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A ground fault circuit interrupter (GFCI) operates under the principle of sensing the difference in current entering and leaving a circuit. This is critical for identifying ground faults, which occur when there is an unintended path for electric current to flow to the ground.

When a GFCI is functioning correctly, the amount of current flowing in the hot wire (the one carrying current to the device) should be equal to the amount of current returning in the neutral wire. If there is any discrepancy, the GFCI detects this imbalance—typically as little as 4 to 6 milliamps—and promptly interrupts the circuit to prevent potential shock hazards. This mechanism is essential for enhancing safety in environments where electrical exposure risks are heightened, such as wet areas (e.g., bathrooms, kitchens).

Other mentioned concepts like detecting changes in temperature, measuring voltage fluctuations, or monitoring electrical resistance do not pertain to the primary function of a GFCI. Such functions are related to different types of electrical devices and not to the protective operation of a GFCI, which directly addresses current imbalances to ensure user safety.