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  #1   Report Post  
dave L
 
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Default should unswitched live should as live on a neon screwdriver?

I've got three tools and I've been using them to test my lamp
pendants.

I've got the faithful neon screwdriver, a digital multimeter (set to
500V) and a 'digital circuit tester' from Blackspur which I bought for
a quid. This last tool has two functions: (a) it shows if the
terminal in contact is live; (b) what the voltage is. It has a
reading of 12-36-55-110-220v and illuminates the value below what the
actual voltage is. It works by the user holding the non-contact end
to complete the circuit a la the neon screwdriver.

When the pendant is switched off, the phase terminal shows (in order
of the tool above):
1. LED lit quite brightly
2. (null reading)
3. between 12V and 36v (36v is slightly illuminated).

When the pendant is switched on, the phase terminal shows (in order of
the tool above):
1. LED lit very brightly
2. 246-250v (depending upon pendant)
3. 220V.

This situation is present on each pendant I have tested. None of the
pendants appear to faulty in day-to-day use.

I'm a little intrigued that the two cheaper tools are showing the the
phase terminal is live when there should be no current. My real
concern is that these tools infer there is current flowing to the
terminal with the pendant is swicthed off and that there's a
possibility of getting electrocuted when changing a bulb.

Are all my pendants wired incorrectly?


Thanks
  #2   Report Post  
Dave Plowman
 
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Default should unswitched live should as live on a neon screwdriver?

In article ,
dave L wrote:
I'm a little intrigued that the two cheaper tools are showing the the
phase terminal is live when there should be no current. My real
concern is that these tools infer there is current flowing to the
terminal with the pendant is swicthed off and that there's a
possibility of getting electrocuted when changing a bulb.


What you're seeing is inductive coupling between the 'live' wire and the
switched one running next to it. Put the bulb back in and you'll see zero,
as the switched line will be connected to neutral via the much lower
impedance of the bulb.

Things like neon screwdrivers - and DVMs - have a very high input
impedance so will read very low currents that the body wouldn't feel.

It's a good idea to use a low value shunt which draws about 0.1 amp when
using a high impedance test device on mains - or alternately make up a
test lamp using a small mains bulb.

--
*Tell me to 'stuff it' - I'm a taxidermist.

Dave Plowman London SW 12
RIP Acorn
  #3   Report Post  
Dave Liquorice
 
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Default should unswitched live should as live on a neon screwdriver?

On 15 Nov 2003 00:22:39 -0800, dave L wrote:

I've got the faithful neon screwdriver, a digital multimeter (set to
500V) and a 'digital circuit tester' from Blackspur ...


snip

When the pendant is switched off, the phase terminal shows (in order
of the tool above):
1. LED lit quite brightly


s/LED/neon/ I think you mean. B-)

2. (null reading)
3. between 12V and 36v (36v is slightly illuminated).


I think you need to be more careful about your descriptions and where
you are actually measuring. "phase" I would take to mean the permenant
live loop through in the ceiling rose and this figures with the
Subject: "unswitched live". But this should be at full mains potential
at all times, if it went on/off with a wall switch then all the lights
in the house would go on and off with that switch (unless you have
some *really* weird lighting wiring).

Just to clarify, do you really mean the above or the switched live
that connects to the bulb?

When the pendant is switched on, the phase terminal shows (in order
of the tool above):
1. LED lit very brightly


s/LED/neon/ I think you mean. B-)

2. 246-250v (depending upon pendant)
3. 220V.

snip
I'm a little intrigued that the two cheaper tools are showing the
the phase terminal is live when there should be no current.


Assuming you mean the switched live, they aren't. You need to
understand the tools you are using and how they interact with what
they are measuring. All of those tools are high impedance and are
voltage measuring devices. What you are seeing is induction from the
permenant live wiring into the open circuit switched live wire. With
high impedance voltage measuring tools it is normal to see this
induction.

--
Cheers
Dave. pam is missing e-mail



  #5   Report Post  
mike ring
 
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Default should unswitched live should as live on a neon screwdriver?

