How does a Ground Fault Circuit Interrupter (GFCI) work?

A GFCI (Ground Fault Circuit Interrupter) is a safety device designed to protect people from electric shock by detecting dangerous current leaks to ground and cutting power within milliseconds.The GFCI uses precise electronic circuitry to sense the imbalance of the load from the hot and neutral lines. In other words, the GFCI monitors the current flow leaving and returning on both the hot and neutral lines of the circuit. In the event of an imbalance, the GFCI immediately releases the holding relay and breaks both the hot and neutral lines simultaneously there by stopping the current flow and preventing human injury. The GFCI is not a circuit breaker in that it does not sense the overall load and disconnect in the event of full or excess balanced current flow. The imbalance in current flow can be very small to “trip” a GFCI. Whenever the current flow “going” and “returning” differs more than 5 mA ( +/-1 mA), the GFCI opens the relay stopping the current flow.

Reverse Phasing occurs when the hot and neutral conductors are wired in reverse at a receptacle or panel connection. Although the outlet may still deliver power, the GFCI's internal circuitry may be energized with unexpected polarity, which can prevent it from tripping correctly — leaving users unprotected. Always verify correct polarity with a plug-in outlet tester after any wiring work.Normally, GFCI receptacles (like those found in your bathroom) can sense ground-faults. However, if the line-side neutral conductor is opened or lifted at a panel, the circuitry in the GFCI receptacles will not have the necessary complete circuit path from which to operate. That means that GFCI is no longer capable of sensing and disengaging. This is called an “open neutral.” Anyone using the receptacles protected by the disabled GFCI will not have GFCI protection. And if a faulted tool is connected to the now-unprotected receptacle, the user will be exposed to a shock or electrocution hazard. If you suspect an open neutral condition, stop using all affected receptacles immediately and contact a licensed electrician to inspect and repair the wiring.