In an electric circuit supplying AC line current to a load (30, 31) from an AC source (26) of given frequency providing repetitive cycles, arc detection is provided by sensing cycle to cycle changes in the AC line current. The AC line current is sampled with a harmonic notch filter (72) at a plurality of n phases .THETA..sub.1 through .THETA..sub.n during each of a plurality of m cycles of the AC source (26), to provide a plurality of currents I.sub.n,m, where n is the phase and m is the cycle. The differences (I.sub.n,m)-(I.sub.n,m-x), where x is a designated number of cycles, provide a plurality of current difference signals. The absolute values of these current difference signals ID are accumulated in a synchronous summer (94) over the m cycles. An arc indicative signal is generated in response to given conditions and given combinations of conditions (116) of the cumulative absolute current difference signals ID. Cycle to cycle changes in AC line voltage are also sensed with a harmonic notch filter (146), and the arc indicative signal is disarmed in response to a given voltage condition. In further embodiments, subcycle changes are sensed.