Field of the Invention:

This invention concerns improvements relating to electric heating elements and controls therefor and, more particularly, concerns planar heating elements of the kind comprising a substrate navmg a resistance heating element formed thereon, for example as a resistance track formed by photolithographic techniques or by printing with electrically conductive ink or as a flame sprayed coating, and heating element overtemperature protection controls adapted to be assembled with the heating element and arranged to switch off the power supply to the heating element in a sensed element overtemperature situation.

Background of the Invention:

Heating elements for electric water heating appliances such as kettles and hot water jugs have conventionally been of the immersion type, in which a wire wound resistance heater is contained within a metal sheath with an electrically insulating material packed into the sheath around the resistance wire, or

of the type wherein such a heating element is clenched or otherwise affixed to the underside of a metal plate, commonly an aluminium plate, which is built into the appliance commonly as a base plate thereof. More recently, however, heating elements of the aforementioned planar type have been developed.

We have consistently been at the forefront of the development of element protection controls for heating elements for electric water heating appliances. Conventionally such controls nave comprised a snap-acting bimetallic switch actuator which is assembled in close thermal contact with the heating element so as to be responsive to the temperature thereof, the bimetallic switch actuator being set to operate at a temperature above the normal operating temperature of the heating element and being arranged to open a switch in the electric current supply path to the heating element when it does operate. We also pioneered the introduction of secondary protection by providing a second snap-acting bimetallic switch actuator in the control, the second bimetallic switch actuator being operable independently of the first and providing an assurance that the heating element will continue to be protected against overtemperature situations even if the primary protection afforded by the first bimetallic switch actuator fails to operate,

for whatever reason. The provision of secondary protection was, and is, important in the context of water heating appliances formed of synthetic plastics materials which could catch fire with disastrous consequences in the event of an unchecked element overtemperature situation.

The current range of otter Controls element protection controls is designated as the X-type of control. The original XI control s described in GB-A-2 194 099 with reference particularly to Figs. 3A, 3B and 3C of the drawings thereof and in GB-A-2 243 724, and a modification of this control, the X2 , is particularly well adapted for use with planar heating elements and is described in GB-A-2 283 156. The X-type control has a snap-acting bimetallic switch actuator which is held in close thermal contact with the heating element for providing primary protection, and for providing secondary or back-up protection, operative in the event of failure of the primary protection, the bimetallic actuator is carried by a plastics material carrier which is arranged to bear against the heating element when the control is assembled thereto. The plastics material of the carrier is selected to soften at a temperature above the normal temperature range of the heating element and above the temperature at which the

bimetallic actuator is set to operate, and the carrier is under spring pressure in the control so that it will collapse when it softens and this collapse opens a set of contacts within the control so as to disrupt the power supply to the element.

The X2 control is the preferred form of element overtemperature protection control for use with planar heating elements as aforementioned which comprise a substrate having a resistance track formed thereon. Heating elements of this k nd are typically constructed as a stainless steel disc substrate of 1.5mm thickness which carries on its undersurface a dielectric insulating layer, for example of glass, the resistance track and a protective coating also of a dielectric insulating material, for example glass. As described in GB-A-2 283 156, openings are provided in the protective insulating layer to provide access for terminal springs of the X2 control to contact terminations of the resistance heating track and to enable the primary bimetallic actuator to be held in close thermal contact with the resistance heater.

A problem has arisen in the mounting of X2 controls to planar heating elements as abovedescribed as a result of the tendency of the planar heating element to deform during normal running and/or during an element overtemperature situation. This

deformation results from the provision of the heating element track and its associated electrically insulating layers on only one side of the substrate. The heating element tends to bow into a convex or concave shape during normal operation and to bow to a greater extent during an overtemperature situation. If the element is formed initially with a convex or concave shape, as is sometimes the case, the convexity or concavity of the element shape tends to change during normal operation and more so during an overtemperature situation. Since the bimetallic switch actuator of the X2 control and the plastics material carrier have to be maintained in proper heat transfer relationship with the heating element in order to be effective, any deformation of the heating element during normal operation and/or in an overtemperature situation is potentially detrimental to the proper functioning of the control.

Objects and Summar ^y of the Invention: Accordingly, it is the principal object of the present invention to overcome or at least substantially reduce the abovementiαned problem.

According to the present invention, the abovementioned object is achieved by use of a metal mounting bracket for the X2 control, the mounting

bracket being adapted to be secured to the planar heating element at three weld locations two of which are disposed symmetrically with respect to a radius of the planar element intersecting the third, the bracket thereby maintaining all points on the underside of the control substantially at the same distance from the heating element surface.

The invention relies upon the assumption that the disc-shaped planar neating element bows into or substantially into tne shape of a segment of a sphere. If the mounting bracket has three legs of closely controlled and similar neight arranged triangularly so that with the end of one leg welded on a radius of the heating element the ends of the other two legs can be welded to the heating element at locations spaced equally on opposite sides of this radius and each at the same radial distance from the centre of the element, then the underside of the control, supported within the triangle formed by the three legs of the bracket, will be substantially at a constant distance from the element even though the element is bowed. Whilst the planar heating element, in practice, may not bow into a precise segment of a sphere, the effect of a bracket as aforedescribed is sufficient to keep an X2 control adequately positioned within its design tolerances.

