Why does the Code require the effective ground-fault current path to be a low-impedance metallic path rather than a connection to the earth?
- Because enough current must flow to open the overcurrent device quickly, and the resistance of soil is far too high to allow that amount of current.
- BBecause connections to earth corrode over time and gradually lose the ability to carry fault current back to the transformer that supplies the building.
Corrosion is a real maintenance concern for buried connections, but it is not why the earth is unsuitable. Even a brand-new, perfectly made earth connection has far too much impedance to clear a fault.
- CBecause the earth conducts current in only one direction and therefore cannot serve as a return path for an alternating current fault.
The earth conducts alternating current in both directions like any other conductive medium. The limitation is resistance, not directionality, and a rod passes only a few amperes at 120 volts.
- DBecause a metallic path is what keeps the voltage on the grounded conductor from rising above the voltage of the utility neutral at the pole.
Stabilizing voltage with respect to earth is a purpose of the grounding electrode system, not the reason the ground-fault current path must be metallic and low impedance.
Why A is correct
Overcurrent devices are current-operated. A path with high impedance limits fault current to a value below the trip threshold, so the faulted metal stays energized indefinitely. Only a low-impedance metallic path back to the source produces a fast trip. A path through soil simply cannot deliver that much current at utilization voltage.
What this question is testing
Whether you understand the physical reason behind the entire equipment grounding scheme, which is the concept that unifies bonding jumpers, equipment grounding conductors and the prohibition on using earth as a fault path. It also checks that you can reject any answer routing fault current back to the source through the soil, which is the single most reliable filter on grounding questions.
On the job
Run the arithmetic once and it sticks: a 120-volt circuit faulted to a metal box that is connected only through 25 ohms of earth passes under 5 amperes. The breaker never moves, the box stays hot, and the first person who touches it becomes part of the circuit. That is why a detached building still needs an equipment grounding conductor run with the feeder even though it has its own rod, and why a bootleg ground on a metal water pipe is not an acceptable substitute.
Memory technique
Dirt does not trip breakers.
Exam tip
Any answer that has fault current returning through the soil is wrong.
Where to look it up
The performance requirements for the effective ground-fault current path are in the general section at the front of Article 250, and the term is defined in Article 100.