Corner-grounded delta grounding

[websparky]

I have a new service installation for a pump station. The part I am not sure of is the 480V wye to 208V delta transformer. The secondary will only have one 230V 3 phase pump and nothing else. The 480V side will be grounded in the usual manner. The 230V delta side has me unsure of the code requirements for grounding the system. I think I will need to corner-ground the delta in order to have a ground-fault path for the secondary side. Right? Is there another code compliant choice?

TIA

 

[jim dungar]

Re: Corner-grounded delta grounding

Is your transformer a 208Y/120 or a 240 delta? You mentioned both voltages. If it is 240V delta then grounding one phase (usually B) is optional.

My preference is to ground a phase if the transformer feeds a distribution panel or several different loads. I will usually recommend an ungrounded delta when there is a single load. My choice is based on because of the high probability of troublshooting difficulties encountered in ungrounded systems. I always recommend a ground detection scheme for ungrounded systems.

 

[augie47]

Re: Corner-grounded delta grounding

Seems ungrounded systems are usually used to reduce shut-down of equipment in fault conditions.
Note in the '05 Code, a monitoring systems is required for ungrounded systems.(with exceptions)

 

[websparky]

Re: Corner-grounded delta grounding

Jim,
Thanks for your reply. The secondary will be 3phase only. This system will be remote to most public and surrounded by a fence. There will be no one present to monitor a detection system and I feel it would be safer to corner-ground the delta for ground-fault puposes. Do you agree?

 

[kiloamp7]

Re: Corner-grounded delta grounding

Is the 480V. side really going to be a wye connection? Is the 480V. side the primary?
What do you mean about the 480V. side being "grounded in the usual manner"?

Am I missing something here?
Or is it a 480 Delta to 240 Delta?

 

[jim dungar]

Re: Corner-grounded delta grounding

Another thing to consider is that "special" circuit breakers are required for corner grounded 3 phase systems. For example, see footnote on page 6-2 of Square D Digest 173.

 

[tepres]

Re: Corner-grounded delta grounding

Originally posted by websparky:
The 480V side will be grounded in the usual manner.

I don't believe you would want to ground the primary (WYE). My understanding is that you would "float" that connection. That is, not connect it to anything because a single phase fault could cause higher reflected fault current on the primary.
Then corner ground your secondary with consideration to Jim's point above.