The invention relates to an on-load tap changer for step transformers, which has one main current branch and one auxiliary current branch for each of the two winding taps to be switched. In each main current branch and auxiliary current branch, switching is accomplished by means of a vacuum switching tube. According to the invention, an additional mechanical contact is provided in each of the main current branches and in each of the auxiliary current branches between the respective winding tap to which said branch is electrically connected and the respective vacuum switching tube in said branch. Said mechanical contacts are switched in such a way that the vacuum switching tubes in the main current branch and the auxiliary current branch of the unconnected winding tap can be galvanically isolated from the unconnected winding tap.