GB401729A - Improvements in or relating to alternating current signalling systems - Google Patents

Abstract

401,729. Substation circuits. STANDARD TELEPHONES & CABLES, Ltd., and SKILLMAN, T. S., Connaught House, Aldwych, London. May 19, 1932, No. 14246. [Class 40 (iv).] In electric signalling systems wherein guard arrangements are incorporated in the receiving equipment to prevent operation of the receiver by currents other than the correct signal, the guard equipment is rendered effective for operation after the receiving equipment has responded to an incoming signal. As shown, a four-frequency signalling system comprises frequency-responsive relays A-D, associated with normally energized relays N, M each having four windings, the normally de-energized windings acting as guard windings. The relays N, M, which are released respectively by frequencies B + D, A + C, are arranged to be operated in turn by incoming signals. When the circuit is seized, relay SZ is operated to prepare a guard circuit for M over sz2, sz3. When N releases on receipt of the first signal, SZ releases and opens the guard circuits. When M releases, SW is operated to prepare alternative guard circuits for M and at contacts sw4, sw5 disconnects the local end of the line from the signalling equipment to prevent reoperation of M by speech. If the signal is correct, M remains released, and slow relay SX releases to energize SY and pass on the signal. If, however, M is operated by a false signal, the guard circuits of M are rendered operative by relays B or D and operation of relays SX, SY is prevented. The system ensures that reception of pure signals is not prevented by interference from the local end of the line. The Provisional Specification describes a system wherein the relay M and its associated circuits only are employed.