Apparatus sensitive to arc amplitude for envelope detection of low current arcs

An arcing fault detector (45) band pass filters the ac current in a protected circuit (67) to generate a sensed current signal with pulses having an amplitude related to the step increases in current produced each time an arc is struck. The envelope of this pulse signal is tracked by a first, slow tracking circuit or envelope detector (51) having a first, long time constant and a second, fast tracking circuit (53) having a second, shorter time constant. When the second tracking signal (121) generated by the second tracking circuit (53) falls below a predetermined fraction of the first tracking signal (119) of the first tracking circuit (51), a charge pulse modulator (59) generates a pulse (137) of a size which is a function of the amplitude of the most recent pulse in the sensed current signal. When a time attenuated accumulation of the charge pulses maintained on an integrating capacitor (139) reaches a preselected level, an output circuit (63) generates an arcing fault indication.