Neon Testers and False Leads - Help Please!!
 

Hi Group, probably old hat, but even a one-liner response would help.

The brother-in-law has a holiday home. Mains electricity - but at a
distance.
I am an electrical engineer (retired - the theory type - not so experienced
on the practice).
He asked me to help with sorting the wiring for the lighting in the house.
A long corridor with dual switches for overhead lights. Been there for many
years.
Not sure if it was ever right - he did some amatuer DIY on it and maybe
changed things . . . screwed it up. . .
Some lights come on when switch A is on, others when switch B is on . . .
A mess.
So, I pulled back the wall switches and did some simple probing with a neon
tester.
Some wires showed a bright neon - obviously live.
Some showed no light - not live.

BUT SOME WIRES SHOWED A DULL NEON LIGHT - when other switches were turned on
these became bright.

I has assumed it would be easy - which wires were live - light or no light -
draw the diagrams etc etc etc.

So, my question please. If a neon tester just glows a bit, can this be due
to seeing a neutral with a long run back
to the local supply trafo and hence with some small voltage drop?
Can a neon tester (the screwdriver handle type) register a few volts on the
neutral?

Or am I losing it totally? Missing some point?

Any advice (be gentle, please) appreciated.

Ger

P.S. Plan B is to switch off the supply to the house, isolate all the wires
from the switches and use a meter to trace the wiring.

Neon Testers and False Leads - Help Please!!
 

Neutral can certainly be a few volts above earth. I remember as a
youngster droping the phase and neutral wires from a bit of equipment
to be moved, out of a 3 phase circuit breaker, not realising it didn't
break neutral - and scaring myself when I accidently sparked the
neutral to earth.

Neon Testers and False Leads - Help Please!!
 

Another explanation is that you probed a section of wire between some
combination of live bulbs - ie bulbs that should be in paralell were
switched into series - so you saw a partial voltage.

Neon Testers and False Leads - Help Please!!
 

On Mon, 17 Apr 2006 22:50:50 +0100, "ger" had
this to say:

Hi Group, probably old hat, but even a one-liner response would help.

The brother-in-law has a holiday home. Mains electricity - but at a
distance.
I am an electrical engineer (retired - the theory type - not so experienced
on the practice).
He asked me to help with sorting the wiring for the lighting in the house.
A long corridor with dual switches for overhead lights. Been there for many
years.
Not sure if it was ever right - he did some amatuer DIY on it and maybe
changed things . . . screwed it up. . .
Some lights come on when switch A is on, others when switch B is on . . .
A mess.
So, I pulled back the wall switches and did some simple probing with a neon
tester.
Some wires showed a bright neon - obviously live.
Some showed no light - not live.

BUT SOME WIRES SHOWED A DULL NEON LIGHT - when other switches were turned on
these became bright.

I has assumed it would be easy - which wires were live - light or no light -
draw the diagrams etc etc etc.

So, my question please. If a neon tester just glows a bit, can this be due
to seeing a neutral with a long run back
to the local supply trafo and hence with some small voltage drop?
Can a neon tester (the screwdriver handle type) register a few volts on the
neutral?

No. A neon takes around 90 volts to strike in the first place. What is
likely happening is either a leakage or induction from a phase wire
into the wire you're "testing".

To be honest, you'd be better off with a really old-fashioned
voltmeter which puts a load on a circuit. A digital meter normally has
a very high input impedance and will give misleading results.

Even a test lamp is useful!

--
Frank Erskine

Neon Testers and False Leads - Help Please!!
 

In message , ger
writes
Hi Group, probably old hat, but even a one-liner response would help.

The brother-in-law has a holiday home. Mains electricity - but at a
distance.
I am an electrical engineer (retired - the theory type - not so experienced
on the practice).

Chuck the ****ing neon tester away and use a half decent meter
(not forgetting that the high impedence of a digi meter can also give
you problems)

I'm tempted to say rip it out and start again with wiring of a known
state - no unpleasant surprises


--
geoff

Neon Testers and False Leads - Help Please!!
 

ger wrote:

So, I pulled back the wall switches and did some simple probing with a neon
tester.

