Abstract: Fiber-optical sensor for detecting electric arc-discharges. An optical waveguide serves to detect electric arc-discharges in an enclosed space, this waveguide being designed, in its detecting or active zone, as a heavily curved spiral. Testing light is fed in at the beginning of the optical waveguide, from a source of testing light. An optical filter, matched to the testing light, is located at the exit from the optical waveguide, this filter directing the light, resulting from a fault, and coming from the arc, up to a first photo-receiver. Sound waves, which are likewise emitted from the arc-discharge, modulate the testing light in the interior ofthe spiral. The modulated testing light passes to a second photoreceiver. The changes in the luminous intensity at the two photoreceivers are recorded in an electronic evaluator, and the time-difference is determined.