TEMPERATURE RESPONSIVE DEVICE AND CIRCUIT BREAKER

Abstract: A temperature responsive device comprises a bimetal element (12) flexible on temperature and current overload conditions to engage a trip latch (14) to open the circuit. The bimetal element (12) is pivotally mounted between knife edges (22) provided on a mounting member (20), whilst one end portion of the bimetal element is connected to an adjustment means (24), by which said end portion may be moved, pivotally to move the bimetal element about its pivotal mounting.

Description: This invention is concerned with improvements relating to thermo responsive devices, particularly of the kind (hereinafter referred to as being of the kind specified) utilising the movement of a bimetal element to effect switching of an electric circuit. Devices of the kind specified conventionally use a bimetal element in the form of a strip of two metals having differing coefficients of thermal expansion, the element being secured at one (fixed) end portion and arranged so that the other (free) end portion moves on variation in temperature, which may be the ambient temperature to which the bimetal element is subjected, or the temperature of the element due to passage of electric current therethrough, or the temperature of the bimetal element when heated by a resistive heating element or a combination thereof. The free end portion of the bimetal element is provided with an engagement member so arranged that when the element moves into a specific position, the engagement member moves into engagement with an operating member, such as a trip latch, which operates to open the electric circuit. Where the device is designed to operate on electrical overload conditions, a circuit branch is connected to the bimetal element, usually at a position separated from the engagement member, such as by a fixed tapping taken from the fixed end portion of the element, and a flexible tapping secured to a central region of the bimetal element. In setting up the device so as to ensure that the circuit is tripped at a desired temperature, it is necessary to adjust the engagement member, conventionally by engaging the member with a tool, such as a screwdriver, and manipulating it so as to vary the position of the engagement member in relation to the bimetal element. Since the bimetal element is flexible this is difficult and time consuming to accomplish, particularly since it is in practice not possible to carry out such adjustment without temporary deformation of the bimetal element. US-A-3213243 discloses a temperature responsive device comprising a housing, a bimetal element carried by a support member, the support member being mounted in the housing for pivotal movement, a screw-threaded adjustment device be provided for moving the support member and hence the bimetal about said pivotal mounting againt the action of a spring. Conversely in accordance with this invention there is provided a temperature responsive device comprising a housing, a bimetal element mounted on a pivotal mounting in the housing, a screw-threaded adjustment device being provided for moving the bimetal element about said pivotal mounting against the action of a spring so that one end portion of the bimetal element is caused to be moved towards or away from an operating element of the device, characterised in that the bimetal element is pivotally supported on a knife edge pivot engaging an intermediate region of the bimetal element, one end portion of the bimetal element being connected to said adjustment device, the opposite end portion of the bimetal element being located adjacent to tripping mechanism of the device. In this manner, since adjustment of the device may be accomplished without deformation of the bimetal element, the device may be set up simply and quickly, in particular by positional adjustment of the screw-thread element under the desired operating conditions until the bimetal element opens or closes. Preferably the adjustment device comprises an abutment surface, against which the bimetal element is urged by a spring. Preferably the pivotal mounting for the bimetal element is afforded by a mounting member secured to the housing of the device. Preferably the bimetal element is one which is adapted to respond both to temperature overload conditions and to current overload conditions. There will now be given a detailed description, to be read with reference to the accompanying drawings, of a temperature responsive device which is a preferred embodiment of this invention, having been selected for the purposes of illustrating the invention by way of example. In the accompanying drawings: FIGURE 1 is a schematic sectional view of a conventional temperature responsive device; FIGURE 2 is a view corresponding to Figure 1 of part of the temperature responsive device which is the preferred embodiment of this invention; and FIGURE 3 is an enlarged view of part of the preferred embodiment. The temperature responsive device shown in Figure 1 is of the kind specified, being a minature circuit breaker (MCB) provided with thermal and magnetic overload protection, the device comprising a housing 6, a terminal 8 comprising a fixed terminal portion 10 within the housing, to which one end portion 12 a of a bimetal element 12 is secured, the opposite end portion 12 b of the bimetal element carrying a screw-thread adjustment device 16. Welded to the bimetal element between the end portions is a flexible conductive element 13, which extends to a circuit branch of the circuit breaker, which is conventional, and not shown. The position adopted by the bimetal element 12 within the housing will be dependent upon the temperature at the element, dependent upon the ambient temperature thereof in conjunction with the current flowing therethrough, namely from the connector 13 to the terminal 8, and under conditions of either thermal or current overload the bimetal will flex in a generally anti-clockwise direction about the fixed end portion 12 so that the adjustment device 16 engages an operating member such as an element 14 of a trip latch, to cause the circuit to open. Disadvantages encountered in such a conventional circuit breaker are as follows: (a) tapping of the bimetal element is difficult due to the small size of the adjustment screw 16; (b) setting up of the device involves adjustment of the screw 16, conveniently through an aperture 18, by the use of a screwdriver, and this necessitates the application of deforming pressure on the bimetal element 12, which makes setting up to a specific condition difficult; (c) the terminal portion 10 tends to remove heat from the bimetal element 12, producing variations in the conditions under which the device will trip. The device which is the preferred embodiment of this invention is similarly a minature circuit breaker, and similar numerals are used to indicate like parts. In the preferred embodiment the bimetal element 12 is pivotaly mounted at an intermediate region thereof on a mounting member 20, provided in the form of a metal pressing (shown in an enlarged view in Figure 2 b ) positioned in channels 19 provided in the housing 6, the mounting element providing flexible knife edges 22 between which the bimetal element 12 may be positioned. At its lower end portion 12 a , the bimetal element 12 is engaged by an adjustment means 24, comprising a spindle 26 extending freely through a boss 28 captive within the housing and through a screw-threaded bore 30 spaced axially from the boss 28. An aperture 18 is provided to enable the spindle 26 to be rotated by a tool such as a screwdriver, and hence moved axially by virtue of its engagement with the threaded bore 30. The spindle 26 is provided with a flange 32 operative to engage one side of the bimetal element 12, the other side of the bimetal being engaged by a spring 34 pressing against the boss 28. In this manner rotation of the shaft 26 in one direction will cause the flange 32 to move the bimetal element pivotally in a clockwise direction about the mounting element 20, against the action of the spring 34, whilst rotation of the shaft 26 in the opposite direction will cause the bimetal element to be rotated in the opposite direction. Alternatively the boss 28 may be substituted by a threaded nut with which the spindle may be threadedly engaged, the bore 26 being unthreaded. Two flexible straps 13 a and 13 b are welded to outer regions of the bimetal element, the strap 13 a being connected to the terminal 8. Thus in the use of the circuit breaker which is the preferred embodiment of this invention, under normal operating conditions the bimetal element will adopt a position similar to that shown in Figure 2, i.e. with its end portion 12 b spaced from the trip latch 14. However on overload conditions the bimetal element will flex, with the engagement portion 12 b moving into engagement with and tripping the latch 14. By the use of the arrangement above described, no tapping of the bimetal element, as was previously required to accommodate the screw 16, is necessary, and loss of heat by means of connection to a fixed terminal portion 10 is avoided. Adjustment of the spindle 26 does not involve direct contact with the bimetal element itself, and thus does not tend to deflect the bimetal element. Thus, the position of the lower end portion 12 a of the bimetal may be accurately adjusted to the point at which the latch 14 is tripped, and the bimetal can then be backed off (with spring assistance) to a desired setting gap. It is to be appreciated that whilst the invention has been described above in relation to a circuit breaker of the kind in which the bimetal element may be deflected by current flowing therethrough to trip a latch to open the circuit on overload conditions, the invention may be utilised in temperature responsive devices in which the bimetal is responding solely to ambient temperature, and in devices in which the upper end portion 12 b of the bimetal element engages or is separated from an operating member in the form of a fixed contact to close or open an electric circuit.