Dave Plowman wrote in
:

It's a good idea to use a low value shunt which draws about 0.1 amp when
using a high impedance test device on mains -


Be a damn good idea if a 5 or 10K shunt was a plugin accessoery for these
things,

mike r


  #6   Report Post  
dave L
 
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Default should unswitched live should as live on a neon screwdriver?

"Dave Liquorice" wrote in message .1...
On 15 Nov 2003 00:22:39 -0800, dave L wrote:


snip

I think you need to be more careful about your descriptions and where
you are actually measuring. "phase" I would take to mean the permenant
live loop through in the ceiling rose and this figures with the
Subject: "unswitched live". But this should be at full mains potential
at all times, if it went on/off with a wall switch then all the lights
in the house would go on and off with that switch (unless you have
some *really* weird lighting wiring).

Just to clarify, do you really mean the above or the switched live
that connects to the bulb?


Sorry about the ambiguity. What I was measuring was the 'live'
terminal of the pendant, ie, that which is connected to the output of
the switch, the switched live.

When the pendant is switched on, the phase terminal shows (in order
of the tool above):
1. LED lit very brightly


s/LED/neon/ I think you mean. B-)

2. 246-250v (depending upon pendant)
3. 220V.

snip
I'm a little intrigued that the two cheaper tools are showing the
the phase terminal is live when there should be no current.


Assuming you mean the switched live, they aren't. You need to
understand the tools you are using and how they interact with what
they are measuring. All of those tools are high impedance and are
voltage measuring devices. What you are seeing is induction from the
permenant live wiring into the open circuit switched live wire. With
high impedance voltage measuring tools it is normal to see this
induction.


Thanks (and to previous respondents); that makes things a bit clearer.
I've been a bit wary of the pendants as I thought they had voltage
when they were switched 'off'. But I couldn't figure where the
voltage was coming from or why the voltage didn't cause current to
flow when I put a bulb in. Very puzzling.
Thanks again.

As a matter of interest do people who know more about electrics than
me (that'll be all of you, then) use these neon screwdrivers? I read
a wiring book the other day which was quite sneery about them, without
being specific about why. I accept your comment that I didn't
understand what it was measuring. As they're sold as a
current-detecting device (ie, to show if a components is live and
likely to shock you) I think they're perhaps mis-sold.

Or is that -more likely, esp in my case- mis-bought?
  #7   Report Post  
mike ring
 
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Default should unswitched live should as live on a neon screwdriver?

(dave L) wrote in
om:

As a matter of interest do people who know more about electrics than
me (that'll be all of you, then) use these neon screwdrivers? I read
a wiring book the other day which was quite sneery about them, without
being specific about why. I accept your comment that I didn't
understand what it was measuring. As they're sold as a
current-detecting device (ie, to show if a components is live and
likely to shock you) I think they're perhaps mis-sold.

Or is that -more likely, esp in my case- mis-bought?

The trouble with neons, and also to some extent digital meters is that
they are fearfully sensitive, and

It's volts wot jolts, but mills wot kills.

Your switched live has travelled close alongside a genuine live inside a
twin and earth all the way from a rose, or junction box.

So 240 v has radiated into it, but only a tiny amount; but enough to
light a neon, which prolly has half a million ohms in series limiting the
current to about half a milliamp, like blackpool front, but not enough
for you to feel.

HTH

mike r

If this wire was connected even very faintly to earth, all these volts
would dribble straight down, but as it is open at bothe ends ( the switch
is off and the bulb which is out - I infer from your last sentence).

But as it isn't quite high voltages may appear, these are completely
harmless, like the static a good old brinylon shirt will generate.

A good oldfashioned meter with a needle is prolly better to work with, or
as Dave said, try a shunt resistor across a DVM.

(sometimes I think they're more trouble than they are worth, it's truee
they will detect a .1 volt change, or check your battery to within about
a microvolt, but they can cause unnecessary alarm and despondency.

mike r

  #8   Report Post  
Dave Plowman
 
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Default should unswitched live should as live on a neon screwdriver?

