An extensible tensioning device, in particular for the protection of overhead power lines, comprising: - an external cylindrical body, extended along a longitudinal direction (x) and delimited at the ends by a pair of end walls (12, 14) facing each other; - a coupling element, integral with a first end wall and adapted to connect the external cylindrical body to a support; - a piston, slidably housed inside the external cylindrical body and comprising a rod slidable within a hole passing through a second end wall, said piston being rotatably translatable between a first retracted position, wherein the rod is in a configuration which is maximally retracted inside the external cylindrical body and in a first angular position with respect thereto, and a second extended position, wherein the rod is in a configuration which is maximally extended outside the external cylindrical body and in a second angular position rotated with respect to the first angular position, said rod having a connection end protruding externally to the second end wall, said connection end being adapted to connect a flexible cable to the extensible tensioning device; and - a first elastic means, housed inside the external cylindrical body and adapted to bias the piston toward said first retracted position, characterized in that it comprises an annular element coaxial with respect to the rod of the piston and radially interposed between said rod and the external cylindrical body, the rod being slidably received in said annular element, said annular element is engaged on one side by one end of the first elastic means, the opposite end of said first elastic means being in abutment against the second end wall of the external cylindrical body, and on the other side by one end of a second elastic means, the opposite end of said second elastic means being in abutment against a projection integral with the rod, said annular element housing at least one hook, integral with the annular element and radially movable, against the action of a secondary elastic means, between an engaged condition with a corresponding recess on the rod, wherein the rod is not free to slide with respect to the annular element, and a disengaged condition from the recess, wherein the rod is free to slide with respect to the annular element. - an external cylindrical body, extended along a longitudinal direction (x) and delimited at the ends by a pair of end walls (12, 14) facing each other; - a coupling element, integral with a first end wall and adapted to connect the external cylindrical body to a support; - a piston, slidably housed inside the external cylindrical body and comprising a rod slidable within a hole passing through a second end wall, said piston being rotatably translatable between a first retracted position, wherein the rod is in a configuration which is maximally retracted inside the external cylindrical body and in a first angular position with respect thereto, and a second extended position, wherein the rod is in a configuration which is maximally extended outside the external cylindrical body and in a second angular position rotated with respect to the first angular position, said rod having a connection end protruding externally to the second end wall, said connection end being adapted to connect a flexible cable to the extensible tensioning device; and - a first elastic means, housed inside the external cylindrical body and adapted to bias the piston toward said first retracted position,