Arc Fault Circuit Interrupter. A device that detects roasting short circuits or arcing open circuits that present a fire hazard, and opens the circuit breaker of that circuit. The device is now mandated by the 2008 NEC for all 15A and 20A outlets in a new-construction residence. "Combination" AFCI's denote units that detect both arcing open circuits and roasting shorts. Not suitable for use in entertainment lighting systems or on dimmers.