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Topic Title: Neon Screwdriver
Topic Summary: Explainantion ?
Created On: 14 November 2013 09:17 PM
Status: Post and Reply
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 14 November 2013 09:17 PM
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Ampman

Posts: 1004
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Evening ,

Had a call tonight regarding a home owner who went to remove his light switches with a neon screwdriver and it lit up when he touch the screws.

He was worried so i went round & took switches off & measured 55 volts to earth in the light switches , I discovered a cut earth in th light fitting so re-connected & got 230 volts at the light switch .

Why would the neon screwdriver light up ? but doesnt when earth connected ?

I would never trust a screwdriver

Cheers
 14 November 2013 09:21 PM
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perspicacious

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Measuring the IR of your circuit should reveal the answer

Regards

BOD
 14 November 2013 10:13 PM
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Ampman

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Please ,

Explain then Bod , So i can rest tonight ,

I was thinking that the 55 volts appeared because of capicitive coupling on the cut earths in light, down to the switch . & when neaon driver touching the screw it would read through the person to earth ?

Please Please Put me out of my misery ?
 14 November 2013 11:35 PM
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mikejumper

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Originally posted by: Ampman
I was thinking that the 55 volts appeared because of capicitive coupling on the cut earths in light, down to the switch . & when neaon driver touching the screw it would read through the person to earth ?

If you were measuring using a DVM then you are presenting a high input impedance between the light switch and earth which is probably why you saw 55 volts. A low impedance moving coil meter would probably have shown nothing.

The neon screwdriver also presented a high impedance, although not to earth. In this case it would have been your customer acting as a capacitor charging and discharging through the resistor and neon.

So I agree with you, it's an induced voltage.
 15 November 2013 07:59 AM
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kengreen

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yes, induced voltages in high impedance circuits can be alarming. I understood that just inside the front doors of the BBC FM transmitter at Wrotham there is/was a wooden plate warning that the building contained high voltages. What made it interesting was that with a pocket screwdriver you could draw a half-inch arc from any of the screws that fixed it to the wall?

Ken Green
 15 November 2013 08:55 PM
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Zs

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Ampman, I remember Geoff Blackwell on the subject of neon screwdrivers reminding us that you put yourself in the circuit as you use one.

I'm amazed that they still come as a freebie in some rather decent tool kits. My Wera kits always come with one in. I take them out and replace with a voltstick ( that'll cause trouble). As it happens, in the hand bag I have a voltstick and a screwdriver just in case. The screwdriver is often one of the discarded neon jobbies.

Even in the hand bag, just rummaging around for the car keys or a hairbrush, neon screwdrivers go off. I'd not trust one for anything but opening a tin of hot chocolate. I prefer the sonic screwdriver for saving the universe.

In this case though, glad it happened and that you sorted it.

Serendipity.

Zs
 15 November 2013 09:46 PM
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mikejumper

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Originally posted by: Zs
Ampman, I remember Geoff Blackwell on the subject of neon screwdrivers reminding us that you put yourself in the circuit as you use one.

Then I'd have to disagree with Geoff Blackwell
 15 November 2013 11:43 PM
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gkenyon

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Originally posted by: mikejumper

Originally posted by: Zs

Ampman, I remember Geoff Blackwell on the subject of neon screwdrivers reminding us that you put yourself in the circuit as you use one.


Then I'd have to disagree with Geoff Blackwell [IMG][/IMG]
If you are using a neon screwdriver with its tip connected to a live conductor, then I'm afraid Geoff is dead right, and if the internal components of your screwdriver become damp, or there were a voltage disturbance at the time you used it, you may be dead (right or not).

In this case, because of the high impedances involved, an induced voltage may be sufficient to light the neon - simple as that. A neon may light with as little as 50 V, and a relatively high impedance (> 120 k?, maybe up to 1 M?). The impedance of the human body is a mere few k?, which is why there is little risk of shock with a neon, in dry conditions and no voltage fluctuations above around 1 kV.

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Eur Ing Graham Kenyon CEng MIET TechIOSH
 16 November 2013 01:18 AM
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Legh

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I'm quite happy to run with the anti neon screwdriver brigade but I'm struggling with the evidence of the probability of danger here.

1/ There are experienced electricians that I know who can justify their use through practical experience
2/ what statistical evidence do we have that neon screw drivers have actually killed people.?
3/ Why should SSE, I believe, issue all their contractors with neon screwdrivers as part of their test kit if they considered them to be a lethal component ?
4/ Why has this knowledge of the use of neon screwdrivers not been brought to the attention of the general public if they are as lethal as certain people say they are?
5/ Why do we still allow the container ships heading for the pound stretcher shops filled with 'magic' screwdrivers continue?

Its almost as bad as the continued perception of the strobotic affect of LPMV discharge lighting affecting rotating machinery. Do you know of anybody or is their actual evidence that persons have been seriously injured or killed by this effect?

