| 摘要 |
<p>1,209,183. Repeater testing. POST OFFICE. 30 May, 1968 [12 June, 1967], No. 26989/67. Heading H4R. In a transmission system including transistor repeaters powered over the line and in which line continuity tests are performed using low values of D.C. or low frequency A.C., insufficient to energize the repeaters and to which the repeaters would present high impedance, the D.C. feed path of the repeater is by-passed by the contacts, 9, of a relay 10 which is energized to open the contacts by the normal supply current of the repeater, but is not energized by the low value testing current. To provide extra protection against failure of the relay contacts to open during normal use of the system a second relay may be provided with its contacts in series with the contacts of the first relay and its coil in series or parallel with the first relay coil. In addition, or alternatively, a fusible link 13, may be arranged in series with the relay contacts arranged to open circuit if the normal supply current flows therein for longer than a predetermined time, e.g. 1 minute. In order to allow the testing system still to be operated if the fusible link opens the link may be shunted by a rectifier back biased by the normal supply current and forward biased by the testing current of opposite polarity. To limit the maximum impedance of the power circuit the relay coil, or coils, is shunted by a low value resistor.</p> |
