What Is An Arc Fault Circuit Interrupter?

     For homeowners of new home construction, you might find some unusual looking yellow circuit breakers in your circuit breaker panel.  According to an online journal report from Lansing, Michigan, that was put out last October, more than 50 percent of the electrical fires that occur every year in the US could have been prevented with the installation of arc fault circuit interrupters or AFCI's, which immediately shut off power when a fire hazard - or an arc fault - is recognized.  Typical household fuses and circuit breakers do not respond to early arcing and sparking conditions in home wiring.  By the time a fuse or circuit breaker opens a circuit to defuse these conditions, a fire may already have begun. AFCI's defend against damaged electrical cords and sparking wires, is currently a residential code requirement in 49 states (Indiana being the exception) and runs about $40 for each device.  As a result, families should be better protected from the threat of electrical fires in the home. So, how does an AFCI work?

     Unlike a standard circuit breaker, which detects overloads and short circuits, an AFCI is a breaker that utilizes advanced electronics to sense certain current waveform characteristics and logic to determine if tripping is necessary, which all sounds very similar to a relay.  Op-amps and transistors perform analog signal processing and a microcontroller performs logic. The end goal of an AFCI is definitive detection of a hazardous arc condition of two types, parallel and series, resulting in breaker tripping.  In the parallel type, an arc will travel from line to line, line to neutral or line to ground and the amount of current available is dependent upon the power source.  In a series type, the arc occurs within the conductor itself and the amount of current available is limited to the load on the circuit.  An example would be a conductor that has pulled apart or a loose connection at a receptacle.  Parallel is the more serious of the two arc types.

     The key to detection of these two arc types is the ability to tell the difference between a normal and a dangerous arc condition.  A normal arc condition would be that of a motor in an electric drill. Arcing that takes place in a drill is established and extinguished at a rate relevant to the revolutions per minute of the drill.  The internal arcing does not have a direct correlation to the AC source, since the arc breaks at each gap in the stator. The electronics of the AFCI detects this normal condition when it compares the periodic function of the current waveform to the voltage waveform.  In a dangerous arc condition, severe broadband noise is generated that can range between tens of kilohertz to 1 gigahertz and exists only during the conduction of current.  The AFCI looks for certain waveform characteristics, such as DC offset and zero crossing behavior, that typically exist whenever noise is being generated to the atmosphere.

     AFCI's resemble Ground Fault Circuit Interrupters (GFCI) in that they both have a test button, but they differ in functionality.  GFCI's protect against electric shock, whereas AFCI's protect against the threat of structure fires, caused by electric hazard. A GFCI detects leakage current, whereas an AFCI detects an abnormal current waveform, by looking for certain characteristics that are indicative of an arc hazard.  AFCI's are of similar shape and construction to a normal circuit breaker and can easily be installed in the home circuit breaker panel, but will have a yellow "AFCI" label and a test button next to the switch.

     Currently, AFCI protection of branch circuit wiring in dwelling unit bedrooms is required on new installation per NEC Code 210.12.  The NEC Code panel wants the industry to gain experience with these devices in bedroom circuits so that in the future their usage might be expanded to other rooms and facilities that could benefit by the added protection they provide.

     Article by Dan Scrobe III