Unbalanced voltage readings Phase to Ground 297/266/281

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Fomophobic

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Location
California
Occupation
Electrician
Phase to phase: 486/487/486
Phase to ground: 297/266/281

Does anybody have any idea why I’m getting these funky readings phase to ground?
 
Are you measuring L-G or L-N? Did you measure N-G?
Sounds like you may have lost your ground reference/bond.
 
There are several electric rooms fed off the same utility. Each with its own 12k-480 transformer. The other electric rooms don’t have the unbalanced voltages. They are around 277v L-G.
 
Fomophobic said:
No neutral. L-G. It’s 3-phase 3-wire. And a balanced load. 338/318/321
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If it is truly 3-wire then why are you even bothering to check L-G voltages? If there is no solid/consistent reference to ground, you readings are pretty much useless. They will vary depending on things like capacitive coupling and circuit loading.

Are you sure the utility is not providing a 4th wire that simply stops at the service entrance?
 
Thanks for all the responses. An engineer had checked it for some reason and brought it to our attention. It’s probably not a serious issue, but just wanted to make sure it doesn’t turn into an issue in the future. We cannot get into the 12k transformer for safety reasons.
 
Fomophobic said:
Thanks for all the responses. An engineer had checked it for some reason and brought it to our attention. It’s probably not a serious issue, but just wanted to make sure it doesn’t turn into an issue in the future. We cannot get into the 12k transformer for safety reasons.
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What you are experiencing could be dangerous. Most newer generation electronic equipment, like VFDs and power supplies, are not rated for ungrounded systems and can sometimes fail fairly spectacularly (e.g. capacitor busting).

I understand this is an existing system, but the NEC recognizes ungrounded systems as being a problem and has recently added requirements for them to have Ground Detection schemes.
 
A totally ungrounded 480V delta source may still exhibit relatively uniform Line to Ground voltages as the result of balanced capacitance to ground in the wiring and loads. Any assymetric leakage from one line ground will easily destabilize the array of line to ground voltages without causing any problems with Line to Line loads.
 
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