How Does a Ground Fault Circuit Interrupter Work?

A ground fault circuit interrupter (also known as ground fault interrupter, GFCI or GFI) has one purpose for existing: to prevent a ground fault.

A woman just out of the shower with wet hair and wet feet could be in for a serious shock by using a hairdryer plugged into an unprotected outlet, especially if the cord is old and worn. A child exploring the world will find certain death if he successfully puts the penny he found in an electrical socket. Both cases create a situation where the path of the current either is or can be altered in a tragic way. Both cases can be avoided by using an outlet equipped with a ground fault circuit interrupter.

Three Modes

In its normal operating mode, a ground fault circuit interrupter completes the electrical circuit, thereby allowing all of your electrical appliances to work as they should. In its test mode, a ground fault circuit interrupter creates a break in the circuit, thereby preventing the same appliances from working. However, unlike a typical light switch, a ground fault circuit interrupter is capable of being “tripped.” When a ground fault circuit interrupter is tripped, it has broken the circuit to prevent damage or electrocution.

Normal Operation

In a properly functioning circuit, the current passes through a circuit breaker and continues on its way through a conductor. In AC circuits, this conductor is typically called the "hot wire" and usually has black sheathing. The hot wire carries the current to the load, which is any resistance the current may encounter on its journey. The load includes any device that draws current. Televisions, radios, computers, major appliances and all light fixtures are just a few of the possible loads a house circuit may contain. After passing through the load, the current then makes a return trip via the neutral wire (usually white), where it finds its way safely to ground.

Ground Fault

A ground fault, on the other hand, occurs when the path the current should be taking is altered somehow. This event provides an alternate conductor and a shorter, potentially dangerous or deadly path to ground.

In the United States, GFCI outlets are rated for 125 volts on either 15- or 20-amp circuits. This means that in most household and business applications, a GFCI outlet will trip if the current exceeds 15 or 20 amps, depending on the individual outlet rating. They also trip if the current going out and coming back from the load differ by as little as 0.005 amps.

A major contributor to ground fault circuit interrupter trips is a short circuit when the current feeds back onto itself. The result is usually a spark, possibly some smoke, and definitely a trip to the electrical service panel.

Leading Cause

The leading cause of ground fault circuit interrupter trips, however, is a ground fault. Like a short circuit, a ground fault results from a diversion of the current from its proper path--when the current is fed directly to ground. In the case of the young boy with the penny, a ground fault circuit interrupter outlet will trip and interrupt the circuit before the boy is seriously injured.

Though most commonly associated with outlets, ground fault circuit interrupters are being utilized in hair dryers, disposal units, power strips, table saws and increasingly more electrical devices to prevent ground-fault occurences.