Claims: A temperature responsive device comprising a housing (6), a bimetal element (12) mounted on a pivotal mounting (20) in the housing, a screw-threaded adjustment device (24) being provided for moving the bimetal element (12) about said pivotal mounting (20) against the action of a spring (34) so that one end portion of the bimetal element (12) is caused to be moved towards or away from an operating element (14) of the device, characterised in that the bimetal element (12) is pivotally supported on a knife edge pivot (22) engaging an intermediate region of the bimetal element (12), one end portion (12 a ) of the bimetal element being connected to said adjustment device (24), the opposite end portion (12 b ) of the bimetal element (12) being located adjacent to said operating element (14). A device according to Claim 1 wherein the adjustment device comprises an abutment surface (32), against which the bimetal element is urged by a spring (34). A device according to one of Claims 1 and 2 wherein the pivotal mounting of the bimetal element (12) is afforded by a mounting member (20) secured to the housing (6) of the device. A device according to any one of the preceding claims wherein the bimetal element (12) responds to both temperature overload conditions and to current overload conditions.

Inventors: PICKETT, BRUCE WILLIAM
Applicants: CRABTREE ELECTRICAL INDUSTRIES LIMITED
Application year: 1988
Main classification: H01H71
IPC codes: H01H71, G05D23, H05B3, H01H37