Field of the Invention:

This invention concerns improvements relating to thermally-responsive electrical switches and more particularly relates to thermostatic electrical switches and thermal cut-outs as used for example in tumble dryers, washing machines, coffee percolators, domestic heaters, cookers, central heating boilers etc. Background of the Invention: A commonly known type of thermally-responsive electrical switch manufactured by many manufacturers comprises a dished bimetal disc mounted at one axial end of a generally cylindrical housing and arranged to open a set of switching contacts via the inter ediacy of a push rod when, in response to an over-temperature condition, the bimetal disc moves to an oppositely curved configuration with a snap action. The bimetal disc is commonly arranged to reset automatically when its temperature falls, thereby enabling the switching contacts to remake, though it could be arranged if desired to require to be manually reset. Such switches in some applications are mounted so as to have the bimetal disc in close thermal contact with the surface of a member whose temperature is to be monitored, for example the head plate of an electric immersion heating

element, and in other applications may be mounted to be responsive to air temperature or to radiant heat.

Hitherto, thermally responsive electrical switches of the type outlined above have been relatively unsophisticated and have not been designed to take advantage of concepts which are becoming relatively commonplace in other situations, such as for example the concept of secondary protection which we have pioneered and successfully developed in the field of providing element over-temperature protection in water boiling vessels, particularly the modern generation of plastics bodied water boiling jugs. As is now well known in the field of element over-temperature protection controls, secondary or back-up protection is provided as a means to ensure proper protective functioning of the control in the event, however unlikely, that the control fails to respond at a primary level to an overtemperature condition. Exemplary arrangements for providing primary and secondary levels of over-temperature protection for the heating elements of electrically powered water boiling vessels are disclosed in our British Patents Nos. 2117568 and 2194099. Objects and Summary of the Invention:

The object of the present invention, in one of its aspects, is thus to provide a thermally responsive switch, generally of the above-described type, which

incorporates secondary protection. In another of its aspects, the present invention aims to provide such a switch, preferably but not essentially incorporating secondary protection, which has the feature that it will go open circuit in the event of the switch becoming detached from its mounting, for example on account of mounting screws becoming loose.

According to the present invention in one of its aspects, therefore, a thermally-responsive switch generally of the type described hereinbefore comprises inner and outer body parts which are movable with respect to each other, spring means urging the inner part towards a predetermined positional relationship with the outer part, and means responsive to the proper mounting of the switch for effecting relative movement between said inner and outer parts against the action of said spring means, the arrangement being such that an electrical circuit through said switch can not be effected with said inner and outer parts in said predetermined positional relationship irrespective of the condition of the bimetal.

In one exemplary embodiment of the above-defined aspect of the present invention, the outer body part of the switch has first and second terminals captured therein and, within the said part, first and second contact-carrying leaf-springs are welded to respective

ones of the terminals, one of said leaf-springs being arranged to be acted upon by the push-rod of the switch when the bimetal moves to its overtemperature condition, and the other of said leaf-springs constituting the aforesaid spring means acting upon the inner body part and being arranged so that its contact is positioned so as to be capable of co-operating with the contact carried by the other spring for switching operations only when the switch is properly mounted and the inner body part urges the respective leaf-spring into the requisite position.

In another, more-preferred embodiment of the above- defined aspect of the present invention, a similar leaf- spring arrangement is utilized but, additionally or alternatively, that one of the two leaf-springs that is acted upon by the inner moulding is arranged to make electrical contact with a respective one of the terminals only when the switch is properly mounted.

In order to provide secondary protection in switches according to the present invention as above- defined, the arrangement may for example be such that the positional relationship between the inner and outer members that is established when the switch is properly mounted is conditional upon the temperature of one or other of said parts being below such an elevated temperature as to cause the respective part to deform

under the action of the aforesaid spring means and consequentially render the switch open circuit.

The above and other features of the present invention are set forth in the appended claims and will become clear from consideration of the following detailed description of exemplary embodiments that is given with reference to the accompanying drawings. Description of the Drawings

Figures 1A to IE show various stages in the manufacture of a first embodiment of the present invention, the various component parts of the switch being separately illustrated;

Figures 2A to 2C show enlarged sectional views of the switch of Figures 1A to IE under different operational conditions;

Figures 3A and 3B show exploded views, from different directions, of a second embodiment of the present invention;

Figures 4A to 4D show sectional views of the switch of Figures 3A and 3B under different operational conditions; and

Figures 5A to 5C schematically illustrate different mounting arrangements for the switch of Figures 3A and 3B. Detailed Description of the Embodiments:

