Abstract: In an overhead electric transmission system in which the phase conductors incorporate optical fibres, a joint between optical fibres of two phase conductors or between optical fibres of a phase conductor and an optical cable extending to a sub-station is enclosed within a fluid-tight metal housing. The housing is electrically connected to each phase conductor, is electrically insulated from earth, and has an external shape of such a form as to provide effective control of electrical stress. Preferably the metal housing is of hemi-spherical shape and is mounted on top of a porcelain insulator containing oil under pressure. Where an optical fibre connected at the joint is that of an optical cable, it passes into the metal housing through the oil filled porcelain insulator.