GFCI

A GFCI — a ground fault circuit interrupter — is the outlet or breaker that watches for current going somewhere it should not. It compares the current leaving on the live wire with the current returning on the neutral, and if the two differ by even a small amount it cuts the power in a fraction of a second.

That difference means current is escaping to earth, which is exactly what happens when a person becomes part of the circuit — through a damaged cord, a wet hand, or a tool with a frayed lead. Because it trips on that imbalance rather than on a heavy overload, a GFCI protects people, while an ordinary breaker protects the wiring.

You will find them in bathrooms, kitchens, garages, outdoors and on construction sites, and each carries a test button and a reset button; testing them monthly is part of the ritual. Its cousin at the panel is the AFCI, which looks for the signature of a failing connection rather than a ground fault.