Patent GB-769324-A (GB)
Title: Improvements in and relating to protecting systems for electrical networks
IPC: H02H7/22
Filed: 1954-05-06
Abstract: 769,324. Protective systems. GENERAL ELECTRIC CO. May 6, 1954 [May 6, 1953], No. 13262/54. Class 38(5) In a protective system for a switch 3 interconnecting two line sections 1, 2, a. pair of secondary current transformer windings 26, 27 are mounted on the switch housing 23 at predetermined distances from the switch terminals 15 ,16 and are shielded with respect to these terminals so that the resulting effect of both individual induction effects upon the transformers caused by normal line currents passing through the switch differs from the resulting effect caused by internal fault currents appearing at different points inside the switch and from the effect caused by external fault currents such as earth faults near the incoming or outgoing sides of the switch. Feeder or distribution circuits 4, 5 and 6, 7 are respectively connected to bus sections 1, 2 through circuit breakers 8, 8a, 8b, 8c all of which are biased towards the open position and held closed by latches 11. Bus sections 1,. 2 are connected to terminals 15, 16 respectively and the switch 3 has a fixed contact 17 and a movable contact 18 which is biased towards the open position but held closed by latch 21. Contacts 17, 18 are enclosed within an insulating cylinder 23 mounted within porcelain shells 25, 25<SP>1</SP>. Between the shells and around cylinder 23 are windings 26, 27 each partially enclosed in metal casings 28, 29, respectively. The outer peripheral portions of the casings are connected to metal plate 30 which is earthed at its outer edge. The inner peripheral portions of the casings are insulated from plate 30. A conductive cylinder 39 is connected to the inner periphery of plate 30. Should an external fault develop at Y at the instant current is flowing from terminal 16 to terminal 15, at least a portion of the current flowing through contact 18 is fed into the fault, resulting in a difference in this current and the sum of the currents in conductors 4, 5. This difference is detected by windings 26, 54, 55 which will effect operation of a differential relay 50 which will energize tripping devices 13, 45 to open circuit-breakers 8, 8a and 3. Differential relay 51 will not be operated and circuit-breakers 8b, 8c remain closed. If this fault occurs at T there will be a difference between the current flowing through contact 18 and the sum of the currents in conductors 6, 7, which difference will be detected by windings 27, 56, 57 to operate differential relay 51 to energize the tripping devices for circuit-breakers 8b, 8c and 3. Differential relay 50 will not be operated and circuit-breakers 8, 8a remain closed. If an internal fault occurs at V, fault current would flow along members 39 and 30 to earth. If, at the instant the fault occurs, current is flowing from 15 to 16, the current in winding 27 will not be equal to the sum of the currents in windings 56, 57, relay 51 will be energized and circuit-breakers 3, 8b, 8c will open. Also the current in winding 26 will not be equal to the sum of the currents in windings 54, 55 and relay 50 will be energized causing circuit-breakers 8, 8a to open. Faults occurring at U and X will similarly cause all the circuit-breakers to open.