I have one circuit in my home that I am getting 28 vols through the bare
copper ground wire. This circuit has severallights on it. Any ideas what the
problem could be? Even with the breaker off I am getting voltage. The
electrical panel looks ok.
Ed

Ed Varin wrote:
I have one circuit in my home that I am getting 28 vols through the
bare copper ground wire. This circuit has severallights on it. Any
ideas what the problem could be? Even with the breaker off I am
getting voltage. The electrical panel looks ok.
Ed

First exactly how are you measuring this voltage? Between the ground
wire and what?? What are you using to measure it? Why did you measure it?
I am suspecting no real problems, but there is one thing that I can think of
that could be a problem depending on the answers to my questions.

--
Joseph Meehan

Dia duit

Also, I am reading 28 volts at the light fixture, this is between the ground
wire and either the white or black wire. There is no reading between the
black and white wire when the breaker is off.
Ed
"Ed Varin" wrote in message
news:hU0Kf.4714$Ug4.433@dukeread12...
Joseph,
This circuit operates a light that can be turned on and off from two
locations. With the power off I was changing a light switch when I kept
getting a little "tickle" every time I toched the ground wire. I took a
multi meter and measure the voltage from the ground to any other wire in
the switch with all wires disconnected from the switch. The ground was the
only wire with power in it. The was no voltage between the other wires. I
go through light bulbs often on this circuit (every 6-8 weeks).
Ed
"Joseph Meehan" wrote in message
. ..
Ed Varin wrote:
I have one circuit in my home that I am getting 28 vols through the
bare copper ground wire. This circuit has severallights on it. Any
ideas what the problem could be? Even with the breaker off I am
getting voltage. The electrical panel looks ok.
Ed

First exactly how are you measuring this voltage? Between the ground
wire and what?? What are you using to measure it? Why did you measure
it? I am suspecting no real problems, but there is one thing that I can
think of that could be a problem depending on the answers to my
questions.

--
Joseph Meehan

Dia duit

Ed Varin wrote:

Also, I am reading 28 volts at the light fixture, this is between the ground
wire and either the white or black wire. There is no reading between the
black and white wire when the breaker is off.
Ed

You probably have a (potentially dangerous) "floating (safety) ground."

If you're a novice, this would be a good time to call an electrician.

"Ed Varin" wrote in message
news:hU0Kf.4714$Ug4.433@dukeread12...

Joseph,
This circuit operates a light that can be turned on and off from two
locations. With the power off I was changing a light switch when I kept
getting a little "tickle" every time I toched the ground wire. I took a
multi meter and measure the voltage from the ground to any other wire in
the switch with all wires disconnected from the switch. The ground was the
only wire with power in it. The was no voltage between the other wires. I
go through light bulbs often on this circuit (every 6-8 weeks).
Ed
"Joseph Meehan" wrote in message
m...

Ed Varin wrote:

I have one circuit in my home that I am getting 28 vols through the
bare copper ground wire. This circuit has severallights on it. Any
ideas what the problem could be? Even with the breaker off I am
getting voltage. The electrical panel looks ok.
Ed

First exactly how are you measuring this voltage? Between the ground
wire and what?? What are you using to measure it? Why did you measure
it? I am suspecting no real problems, but there is one thing that I can
think of that could be a problem depending on the answers to my
questions.

--
Joseph Meehan

Dia duit







--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

"Ed Varin" wrote in message
news:g90Kf.4710$Ug4.3161@dukeread12...
:I have one circuit in my home that I am getting 28 vols through
the bare
: copper ground wire. This circuit has severallights on it. Any
ideas what the
: problem could be? Even with the breaker off I am getting
voltage. The
: electrical panel looks ok.
: Ed
:
:
It sounds like you're reading a phantom voltage, one which is
induced into the wiring by stray fields, and quite harmless. But
you really should prove that. You really didn't give enough
details to make a sound judgement.

One way to prove phantom voltage would be to turn on one of the
light switches on the ckt. If it goes completely away, it's just
a phantom voltage. If there is still any voltage left, then it's
time to call a repairman or at least a friend who knows how to
tell when he's found a phantom voltage: It's very, very common,
especially when high impedance meters are used for the
measurement.
Someone asked a bunch of questions that I don't yet see
answered; that would help a lot.

