I'm not sure what a ring crane is. However a concern with grounding any crane in the vicinity of large power installations is the ground potential at the hook, relative to the ground potential at the crane grounding site. There have been incidents and injuries due to a potential difference between the crane ground and the landing area of the hook. If the potential for step and or touch potentials exist consider that the reach of the crane may result in a "step" of many feet instead of the normal 3 foot step. It doesn't take much of a ground potential gradient to result in a dangerous potential difference at the hook.
If this crane is running on rails you may consider grounding the rails every 50 feet or so. This would be with grounding electrodes and or bare cables, not an insulated grounding conductor from remote electrodes.
Bill
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"Why not the best?"
Jimmy Carter
