I used to work on a mainframe computer system that was updated to isolated grounding. There were a few pieces of hardware on which it was impossible to isolate the grounds but the engineers decided this was OK. I have never seen so much blue smoke in my life as when they applied power. This happened nearly 20 years ago and the essence of ohms still lingers in the air. A few years ago all the equipment wa upgraded and it was all designed for IG and the appropriate IG system was installed. A few months ago the equipment was expanded and it was decide that IG was not needed so now we have bare ground wires laying in bare metal cable trays attached to IG ground points, cable trays are fastened to earth ground.
Jimmie