Pop

Pop wrote:
"Ed Varin" wrote in message
news:g90Kf.4710$Ug4.3161@dukeread12...
:I have one circuit in my home that I am getting 28 vols through
the bare
: copper ground wire. This circuit has severallights on it. Any
ideas what the
: problem could be? Even with the breaker off I am getting
voltage. The
: electrical panel looks ok.
: Ed
:
:
It sounds like you're reading a phantom voltage, one which is
induced into the wiring by stray fields, and quite harmless. But
you really should prove that. You really didn't give enough
details to make a sound judgement.

One way to prove phantom voltage would be to turn on one of the
light switches on the ckt. If it goes completely away, it's just
a phantom voltage. If there is still any voltage left, then it's
time to call a repairman or at least a friend who knows how to
tell when he's found a phantom voltage: It's very, very common,
especially when high impedance meters are used for the
measurement.
Someone asked a bunch of questions that I don't yet see
answered; that would help a lot.

Pop

Perhaps but is not an induced "phantom voltage" not enough to give one
the "tickle"

IMO anytime you get a stray voltage reading AND the "tickle / tingle"
feeeling, the problem is real.

cheers
Bob

"BobK207" wrote in message
oups.com...
:
: Pop wrote:
: "Ed Varin" wrote in message
: news:g90Kf.4710$Ug4.3161@dukeread12...
: :I have one circuit in my home that I am getting 28 vols
through
: the bare
: : copper ground wire. This circuit has severallights on it.
Any
: ideas what the
: : problem could be? Even with the breaker off I am getting
: voltage. The
: : electrical panel looks ok.
: : Ed
: :
: :
: It sounds like you're reading a phantom voltage, one which is
: induced into the wiring by stray fields, and quite harmless.
But
: you really should prove that. You really didn't give enough
: details to make a sound judgement.
:
: One way to prove phantom voltage would be to turn on one of
the
: light switches on the ckt. If it goes completely away, it's
just
: a phantom voltage. If there is still any voltage left, then
it's
: time to call a repairman or at least a friend who knows how
to
: tell when he's found a phantom voltage: It's very, very
common,
: especially when high impedance meters are used for the
: measurement.
: Someone asked a bunch of questions that I don't yet see
: answered; that would help a lot.
:
: Pop
:
: Perhaps but is not an induced "phantom voltage" not enough to
give one
: the "tickle"
:
: IMO anytime you get a stray voltage reading AND the "tickle /
tingle"
: feeeling, the problem is real.
:
: cheers
: Bob
:

True enough, but ... 28Vac shouldn't be anything most people
would feel, but ... that would be a good indication of another
problem. I don't recall anything about it "tickling" the OP, so
I must ahve missed that part.
Putting a load on the ckt, eg turning on one of the lights
should show whether it's got any power behind it though. A solid
28Vac wouldn't light it up, but it would/should draw the voltage
down to nothing if it's phantom; if not, then it's a very real
problem, for sure.
IMO, there's too little info to make any good analysis.

Pop

Pop wrote:

"Ed Varin" wrote in message
news:g90Kf.4710$Ug4.3161@dukeread12...
:I have one circuit in my home that I am getting 28 vols through
the bare
: copper ground wire. This circuit has severallights on it. Any
ideas what the
: problem could be? Even with the breaker off I am getting
voltage. The
: electrical panel looks ok.
: Ed
:
:
It sounds like you're reading a phantom voltage, one which is
induced into the wiring by stray fields, and quite harmless. But
you really should prove that. You really didn't give enough
details to make a sound judgement.

On the safety ground? That seems pretty unlikely to me. The safety
ground should have a low impedance path to earth.

One way to prove phantom voltage would be to turn on one of the
light switches on the ckt. If it goes completely away, it's just
a phantom voltage. If there is still any voltage left, then it's
time to call a repairman or at least a friend who knows how to
tell when he's found a phantom voltage: It's very, very common,
especially when high impedance meters are used for the
measurement.
Someone asked a bunch of questions that I don't yet see
answered; that would help a lot.

Pop






--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

Pop wrote:

snip28Vac shouldn't be anything most people
would feel
snip

I disagree. You can get quite a jolt from even a 12 volt source.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

How? I have been handling 12 volts and more for all my life and never felt
anything. However, I understand that if you inject or puncture the skin with
the wire, you can feel it, it can even be dangerous in this manner.

"CJT" wrote in message
...
Pop wrote:

snip28Vac shouldn't be anything most people
would feel
snip

I disagree. You can get quite a jolt from even a 12 volt source.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

"Pop" wrote in message
news:9Q2Kf.4940$Td2.488@trndny06...

