GB384331A - Improvements in and relating to protective apparatus for electric circuits - Google Patents

Abstract

384,331. Protective cut-out arrangements. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London. -(Assignees of Kennedy, L. F. ; 58th Street, Philadelphia, U.S.A.) May 27, 1931, No. 15523. Convention date, May 27, 1930. [Class 38 (v).] Electromagnetic switches.-A protective relay is provided with operating and restraining windings for controlling its movable member in accordance with the relative values of current at two parts of a system, and with means for varying the minimum operating value of current in accordance with the voltage of the system, whereby the sensitiveness of the relay is changed to meet a condition of the system such as a fault or switching surge. As applied to parallel feeders 7, 8 having latchedin circuit-breakers 9, 10, each feeder has associated with it a relay 15 having a centrallypivoted armature 16 which is acted on by an operating winding 17 energized by its associated feeder, restraining winding 18 energized by the other feeder, a winding 24 for holding the armature in its displaced position, and a further restraining winding 20 energized by, the voltage of the associated feeder. Further restraint is imposed by a spring 39 and the contacts are shown at 22, 23. The adjustable cores 25, 26 of the windings 17, 18 have short-circuited windings 30 to prevent the restraint from becoming zero, and have a magnetic member 28 arranged between them to which the armature is pivoted. The winding 20 also has an adjustable core 31 and is surrounded by a magnetic screening shell 33 with which the armature 34 co-operates. The latter is loosely mounted on the armature 16 by means of a countersunk hole larger than the countersunk head of the fixing screw 35 that screws into a non-magnetic stud 36 adjustably carried by the armature 16. The windings 17, 18 are connected so that their fluxes are vertically upwards or downwards with normal current directions, and the relation between the operating and restraining currents is a straight-line one. The effect of the voltage restraint is to decrease the sensitivity of the relay on normal, as opposed to reduced, voltage. Similarly the sensitivity is increased by the cutting out of a winding 18 when one feeder is out of service, but so long as the live feeder is on normal voltage there will be sufficient restraint against operation of the relay. A change in the relative directions of currents causes a redistribution of flux in the relay, the effect of which is to give increased sensitivity. In Fig. 3 such a relay 15 is shown applied to a transformer 50, the winding 20 being energized so as to provide restraint when switching in, and so to prevent the rush of magnetizing current from operating the relay. The winding 20 is accordingly connected to the contacts of an instantaneously-closing, slow-opening, relay 56. The operating winding 17 is connected across normally equipotential points of a circuit 51 so as to carry the difference currents between the two sides of the transformer 50, the sum passing through the winding 18.