Lost leg on Delta Transformer?

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Psychlo

Psychlo

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Melissa, TX
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Master Electrician
Recently, I looked at a 30kVA, 3Ph, 480 - 240, 3wire Delta transformer that apparently had lost a leg about ten years ago. I measured the voltage at the panel being supplied by the xfmr:

A-B=240v
A-C=240v
B-C=240v
A-Grnd=240v
B-Grnd=0v
C-Grnd=240v

Can someone explain how these readings are possible? Obviously, something happened to B phase, but wht would cause these voltages?
 
Psychlo said:
Recently, I looked at a 30kVA, 3Ph, 480 - 240, 3wire Delta transformer that apparently had lost a leg about ten years ago. I measured the voltage at the panel being supplied by the xfmr:

A-B=240v
A-C=240v
B-C=240v
A-Grnd=240v
B-Grnd=0v
C-Grnd=240v

Can someone explain how these readings are possible? Obviously, something happened to B phase, but wht would cause these voltages?
Click to expand...
B-phase was purposlly connected to ground.

This is technically called a corner grounded system, but a lot of people call it a Grounded B Phase.

There is nothing wrong with your installation.
 
240 corner ground would be a rare bird around here, but then again we 240/208/120 high leg deltas everywhere.


Nothing wrong with what you have unless you need 120, You might even find you can change the taps and make it into a high leg if you need 120
 
Psychlo said:
Recently, I looked at a 30kVA, 3Ph, 480 - 240, 3wire Delta transformer that apparently had lost a leg about ten years ago. I measured the voltage at the panel being supplied by the xfmr:

A-B=240v
A-C=240v
B-C=240v
A-Grnd=240v
B-Grnd=0v
C-Grnd=240v

Can someone explain how these readings are possible? Obviously, something happened to B phase, but wht would cause these voltages?
Click to expand...

If you go to the point where the GEC terminates, probably the transformer in this case, you should see the connection between the B phase and the supply side bonding jumper.
 
Joethemechanic said:
Nothing wrong with what you have unless you need 120, You might even find you can change the taps and make it into a high leg if you need 120
Click to expand...
Most likely this is not a feasible recommendation.
The equipment is probably all rated for corner grounded 3W and not 4W, so it is possible the entire facility would need to be gutted and rewired.

The system has been running for 10 years, according to the OP. I am pretty sure they already have a method of getting 120V where it is needed.
 
There was only one 3-phase load in the panel, and that piece of equipment no longer exists. The rest are 1-phase loads (heaters, A/C, etc.).

The owner said that it lost a leg about 10 years ago and that they have "gotten by" ever since. Don't know what makes him think he lost a leg, but apparently something is not as it was originally.
 
jim dungar said:
Most likely this is not a feasible recommendation.
The equipment is probably all rated for corner grounded 3W and not 4W, so it is possible the entire facility would need to be gutted and rewired.

The system has been running for 10 years, according to the OP. I am pretty sure they already have a method of getting 120V where it is needed.
Click to expand...

OP said:
that apparently had lost a leg about ten years ago
Click to expand...

I took it to mean the transformer was deemed to be defective by someone who was unfamiliar with a corner ground. It sounds to me like this trans has been abandoned for 10 years and now he is starting from scratch. If a transformer had "lost a leg" would you have continued to keep it in service for an additional 10 years after the diagnosis (although incorrect) was made?
 
The transformer has been in use the whole time. As far as I can tell, the 1-phase loads have remained unaffected. This is an older fab shop that has had a lot of mickey-mouse repairs and add-ons over the years. That is why I didn't trust the "grounded B system", having had no prior experience with them.

The crux of the problem: He wants to add another 100 amp panel off of this transformer for some 3-ph, 240v loads for his new tenant. The existing panel is 150 amp. Location prohibits taking the new loads from there.

That being said, does a guy leave it as a corner grounded system and add the new panel? Or change it over to a standard Delta configuration (since existing loads are all 1-phase anyway)?
 
Not to mention, no need to change the rest of the building if the 120 loads are comparability small.
 

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The 120v loads are all served by another 1-phase panel and transformer, so that is not an issue. Also, the new loads will not require a neutral.
 
Psychlo said:
The transformer has been in use the whole time. As far as I can tell, the 1-phase loads have remained unaffected. This is an older fab shop that has had a lot of mickey-mouse repairs and add-ons over the years. That is why I didn't trust the "grounded B system", having had no prior experience with them.

The crux of the problem: He wants to add another 100 amp panel off of this transformer for some 3-ph, 240v loads for his new tenant. The existing panel is 150 amp. Location prohibits taking the new loads from there.

That being said, does a guy leave it as a corner grounded system and add the new panel? Or change it over to a standard Delta configuration (since existing loads are all 1-phase anyway)?
Click to expand...


Is the new panel only going to supply 240V loads?
 
