I was mystified by something I experienced today. I'm sure any
electrician will know the answer. Sorry if its a naive question.

I was using one of those electrical screwdrivers with abuilt-in diode
for identifying live wires. I wanted to disconnect a security light on
my house but I wasn't sure which circuit it was connected to. I
concluded the light was wired into the lighting circuit, because it
failed to come on after I pulled the main fuse from that circuit. But
just to play safe, I touched the screwdriver onto the terminals in the
junction box which the light was connected to. The diode didn't light
up. However, when I touched the brass end of the screwdriver's handle,
it lit up, quite brightly. So I found my volt meter and tested the
wires to see if there was any voltage there. No voltage registered.
Can anyone explain why the diode lit up when I touched the brass end?
And it lit up the same, whether I touched the screwdriver onto the
live, the neutral or the earth terminal.

Regards,

Drake

On 10 May, 20:30, Drake wrote:
I was mystified by something I experienced today. I'm sure any
electrician will know the answer. Sorry if its a naive question.

I was using one of those electrical screwdrivers with abuilt-in diode
for identifying live wires. I wanted to disconnect a security light on
my house but I wasn't sure which circuit it was connected to. I
concluded the light was wired into the lighting circuit, because it
failed to come on after I pulled the main fuse from that circuit. But
just to play safe, I touched the screwdriver onto the terminals in the
junction box which the light was connected to. The diode didn't light
up. However, when I touched the brass end of the screwdriver's handle,
it lit up, quite brightly. So I found my volt meter and tested the
wires to see if there was any voltage there. No voltage registered.
Can anyone explain why the diode lit up when I touched the brass end?
And it lit up the same, whether I touched the screwdriver onto the
live, the neutral or the earth terminal.

Regards,

Drake

Did you read the instructions? The light will only light when you
touch the end as you're completing the circuit. Why it lights on all
three wires is somehting else though!

On 10 May 2007 12:30:48 -0700, adder1969
wrote:

Did you read the instructions?

No, I've never read the instructions. I don't think it came with any.

The light will only light when you
touch the end as you're completing the circuit.

Not actually true with this particular screwdriver. If you touch it
onto a genuinely live contact, the red light lights up brightly
whether you have your finger on the end or not.

Why it lights on all
three wires is somehting else though!

Yes, I hope someone here can offer a possible explanation.

Drake

On Thu, 10 May 2007 20:30:52 +0100, Drake
mused:


I was mystified by something I experienced today. I'm sure any
electrician will know the answer. Sorry if its a naive question.

I was using one of those electrical screwdrivers with abuilt-in diode
for identifying live wires. I wanted to disconnect a security light on
my house but I wasn't sure which circuit it was connected to. I
concluded the light was wired into the lighting circuit, because it
failed to come on after I pulled the main fuse from that circuit. But
just to play safe, I touched the screwdriver onto the terminals in the
junction box which the light was connected to. The diode didn't light
up. However, when I touched the brass end of the screwdriver's handle,
it lit up, quite brightly. So I found my volt meter and tested the
wires to see if there was any voltage there. No voltage registered.
Can anyone explain why the diode lit up when I touched the brass end?
And it lit up the same, whether I touched the screwdriver onto the
live, the neutral or the earth terminal.

Basically, those neon screwdrivers are absolutely crap. Don't ever use
it as a means of testing for power, ever. It's as safe as the wet
finger test.
--
Regards,
Stuart.

Lurch wrote:

Basically, those neon screwdrivers are absolutely crap. Don't ever use
it as a means of testing for power, ever. It's as safe as the wet
finger test.

What do you think of these?
http://www.maplin.co.uk/module.aspx?...45560&doy=10m5


--
Dave
The Medway Handyman
www.medwayhandyman.co.uk
01634 717930
07850 597257

On Thu, 10 May 2007 22:09:55 UTC, "The Medway Handyman"
wrote:

Lurch wrote:

Basically, those neon screwdrivers are absolutely crap. Don't ever use
it as a means of testing for power, ever. It's as safe as the wet
finger test.

What do you think of these?
http://www.maplin.co.uk/module.aspx?...45560&doy=10m5

Wouldn't trust them either. (was waiting for someone to mention them!)

--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
http://www.diybanter.com

The Medway Handyman wrote:

What do you think of these?
http://www.maplin.co.uk/module.aspx?...45560&doy=10m5

They are not quite as evil as the neon based contact type, but I am not
sure I would trust them as a sole means of identifying energised circuits.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"John Rumm" wrote in message
...
The Medway Handyman wrote:

What do you think of these?
http://www.maplin.co.uk/module.aspx?...45560&doy=10m5

They are not quite as evil as the neon based contact type, but I am not
sure I would trust them as a sole means of identifying energised circuits.

