Patent GB-1026296-A (GB)
Title: Electrical protection system
Inventor: HEWLETT ARTHUR JAMES
IPC: G08B25/04
Filed: 1964-08-18
Abstract: 1,026,296. Selective signalling systems. AMERICAN DISTRICT TELEGRAPH CO. Aug. 18, 1964, No. 33777/64. Heading G4H. In a protection system, code signalling devices 11 (Fig. 1, not shown) are connected in series in a loop circuit 10 which has an impedance element connected to each of its ends, and a capacitor 29 (Fig. 2, not shown) is charged in dependence on the voltage drop across each of the impedance elements, the charging current causing operation of a signal registration device. Alarm conditions.-An alarm condition causes a signal wheel 11 to rotate, thereby imposing alternate ground and break conditions on loop 12 in a coded sequence indicating the location of the alarm. When a wheel 11 grounds loop 10, current through resistor 23 and therefore the voltage across it falls, and the change in voltage is applied to capacitor 29 over lead 35. A break signal likewise reduces the current through resistor 23, but also reduces the current through resistor 19 thereby reducing the current through transistor 26 and potentiometer 28, the voltage drop across the latter being applied over lead 30 to capacitor 29. Voltage charges applied to capacitor 29 result in charging currents which alter the bias of transistor 39 so that it conducts and a pen recorder 38 records the signal train. To avoid both ground and break signals operating register 38, the first break condition, indicated by a voltage drop on lead 30, causes transistor 55 to saturate, the voltage drop across resistor 56 charging capacitor 54, and emitter follower 51 operates a relay 52 to open contact 36, Fig. 1, thereby isolating ground detector resistor 23 from capacitor 29, so that only following break conditions cause operation of register 38. The first break signal is preceded by a ground signal, but this merges with the first break to give an elongated first trace. Operation under fault conditions. General.-A line fault would normally cause continuous operation of register 38 until capacitor 29 was discharged, but a fault indicator is provided whereby an operator's attention may be drawn to the fault condition and steps taken to restore loop supervision. As above described, a break condition causes transistor 55 to saturate, and a warning lamp 68 lights. Under normal conditions lamp 68 flashes on and off as a signal wheel 11 transmits a code sequence, but when a continuous fault exists, lamp 68 is lit continuously to indicate the fault. Transistor 60 operates to light lamp 68 in the event of a ground fault. To determine the nature of the fault and restore supervision, a switch 70 is moved to its various positions in turn until lamp 68 is extinguished by reapplication of base bias to transistor 55 and/or 60. In each fault position of switch 70, positive potential is applied to both ends of the logs 10. Line grounded.-With switch 70 in position G, contacts SW1-SW4 are transferred. Break conditions imposed on the loop by a wheel 11 located between jack 15 and a ground fault 77 operate register 38 as normal. Break signals from the other side of the fault 77 reduce the current through resistor 74, in turn reducing the current through transistor 80 and potentiometer 76 to operate register 38 via leads 83, 35. Line open.-With switch 70 in position O, contacts SW1-SW6 are transferred. Ground signals from a wheel 11 located between jack 15 and an open line fault 96 increase the current through resistor 25 and reduce the collector current of transistor 26 to operate register 38 via lead 30. Ground signals from the other side of fault 96 increase the current through resistor 81 and decrease the collector current of transistor 80. Line open grounded.-With switch 70 in position OG, contacts SW1-SW5 are transferred, ground signals from a wheel 11 located between jack 15 and the fault operate as in the line open fault condition, and break signals from a wheel 11 on the other side of the fault operate as in the line grounded fault condition. Line grounded open.-With switch 70 in position GO, contacts SW1-SW4 and SW6 are transferred, break signals from a wheel 11 between jack 15 and the fault operate as normal, and ground signals from a wheel 11 on the other side of the fault operate as in the line open fault condition.