John - I'm kinda confused about something. Maybe I'm misreading here, but how are you gonna make a 1:1 isolation out of a step-down transformer?
Nevertheless - just because it's an isolation transformer does not preclude you from grounding it (the secondary, that is). And grounding the case won't (or shouldn't) have any affect on the secondary, separately derived, electrical system. I'd like to believe there's no connection there.
My suggestion, if you really really want it to be floating and ungrounded, is to look into high resistance grounding. Caution here - although it will sort of establish a reference for the system, it would probably not help in clearing a fault. I'm sure other members here can give more info on this.
Back in my Navy days on a submarine (SSN), all the electrical systems were ungrounded. Equipment grounds were continuous everywhere, but the systems themselves were ungrounded. They consisted on 120V and 450V AC, 120V 400Hz AC, and around 300VDC - all completely ungrounded.
Can you provide a little more info regarding your transformer and your application?
Radar
