What is "Phantom Voltage"?

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jbs948

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I just returned from a service call for a customer that was blowing a 200A fuse on new feeder conductors. I meggered all the wires and there is no insulation resistance problem, so I turned on the disconnect and all is well. I believe it was user error in the first place, but that?s not the problem now.

Now here?s something I?ve never seen before. With the feeder conductors installed in the underground RMC and not landed on any terminals at neither the source nor the branch panel; I read 277V on my DMM between any phase at the source and any of the conductors. What is going on? I spoke with an engineer and a senior electrician and they both referenced ?Phantom Voltage?, but with no explanation.

The feeder conductors are fed out of an 800A 480/277V Switchboard. The grounding and bonding appear to be appropriate, but I did not perform a ground-resistance check.

I?m looking forward to hearing about this.
 
480sparky

480sparky

Senior Member
Location
Iowegia
Do a site search for "phantom voltage" or "ghost voltage".

But I doubt the two (the phantom voltage & trip problem) are related.
 
brian john

brian john

Senior Member
Location
Leesburg, VA
Was it 277 VAC or 277 mv?

I have responded to more than one call where the contractor or maintenance guy saw 270 (or close) mv on a de-energized conductor and freaked out.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
catchtwentytwo said:
Can you use or borrow an analog voltmeter like a Simpson 260 to take the measurement?
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A Simpson may show a voltage as well but somewhat less than 277. Notice where the needle is as you cycle thru the ranges. If it stays in about the same position on each scale, then it is ghost voltage. Always check with two meters when in doubt.
 
quogueelectric

quogueelectric

Senior Member
Location
new york
jbs948 said:
I just returned from a service call for a customer that was blowing a 200A fuse on new feeder conductors. I meggered all the wires and there is no insulation resistance problem, so I turned on the disconnect and all is well. I believe it was user error in the first place, but that?s not the problem now.

Now here?s something I?ve never seen before. With the feeder conductors installed in the underground RMC and not landed on any terminals at neither the source nor the branch panel; I read 277V on my DMM between any phase at the source and any of the conductors. What is going on? I spoke with an engineer and a senior electrician and they both referenced ?Phantom Voltage?, but with no explanation.

The feeder conductors are fed out of an 800A 480/277V Switchboard. The grounding and bonding appear to be appropriate, but I did not perform a ground-resistance check.

I?m looking forward to hearing about this.
Click to expand...
I would suspect a different panel source has been tied into the panel feeders you have shut down somewhere.
See if you have 480 phase to phase between conductors and see if you can put some type of a load on this feeder and it will hold. Do not tie the source end in and maybe see if you can power up a lightly loaded lighting load using this as a source.
It could be some sort of capacitive coupling of other feeders with a heavy load inducing a voltage that really doesnt have the ability to drive a load but will show a voltage under a noload condition.
Cows dont believe in ghosts. There is always an explanation. You just havent found it yet.
Maybe metering cts tied in to feeders.
 
R

realolman

Senior Member
quogueelectric said:
.... There is always an explanation. You just havent found it yet.
.
Click to expand...

I agree whole heartedly. I guess it's just some sort of jargon, but those kinds of phrases bug the heck outta me.... like "backfeed".... like it's some sort of mysterious unexplainable phenomenon.
 
electricalperson

electricalperson

Senior Member
Location
massachusetts
you need tto use a low impedance meter and you wont find ghost voltage. i have the fluke 117 electrician multimeter with the loz function. your actually reading a capacitive effect on the unenergized conductor
 
cschmid

cschmid

Senior Member
Location
Northern cold country
all good Ideas but are we talking 277 vac or 277 mv..I am with brian here more info please..

If both ends are disconnected I would assume induced voltage from another source..with not being connected I would assume no way to drain induced voltage..but all assumption and we all know what assume means..
 
robbietan

robbietan

Senior Member
Location
Antipolo City
jbs948 said:
I just returned from a service call for a customer that was blowing a 200A fuse on new feeder conductors. I meggered all the wires and there is no insulation resistance problem, so I turned on the disconnect and all is well. I believe it was user error in the first place, but that?s not the problem now.

Now here?s something I?ve never seen before. With the feeder conductors installed in the underground RMC and not landed on any terminals at neither the source nor the branch panel; I read 277V on my DMM between any phase at the source and any of the conductors. What is going on? I spoke with an engineer and a senior electrician and they both referenced ?Phantom Voltage?, but with no explanation.

The feeder conductors are fed out of an 800A 480/277V Switchboard. The grounding and bonding appear to be appropriate, but I did not perform a ground-resistance check.

I?m looking forward to hearing about this.
Click to expand...

"phantom voltage" to us here means that the voltage may read 277V but it will not power anything. to check it, try powering a light bulb with it, chances are, the bulb would not light. if your loads are mostly motors and heaters, no problem. electronic loads may have problems

what you described may be a grounding / bonding problem. somewhere is a grounded conductor and the fuse is blowing as the current is still affecting the ground connections.

had the same trouble here once, when they located the grounded conductor (from a motor's supply) the problem was solved.
 
480sparky

480sparky

Senior Member
Location
Iowegia
brian john said:
Was it 277 VAC or 277 mv?

I have responded to more than one call where the contractor or maintenance guy saw 270 (or close) mv on a de-energized conductor and freaked out.
Click to expand...


Very common mistake on DMMs.

When you're used to seeing
LCD1.jpg




you tend to miss the little 'm' and see 30 volts
LCD2.jpg
 
76nemo

76nemo

Senior Member
Location
Ogdensburg, NY
realolman said:
I agree whole heartedly. I guess it's just some sort of jargon, but those kinds of phrases bug the heck outta me.... like "backfeed".... like it's some sort of mysterious unexplainable phenomenon.
Click to expand...


Chris already explained it, it's dependent on the impedance input of your DMM. High impedance(10K+), will certainly see stray voltage. As you lighten the impedance, like an old solenoid tester, the tester won't pick that up. Now, in this day and age we have dual impedance meters like Chris mentioned in post#20. They are worth their weight in gold.

Little taken back here though. A solid 277V reading of "phantom" voltage? I am second guessing that. You'll never see a stray voltage of that sort anywhere near or dead even with line voltage unless something is very wrong:confused:
 
R

realolman

Senior Member
76nemo said:
Chris already explained it, it's dependent on the impedance input of your DMM. High impedance(10K+), will certainly see stray voltage. As you lighten the impedance, like an old solenoid tester, the tester won't pick that up.
Click to expand...

I agree whole heartedly. That' s why I suggested a wiggie back in post #8.

It's not "Phantom" at all, is it? So why give it this mysterious name like it's some strange unexplainable phenomenon?

This is deja vu all over again.:smile:
 
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