An apparatus (14) for detecting a zero-crossing of an alternating current after occurrence of a fault in a current path (2) for determining a suitable time for opening an electric switching device (2) arranged in the current path for breaking the current in the current path comprises members (15) adapted to detect the current in the current path. An arrangement (19) is adapted to calculate the dc-level of the current and the decay of the dc-level with time on the basis of values of the alternating current detected and also predict the time for a future zero-crossing of the alternating current on the basis of at least current values obtained through said current detection, the dc-level calculated, the dc-decay calculated and information about the period time of the alternating current.