Harvey51 wrote:
I wired my garage according to the unofficial wiring code book. The sub box ground is a heavy plate buried in the ground (the rod couldn't be pounded deep enough). The only explanation I could find for not connecting ground to ground or neutral wires from the house was that someone might accidentally dig up the cable to the house and cut just the ground and neutral. Is there a more likely scenario that would cause a problem in the garage or RV?
"Bonding" is another word for connecting, right?
Bonding, in this case, does mean connecting neutral and ground together--more specifically, in one spot, at the main panel.
Bonding and grounding/earthing are related, but serve somewhat different purposes. The neutral line is bonded to the safety ground to provide a low impedance return path for current from the chassis of whatever is attached to the circuit, so that a fault will safely go back that way (and, if the fault is severe enough, cause the breaker to trip and shut off power) rather than through your body when you touch the device. This works independently of an actual earth ground.
Tying the neutral (and safety ground) to a grounding rod serves to keep the overall voltages in the system in line with earth potential so you don't get shocked; without it, particularly in large distributed power systems, the voltage levels could float rather far from ground potential.
For outbuildings with their own panels, there are two basic options permitted by the NEC: either the connection to the outbuilding can be made with no ground connection (a wire or metallic conduit or metal water pipe or similar) and the service at the outbuilding treated more or less as a standard service entrance with neutral and ground bonded together at the outbuilding panel; or the connection can be made with a proper ground conductor of some sort (usually a wire) and the panel in the outbuilding must not have ground and neutral bonded. In either case, the ground at the outbuilding generally has to have an earth connection to a grounding rod.
The reason for not bonding neutral and ground at the outbuilding is that in some fault conditions (such as an open neutral between the outbuilding and the main building) it can cause ground connections to be current carrying and lead to all sorts of problems, particularly with other common grounds (such as metallic conduit, cable TV coax connections, etc.). It also makes for parallel current return paths that can cause other lesser difficulties.
