ground fault detectors

[marcs11]

I have a machine shop currently being served by the utility co. by a 200 amp three phase 240 volt un grounded delta service. I propose to install a new 400 amp 120/208 three phase service. Since the new machines require 120/208 currently they small transformers converting 240 to 120/208. We will be installing a new owner provided 75 kva 208 delta to 240 delta step up transformer. This will allow them to keep the older machines running the same. The question is DO we need to provide ground fault detection per Article 250.21 B. Although technically being a separately derived system un grounded. I can't believe that we could have a ground fault due to if a phase conductor on the secondary of the transformer were to short to the equipment ground the primary circuit breaker would trip, since it is solidly grounded. I can't find in the code any exception or language that would allow me NOT to install Ground fault detection.

Thanks in Advance
Marc

 

[ActionDave]

I can't believe that we could have a ground fault due to if a phase conductor on the secondary of the transformer were to short to the equipment ground the primary circuit breaker would trip, since it is solidly grounded...

That is not correct, the primary breaker would not trip. You need to ground to do a corner ground or install ground fault detectors.