The invention will best be appreciated by consideration of the following description given with reference to the accompanying drawings.

Description of the Drawings: Figures 1A and 13 show, respectively, an exploded perspective view of an X2 element protection control, a fixing bracket and a planar heating element and a perspective view of these three components in assembled condition; Figure 2 is a side elevational view showing an X2 control affixed to a planar heating element by means of a bracket, all as shown in Figures 1A and IB; and

Figures 3A and 3B are, respectively, a plan view of the assembled X2 control, mounting bracket and planar heating element, all as shown in Figures 1A and IB, and a side elevation view of a planar heating element with the bow that is developed during its operation shown to an exaggerated extant.

Detailed Description of the Embodiment: Referring first to Figure 1A, shown therein is a planar heating element 1, an X2 element protection control 2 and a metal bracket 3 serving for the attachment of the control 2 to the element 1.

The underside of the element 1 is the side that

is viewed in Figures 1A and IB and it is this side of the element that carries the resistance heating track or layer, the other side of the element customarily being plain. The element, as aforementioned, might for example consist of a stainless steel substrate carrying on its underside a glass insulating layer onto which the resistance heating track or layer is formed, and a further electrically insulating protective glass layer overlying the resistance heater. As described n GB-A-2 233 156, openings are provided in the protective glass layer to enable spring terminals of the X2 control to contact terminal pads of the resistance heater and to enable the bimetallic switch actuator of the X2 control to make close thermal contact with the resistance heater; these openings are not shown in any of the accompanying drawings.

The X2 control, as described in GB-A-2 283 156 for example, has a generally rectangular plastics body 4 with a planar underside from which projects the aforementioned thermally-defor able plastics material carrier, and the aforementioned bimetallic switch actuator is received in the carrier. More particularly, the carrier has four legs which, in use, stand upon the protective glass layer of the heating element 1 just outside of the opening that is formed

therein for enabling the bimetallic switch actuator to access the resistance heater, and the four legs are shaped at their feet portions to receive respective corners of the bimetal which has a dished generally rectangular shape.

The bracket 3 defines a planar platform 5 having a cut-out 6 through which extend the carrier and the associated bimetallic switch actuator of the X2 control 2 when the two are operativeiy assembled. The bracket 3 has three legs 7 , 3 and 9 which are of carefully controlled length such that when the bracket is stood, on ts legs, on a planar surface, the platform 5 will be parallel to the surface. The legs 7, 8 and 9 end in feet 10 serving for welding the bracket 3 to the undersurface of the heating element 1, weld pads 11 being provided on the element for this purpose and constituting openings through the various layers formed on the element substrate down to the substrate metal itself. Openings 12 are provided in the bracket 3 for co-operation with openings 13 formed in the X2 control 2 for securing the bracket to the control, and front and rear upstands 14 and 15 are provided on the bracket for serving a control locating function, particularly the rear upstand 15 which can be located between a pair of opposed formations 16 provided on a side surface of the control 2 in the

manner shown in Figure 3A.

Figure 2 is a side elevation view showing the X2 control 2 secured to the bracket 3 which in turn is secured to the heating element l. The location of the thermally-deformable plastics material carrier 20 of the control 2 and of the bimetallic switch-actuator 21 can be seen in this figure and it can be seen that the rearmost legs 3 and 9 of the carrier are aligned centrally with the carrier 20 and with the bimetal 21. Figure 3A is a plan view showing the X2 control

2 assembled with the bracket 3 and with the latter spot welded to the heating element 1. As shown, the weld positions A of the legs 3 and 9 are symmetrically disposed about a radial line through the weld position B of the leg 7 at equal distances X on either side of such radial line and at the same radial distance from the centre of the heating element 1. By virtue of this arrangement and as illustrated in Figure 3B, which shows the bowing of the element 1 in greatly exaggerated form, the heights H of the weld points A above the datum plane 30 of the element will be the same. By forming the legs 7, 3 and 9 of the bracket

3 so that they are of closely controlled similar height, then all points on the planar underside of the body 4 of control 2, being within the triangle defined by the three legs of the bracket, will be

substantially at the same distance from the element, even though the element is bowed, and well within the operating tolerances allowed for the control 2. This is particularly the case in view of the location of the carrier 20 and bimetal 21 symmetrically between the legs 8 and 9 of the bracket .

The welding of the forward leg 7 of the bracket to a weld point 11 close to the peripnery of the disc element 1 is preferred since at this location there is little movement of the weld point as the heating element bows and, furthermore, the greatest amount of space is provided behind (on the opposite side of) the control 2 for accommodating an associated electrical connector part if the heating element 1 is to be incorporated into a cordless appliance. Whereas a single forward weld point B as shown m Figure 3A is preferred, two spaced-apart forward weld points having symmetrical locations on either side of weld point B could alternatively be employed. Having thus described the invention by reference to an exemplary embodiment, it is to be well understood that modifications and variations thereto are possible without departure from the spirit and scope of the appended claims. For example, the forward leg 7 could be omitted from the bracket 3 in the described embodiment if the bracket was formed of

97/02526

12 su ^f ficient ^l y s ^t iff _m etal and/or i _f the feet ₁₀ o _f legs ⁸ and ⁹ were enlarged somewhat.