Ungrounded Delta Ground Fault Through a Transformer

[thomasmwilson89]

I have searched pretty hard for this scenario and haven't had any luck, hopefully you guys can help.

I work at an industrial plant with 480V three phase ungrounded deltas for most MCCs. The MCCs all have ground fault indicating lights and the other day we had B Phase light go out in MCC #1 and MCC #2, Phase A and C lights lit brighter, as they were obviously getting higher voltage due to the ground fault. We began shutting off buckets until we got to a 480V to 240V transformer, which corrected the ground fault. This transformer then feeds a main breaker panel, which then feeds a sub panel. On this sub panel circuit #13 is a 2 pole 240V breaker for a blower fan for an air conditioner (not the condenser). Upon turning this breaker off, the ground fault disappeared. We disconnected the fan motor and turned the breaker back on, and everything went back to normal.

So as most of you know, the reason the ground fault lights act as they do is that when a ground fault occurs you are now reading 480V (in this case) on A and C phase to ground (which is now essentially B Phase), and 0V on B, as there is no longer any potential. In this particular case though, there isn't 480V going to ground, there is only 120V going to ground. In theory I would assume the breaker should have tripped, but there's a chance it's oversized (somewhat beside the point). So the question is, how and why did this happen? We don't technically have a 480V short to ground, so why are the lights acting as if we do (voltage across A phase light and C phase light was 480V)? I've talked this over with a few coworkers and no one has had a solid concrete answer other than "PFM". Some people speculate there is some sort of back feeding, but they don't really know, or can't explain it.

Thanks in advance!

 

[winnie]

What you describe suggests that the fault was galvanically connected to the 480V system.

Is it possible that the 480V to 240V transformer was an autotransformer rather than a more common isolation transformer?

Jon

 

[__dan]

My earlier post was deleted.

The 240 V secondary should be only magnetically coupled to the service's floating delta.

The fact that the secondary load, fan breaker, did not trip, would indicate that transformer secondary is not properly grounded. I would check that first, if the 240 is a grounded system and the system bonding jumper is installed. I would physically look at the system bonding jumper, and verify the secondary line to ground and line to neutral Voltage.

The fan breaker was supposed to trip, if that system is grounded.

 

[thomasmwilson89]

- Just checked, it is not an autotransformer, so no physical connection there
- I agree that the breaker should have tripped, and hopefully once we fix the fan motor, we'll also take a look at why it didn't trip (breaker size, transformer connections, etc.). That isn't necessarily what I'm looking for the answer to though (although i appreciate your input). I guess if you boil it down to a very basic questions, I'm just wondering how 120V going to ground can look like 480V going to ground according to the ground fault lights.

 

[wwhitney]

Seems like there must be a fault connecting a 480V system conductor to a 240V system conductor. Presumably that could be tested for via a temporary set of "ground" fault detectors detecting against the 240V system conductors (one at a time) instead of ground.

Cheers, Wayne

 

[__dan]

120 / 240 is usually either red leg delta 3 phase or single phase. Either would be a grounded system for your application. Do you know if the system bonding jumper is installed and effective. Really need to know this first. Is that 240 secondary solidly grounded, then if so why did the fan breaker not trip (assuming that motor shorted to ground).

The light indicator on the service indicates it is working properly. The problem is the secondary may not be grounded. Another possibility is not indicated until this is known.

 

[winnie]

Agreed. If the 240V system is also not grounded, and there is a connection from a 480V phase to a 240V phase, a ground fault on the 240V side would show up on the 480V side.

If there is true galvanic isolation between the two systems, then perhaps some very strong capacitive coupling between the two systems could make the 480V system 'look grounded'.

Remember that these things we call 'ungrounded' are really capacitively grounded to surrounding metal, and also resistance grounded through the ground detection lamps.

Jon

 

[thomasmwilson89]

- My only issue with saying that there is any sort of other problem is that when we removed the 240V motor from the equation, the problem was fixed. That's not to say its not possible, but it would be more coincidental if something else other than that motor was the issue.

 

[jim dungar]

...just wondering how 120V going to ground can look like 480V going to ground according to the ground fault lights.

Where are you getting 120V? I thought your transformer was a 240V ungrounded delta secondary.

 

[winnie]

120 / 240 is usually either red leg delta 3 phase or single phase. Either would be a grounded system for your application.

Change 'would' to 'should' and I agree 100%.

Jon

 

[wwhitney]

- My only issue with saying that there is any sort of other problem is that when we removed the 240V motor from the equation, the problem was fixed.

You can't really say that the problem was fixed, just that the indicators you have installed to check for certain problems went back to normal. There could still be a silent problem, and the odd behavior of the 240V motor failure suggests there is a silent problem.

Cheers, Wayne

 

[winnie]

- My only issue with saying that there is any sort of other problem is that when we removed the 240V motor from the equation, the problem was fixed. That's not to say its not possible, but it would be more coincidental if something else other than that motor was the issue.

If the 240V system is not grounded and connected to the 480V system, then both would appear to function without apparent problem. Thus there may be 2 faults without it being a wild coincidence.

An analogy is a supposedly ungrounded system without fault detection. You wouldn't know there was a problem until the second fault.

Jon

 

[wwhitney]

Where are you getting 120V? I thought your transformer was a 240V ungrounded delta secondary.

If the secondary is 240V ungrounded delta, then it should have its own ground fault detectors, yes?

If guess if the secondary is grounded, and one of the primary conductors is faulted to one of the secondary conductors, that will (conductively) reference the primary conductors to ground, which should show up on the primary ground fault detectors.