They are not bad. I trust them on T&E cables if used properly.

Adam

On Fri, 11 May 2007 16:44:31 GMT, "ARWadsworth"
mused:


"John Rumm" wrote in message
...
The Medway Handyman wrote:

What do you think of these?
http://www.maplin.co.uk/module.aspx?...45560&doy=10m5

They are not quite as evil as the neon based contact type, but I am not
sure I would trust them as a sole means of identifying energised circuits.

They are not bad. I trust them on T&E cables if used properly.

Yeah, although I've only ever used the MK and Fluke ones. I've seen
people with unbranded\cheap ones and they are somewhat erratic.

Just remember not to use it as a sole means of testing when your
having an NICEIC inspection.
--
Regards,
Stuart.

On Thu, 10 May 2007 19:30:52 UTC, Drake wrote:

I was mystified by something I experienced today. I'm sure any
electrician will know the answer. Sorry if its a naive question.

Several naive bits actually. Did you read the instructions?

I was using one of those electrical screwdrivers with abuilt-in diode
for identifying live wires.

Not a diode - a neon. A fairly massive difference. If it were a diode
you'd get a shock!

I wanted to disconnect a security light on
my house but I wasn't sure which circuit it was connected to. I
concluded the light was wired into the lighting circuit, because it
failed to come on after I pulled the main fuse from that circuit. But
just to play safe, I touched the screwdriver onto the terminals in the
junction box which the light was connected to. The diode didn't light
up.

The NEON wouldn't light without sometjing else to complete the circuit.
You're only giving it one contact - the blade.

However, when I touched the brass end of the screwdriver's handle,
it lit up, quite brightly.

That's the other contact. It needs only a tiny current - which is just
as well. That's what you are meant to do.

So I found my volt meter and tested the
wires to see if there was any voltage there. No voltage registered.

The neon is very high resistance and so nothing will show.

Can anyone explain why the diode lit up when I touched the brass end?
And it lit up the same, whether I touched the screwdriver onto the
live, the neutral or the earth terminal.

As explained above.

These things can be lethal:

1) They fail without warning. Don't ever rely on them if they say there
ISN'T a voltage.
2) If they get damaged and short (unlikely, but...) you can get a
serious shock.

Use a voltmeter. You had one, so use it!
--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
http://www.diybanter.com

"Bob Eager" wrote in message
...
Use a voltmeter. You had one, so use it!


Whist I agree with everything you say, modern voltmeters can be misleading
too.

Modern DVMs have such a high input impedance that they can find stray
voltage on longish runs of cable which is actually of such high source
impedance that it will drop to zero volts as soon as any ( even megohm )
load is put on it.

So they can indicate a voltage as present when it's not really.
( Safer than the reverse, of course! )

I trust my old 20Kohm/v AVO-8 for this kind of thing more than my fancy
all-singing Fluke DVM.

--
Ron

"Ron Lowe" ronATlowe-famlyDOTmeDOTukSPURIOUS wrote in
:


I trust my old 20Kohm/v AVO-8 for this kind of thing more than my
fancy all-singing Fluke DVM.

AVO-8 even....

Respect

mike

On Thu, 10 May 2007 20:37:40 UTC, "Ron Lowe"
ronATlowe-famlyDOTmeDOTukSPURIOUS wrote:

"Bob Eager" wrote in message
...
Use a voltmeter. You had one, so use it!

Whist I agree with everything you say, modern voltmeters can be misleading
too.

I agree...good point. I was rather assuming a cheap low impedance
one...!

I trust my old 20Kohm/v AVO-8 for this kind of thing more than my fancy
all-singing Fluke DVM.

Indeed. I use a cheap one I got from Maplin many years ago. Friend of
mine has an 8, but then he had a summer job at Avo for years!

--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
http://www.diybanter.com

Ron Lowe wrote:


"Bob Eager" wrote in message
...

Use a voltmeter. You had one, so use it!



Whist I agree with everything you say, modern voltmeters can be
misleading too.

Modern DVMs have such a high input impedance that they can find stray
voltage on longish runs of cable which is actually of such high source
impedance that it will drop to zero volts as soon as any ( even megohm )
load is put on it.

So they can indicate a voltage as present when it's not really.
( Safer than the reverse, of course! )

I trust my old 20Kohm/v AVO-8 for this kind of thing more than my fancy
all-singing Fluke DVM.

