The inspection of high-voltage transmission lines is a dangerous and time-consuming job, that relies on especially trained workers operating tens of meters above the ground, and close to live lines with thousands of volts. This paper presents the development of a tele-operated robot designed for preventive maintenance of high-voltage lines. The robot was designed with mobility in mind, so that cable spacers, suspension clamps and other obstacles, which so far have prevented the use of robots for inspection of high-voltage lines, would not hamper the operation of the machine. After careful considerations on mobility, the robot was designed, and a prototype was built. Results of tests, performed on facilities reproducing real field conditions, are also presented in this paper.