Re: Sub panel question
eprice:
I still think that you are visualizing paths that cannot carry current in the direction that you are describing. So let?s pin down a couple of basic principles, and see where we can go from there.
First, no current EVER flows in the GEC (i.e., the wire that connects the main panel?s ground bus to the grounding electrode ? be that electrode a ground rod or a water pipe or whatever), and no current EVER flows through the grounding electrode itself, UNLESS some piece of equipment has a fault to ground (e.g., hot wire to case), or unless there is a lightning strike. Reason: There is not a complete path. The resistance between all current-carrying components (e.g., the transformers, panels, wires, and loads) and planet Earth is essentially infinite.
Secondly, if there is a fault within a component (e.g., hot wire to case), the current flowing in the fault path (i.e., source to panel to fault point to case to EGC to panel and back to source) should be high enough to trip the breaker and terminate the event.
Third, although it is true that current will take all available paths, it does not travel in all available directions. The direction of the push (i.e., from source hot to source neutral) is the only direction that the current can take. No matter what fault conditions exist, no matter if an unfortunate person is touching the wrong component at the wrong time, current cannot flow from the main panel, via the feeder EGC, in a direction that takes it to the branch circuit EGC. The voltage source will push any and all current within the EGC in the opposite direction ? towards the main panel. This is not a matter of ?how much resistance does one path have, in comparison to that of another path.? It is a matter of current goes in the direction in which it is pushed.
As to the potential danger of a fault path that includes planet Earth, there are (sad to say) a vast number of documented cases in which a person holding a device receives a shock, with the only possible path through their body necessarily including the dirt beneath their feet, and with fatal consequences. It only takes 0.1 amps to cause a fatality. On a 110 volt system, that means the total path resistance would have to be less than 1100 ohms. The resistance of the body itself can easily be under 300 ohms, and the resistance of the ground rod to earth should be under 25 ohms. Unless the person is wearing rubber boots and is standing on dry desert sands, the resistance of the dirt can easily be under 700 ohms. As I say, it has happened many times.