Self-powered apparatus and method for optically detecting arcing faults in electric power systems in the presence of other light sources

Arcing faults in an electric power system are detected by sensors (1, 1') powered by the light generated by the arcing even in the presence of background light. The sensors (1, 1') have one or more first photovoltaic cells (5) energized by light passed through a first filter (7) having a passband which includes a characteristic wavelength of the arcing material to generate a sensed light electrical signal. One or more second photovoltaic cells (11) are energized by light filtered by a second filter (13) having a passband not including the characteristic wavelength to generate a background light electrical signal. These two electrical signals are applied in opposition to an LED (17) which is turned on when arcing is present. The light signal from the LED (17) is transmitted through an optic fiber (29) to a photoelectric circuit (25), which generates an arcing signal when energized. The LED (17) may be biased toward the on condition by unfiltered additional photovoltaic cells (21).