a very visible indication of 'live'! (It's a fluorescent one so, while bulb failure is possible, it's way less likely than for a filament bulb)

OK, there are times when the above won't work (lighting circuits, etc.) and you also need to be sure the bit you are working on is the same circuit as you have the device plugged in but most of the time I still think my way is better than either neon screwdrivers or meters.

I do have both analogue and digital meters and use them when appropriate. FWIW for checking 'liveness' an analogue meter has its advantages over a digital one, it won't indicate due to capacitive coupling and you don't have to stare at it to check whether it's indicating 250 volts or 250 millivolts. (On the other hand it will break if you try and check 250 volts on the 250 millivolt range!)

[<me9]

you should always test the tester before /and/ after use, which will minimise false negatives. If you understand how they work, /and/ their limitations you can't go far wrong. This applies to all test proceedures.

[John Evans]

The 'driver body contains a neon capsule in series with a large value resitor.

When a live terminal is touched a voltage is applied across a series circuit comprising the neon (a high resistance when not conducting), the resistor , your body and the earth path.

At some point of the mains cycle the neon gas conducts causing a low resitance path through it and the gas glows.

The current that flows is limited to a very low value by the series resistance and the rest of the return path. This current is low but sufficient to keep the neon conducting.

Unless there is fault with the 'driver there isn't any danger to the user. I've never known of any problems caused by one going faulty.

[George (dicegeorge)]

neon screwdrivers- holding them in my mouth whilst doing something else with my hands is probably a bad idea cos they can get damp and i could get poisoned!

[g]

[Harry Bloomfield]

Pete Verdon presented the following explanation :

The neon draws a relatively tiny current and to restrict it even more there is an high value resistor in series with it. The idea is that the current passes through your body to ground. Problem - you may not always be at ground potential so it may not light up or lights up dimly. So one finger on the top and make sure you are touching something else with a reasonable ground on it.

Problem 2 - if the neon should become damp or faulty internally, you may be electrocuted.

Problem 3 - They are dim and may not be be seen, which may lead you to believe something is dead when it is not.

Solution - Don't use it, buy a volt stick which responds to the field around a live cable and no current needs to flow through you. But test it before every use.

[Harry Bloomfield]

The Medway Handyman formulated on Wednesday :

They detect the field around the cable. Your waving it about moves it through a field, which is a good before and after check of the volt stick.