Ungrounded Delta

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hpec

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A friend of mine called me today and ask me about an ungrounded delta. The voltage is 240 volts. He told me he took readings from phase to to?ground and come up with the following voltage readings.

A-B 450 volts
B-C 435 volts
C-A 215 volts

Is this possible? Can the voltage to ground be higher than the phase to phase voltage? I was always told that the voltage to ground on an ungrounded delta can be anything, but I did not think the voltage could be higher than the phase voltage.

Hpec
 
Re: Ungrounded Delta

The voltages you gave were listed as phase to phase, but I assume that was a typo.

But yes, the phase to ground on an ungrounded system could be as high as twice the line voltage.
 
Re: Ungrounded Delta

Jim,
Can you expand on your answer? Is it based on normal or fault conditions? Thanks.
Don
 
Re: Ungrounded Delta

Under normal conditions an ungrounded delta system is actually capacitively coupled to ground. The use of high impedance meters (expensive digital ones) can result in reading "ghost" voltages. The level of these voltages is dependent on the size of the distribution system as well as the impedance of the meter and leads. A low impedance (ie. Wiggy or solenoid type) tester is the preferred type for ungrounded systems.

During fault conditions, especially arcing type, the voltage to ground on the unfaulted conductors can rise to at least 1.73 times normal.
 
Re: Ungrounded Delta

So with those measurements listed in the original post, there could be a fault on phase C to ground? Is 215 V on C phase possible if there is a ground fault on it?
 
Re: Ungrounded Delta

Hello Jim,
Under normal conditions, on a 240 volts ungrounded delta, what should the voltages be to ground on each leg using a wiggy testers?
Will it read 240 to ground?

Should we just read it phase to phase to check that all the legs have 240 volts between them?

I remember having no luck trying to read phase to ground voltage with a digital multi meter on a 575 volts delta system some years back. We were having the same types of strange ghost voltages as decribed in the original post.

Look forward to your comments.
Phil
 
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