240v phase to ground

[nixin9]

I recently had and issue with a 240v three phase Delta system while on the job. While testing a condensing units 3 pole breaker for voltage I found 235v A phase to B phase and 0v C phase. So then tested phase to ground and found 235v phase to ground on both A and B phases. Never ran into that before. I ended up tracing out the C phase to a blown 400A fuse. Installed a new fuse and to my surprise I tested 235v phase to phase across all three phases but still 235v phase to ground on two phases. I ended up finding the fault in a wire way down into the system. The fault went through three sets of fuses before the 400A blew. I cleared the fault and phase to ground voltage went back to normal 120v across the 2 remaining phases. Any ideas on the 240v phase to ground?

Thanks.

 

[ActionDave]

Where is this 240V coming from, the utility or a transformer? Whichever it is, it sounds like you have an unintentional ungrounded system that should be solidly grounded. The first fault makes the system grounded and the second fault blows a fuse.

 

[ptonsparky]

The system could be an ungrounded system from the the start but usually there are ground fault detection lights involved. Was this a mfg plant at some time or another?

Problem you have is there should have been two faults. The first made it a grounded system, the second blew the fuse. Where is the second one?

 

[roger]

I cleared the fault and phase to ground voltage went back to normal 120v across the 2 remaining phases. Any ideas on the 240v phase to ground?

Thanks.

What was the fault you found?

Roger

 

[kwired]

I cleared the fault and phase to ground voltage went back to normal 120v across the 2 remaining phases.

This statement seems to indicate you have a neutral conductor in the system - should also have 208 to ground on the other leg when everything is "normal".

If you blew fuses in two lines and had the high leg as the only remaining working line - you would see over 120 volts to ground on pretty much everything if a three phase load is "actively connected" But you should see 200-maybe 215 volts and not 235.

Blown fuses/open circuits can leave you with what seems to be strange voltages, one needs to draw out the circuit and then add the "open" point as well as a fault into the drawing and sometimes that can help make more sense of why you read what you read. Common trip circuit breakers open all ungrounded conductors and leave everything dead - and this makes troubleshooting seem strange if you seldom run into fuses as the overcurrent protection.

 

[nixin9]

The system could be an ungrounded system from the the start but usually there are ground fault detection lights involved. Was this a mfg plant at some time or another?

Problem you have is there should have been two faults. The first made it a grounded system, the second blew the fuse. Where is the second one?

Well this system is a pretty old one. It used to be DC and was converted to AC. That being said it is an ugrounded system but there aren't any ground fault detection lights. The only fault I found was a grounded phase conductor in RMC. It's old "rag" wire but I haven't figured out what caused it to fail. Could be it was just that old. There are a lot of parts to this system though. It's hard to explain it all. Maybe I'll have to draw it. I'm really just trying to figure out how I tested 235v phase to ground.

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[nixin9]

What was the fault you found?

Roger

One of the old "rag wire" has continuity to the RMC it's in.

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[nixin9]

This statement seems to indicate you have a neutral conductor in the system - should also have 208 to ground on the other leg when everything is "normal".

If you blew fuses in two lines and had the high leg as the only remaining working line - you would see over 120 volts to ground on pretty much everything if a three phase load is "actively connected" But you should see 200-maybe 215 volts and not 235.

Blown fuses/open circuits can leave you with what seems to be strange voltages, one needs to draw out the circuit and then add the "open" point as well as a fault into the drawing and sometimes that can help make more sense of why you read what you read. Common trip circuit breakers open all ungrounded conductors and leave everything dead - and this makes troubleshooting seem strange if you seldom run into fuses as the overcurrent protection.

Well the system is not a high leg delta and there is no neutral. However I did test the phase to ground again after fixing the fault and I said earlier I got 120v to ground but it is actually 135v phase to ground a,b and c. 235v phase to phase a,b and c. I saw something about a corner grounded delta system? It has voltages simular to the ones I was getting when I had the fault. 240v phase to ground for A and B phase and 0v on C phase. Is it possible it created a corner grounded delta system with the C phase wire going to ground?? It is an old ungrounded system though. Guess I'll have to do some more research on corner grounded deltas.

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[nixin9]

Where is this 240V coming from, the utility or a transformer? Whichever it is, it sounds like you have an unintentional ungrounded system that should be solidly grounded. The first fault makes the system grounded and the second fault blows a fuse.

The 240v comes from 3 single phase oil filled transformers. I don't see a service ground only mechanical bonds.

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[petersonra]

Well the system is not a high leg delta and there is no neutral. However I did test the phase to ground again after fixing the fault and I said earlier I got 120v to ground but it is actually 135v phase to ground a,b and c. 235v phase to phase a,b and c. I saw something about a corner grounded delta system? It has voltages simular to the ones I was getting when I had the fault. 240v phase to ground for A and B phase and 0v on C phase. Is it possible it created a corner grounded delta system with the C phase wire going to ground?? It is an old ungrounded system though. Guess I'll have to do some more research on corner grounded deltas.

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that would be about right if it was a 240V delta system. the voltage to ground is from impedance created by stray capacitance. 235/sqrt(3) = 136. So the math matches what you are reading. I suppose it could be a 230V wye system which would show the same voltages but I can't say I have ever heard of one and it would have tripped something if one of the phases was grounded.

 

[ActionDave]

..... Is it possible it created a corner grounded delta system with the C phase wire going to ground??

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Yes, it is. That is what I was trying to explain in my first post.

With an ungrounded system the first fault establishes your ground potential. Electrically speaking, the only difference between a corner grounded system and an ungrounded system is that the corner ground tied one phase to ground from the first day the power was turned on. That's why proper ungrounded systems have a light or alarm to alert folks that there is a fault, and proper corner grounded systems don't have fuses on the grounded phase.

 

[nixin9]

Yes, it is. That is what I was trying to explain in my first post.

With an ungrounded system the first fault establishes your ground potential. Electrically speaking, the only difference between a corner grounded system and an ungrounded system is that the corner ground tied one phase to ground from the first day the power was turned on. That's why proper ungrounded systems have a light or alarm to alert folks that there is a fault, and proper corner grounded systems don't have fuses on the grounded phase.

Thanks for the help! I guess I'll have to look for an alarm to this system.

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[kwired]

If your 135 volts is pretty stable on all three lines, there may be something wye connected somewhere with the wye point grounded for some reason. Such a condition would create a neutral for the system.

Another possibility is you do have an impedance grounded system, though I think it would be pretty rare to see such a thing on a 240 volt nominal system, and if done properly should still be a 4 wire supply, if you only have three wire supply it has to be corner ground or ungrounded delta or else something is not done correctly.