"Ed Varin" wrote in message
news:g90Kf.4710$Ug4.3161@dukeread12...
:I have one circuit in my home that I am getting 28 vols through
the bare
: copper ground wire. This circuit has severallights on it. Any
ideas what the
: problem could be? Even with the breaker off I am getting
voltage. The
: electrical panel looks ok.
: Ed
:
:
It sounds like you're reading a phantom voltage, one which is
induced into the wiring by stray fields, and quite harmless. But
you really should prove that. You really didn't give enough
details to make a sound judgement.

One way to prove phantom voltage would be to turn on one of the
light switches on the ckt. If it goes completely away, it's just
a phantom voltage. If there is still any voltage left, then it's
time to call a repairman or at least a friend who knows how to
tell when he's found a phantom voltage: It's very, very common,
especially when high impedance meters are used for the
measurement.
Someone asked a bunch of questions that I don't yet see
answered; that would help a lot.

Pop

Yes, phantom voltage can be puzzling to some. I can remember my first high
impedence VTVM, I found that it could read voltage between my left and and
my right hand. I guess this is one of the principals of a lie detector.

Ed Varin wrote:
Also, I am reading 28 volts at the light fixture, this is between the
ground wire and either the white or black wire. There is no reading
between the black and white wire when the breaker is off.
Ed

I don't like that tingle. It is a sign that you could be in for a real
shock, maybe the shock of your life. There are several possibilities and
two of them can be dangerous and one is safe. I wanted to know exactly what
kind of meter you used. The older multi meters would not have picked up on
the third type of voltage but most of the new digital meters will. This one
can give you a tingle but is very unlikely to do any damage.

I will have to go on with the other two. One is a floating neutral that
is transferring over to the ground some how. How I don't know but it would
tend to indicate a second fault as well. This one is serious and is often
accompanied by lights that seem to brighten and dim for no reason. The
other is a failed ground and that could kill if you get between it and a
proper ground.

Finding these faults is not all that difficult if you have a good idea
of wiring, but if you don't it is best left to the professional. Remember
that it is an indication of something that is not right in two out of three
cases. That means you can't count on things that might usually be safe
being safe. I suggest you call in the pro.


"Ed Varin" wrote in message
news:hU0Kf.4714$Ug4.433@dukeread12...
Joseph,
This circuit operates a light that can be turned on and off from two
locations. With the power off I was changing a light switch when I
kept getting a little "tickle" every time I toched the ground wire.
I took a multi meter and measure the voltage from the ground to any
other wire in the switch with all wires disconnected from the
switch. The ground was the only wire with power in it. The was no
voltage between the other wires. I go through light bulbs often on
this circuit (every 6-8 weeks). Ed
"Joseph Meehan" wrote in message
. ..
Ed Varin wrote:
I have one circuit in my home that I am getting 28 vols through the
bare copper ground wire. This circuit has severallights on it. Any
ideas what the problem could be? Even with the breaker off I am
getting voltage. The electrical panel looks ok.
Ed

First exactly how are you measuring this voltage? Between the
ground wire and what?? What are you using to measure it? Why did
you measure it? I am suspecting no real problems, but there is one
thing that I can think of that could be a problem depending on the
answers to my questions.

--
Joseph Meehan

Dia duit

--
Joseph Meehan

Dia duit

On Sun, 19 Feb 2006 19:49:28 GMT, "Pop"
wrote:
"BobK207" wrote in message :
: Perhaps but is not an induced "phantom voltage" not enough to
:give one: the "tickle"

Glad you have the second sentence, because this one had too many
"not"s for me to understand it.
:
: IMO anytime you get a stray voltage reading AND the "tickle /
tingle" feeeling, the problem is real.
:
: cheers Bob
:
Putting a load on the ckt, eg turning on one of the lights
should show whether it's got any power behind it though. A solid
28Vac wouldn't light it up, but it would/should draw the voltage

So all I would have to do in this situation would be to connect a 110V
lightbulb from the ground wire to {another actual ground / the white
wire / the black wire / ??} and keep the meter at the same points?

If the voltage drops to zero, no problem. If it stay about 10v real
problem. ??

I've never noticed this problem so I don't want to buy a wiggy
(willy?) for only one use.

Could I just use a low impedance meter, like one with a needle that is
20,000 or 50,000 ohms per volt. Would that load make the phantom
voltage disappear?


down to nothing if it's phantom; if not, then it's a very real
problem, for sure.
IMO, there's too little info to make any good analysis.

Pop



Remove NOPSAM to email me. Please let
me know if you have posted also.

