480V Ungrounded Delta System

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350MCM

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Not sure if this should go into the grounding and bonding section.
I am working on 3 phase / 3 wire 480V delta system. I believe there is a grounded leg, but the plant doesn’t think so. I have 480V across all 3 phases. Phase A to ground 0V, the other 2 480V on one feed bus. If I go to a different bus I get 480V across all 3 phases and 277 to ground on all 3 to ground, which I normally would expect. Does this sound like a ground, as I said there is no neutral in this system.
 
Yes.. You have a grounded leg if you read a voltage to ground of zero. You have a Corner Grounded Delta System and a 3 Phase Y system. Neutral is not required unless you need to power single phase loads. I am assuming that neither of these is the Service.
 
Thanks, looks like we need to hunt for a ground. Any tips on finding ground on an ungrounded delta system. We were just going to start turning off panels.
 
True... ungrounded systems require ground detectors. From what the OP indicated... its a grounded system, a corner grounded system.
 
350MCM said:
Thanks, looks like we need to hunt for a ground. Any tips on finding ground on an ungrounded delta system. We were just going to start turning off panels.
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The grounded leg your looking for is in the transformer. Its supposed to be grounded. You dont have an UNgrounded system.
 
No sorry, it’s an ungrounded 3 phase system, with one leg grounded due to a fault. All the bus ways have grounding lights. But this section the ground indicators are not working, they are in the process of replacing.
 
350MCM said:
No sorry, it’s an ungrounded 3 phase system, with one leg grounded due to a fault. All the bus ways have grounding lights. But this section the ground indicators are not working, they are in the process of replacing.
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Beat me to it. One of the main reasons WHY people will use an ungrounded delta system is so that the first ground fault does not shut down the entire system; you just go from ungrounded delta to corner grounded delta. When I worked at a steel mill, that was the case for us; if the main breaker were to trip on a ground fault, the arc furnaces would cool down and if it went long enough, that could be a "massive lay off" level event. So by sticking with ungrounded delta, the ground lights would tell us which phase went to ground, but leave the plant up and running. Then we (electricians) basically had to drive around in our golf carts looking for a column of smoke or a production manager pulling his hair out and screaming for an electrician because his part of the plant was not working right and he wasn't going to meet quota.

The thing about your original post that vexes me is your saying you had 277V to ground on all 3 legs. Most likely that was either fed from a different transformer, or that was just coincidental. With an ungrounded delta system (where there is no fault), measurements to ground are going to be based on capacitance because there IS NO reference to ground, so sometimes it will read one voltage, sometimes another. I have never seen it be exactly 277V on all 3 however.
 
Are there separate transformers supplying the two buses where you measured the voltages? If everything is supplied from a common transformer, I don't see how you can have one bus with a grounded phase and one without. I guess if you have an intermittent grounded phase, those two measurements would be possible.

As far as the ground detectors, those are a realtively recent addition to the code, added in 2008, so an older system may not have the. They were common before the code required them, but since they were not required, some older systems do not have any ground detection.
 
Jraef said:
Beat me to it. One of the main reasons WHY people will use an ungrounded delta system is so that the first ground fault does not shut down the entire system; you just go from ungrounded delta to corner grounded delta. When I worked at a steel mill, that was the case for us; if the main breaker were to trip on a ground fault, the arc furnaces would cool down and if it went long enough, that could be a "massive lay off" level event. So by sticking with ungrounded delta, the ground lights would tell us which phase went to ground, but leave the plant up and running. Then we (electricians) basically had to drive around in our golf carts looking for a column of smoke or a production manager pulling his hair out and screaming for an electrician because his part of the plant was not working right and he wasn't going to meet quota.

The thing about your original post that vexes me is your saying you had 277V to ground on all 3 legs. Most likely that was either fed from a different transformer, or that was just coincidental. With an ungrounded delta system (where there is no fault), measurements to ground are going to be based on capacitance because there IS NO reference to ground, so sometimes it will read one voltage, sometimes another. I have never seen it be exactly 277V on all 3 however.
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That’s what is throwing me off, I didn’t use the meter myself, so they could be a little different.
 
don_resqcapt19 said:
Are there separate transformers supplying the two buses where you measured the voltages? If everything is supplied from a common transformer, I don't see how you can have one bus with a grounded phase and one without. I guess if you have an intermittent grounded phase, those two measurements would be possible.

As far as the ground detectors, those are a realtively recent addition to the code, added in 2008, so an older system may not have the. They were common before the code required them, but since they were not required, some older systems do not have any ground detection.
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All the bus work is fed from one 13.2 to 480 transformer, with separate 1600A feeder breakers for each bus way. Common switchgear line up.
 
Jraef said:
The thing about your original post that vexes me is your saying you had 277V to ground on all 3 legs. Most likely that was either fed from a different transformer, or that was just coincidental. With an ungrounded delta system (where there is no fault), measurements to ground are going to be based on capacitance because there IS NO reference to ground, so sometimes it will read one voltage, sometimes another. I have never seen it be exactly 277V on all 3 however.
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agreed.
measured one a couple of weeks back. Something like 187,316,504...
Had a heck of a time explaining to a lineman that was fine...
 
I don't think the system is fully understood by the people in the plant.

An 'ungrounded' system actually does have ground connections, via the ground detection lights (if that is what is used) and via capacitive coupling.

As noted when a ground fault occurs you end up with an unintended corner grounded system that continues to function.

But if you have a _single_ system (implied by a single transformer) you can't have corner grounding in on location and no grounding elsewhere...unless we are talking transmission line huge.

The _different_ voltage readings imply multiple systems or something uncommon like a transformer with two independent secondary outputs.

Jon
 
winnie said:
I don't think the system is fully understood by the people in the plant.

An 'ungrounded' system actually does have ground connections, via the ground detection lights (if that is what is used) and via capacitive coupling.

As noted when a ground fault occurs you end up with an unintended corner grounded system that continues to function.

But if you have a _single_ system (implied by a single transformer) you can't have corner grounding in on location and no grounding elsewhere...unless we are talking transmission line huge.

The _different_ voltage readings imply multiple systems or something uncommon like a transformer with two independent secondary outputs.

Jon
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I think they got it now, there is a ground, but it’s not bonded to a neutral. I have had a hard time explaining to them, the system is designed so that you can run with a leg grounded. People were convinced it was a corner grounded system, which now a days is rare, and this plant was built in the 90’s. The 270 was just throwing me off, just need led to confirm my suspicions.
 
350MCM said:
Phase A to ground 0V, the other 2 480V on one feed bus. If I go to a different bus I get 480V across all 3 phases and 277 to ground on all 3 to ground, which I normally would expect.
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All the bus work is fed from one 13.2 to 480 transformer, with separate 1600A feeder breakers for each bus way.
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This is not a possible combination.
 
LarryFine said:
This is not a possible combination.
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It’s a AKD-8 switch gear line up, 13.2kV through a high voltage switch to a 13.2KV to 480V delta delta transformer, through the main switch gear breaker, which feeds all the 1600A feeder breakers. Those 1600A breakers feed various busway sections.
 
If one line conductor is grounded anywhere, that conductor will measure as grounded everywhere.
 
If the A phase of the bad buss is disconnected (fuse?) and grounded somewhere out on the buss, then past that point, it would read 0 volts to ground. That still doesn't explain the 277 readings elsewhere.

I to wonder if the original readings are really what the OP was told or if they were confused.
 
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