A branching unit for an optical transmission system, the branching unit comprising - 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; wherein the second incoming fiber is part of an optical cable comprising an electrical conductor; wherein the branching unit is configured to receive electrical power via the optical cable; and - an optical by-pass switch configured to switch between a first mode and a second mode; wherein in the first mode, the by-pass switch is configured to couple the second incoming fiber to the first outgoing fiber; and wherein in the second mode, the by-pass switch is configured to couple the first incoming fiber to the first outgoing fiber; - wherein - the first and second incoming fiber and the first outgoing fiber (320, 321, 322) are configured to transmit wavelength division multiplex, WDM, signals comprising a plurality of wavelengths; - the branching unit further comprises a first WDM drop unit configured to extract a first set of wavelengths from the first incoming fiber; a second WDM drop unit configured to extract the first set of wavelengths from the second incoming fiber; and a first WDM add unit configured to add the first set of wavelengths to the first outgoing fiber; and - the optical by-pass switch is configured to pass the first set of wavelengths from the second incoming fiber to the first set of wavelengths of the first outgoing fiber, when in the first mode; and to pass the first set of wavelengths from the first incoming fiber to the first set of wavelengths of the first outgoing fiber, when in the second mode; wherein the first mode is a powered mode of the optical by-pass switch, subject to the optical by-pass switch receiving electrical power via the optical cable above a power threshold, and wherein the second mode is an un-powered mode of the optical by-pass switch, subject to the optical by-pass switch receiving no electrical power via the optical cable or electrical power below the power threshold. - 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; wherein the second incoming fiber is part of an optical cable comprising an electrical conductor; wherein the branching unit is configured to receive electrical power via the optical cable; and - an optical by-pass switch configured to switch between a first mode and a second mode; wherein in the first mode, the by-pass switch is configured to couple the second incoming fiber to the first outgoing fiber; and wherein in the second mode, the by-pass switch is configured to couple the first incoming fiber to the first outgoing fiber; - wherein - the first and second incoming fiber and the first outgoing fiber (320, 321, 322) are configured to transmit wavelength division multiplex, WDM, signals comprising a plurality of wavelengths; - the branching unit further comprises a first WDM drop unit configured to extract a first set of wavelengths from the first incoming fiber; a second WDM drop unit configured to extract the first set of wavelengths from the second incoming fiber; and a first WDM add unit configured to add the first set of wavelengths to the first outgoing fiber; and - the optical by-pass switch is configured to pass the first set of wavelengths from the second incoming fiber to the first set of wavelengths of the first outgoing fiber, when in the first mode; and to pass the first set of wavelengths from the first incoming fiber to the first set of wavelengths of the first outgoing fiber, when in the second mode;