A main switch for electrical systems of vehicles, comprising an outer casing projecting from which is a plurality of electrical contacts that are to be connected to cables of the electrical system, wherein the casing has a base that is to rest on a supporting plate of the vehicle, said base being fixed by means of screws to the supporting plate, wherein the base is provided with two pairs of fixing elements having respective holes that are to be engaged by said screws, said fixing elements being insertable in the respective seats of the base in two different positions to which there correspond different distances between the centers of the fixing elements of each pair.