Ok, let me back up again (for educational purposes). If we consider a house, no detached structure, it feeds from the mains via 3 wires & a ground to earth right, or am I missing something? Yes, but the "ground to earth" you mention is a grounding electrode conductor, not a neutral nor an equipment grounding conductor (neutral). It's purpose is NOT to be part of normal circuit operation or used during a circuit malfunction.
So in the 3 wire detached scenario, it would be 3 wires & a ground to earth (as if the main structure was the utility). I get that in the sub you'd probably have to run the neutral to ground as you would in a house (which I understand is disallowed now since any neutral feeding from the main can't be bonded). See point 2 below.
In the above scenarios, what makes one structure functionally different than the other (for breaker tripping purposes)? And for safety, if I'm reading you correctly it creates a hazard that a lineman would be cognizant of coming from a house but a homeowner would not be in the detached? Nothing to do with a lineman.
Much confusion comes from the names of equipment grounding conductor (EGC) used as a low resistance return path for fault current to trip the cb that the damaged circuit is part of
VS. grounding electrodes( e.g. - ground rods) and grounding electrode conductors (GEC) that are used to (hopefully) dissipate static electrical charge built up on a building due to wind, moisture and passing clouds
VS. that of a grounded conductor (neutral) used purposely and identified as the return path for normal circuit operation.
In this second named usage, the
ground rods hopefully
dissipate the static charge on the structure to the earth possibly collected through the multiple paths created by equipment ground wires from anything conductive they are attached to and brought back to the ground bar from the branch circuits and then out to the rods via the grounding electrode conductor. Moisture on the structure, humidity and other conductive objects with some random path to earth can also help drain off any excess charge. I use the word hopefully as resistance between the soil and the ground rods varies by location and soil moisture content. Not every EGC touches something "charged".
The
variable soil resistance is usually
not sufficient to provide a low enough fault path the trip the cb of a branch circuit experiencing a ground fault (hot - EGC). If the branch cb did not trip, then the cb for the feeder is next in line to trip. The conductivity of the soil between the 2 buildings is unpredictable but generally too low to trip the cb quickly or at all. This is why the
EGC is run between the 2 buildings to
ensure a low resistance path allowing high fault current to be created to
cause the upstream cb to immediately trip.
Point 2
In the house where the
main panel is and in this example, the first place of overcurrent protection,
the neutral (grounded conductor) is bonded to the equipment enclosure and only here. Multiple bonding points between grounded surfaces and neutral wires are not permitted as it creates multiple paths for neutral current to split between the neutral and the paralleled ground wire. Someone opening equipment grounding conductors in this paralleled path can get shocked. (In this example, the EGC is carrying current potentially all the time and not just for the instant of a fault.)
These parallel paths can be created by other paths between buildings via interconnected water pipes, drain pies, air lines, etc. The expectation of the 4th wire (EGC) run with the feeder will be such a low resistance compared to other paths. So isolating the neutral wires (currents) from the EGC, eliminates any unintended conductive paths. This also further accomplished with isolated neutral bars from ground bars in sub-panels.