In article 0,
mike ring wrote:
A good oldfashioned meter with a needle is prolly better to work with,
or as Dave said, try a shunt resistor across a DVM.


(sometimes I think they're more trouble than they are worth, it's truee
they will detect a .1 volt change, or check your battery to within about
a microvolt, but they can cause unnecessary alarm and despondency.


They're really made for electronics use where a high input impedance is
near essential.

There could easily be a case made for selling a much lower impedance
device for mains work.

--
*Errors have been made. Others will be blamed.

Dave Plowman London SW 12
RIP Acorn
  #9   Report Post  
Dave Plowman
 
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Default should unswitched live should as live on a neon screwdriver?

In article ,
dave L wrote:
As they're sold as a current-detecting device (ie, to show if a
components is live and likely to shock you) I think they're perhaps
mis-sold.


Think of them more as a voltage detecting device as they draw so little
current.

And as the old saying goes, it's better to be safe than sorry - if you
switch off at the main when it lights up *anywhere*, you'll be doing the
safe thing.

--
*How many roads must a man travel down before he admits he is lost? *

Dave Plowman London SW 12
RIP Acorn
  #10   Report Post  
Dave Liquorice
 
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Default should unswitched live should as live on a neon screwdriver?

On 15 Nov 2003 14:40:39 -0800, dave L wrote:

As a matter of interest do people who know more about electrics than
me (that'll be all of you, then) use these neon screwdrivers?


I have one and use it occasionally when I can't find my prefered
tester a "Volt Stick". The big plus about the volt stick is that is
non-contact, you can see if a cable is live without needing to be at
an end or cut into it.

As they're sold as a current-detecting device


Voltage detecting device, you could touch it against a cable carrying
a thousand amps at a low voltage and it wouldn't light.

(ie, to show if a components is live and likely to shock you) I
think they're perhaps mis-sold.


Cheap modern ones seem to be much more prone to this "false"
indication than an old (40years) one that my father has. I have never
known that give a false indiaction and I've never felt a tingle from
it either. Both of which I have had from a cheap modern one.

--
Cheers
Dave. pam is missing e-mail





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ARWadsworth
 
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Default should unswitched live should as live on a neon screwdriver?


"Dave Liquorice" wrote in message
. 1...
On 15 Nov 2003 14:40:39 -0800, dave L wrote:

As a matter of interest do people who know more about electrics than
me (that'll be all of you, then) use these neon screwdrivers?


I have one and use it occasionally when I can't find my prefered
tester a "Volt Stick". The big plus about the volt stick is that is
non-contact, you can see if a cable is live without needing to be at
an end or cut into it.


There are times when a neon can be more useful than the volt stick e.g. when
testing inside a central heating control centre. The volt stick picks up too
many signals from adjacent cables.


--
Adam




  #12   Report Post  
dave L
 
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Default should unswitched live should as live on a neon screwdriver?

mike ring wrote in message . 252.50...

A good oldfashioned meter with a needle is prolly better to work with, or
as Dave said, try a shunt resistor across a DVM.

(sometimes I think they're more trouble than they are worth, it's truee
they will detect a .1 volt change, or check your battery to within about
a microvolt, but they can cause unnecessary alarm and despondency.

Can you explain how to use a shunt resistor in this context ? I've
had a search and most the hits are quite advanced enginnering or
electronics references.

However what I thought they meant was that I get a 0.1 ohm resistor
and wire it up between the live and neutral terminals of the pendant
and then measure the current that flows across the resistor. By my
maths (I=V/R = 240/0.1) this would measure (um) 2400A. I'm *know*
that's not what I want to do.

One post (http://tinyurl.com/v744) -which Dave Plowman contributed to
in '01- mentions adapting the multimeter. Is this the only way I can
use the stunt ? That's going too far for me, I think, as I want to
master (ahem) wiring before I get into electronics.

Whatever the method, if it does inolve measuring the current then I
need to head to Maplin as my multimeter doesn't measure AC current
(Gunson Pocketmeter2).

