Patent GB-313667-A (GB)
Title: Improvements in or relating to electrical signalling systems
IPC: H02J13/00
Filed: 1928-03-23
Abstract: 313,667. Aitken, I. M. E., Gordon, D. A., and Associated Electrical Industries, Ltd. March 23, 1928. Tell-tale apparatus.-Relates to an electric signalling system wherein a step-by-step switch associated with apparatus, such as a measuring instrument and circuit breakers, at a remote station is maintained in step with a similar switch at a central station in order to effect a plurality of signals over a pair of high resistance wires. Stepping of the switches is under the control of a Wheatstone bridge circuit one arm of which includes the line wires and the arrangement is such that upon the central station switch advancing one step, the bridge is unbalanced and the sub-station switch is caused to move one step to rebalance the bridge, this cycle being repeated as stepping proceeds. The various measuring instruments and circuit breakers at the substation control corresponding resistances which as the cycle proceeds are successively associated with indicating apparatus at the control station over the contacts of the switches and the line wires in a subsidiary Wheatstone bridge circuit. The signalling operation is such that in each position of the switches the corresponding signalling bridge is tested for balance. If the instrument or circuit breaker has changed its position the corresponding resistance is varied to unbalance the bridge and the indicating apparatus is driven step-by-step until an associated variable resistance at the central station rebalances the bridge. During signalling the stepping operation of the selective, switches is held up. Fig. 1 shows a simplified arrangement for effecting the stepping of the central and substation switches. Figs. 2 and 4 show apparatus at the central station and substation. Stepping arrangements, Fig. 1. The central station switch 10 is driven step-by-step by a magnet 12 and alternate contacts of the switch are connected respectively through resistances 14, 16 of different value to the line wire 18. Similarly, resistances 22, 24 are connected to alternate contacts of the substation switch 26 and the arrangement is such that, together with the fixed resistances 30, 34, a Wheatstone bridge cixcuit is formed which is supplied from a battery 32 and has a relay 36 connected across a diagonal. When the switch 10 is moved one step the bridge is unbalanced and the relay 36 is energized to operate a slowacting relay 38 and the stepping magnet 40. After an interval the magnet 40 is deenergized at the contact of relay 38 to move the switch 26 one step and simultaneously re-balance the bridge. Thereafter the cycle is repeated and the switches 10, 26 are operated in step. Stepping arrangements, Figs. 2 and 4. The selecting switches 50, 60 at the central and substation respectively have banks of contacts 51, 61 connected to resistances R4, R3 and adapted to control the stepping of the switches by unbalancing and re-balancing a Wheatstone bridge circuit which includes resistances R1, R2. The arrangements are such that provided a key K is closed, the stepping switches operate continuously but if the key is opened the switches come to rest on the zero contact. Assuming that the key K is closed and the switch 50 is a step in advance of the switch 60 the arrangements are such that at the instant the relays d, e are energized the relay d opens the circuit of relay e which, however, is of slow-release type. The relay e energizes relay f which closes the arm of the Wheatstone bridge containing the line wires 18, 19 and a relay C. As the bridge is unbalanced, the bridge relay g is energized to cause operation of relays h, j and consequently stepping magnet a of switch 60. Relay j de-energizes relays g, h and in sequence causes its own release and re-energization of relay g and de-energization of the stepping magnet a to advance the switch 60 into step with the switch 50 and re-balance the bridge. Relay e now releases its contacts and causes deenergization of relay C at the central station to operate relay E and slow-acting relay D. Relay E energizes stepping magnet A of switch 50 and relay F which connects the line wires 18, 19 to a subsidiary Wheatstone bridge including a relay G at the central station and relay c at the remote station. Relay c de-energizes relay d. When relay D pulls up, relay E is released to reconnect the main Wheatstone bridge in circuit and deenergize the stepping magnet A to advance switch 50 a further step. Relay E also releases relay c and in consequence relays d, e are re-energized and the apparatus is in a condition to cause repetition of the cycle of operations previously described. In this manner the switches 50, 60 are maintained in step. Supervisory signalling. The resistances 71, 91 at the substation are associated with measuring instruments, circuit breakers, &c. which are adapted to vary the resistance in accordance with the readings or positions and are connected to the contacts of a bank 62 of the switch 60. When the switches 50, 60 lie on contacts corresponding to one such resistance, the resistance is included in the subsidiary Wheatstone bridge which is switched in by relay F as previously described. This bridge is formed by one or other of the resistances 71, 91, resistances r1, r2 and resistances such as 72, 82 or 92, Fig. 2, which are connected to contacts of a bank 52 of the switch 50. Relay G forms the bridge relay and is energized when the bridge is unbalanced. If when corresponding apparatus are connected over the bank 52, 62 the bridge remains balanced and the relay G remains inoperative the stepping operation of the switches 50, 60 proceed with. cut interruption. If, however, a measuring instrument has altered the value of its resistance 71, the relay G is operated to energize relay H which maintains relay E operated and thereby prevents further stepping. Relay H also energizes relay J which operates stepping magnet B corresponding to the measuring instrument over the selected contact in the bank 54. Relay J causes release of relay H to interrupt its own circuit and thereby release the magnet B to cause the wiper of the balancing resistance, such as 92, to advance one step. This operation is repeated until the subsidiary Wheatstone bridge is rebalanced and the relay G de-energized, the relay E meanwhile being maintained operative to prevent stepping of the switches 50 60 during signalling. After signalling, relay E releases and the switches advance a further ' step to complete the next signalling circuit. The resistances 91 are associated with circuit breakers and have in consequence only two values which are adapted to be balanced by corresponding values in the corresponding indicators such as 92. The readings of the measuring instruments may be indicated directly by the positions of the balancing switches at the central station or these switches may control further resistances connected to indicating instruments. Similarly the positions of the circuit breakers may be indicated by the positions of the balancing switches or by indicators such as 94 controlled by the switches. Calling substation. A slow-acting relay X at the substation may be connected in circuit with the zero contact and wiper arm of a third bank of contacts (not shown) of the switch 60. Thus if the switch K at the central station is opened for a sufficient time period the switches will rest in their zero positions and permit the relay X to operate a signalling device, which, for example, may be used for effecting a telephone call. Telephone instruments may be connected to the lines through condensers. In order to effect a call from the substation a special resistance 111 may be associated with the zero contact of the bank 62 and adapted to be balanced by a resistance 112 at the central station controlled by a stepping switch similar to the switches 72, &c. Synchronization. The switches 50, 60 are synchronized once per revolution by special resistances connected to the zero contacts on the banks 51, 61. Recording. A printed record of the conditions prevailing at the substation may be obtained once per revolution by the energization of recording magnets over the zero contact of bank 54. Repeater systems. If it is required to transmit over long distance two systems may be used in tandem by arranging that the switches, such as 72, 82, of one system vary the resistances 71, &c. of the second system.