I have a detached garage whose wiring is fed by just two conductors running back to the main panel. There's no ground wire in the feeder and no separate ground rod at the garage. The feeder has only one black and one white conductor. Can I add a ground rod at the garage and rewire to update the wiring to support a bare ground wire in the new wiring? The garage subpanel is modern, replacing old glass screw-in fuses with circuit breakers. That subpanel also feeds a separate shed. In both buildings a circuit tester shows a missing ground, I assume that is because there's no ground rod at the garage and no bare ground wire running back to the main panel. Basically, is it OK to have each building equipped with separate ground rods used just for the bare ground wires in those buildings?
The distance between the main panel and the garage is about 160 ft. It is underground, probably without a conduit, as none of the other wiring uses a conduit and the feeder wire just appears out of the concrete floor of the garage. Most of the construction was done in the 1960s by a non-professonal. No 240v is involved. There's a single 30 amp breaker on the feed to the garage. I don't need more than 15 amps on any of the feeds going from the garage subpanel. I realize that feeds off that garage subpanel could add up to higher than 30 and trip the breaker back at the main panel.
I have looked at the previous, similar questions but see conflicting answers. Some folks say that you cannot have too many grounds. A satellite tech recently put in a new antenna on the distant shed. He grounded that to some rebar about two feet long. After he got done that particular circuit no longer showed a missing ground with a tester. Another circuit in the same building shows open ground. I realize that two feet of rebar is not much but don't see how that could be a hazard when used just for the bare wire. I just wanted to expand on that solution.