Would I use neon screwdrivers? well, I have a 'gold plated' one in my kit, but as yet I've never felt the need to use it in earnest. Its a bit like keeping old videos just in case I might want to watch them. But at present I do not consider them as dangerous as I once did and not as superfluous as watching old videos

My counter argument would be that they hadn't been tested to a 'standard' to ensure that even when full of water they were unable to electrocute the user.
But it still brings me back to the fact that:

So we can proudly announce that there is no recorded evidence of injury due to the use of a neon screwdriver.

Would I use them in a non-domestic situation? ... Like hell I would ! I value my life a little more than that ..... now look at the paranoia and superstitious nonsense you have created !!!

Legh

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Why do we need Vernier Calipers when we have container ships?

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"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 16 November 2013 07:40 AM
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Thripster

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Assume for a moment that you are testing for dead with an approved voltage tester. Also assume, for whatever reason, that on the machine you are testing, it has a single pole switch wired in the neutral and that the CPC is discontinuous (and has been that way for many years). At first knockings, the approved, two lead voltage tester would indicate safe - the neon screwdriver/volt stick would show voltage present. So I carry the latter (as well). Using a water filled neon screwdriver would be idiotic in the extreme, as would using a water filled, mains powered hand drill. Improve the standard of education - don't try to legislate for all moronic actions - it cannot be achieved and, besides, would be too expensive.

Regards
 16 November 2013 12:15 PM
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alancapon

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Originally posted by: Legh
I'm quite happy to run with the anti neon screwdriver brigade but I'm struggling with the evidence of the probability of danger here. . .

If you look at the construction, there is a neon lamp that illuminates at about 50V (when the neon gas in the glass capsule begins to conduct) and would reach full brightness at about 75V. This lamp is put in series with a resistor that limits the current through the lamp to a low value that cannot be perceived as an electric shock, based on the resistance of a human wearing sensible footwear who is also in the series circuit to earth.

If you remove or reduce the resistance of the series resistor by either dirt or moisture, you will receive an electric shock. A neon lamp is a good regulator, and will attempt to draw enough current for its terminal voltage to drop to 80 to 90V. If you connect the lamp straight across the mains without any series resistors, the current draw will be so high that the lamp capsule will shatter due to the heat buildup.

. . . 3/ Why should SSE, I believe, issue all their contractors with neon screwdrivers as part of their test kit if they considered them to be a lethal component ? . . .

You would have to ask them. Our safety rules (another DNO) ban them from use.

. . . My counter argument would be that they hadn't been tested to a 'standard' to ensure that even when full of water they were unable to electrocute the user. . .

It would be difficult to approve a product that involves the user being placed in series with the mains supply by the test instrument.


Regards,

Alan.
 16 November 2013 01:09 PM
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Legh

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Originally posted by: alancapon
Originally posted by: Legh
I'm quite happy to run with the anti neon screwdriver brigade but I'm struggling with the evidence of the probability of danger here. . .

If you look at the construction, there is a neon lamp that illuminates at about 50V (when the neon gas in the glass capsule begins to conduct) and would reach full brightness at about 75V. This lamp is put in series with a resistor that limits the current through the lamp to a low value that cannot be perceived as an electric shock, based on the resistance of a human wearing sensible footwear who is also in the series circuit to earth.

If you remove or reduce the resistance of the series resistor by either dirt or moisture, you will receive an electric shock. A neon lamp is a good regulator, and will attempt to draw enough current for its terminal voltage to drop to 80 to 90V. If you connect the lamp straight across the mains without any series resistors, the current draw will be so high that the lamp capsule will shatter due to the heat buildup.


I don't have a problem the theory of volt drops across series resistances as this is the case here...and what if we had a situation where some of the series resistances were removed or reduced by the effects of water.
We do have evidence that as a rule of thumb 95% of the population can withstand a serious shock from a supply less than 50V and we make provision for that by using SELV systems and equipment. The one advantage of using a Neon screwdriver is that if the neon were to light then we would have an indicator of a dangerous voltage.

. . . 3/ Why should SSE, I believe, issue all their contractors with neon screwdrivers as part of their test kit if they considered them to be a lethal component ? . . .


You would have to ask them. Our safety rules (another DNO) ban them from use.

Ok, but why the difference in policy thinking? If it was so dangerous it would be universally legislated for?

. . . My counter argument would be that they hadn't been tested to a 'standard' to ensure that even when full of water they were unable to electrocute the user. . .


It would be difficult to approve a product that involves the user being placed in series with the mains supply by the test instrument.

Now, I would agree, theoretically, this to be the case, but it's a 'what if' scenario and doesn't appear to be based on any hard evidence.

We know what happens when somebody fractures a gas pipe but we only have received theoretical wisdom about what might happen when we place ourselves in series with other resistances. In fact it happens from time to time when we get micro shocks



Regards,

Alan.


I think my mind is made up regarding the use of neon screwdrivers.
Judicious use could prove useful. Certainly better than striking the back of your hand or flashing your finger against an unknown terminal or wire. I can and have done that accidentally on occasion and its always an unpleasant experience.

