Overhead electric and optical transmission system

In an optical guide joint in an overhead electric transmission system of the kind in which overhead electric conductors 1, which include an optical guide 18 comprising at least one optical fibre for the transmission of light, are freely supported in long lengths between spaced towers, each optical guide 18 connected at the joint passes into a housing 21 which is mounted above the ground on a tower, which accommodates the optical fibre joint 30 and which is at least partially filled with an electrically insulating medium in a fluid or semi-fluid state to an extent sufficient to surround the optical fibre joint. Each optical guide 18 effects a fluid-tight seal with a wall of the housing or enters the housing in such a way that risk of water entering the housing is eliminated.