The present document relates to optical transmission systems. In particular, the present document relates to optical submarine transmission systems. A branching unit (210) for an optical transmission system (200) is described. The branching unit (210) comprises a first and a second input port configured to be coupled to a first and a second incoming fiber (320, 321), respectively; a first output port configured to be coupled to a first outgoing fiber (322); wherein the second incoming fiber (321, 323) is part of an optical cable comprising an electrical conductor; wherein the branching unit (210) is configured to receive electrical power via the optical cable; and an optical by-pass switch (345) configured to switch between a first mode, and a second mode; wherein in the first mode, the by-pass switch (345) is configured to couple the second incoming fiber (321 ) to the first outgoing fiber (322); and wherein in the second mode, the by-pass switch (345) is configured to couple the first incoming fiber (320) to the first outgoing fiber (322).