Abstract: Elastic devices (3) abruptly actuate a contact carrier (9) in the upward direction when a push-member (1) reaches a tensioning position (E) from a position of rest (R). In the event of any blockage preventing such abrupt movement a lever (24) constrainedly produces the movement when the push-member (1) continues its movement as far as a constrained actuation position (A). The push-member carries colored zones (101, 102). The contact carrier carries a colored zone (104). From the relative position of these zones viewed through a window in the casing (100), the user can adjust the positioning of the limit switch with respect to a movable member intended to actuate it so that the two phases of the actuation take place at the required points. The user can also check whether the contact carrier moves in response to the elastic devices or in response to the lever.