Optical fibre communication systems for long haul operation normally employ electrical equipment such as repeater amplifiers and supervisory circuits along their length which require the supply of electrical current to power them.  In such systems the optical fibre cable includes one or more electrical conductors for supplying such power.  Communications systems of this type can have a main trunk cable which extends between remote transmit/receive stations and one or more similar optical fibre spur cables which are each coupled to the trunk cable by individual branching units.  In a simple configuration with a single spur, one system of providing a power feed known to us is completely symmetrical in that the application of power to any two of the three cable ends actuates a switching configuration which selects the cable sections from those two ends as being the main feed path and allows the third section to power through an earth at the branching unit.  In the case of underwater systems the earth may be a sea earth.  Such a system requires 4 or 5 vacuum relays and several other components. 
In British Patent Application number GB 2275 834A there is described a branching unit for use in a submarine telecommunications optical cable system which employs electrical power feeding for repeaters/regenerators, in particular a system employing multiple branching units.  The unit terminates three line cables 1,2,3 and a sea earth includes three high voltage relays A, B, C only one of which is energised at a time.  When electrical power is supplied between any two line terminations.  e.g. 1 and 3, an associated relay coil, eg.  C is energised so that its contacts isolate the third line termination.  e.g. 2, and connect it to the sea earth termination 4 via contacts of a relay D if the power feed direction is from 1 to 3, or via contacts of a relay E if the power feed direction is from 3 to 1. 
In British Patent Application number GB 2252 686A there is described a branching unit for use in a submarine telecommunications cable system which employs electrical power feeding for repeaters/regenerators of an optical fibre cable system, and in particular a system employing multiple branching units.  The unit terminates three lines cables (at 1, 2, 3) and a sea earth and includes three high voltage relays, A, A1, B, B1, C, C1, only one of which is energised at a time.  When electrical power is supplied between any two lines cables e.g. 1 and 3, an associated relay coil, e.g. C, is energised so that its contacts C1 isolate the terminal and connect it to the sea earth. 
In British Patent Application number 0495509A2 there is described a feeding system for supplying a current to repeaters (3) on communication cable including a feed line (4) for conducting a current to the repeaters, comprising: a plurality of branching units (1.sub.j, j=1, 2, . . . )  provided on the communications cable; terminal stations (2.sub.i, i=1, 2, . . . )  each being connected through at least one of the repeaters to one of the branching units (1.sub.j), each of the terminal stations feeding a current through the feed line to the repeaters; each of the plurality of branching units having at least one switching circuit for switching to connect the at least one of the repeaters to the communication cable or to the ground, depending when a predetermined threshold current is exceeded conducting through the feed line in the communication cable; the current thresholds of the switching circuits in the plurality of branching units being different from each other. 
A requirement has arisen, mainly in Wavelength division multiplex systems for different and potentially simpler power switching in branching units maintaining current supply to the branching units when there is break in the trunk.  This invention has resulted from a consideration of these requirements.