Method and device for prediction of zero-crossing of alternating current

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.