It appears the most i can tell the students that 'no' you shouldn't use neon screwdrivers because of the theory and what your coursework book states but 'maybe yes' in certain situations where there is an unreliable earth, it would be better than using your fingers without a 'known' resistance in series

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 16 November 2013 01:35 PM
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geoffsd

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Just out of interest, how do THESE work?

You don't have to touch the end for voltage indication but when testing continuity.
 16 November 2013 01:48 PM
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alancapon

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Originally posted by: Legh
I don't have a problem the theory of volt drops across series resistances as this is the case here...and what if we had a situation where some of the series resistances were removed or reduced by the effects of water. . .

Unfortunately, most neon screwdrivers only have two resistances as far as I am aware - one is in the screwdriver handle, the other is the user.

. . . The one advantage of using a Neon screwdriver is that if the neon were to light then we would have an indicator of a dangerous voltage. . .

Under most circumstances. A better option would be a volt-stick, which are specifically designed such that direct contact with a live terminal is not required. These also give a much brighter light, making it easier to see. Unfortunately the technology makes them more expensive and a lot of people will not pay the extra.

. . . why the difference in policy thinking? If it was so dangerous it would be universally legislated for? . . .

I am not familiar with the SSE rules. Our rules state that only approved devices can be connected to a live terminal or conductor. As a neon screwdriver has not been approved, it cannot be used.


Regards,

Alan.
 16 November 2013 02:13 PM
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Legh

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Well, I thought I'd better do a little research as it appears that there have been many posts and threads, Thripsters one several years ago, as an example, where I took a less sympathetic stance.

Try this as it seems a reasonable explanation: borrowed from DIYWIKI

How they work

Mains power flows down the screw driving tip to the resistor and neon in the handle. When you touch the brass cap, the bodies natural capacitance allows enough power to flow through the screwdriver and the user to cause the neon to light.

This small current is entirely harmless, there is no electric shock under normal circumstances.


The Problems

Direct shock

Water or condensation on the resistor can result in the tool shocking the user, maybe causing a fall from a ladder. Hence never hold one of these screwdrivers in your mouth, eg when working with both hands.

Certain tool faults such as resistor a failure can create similar risks.

False negative

The lighting of the neon is sometimes too dim to see, especially in daylight. The result is shock from touching a live wire, and this is probably the most common failure mode

Not being connected to or near earth, insulated from ground, can reduce current flow further, making the neon even dimmer. The body being capacitively coupled to some other nearby live conductor, can prevent the light lighting at all, even when the screw driver tip is touching a live wire or contact.

False Positive

A user capacitively connected to a live supply will light a neon screwdriver touched to a non-live wire. Its quite easy to be capacitively coupled in this way without noticing. Holding an inspection light, standing on a cable, even leaning on a wall in some cases, and so on. Also touching a "floating" conductor (i.e. an unconnected wire that is routed in close proximity to other live wires) may also yield a false positive test.

Minimising risk / safe use

If you must use one of these devices, then adopt a sound "proven dead" approach to testing. Before making a test, test the screwdriver on a known live conductor and ensure it lights up. Next test the unknown conductor, and ensure it does not light, Finally test again on a known live conductor.

Avoid putting the tool anywhere that may expose it to allow water or condensation, such as a shed, outdoors, in one's mouth etc.

Legality

There is some question as to whether these tools, are legal to use in the workplace, since they don't possess a number of the attributes deemed necessary in electrical test gear by the health and safety executive. See the document GS38


I'm quite happy with that as an explanation...what do you lot think?

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 16 November 2013 07:55 PM
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stateit

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My superior half's handbag resembles Zs's by the sound of it:

Her larger 'holiday-size' handbag includes (amongst various female ephemera): An Allen-key set, a Multi-Tool, one of those widget sized screwdriver-bit sets and one of my cast-off neon screwdrivers.

Having just written this has made me scratch my head
All those years ago - Yes - It was love at first sight!

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S George
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 16 November 2013 08:04 PM
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Thripster

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I'll go with that Legh.

Regards
 16 November 2013 10:10 PM
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gkenyon

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A number of years ago, I had a belt ofd a damp neon screwdriver - an undetected leak from a machine into the top cantilever of my toolbox. Not too nasty, but it does make you think.

Pre-1989 before anyone asks.

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Eur Ing Graham Kenyon CEng MIET TechIOSH
 17 November 2013 12:25 AM
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stateit

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Graham:

Is your avatar picture of that very same incident ?

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S George
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 17 November 2013 01:22 PM
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rocknroll

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Most electricians I come across carry one in their pocket or walk in with one in their hand if you call them for a fault, simply, its just another tool or aid in your box and like all tools if you use them incorrectly then you could cause yourself and others harm, but if used correctly as most do then there is no danger to anyone.

regards

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"Take nothing but a picture,
leave nothing but footprints!"
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"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
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IET » Wiring and the regulations » Neon Screwdriver

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