New methods are used for detecting arcs in a plasma process that is fed by, for example, a freely oscillating AC generator with an output signal of the AC generator for supplying power. The method includes measuring or determining a reference value that is based on a time response of the output signal or of an internal signal of the AC generator relating to the output signal. The methods also include comparing the reference value with a threshold value and detecting an arc when a predetermined result of the comparison is obtained.