Re: Isolated ground
Planet Earth is not involved in the current path. The ground rod is not relevant. Current leaves the source, and seeks its way back to the source.
If there is a leak from a hot conductor to case in any item of equipment, current will follow the EGC back to the source. If the leakage current is high enough, it will trip the breaker and terminate the event. If the leakage current is not that high, then there could be a leakage current flowing along the EGC for a long time. That will cause there to be a voltage drop along the length of the EGC. Any other equipment that has its EGC connected to the one with the leak will have the same voltage imposed upon it. Some special items of equipment could be damaged, or at least might not work right, if their metal cases have that voltage imposed upon them.
By installing a separate EGC from the ?special equipment? back to the source panel (not just to any sub-panels along the way), then the leakage current will not impose a voltage on the isolated ground wire. Thus, the leak will not have an adverse impact on the ?special equipment.?