There are some Cahier Techniques papers on this subject over at Group Schneider's website that specify different and much more robust ratings for grounding resistors and transformers.

These documents are:

ect062.pdf neutral earthing techniques

ect190.pdf ferroresonance

Possibly also ect181.pdt directional protection

One of these documents recommends that the available current from the neutral grounding resistor be at least 2 times charging current, twice as much as the I-gard people recommend.

A resistor that is on the secondary of a wye primary broken 120 volt delta secondary should be rated for continuous use at 360 volts, not the 208 volts that I-gard recommends.

MY recommendation is that a neutral grounding resistor and any grounding transformer has to withstand what happens when there is a power cross between the resistance grounded system and the next lower voltage such as 120 volt control power. This can happenvery easily if somebody is troubleshooting using a jumper wire or if a forklift truck shears off a power feed that contains say 480 volts resistance grounded and 120 volt auxiliary power. A zero phase shift ( delta primary zig-zag secondary ) transformer creates the most amount of voltage across a neutral grounding resistor when a power cross occurs.

MY opinion is that the primary windings of a wye primary broken delta secondary transformer should be rated 600 volts on a 480 volt resistance grounded system. In this case a power cross to 120 volts from a more common delta primary wye secondary transformer could put 600 volts across one of the grounding transformer primary windings. Obviously, we would want the transformer to handle this without saturation until protective relays or manual action removes the fault.

This is just a difference in opinion between different electrical engineers as to what values are required.

Michael R. Cole, mc5w at earthlink dot net