Paul;
that would be NEC 250-32,
#1-
we can either run a separate N & G wire, and run G-rods off the G-bar ( N would be isolated, or 'floating')
OR......
#2-
we can run one conductor serving the purpose of both N & G, and run G-rods off the common bar.
this is the basic jist of the article, it is debated regularly here