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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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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On 2015-06-05, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

Bus to bus and bus to ground

All of them had voltage and to ground also.

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

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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On Thursday, June 4, 2015 at 9:57:21 PM UTC-4, Ignoramus22165 wrote:

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 here: "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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On Thursday, June 4, 2015 at 9:57:21 PM UTC-4, Ignoramus22165 wrote:

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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On Fri, 5 Jun 2015 10:46:30 -0700 (PDT), walter snipped-for-privacy@post.com wrote:

Like the time I replaced the fuses on our incoming 600V feed that had been hit by lightning. Threw the handle to "ON" and hit the opposite wall Lucky I guess!
--

Gerry :-)}
London,Canada
  Click to see the full signature.
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On 6/4/2015 8:26 PM, Lloyd E. Sponenburgh wrote:

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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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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It was phase to phase and phase to ground.

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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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What's the proper move for this situation? Stand on wood or ground the stuff being worked on, then proceed?
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On Friday, June 5, 2015 at 1:01:39 PM UTC-4, Cydrome Leader wrote:

00

I don't see in Iggy's post that all the breakers in the panel were turned o ff. He did say that the panels weren't being back-fed, but... I had a coupl e of APC Smart-UPS boxes that would, under battery power, put out enough vo ltage from their line cords to give a pretty decent shock. I did not measur e the voltages, nor did I test them under load. We just replaced the UPS bo xes (which was scheduled anyway) and sent the old ones off to a local guy w ho refurbishes them.
So, there COULD have been something connected elsewhere that was unintentio nally back-feeding the panel.
On a sort of related note:
Last night I watch a show on National Geographic about working on HIGH volt age lines - somewhere around half a million volts. They were working on one of the big "T" shaped towers. The lines on one side of the T were deactiva ted while being worked on, but the lines on the other side were still live.
They said that they still had to ground the otherwise "dead" wires to the t ower because there was an induced voltage of something over 8KV at 15 amps. And no, I don't think that's what was going on in Iggy's panel.
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wrote:

I don't see in Iggy's post that all the breakers in the panel were turned off. He did say that the panels weren't being back-fed, but... I had a couple of APC Smart-UPS boxes that would, under battery power, put out enough voltage from their line cords to give a pretty decent shock. I did not measure the voltages, nor did I test them under load. We just replaced the UPS boxes (which was scheduled anyway) and sent the old ones off to a local guy who refurbishes them. ============= On battery my APC1400 measures nothing line-to-line, but 20VDC from either line to ground. -jsw
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If the cable runs were running parallel to other still energized cable runs then they could very well have had voltage induced onto them. When I worked for Arizona Telephone Company we had issues with that even with underground telephone cable running a couple hundred feet away but parallel to high voltage cross country lines. (Other side of the road for about 10 miles)
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Bob La Londe wrote:

There was a local guy who put that to good use for a while. He ran loops of wire parallel to the HV lines overhead. The power was fed into a self built regulator. The company caught him when they were trimming the ROW. Charged him with theft of services and trespassing. Strange part of it was that the judge actually told the guy that if he had ran the wires only on his own property that the utility couldn't have done a thing!
--
Steve W.

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This is very smart, any idea of the area he looped and the wattage he was getting?
i
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