Field of the Invention:

This invention relates to thermal controls for electrically powered liquid heating vessels such as electric kettles, hot water jugs, pots, pans, urns, laboratory equipment and the like, the control serving an element protector function for switching off or reducing the supply of electricity to the heating element of the vessel in response to a sensed element overtemperature condition caused for example by switching the element on without ensuring that the vessel has first been filled. Controls of this kind are well known and are commonly referred to as element protector controls.

Background of the Invention:

Disclosed in our British Patent Specification No. GB-A-2 283 156 is an element protector control for use with a planar electric heating element comprising a patterned electrical resistance heating element formed on an electrically insulating substrate, the substrate comprising a thin sheet of stainless steel on the underside of which there is provided a thin layer of

glass or other temperature-resistant electrically-insulating material, and the resistance heating element being formed on the surface of the glass layer with a further thin layer of glass or other temperature-resistant electrically-insulating material formed over the resistance heating element. The control that is described in GB-A-2 283 156 is a modified form of the control that is described in our British Patent Specification No. GB-A-2 194 099 with particular reference to Figures 3A, 3B and 3C thereof and in our British Patent Specification No. GB-A-2 248 724. The disclosures of GB-A-2 283 156, GB-A-2 194 099 and GB-A-2 248 724 are incorporated herein by reference. As described in GB-A-2 283 156, the element protector control is mounted to the planar heating element by providing mounting studs on the heating element and affixing the control to these. A problem has arisen however on account of thermal expansion/contraction effects associated with the heating element construction which can exceed the designed mounting tolerance of the control. This situation arises on account of the very high power density of the heating element which gives rise to thermal expansion/contraction effects, particularly differential expansion between the wet (water) side of

the heating element and its dry (resistance heating element) side, which can result in quite significant movement of the element in use and especially under dry boil conditions, namely the heating element being powered in a dry condition, which is when the element protector control is required to operate. The element protector control that is described in GB-A-2 283 156 is susceptible to be affected by these thermal movements of the planar heating element since it comprises a secondary or back-up protection system in the form of a moulded plastics carrier for a primary protection bimetal, the carrier seating on the heating element and being adapted to deform under spring pressure and thereby cause electrical contacts within the control to open when the plastics material of the carrier softens under exposure to excessive heating element temperature. To enable the secondary protection carrier to function properly a predetermined minimum distance is required to be maintained between the body of the control and the planar heating element and we have found that this cannot always be guaranteed to be the case with the arrangement that is described in GB-A-2 283 156.

The problem outlined above does not only arise in situations where the element protector control is fixed to the planar heating element by means of

mounting studs provided on the element. It also arises where other means are employed to fixedly clamp the control to the element. Further, the problem is not restricted to planar elements of the kind described above, it can also arise with planar heating elements of the alternative kind in which a wire wound resistance heating element contained within a tubular metal sheath is cast or clenched into an aluminium plate.

Objects and Summary of the Invention:

It is thus the principal object of the present invention to overcome or at least substantially reduce the abovementioned problem.

According to the present invention, the abovementioned object is attained by means of a resilient mounting of the element protector control to the planar heating element in which the control is pressed against the element by means of a spring strip, and by providing spacers formed of heat-resistant material, preferably a ceramics material, on the body of the control to assure that it is maintained at a proper spacing from the planar heating element.

More generally, in one of its aspects the present invention provides an element protector control for

assembly with a planar electric heating element to protect the heating element against overheating by switching off the power supply thereto, the control comprising a body portion to be juxtaposed with the planar heating element at a certain predetermined spacing therefrom, spacers formed of heat-resistant material being provided on said body portion to ensure said spacing, and a thermally-responsive member of said control being arranged in use of the control to bear against the planar heating element and to deform under spring pressure when the heating element temperature exceeds a certain level, such deformation being arranged to open a set of contacts within the control thereby to disconnect the heating element from its power supply.

The invention also extends to the combination of an element protector control as above described with a planar electric heating element, the control being mounted to the heating element by resilient means urging the control towards the heating element such that said spacers bear against the heating element surface.

The above and further features and aspects of the present invention are set forth with particularity in the appended claims and will be explained more fully in the following description which is given with

reference to the accompanying drawings.

Description of the Drawings:

Figures lA to IF show an exemplary element protector control embodying the present invention, Figure 1A being a plan view of the control from the side which juxtaposes with a heating element in use, Figure IB being a plan view of the other side of the control, Figure 1C being a short side elevation view, Figure ID being a long side elevation view, and Figures IE and IF being perspective views;

Figure 2 is an exploded perspective view corresponding to Figure 1 of GB-A-2 283 156 and showing the construction of a prior art element protector control which is very similar, particular as regards its internal construction, to the control shown in Figures 1A to IF hereof; and

Figure 3 schematically illustrates the attachment of the control of Figures 1A to IF to a planar heating element.

