While the NEC doesn't give a measurment of the proximity of a grounding electrode to a building or structure, it does state "present at each building or structure" indicating more than one single grounding electrode. It says one of the electrodes at each building or structure, not one grounding electrode per parcel of land. A distance of 1-6 feet would be reasonable and as was stated in an earlier post, if that was the distance between the 2 buildings you're talking about I think it would be reasonable. But 50-500 feet of distance I believe is clearly not "at the building or structure". By your reasoning, why don't we just use the PoCo's incoming neutral? It's grounded to the copper on the pole which consists of a butt wrap 6-8' deep on the bottom of the pole and is probably a better ground than any 2 driven rods. Because it's not "at each building or structure", that's why. The earth is considered to be at zero potential. In some cases, the grounding electrode serves to ground the electrical system. In other instances, the electrode is used to connect noncurrent carrying metallic portions of electrical equipment to the earth. In both situations, the grounding electrode attempts to maintain the electrical equipment at the earth potential present at the grounding electrode. Earth potential can vary over a distance of, say 10'. What good does it do to maintain earth potential of one building at another, detached building? You would likely have objectionable current on the GEC at the second building. To appreciate and better understand the purpose and function of grounding electrodes, the Soares Book on Grounding is the best place to learn.
