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Old June 5th 15, 03:20 AM posted to rec.crafts.metalworking
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Default Phantom voltage in a disconnected busbar?

I was recently pulling some extremely small amount of MCM 500 copper
cables. Of course, it was legitimately purchased from the owner of
that building.

There was this one particular panel that was always live (hooked up to
service drop) and under power.

The panel had two 800A switch boxes. The boxes were fed through panel
busbars, with a big handle switch, then fuses, then terminals for MCM 500
cables.

Both boxes were in the same condition, namely:

1) The switch was in the OFF position
2) The outgoing cables were already removed.
3) The whole panel was 480 volts

There was literally NO incoming power that was not disconnected,
back-fed, or anything.

And yet, on one box out of two, my "Sperry voltage tester" registered
live voltage.

I then re-checked it with a voltmeter and found "40 volts" (sic). And
thi sis in a 480 volt panel!

I needed to touch those busbars (switched off, remember) to unbolt and
remove the fuses, since the fuses are $100 for 3. Out of abundance of
caution, I brought in a wooden pallet, stood on wood, used a socket
extension and was very careful not to touch anything, and everything
worked out OK.

But I am still wondering how, on a disconnected busbar, I would read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.

i

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Old June 5th 15, 03:26 AM posted to rec.crafts.metalworking
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Posts: 4,632
Default Phantom voltage in a disconnected busbar?

Ignoramus22165 fired this volley in
:

But I am still wondering how, on a disconnected busbar, I would read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.


The real first question is, "what was your reference"?

Then, following that up some:
Were you measuring bar to 'local ground', bar to bar, bar to box neutral?
Was this single-phase, or three-phase?

What's the input impedance of your meter? Did you try a microscopic load
-- like (say) a couple-hundred K-ohms across the meter leads? You might
have a very high input impedance meter, and just be measuring "antenna
effect" to some nearby physically parallel live circuit.

There might be the residue from an old arc in that box that left some
slightly-conductive metal/metal oxides plated in and around the
insulators of the box.

Lloyd
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Old June 5th 15, 03:50 AM posted to rec.crafts.metalworking
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Posts: 5,532
Default Phantom voltage in a disconnected busbar?


"Ignoramus22165" wrote in
message ...
I was recently pulling some extremely small amount of MCM 500 copper
cables. Of course, it was legitimately purchased from the owner of
that building.

There was this one particular panel that was always live (hooked up
to
service drop) and under power.

The panel had two 800A switch boxes. The boxes were fed through
panel
busbars, with a big handle switch, then fuses, then terminals for
MCM 500
cables.

Both boxes were in the same condition, namely:

1) The switch was in the OFF position
2) The outgoing cables were already removed.
3) The whole panel was 480 volts

There was literally NO incoming power that was not disconnected,
back-fed, or anything.

And yet, on one box out of two, my "Sperry voltage tester"
registered
live voltage.

I then re-checked it with a voltmeter and found "40 volts" (sic).
And
thi sis in a 480 volt panel!

I needed to touch those busbars (switched off, remember) to unbolt
and
remove the fuses, since the fuses are $100 for 3. Out of abundance
of
caution, I brought in a wooden pallet, stood on wood, used a socket
extension and was very careful not to touch anything, and everything
worked out OK.

But I am still wondering how, on a disconnected busbar, I would read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.

i


Perhaps you saw a difference in ground potentials. The ground (Earth)
is a return path and not always a good one.

I've measured 30A flowing through a water pipe when chasing a problem
that turned out to be a corroded neutral splice at the weatherhead.

-jsw


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Old June 5th 15, 03:57 AM posted to rec.crafts.metalworking
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First recorded activity by DIYBanter: Jun 2015
Posts: 3
Default Phantom voltage in a disconnected busbar?

On 2015-06-05, Lloyd E. Sponenburgh lloydspinsidemindspring.com wrote:
Ignoramus22165 fired this volley in
:

But I am still wondering how, on a disconnected busbar, I would read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.


The real first question is, "what was your reference"?


Bus to bus and bus to ground

Then, following that up some: Were you measuring bar to 'local
ground', bar to bar, bar to box neutral? Was this single-phase, or
three-phase?


All of them had voltage and to ground also.

What's the input impedance of your meter? Did you try a microscopic load
-- like (say) a couple-hundred K-ohms across the meter leads? You might
have a very high input impedance meter, and just be measuring "antenna
effect" to some nearby physically parallel live circuit.


