Inside the casing (1), there are three or more ribs (8) arranged with their longitudinal axes parallel to the closing body's direction of movement when closing so that each rib forms a slide-way for the closing body (5) moving from open into closed position. Into each of these ribs there is cut a first groove (10) in which is placed a shaped part (11) that supports the closing spring (9). At least the ribs arranged approximately opposite to the tappet (20) have a second groove whose side facing the seat (7) forms a rest edge (18) on which the control edge may rest when the closing body is in open position. When the fusible soft solder component (21) is melting away, the tappet changes its position in axial direction under the force exercised by the closing spring. Due to this change of position, the control edge is no longer resting on the rest edge, and the closing body driven by the dosing spring switches over into its close position.