Your not in one of those situations if this is the same system you were talking about in your other thread ( http://forums.mikeholt.com/showthread.php?t=107872&page=3 )
Not grounding a system is more uncommon then you may think, most electricians will never see a "properly installed" ungrounded system over 150v to ground in there career.
Let me re-phrase the beginning of this " you are in a situation where you need to ground the system almost always"