GB252697A - - Google Patents

Abstract

252,697. British Thomson-Houston Co., Ltd., (Assignees of McDonald, G. R.). May 28, 1925, [Convention date]. Automatic circuit-breakers for direct current which open at a lower value of current when the current is rising rapidly (as in the case of a fault) than when the current is rising slowly (as on ordinary overload), as described in Specification 225,963, are retained set by a latch 14, Fig. 1, engaging the underset electromagnetically-operated closing toggle 13, 12, the latch 14 in turn being locked by a second latch 16 which is tripped on overload. The tripping is effected by an extension 21, Fig. 2, of the tripping armature 20 which is under the action of two opposing fluxes both generated by the current in the terminal stud (i) the main flux through the core 22, 23 tending to attract the armature to the pole-pieces 25 and (ii) a shunt flux through the back plate 28, the bolt 24 and the adjustable pole-piece 26 tending to attract the armature to the pole-pieces 26. The bolt 24 is surrounded by a conducting sheath 33. With slowly-rising excess current, the shunt flux path becomes saturated and the main flux finally draws the armature over from its normal position, shown, to trip the switch. With rapidly-rising excess current, the sheath 33 chokes off the opposing flux in the shunt flux path and the circuit-breaker is tripped at a lower value of current.