First, excercise caution. Avoid touching wires, ensure your circuits
are properly disconnected. Sounds like common sense, but a lot of
people (especially the experienced ones) ignore it.

I'm not certain what the required methods of running multi-phased
circuits using single neutrals are. Perhaps someone can let us know if
this is/was common practice for house hold wiring. I have gotten the
general impression its not. Just so your informed, it is possible to
run multiple hots on various phases using 1 neutral. If you open 1 hot
this will not cut the power on the neutral completly.

At this point I'll assume your taking an interest in how to solve the
dilemma, or find out exactly what is wrong. So I won't pester you with
the "shouldn't do it" concerns others may have.

A suggestion which no one seems to have offered is to compare the
resistance of 1 ground with another seperate ground run. It would seem
to me that if I had an open on a ground wire then I wouldn't see a low
impedance if I checked against another ground wire.

Seeing how I can't stand dealing with stray voltages at the best of
timies, I'll simply stay away from this item. (I generally deal with
low voltage signalling wire, and find that using a drain wire helps
reduce stray voltages for my applications.)

JW

On Sun, 19 Feb 2006 20:41:00 GMT, CJT wrote:

Pop wrote:

snip28Vac shouldn't be anything most people
would feel
snip

I disagree. You can get quite a jolt from even a 12 volt source.

Under what conditions?
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Democracy is two wolves and a lamb voting on what
to have for lunch. Liberty is a well armed lamb
contesting the vote." - Benjamin Franklin

Mark Lloyd wrote:

On Sun, 19 Feb 2006 20:41:00 GMT, CJT wrote:


Pop wrote:

snip28Vac shouldn't be anything most people

would feel

snip

I disagree. You can get quite a jolt from even a 12 volt source.


Under what conditions?

moist skin (e.g. sweat)

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

"Ed Varin" wrote in message
news:g90Kf.4710$Ug4.3161@dukeread12...
I have one circuit in my home that I am getting 28 vols through the bare
copper ground wire. This circuit has severallights on it. Any ideas what
the
problem could be? Even with the breaker off I am getting voltage. The
electrical panel looks ok.
Ed



A few things I would do: Check the voltage between that suspect ground wire
and the nearest water pipe. Also check the voltage between the other wires
in the switch box and the water pipe. Next I would shut each circuit
breaker off in the panel one at a time and check to see if the voltage
reading changes.

I would also go to a different part of the house on a different circuit and
check the voltage in the same manner with the circuit off.

Let us know what you find.

How many wires are in the switch box?

I'm thinking that this particular circuit may be part of a three wire
circuit. The voltage that you are reading is actually coming from the
neutral because there is a load on the other leg. By shutting off the other
circuit breaker the voltage should almost disappear. If this is the case I
recommend that you tighten all of the neutral and ground connections in the
electrical panel. There could also be a neutral connection somewhere on the
circuit that needs tightening such as a receptacle.


John Grabowski
http://www.mrelectrician.tv

Ed Varin wrote:
Also, I am reading 28 volts at the light fixture, this is between the ground
wire and either the white or black wire. There is no reading between the
black and white wire when the breaker is off.
Ed

That is a clear indication that your ground wire is not properly
connected. If the breaker is on you should see 110V between ground and
the black wire, and 0 volts between ground and the white wire. Trace
that ground wire back and see if you still get the voltage at upstream
boxes.




"Ed Varin" wrote in message
news:hU0Kf.4714$Ug4.433@dukeread12...

Joseph,
This circuit operates a light that can be turned on and off from two
locations. With the power off I was changing a light switch when I kept
getting a little "tickle" every time I toched the ground wire. I took a
multi meter and measure the voltage from the ground to any other wire in
the switch with all wires disconnected from the switch. The ground was the
only wire with power in it. The was no voltage between the other wires. I
go through light bulbs often on this circuit (every 6-8 weeks).
Ed
"Joseph Meehan" wrote in message
m...

Ed Varin wrote:

I have one circuit in my home that I am getting 28 vols through the
bare copper ground wire. This circuit has severallights on it. Any
ideas what the problem could be? Even with the breaker off I am
getting voltage. The electrical panel looks ok.
Ed

First exactly how are you measuring this voltage? Between the ground
wire and what?? What are you using to measure it? Why did you measure
it? I am suspecting no real problems, but there is one thing that I can
think of that could be a problem depending on the answers to my
questions.

--
Joseph Meehan

Dia duit







--
Thank you,



"Then said I, Wisdom [is] better than strength: nevertheless the poor
man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16

According to Justin West :
First, excercise caution. Avoid touching wires, ensure your circuits
are properly disconnected. Sounds like common sense, but a lot of
people (especially the experienced ones) ignore it.

