The device comprises a control organ (4) slidingly mounted on a shaft (6) able to be actuated in rotation by a motor of a ventilator. The control organ (4) rigidly bears a fan (2) and is movable in contrast to a helical spring (18) by effect of an axial thrust produced by the functioning fan. The translation of the control organ (4) between a first and a second operative position causes, through a transmission kinematic mechanism (12), an air lock (2a) of the ventilator to switch from a closed condition to an open condition. A bell element (14) and an extensible protecting shroud (20) safeguard the components of the device against the entry of dust and against the action of atmospheric agents.