A method of responding to voltage or current transients during processing of a wafer in a plasma reactor at each of plural RF power applicators and at the wafer support surface. For each process step and for each of the power applicators and the wafer support surface, the method includes determining an arc detection threshold lying above a noise level. The method further includes comparing each transient with the threshold determined for the corresponding power applicator or wafer support surface, and issuing an arc detect flag if the transient exceeds the threshold.