Electrical Panel Grounding


Beyond electrical code requirements, why does the sub panel earth ground, with its own earth ground, need to be tied to the main electrical panel ground?
wgutz
Post removed 
Post removed 
Thanks for the info. I'll test the phase of my PS units when I have them close enough for my test leads.  

I have thought of pouring a concrete pier say 6' deep and 12" diameter mixing the concrete with an additive for conductivity. Do you know if graphite would work? I once knew but have since forgotten the correct additive.

There was an electrical testing facility here that dug out the entire parking lot about 12' deep and put in a conductive 4' thick conductive concrete pad full of rebar all tied together with what looked like bronze or copper clamps. It was at least 300' by 300' in size. I guess they wanted a low resistance ground.

Bill
Post removed