575 volts

[JJWalecka]

Pardon my ignorance.

Hypothetical speaking.
In an industrial environment, 575 volts supplies a disconnect. A volt meter reads 575ish between all three phases. Two of the three phases read 600+volts to ground and one phase measures 75volts. Why is that?

I?m a researching on my own but feedback is welcome.

Is 575 conceived through a 3 wire delta?

 

[augie47]

Your analysis is most likely correct.
An ungrounded delta system can provide interesting voltage readings to ground.
You might find 250.21(B) interesting

 

[philly]

Pardon my ignorance.

Hypothetical speaking.
In an industrial environment, 575 volts supplies a disconnect. A volt meter reads 575ish between all three phases. Two of the three phases read 600+volts to ground and one phase measures 75volts. Why is that?

I?m a researching on my own but feedback is welcome.

Is 575 conceived through a 3 wire delta?

Just to venture a guess it could be possible there is a L-G fault on one of the phases on an ungrounded system. This would result in two of the phases reading L-L values on the two ufaulted phases and 0V on the faulted phase. Maybe its possbile the 75V on the faulted phase are due to an impedance in the fault.

 

[JJWalecka]

Thank you both for your feedback

"Just to venture a guess it could be possible there is a L-G fault on one of the phases on an ungrounded system." Hypothetically?. the three phases where continuity checked and all three phase didn?t register a fault. Megging out the phases might have produced the fault better than a continuity tester.

I wonder if the fault could be in the buss of the switch gear?

 

[big john]

We've got a lot of nominal 600V ungrounded systems. The voltages are generally pretty stable; I attribute this to long runs and capacitive coupling. The voltage to ground on each phase is usually somewhere around 350-390.

If you're reading 600 on two phases, and 75 on the third, I'd bet a fair bit of money on that third phase having a high-resistance ground fault. You have a ground-fault detection lights? Check them out, I'll bet one isn't glowing so bright anymore.

EDIT: I type slow, just saw the post about the continuity tester. You're right, you do need a megger to find this fault, especially because it does sound like high-resistance. The battery in a multimeter just can't provide enough juice to test for it. You could have a fault anywhere, but I think it's unlikely to be in the buswork. Do you know how long the voltages have been this way? What has changed?

-John

 

[rcwilson]

The fault could be anywhere, including the bus of the switchgear. It does sound like you have an ungrounded 600V delta system. If you can shut off the loads in the system while monitoring the phase-ground voltages, you might be able to locate the fault. Theoretically, when you open the breaker on the faulted circuit, the phase-ground voltages will go back to being more balanced.

Open a circuit, if nothing changes, close it back in again.

 

[jim dungar]

You have a ground-fault detection lights? Check them out, I'll bet one isn't glowing so bright anymore.

Most ground detector lights are at equal, but dim, brightness, during normal (no ground) conditions. During a ground faul, the grounde phase goes out and the other 2 burn brighter. So I would expect if the OP has lights 2 of them are now brighter than the third one which maybe just barely glowing

 

[K2500]

Is 575 conceived through a 3 wire delta?

Suppose it could be an impedance grounded 600Y/347.
Not sure how common that is, though.

 

[big john]

Suppose it could be an impedance grounded 600Y/347.
Not sure how common that is, though.

Could it be, though? I've never taken voltages on an impedance-grounded system.

While I would expect to find voltage to ground, I'd imagine the system to be wye connected, and the voltages would be the same for all three phases, right?

The fact that he has a "low" voltage on one phase, and the other two are through the roof, is what makes me think it's a fault.

-John

 

[kwired]

The phase with 75 volts to ground seems to indicate that there is a problem with something, but one needs to know if the system is grounded or ungrounded first, and if it is grounded is it a wye, likely not from voltage readings but possibly could be if a bad neutral or an impedance grounded system , a corner grounded delta, or a mid phase grounded delta.

If pole mounted transformers you can see the connections and know what the configuration is, you just will not know what the voltage is supposed to be unless you can read the transformer nameplates from 25 - 30 feet away. With a padmounted transformer the nameplate is usually inside the doors behind the POCO lock on the door so you are out of luck without calling them.

 

[K2500]

Could it be, though? I've never taken voltages on an impedance-grounded system.

While I would expect to find voltage to ground, I'd imagine the system to be wye connected, and the voltages would be the same for all three phases, right?

The fact that he has a "low" voltage on one phase, and the other two are through the roof, is what makes me think it's a fault.

-John

The voltages would be the same as a solidly grounded wye with out fault.
With a fault, the impedance grounded voltages can and will vary to ground, in a manner similar to what the OP described, depending on the impedance of the fault to ground.



Most grounding impedance equipment that I've seen will have a pulse switch, something that most likely will not be present on ungrounded systems. Might be a quick way to tell, lacking any other ID.

 

[K2500]

Double post

 

[JJWalecka]

Thank you all for your posts. I appreciate it.

Hypothetically, there was a fault in the distribution system itself, that I was just not aware of.