The power supply of telecommunication plants and FM/TV relay stations has always entailed numerous problems and specific needs which are difficult to meet. We refer in particular to the difficulty of reaching the installation sites for carrying out the technical service, to the exposure to atmospheric discharges and to the length of electric lines with the consequent voltage variation. The operators of TLC plants are well aware of the direct and indirect costs deriving from a bad power supply. As a matter of fact, the costs deriving from service interruption and from "low audience efficiency" must be added to the failures which may occur to the plants. In order to obtain the maximum economic yield from the service, it is fundamental to build an electric plant assuring a good protection of the transmission plants and the safety of operators employed in plant maintenance. Since the isolating transformer is one of the most important components of the electric plant, the use of a specific product is essential.