A method for detecting electrical arcing in a plasma process powered by an AC source comprises the steps of sampling at least one Fourier component of the AC source waveform distorted by the non-linear response of the plasma, determining when a change in amplitude of the component, irrespective of the direction of the change, exceeds any one of a plurality of different threshold levels, and determining the duration that each such threshold is exceeded. Each threshold is a predetermined fraction of a running average of the amplitude of the component.