What must be true about the supply system for a ground-fault circuit interrupter to function effectively?

For a ground-fault circuit interrupter (GFCI) to function effectively, it is essential that the equipment is properly grounded. The grounding provides a reference point for the electrical system and enhances the safety and reliability of the circuit. When a GFCI detects an imbalance in the current—typically indicating a ground fault such as when current escapes to the ground through an unintended path—it relies on a grounded system to quickly interrupt the flow of electricity. This rapid response is vital to prevent electric shocks or other hazards.

While insulation and circuit independence may be relevant in various electrical safety contexts, they do not directly influence the operational capability of a GFCI like proper grounding does. In addition, while the presence of grounding is crucial for the GFCI's mechanism, the option suggesting no grounding is necessary contradicts the fundamental operation of GFCIs, which require grounding to ensure accurate detection of faults. Thus, proper grounding is not just a recommendation; it's a necessity for the effective functioning of the device.