GB541542A - Improvements in electrical protective devices - Google Patents

Abstract

541,542. Protective cut-out arrangements; thermal switches; resistances ; temperature responsive actuating elements. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 30, 1940. No. 18194. Convention date, Dec. 29, 1939. [Class 38 (v)] [Also in Group XXXVI] An over-voltage protective device, suitable for current transformers that have inadvertently been relieved of their burden, comprises a resistance unit of non-linear characteristic which decreases in resistance with increase of current or voltage, in the form of a pair of discs 19, 20 clamped on either side of a metal plate 21 by means of metal plates 22, 23 threaded to receive a bolt 24 which also fastens the whole to a support 18. Plate 21 is connected to one pole of the transformer secondary and a bolt 24 to the other pole, and an insulating sleeve 25 insulates the bolt from the discs 19, 20 and plate 21. whereby the discs are in parallel paths across the secondary. Plate 21 is grooved at 27 to receive a bi-metal strip so as to be responsive to the internal temperature of the unit. The strip is riveted at 42 and brazed or welded to the face of the groove. Its free end 29 co-operates with a pin 32 passing into the housing 31 of a snapaction switch adapted, when closed by excessive movement of the strip 28, to short-circuit the resistor. The switch comprises an E-shaped member, the outer limbs 33 of which are bowed apart from the inner limb 34, the outer ends of all the limbs being rigidly supported to the apparatus. When a small movement is imparted to limb 34 near its point of support, it snaps a movable contact on the yoke into engagement with a co-operating contact, and the contact opens again when the strip cools and removes the pressure. The distance between the housing 31 and strip is adjusted by mounting the housing on the outer arm 36 of a split shelf 30. By means of a pair of screws 39, 40 the distance between the arms 36, 37 is adjusted, the screw 39 passing freely through a hole in arm 37 and is threaded into arm 36, whereas screw 40 is threaded through arm 37 and carries a nut 41 bearing on arm 36. The discs 19, 20 are of a mixture of silicon carbide and carbon with a binder, or of silicon carbide with, for example, tungsten and molybdenum their surfaces being copper-sprayed.