GB495888A - Improvements in thermally responsive protective devices for electrical apparatus - Google Patents

Abstract

495,888. Thermal switches. ENGLISH ELECTRIC CO., Ltd., ROBERTS, L., and BALL, R. D. June 21, 1937, Nos. 17200 and 26516. [Class 38 (v)] A protective device for opening the circuit of an electric machine on excessive heating thereof by the passage of current therethrough, comprises a bimetal strip type of heat-sensitive element mounted on the machine in heatreceiving association with a heated part thereof and adapted to actuate switch contacts which separate with a snap action. The bimetal strip 3 is anchored at one end by stud 5 between a metal block 4 on the core 1 and a moulded insulating casing 2, and at the other end engages a groove 6 in the casing. When overheated, the strip, after " creeping " a short distance, snaps over in the other direction. Contact 7 due to its supporting spring 9, follows the strip until arrested by stop 10 at the dead-centre position, the circuit with contact 11 being then broken. When the contacts are directly in the circuit of the controlled machine, contact 11 is insulated from the strip, but in large machines an intermediate circuit is employed. A heater coil for strip 3 may be provided in the main circuit. In a modification, Fig. 2 (not shown), the strip is adapted to carry the main current by being insulated from the block and stud by means of mica layers arranged to offer small thermal resistance, the device thus operating on both prolonged overload causing overheating and brief heavy overload. Spring snap-action mechanism and Specifications 384,777 and 418,688 are referred to. The second Provisional Specification states that a contact may be placed on the opposite side of the strip to that shown, so as to close a circuit when the strip is heated, and that the contact may be on both sides of the strip so as to serve as a change-over switch.