Psychlo said:
The 120v loads are all served by another 1-phase panel and transformer, so that is not an issue. Also, the new loads will not require a neutral.
Click to expand...


Then I'd leave it alone and just grab your 240 3 phase from A, B &C

In a hacked up shop like that, you could be opening up a can of worms by changing grounding points.

I've been in places like that with a 3 phase trans and all single phase equipment hooked up on romex and SO cord. You can end up with a project on your hands
 
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Psychlo said:
The crux of the problem: He wants to add another 100 amp panel off of this transformer for some 3-ph, 240v loads for his new tenant. The existing panel is 150 amp. Location prohibits taking the new loads from there.

That being said, does a guy leave it as a corner grounded system and add the new panel? Or change it over to a standard Delta configuration (since existing loads are all 1-phase anyway)?
Click to expand...

Time to start over.

You gave us voltage readings that conform with a corner-grounded system.
You talk about 1-phase loads, are they all 240V? Heaven forbid, have people been running 120V loads off this existing transformer?
You want to add a new panel, will it need 120V?


A corner grounded 240V system is a 'standard delta', you are just more familiar with center-tapped systems.
Changing the system from a corner-grounded 240V 3PH 3W to a center tapped 240/120 3PH 4W may create a need for a major rewire job based on how the grounded conductor was previously installed (it should have been white even though it is a phase conductor). It is possible that all of the protective devices are 2-pole instead of 3-pole.

If three phase is no longer needed I would consider replacing this transformer with a 120/240V single phase unit.
 
Is the trans even bad? If one winding burned out, and was completly open, it would still be functioning as an open delta.


Who said this transformer "lost a leg"? And why did they say it?

I say, forget anything the Oompa_Loompas told you, open up the delta and test each winding separately.
 
The existing panel is the only load on the transformer. And it has only 1-phase, 240v loads.

There will be no 120v loads in the new panel - just 3-phase, 240v.

Yes, it was the Oompas-Loompas that said there was a leg lost. The owner is not sure what happened. He was told by "someone who looked at it" that the transformer dropped a leg.

I guess the question becomes: Assuming the trans is indeed fine, is there a practical way to achieve 3-phase, 240v (no neut.) power - other than using the corner grounded setup?
 
Psychlo said:
The existing panel is the only load on the transformer. And it has only 1-phase, 240v loads.

There will be no 120v loads in the new panel - just 3-phase, 240v.

Yes, it was the Oompas-Loompas that said there was a leg lost. The owner is not sure what happened. He was told by "someone who looked at it" that the transformer dropped a leg.

I guess the question becomes: Assuming the trans is indeed fine, is there a practical way to achieve 3-phase, 240v (no neut.) power - other than using the corner grounded setup?
Click to expand...

It depends on the trans. How many secondary leads are coming out of it, and how are they connected now?



Why don't you want to stay with the corner ground?

My thought is, if you change the grounding point, you have to check over all the other circuits, to make sure they will work if grounded in a different way. You might end up having to bring the whole place up to code.
 
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It would take a set of double lugs to come off of the trans again. But I may need to upsize the tranny anyway. It's a 30kVA. I'll have two panels coming off of it - 150amp and 100amp. I don't know the exact connected load, yet.

In the articles I've been reading, I'm finding that most people are recommending to steer clear of corner ground setups. If I have to change out the tranny anyway, I thought it might be easier (and safer) to switch it over to a more standard scenario. Since the existing loads are all 1-phase, 240v, it wouldn't require rewiring of the branch circuits, right? Or is there a possibility that they tied B-phase to ground at the equipment also?

As you may have noticed, I'm no transformer wiz. Even so, this whole system seems counter-intuitive. Identifying B-phase as a neutral, even though it is a phase conductor? Seems like we're in the Twilight Zone! But that's just my inexperience talking.
 
Psychlo said:
In the articles I've been reading, I'm finding that most people are recommending to steer clear of corner ground setups. If I have to change out the tranny anyway, I thought it might be easier (and safer) to switch it over to a more standard scenario. Since the existing loads are all 1-phase, 240v, it wouldn't require rewiring of the branch circuits, right? Or is there a possibility that they tied B-phase to ground at the equipment also?

As you may have noticed, I'm no transformer wiz. Even so, this whole system seems counter-intuitive. Identifying B-phase as a neutral, even though it is a phase conductor? Seems like we're in the Twilight Zone! But that's just my inexperience talking.
Click to expand...

Be careful about using terms incorrectly.
Stop believing what an uninformed layperson tells you, trust your training and the experts on this forum.

A grounded conductor is not always a neutral.
Corner grounded 3-phase systems have been used for almost 100yrs. Typically the only special requirement is that 2-pole breakers can not be 'slash rated' (in the Square D world you cannot use a QO220, instead you need a QO220H).

Corner grounded systems are not necessarily more unsafe than any other system just because they are 240V L-G, after all a center-tapped 240/120V system has one leg that is 208V L-G.
 
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