GB700472A - Improvements relating to electric control systems - Google Patents

Abstract

700,472. Protective cut-out arrangements. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. March 5, 1952 [April 17, 1951], No. 8965/51. Class 38(5) A protective control system has a circuit-breaker or contactor 1 remotely disposed from a controlled apparatus, e.g. a motor 2, and controlled by a relay 3 which has a coil 18 in series with a pilot conductor 5, a starting switch 6, a half-wave rectifier 17 and an a.c. supply source 4, the relay also having a second coil 19 short-circuited on itself. To start the motor, the switch 6 is moved to the " start " position 8 and the relay coil 18 is energized by the half-wave rectified current flowing in the pilot circuit. The switch is then moved to the " run " position 9 to insert a resistance 10 into the pilot circuit to reduce the excitation of the coil 18 to a level sufficient to keep the relay 3 closed, but insufficient to reclose it on the restoration of the supply after a failure. The short-circuited coil 19, which may be a tube or single ring of copper of heavy section, is at this stage effective to prevent chattering. In the event of an earth fault in the pilot conductor 5, the rectifier 17 is shorted out of the pilot circuit and a full-wave alternating current flows through the coil 18. Under these conditions, the impedance of the relay is such that the relay is not sufficiently energized for it to remain closed and the circuit-breaker is opened. A resistance 20 limits the current which would flow when the pilot wire is shorted to earth.