Odd voltage readings

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nizak

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Got a question regarding 208/120 Voltage

I've got a piece of 3 phase equipment that's being supplied by 208/120 voltage that's derived from 480V and a transformer.

Voltage at machine disconnect measures 212 / 209 / 212 between phases.

Voltage from each phase to ground measures 99 / 97 / 99 to ground.

I can't access the transformer at this point as it sits 30' up on a rack surrounded by equipment placed after the install 30+ years ago.

The equipment has been operating for many years I'm assuming as is.

Where would I start looking for the issue?

Thanks
 
retirede said:
What is “ground” you are measuring to and why do you care what it is?

Is there a neutral?
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Equipment ground in disconnect is reference point. Same reading to building steel.
No grounded conductor present in disconnect.

More curiosity than anything.
Just seemed like odd values.
 
I agree that a likely cause of those voltage readings is an ungrounded secondary. But it could also be something like waveform distortion from transformer saturation or non linear loading.

A simple test is to make the same voltage measurements with a low impedance meter such as a solenoid tester. If the secondary is not grounded, then a low impedance meter will bring the L-G voltage down to near zero.

Jon
 
nizak said:
Equipment ground in disconnect is reference point. Same reading to building steel.
No grounded conductor present in disconnect.

More curiosity than anything.
Just seemed like odd values.
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Looks like you'll need to open the transformer to confirm that there is no SBJ present. Your voltage to ground readings should be near or at 120 volts.
 
In my opinion if we ground the secondary neutral-I presume it is Y connected- no change in the unbalance between phase-to-phase voltages would happen. The
unbalance it is due to different no-load current per magnetic columns: the central column presents an air gap at the junction point so the no-load current is greater.
However, the unbalance goes away with a balanced load.
 
Bonding the transformer neutral point would have no effect on the line to line voltages. However it absolutely impacts the line to ground voltages, by providing a solid reference point. With an unbonded neutral point the line to ground voltage readings are dependent only on the capacitive coupling, leakage current, between the conductive paths.
 
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