Thanks for your thoughts.
  #13   Report Post  
mike ring
 
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Default should unswitched live should as live on a neon screwdriver?

Dave Plowman wrote in
:

They're really made for electronics use where a high input impedance is
near essential.

There could easily be a case made for selling a much lower impedance
device for mains work.


Absolutely agree; a gorilla's version for Sparks ;-), and a separate shunt
accessory for hi impedance jobbies would do nicely.

Of course as this is ukd-i-y why am I whining instead of making one?

mike r
  #14   Report Post  
Tony Williams
 
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Default should unswitched live should as live on a neon screwdriver?

In article ,
Dave Plowman wrote:

There could easily be a case made for selling a much lower
impedance device for mains work.


How about a Radiospares 259-533?
A 300Vac panel meter, with about a 20k resistance.
13-67 each, but no other components needed.

--
Tony Williams. Change "nospam" to "ledelec" to email.
  #15   Report Post  
Dave Plowman
 
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Default should unswitched live should as live on a neon screwdriver?

In article ,
dave L wrote:
However what I thought they meant was that I get a 0.1 ohm resistor
and wire it up between the live and neutral terminals of the pendant
and then measure the current that flows across the resistor. By my
maths (I=V/R = 240/0.1) this would measure (um) 2400A. I'm *know*
that's not what I want to do.


That would be a shunt for measuring current using a sensitive meter - you
'shunt' the majority of the current across the resistor and the meter
reads the rest in proportion. These sort of shunts will be fitted to the
amps range of any meter, but are rather specialised to make since the
ohmic value can be extremely low and needs to be accurate. They're often
simply a strip of metal.

However, for increasing the current drawn while measuring mains, you need
wire the resistor in parallel with the meter in the same way, but of
course for measuring voltage the combination is wired in parallel with the
mains rather than in series as for current measurement.

If you want to experiment, I'd use a 22000 ohm (22k0) 3 watt resistor from
Maplin (W22K) at 0.37 gbp. This will draw about 10 milliamps and is within
its 3 watt rating.

Obviously, there are safety implications since you'll be measuring mains,
so don't attempt this if you're unsure about how to do it safely. I'd buy
a second set of test leads and incorporate it into these, enclosing it in
a box suitable for the fact that it will also get quite hot with prolonged
use.

--
*Horn broken. - Watch for finger.

Dave Plowman London SW 12
RIP Acorn


  #16   Report Post  
Dave Liquorice
 
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Default should unswitched live should as live on a neon screwdriver?

On 16 Nov 2003 02:03:57 -0800, dave L wrote:

Can you explain how to use a shunt resistor in this context ?


However what I thought they meant was that I get a 0.1 ohm resistor
and wire it up between the live and neutral terminals of the pendant
and then measure the current that flows across the resistor. By my
maths (I=V/R = 240/0.1) this would measure (um) 2400A. I'm *know*
that's not what I want to do.


Certainly. B-)

You need to choose a value that won't drawn exceesive current if you
connect it across the full supply by accident but will draw enough to
generate better readings on open circuit cables.

Whatever the method, if it does inolve measuring the current then I
need to head to Maplin as my multimeter doesn't measure AC current
(Gunson Pocketmeter2).


Generally you rarely need to measure current. Most times you are only
interested in the voltage.

--
Cheers
Dave. pam is missing e-mail



  #17   Report Post  
 
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Default should unswitched live should as live on a neon screwdriver?

Dave Plowman wrote:
In article ,
dave L wrote:
I'm a little intrigued that the two cheaper tools are showing the the
phase terminal is live when there should be no current. My real
concern is that these tools infer there is current flowing to the
terminal with the pendant is swicthed off and that there's a
possibility of getting electrocuted when changing a bulb.


What you're seeing is inductive coupling between the 'live' wire and the


Capacitive more likely.

switched one running next to it. Put the bulb back in and you'll see zero,
as the switched line will be connected to neutral via the much lower
impedance of the bulb.

Things like neon screwdrivers - and DVMs - have a very high input
impedance so will read very low currents that the body wouldn't feel.

