No problem Secret, disagreement is over. I apologize if I made you feel I don't think your qualified. You are obviously better qualified than I in this area of design.
I don't see much in 250.30 that really helps much. Other than if seperately derived you would have to be not connected to the building supply neutral. I believe this would just be a plug in transformer that is connected to the buliding electrical supply neutral , however if I am understanding correctly the isolation comes from a neutral to ground bond on the secondary. This to me would indicate a seperately derived system. If the transformer is dedicated to a load and is a 1 to 1 ratio then it would seem that the equipment being supplied would have its own neutral to ground reference independent of the building supply. Sooooo it looks like you would effectively eliminate any coupled conductor noise from a common building supply ground.
Now I believe BCK is asking which type isolation transformer does he need. So to be truly isolated you would want one that only has ground on the output of the transformer. Thats how I see it.
Thanks Stubbie... your explanation makes total sense to me and very well said. I think one of the hardest things to explain to a layperson is the concept of bonding a neutral. All common sense and intuition tends to make one think that this connection sets the return path for current to go straight to ground.
Admittedly, I am very rusty in my code recitation. I appreciate you looking up that article. I'm holding out until September when they release the new 2008 NEC and then perhaps I can at least be a little better informed.