There have been several works proposing mobile robots for inspection of high-voltage transmission lines. Those studies addressed mostly locomotion issues, with little attention paid to the sensing task. However, the deployment of sensors around the cables is not an elementary operation. This work indicates which sensors may be most suitable for the inspection of transmission lines, and describes their assembly in a simple and compact structure (Sensor-Unit). In addition, it proposes several manipulator mechanisms (Sensor-Arm) to deploy sensors around the cables and to remove them when necessary. The mechanisms are compared taking into account practical aspects such as assembly, reliability, functionality and results of field experiments. Finally, the design of a high-quality camera for detailed inspection is also described.