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I have a ground problem that I can't quite get my head around. I've installed a 50KVA 240 / 600V step up - step down system to get a 200A service into the back of a customers property. I don't have a lot of experience with transformers so I gave the local inspector a call to ask about grounding and bonding requirements. He referred me to a recent Technical Safety BC bulletin that has a typical detailing minimum bond and ground requirements which I've attached.

My problem - the typical clearly details a #6 ground wire connection on the high side of the system. Every time I look at this typical and think about connecting a ground directly to one leg of a 600V secondary my brain screams that this will be a direct short and a big boom. Anyways, I went ahead and made the connection and took a leap of blind faith only to immediately trip the 200A main. Luckily no big boom.

What am I missing?
Transformer Ground.PNG

 

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Matching transformers?
 

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Bilge Rat
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A transformer, by its nature, is an isolating device. This means there is no direct connection between the low volt side and the high volt side.

The diagram is correct, you need to ground one side of the 600 volt line and only one side and only one place. Normally, there will be no current flowing in the ground jumper but in case the ungrounded side of the 600 volt line becomes grounded, the feeder breaker twill trip.
 

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Also make sure it is isolated from the bonding that is happening. That conductor need to go to the system ground and not be bonded with anything. A separate bonding conducting conductor is required for that.

Cheers
John
 

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Discussion Starter #6
A transformer, by its nature, is an isolating device. This means there is no direct connection between the low volt side and the high volt side.

The diagram is correct, you need to ground one side of the 600 volt line and only one side and only one place. Normally, there will be no current flowing in the ground jumper but in case the ungrounded side of the 600 volt line becomes grounded, the feeder breaker twill trip.
That's exactly what is happening, my feeder breaker is tripping. Sounds like I have a pre-existing ground and the one I'm adding is completing the circuit.
 

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Also make sure it is isolated from the bonding that is happening. That conductor need to go to the system ground and not be bonded with anything. A separate bonding conducting conductor is required for that.

Cheers
John
I'll check over my bonding again. I've split-bolted a run of #6 insulated copper back to the system ground.
 

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Delete: brain fart.
 

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No factory installed ground on the 600V?
 

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also, check if the transformer is not damaged before installing it and powering the circuit

electrical equipment that is either new or refurbished can still be damaged due to insulation failure
 

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If you could provide some photos of the termination points , transformer tag and connection diagram, it might be easier to deduce where the problem is.

If it were my job, I would want a disconnect on the output of the 1st transformer, with an over-current device. If you had one there you could isolate the 600V downstream feeder, transformer, and determine if the problem is on the service end or the load end. Could it be that the 600V feeder to the load transformer has an issue. What method was used to run the feeder to the second building? Was the boom concealed underground?

The diagram may be correct, but does your connections match it exactly, does the transformers have only a two wire isolated secondary as the drawing shows, is there a center tap or a factory bonding connection on either side of them?

CMP Ω
 

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What's the breaker size? A 50 KVA transformer has a pretty heavy inrush, there's a fair chance that the breaker is seeing a fault and tripping.

If it tripped with both transformers connected, try it with only one then close the second one in.
 

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Just food for thought. Sometimes bigger transformers will trip the breaker the first couple of times you turn them on, especially if the breakers a little on the small side. Then they will run fine. Did you only try to turn it on once?
 

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If you could provide some photos of the termination points , transformer tag and connection diagram, it might be easier to deduce where the problem is.
I second this, pics would be helpful.
 

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What's the breaker size? A 50 KVA transformer has a pretty heavy inrush, there's a fair chance that the breaker is seeing a fault and tripping.

If it tripped with both transformers connected, try it with only one then close the second one in.
We did something similar a few years ago and had the same problem. The inrush on the step up is huge.
 

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Lift your grounds you put on the 600v side and try again. Then read voltage to ground with a low z meter to each side. If you get 600 volts on one side and 0 volts on the other the system has a bond/ground already somewhere installed.

Cowboy
 
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If my memory serves correctly we were feeding a single phase 7200 x 480 utility style transformer with 480 2 hots and ground, stepping up to 7200 for a long feeder ryun. When we turned on the 480 breaker the transformer hummed and howled like crazy until the breaker tripped. The transformer was designed to be fed 7200 then stepped down. The 7200 side is bonded internally. The only way we could get it to work was by disconnecting the incoming ground. The engineers from the poco were able to explain it. The inspectors were not happy but after much going around every body was happy and it is still working today. There is something about the ground on the 7200 side that causes a back feed. Perhaps Paul can explain it better.
 
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