AC MCP servocable-reels are used where telescopic boom extensions over 12 meters are needs. The unwinding transducer (or transducers) is formed by drum cable servo-winders on which a cable is wound that is attached to the arm head (or to intermediate elements). With the unwinding extension, the wire unwinds measuring the unwinding length thanks to a potentiometric sensing device. In the use of the crane, it is always necessary to carefully survey the cable unwinding along the arm, because it can be subjected to accidental break if collided. The replacement of this element is to be done by qualified personnel only. The angle transducer (inside to cable reel) detects the absolute angle of the crane arm thanks to a potentiometric sensing device. The signal is used by the limiter to compute the geometry. The replacement of this element is delicate and for this reason it must be done by qualified personnel.