Final answer:
A GFCI detects differences in current between the live/hot and neutral wires and interrupts the circuit if leakage current exceeds 5 mA to prevent electric shock.
Step-by-step explanation:
The primary function of a ground fault circuit interrupter (GFCI) in an electrical circuit is to detect differences in current between the ungrounded conductor (live/hot wire) and the grounded conductor (neutral wire). A GFCI prevents shock by sensing the loss of current to unintentional paths. If there is an imbalance, this usually indicates that some of the current is returning to the voltage source through a path other than the neutral wire, known as a leakage current. GFCIs are designed to trip and interrupt the circuit if the leakage current exceeds the threshold of safety, typically set at 5 mA, which is the accepted maximum for a harmless shock. This action forces the repair of the leakage to ensure that no hazardous conditions persist.