A low and medium voltage electrical enclosure comprising a supporting structure which defines an internal space delimited by walls forming one or more compartments housing one or more electrical apparatuses. At least one of said walls is provided with one or more venting openings that put into communication said internal space with the outside of said low voltage electrical enclosure and/or one of said compartments with a second compartment. The venting openings are provided with a cover closing a corresponding venting opening in a first—closed—operating condition at first predetermined temperatures and the enclosure is provided with an actuating device adapted to open said cover in a second—open—operating condition at second predetermined temperatures. The actuating device comprises an actuating mechanism having a Shape Memory Alloy-based (SMA-based) actuating element having a first dimension at said first predetermined temperatures reversibly changeable into a second dimension at said second predetermined temperatures, the reversible change of dimension of said SMA-based actuating element from said first dimension to said second dimension determining the reversible movement of said cover from said first—closed—operating condition to said second—open—operating condition, and vice-versa.