480 volt 3 phase delta ungrounded question

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stardust

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have a 4160 delta to 480 delta 3 phase ungrounded transformer. reading approximately 480 volts phase to phase (actually 497,499,497). reading A-G 483V, B-G 504V AND C-G 39V. i know that readings to g are unreliable with this type transformer. My question is could this C leg be partially grounded and what would be a simple way to check for it?
Thanks!
 
Make sure you are using a LoZ or low impedance meter to measure the voltages. A high impedance meter with 10meg-ohms of input impedance on the voltmeter can cause incorrect readings under some circumstances like yours.
 
stardust said:
have a 4160 delta to 480 delta 3 phase ungrounded transformer. reading approximately 480 volts phase to phase (actually 497,499,497). reading A-G 483V, B-G 504V AND C-G 39V. i know that readings to g are unreliable with this type transformer. My question is could this C leg be partially grounded and what would be a simple way to check for it?
Thanks!
Click to expand...

Just curious, if you are measuring basically 480v from A and B to ground and C to ground as basically 0v one would assume that the C phase is in fact grounded.
 
SG-1 said:
Make sure you are using a LoZ or low impedance meter to measure the voltages. A high impedance meter with 10meg-ohms of input impedance on the voltmeter can cause incorrect readings under some circumstances like yours.
Click to expand...
Just curious - why wouldn't a low impedance meter be more likely to shunt current to ground from a phase and thereby ground it than a high Z meter?
 
ggunn said:
Just curious - why wouldn't a low impedance meter be more likely to shunt current to ground from a phase and thereby ground it than a high Z meter?
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Actually, that is the idea, if the voltage is not really there, but is being generated by capactive coupling. The LoZ meter will "drain" the capacitor and show a true voltage reading.

High impedance meters are ideal for use with electronics & generally have an input impedance of 10 mega-ohms. Their voltage readings are so sensitive that capactive coupling can render the reading useless. The readings giving suggest capactive coupling is occuring, at least to me.

LoZ meters or meters with a LoZ setting still have between 3 & 5 thousand ohms of input resistance, just like the analog meters of yesterday or a wiggy. More current flows during the reading. Since this lightly loads the circuit the instrument is less likely to read high because of capacitive coupling.

Because more current flows using a LoZ meter on an electronic circuit board can result in disaster. Say for instance I were to take a reading across a 100K resistor. My meter would short circuit the resistor & could destroy the componets that try to deliver twenty times more current because I replaced 100K resistor with a 5K meter impedance.

I only suggest that the proper tool be used. I have no bias aginst or for a high impedance meter. They are great tools for measuring voltage. One must be aware that it can measure a phantom voltage, that is not really there. This can send troubleshooting down the primrose path. When the readings get weird, reach for a LoZ meter. Fluke makes an attachment to make their multimeters LoZ, I think it is called an SV225.
 
SG-1 said:
I only suggest that the proper tool be used. I have no bias aginst or for a high impedance meter. They are great tools for measuring voltage. One must be aware that it can measure a phantom voltage, that is not really there. This can send troubleshooting down the primrose path. When the readings get weird, reach for a LoZ meter. Fluke makes an attachment to make their multimeters LoZ, I think it is called an SV225.
Click to expand...

I didn't know Fluke made these. We had some home made ones for finding ground faults on delta services to fit our Fluke meters. Our lab made them for us. We tried using one of them to find a ground fault and when we put the meter with the shunt resistor across 240v the resistor and the Fluke exploded. Everybody had all their safety gear on so we just laughed about it, but it could have been bad. Needless to say, most of them went in the trash. I only use it for low levels of stray voltage once in a while and a Wiggy for finding grounded delta. The Fluke resistor has all the right ratings that we need as well. Gonna see if I can get some ordered when when I go to work.

Thanks
 
UPDATE

UPDATE

Found 1 phase of feeder going under basement of switchgear grounded. when we shut breaker off to this circuit and left others energized had one phase to ground read 247v, one 396v and last one 249v. does anyone think we still have a problem in the remaining energized circuits? or could this be a normal reading?
Thanks for the help
 
The readings look better now, more balenced.

The only way to know the condition of your conductors would be to megger or otherwise test their dielectric strength.

You should strongly consider installing ground fault detection equipment on your system. Ungrounded systems can produce catastropic failures & burn downs of biblical porportion or they run unnoticed for years with a ground fault present.
 
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