I'm not certain what the required methods of running multi-phased
circuits using single neutrals are. Perhaps someone can let us know if
this is/was common practice for house hold wiring. I have gotten the
general impression its not.

It's fairly common. At least until recently, all Canadian kitchen counter
outlets HAD to be done with "multiwire branch circuits" (CEC terminology
for multiple hots and one neutral).

The main techniques of importance with multiwire branches is that you
ALWAYS pigtail neutrals.

Just so your informed, it is possible to
run multiple hots on various phases using 1 neutral. If you open 1 hot
this will not cut the power on the neutral completly.

If your ground and neutral are properly connected back to the panel,
the most you're going to see (in terms of a neutral voltage relative
to ground) is about 2-3V (circuit delivering max load). The ground should
be unaffected.

If you _ever_ see a neutral-ground voltage of more than 2 or 3 volts,
then something is definately wrong. "stray" (aka "induced" aka "phantom")
voltages are indeed common when you use high impedance voltmeters - but these
will _never_ occur between conductors that are currently "in circuit".

In a correctly operating circuit, the only time you'll see a phantom voltage
is on a switched off hot. You can't conclude anything from such a measurement
unless you can load the hot enough to short out the stray component.

In contrast, ground and neutral are supposed to be interconnected (back at the
main), and neither are EVER supposed to be switched. Thus, if you see more than
a couple volts between a ground and a neutral, then one of them is broken.
The voltage _may_ be "stray impedance". Or it may be something with
real punch behind it. Either way, there _is_ a problem.

One way to distinguish whether the problem is with the ground or the neutral
is to measure the voltage between the offending ground, and something
that's grounded on another circuit (or often a copper pipe). If you still
see voltage, the ground is broken. If not, it's the neutral. If it's the
neutral, the circuit won't be working at all. If it's the ground, certain
devices on it (eg: metal-cased 3-wire devices) may have live cases.

Further, anything that gives you a tingle ain't stray impedance. It's
been my experience that "tingle" is almost always a sign of "[near] full
line current available, the resistance of your shoes are saving you
from being thrown across the room".
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

According to mm :

Could I just use a low impedance meter, like one with a needle that is
20,000 or 50,000 ohms per volt. Would that load make the phantom
voltage disappear?

Yes. But as I mentioned elsewhere, phantom voltage or not,
measuring voltage between ground and neutral means there's a very
real problem.

Grabowski mentions the possibility of a loose neutral in a shared
neutral circuit. That indeed is possible. However, I think it
more likely that somewhere upstream of that outlet there's
a poor ground connection. Many not-quite electricians are quite
sloppy at connecting grounds together.

A loose neutral in a shared neutral circuit has other side effects.
Like having lights brighten when you turn something else on. The
circuit will seem "sick" in other ways. Besides, shared neutral
circuits are usually only a tiny fraction of the circuits in
a home.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

Chris Lewis wrote:
According to mm :

Could I just use a low impedance meter, like one with a needle that is
20,000 or 50,000 ohms per volt. Would that load make the phantom
voltage disappear?

Yes. But as I mentioned elsewhere, phantom voltage or not,
measuring voltage between ground and neutral means there's a very
real problem.

Grabowski mentions the possibility of a loose neutral in a shared
neutral circuit. That indeed is possible. However, I think it
more likely that somewhere upstream of that outlet there's
a poor ground connection. Many not-quite electricians are quite
sloppy at connecting grounds together.

A loose neutral in a shared neutral circuit has other side effects.
Like having lights brighten when you turn something else on. The
circuit will seem "sick" in other ways. Besides, shared neutral
circuits are usually only a tiny fraction of the circuits in
a home.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

This could be the result of an amateur "fix" to a serious problem. As I
mentioned before, (if you've heard this stop me) I discovered the hard
way that the pigtail ground on the outlet in the light fixture above
the kitchen sink in a place I rented had been connected to the hot
lead; puzzled me no end since the mounting of the outlet connected the
whole fixture to that ground wire, which connected the whole bx cable,
etc. until I went into the basement and found that the BX had been
disconnected from the box at the other end, so it was all hot. Either
an attempted murder, or a really bad amateur. Knowing my landlord, I
decided the latter was more likely.

So, in your case somebody elxe could have "fixed" a problem of a leaky
partial short to the ground wire somewhere by simply disconnecting the
ground.

