Ground Fault Circuit Interrupters
The Ground Fault Circuit Interrupter (GFCI) was developed to help save people from getting shocked. The easiest way to think of a GFCI is to remember that a normal circuit breaker protects property, while a GFCI protects people.
When a short or ground fault occurs, the GFCI detects it. Any variation indicates some of the current is going where it's not supposed to go and is creating a shock hazard. When this occurs, the GFCI trips in one-fortieth of a second -- a short enough period of time so that most healthy people aren't injured.
GFCIs should be installed at all receptacles within 4 feet of a sink, at all exterior and garage receptacles, and at all electric fixtures over showers and tubs.
All GFCI receptacles have test buttons. You should test each GFCI receptacle in your home at least once a month. If the test doesn't trip the breaker, replace the GFCI immediately.