Most homeowners are familiar with the common three-hole electrical receptacle. Two slots accept the flat prongs of a standard plug. These, the hot and neutral, allow the current to flow through an ap-pli-ance and return to the ground, making an electrical circuit. The third hole accepts a half-round prong. This is for the ground circuit. It provides a safe escape for stray electricity that may leak through loose connections or frayed wires in the appliance.
Even though circuits had the added ground wire, safety experts found that electrical shocks were still common around damp or wet areas - the kitchen and bathroom. This happened because of the proliferation of electrical appliances in use near water and plumbing (both providing an excellent alternate ground). Fatal electrical shocks could easily occur when someone holding a faulty hair dryer reached out to turn off the water faucet.The fuses and circuit breakers in the electrical system offer no protection against this type of accident. These devices are only designed to safeguard the house wiring in the event of a short circuit. To protect individuals, electrical component designers and manufacturers developed the ground fault circuit interrupter (GFCI for short). The GFCI is essentially an electrical outlet with a fault sensor and circuit breaker.
The sensor is a miniature transformer that monitors the current flowing through the outlet. As long as there are no ground faults, current flowing out of the circuit directly to a nearby ground, the current passing through the hot wire is equal to that passing through the neutral wire. When a current leak occurs, the balance between the two is upset. If the difference reaches .005 amperes (.006 amperes is enough current to stop the human heart), the sensor breaks the circuit and stops the flow of electricity.
The first GFCIs were expensive - more than $25 - so their installation was limited to obvious danger locations like in circuits near swimming pools. As the price of the components dropped, GFCIs were introduced into interior circuits. In 1993, the National Electrical Code was revised, and now GFCIs are required in garages, crawl spaces, unfinished basements, kitchens, bathrooms and wherever an outlet will be positioned within 6 feet of a sink.
In addition to the internal sensor, GFCIs also have a test and reset button. These allow homeowners to periodically (the test should be performed once a week) test the receptacle. When the test button is pushed, the GFCI introduces a small electrical fault into the circuit. If the unit is working properly the sensor will immediately detect the fault, open the circuit and shut off the power. Pressing the reset will restore power.
When performing the test, plug a small appliance, like a hair dryer, into the outlet. Turn the appliance on, then press the test button on the GFCI; the appliance should immediately go off. Press the reset button; the appliance should come back on.
GFCIs are easy to wire and can be installed to replace a standard duplex outlet. There are two types of hookups. The GFCI may be installed at a single location, so that it protects only itself. It can also be wired in the midpoint of a circuit (a multiple location hookup). In this position it will protect itself and all the switches and receptacles to the end of the circuit..
At first glance, the multiple location hookup may seem ideal because it protects a number of receptacles. There is a down-side to this configuration. GFCIs are extremely sensitive to minute fluctuations in power. Plugging an appliance into one of the receptacles in the circuit may cause a tiny power fluctuation that will trip the GFCI breaker and shut off the power. This phenomenon, known as "phantom tripping," can happen even if the appliance is wired properly. It is a good reason to wire a GFCI for single location protection only.
Before attempting to install the GFCI, shut off all power to the circuit at the main service panel. Next use a neon circuit tester to make sure that you've tripped the correct breaker and that the circuit is cold (without power). Remove the outlet cover plate, then loosen the mounting screws that hold the receptacle in place. Gently pull the receptacle from the box. Avoid touching the connecting wires. Once the receptacle is out of the box, perform another circuit test - this time touching the probes to the terminals - to confirm that the power is off.
Disconnect the two neutral (white) wires from the silver terminals. Connecting these to the GFCI requires adding a third white wire, called a pigtail. Twist the three wires together and cap the connection with a solderless connector (a wire nut). Connect the free end of the pigtail to the terminal on the GFCI marked "white line."
The hot (black) wire hookup also requires a pigtail that will be joined at one end to the two black wires and to the "hot line" terminal at the other end. If there is a ground wire on the receptacle, it should be removed and connected to the green grounding screw on the GFCI. After connecting all the wires to the terminals, double check to make sure that all connections are secure. Gently push the GFCI into the junction box, secure the mounting screws, then replace the cover.