Can’t cite code, but “general rules” are to solidly ground if voltage-to-ground can be limited to 150V, or solidly ground if ø-n {120, 277 or 347V} loads are served. Ungrounded low-voltage distribution {Y or ∆} is permitted, but can be unreliable and disastrous from insulation damage through transient overvoltages that develop across the inherent ø-g capacitance in each phase.
In non-utility-owned, 240-600V systems where service continuity is desired in non-solidly-grounded systems, high-resistance grounding and ground detection are recognized as a reasonable compromise. Typically, non-solidly-grounded 480, 600, 2400 or 4160V are amenable to hi-r grounding.
99NEC450-5(c) Ground Reference for Damping Transitory Overvoltages is a salient reference.
[This message has been edited by Bjarney (edited 07-23-2004).]
