A excitation system and a generator arrangement with the excitation system is proposed. The excitation system comprises a converter adapted for converting an AC current to a DC current, a switching device for short circuiting an AC input of the converter, and an arc detection device for detecting an arc fault in the excitation system and for actuating the switching device upon detecting the arc fault. Therein, the switching device comprises an irreversible switch adapted for short circuiting the AC input such that the arc fault is quenched. This provide a comprehensive protection against arc faults.