Detailed Description of the Embodiment:

Referring to Figures 1A to IF, the element protector control that is shown therein is substantially as described in GB-A-2 283 156 with reference to Figure 1 thereof and, for ease of

reference, the same reference numerals are used in the following description as were used in GB-A-2 283 156 to refer to like parts. Figure 2 of the accompanying drawings corresponds to Figure 1 of GB-A-2 283 156. Referring to Figure 2 , the element protector control shown therein comprises first and second moulded plastics body parts 1 and 2 which are adapted to fit together and to capture therebetween first and second terminals 3 and 4 and spring metal conductors 5, 6 and 7. On the other side of body part 2 there is provided a bimetallic primary switch actuator 8 which, in similar manner to the control that is described in GB-A-2 194 099 and in GB-A-2 248 724, is adapted to be mounted in a plastics material carrier 9 which is adapted in turn to locate in predetermined manner with the front (not visible) face of body part 2.

The bimetallic primary switch actuator 8 is arranged, in a manner substantially identical to that described in GB-A-2 194 099 and GB-A-2 248 724, to determine the status of a switch defined by the parts 10 and 11 of the spring metal conductors 5 and 6, a push-rod 12 being provided for this purpose. the parts 10 and 11 normally make electrical contact with each other and are opened by push-rod 12 when the bimetallic primary switch-actuator 8 responds, in use, to an element overtemperature condition. In the event

that the primary switch actuator fails, for whatever reason, to open the contact between the parts 10 and 11, the heating element will continue to be powered so that its temperature will rise to a level whereat the secondary thermally-responsive actuator constituted by the plastics material carrier 9 will become operative. The carrier 9 has rearwardly-facing projections 13 and 14 which are designed to extend through the body part 2 and into contact with the parts 15 and 16 of the spring metal conductors 5 and 7 so that, when the control is operatively coupled to a planar heating element and the carrier 9 is urged rearwardly towards the body part 2, the spring metal parts 15 and 16 are urged by the projections 13 and 14 into contact with the terminals 3 and 4 respectively. The carrier 9 has four legs 17 with feet 18 which sit upon the rear surface of an associated heating element and, when the element temperature rises to such a level as to cause the legs 17 to begin to melt, the carrier 9 is collapsed in a forwards direction towards the heating element by the spring action of the conductor parts 15 and 16 so that the conductor parts 15 and 16 eventually move out of contact with the terminals 3 and 4 of the control. The element protector control of Figure 1 is adapted for use with a generally planar heating

element and it will be seen that the spring metal conductors 6 and 7 have forwardly extending portions 19 and 20 which are adapted to project forwardly of the control body part 2 and make electrical contact with terminal portions of the planar heating element for supplying electrical energy thereto through the control.

Referring to Figures 1A to IF, it will be seen that the surface of the control which is juxtaposed with a planar heating element in use is provided with three ceramics material spacers 25 which are received in hollow bosses formed on the element protector part 2 so as to be coplanar with the bimetallic primary switch actuator 8 and with the feet 18 of the plastics material carrier 9. The spacers 25 ensure the correct positioning of the bimetallic primary switch actuator 8 and of the plastics material carrier 9 in relation to the surface of a planar heating element upon which the spacers are seated and, by virtue of being formed of heat resistant ceramics material, preferably a material having a low thermal conductivity such as a fired steatite material, the moulded plastics body of the control is protected from being overheated.

Figures IB and IF, show the provision of formations 30 on the side of the control which, when the control is juxtaposed with a planar heating

element, is furthest from the heating element. These formations 30 provide for the positive location of the control with respect to a cantilevered clamping spring 35 which, as shown in Figure 3, is formed at its free end for positive engagement with the formations 30 and is mounted at its other end to a mounting stud 40 formed at a convenient location on the planar heating element 45 or, alternatively, formed on part of the associated vessel or on another component associated with the vessel.

By virtue of the arrangement described with reference to Figures 1A to IF and Figure 3 of the accompanying drawings, the element protector control is held by means of a resilient bias at a predetermined position of the planar heating element and the requisite spacing of the control from the element surface is assured. This arrangement can accommodate thermal expansion/contraction of the heating element without prejudicing the proper positioning of the control in relation to the heating element and without prejudicing the proper positioning of the collapsible plastics material carrier of the control relative to the heating element.

The invention having been described in the foregoing by reference to a specific embodiment, it is to be well appreciated that the invention is not

limited to the described embodiment and that modifications and variations are possible without departure from the spirit and scope of the invention as set forth in the appended claims. For example, instead of a cantilevered spring retaining the element protector control in operative relationship with the planar element, the spring could be in the form of a beam supported at both of its ends and bearing upon the control at an intermediate location.