I had this problem when measuring the flap/slat asymmetry detection
voltage on an aircraft I used to work on (it was an AC voltage wave
form. 400 Hertz) The modern volt meters used to measure the residual
voltages in the wiring. Reverting back to the milliamp hungry meters
cured this and I could get a proper reading

Never believe a reading from a modern meter, unless you know what you
are doing.

Dave

In article ,
Ron Lowe ronATlowe-famlyDOTmeDOTukSPURIOUS wrote:
I trust my old 20Kohm/v AVO-8 for this kind of thing more than my fancy
all-singing Fluke DVM.

It's a bit big for simple volt measurements like this, though. Pretty well
any analogue meter will do - or simply fit a shunt to the Fluke. My Model
8 stays on the shelf these days - too expensive for general use outside
the workshop.

For general 'are there volts present and how many' type uses, this is my
device of choice:-

http://www.tlc-direct.co.uk/Main_Ind...741/index.html

It avoids having a high input impedance where it's not needed.

--
*When cheese gets its picture taken, what does it say? *

Dave Plowman London SW
To e-mail, change noise into sound.

On 10 May 2007 20:20:19 GMT, "Bob Eager" wrote:

I wanted to disconnect a security light on
my house but I wasn't sure which circuit it was connected to. I
concluded the light was wired into the lighting circuit, because it
failed to come on after I pulled the main fuse from that circuit. But
just to play safe, I touched the screwdriver onto the terminals in the
junction box which the light was connected to. The diode didn't light
up.

The NEON wouldn't light without sometjing else to complete the circuit.
You're only giving it one contact - the blade.

Funnily enough, as I mentioned to adder, it does light up if you touch
it on a genuinely live conductor - even if you don't have a finger on
the end!

Anyway, thanks to all for the input. I'd still like to know why the
thing lit up when touched on junction box terminals after the fuse for
that circuit was removed, and on all three too (including earth) -
when a volt meter registered 0 volts.

Drake

On Thu, 10 May 2007 21:39:35 UTC, Drake wrote:

On 10 May 2007 20:20:19 GMT, "Bob Eager" wrote:

I wanted to disconnect a security light on
my house but I wasn't sure which circuit it was connected to. I
concluded the light was wired into the lighting circuit, because it
failed to come on after I pulled the main fuse from that circuit. But
just to play safe, I touched the screwdriver onto the terminals in the
junction box which the light was connected to. The diode didn't light
up.

The NEON wouldn't light without sometjing else to complete the circuit.
You're only giving it one contact - the blade.

Funnily enough, as I mentioned to adder, it does light up if you touch
it on a genuinely live conductor - even if you don't have a finger on
the end!

It doesn't require much leakage for the neon to light.

Anyway, thanks to all for the input. I'd still like to know why the
thing lit up when touched on junction box terminals after the fuse for
that circuit was removed, and on all three too (including earth) -
when a volt meter registered 0 volts.

Leakage again...voltmeter is more reliable (but note that a cheap one is
probably better here).
--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
http://www.diybanter.com

In article ,
Bob Eager wrote:
Funnily enough, as I mentioned to adder, it does light up if you touch
it on a genuinely live conductor - even if you don't have a finger on
the end!

It doesn't require much leakage for the neon to light.

If it's a red light it's likely one of those volt stick devices which is
active in operation. It has a small amp built in. With these you really do
need to read the instructions.

--
*Bigamy is having one wife too many - monogamy is the same

Dave Plowman London SW
To e-mail, change noise into sound.

Drake wrote:

Anyway, thanks to all for the input. I'd still like to know why the
thing lit up when touched on junction box terminals after the fuse for
that circuit was removed, and on all three too (including earth) -
when a volt meter registered 0 volts.

Its called inductive coupling. The wires probably run some distance in
proximity to other wires which are live. Although there is no physical
connection between them the alternating magnetic field produced by the
current flowing in the live circuit will induce a voltage in the
disconnected one. (same principle by which a transformer works)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Its called inductive coupling. The wires probably run some distance in
proximity to other wires which are live. Although there is no physical
connection between them the alternating magnetic field produced by the
current flowing in the live circuit will induce a voltage in the
disconnected one. (same principle by which a transformer works)


--
Cheers,

John.



Sense at last!

Yes, you need to be careful with neon screwdrivers. A false negative may
mean a broken screwdriver but if you check that it is working every time you
use it the chance of getting zapped is very low (and if you short every
potential live to earth before touching it it is even lower). You can often
recognise a false positive caused by induction because they don't light
quite so brightly. I prefer my cheapo "electronic" screwdrive which buzzes
on a live because it will do this through insulation, but I still use neons.
And I always check that it's working before trusting it.

