Patent GB-681726-A (GB)
Title: Improvements in and relating to tripping means for electric circuit breakers
IPC: H01H71/44
Filed: 1949-12-22
Abstract: 681,726. Automatic circuit-breakers. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 22, 1949 [Dec. 29, 1948], No. 32839/49. Class 38 (v) Electromagnetic trip mechanism for circuit breakers is capable of operating with long or short inverse time delay or instantaneously, depending upon the value of the overload current, whereby close discrimination between a number of series-connected breakers can be obtained so that only the breaker nearest the fault is opened. A core 10 carrying a serieswinding 20 has pole pieces 11,12 which attract armatures 13,14, respectively, both pivoted on a pin 25. A trip member 26, also pivoted on the pin 25, and biassed away from the core 10 by a spring 27, can be engaged by arms 29, 30 on the armatures 13, 14, respectively. The armature 13, which effects the short-time operation, is biased by a spring 33 attached thereto by an arm 34, the other end of the spring being connected to a member 85 which can be.adjusted up and down and clamped by a nut 89 to the casing 76. An extension 36 on the armature 13 has a rack gear 37 co-operating with a pinion 38 of the ratchet wheel 39 of the short inverse time delay device 45. Teeth 40 on the wheel 39 are engaged by an oscillating escapement pallet 41 carried by a bracket 43 pivotal about the axis 44 and adjustably clamped by a screw 96 to vary the number of teeth 40 engaged by the pallet 41 before free run-on occurs. The long-time and instantaneous armature 14 is connected through an arm 54 to a strong spring 48 having its other end attached through an adjustable link 55 to an arm 56 of a bell-crank lever 49. The latter is pivoted on the pin 25 and its end 58 is connected to a weak spring 57, the other end of which is anchored to an adjusting member 86 clamped by a .nut 92. A pin 50 on the lever 49 co-operates with a slotted arm 51 secured to a shaft 67 having an arm 66 connected by a link 65 to a piston 60 working in an oil-cylinder 61 of a long inverse time delay dashpot 52. The tension of the spring 57 is so adjusted that an over-current within the range of 115-140 per cent of full-load current attracts the armature 14, the strong spring 48 acting solely as a link connecting the armature 14 to the lever 49. After a delay of, say, 20 to 30 secs., determined by the size of an orifice 62 in the piston 60, the arm 30 engages the trip member 26 so that a finger 28 thereon engages the circuit-breaker releasing arm 31. The strength of the spring 48 is so adjusted that for overloads of about 15 timesfull-load current, its tension is overcome by the armature 14 whereby the time-delay dashpot 52 is overridden and the trip member 26 operated instantaneously. The tension of spring 33 is adjusted so that an overcurrent, of about 5 times that necessary to attract the armature 14 for long-time operation, attracts. the armature 13 which, after a short delay determined by the device 45, operates the trip member 26, the long-time device 52 thus being overridden. Fig. 11 (not shown) illustrates the operating characteristics of number of closely-graded trip mechanisms.