A method for processing a semiconductor wafer in a plasma reactor comprises sensing transient voltages or currents on a conductor coupled to the wafer and providing a first comparator for comparing the transient voltages or currents with a threshold level stored in the comparator. The method further includes transmitting from the comparator an arc flag signal whenever a transient voltage or current is sensed that exceeds the threshold level, and deactivating the power generator in response to the arc flag signal.