An AFCI looks for a variation in the number of load current step pulses, i.e., di/dt pulses, occurring in a succession of power line intervals. A number of di/dt pulses occurring during one cycle of the power wave is compared to a number of di/dt pulses occurring in a subsequent cycle of the power wave. The AFCI activates when the number of times that the di/dt count varies from line cycle to subsequent line cycle exceeds a predetermined number within a predetermined number of line cycles. A counting window within each line cycle preferably extends from approximately 150 degrees after the start of the first half wave cycle to approximately 30 degrees after the start of the following half wave cycle, thereby placing the counting window in the band where light dimmers typically are not designed to operate.