Incorrect hookup of Transformer

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diamonddean

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Location
Oak Ridge TN
Background
We have a 150KVA pad mounted utility transformer (12470/480V) to feed a building.
During construction, contractor inadvertantly connected building nuetral to Phase C of Transformer. The only load connected at the time was a small construction panel and a couple HVAC units. When the 480V AC units wouldn't run, troubleshooting discovered the problem.

Question: Shouldn't a transformer primary fuse (15A Expulsion link and 80A current limiting) have blown when they energized the Xfmr? If not then what is the potential for someone getting a shock?
Thanks
 
Hard to tell without having an accurate diagram but it is entirely possible the only fault path back to the source was via the earth between the building and the transformer. If this was the case the load on the transformer would be insignificant and not blow the fuse.

The shock hazard could be high.
 
The electrical Inspector concluded there was no shock hazard as all of the building steel and building ground was connected to the neutral. The distance (100') from the bldg to the xfmr with dry sandy soil between yielded no current path to the transformer ground.
Where would the shock hazard be?
 
diamonddean said:
The electrical Inspector concluded there was no shock hazard as all of the building steel and building ground was connected to the neutral.
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Which would mean that all the building steel and grounds would be at 277 volts above the potential of the earth.

Had someone grabbed building steel while standing on dirt I think they could get blasted.
 
But the building was grounding to the earthat multiple locations causing the earth potential near the building to also be at 277V. ie., no potential difference therefore no shock. The disconnect downstream of the city transformer is mounted on the outside of the bldg. It was operated in this condition without incident
 
Off the top of my head, the shock hazard would have been similar to that of a corner-grounded system.

But equipment that expected to see 277V L-N would have been exposed to 480V instead.
 
diamonddean said:
But the building was grounding to the earthat multiple locations causing the earth potential near the building to also be at 277V. ie., no potential difference therefore no shock. ...
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It really doesn't work that way. If you are more than 3 feet from the grounding electrode you will be sujected to about 85% of the votlage that is connected to the grounding electrode. I think eveyone was just very luck here.
 
So your saying that someone standing 3' away that touched the ground rod would see 85% of the 277V? The entire area around the switch is concrete covered. What would someone have to touch or come between to see the potential? the utility transformer was solidly gounded
 
iwire said:
Except that in this case the wye transformers XO was likely already connected to earth out at the pad.
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Yes that makes a difference.
I can assume conditions that would not cause problems as well as those that would.

We need more information from the OP as to all of the transformer, grounding, and service entrance equipment connections.

I would be more concerned with what equipment got abused rather than what hazard may have been created.
 
Kind of a silly graphic but it is relevant.


touch.gif


Grounding electrodes do not do that much to protect people from shocks at these low voltages.

With high voltages they can be more effective as they can cause high voltage overcurrent devices to open.
 
This condition lasted nearly 2 weeks, with alot of people going in and out building with no problems. construction power was via a small transformer hooked between the A&B phases , therefore no issues there. I have been told that since the building was well grounded thoughout and the utility transformer was grounded at the center tap that no one was in danger to getting shocked, unless they somehow got between the nuetral and ground. Is this condition similar to a corner grounded Delta transformer?
 
There are lots of varibles that can affect the shock level, I had a service call on a big box store where their pylon sign was not working, I found the voltage from the neutral to ground was 480 volts, as well as hot to ground, but hot to neutral was 0 volts. Traced it back, and found a improperly marked ground for the parking lot lights was terminated on an "A" phase breaker, while the hot and neutral was terminated on two different "B" phase breakers. Even though 30 parking lot poles with rebar and ground rods in each one, along with a metal pylon sign pole buried at least 15-20 feet did not have a low enough resistance to trip a 20 amp 277 volt circuit. This store had been open about a month, with no one complaining about getting shocked. Soil conductivity has a lot to do with it.
 
I think you just got lucky it was still under construction. No barefoot people.

In the graphic above it shows the knee on the ground but if you were standing there touching the pole in the same scenario with shoes on, you wouldn't notice a thing. Like a bird on a wire.
 
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