Xformer, no X0

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Mgraw

Senior Member
Location
Opelousas, Louisiana
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Electrician
Recently I looked at a transformer that kept tripping the breaker feeding it. The transformer is a 45 kva 480/277 3ph to 208 3ph with no X0 terminal. Originally the transformer feed a panel with four 208 3ph breakers. The transformer housing is bonded to building steel with a #6 solid. This was installed around 40 years ago.

About 20 years ago one of the 208 3ph breakers was used to feed a 208/120 3ph panel. FMC was run from the transformer to the 208/120 3ph panel. The Ground wire from the line side disconnect was removed from the grounding terminal in the transformer and spliced to the grounded conductor from the 208/120 3ph panel. This is a direct path back to the MDP.

The problem I found is the mechanical lug connected to building steel had become loose and was arcing.

How does this work? My guess is the "neutral" path is from the 208/120 3ph panel to the MDP then to building steel and then from building steel to the transformer housing and then finally to the transformer.
 
It doesn't work.

The only way it could work is if they had roughly balanced 120 volt loads on all three phases.

Ground does not create a neutral from a delta transformer.
 
iwire said:
It doesn't work.

The only way it could work is if they had roughly balanced 120 volt loads on all three phases.

Ground does not create a neutral from a delta transformer.
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While I don't disagree with you, It has been working for 20 years.

Also why did the ground lug arc?
 
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Mgraw said:
How does this work? My guess is the "neutral" path is from the 208/120 3ph panel to the MDP then to building steel and then from building steel to the transformer housing and then finally to the transformer.
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Is this a WYE/DELTA or DELTA/WYE setup, what is the configuration of the 208 volt side, is it WYE? How is there any connection between the 208 volt coils and the housing if there is no common point?
 
infinity said:
Is this a WYE/DELTA or DELTA/WYE setup, what is the configuration of the 208 volt side, is it WYE? How is there any connection between the 208 volt coils and the housing if there is no common point?
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Unfortunately the tag is gone. We, myself, another EC and an engineer, could only see the front and bottom inside. A cage has been built around the transformer and has limited our view. The cage is going to be removed and the transformer changed. I will ask the other EC about it when he changes it out. It is not my job, I was there as a second opinion. The engineer was the third opinion.
 
It's difficult to break laws of physics :D
If you have 120v, somewhere you will discover an XO connection...possibly to steel.
 
augie47 said:
It's difficult to break laws of physics :D
If you have 120v, somewhere you will discover an XO connection...possibly to steel.
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I agree. Been doing some research and it appears there is a way to create a neutral using a buck-boost transformer. I did notice a buck-boost transformer on the side of the panel. I assumed it was there for a 240v load but I didn't look in to it. But would that explain neutral current on the ground?
 
Mgraw said:
I agree. Been doing some research and it appears there is a way to create a neutral using a buck-boost transformer. I did notice a buck-boost transformer on the side of the panel. I assumed it was there for a 240v load but I didn't look in to it. But would that explain neutral current on the ground?
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Maybe you are thinking of creating a neutral with a zig-zag transformer. A buck boost is just an autotransformer with no method to create a grounded (neutral) conductor.
 
Mgraw said:
Recently I looked at a transformer that kept tripping the breaker feeding it. The transformer is a 45 kva 480/277 3ph to 208 3ph with no X0 terminal. Originally the transformer feed a panel with four 208 3ph breakers. The transformer housing is bonded to building steel with a #6 solid. This was installed around 40 years ago.

About 20 years ago one of the 208 3ph breakers was used to feed a 208/120 3ph panel. FMC was run from the transformer to the 208/120 3ph panel. The Ground wire from the line side disconnect was removed from the grounding terminal in the transformer and spliced to the grounded conductor from the 208/120 3ph panel. This is a direct path back to the MDP.

The problem I found is the mechanical lug connected to building steel had become loose and was arcing.

How does this work? My guess is the "neutral" path is from the 208/120 3ph panel to the MDP then to building steel and then from building steel to the transformer housing and then finally to the transformer.
Click to expand...
Would it be asking too much if you were to attach a picture of the nameplate of the transformer? It will provide the primary and secomdary winding configuration as well as the transformer taps and primary and secondary connections which would be quite helpful.
 
templdl said:
Would it be asking too much if you were to attach a picture of the nameplate of the transformer? It will provide the primary and secomdary winding configuration as well as the transformer taps and primary and secondary connections which would be quite helpful.
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You probably missed this earlier post??


Mgraw said:
Unfortunately the tag is gone. We, myself, another EC and an engineer, could only see the front and bottom inside. A cage has been built around the transformer and has limited our view. The cage is going to be removed and the transformer changed. I will ask the other EC about it when he changes it out. It is not my job, I was there as a second opinion. The engineer was the third opinion.
Click to expand...
 
Since the nameplate is gone a photo of the transformer without the cover would help.
 
infinity said:
Since the nameplate is gone a photo of the transformer without the cover would help.
Click to expand...

Yes, a good picture of the terminations which may show how they are marked, the coil faces which would show the taps and identification, and a picture as to how the leads are brought out of the top and or bottom of the coils which may be difficult to get but would also be helpful.
I find it to be unusual that you would have a 480y/277 primary and a 208d secondary which the 208d would not have an X0. If the secondary was a wye it would be a Y-Y transformer which would be very unusual. It would be an assumption on my part the you could have a 208d-480y/277 step up transformer being used as a step down and if so the 480y/277 would have an X0 and as such if fed from a 480y/277 the neutral should not have been brought out just the lines and the X0 termination on the transformer left insulated/isolated with no connections made to it what so ever.
But some good pictures may explain a lot.
 
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