As a follow on question, how does the use of 'line to neutral' loads affect the installation?
I understand that NEC only permits high resistance grounding when line to line loads are served. I'd like to understand the reason for this restriction.
The text that Bjarney pointed out notes that an 'ungrounded' system is really a mis-named; instead it should be called a capacitively grounded system. A balanced, ungrounded 'wye' system will pretty much have its neutral near ground potential, until a fault occurs. Once a fault occurs, the system will 're-reference' and the neutral will be at elevated potential. The same thing happens in a resistance grounded system.
I can understand that any equipment which is designed with the expectation that neutral will be near ground potential might have a problem with this. In the event of a phase to ground fault in a resistance ungrounded 480V wye system, the neutral will suddenly go from approximately 0V to approximately 277V. At the same time, I could see that this only depends upon the quality of the insulation system; a system designed to operate with 277V between its supply terminals, with good enough insulation to operate with 480V to ground at any of its supply terminals, should have no problem operating on a resistance grounded system, even with a fault.
Am I missing other aspects of the interaction between line-neutral loads and resistance grounded systems?
The reason that this comes up is that it turns out that the control power supplies in the inverter systems are supplied line to neutral. This means that the bulk of the load is line to line, but there will be a couple of amps of line-neutral loading.
-Jon