Poxy dangerous things! (as you have discovered)

Some wires showed a bright neon - obviously live.

Not always...

Some showed no light - not live.

Not always...

BUT SOME WIRES SHOWED A DULL NEON LIGHT - when other switches were turned on
these became bright.

Not uncommon...

So, my question please. If a neon tester just glows a bit, can this be due
to seeing a neutral with a long run back

It can be seeing a unconnected wire that is running a distance close to
a live wire and is hence capacitively coupled to it.

Two way lighting circuits are exactly the sort of place you tend to find
this and hence these things are no use to man nor beast in fault
finding. Even a digital volt meter can give you spurious results. An old
analogue one will tend to be more accurate since it will normally place
enough load on the line to dicharge the (tiny) capacitance.

to the local supply trafo and hence with some small voltage drop?
Can a neon tester (the screwdriver handle type) register a few volts on the
neutral?

Unlikely to be enough voltage on a neutral to light it. You are correct
that a neutral can and should be treated as a live - in that it can be
held several volts above ground and with a reasonably low impedance - so
significant current can flow from say neutral to earth in some
circumstances.

Or am I losing it totally? Missing some point?

Only that neon screwdrivers are the work of santa!

Any advice (be gentle, please) appreciated.

Dig out a circus diagram for a two way lighting circuit (there are a few
ways of doing it - my favourite wires the live and swtich return across
L1 and L2 on the nearest switch and then simply joins L1, L2, and COM,
to the other switch using some three core and earth). Work through it
methodicaly with a clunky old multimeter (in fact a digitial will often
work in that you can usually see the difference between real 240V live
and a coupled but floating wire)

P.S. Plan B is to switch off the supply to the house, isolate all the wires
from the switches and use a meter to trace the wiring.

That might work as well. ;-)

--
Cheers,

John.

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

Neon Testers and False Leads - Help Please!!
 

"John Rumm" wrote in message
...
ger wrote:

So, I pulled back the wall switches and did some simple probing with a
neon tester.

Poxy dangerous things! (as you have discovered)

Some wires showed a bright neon - obviously live.

Not always...

Some showed no light - not live.

Not always...

BUT SOME WIRES SHOWED A DULL NEON LIGHT - when other switches were turned
on these became bright.

Not uncommon...

So, my question please. If a neon tester just glows a bit, can this be
due to seeing a neutral with a long run back

It can be seeing a unconnected wire that is running a distance close to a
live wire and is hence capacitively coupled to it.

Two way lighting circuits are exactly the sort of place you tend to find
this and hence these things are no use to man nor beast in fault finding.
Even a digital volt meter can give you spurious results. An old analogue
one will tend to be more accurate since it will normally place enough load
on the line to dicharge the (tiny) capacitance.

to the local supply trafo and hence with some small voltage drop?
Can a neon tester (the screwdriver handle type) register a few volts on
the neutral?

Unlikely to be enough voltage on a neutral to light it. You are correct
that a neutral can and should be treated as a live - in that it can be
held several volts above ground and with a reasonably low impedance - so
significant current can flow from say neutral to earth in some
circumstances.

Or am I losing it totally? Missing some point?

Only that neon screwdrivers are the work of santa!

Any advice (be gentle, please) appreciated.

Dig out a circus diagram for a two way lighting circuit (there are a few
ways of doing it - my favourite wires the live and swtich return across L1
and L2 on the nearest switch and then simply joins L1, L2, and COM, to the
other switch using some three core and earth). Work through it methodicaly
with a clunky old multimeter (in fact a digitial will often work in that
you can usually see the difference between real 240V live and a coupled
but floating wire)

P.S. Plan B is to switch off the supply to the house, isolate all the
wires from the switches and use a meter to trace the wiring.

That might work as well. ;-)

--
Cheers,

John.

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

Many thanks to all who replied. Problem now fully understood.
Neon testers now reduced to the role of screwdriver - intestines removed.

Ger - a happy camper!