On Mon, 20 Feb 2006 01:50:09 GMT, CJT wrote:

Mark Lloyd wrote:

On Sun, 19 Feb 2006 20:41:00 GMT, CJT wrote:


Pop wrote:

snip28Vac shouldn't be anything most people

would feel

snip

I disagree. You can get quite a jolt from even a 12 volt source.


Under what conditions?

moist skin (e.g. sweat)

12 volts can give you an even stronger feeling if applied directly to
the tongue. 12V can hurt, but this is a common way of testing 9V
batteries. A good one tastes like lemon juice without the lemon.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Democracy is two wolves and a lamb voting on what
to have for lunch. Liberty is a well armed lamb
contesting the vote." - Benjamin Franklin

On Mon, 20 Feb 2006 11:56:46 -0600, Bud--
wrote:

Chris Lewis wrote:
According to mm :


Could I just use a low impedance meter, like one with a needle that is
20,000 or 50,000 ohms per volt. Would that load make the phantom
voltage disappear?


Yes. But as I mentioned elsewhere, phantom voltage or not,
measuring voltage between ground and neutral means there's a very
real problem.

OK, you did say that.

Disappear? Not necessarily. On a 150 volt scale a 20,000 ohm per volt
meter has a resistance of 3,000,000 ohms. A hot-ground 'leak' of 12 meg
ohms would give reading of 28 volts from 'ground' to ground!.

OK. I'll abandon that idea. Thanks

bud--


Remove NOPSAM to email me. Please let
me know if you have posted also.

Very good response. At least for me, as I was actually able to
understand it.

However, I do have a followup question. I recently replaced a fire
alarm system in a condo tower which had the amplifier stack, fire alarm
system, and security panel on 3 seperate hots but used one neutral.

I opened the disconnect for the ampifier stack, and fire alarm panel,
and measured from hot to neutral (0V), Hot to ground (0 volts), neutral
to ground (0v). When my assistant opened the neutral he noticed some
arcing as the security system was still live. Thus I learned about the
multi-wire branched circuits.

How would i go about knowing if a neutral is live or not? As all my
measurements above, and (gasp, I know it's not a good method) my magic
wand fluke voltage checker all showed 0V.

Thanks, JW

According to Justin West :
Very good response. At least for me, as I was actually able to
understand it.

However, I do have a followup question. I recently replaced a fire
alarm system in a condo tower which had the amplifier stack, fire alarm
system, and security panel on 3 seperate hots but used one neutral.

I opened the disconnect for the ampifier stack, and fire alarm panel,
and measured from hot to neutral (0V), Hot to ground (0 volts), neutral
to ground (0v). When my assistant opened the neutral he noticed some
arcing as the security system was still live. Thus I learned about the
multi-wire branched circuits.

How would i go about knowing if a neutral is live or not? As all my
measurements above, and (gasp, I know it's not a good method) my magic
wand fluke voltage checker all showed 0V.

You can't, really. The best you could do is use some sort of current
detector (eg: clamp-on ammeter) to look for current flow in the neutral
before you disconnect it. Alarm systems may be carrying quite low current,
and a clampon ammeter might not register that low.

"Shared neutral" is usually used to refer to a 4 wire 240V circuit, where
you share the neutral between the two hots. Or a 5 wire 3 phase circuit,
where you share the neutral between the three hots. In these circuits,
the neutral is only carrying the _difference_ between the hot currents.

When you share neutral between two hots on the same leg of the panel,
it's possible to overload the neutral, because the neutral current is
the sum of both hots, not the difference.

The two paragraphs above are because I _suspect_ that you're not really
seeing a shared neutral - especially because of the reference to three
hots - unless the alarm system is hanging off a 3 phase circuit.

What I more suspect that it's actually a _single_ 120V circuit, you
disconnected an intermediate junction, and the "disconnect" only
disconnected _part_ of the circuit, whereas the neutral fed thru
to more equipment that got its feed _before_ the disconnect. Eg:
the downstream equipment "feeds through" where you disconnected, but
the downstream hot didn't.

Mind you, I've not worked with large multi-unit buildings, so maybe
they are wired differently.

Shared neutral circuits are supposed to be wired such that you
can remove a device in the middle of the circuit without
interrupting the neutral or ground. Eg: pigtails.

If you're working on a suspected shared neutral circuit, when you have
the slightest doubt, you should take the time necessary to track down
where all the hots are, and kill them all at the panel. I personally
will not wire shared neutral circuits without a tied breaker, despite the
fact that code doesn't always require it.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.