# 2 lines show voltage to ground, but no voltage between lines



## erics37 (May 7, 2009)

BobC-64 said:


> On a 3 phase 480/277V system, how is it possible for line A and line B to measure 277V to ground, but to measure 0 volts between line A and line B?


Is it a switch loop?


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## Switched (Dec 23, 2012)

Is the system ungrounded? Been a while since I worked on them, but they have different readings?


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## erics37 (May 7, 2009)

Switched said:


> Is the system ungrounded? Been a while since I worked on them, but they have different readings?


That wouldn't have any effect on the potential between phases.

The only scenario I can think of that could explain this reasonably is that he's putting his volt meter across a closed switch.


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## sparkywannabee (Jan 29, 2013)

If you trace them back to the panel, you will find they are off breakers that are on the same bus.


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## erics37 (May 7, 2009)

sparkywannabee said:


> If you trace them back to the panel, you will find they are off breakers that are on the same bus.


Or that.


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## Switched (Dec 23, 2012)

erics37 said:


> That wouldn't have any effect on the potential between phases.
> 
> The only scenario I can think of that could explain this reasonably is that he's putting his volt meter across a closed switch.


It's been while! I'm having a hard time remembering the measurement problems with an ungrounded system. I'll figure it out next time I run across it!


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## 480sparky (Sep 20, 2007)

Where are you measuring at?... in a box somewhere, or at the panel?


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## dmxtothemax (Jun 15, 2010)

BobC-64 said:


> On a 3 phase 480/277V system, how is it possible for line A and line B to measure 277V to ground, but to measure 0 volts between line A and line B?


 
One phase down and reading backfeed voltage !


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## BobC-64 (Mar 1, 2013)

It is being measured at the first parking lot light fixture from panel room, wires have been separated from wires going to next pole light.


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## A Little Short (Nov 11, 2010)

dmxtothemax said:


> One phase down and reading backfeed voltage !


:thumbsup:
Plus one here!


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## BobC-64 (Mar 1, 2013)

can you explain how the backfeed is being created?


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## HARRY304E (Sep 15, 2010)

BobC-64 said:


> On a 3 phase 480/277V system, how is it possible for line A and line B to measure 277V to ground, but to measure 0 volts between line A and line B?


They've got to be on the same phase and at the point you are getting this measurement , your wires are miss marked and spliced together between the pole and the panel.


Welcome to the forum....:thumbup:


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## BobC-64 (Mar 1, 2013)

thanks to everyone who responded.


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## HARRY304E (Sep 15, 2010)

BobC-64 said:


> thanks to everyone who responded.


You're Welcome...:thumbup:


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## 480sparky (Sep 20, 2007)

BobC-64 said:


> can you explain how the backfeed is being created?


Contactor used as controller...... Breaking only one leg of the 480v circuit.


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## Southeast Power (Jan 18, 2009)

BobC-64 said:


> On a 3 phase 480/277V system, how is it possible for line A and line B to measure 277V to ground, but to measure 0 volts between line A and line B?


It's not. What type of equipment are you testing? Is it a contractor or relay?
Maybe a blown fuse upstream?


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## guest (Feb 21, 2009)

sparkywannabee said:


> If you trace them back to the panel, you will find they are off breakers that are on the same bus.


Bingo. :thumbup:


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## 220/221 (Sep 25, 2007)

BobC-64 said:


> can you explain how the backfeed is being created?


From the downstream connected load.

You are not reading 480 because it's the same phase backfeeding.


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## Big John (May 23, 2010)

I'm gonna go with blown fuse or open conductor for $1000, please, Alex.


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## Awg-Dawg (Jan 23, 2007)

HARRY304E said:


> your wires are miss marked and spliced together between the pole and the panel.


 
I had a service call this week that had something similar.

The owner said the first EC tried to fix it but couldnt figure it out.

This is what I found.


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## green light (Oct 12, 2011)

Awg-Dawg said:


> I had a service call this week that had something similar.
> 
> The owner said the first EC tried to fix it but couldnt figure it out.
> 
> This is what I found.


Seriously? That's pretty bad...


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## Big John (May 23, 2010)




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## Awg-Dawg (Jan 23, 2007)

green light said:


> Seriously? That's pretty bad...


 
I almost forgot.

I lifted a skirt up on one of the poles and found a geocache.

I had never heard of it before.:laughing:

http://www.geocaching.com/


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## bobelectric (Feb 24, 2007)

Open conductor and you are reading to ground through the ballast.


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## jza (Oct 31, 2009)

Awg-Dawg said:


> I had a service call this week that had something similar.
> 
> The owner said the first EC tried to fix it but couldnt figure it out.
> 
> This is what I found.


Holy ****.


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## dmxtothemax (Jun 15, 2010)

BobC-64 said:


> can you explain how the backfeed is being created?


Maybe thru indicator lights on panel ?


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## 480sparky (Sep 20, 2007)

dmxtothemax said:


> Maybe thru indicator lights on panel ?



Or the 480v pole light ballasts.


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## dmxtothemax (Jun 15, 2010)

480sparky said:


> Or the 480v pole light ballasts.


Wouldn't post no - 10, rule that out ?


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