Abstract: A branching unit for an optical transmission system, an optical transmission system and a method of operation of said branching unit (30) is disclosed having a first branch (38) and a second branch (40), each connected to a main branch (42). First directional current flow means (54) and second directional current flow means (58) are operable to allow electrical current to flow substantially in one direction only along the first branch (38) and the second branch (40) respectively such that electrical current flow from the first branch to the second branch and from the second branch to the first branch is substantially prevented and a simultaneous conduction of current between the first branch and the main branch and between the second branch and the main branch is prevented.