Patent GB-649742-A (GB)
Title: Improvements in or relating to electrical signalling systems
Inventor: BURNS GEORGE ARTHUR
IPC: H02H7/26
Filed: 1948-07-13
Abstract: 649,742. Protective cut-out arrangements. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. July 13, 1948, No. 18846. [Class 38(v)] In a system for protecting a section of a distributing network, the intertripping of the circuit-breakers under fault conditions is controlled by means of a pilot channel using currents of four different frequencies, the equipments at the stations at the ends of the protected section transmitting on their own frequencies under normal conditions and on further frequencies under fault conditions. In the Figure, the equipments at the two stations A, B are connected by a pilot line L which may be a rented telephone line, and the contacts of the relays of the equipments are shown in the positions occupied when the relays are de-energized. When power is applied to the system, for example to station A, a relay COA is energized over contacts RA1, BA1 and it closes contacts COA1 to prepare a holding circuit, closes contacts COA2 to prepare a circuit for a relay BA and changes over contact COA3 to connect a filter Fa to the pilot line. At the same time an oscillator OA is set into operation to apply to the pilot line L a current of frequency c. The corresponding relay at station B is operated and the oscillator OB supplies a current of frequency a to which the filter Fa is tuned. A receiver REA now receives current from filter Fa and operates relay RA which changes over its contacts RA1 to complete the holding circuit for relay COA and to energize relay BA, which closes its contacts BA2 to prepare a circuit for a relay TA. The relays RB, BB at station B are operated in a like manner due to 'the receipt of frequency c. The system is now in its normal operating condition. If the circuit-breaker at station A is tripped due to a fault, a potential is applied to a connection TCA to energize a relay SA which closes its contacts SA1 to alter the tuning circuit of the oscillator OA. The oscillator OA now produces a current of frequency d and relay RB at station B is released, which in turn causes relay COB to release and changeover its contacts COB3 to connect the filter Fd in place of filter Fc. Relay RB is now re-energized and as relay BB is slow to release, contacts BB2 have not re-opened by this time, so that relay TB is energized over contacts RB1, COB2, BB2, and changes over its contacts TB1 to apply current to a connection TCB to cause the tripping of the circuitbreaker (not shown). Similarly, if the circuit-breaker at station B had been tripped first, the change of frequency of oscillator OB from a to b would cause the tripping of the circuit-breaker at station A. An interruption of the pilot circuit would not cause incorrect operation as relays BA, BB would have opened the circuits prepared for the relays TA, TB before the restoration of the pilot circuit could bring about the reclosure of relays RA, RB. In the circuit shown some interaction may occur between the relays COA, BA when the power is first supplied to the system and before the relay RA has been operated. but it is stated that this interaction may be prevented by providing relay COA with separate energizing and holding windings.