GB437766A - Improvements relating to electric selective systems - Google Patents

Abstract

437,766. Automatic exchange systems. SIEMENS BROS. & CO., Ltd., Caxton House, Tothill Street, Westminster, and HUMPHRIES, H. E., 61, Rochester Way, Blackheath, London. Aug. 18, 1934, No. 23839. [Class 40 (iv)] Different classes of subscribers' lines are distinguished by the speed of operation of their cut-off relays, and the class of a called line is recognized in the final selector by whether or not a relay connected up upon energization of the test relay has time to operate before the cut-off relay pulls up. When the marking switch MS has been set by the incoming digits, relay C (not shown), falls back and connects up latch magnet L in series with U(ii) to advance the final selector to the marked line, relay TA being short-circuited. Test relay T, which may be a high-speed relay of the kind described in Specification 405,436, [Group XXXVII], energizes in series with cut-off relay CO, opening the circuit of L and U(ii) and removing the short-circuit from TA, which energizes in series with L. Relay CO upon operating removes the shunt resistance CR, thus increasing the impedance of the test circuit sufficiently to cause T to relapse momentarily. If this occurs before TA has operated, a circuit is closed over back contact ta1 for relay V, which locks, while if TA has already operated, V remains inoperative. A relay similar to V may be connected to front contact ta1 to respond in the latter case. While relay T is back, the line is guarded by virtue of the fact that the increase of impedance which causes its release also prevents the operation of a second relay T. The arrangement may be used to determine whether or not ringing is to take place.