Referring first to Figures 1A to IE of the

drawings, the construction of a first embodiment is there shown. Figure 1A shows first and second brass terminal parts 1 and 2 which are captured in an outer moulded plastics switch body part 3 by insert moulding, portions of the terminals 1 and 2 extending into the interior of the part 3 and being exposed therein. Figure IB shows first and second leaf-springs 4 and 5 which are spot welded to the terminal parts 1 and 2 respectively where they are exposed within the body part 3. The leaf-springs 4 and 5 carry respective switch contacts 6 and 7 only one of which can be seen in Figure IB. Figure 1C shows a moulded plastics inner body part 8 which is sized to make an easy sliding fit within outer body part 3 of the switch, a push-rod or slide 9 which is adapted to be inserted into a rectangular passageway formed in inner body part 8, and a dished bimetal disc 10. As shown, the inner body part 8 is formed with three legs 11 which themselves are formed with enlarged portions 12 which define seats for the edge of the bimetal disc 10. The slide 9 is bifurcated at one end, the end which extends inwardly of the switch, and this end of the slide sits astride the radially-inwardly directed limb 4' of the leaf-spring 4. At its other end, the slide 9 has a protrusion which is adapted to engage the centre of the bimetal disc 10. Finally, a metal retaining cap 13 is shown in Fig. ID,

such cap 13 being arranged to be fitted over the open end of the outer body part 3 and clenched thereto and having a central opening 14 providing access to the bimetal disc 10 and a plurality of circumferential apertures 15 providing an easy clearance fit for the legs 11 of the inner body part 8. As shown in Figure 1E-1 the ends of the legs 11 protrude through the apertures 15 in the free standing condition of the switch. Figure 1E-2 shows the completed switch as viewed from the side opposite to the view of Figure 1E- 1.

Figures 2A to 2C show cross-sectional views of the completed switch, Figure 2A showing the switch in free standing condition, Figure 2B showing the switch properly mounted to a surface whose temperature is to be monitored, and Figure 2C showing the overtemperature condition of the switch. As shown in Figure 2A, in the free standing condition of the switch the contacts 6 and 7 carried by the leaf-springs 4 and 5 are broken and the inner part 8 of the switch takes up a position which, as viewed in the figures, is towards the bottom of the switch and has the legs 11 projecting through the apertures 15 in the retaining cap 13. When the switch is properly mounted, as shown in Figure 2B, the contact established between the legs 11 and the switch mounting surface drives the inner body part 8 of the switch

upwardly into the outer body part 3 and a pip 16 on the inner body part 8 urges the contact carrying part of leaf-spring 5 upwardly so as to cause its contact 7 to make with contact 6 carried by leaf-spring 4. The movement of the inner body part 8 releases the edge of the bimetal disc 10 from its seats on the enlarged parts 12 of the legs 11, but nonetheless the bimetal remains positively located between the legs 11 by the action of leaf-spring 6 on push-rod 9. In Figure 2C the condition of the switch is shown after it has switched in response to a sensed over-temperature and it can be seen that the bimetal disc 10 has snapped into its oppositely dished configuration thereby moving push-rod 9 so as to lift leaf-spring 4 and move contact 6 away from contact 7; this is the normal operation of the switch.

In the event of the switch failing to adopt the condition shown in Figure 2C in an overtemperature situation which should normally cause operation of the switch, for example because of failure of the bimetal disc 10 to operate, the temperature of the surface sensed by the switch will most likely continue to rise. At some predetermined temperature, the legs 11 of the inner moulding 8 will begin to soften and forced deformation of these legs will occur with the inner body part 8 of the switch being urged downwardly by the action of leaf-spring 5. As the legs 11 deform, so the

switch contact 7 would eventually move away from contact 6 thereby rendering the switch open circuit. By this means the switch provides secondary or back-up protection. The thus-described switch thus provides both primary and secondary levels of protection, and has the further advantage that in the event of its becoming separated from its proper mounting surface it will go open circuit. The objects of the present invention are thus achieved by this switch.

Figures 3A and 3B show an alternative and presently preferred embodiment of the present invention, and the same reference numerals are used to designate the parts in these figures as were used to designate similar parts in the foregoing description of the first embodiment. Thus a main or outer moulded plastics body part 3 has captured terminals 1 and 2 which extend into the hollow interior of the body part 3 where they contact leaf- springs 4 and 5 respectively. An inner moulded plastics body part 8 is adapted to be slidably received within the outer body part 3 and is formed with a through- aperture for accommodating a push-rod or slide 9, and a dished bimetal disc 10 and a retaining cap 13 complete the component count for the switch. The leaf-spring 4 is generally D-shaped or semi-circular and carries the switch contact 6 at one end of its diametral part, an

