Multi-SDS and 3P xfer switch...its in my lap

[Jimmay]

I was recently called in to evaluate a multiple building/multiple service (12.47kV delta - 480V/277Y 3P, 4W) facility that had an emergency generator (1500kw) installed to serve loads in both buildings.
The rub is 3-pole transfer switches were installed in both buildings. Short of recommending the replacement of all transfer switches with 4-pole switches is another solution available? I?m currently digging into 250.30(A)(4) as a last hope, or at least I have not been able to poke holes in applying it...yet. I?m open to anything you can throw at me (no sharp objects please).

 

[jghrist]

You could leave the generator neutral unbonded at the generator. Then it would not be a separately derived system and the neutral would be bonded to ground only at the service entrance. The problem with this solution is ensuring that there is enough ground current to trip the breaker when running on the generator. The ground fault path will be through the main bonding conductor at the service entrance and could be quite long, depending on the location of the generator.

See http://ecmweb.com/mag/electric_grounding_nonseparately_derived/

 

[steve66]

You could leave the generator neutral unbonded at the generator. Then it would not be a separately derived system and the neutral would be bonded to ground only at the service entrance. The problem with this solution is ensuring that there is enough ground current to trip the breaker when running on the generator. The ground fault path will be through the main bonding conductor at the service entrance and could be quite long, depending on the location of the generator.

See http://ecmweb.com/mag/electric_grounding_nonseparately_derived/

I don't think that will help with two transfer switches. You still need two Neutral-to-Ground bonds (one at each building). That will allow the neutral current to flow on the ground wires.

The only other solution I can think of would be a delta to wye transformer prior to one of the transfer switches. That would break the neutral wire, and put one of the N-G bonds on the transformer secondary. That has got to be as expensive as transfer switches, though.

Are they having problems?? How did they realize they should have gone with 4 pole switches?

Steve