transformer question

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buck33k

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Washington state and North Carolina
Here is a transformer I saw today in a metal fabricaion shop.
3 phase 460 delta primary, 230/133 Y secondary (see nameplate.) It has been in operation for several years. 35' secondary conductor length.
Loads are 3 ph. 230v. only.
Secondary overcurrent protection are fuses in a fuseblock bolted to the inside of the transformer.
LV conductors are color coded the same as the HV.
Xo is not bonded and there is a sign near the secondary terminals "do not ground"(see photo). What was the reasoning for this? A ground fault would energize the machinery, conduit, etc.

http://i158.photobucket.com/albums/t108/buck33k/009.jpg
http://i158.photobucket.com/albums/t108/buck33k/007.jpg
http://i158.photobucket.com/albums/t108/buck33k/006.jpg
 
buck33k said:
Here is a transformer I saw today in a metal fabricaion shop.
3 phase 460 delta primary, 230/133 Y secondary (see nameplate.) It has been in operation for several years. 35' secondary conductor length.
Loads are 3 ph. 230v. only.
Secondary overcurrent protection are fuses in a fuseblock bolted to the inside of the transformer.
LV conductors are color coded the same as the HV.
Xo is not bonded and there is a sign near the secondary terminals "do not ground"(see photo). ...
http://i158.photobucket.com/albums/t108/buck33k/006.jpg
Click to expand...


buck33k said:
What was the reasoning for this? ....
Click to expand...
Feeds a VFD?
Feeds a critical circuit that requires high reliability?
Feeds a piece of equipment that doesn't like a grounded supply?
Trying to get some isolation to inhibit galvanic corrosion?
Original installer screwed up?

buck33k said:
... A ground fault would energize the machinery, conduit, etc.
Click to expand...
Nope - it won't. But it should have ground fault detectors.

And the bonding has to be right - but this is equally important - grounded or ungrounded system, doesn't matter.

cf
 
ungrounded or impedance grounded systems operate on the principal that if a phase is faulted to ground the overcurrent protection will not open because it is not solidly grounded and it will not shut down a process that you maybe would not want to shut down immediately but rather after it is safer to shut the system down. These systems usually require some method to detect that there is a fault and to alert the operator of the equipment that there is a problem.

BTW if you allow the faulted phase to ground to continue to operate and then a second phase faults to ground you will operate the OCPD in at least one of the phases. The first phase essentially becomes a grounded conductor with the fault being the bonding jumper.
 
A Wye secondary transformer such as this one is required to be grounded. You can't leave this transformer ungrounded even with a ground detector system.

See 250.20(B)(1) and (2).

Chris
 
Chris,
I was mistaken on this for a long time.
Side Question:
Based on 250.20(B), if this transformer had a 480/277 secondary (or any voltage with a >150 to ground secondary) AND the neutral was not used as a circuit conductor, then one could elect not to ground; Correct ?
 
augie47 said:
Chris,
I was mistaken on this for a long time.
Side Question:
Based on 250.20(B), if this transformer had a 480/277 secondary (or any voltage with a >150 to ground secondary) AND the neutral was not used as a circuit conductor, then one could elect not to ground; Correct ?
Click to expand...

Correct, 250.20(B)(1) would not apply as the grounding would not limit the voltage to ground to less than 150, and 250.20(B)(2) would not apply as the neutral conductor is not being used as a circuit conductor.

In the OP they mentioned that the secondary side of the transformer was a 230/133 volt Wye so 250.20(B)(1) would apply because we can ground the system so the voltage to ground is less than 150 volts.

Chris
 
raider1 said:
A Wye secondary transformer such as this one is required to be grounded. You can't leave this transformer ungrounded even with a ground detector system.

See 250.20(B)(1) and (2).

Chris
Click to expand...
Yes, 250.20(B)(1) and (2) does say that.

However, 250.21.A.1 and .2 appear to allow not grounding certain systems. (edit to add) But this one likely is not one of these.

cf
 
Last edited:
(I didn't intend to "muddy" the OP. I agree, due to his 133 volt to ground he needs to ground his secondary. I was just making sure I understood this section correctly)
Thanks for verifying.
 
augie47 said:
(I didn't intend to "muddy" the OP. I agree, due to his 133 volt to ground he needs to ground his secondary. I was just making sure I understood this section correctly)
Thanks for verifying.
Click to expand...

Not a problem Gus, I don't think you were muddying anything.:)

Chris
 
Cold Fusion said:
Yes, that's true - in fact I think I said that - oh, you deleted that part from my quote.

But, I'm curious, how would you have known that from the OP?

cf
Click to expand...

I was going on my own experiences with wiring of metal fabrication shops.

That is why I said most likely.:)

Chris
 
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