extension or enlargement 20 at the other end of such part being provided for contacting switch terminal 1 within the hollow interior of outer body part 3. The other leaf-spring 5 is generally annular with external projections 21 (the purpose whereof will be explained below) , an enlarged portion whereat it carries the switch contact 7, an enlarged portion 22 with a cut-out designed to enable the leaf-spring 5 to be held captive on a post 23 formed as an integral part of the inner switch body part 8, the post 23 passing with interference through the cut-out, and an internal projection 24 (the purpose whereof will also be explained below) . Mounting lugs 25 are provided on the outer switch body part 3 as shown, and co-operating mounting lugs 26 are provided on the inner body part 8, the lugs 26 being guided by slots 27 formed in the main body part 3 and feet 28 being formed on the lugs 26. A post 29 is formed on the inner body part 8 underlying the internal projection 24 of the leaf spring 5. The interior of the main switch body part 3 is formed to define a seating for the leaf-spring 4 -which has its enlarged part 20 spot welded into electrical contact with the switch terminal 1. The remainder of the leaf-spring 4 is constrained against movement in its own plane but can be flexed by the action of push-rod 9 which is responsive to the condition of bimetal disc 10.

The leaf-spring 5 is designed so that its outwardly projecting portions or ears 21 locate in grooves 30 formed in the main body part 3 and its curved circumferential periphery on both sides of the ears 21 are unsupported and have a clearance from the adjacent internal wall of the body part 3; the leaf-spring 5 is thus capable of being bowed between mounting locations defined by engagement of the ears 21 with the inner ends of the grooves 30, that is to say generally about a notional straight line extending between switch contact 7 and the mounting enlargement 22.

Referring to Figure 4A, this cross-sectional view shows the free-standing condition of the switch. Leaf- spring 4 is welded at its enlarged portion 20 to switch terminal l and is held generally in a position to be moved by the action of push-rod 9 when it is operated by temperature-responsive operation of bimetal disc 10. Leaf-spring 5 is held with its ears 21 lodged in the bottoms of the grooves 30, and the post 29 of the inner body part 8 bears against the underside of the inwardly- directed spring extension 24 with a sufficient spring force to keep the body part 8 in contact with the periphery of the bimetal disc 10. In this condition of the switch the switch contacts 6 and 7 are spaced apart, and additionally there is a spacing between the spring extension 24 and a contact defining portion 31 of switch

terminal 2. As the switch is mounted in its desired operating position by use of fixing screws passed through the mounting lugs 25 and 26 of the switch body parts 3 and 8, so the legs 28 of the inner body part 8 contact the mounting surface and cause the body part 8 to be forced inwardly of the main body part 3; Figure 4B shows the switch in the condition where the inner body part 8 has been raised through approximately half of its movement, and in this condition the switch contacts 6 and 7 on the two leaf-springs 4 and 5 make contact but there is still no contact made between the extension 24 of leaf-spring 5 and the contact defining portion 31 of switch terminal 2.

As the movement of inner switch part 8 into main body part 3 continues, so the condition shown in Figure

4C is eventually reached where the post 29 on switch body part 8 urges the extension 24 of leaf-spring 5 into electrical contact with contact defining portion 31 of switch terminal 2. With the switch in this condition, operation of bimetal disc 10 in response to an overtemperature condition will normally cause switch contacts 6 and 7 to be opened as shown in Figure 4D by the action of push-rod 9 upon leaf-spring 4, and in the event that this primary response fails to occur an increase in the temperature of the monitored surface will initiate a secondary or back-up action caused by

softening and forced deformation of the legs 28 of the switch part 8 under the action of leaf-spring 5 until the electrical connection between the extension 24 of the leaf-spring 5 and the contact defining portion 31 of switch terminal 2 is also broken.

The switch of Figures 3A and 3B which operates as described with reference to Figures 4A to 4D thus also achieves the preferred objects of the present invention, namely to provide secondary or back-up protection in a switch package wherein the switch will go open circuit in the event that it becomes inadvertently detached from its intended mounting location. Figures 5A to 5C show schematic views of the switch in stand free, surface mounted and bulkhead mounted situations and it will be seen that versions of the switch responsive to surface temperature or to the air temperature within a duct for example need only have different length legs 28 on the inner switch body part.

The invention having been described hereinbefore with particular reference to two embodiments, it is to be appreciated that the described embodiments are in all respects exemplary and that modifications and variations will occur to those possessed of the appropriate skills without departure from the spirit and scope of the present invention. For example, whereas in the described embodiments secondary protection is afforded

by providing for forced thermal deformation of parts of the inner switch body part in the event of the switch being subjected to a high overtemperature situation without operation of the primary protection, the secondary protection could be provided by other means employing, for example, a thermally deformable pillar or other fusible material appropriately arranged within the switch.