Abstract: A fault detector processes half cycles of current in variable frequency ac power systems by weighting half cycle magnitude values by the period of the half cycle. Time attenuated accumulations of the weighted half cycle peak values exceeding a first threshold for short delay protection, and of weighted positive differences between the peak values of successive half cycles and the preceding half cycles for a selected period of time after a peak value exceeds a second threshold for arc faults, generate trip signals when they reach specified levels.