Patent GB-792505-A (GB)
Title: Improvements in or relating to electrical circuits including automatic switches
Inventor: WHEATCROFT LEWIS
IPC: H04Q3/00
Filed: 1955-03-03
Abstract: 792,505. Automatic exchange systems. GENERAL ELECTRIC CO., Ltd. March 1, 1956 [DZarch 3, 1955], No. 6321/55. Class 40 (4). A Strowger switch which steps vertically in response to a dialled digit and hunts over two sets of wipers by interaction of the rotary magnet with mechanically operated interrupter contacts has a test relay C which responds when the switch finds an idle outlet marked by battery and disconnects the rotary magnet while energizing appropriate one of two line switching relays HA and HB, each of which is associated with one of the wiper sets. The selector switch hunts simiiltaneously with alternative sets of wipers, 13, 15, PI and 14, 16, P2, the test wipers P1, P2, being associated in common with relay C, and separately with relays HA and HB, over rectifiers 22 and 23 which block the test wires to positive potential used for metering. In the circuit shown in Figs. 1 (Comp.) and 2, the selector is marked idle by battery 20 and when taken into use by closing a loop across speech wires 1 and 2 relays A, B and C respond in turn. Vertical magnet V does not operate while B is connected in series but dialling interrupts relay A which operates V by bridging B. At the end of the digit, continued operation of relay A shunts out C which when released completes a circuit prepared by offnormal contact N1 to energize rotary magnet R. The switch hunts for an outlet marked by negative battery which if found by either PI or P2 re-energizes relay C over off-normal contacts NR3, NR2, respectively, and thereby disconnects and stops magnet R. If the mechanical interrupter contacts of R are of a toggle type as described in Specification 657,410, relay C need not be fast acting to stop R. Suppose both P1 and P2 find outlets simultaneously, then in energizing C earth is connected to windings 9 and 12 of switching relays HA, HB, which both respond and disconnect C. HA locks up over ha5 and prevents HB from locking up over hb5. C in falling back disconnects windings 9 and 12 of HA, HB, so that HB falls back. Should P 1 alone find an idle outlet HA pulls back to switch the line while if P2 alone finds an outlet HB alone responds. To release the circuit earth is taken from the test wire so that HA or HB, which ever is made, falls back to energize release magnet Z, a contact z1 of which puts guard earth on the test wire until release is complete when it marks the line idle with battery 20. If all outlets are busy, R moves the wipers to 11th contacts to operate contacts S1, S2 and S4, which respectively connect earth to wiper P1 to operate an overflow meter, stop magnet R, and connect busy tone to the line over A. In Fig. 1 (Prov.), an alternative arrangement of the relay C is shown in which, after the operation of HA or HB, C is released quickly by reverse energization from battery 24 to earth on contact c3 in order to disconnect the unwanted switching relay. When released, relay A releases B slowly and re-operates C. To release the circuit earth is removed from the test wire releasing HA or HB, whichever is opeated; which disconnects relay C and connects up magnet Z. Off-normal contact N2, in conjunction with contact c3, holds a guard earth on the test wire.