This is my own explanation. I did not want to stick anything across
the leads, however.

There might be the residue from an old arc in that box that left some
slightly-conductive metal/metal oxides plated in and around the
insulators of the box.


This is something that I did not consider, but it also makes good
sense.

It would also provide for serious arc flash hazard, if
disconnected under load.

480 volts + 500 kVa + big busbars == arc flash hazard

i
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Old June 5th 15, 04:04 AM posted to rec.crafts.metalworking
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Posts: 3
Default Phantom voltage in a disconnected busbar?

On 2015-06-05, Jim Wilkins wrote:

"Ignoramus22165" wrote in
message ...
I was recently pulling some extremely small amount of MCM 500 copper
cables. Of course, it was legitimately purchased from the owner of
that building.

There was this one particular panel that was always live (hooked up
to
service drop) and under power.

The panel had two 800A switch boxes. The boxes were fed through
panel
busbars, with a big handle switch, then fuses, then terminals for
MCM 500
cables.

Both boxes were in the same condition, namely:

1) The switch was in the OFF position
2) The outgoing cables were already removed.
3) The whole panel was 480 volts

There was literally NO incoming power that was not disconnected,
back-fed, or anything.

And yet, on one box out of two, my "Sperry voltage tester"
registered
live voltage.

I then re-checked it with a voltmeter and found "40 volts" (sic).
And
thi sis in a 480 volt panel!

I needed to touch those busbars (switched off, remember) to unbolt
and
remove the fuses, since the fuses are $100 for 3. Out of abundance
of
caution, I brought in a wooden pallet, stood on wood, used a socket
extension and was very careful not to touch anything, and everything
worked out OK.

But I am still wondering how, on a disconnected busbar, I would read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.

i


Perhaps you saw a difference in ground potentials. The ground (Earth)
is a return path and not always a good one.


It was phase to phase and phase to ground.

I've measured 30A flowing through a water pipe when chasing a problem
that turned out to be a corroded neutral splice at the weatherhead.



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Old June 5th 15, 05:04 AM posted to rec.crafts.metalworking
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Posts: 678
Default Phantom voltage in a disconnected busbar?

On 6/4/2015 8:26 PM, Lloyd E. Sponenburgh wrote:
Ignoramus22165 fired this volley in
:

But I am still wondering how, on a disconnected busbar, I would read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.


The real first question is, "what was your reference"?

Then, following that up some:
Were you measuring bar to 'local ground', bar to bar, bar to box neutral?
Was this single-phase, or three-phase?

What's the input impedance of your meter? Did you try a microscopic load
-- like (say) a couple-hundred K-ohms across the meter leads? You might
have a very high input impedance meter, and just be measuring "antenna
effect" to some nearby physically parallel live circuit.


I agree here, if you had put a lightbulb across the two points, you
would not read a voltage, or maybe millivolts.

Mikek
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Old June 5th 15, 01:38 PM posted to rec.crafts.metalworking
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First recorded activity by DIYBanter: Jun 2011
Posts: 5,532
Default Phantom voltage in a disconnected busbar?

"Ignoramus22165" wrote in
message ...
On 2015-06-05, Jim Wilkins wrote:

"Ignoramus22165" wrote in
message ...
I was recently pulling some extremely small amount of MCM 500
copper
cables. Of course, it was legitimately purchased from the owner of
that building.

There was this one particular panel that was always live (hooked
up
to
service drop) and under power.

The panel had two 800A switch boxes. The boxes were fed through
panel
busbars, with a big handle switch, then fuses, then terminals for
MCM 500
cables.

Both boxes were in the same condition, namely:

1) The switch was in the OFF position
2) The outgoing cables were already removed.
3) The whole panel was 480 volts

There was literally NO incoming power that was not disconnected,
back-fed, or anything.

And yet, on one box out of two, my "Sperry voltage tester"
registered
live voltage.

I then re-checked it with a voltmeter and found "40 volts" (sic).
And
thi sis in a 480 volt panel!

I needed to touch those busbars (switched off, remember) to unbolt
and
remove the fuses, since the fuses are $100 for 3. Out of abundance
of
caution, I brought in a wooden pallet, stood on wood, used a
socket
extension and was very careful not to touch anything, and
everything
worked out OK.

But I am still wondering how, on a disconnected busbar, I would
read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.

i


Perhaps you saw a difference in ground potentials. The ground
(Earth)
is a return path and not always a good one.


It was phase to phase and phase to ground.


