Abstract: A converter arrangement connects to a DC source or load and to a plurality of AC terminals. Each converter arrangement includes a DC part with two capacitors arranged in parallel to the DC source or DC load and a neutral DC point between them. At least one AC leg is connected to a neutral AC point via an AC leg capacitor. A pre-charge unit is arranged at each leg of the AC terminal. The method includes opening a decoupling switch and all second AC switches; loading the pre-charge unit; sequentially closing each second AC switch; measuring at each closing step a voltage between each AC leg of the second converter arrangement; and determining the stuck AC switch of the second converter arrangement based on the measured voltage.