Cheers, Wayne

 

[__dan]

- My only issue with saying that there is any sort of other problem is that when we removed the 240V motor from the equation, the problem was fixed. That's not to say its not possible, but it would be more coincidental if something else other than that motor was the issue.

The only issue is the faulted motor load on the secondary did not trip the breaker.

The fact that the problem was observable on the primary side Voltage imbalance detector lights is relevant but not necessary. For your application, fault detection on the secondary side did not work. Either the breaker or the system bonding jumper.

If you find that secondary is also floating, you are advised to ground it per code. A quick Voltage check of secondary line to ground, line to neutral, ground to neutral Voltages should tell you how close you are to the problem.

 

[thomasmwilson89]

Where are you getting 120V? I thought your transformer was a 240V ungrounded delta secondary.

The transformer feeding the MCC is an ungrounded delta, the transformer that is fed from the MCC feeds a 120/240V panel from a single phase transformer. I assume the motor in question was having issues with only one phase going to ground otherwise we'd have a phase to phase issue which i can only assume would have caused MUCH bigger issues. Would 120V not be the right way to describe only one phase from a 240V circuit?

You can't really say that the problem was fixed, just that the indicators you have installed to check for certain problems went back to normal. There could still be a silent problem, and the odd behavior of the 240V motor failure suggests there is a silent problem.

The indicators are where my head is really at right now. By the fact that the B phase light was out (AKA zero potential), and A and C phases were reading 480, that tells me that my ground wire had to have had 480 on it. When the indicator lights returned to their proper state, the only thing that was changed was a 240V motor being removed and it still just doesn't make sense to me.

I'm not at the plant currently (Graveyard last night), so I can't check or test anything unfortunately. I can only assume that everything on the secondary transformer is properly grounded/bonded as everything in the main panel and secondary panel function and have been functioning without any issues.

Not to say there isn't/couldn't be other issues with the system, but in strict terms of the indicator lights showing 480V, does the grounding really matter? Unless the ground was PHYSICALLY connected to my 120V "problem phase" and some other 480V phase, i dont see why that ground had 480V on it. And if it did happen to be connected to a 480 phase, then I'd imagine we would still have the indicator lights acting as though they saw a fault. Am i totally missing something here?

 

[wwhitney]

The indicators are where my head is really at right now. By the fact that the B phase light was out (AKA zero potential), and A and C phases were reading 480, that tells me that my ground wire had to have had 480 on it.

It tells you the ground wire was faulted to B, that B became grounded.

Which two phases of the 480V delta supply the single phase 120/240V transformer? Are you certain that the the 120/240V secondary has a main bonding jumper installed, connecting the neutral to the EGC?

Cheers, Wayne

 

[thomasmwilson89]

It tells you the ground wire was faulted to B, that B became grounded.

Which two phases of the 480V delta supply the single phase 120/240V transformer? Are you certain that the the 120/240V secondary has a main bonding jumper installed, connecting the neutral to the EGC?

Specifically that B was grounded with 480V, correct?

Unfortunately I don't know which phases supply the single phase transformer and cannot confirm right now that the main bonding jumper is installed, but can only assume (sorry, i know assuming is bad) that it is.

 

[wwhitney]

Specifically that B was grounded with 480V, correct?

480V refers to a voltage between two points. Grounded means 0V to ground. So with B grounded, A and C will be at 480V to B and to ground. I wouldn't say B was grounded with 480V, but you could say it was grounded in a 480V system.

Unfortunately I don't know which phases supply the single phase transformer and cannot confirm right now that the main bonding jumper is installed, but can only assume (sorry, i know assuming is bad) that it is.

Next things to check when you have the chance. One indirect way to check if the 120/240V system has a system bonding jumper (I used the wrong term, MBJ I think is reserved for services) is if there's a GFCI receptacle that definitely has a proper EGC, an external GFCI tester would trip the receptacle only if there's an SBJ.

Cheers, Wayne

 

[thomasmwilson89]

480V refers to a voltage between two points. Grounded means 0V to ground. So with B grounded, A and C will be at 480V to B and to ground. I wouldn't say B was grounded with 480V, but you could say it was grounded in a 480V system.

I suppose I didnt write it up exactly how i should have, but yeah what i ultimately meant was that in order for B Phase to be at 0V to ground and A and C Phase to be at 480V to ground, B phase would have to be faulted somewhere in the 480V system. The reason I find this important is that when we isolate a 240V circuit that occurs after an inductance based transformer (AKA, no physical connection on phases), we suddenly go back to 277V on A, B, and C to ground (normal lights). Not to discount any advice put in this thread so far, but the way my brain is seeing this right now is that unless my 240V system has a physical connection with the 480V system (phases, not grounds), then i dont understand how I get 480V/0V at the lights when a 240V circuit grounds itself.

I know electricity does goofy stuff, especially on ungrounded deltas, but let's just play devil's advocate and say that grounding is messed up in some fashion, but there is no physical connection between the 480V phases and the 240V phases. How could this yield the 480V system being grounded, when it's actually a 240V circuit that is grounded?

 

[jim dungar]

I know electricity does goofy stuff, especially on ungrounded deltas, but let's just play devil's advocate and say that grounding is messed up in some fashion, but there is no physical connection between the 480V phases and the 240V phases. How could this yield the 480V system being grounded, when it's actually a 240V circuit that is grounded?

Check the 120/240 transformer grounding.
Rule out a physical connection between the systems.
Measure the leakage, or capacitive coupling, current on your 480V system with the small transformer connected and disconnected (I don't expect it to make a difference).