| 摘要 |
<p>Incoming telephone circuits terminate (2A,2B) on protection modules (1) interposed before the jumper connections (3A,3B). Each line is connected through a surge diverter (5A,5B), to a common bar (8,9) earthed (4) through a thermal relay (10). The diverters, e.g. gas discharges tubes, have short-circuiting contacts (6A,6B) cross-connected (46A,46B) so that break-down of one line necessarily earths the other also. The thermal relay's bimetallic delay element (11) operates main contacts (13), a by-pass resistor (12) enduring continuity. An auxiliary contact (16) operates a visible alarm 923) for each group of circuits (I) and an audible alarm (25) shared with other groups (16',23'). The thermal delay avoids alarm operations on transient faults. The test module (39) has a light-emitting diode (32) and a wander-lead (31) successively plugged into the line jumper points (31A,31B) to detect faulty circuits.</p> |
