The objective of a safety ground (tied to earth) is to give a true 0V reference. That in turn should provide a non current carrying path throughout your electrical subsystem for safety-fault conditions. If you have not installed isolation transformers or power subsytems, then one should be enough. If there are isolation transformers, or power subsystems, there needs to be a ground rod for each. Safety ground should never carry current except in a fault condition. The metal frame on each of your machines should be tied to the safety ground.
Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
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