A GFCI is not the same as a breaker or fuse. It exists to protect people whereas a fuse or breaker exists to protect the conductors in your house. A GFCI works on the principle that the current moving through a circuit should be the same at all points of the circuit. The GFIC monitors current going from hot to neutral. If the current going from hot differs from neutral, the GFIC trips and stops the flow of current.
The GFCI receptacle believes that if there’s an imbalance between the two currents, hot and neutral, there’s a chance the current could be going through you. The GFCI will trip on as low as 4 or 5 milliamps and in about 1/30th of a second. It takes about 30 milliamps to kill a person.
Example: You’re drilling a hole through a wall outside. It’s a bit wet out, so this provides a perfect path to ground for current if anything goes wrong. There’s an open hot wire inside the drill to the casing. If current goes from hot, through the drill, through you to ground, the GFCI will sense an imbalance from hot to neutral and stop the flow of current.
GFCI receptacles are usually mandatory in kitchens, bathrooms, outside, garages and anywhere around water, but you’ll need to check local building codes to be sure.
GFCI protection also is available with GFCI circuit breakers and GFCI cords (like an extension cord).