I've encountered unwanted stray voltages several times and always had
trouble finding the problem because there isn't a readily available
safe variable load to measure the source impedance, which can be
anywhere from a hot power line to MegOhms. Sometimes voltage sneaks in
through noise reduction capacitors that aren't properly grounded.

The $5 HF meter has a lower input impedance than a good meter and may
show a lower voltage reading from stray leakage but the same from a
solid connection.

My older yellow one reads 0.5 Meg on AC Volts and 1 Meg on DC Volts
between the probes with a 1000V Megger, though a low voltage (0.15V)
Ohms measurement with another HF meter shows only the 1 Meg on all the
DC ranges. My Fluke shows 10 Megs on AC and DC Volts.

-jsw


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Old June 5th 15, 07:01 PM posted to rec.crafts.metalworking
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Posts: 1,832
Default Phantom voltage in a disconnected busbar?

Ignoramus22165 wrote:
I was recently pulling some extremely small amount of MCM 500 copper
cables. Of course, it was legitimately purchased from the owner of
that building.

There was this one particular panel that was always live (hooked up to
service drop) and under power.

The panel had two 800A switch boxes. The boxes were fed through panel
busbars, with a big handle switch, then fuses, then terminals for MCM 500
cables.

Both boxes were in the same condition, namely:

1) The switch was in the OFF position
2) The outgoing cables were already removed.
3) The whole panel was 480 volts

There was literally NO incoming power that was not disconnected,
back-fed, or anything.

And yet, on one box out of two, my "Sperry voltage tester" registered
live voltage.

I then re-checked it with a voltmeter and found "40 volts" (sic). And
thi sis in a 480 volt panel!

I needed to touch those busbars (switched off, remember) to unbolt and
remove the fuses, since the fuses are $100 for 3. Out of abundance of
caution, I brought in a wooden pallet, stood on wood, used a socket
extension and was very careful not to touch anything, and everything
worked out OK.


What's the proper move for this situation? Stand on wood or ground the
stuff being worked on, then proceed?
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Old June 5th 15, 07:34 PM posted to rec.crafts.metalworking
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Posts: 125
Default Phantom voltage in a disconnected busbar?

On Thursday, June 4, 2015 at 9:57:21 PM UTC-4, Ignoramus22165 wrote:
On 2015-06-05, Lloyd E. Sponenburgh lloydspinsidemindspring.com wrote:
Ignoramus22165 fired this volley in
:

But I am still wondering how, on a disconnected busbar, I would read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.


The real first question is, "what was your reference"?


Bus to bus and bus to ground


If I were there, I'd call service and ask them if they are supplying power to that address at that moment, then wait to have the utility people come by.

Then tell them just as you said he "There was literally NO incoming power that was not disconnected, back-fed, or anything. "

Then they'd test to see if that was the case.

They'd find out if: "on one box out of two" your "'Sperry voltage tester' registered live voltage" or not.
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Old June 5th 15, 07:46 PM posted to rec.crafts.metalworking
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First recorded activity by DIYBanter: Jun 2014
Posts: 125
Default Phantom voltage in a disconnected busbar?

On Thursday, June 4, 2015 at 9:57:21 PM UTC-4, Ignoramus22165 wrote:
On 2015-06-05, Lloyd E. Sponenburgh lloydspinsidemindspring.com wrote:
Ignoramus22165 fired this volley in
:

But I am still wondering how, on a disconnected busbar, I would read
that phantom voltage. Was that just electromagnetic induction?

It is still a mystery to me.

The biggest question is how come it was only on one of the two
identical boxes.


The real first question is, "what was your reference"?


Bus to bus and bus to ground

Then, following that up some: Were you measuring bar to 'local
ground', bar to bar, bar to box neutral? Was this single-phase, or
three-phase?


All of them had voltage and to ground also.

What's the input impedance of your meter? Did you try a microscopic load
-- like (say) a couple-hundred K-ohms across the meter leads? You might
have a very high input impedance meter, and just be measuring "antenna
effect" to some nearby physically parallel live circuit.


This is my own explanation. I did not want to stick anything across
the leads, however.

There might be the residue from an old arc in that box that left
some slightly-conductive metal/metal oxides plated in and around
the insulators of the box.


This is something that I did not consider,


Well one person can't consider everything at once, that's just the problem. So you get the people from the utility to come and see if they've truly stopped supplying power or not.

Instead, I mean who wants to suddenly launch skyward out of an electrical room at light speed?


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