What does a Ground Fault Circuit Interrupter (GFCI) monitor?

A Ground Fault Circuit Interrupter (GFCI) is designed to monitor the difference in electrical current between the hot and neutral wires in a circuit. It detects any imbalance in current flow, which can indicate that current is leaking through an unintended path, such as a person who is grounded, thus posing a shock hazard.

When a GFCI senses that the current flowing through the hot wire differs from the current returning through the neutral wire by a certain threshold, typically 4-6 milliamps, it quickly interrupts the circuit to prevent serious injury or even death from electric shock. This critical safety feature is particularly important in environments where water is present, as the risk of electric shock is greatly increased.

The other options, while important aspects of electrical systems, do not pertain directly to the primary function of a GFCI. The length of the electrical circuit, voltage of the circuit, or the temperature of the wiring are not monitored by a GFCI, making the focus on current difference the key aspect of its operation and protective functionality.