Abstract: A swich comprises pack-aligned switching units 9, each provided with fixed 30 and movable contacts 36 operable by a cam shaft 11 rotatably passing through the switching units 9 to close and break electric circuits to which said contacts lead off. The shaft is supported, at one free end thereof, by a terminal closure element 10 fixed to the switching units 9 and is connected, at its opposed end, to a trip mechanism 12 located in a trip unit 8 fastened to said switching units. The switching units, trip unit and terminal closure element constitute each a modular unit engageable with other modular units by restrained fixing and disengageable therefrom without bringing about the disassembly and division of said unit into its component parts. The cam shaft is comprised of several hub-shaped elements 40 consecutively engaged in a rotational direction and associated each with a switching unit.