In an electrical vault situation is a ground conductor required on the secondaries to the first point of disc.

Answer 1

A ground is required on all systems except for deliberately designed isolated systems where use of a ground would introduce a hazard rather than protect against it (eg. hospital ICU's).

Answer 2

I'm unclear what "disc" means. However, one of the secondary terminals must always be grounded and becomes the neutral line for the rest of the circuit. Exactly which secondary terminal is grounded depends on the nature of the transformer -- three phase wye, split secondary, or single phase.

Reading your question a second time, I see that I may have overlooked the key point, "in an electrical vault situation." Perhaps your question is more about meeting the NEC regulations for electrical vaults and less about practical circuit theory than I first imagined. If so, I have nothing to pertinent offer. Sorry.......

Answer 3

It is normally so if u don't and the transformer shorts u could have several thousand volts there.