Strange Line to ground voltage

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mull982

Senior Member
I have a center tapped 480V to 240/120V 5kV transformer that I am using to supply both 120V and 240V loads. On my 120V circuits, I read 120V line to nuetral, however I only read 90V line to ground. Does anybody have any ideas why this may be.

We have burned out a power supply and are not sure if this faulty line to ground voltage may be the cause. Would a faulty line to ground reading lead to the destruction of a power supply even with the L-N reading being 120V.

With an ungrounded transformer on the secondary, what would you expect to see for the L-G voltage?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
mull982 said:
On my 120V circuits, I read 120V line to nuetral, however I only read 90V line to ground. Does anybody have any ideas why this may be.

With an ungrounded transformer on the secondary, what would you expect to see for the L-G voltage?
You've answered your own question. It sounds like you never grounded the secondary neutral, which you should.
 

spsnyder

Senior Member
To expand... If the loads were balanced the voltage would be 120V. Search for lost neutrals in this forum and you'll find great discussions and examples.

edited spelling.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
mull982,
Do you read 90 volts on both lines to ground? If the system has the neutral floated, and you read 90 volts to ground from one hot, I would expect to see 150 volts to ground on the other hot. Note this is not the same as an open neutral and should not cause any problems with the operation of anything connected to the system. The connected loads see the line to neutral voltage and not the line to ground voltage.
Don
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
spsnyder said:
To expand... If the loads were balanced the voltage would be 120V. Search for lost neutrals in this forum and you'll find great discussions and examples.

edited spelling.
It didn't sound like the OP had a lost neutral, just a floating (ungrounded) neutral.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
mull982 said:
I have a center tapped 480V to 240/120V 5kV transformer that I am using to supply both 120V and 240V loads. On my 120V circuits, I read 120V line to nuetral, however I only read 90V line to ground.
Did you take a neutral to ground voltage reading? If so, what was it?

Does anybody have any ideas why this may be.
Like Larry said, an ungrounded neutral.

We have burned out a power supply and are not sure if this faulty line to ground voltage may be the cause. Would a faulty line to ground reading lead to the destruction of a power supply even with the L-N reading being 120V.
My quick answer would be no, the power supply should be ok. The only thing that might be an issue would be some kind of surge suppressor that shunts out of spec voltages to ground (a problem since the ground is not referenced to the secondary side of the transformer).

With an ungrounded transformer on the secondary, what would you expect to see for the L-G voltage?
Dynamic.
 
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