In order to perform measurements of "transient phenomena in high level electromagnetic environments (lightning stroke) a data transmission system using optical fibres has been developed. Beside, this system must also permit the remote control of auxiliary devices installed in the critical areas. As the transmission system is mainly intended for lightening current measurements, the security distance between the emitter and the receiver has been chosen to be of 200 meters. An optical cable composed of two mono-fibres is used for transmitting information over the above distance.