What principle does a GFCI protect against?

A Ground Fault Circuit Interrupter (GFCI) is specifically designed to protect against ground faults, which occur when electricity unintentionally flows through an unintended path, typically when a person comes into contact with a live wire and the ground. The GFCI continuously monitors the current flowing through the circuit. If it detects an imbalance—indicating that some current is leaking to the ground (which could pose a risk of electric shock)—it quickly shuts off the power, thus preventing a potentially lethal situation.

While electric shock can occur due to ground faults, it is the detection of these faults that the GFCI is designed to guard against. Therefore, the principle that a GFCI protects against is primarily ground faults, making that the most accurate response. The protection from electric shock is a consequence of the GFCI's primary function of detecting ground faults in the circuit.