Safety device for cable stretching units, particularly for electric contact lines of railways

A safety device for cable stretching units, particularly for electric contact lines for railroads, comprising a first element (1) which is connected to an end portion of a stretched cable (3) and a second element (2) which is substantially unaffected by longitudinal movements of the stretched cable, the first element (1) being movable with respect to the second element (2) as a consequence of longitudinal movements of the end portion of the stretched cable (3), inertial locking elements (5) are interposed between the first element (1) and the second element (2), and are suitable to lock the first element (1) with respect to the second element (2) when the first element (1) exceeds a preset acceleration in its movement with respect to the second element (2).