Bonding separate power systems

[Fnewman]

Situation involves two large belt conveyors, each fed from a separate industrial power system, that will meet at a property line transfer tower. What are the applicable code requirements for making sure no potential exists between The two grounding systems?

 

[paulengr]

No Code. The resistance between two grounds is proportional to the inverse of the distance.

The one exception I know of is that in mines the substations are normally grounded with two grounds. The HV side, fences, lightning arresters, frame, etc., are tied to one ground. A second ground at least 25 feet away goes to the transformer neutral and supplies the neutral and grounds for the mining equipment (LV) side. This is done to prevent power line disturbances and lightning entering the mine.

You want the grounds to be far enough apart to avoid near field coupling (one or two ground rod lengths) but not so far that it becomes remote Earth.

Regardless why would you be concerned with coupling? If we have two transformers the return path on a fault is back to the source (complete the circuit).

 

[Fnewman]

I think the concern expressed by the client is about the possibility of different touch potentials during a major fault on one of the two systems. I am certainly not an expert on that subject, but looking for some concrete information for a reply.