Patent GB-1336071-A (GB)
Title: Power feed for a communications system
IPC: H04B3/44
Filed: 1972-01-24
Abstract: 1336071 Repeater station power supply POST OFFICE 24 Jan 1972 [1 Jan 1971] 113/71 Heading H4R Power is fed to repeaters situated in separate incoming and outgoing trans mission paths connected between two terminal stations by using two voltage sources connected between the paths at one station and two current sources connected in series between the paths at the other station, the arrangement being such that there is on each transmission path a point which is at zero potential with respect to earth, this point being variable along the length of each transmission path by varying the output of the voltage sources at the one station. The arrangement allows the potential to be reduced to earth at any particular point along each transmission path without removing the power,from the paths, thereby enabling a person to work on the path at that point in safety. As shown power is supplied to repeaters A1-A4 in a path L1 and to repeaters A5-A8 in a return path L2. Signals transmitted from East to West are fed to path L2 via line 5 and are fed out to a line 7 at the West terminal. Likewise West-East signals are fed to the path L 1 via line 1 and are fed out at the East terminal to line 3. The path between repeaters, e.g. L1A-L1E, L2A-L2E may comprise coaxial cable with the outer conductor Dl or D2 earthed. At the West terminal there are included are two voltage supply batteries B1, B2, arranged with their common point earthed, for applying a positive potential to the West end of path L1 and a negative potential to the West end of path L2 via respective current limiters C1, C2. At the East terminal there are included two current sources S1, S2 arranged with their common point connected to earth via a voltage limiter 9. The purpose of the limiter 9 is to allow the junction of sources S1, S2 to float until it reaches a predetermined potential, either positive or negative relative to earth, at which one or other of the semi-conductor controlled rectifiers 10, 11 becomes conductive. Under normal operation each battery has an output of 250 volts resulting in the West end of path L1 and the East end of path L2 being at + 250 V. and the other end of each path being at. -250 V. In such conditions the point of zero potential will be mid way along each path assuming a uniform distribution of repeaters along each path. If both batteries are reduced to 130 V., the positive end of path L1 drops to + 130 V. and the negative end of path L1 to - 370 V. (the voltage generated by source S1 must increase in magnitude in order to maintain a drop of 500 V. along the path). Likewise the ends of path L2 are at - 130 V. and + 370 V. respectively. The earth potential point on each path is therefore shifted towards the West terminal and by arranging the batteries B1, B2 to be varied together, the two points which are at earth potential can be arranged to be at adjacent positions on the paths. To protect a person working on the paths at the earth potential point against a break in either path there is connected in series between the two paths, at the beginning and end of a section, e.g. L1A, to be worked on two back to back Zener diodes interconnected by an inductor (Fig. 3, not shown). Such a circuit is only connected after the potential on the section has been reduced to near earth potential as described above. If the potential across the Zener diodes rises above the threshold limit a short circuit will occur, whereby the section of the path remains near earth potential. A typical path current is quoted as 100 mA.