Where You Should Use GFCIs
GFCIs should be installed instead of regular wall outlets wherever electric appliances may inadvertently come into contact with water, such as the bathroom, kitchen and outdoors. Once installed, they should be tested regularly. Most new homes built in the last 25 years already have them installed.
How GFCIs Work
As the name suggests, a Ground Fault Circuit Interrupter guards against a ground fault. A ground fault happens when the electrical current in a product strays or "leaks" outside of the path where it should flow. When electricity leaks, it seeks the path of least resistance to the ground. If you're in the path of the electrical circuit flowing to the ground, you can be seriously injured. For example, without GFCI protection, using a lawnmower with a frayed cord on wet grass could result in a severe electric shock.
A GFCI works quite simply. It constantly monitors electricity flowing into a circuit to ensure the same amount is flowing out to the appliance and back to the GFCI outlet. If the electricity flowing through the circuit differs by a slight amount from that returning - and is, instead, leaking - the GFCI will quickly shut off the power to the outlet to prevent possible electrocution. It even detects leaks that are too small to cause a fuse or circuit breaker to shut off the circuit.