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A GFCI, or Ground Fault Circuit Interrupter, is designed to protect against electrical shock by monitoring the current flowing through the circuit. It is typically required to operate at a voltage level of 120 volts in most residential applications in North America. The GFCI continuously compares the current going in and out of a circuit; if it detects any imbalance that may indicate a ground fault, it will interrupt the flow of electricity, effectively reducing the risk of electrical shock.

Using a GFCI at this voltage level is critical for ensuring safety in areas where electrical devices may be exposed to moisture, such as kitchens, bathrooms, and outdoor settings. In contrast, options representing higher voltages, such as 240 volts or 220 volts, are more common in industrial or specialized applications and are not standard for general residential GFCI usage. Therefore, the requirement for a GFCI to be effective is primarily at 120 volts, which is the standard voltage for most household outlets in the U.S.