WYE WYE

[brian john]

I have a UPS that is 480 (3 phase 3 wire input) and 208/120 3 phase 3 wire output. There is an external 2 circuit breaker bypass cabinet and a factory furnished 208/120 wye to 480/277 wye step up transformer.

This facility is 208/120 wye throughout. The Y/Y transformer is for stepping up the input to the UPS. XO/HO connection is a welded bus common connection.

Normally with a 208/120 wye to 480 delta (back fed transformer) one does not install a neutral to the XO. Does anyone feel a neutral is need for this installation?

Without a neutral the XO/HO terminal would need to be grounded, with a neutral on the load side this connection would be grounded at the main service for this facility.

 

[earlydean]

Re: WYE WYE

Are you saying you feel you need to ground the primary side of your step-up transformer, and not the secondary side because it is 480 volts delta?

 

[brian john]

Re: WYE WYE

What I am asking is:

1. Do I need a neutral on the low voltage side (line)?

2. If I do not install a neutral on the low voltage side then I will need to ground the high voltage side (load).

3. The manufacture was contacted and they basically said to do what I like. Problem with this is the low voltage side (line) may not have a neutral (the way I think it should be connected), but by grounding the high voltage side (load) I will be grounding the line and load primary/secondary of the transformer. Could this cause any problems with current from the low voltage side (line) on the grounding conductor?

 

[hurk27]

Re: WYE WYE

Brian 250.20(B)(2) only requires the XO/HO to be grounded if the neutral is a circuit conductor, and as long as no current is being utilized from it it is not a circuit conductor. this would be using the transformer as a delta 208 to delta 480 to feed the UPS delta 480.

But now you have a Separately Derived System.

250.30(B)(1) only requires that the metal enclosures of the derived system be connected to the grounding electrode. but allows us to make this connection at any point on the separately derived system from the source to the first system disconnecting means.

Now for fault path:
you must supply fault path from anywhere in the system where a fault might occure back to it's source.
The conductors between the supply to the transformer then from the transformer are only required to meet this when a second fault occures as in 250.4(B) (4)

To facilitate the operation of overcurrent devices should a second fault occur on the wiring system.

But you must also supply a fault current path on the output of the UPS of course this would be the bonding of the neutral to the system grounding. any fault occuring in this system will only return to this source. the rest I think you know.
I hope this was what you were looking for. Wayne

[ June 06, 2004, 12:20 PM: Message edited by: hurk27 ]

 

[wthomas]

Re: WYE WYE

Recommend consulting the Eaton/Cutler-Hammer 2003 Consulting Application Guide, 2003 Edition,
Page 1.1-5 for recommendations and effects of gounding one or both sides of a Y-Y connection. Note that Y-Y grounded on secondary side only is not suitable for line-to-neutral loads!