Newshound wrote:


use it the chance of getting zapped is very low (and if you short every
potential live to earth before touching it it is even lower). You can often

I gave up using my one after confirming that I had pulled the correct
fuse with the thing, and then did the live to earth check "just to be
sure". The big blue flash and the vaporising of the end of the
screwdriver was enough to convince me never to trust one again! (the
lack of a blade also rendered that one less useful as well)



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Thu, 10 May 2007 23:57:52 +0100, John Rumm
wrote:

Drake wrote:

Anyway, thanks to all for the input. I'd still like to know why the
thing lit up when touched on junction box terminals after the fuse for
that circuit was removed, and on all three too (including earth) -
when a volt meter registered 0 volts.

Its called inductive coupling. The wires probably run some distance in
proximity to other wires which are live. Although there is no physical
connection between them the alternating magnetic field produced by the
current flowing in the live circuit will induce a voltage in the
disconnected one. (same principle by which a transformer works)

Ah, yes. I know for a fact that the cable leading to that circuit does
run close to other cables for some distance. Thanks for the education!

Drake

John Rumm wrote:

Its called inductive coupling. The wires probably run some distance in
proximity to other wires which are live. Although there is no physical
connection between them the alternating magnetic field produced by the
current flowing in the live circuit will induce a voltage in the
disconnected one. (same principle by which a transformer works)

Sorry John, that's completely wrong. There's no way you'd get enough
inductive (electro_magnetic_) coupling in this sort of situation to
induce a voltage high enough to light a neon.

Capacitive coupling is the mechanism involved here, aka electro_static_
induction (note induction, not inductive). What was probably happening
was that the OP's body was near enough to a live cable to pick up a
significant voltage (there's no shock felt of course because any current
flowing is tiny (microamps) and there's no point of skin contact). The
sharp end of the screwdriver was in contact with an earth or neutral
terminal and it was the user that provided the 'live' connection to the
blunt end. You can get similar 'false positive' effects when using a
'volt-stick' type of indicator with your body near to a live cable.

Moral: always use a proper voltage indicator or test lamp and test it on
a known live supply (or proving unit) both before and after isolating &
checking the circuit you intend to work on.

--
Andy

Andy Wade wrote:

Sorry John, that's completely wrong. There's no way you'd get enough
inductive (electro_magnetic_) coupling in this sort of situation to
induce a voltage high enough to light a neon.

Yup, you could well be right thinking about it... different case from
false positives with a high impedance DVM.

What sort of strike voltage does a neon need?

Capacitive coupling is the mechanism involved here, aka electro_static_
induction (note induction, not inductive). What was probably happening
was that the OP's body was near enough to a live cable to pick up a
significant voltage (there's no shock felt of course because any current
flowing is tiny (microamps) and there's no point of skin contact). The
sharp end of the screwdriver was in contact with an earth or neutral
terminal and it was the user that provided the 'live' connection to the
blunt end. You can get similar 'false positive' effects when using a
'volt-stick' type of indicator with your body near to a live cable.

That would better explain the thing lighting on the neutral and earth
wires since unlike the live they would not have been left floating by
pulling the fuse.

Moral: always use a proper voltage indicator or test lamp and test it on
a known live supply (or proving unit) both before and after isolating &
checking the circuit you intend to work on.

Can't argue with that!

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Andy Wade wrote:


Moral: always use a proper voltage indicator or test lamp and test it
on a known live supply (or proving unit) both before and after
isolating & checking the circuit you intend to work on.

I only do minor electrical stuff like changing lights. My proceedure is;

Switch light on, check with volt stick to ensure volt stick is working.
Remove fuse/turn off breaker for relevant circuit. Check again with volt
stick.

I assume thats safe enough?


--
Dave
The Medway Handyman
www.medwayhandyman.co.uk
01634 717930
07850 597257

Andy Wade wrote:


Moral: always use a proper voltage indicator or test lamp and test it on
a known live supply (or proving unit) both before and after isolating &
checking the circuit you intend to work on.

I wish I had taken that advice a few years ago. The cable tv man was
coming so I had pulled the TV and stand etc out and decided to spend the
time figuring out why the socket behind it didn't work. So I pulled the
fuse for the downstairs ring, noted that nothing was on but did not test
the recalcitrant socket. So there I was poking about in the switch
seeing if the connections were good when there was a flash and the
lights went out (it was an overcast winters day). Turned out that socket
had been extended not from the ring, but from a port for a hard wired
standard lamp, so when I shorted it the lighting fuse blew. I have been
much more careful since and will take Andy's advice to heart for all
further such excursions.

Peter
--
Add my middle initial to email me. It has become attached to a country
www.the-brights.net