Patent GB-481982-A (GB)
Title: Improvements in and relating to protective devices for electric apparatus
IPC: H01H37/00
Filed: 1936-10-28
Abstract: 481,982. Electrically - tripped switches ; thermal switches ; protective cut - out arrangements. BRITISH THOMSON-HOUSTON CO., Ltd. Oct. 28, 1936, No. 29322. Convention date, Oct. 28, 1935. [Class 38 (v)] A protective device for an electrical apparatus comprises a thermal element movable to release a circuit-breaker tripping device in accordance with the current flowing in the apparatus, a second thermal element movable in accordance with the temperature of the apparatus, means for combining the individual movements of the thermal elements, and means responsive to a predetermined total movement of these elements for tripping the circuit-breaker. In the construction shown in Figs. 1 and 4, the invention is applied to a circuit-breaker 10 mounted on the wall of an oil-filled transformer tank 11. The circuit-breaker comprises a fixed contact 15, and a movable contact 16 secured by a pin 17 to the end of a guide link 18 pivoted to the framework at 19. A handle 21, also pivoted at 19, is arranged to operate a pin 22 supported on the free ends of a pair of levers 23 pivoted to the frame at 24. The pins 17 and 22 are connected by a pair of overcentre springs 25, so that the movable contact can be'operated on and off with a snap action by means of the handle. A tripping member 26 pivoted at 19 is provided with an arm 28 which is adapted to engage a member 29 on the link 18 and open the circuit-breaker under the action of a spring 27 when the tripping member is released. The tripping member is normally held by a spring catch 30 supported on conducting and insulating blocks 31, 32, which also support a U-shaped bimetallic element 33 which is included in the circuit of the secondary winding 14 of the transformer. The free end of the bimetallic element supports an insulating block 34 carrying the second thermal element 35, which comprises an expansible bellows connected by a capillary tube 43 to a bulb 44 immersed in the oil in the casing 11. One end of the bellows is secured to an insulating-disc 36 connected by springs 37 to the block 34. The disc 36 is connected to the free end of the latch spring 30 by a pair of levers 38. The bulb 44 and bellows 35 are filled with an expansible medium, so that on increase in the temperature of the oil, the bellows expands against the bias exerted by the springs 37. The release of the catch 30 to trip the circuit-breaker is therefore determined by the combined movement of the bimetallic element 33 and bellows 35, and takes place with a relatively small overload when the transformer is hot, while it requires a relatively large overload when the transformer is cold. In a modification, Figs. 8-10, for a double-pole circuit-breaker, three units are mounted side by side on bases 48, the outer units carrying the contacts 52, 66 which separate in a compression chamber 49. The centre unit carries the bellows 56 and an operating member provided with an insulating handle 57, the operating members 50 of all the units being connected together by a cross-bar 58. The tripping members 54 are also connected by a cross-bar 59, which is provided with a latching finger 60 carrying a roller which engages a slot 62 in a pivoted latch member 63, Fig. 8. The free end of the latch member engages a trip shaft 75 and is thereby restrained from rotation in a counterclockwise direction to permit the roller to pass to the end of the slot 62 until the trip shaft has been rotated sufficientlv to allow the end of the latch member to pass through a cutaway portion 77 of the shaft. The shaft 75 is mounted between a pair of side plates 74, the ends of which carry another shaft 78, both shafts being biassed to the position shown by springs. The shaft 78 carries a bifurcated member 80, which supports a pin 81 to which one end of a lever 82 is pivoted. The other end of the lever carries an adjustable screw 83 which engages a lever 84 which is movable, against the bias of a spring, by an adjustable sleeve 92 mounted on a rod 91 which is connected to the movable end of the bellows 56 and passes through a hole in the lever 84. The circuit through each pole of the circuit-breaker includes a thermal element comprising a pair of bimetallic stops 69 connected at their free ends by a conducting member 70 which engages an adjustable screw 89 carried by a trip finger 88 secured to the shaft 78. With this arrangement the movement of one end of the lever 82 is dependent on the current flowing through the bimetallic strips, while the movement of the other,end of the lever is dependent on the expansion of the bellows ; the resulting combined movement is transmitted to an adjustable screw 86 carried by an arm 87 secured to the trip shaft 75. A predetermined movement of the lever 82 therefore rotates the trip shaft sufficiently to release the latch lever 63 and trip the circuit-breaker. The circuit-breaker is reset by moving the handle 57 in a clockwise direction to engage and depress the cross-bar 59, so that the roller 61 is moved along the slot 62 to return the latch member 63 to its operative position. The bellows 56 can be disconnected temporarily from the trip mechanism by moving the rod 91 to one side against the bias of a spring 95 so that the sleeve 92, which normally engages the edge of the hole 90 in the lever 84, is able to pass through the hole.