Arc-Fault Circuit Interrupters (AFCIs)

Jimmy H. Beard, M.E., P.E., CFEI

Arc-Fault Circuit Interrupters (AFCIs)

An electrical arc is the flow of electrical current through a gap between two separated conductors.  One very familiar form of electrical arc is lightning. Lightning is an electrical arc through the atmosphere and is well-known for its ability to start fires.  But there are other, less well-known sources of electrical arcs that also have the potential for starting fires.  The electrical arcs that can form in the wiring in our homes can also be a potential fire hazard and cause up to 82% of all electrical fires (Siemens Incorporated, 2012). Electrical arc

Arcs between electrical conductors (wires) can occur when the conductors are energized and in close proximity to each other.  If the insulation between the wires is damaged, electrical current can begin to flow through the gap between the wires, creating an electrical arc.  Electrical arcs typically emit bright light and can generate temperatures of several thousand degrees. The heat that is generated by the arc creates a potential fire hazard.

Because traditional electrical protection devices like fuses and circuit breakers do not provide adequate protection against arc-induced fires (Douglas A. Lee, 2002), a new type of device is needed.  Recent advances in technology have resulted in the development of the Arc-Fault Circuit Interrupter (AFCI). This type of device is microprocessor-controlled and monitors the current flow in a protected circuit (MacBeth, 2003).  The microprocessor in an arc-fault circuit interrupter will continuously monitor the voltage and current flowing in a circuit and interrupt the current flow if an arc is detected.   Simplified AFCI Block Diagram

Arc-fault circuit interrupters have been required as part of the National Electrical Code since 1999 and were initially required only in bedrooms.  However, the National Electrical Code requirements have been updated several times to include all 15- and 20-ampere, 120-volt circuits that supply lighting, receptacles, and other outlets. These requirements generally do not include bathrooms, unfinished basements, garages, and outdoor areas where ground fault protection is typically required (National Fire Protection Association, 2017).  Arc-fault circuit interrupters are also required where certain modifications are made to existing wiring.  AFCI Circuit BreakerBuilding codes vary from state to state, and many states have not adopted the most recent National Electrical Code requirements for arc-fault circuit interrupters.  You should always check with your local code enforcement office for requirements in your area before undertaking any electrical project.




Douglas A. Lee, A. M. (2002). New technology for Preventing Residential Electrical Fires: Arc-Fault Circuit Interrupters (AFCIs). Retrieved from

MacBeth, P. R. (2003). AFCI device which detects upstream and downstream series and parallel ARC faults. Retrieved from Google Patents:

National Fire Protection Association. (2017). National Electric Code Handbook.
Shu-Chen Wang, C.-J. W.-J. (2012). Detection of Arc Fault on Low Voltage Power Circuits in Time and Frequency Domain Approach. INTERNATIONAL JOURNAL OF CIRCUITS, SYSTEMS AND SIGNAL PROCESSING.

Siemens Incorporated. (2012). History of the AFCI. Retrieved from