It's a good idea to use a low value shunt which draws about 0.1 amp when
using a high impedance test device on mains - or alternately make up a
test lamp using a small mains bulb.

This is one of the reasons that I use my 'cheap' analogue meter when
checking for mains, it's much more likely to give me a useful reading
than a digital meter and is also easier to quickly glance at and
check.

--
Chris Green )
  #18   Report Post  
Zymurgy
 
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Default should unswitched live should as live on a neon screwdriver?

usenet wrote
Dave Plowman wrote:
dave L wrote:
I'm a little intrigued that the two cheaper tools are showing the the
phase terminal is live when there should be no current. My real
concern is that these tools infer there is current flowing to the
terminal with the pendant is swicthed off and that there's a
possibility of getting electrocuted when changing a bulb.


What you're seeing is inductive coupling between the 'live' wire and the


Capacitive more likely.


I saw these floats on unterminated earth wires whilst doing my
electrical installations City & Guilds.

But it took completion of my HND in Electrical and Electronic
Engineering to understand why ;-) [1]

I too reckon it's inductive coupling, not capacitive. Although i'm not
saying that metres and metres of dilectric wouldn't hold "some"
charge.

Cheers,

Paul.

[1] Hell, I could probably calculate it !
  #19   Report Post  
Dave Plowman
 
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Default should unswitched live should as live on a neon screwdriver?

In article ,
Zymurgy wrote:
What you're seeing is inductive coupling between the 'live' wire and
the


Capacitive more likely.


I saw these floats on unterminated earth wires whilst doing my
electrical installations City & Guilds.


But it took completion of my HND in Electrical and Electronic
Engineering to understand why ;-) [1]


I too reckon it's inductive coupling, not capacitive. Although i'm not
saying that metres and metres of dilectric wouldn't hold "some" charge.


I'm not even guessing which it actually is, but if you lay an audio cable
alongside a mains cable - especially one fed from with a dimmer - the
resultant buzz is always called induction.

Perhaps I should have said induced which is suitably vague. Even for this
group. ;-)

--
*Why is the man who invests all your money called a broker? *

Dave Plowman London SW 12
RIP Acorn
  #20   Report Post  
Andy Wade
 
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Default should unswitched live should as live on a neon screwdriver?

"Dave Plowman" wrote in message ...

I'm not even guessing which it actually is,


Definitly capacitive, but the terminology can be confusing because the
effect that we call capacitance comes about because of induced charge
(called "electrostatic induction" in days gone by).

but if you lay an audio cable alongside a mains cable - especially
one fed from with a dimmer - the resultant buzz is always called
induction.


That is inductive coupling since it's the current flowing in the mains
cable, rich in harmonics due to the dimmer, that is causing a small EMF to
be induced in series with your audio circuit.

Perhaps I should have said induced which is suitably vague. Even for
this group. ;-)


There'd still have been a debate.

*Why is the man who invests all your money called a broker? *


Stockbroker: someone who keeps investing your money until it's all gone
[Woody Allen].

--
Andy




  #21   Report Post  
Zymurgy
 
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Default should unswitched live should as live on a neon screwdriver?

"Andy Wade" wrote
"Dave Plowman" wrote

I'm not even guessing which it actually is,


Definitly capacitive


Well, we'll know soon enough, as i've X posted a mail over to
sci.electronics for the definitive answer !!!!

HTH

Paul.
  #22   Report Post  
Andy Wade
 
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Default should unswitched live should as live on a neon screwdriver?

"Zymurgy" wrote in message
om...

Well, we'll know soon enough, as i've X posted a mail over to
sci.electronics for the definitive answer !!!!


I've given you a definitive answer.

--
Andy


  #23   Report Post  
Zymurgy
 
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Default should unswitched live should as live on a neon screwdriver?

"Andy Wade" wrote
"Zymurgy" wrote

Well, we'll know soon enough, as i've X posted a mail over to
sci.electronics for the definitive answer !!!!


I've given you a definitive answer.


Hardly, if you consider a two word answer definitive !

But from what i've read so far, I concede, you're right ;-)

P.
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