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Single Phase Ungrounded Power line voltage questions

Single Phase Ungrounded Power line voltage questions

Single Phase Ungrounded Power line voltage questions

(OP)
Okay so here is the scenario

At work we have a single phase ungrounded 6.9kv circuit. From each leg to ground is 3.45kv. When a ground fault occurs the leg with the fault goes to 0v and the other leg to 6.9kv. I've been told the reason for this circuit design is so we never lose power.

It was always my understanding that the basic definition of voltage was difference in potential. Since you are using the same ground as a reference point in metering why is each leg 3.4kv to ground and not 6.9kv in normal circumstances? I understand 3.45kv per phase is 6.9kv phase to phase. But I don't understand how it goes from half voltage to ground to full voltage to ground with a ground fault. Everyone at work just tells me its because it is a ungrounded circuit but that doesn't explain much to me. I've tried looking around for more information but their doesn't seem to be a lot out there on single phase ungrounded systems. I was hoping someone here could explain it to me or at least point me in the right direction of some material to read up on. Thanks in advance!

RE: Single Phase Ungrounded Power line voltage questions

Your line has capacitance to ground and a high resistance to ground. So each hot line is in effect connected to ground by a high impedance circuit. The impedance of each line to ground is approximately equal. You have a voltage divider with the 50% point connected to ground. The impedance is high and the capacitance current and the leakage current of the resistance to ground is very low. There is enough current that you can measure the voltage to ground from each line with a high impedance meter without loading the circuit enough to make much change to the voltage to ground.
If one line goes to ground that effectively shorts out the impedance to ground. The leakage current and the capacitive current from the other line to ground will double but will still be very low. The leakage current is still probably high enough to be lethal.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Single Phase Ungrounded Power line voltage questions

(OP)
Bill thanks for the response!

I have some follow up questions if you don't mind.

Does the length of line change the line capacitance and impedance to ground? The readings I did were at the open disconnect switch about 10ft from the transformer. Would it also be the same 10 miles down the road? Also if I was to disconnect the lines at the transformer and meter right off of the taps would I still have the same voltages? I will definitely be doing some more reading on it.

Thanks again for the previous response!

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