Why can't the earth be used to clear a line-to-case ground fault for premises wiring?

The reason the earth cannot be used to clear a line-to-case ground fault in premises wiring is primarily due to the high resistance to current flow in the ground. When a ground fault occurs, the fault current needs a low-resistance path to flow effectively in order to trip a protective device, such as a circuit breaker. If the resistance is too high, the fault current may not be sufficient to trigger these protective devices, which could lead to a dangerous situation, including potential electric shock hazards and equipment damage.

Proper grounding systems are designed to ensure that fault currents can quickly and effectively return to the ground, minimizing the chances of electrical hazards. However, relying on the earth without a dedicated grounding conductor can result in inadequate fault current paths, ultimately failing to provide the necessary protection. Thus, using the earth this way is not a reliable practice for ensuring safety in electrical installations.