Briefly stated, and in accordance with a presently preferred embodiment of the invention, an arc fault detecting circuit using, a pulse width modulation, or PWM, technique is used to sense series or parallel arc faults. When arc faults occur, typically an arc step in current is followed by broad band arc noise which is caused by the random fluctuations in arc column resistance. In this invention, a logic signal is created which has a duration or width that corresponds to the time interval during which the broad band arc noise generated by the arc is present. Typically an arc fault randomly starts, generates broad band arc noise, and then extinguishes as the next current zero cross of the AC load current is approached. The random start causes PWM of the logic pulse whenever the start of the arc randomly moves with respect to the current zero cross. An arc fault that randomly starts, and then randomly stops, will also cause PWM with respect to the zero cross. The PWM logic pulse is coupled to the input of a microprocessor which measures and stores the length of the present PWM pulse under measurement, and compares that pulse to the pulse length of the last stored PWM pulse, and increments a counter if the difference in pulse length is greater than a predetermined amount. When the counter number is greater than or equal to a predetermined number held in the microprocessor memory, an output signal is sent to a controller which activates the circuit interrupter. Another input to the microprocessor, corresponding to the magnitude of the arc current, is used to adjust the predetermined number value down or up depending on the intensity of the arc fault for faster or slower interrupts respectively.