GB819717A - Improvements in electric switches and circuit breakers - Google Patents

Abstract

819,717. Arcing preventing in switches; air blast switches; automatic circuit-breakers; protective cut-out arrangements. BRITISH THOMSON-HOUSTON CO. Ltd. Aug. 24, 1955 [Sept. 3, 1954; Nov. 1, 1954], Nos. 25655/54 and 31536/54. Class 38 (5). An A.C. circuit-breaking arrangement comprises a half-wave rectifier connected in shunt with main contacts of a circuit-breaker and control means responsive to the current in the protected circuit to open the contacts during a half cycle of the current during which the rectifier is conducting and then to introduce a further break in series with the rectifier during a half cycle in which the rectifier is non-conducting. In Fig. 3, when an over-current occurs, a saturating current transformer 39 with resistive burden 40 applies an impulse to the grid of a thyratron 41 causing it to fire and energize a coil 38 to release a latch 35. This enables a spring 32 to move a switch blade contact 20, the constants of the releasing circuit and the inertia of the moving parts being such that the contact 20 after engaging the contact 22 to connect the rectifier 25; separates from the contact 21 during the positive half-cycle in which the rectifier 25 is conducting so that the contacts open without current therein. The contacts 20 and 22 then separate during the following half cycle in which the rectifier 25 is non-conducting so that they also open without current therein. In the air blast switch of Fig. 4, a blast valve (not shown) which would be operated by a tripping circuit similar to that of Fig. 3 applies pressure air to a piston 65 to open nozzle contacts 51, 59 during the half cycle in which the rectifier 74 is conducting. Pressure air is also applied to a piston 64 through a conduit 77 of such length that the piston 64 opens the nozzle contacts 50, 58 during the following half cycle in which the rectifier 74 is non- conducting.