# 480 volt corner ground lost one phase



## tbain077 (Nov 17, 2011)

Need help understanding why the voltage readings from phase to phase (a-b,a-c,b-c) has increased to 499 volts when i finally looked up and found an open fuse on the utility side.


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## Big John (May 23, 2010)

Corner grounded delta will read 480 anywhere you put a meter, except one phase will be 0 volts to ground. Not sure why you would see a voltage increase if you lost a primary phase, though.

-John


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## joethemechanic (Sep 21, 2011)

No load, no voltage drop?


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## jmsmith (Sep 10, 2011)

Big John said:


> Corner grounded delta will read 480 anywhere you put a meter, except one phase will be 0 volts to ground. Not sure why you would see a voltage increase if you lost a primary phase, though.
> 
> -John


Hey John-
Would this be possible if the phase coil (or transformer, using 3 single-phase units) that is NOT connected to ground opened or lost feed at either end? I know that two transformers (or coils) can serve a three phase load, but at around 57%(?) capacity. To be honest though, I have no earthly idea what would be read voltage wise, but there would be a voltage to ground due to the unbalanced conditions.
Is that right?


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## jmsmith (Sep 10, 2011)

jmsmith said:


> Hey John-
> Would this be possible if the phase coil (or transformer, using 3 single-phase units) that is NOT connected to ground opened or lost feed at either end? I know that two transformers (or coils) can serve a three phase load, but at around 57%(?) capacity. To be honest though, I have no earthly idea what would be read voltage wise, but there would be a voltage to ground due to the unbalanced conditions.
> Is that right?


Sorry, meant the coil or transformer that MAKES the corner ground, creating an open-delta as in the above case. Losing the grounded phase will then allow you to see other than 0 volts at the "grounded" phase via the other coils or transformers.


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## Big John (May 23, 2010)

I see what you're saying, what if you had a corner-grounded open delta? I don't see off the top of my head why the voltages would be different, just reduced capacity, like you said.

What makes this goofy, it sounds like the OP is describing an open delta primary, and but kept a closed delta secondary. I have no clue what, if anything, that would do to voltages/capacity.

-John


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## jmsmith (Sep 10, 2011)

Big John said:


> I see what you're saying, what if you had a corner-grounded open delta? I don't see off the top of my head why the voltages would be different, just reduced capacity, like you said.
> 
> What makes this goofy, it sounds like the OP is describing an open delta primary, and but kept a closed delta secondary. I have no clue what, if anything, that would do to voltages/capacity.
> 
> -John


I had to bank transformers in the oilfields throughout the south. In a pinch, we have had to wire open-delta. From what I recall, secondary had to be reduced to 57%, also, unless the remaining units were fairly identical to each other (impedance, etc.). Then you could use up to 87% capacity with relative safety. Both units also had to be either additive or subtractive in polarity. Do you think that could act like using a booster, and you could see an increase in secondary voltage, or that phase shift could have anything to do with this? Just wondering...
:blink:


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## Big John (May 23, 2010)

jmsmith said:


> ...From what I recall, secondary had to be reduced to 57%, also, unless the remaining units were fairly identical to each other (impedance, etc.). Then you could use up to 87% capacity with relative safety....


 I'm sure having mismatched impedance will affect voltage at high output, but the 57% and 87% are actually two ways of looking at the same equation: If you bank two transformers in an open delta, you can only run them at 87% of their combined kVA. And if you have a closed delta, but loose one winding, you're subtracting 1/3 of 87%, so you get a 58% capacity of the capacity for _three transformers_.


> ...Both units also had to be either additive or subtractive in polarity. Do you think that could act like using a booster, and you could see an increase in secondary voltage, or that phase shift could have anything to do with this?


 I'd have to do some reading, that's definitely outside what I know off the top of my head. :laughing:

-John


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## jmsmith (Sep 10, 2011)

Big John said:


> I'm sure having mismatched impedance will affect voltage at high output, but the 57% and 87% are actually two ways of looking at the same equation: If you bank two transformers in an open delta, you can only run them at 87% of their combined kVA. And if you have a closed delta, but loose one winding, you're subtracting 1/3 of 87%, so you get a 58% capacity of the capacity for three transformers. I'd have to do some reading, that's definitely outside what I know off the top of my head. :laughing:
> 
> -John


Yeah, right on the math... I kinda made a stuttering move there! As for the other, it's been almost 31 years since school, part of theory class I thought I'd never have a use for!! Guess I'll hit the books with ya!
:blink: :laughing:


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## RGH (Sep 12, 2011)

If you lost part of the ground on the grounded phase you could be getting transient voltage back from a bad harmonic. Is it 480 primary to 277 secondary? Check all grounds and neutrals, if neutrals are crossed down stream on secondary this could increase voltage.


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## Big John (May 23, 2010)

RGH, have you been watching this video?




-John


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## McClary’s Electrical (Feb 21, 2009)

I'm seen the same thing in production facilities that have opened a primary fuse on one phase during production. With all the 3 phase motors running , and closed 3 phase contactors, you'll still read voltage on all 3 phases to ground, through all the motor windings in the plant.


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## RGH (Sep 12, 2011)

Big john he lost me after hello lol:blink:. Been a while since I tackled these problems for sure. Just remember the issues I stated. Delta, open,closed, corner grounded ect., to y config's always a pain in the arse when chasing the ghost. I was luckey to have a bunch of eng's on our staff to help, and then I would say of yeah that lol.:whistling2:


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## jmsmith (Sep 10, 2011)

Hey John--
Just a thot here.... Losing a phase and creating an open delta would reduce the available turns in the primary, right? We would still have the same turns in the secondary. Not taking core losses into account, would reducing the amp-turns in the primary actually induce a higher voltage in the secondary if this hasn't changed by the same amount? To me that would also account for the reduced load that you could put on the system. I guess what I'm asking here is if primary voltage is still the same, would reducing pri/sec turns ratio actually create a higher voltage in the secondary? Looking forward to your take on this, from you and RHG.
-Jim


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## jmsmith (Sep 10, 2011)

RGH... Sorry about that.


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## RGH (Sep 12, 2011)

Be sure to post your findings. So much hot air in here sometimes guys forget can can learn from each other..Not that I don't mind the total and mindless bantter..but this is a really really good problem for a change!!!


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