Ungrounded system questions

[jerryatric]

These questions are pertaining to utilizing a shore power system (480 volt ungrounded delta ) to power painting , blasting and hvac equipment on a construction site. While I have installed these systems I have never used one in this manner. I believe said usage would be code compliant : all equipment grounding conductors will be in place and the building has an extensive ground grid with many grid to building steel connections , transformer case to ground grid to building steel to EGC's will be verified , double ended sub station has a ground detector at each end , signage as per code will be
installed. Electrical personnel will be on site at all times . Some vendor supplied temporary panels will be utilized , installed breakers in panelboards are straight 480 or 600 volt rated. Worker safety is my greatest concern , however if I understand correctly this system would be comparably safe to a grounded wye system providing all equipment grounding is installed as per code. On reading older posts it seems damage to electronics can be more of a problem on this system , which is a concern since most of said equipment employs vfd or soft start control. The substation has surge suppressors at each end ; these we will upgrade . Any thoughts , help or advice is greatly appreciated.

 

[zbang]

Yes, be wary of equipment designed for wye-connected 480v, if they have any components from phase to ground, the bad kind of hilarity will ensue. When in doubt, check with the manufacturer.....

Why ungrounded delta at all? Could you add a zigzag transformer to establish a ground reference?
Also, are there any OSHA rules that would apply? I don't think there's anything relavent in art. 590.

 

[iceworm]

I reviewed my response. I did not add much to Zbang's response. Although I would not bother with zig-zag. Not much different from using grounded Y. Perhaps consider my response as curiosity questions about how you got stuck with ungrounded Delta.

Assumption: "shore power" and "construction site" indicate temporary power.

As zbang said, I don't see any code issues with using ungrounded 480D. Art 590 doesn't have anything to say.
That said, I put in a lot of temporary power. It is never 480D. It is all grounded Y. Most of my customers systems are HRG, but not the temporary power.

I highly suspect your concerns (and zbang response) about the VFDs and soft starts are well founded.

My question would be, "What is the advantage of 480D over 480Y (grounded)?

My general dislike of 480D is influencing my response. I have never seen any advantage for 480D - unless the specific equipment (such as an alien containment field) requires it.

You did not mention it, but ground fault indication is required. Unless there is an overriding reason for continuity of service (unusual in temporary power), my inclination would be to trip on GF - not just indicate. Maybe the ground detection at the substation takes care of this

double ended sub station has a ground detector at each end

This ones puzzles me a bit.
Assuming the line up is:

xfm - Main CB - Tie CB - Main CB - xfm​

And the xfm secondary's are 480D (ungrounded)​

If so, 480D is sufficiently rare, I have never seen one.

Q1: Is the completed project power source to be 480D (ungrounded)? if so - yuck (engneeringly speaking, of course)
Q2: Can you parallel the two transformers, as in run with both Main CBs closed and the Tie CB closed?

If so, how did you do the initial phase verification across the tie? Ungrounded D can float anywhere from zero (grounded) to 480V. Measuring across the open Tie CB could be anything. I guess one could open all the feeders and the phase to ground voltage could hover around 277V. I don't know, I have never tried to sync two ungrounded Deltas​

Your description looks safe - just flat freekin' strange.