What I have read elsewhere is that the grounding resistor should be able supply a minimum of 2 times the capacitive that would flow through a ground fault but no less than what it takes to actuate ground fault alarms and ground fault protection for equipment.

However, ground fault current also needs to be limited to a level that will not cause ground faults to burn clear. If you are using a shielded extension cord so that cord cuts a nail punctures will be shut off by ground fault protection, it makes no sense if the fault burns clear. Actually, one failure mechanism for a cord is that a wire strand breaks and then works its way to the surface or to the other conductor in which case the conductor shielding will cause that strand to ground fault and the ground fault MUST NOT burn clear.

One thing that electrical engineers sometimes forget is that the grounding resistance also needs to withstand a power cross with a control voltage such as 120 volts. This would mean a 397 volt resistor for 277Y480 impedance ground and 267 read 480 volts for 347Y600 volts. The ground fault current may or may not be enough to blow control circuit fuses. Control circuit fuses also need to be able to interrupt the higher voltage but not necessarily at short circuit levels.

Mike Cole, mc5w at earthlink dot net