The Importance of Ground Fault Circuit Interrupters (GFCIs)

Edgardo López, P.E., CFEI


Electricity is defined as the flow of current through a conductor.  That flow takes place when there is a voltage difference between two points, for instance, between a hot wire and a neutral wire to which a light fixture in a home is connected.  This voltage difference results in a flow of current to create light.  When thinking about current flow, think about the pins on the plug at the end of an appliance electrical cord. The pair of pins correspond to the hot wire and the neutral wire. The flow of current takes place when the plug is connected to a receptacle, thus completing the circuit between the hot and neutral wire.

Ground Fault Circuit Interrupters (GFCIs)

With electricity, there is the hazard of an electric shock or even an electrocution. Minimization of the risk associated with this hazard is what gave rise to the development of Ground Fault Circuit Interrupters (GFCIs).  

Since their introduction dating back to 1961, GFCIs have served to reduce the risk of electric shocks inside and around homes.  A GFCI is a device that is installed in a receptacle or a circuit breaker, or built into an extension power cord. 

This device protects us from electric shocks. The device makes a comparison between the current from the receptacle to the appliance and the current going through the system’s neutral wire. When everything is operating as intended, the same amount of current flows from the receptacle to the appliance and from the appliance to the receptacle. Should there be a difference in these current values, the GFCI stops the flow of current, creating an open circuit within the receptacle, the breaker, or the extension power cord. The interruption occurs very fast – on the order of 25 milliseconds (0.025 seconds) or less.   

Consider, for example, a power drill that malfunctions while in use, energizing the metal casing of the drill. The amount of current from the receptacle to the drill will equal the sum of the current that returns from the drill to the receptacle and the current flowing through the person using the drill to ground.  An electrical shock would be felt by the person due to the current flowing through him or her to ground.  However, had the drill been connected to a GFCI, the GFCI would have sensed the amount of current difference returning to the receptacle and would have tripped, opening the circuit and stopping the current flow. The response would be fast enough to prevent the electric shock.

GFCIs  contain the following components: a differential current transformer (current transformer), a trip solenoid, a trip mechanism, and test and reset buttons.  The current transformer serves to compare the current flowing in and out of the appliance. When there is a difference in current flow, the trip solenoid reacts and sends a signal to the trip mechanism to open the circuit.  The test and reset buttons in GFCIs are used to provide a means to externally test the functionality of the device.  When the test button is depressed, a GFCI acts as though the device has detected a current flowing to ground. A GFCI in working order will stop the current flow. The reset button is used to return the GFCI to operating condition after the GFCI has been tested. 

A requirement for GFCIs was first added to the National Electric Code (NEC) in 1971.  The requirement began with single-phase receptacles in use at construction sites. Requirements expanded over time. In subsequent NEC editions, many other locations were addressed where the hazard of an electrical shock could be present.  Today, GFCIs are required in locations such as kitchens, bathrooms, outdoor areas, garages, crawlspaces, basements, boathouses, and laundries.  

It would be an understatement to say that many lives have been saved or injuries avoided since GFCI protection requirements were introduced. From an electrical engineering perspective, the GFCI is a valuable protective device that has accomplished its purpose of safeguarding people from the hazards of using electricity in their daily lives.

GFCI Wall Outlet
Figure 1.0- GFCI Wall Outlet.