This invention relates to electric immersion heaters.

Traditionally, electric immersion heaters which are used, for example, in kettles, hot water jugs and other water heating appliances comprise a metal sheathed element which is brazed to a metal element head which is mounted to overlie and is sealed with respect to an opening in a wall of a liguid containing chamber of an appliance. The element is brazed to the head at both its end portions, where the element sheath extends through the head to project by a small amount into the dry side of the head. The element is formed so that a so-called hot return portion, which is a portion intermediate the ends, is bent back against the ^' head and brazed thereto. As shown, for example in GB-B-2181598, a thermally-sensitive actuator of a control is commonly arranged in thermal contact with the side of the head opposite the hot return i.e. the dry side, so that should the element overheat, for example when the appliance is switched on dry or boils dry, the temperature rise in the hot return portion of the element is conducted through the head to the actuator which, at a predetermined temperature, operates the control to interrupt the power supply to the element and thus de-energises the element.

The control may also comprise a back-up protector which operates to disconnect the power in the event that the primary protector fails to operate.

A water boiling vessel may also be provided with a steam-sensitive control which operates to disconnect the power to the element when liguid in the vessel boils. The assembly of such elements and controls to the vessel is time consuming, it being necessary first to position the back of the element head against the

opening in the wall from inside the vessel, with suitable sealing means interposed therebetween, and then mounting the control to the back of the head from outside the vessel, and securing the two together to mount the heater and control in the vessel wall. At the same time electrical connections need to be established between the control and heater. The assembly must then be tested in situ. The present invention seeks to provide an immersion heater which overcomes these problems.

According to the invention there is provided a combined immersion heater and control unit for a liguid boiling vessel, comprising a control unit adapted to be mounted in relation to a vessel wall in or adjacent an opening therein, and a heating element mounted to said unit and being of lesser radial extent than both the unit and the opening so as to be introducable into the vessel via said opening from the outside as the unit is mounted to the vessel, said control unit incorporating a primary overheat protector, a back-up overheat protector, and a vapour sensitive boiling control for said vessel.

Thus in accordance with the invention, a single, factory pre-assembled and fully tested heater and control unit containing all the controls and electrical parts necessary for satisfactory operation of the vessel may be mounted in or adjacent an opening in the vessel wall without the need for the subsequent assembly and testing steps discussed above. In a preferred embodiment, the primary overheat protector comprises a thermally responsive actuator, for example a bimetallic element, preferably snap acting, mounted on a high thermal conductivity link attached at one end to and extending at least partially around a return portion of the element in good thermal contact therewith. Such an arrangement is described in our co- pending International Patent Application No.

PCT/GB91/01579 ( O92/05675) , the disclosure of which is incorporated herein by reference.

The control unit preferably comprises a plastics housing, through an opening in a front face of which the end portions, ie. the cold leads, of the element and the thermal link extend, with sealing means engaging not only between the housing and the opening but also between the housing and the link and the element end portions. The movement of the actuator of the primary overheat protector may be used to trigger a pre-loaded spring mechanism which acts to open a set of contacts within the control. In one embodiment, the actuator acts on a preferably slidably mounted lever having a portion which engages a leaf spring mounting a movable contact, so as to move it away from a fixed contact, provided on, for example, a terminal pin of the control. • The spring force may conveniently be provided by an over-centre spring, for example a B-spring, mounted in the control housing and engaging the lever. Where double pole protection is sought, the lever may engage two movable contacts provided in the line and neutral sides of the supply to the element with the fixed contacts preferably being mounted to or forming part of the respective terminal pins. Conveniently the leaf springs mounting the movable contacts extend directly between the cold leads of the element and the terminal pins, and most preferably are mounted directly onto the cold leads of the element although they may be mounted on the terminal pins, if desired.

This provides a particularly desirable construction, since it reduces the number of components in the control. Furthermore, such a construction is rendered possible, or at least considerably facilitated by the pre-assembly of the control and element prior to its mounting on the vessel wall.

The vapour sensitive control preferably comprises a

thermally-sensitive actuator, such as a bimetallic actuator mounted on or in the control unit housing. Preferably the actuator, which may be snap-acting, is mounted externally on said housing for example on an upper part thereof so as to avoid vapour, which is conducted via a suitable steam passage of the vessel to the actuator for its operation, entering the housing.

In a preferred embodiment, the steam sensitive actuator causes opening of the same contacts as the primary overheat protector. In an arrangement described above, therefore, the slidable lever may extend out of the top of the control unit housing and engage the actuator. The lever may also have a portion for engagement with an operating member which is coupled with or forms an on-off knob of the appliance. Thus operation of either the steam control or the primary overheat protector will act to move the knob to an off position, the subsequent act of turning the knob to an on position then serving to re-set the spring mechanisii for the next operation.

In a preferred embodiment, the back-up protector comprises a thermal fuse which is in thermal contact with the element and which melts or thermally distorts when the element overheats. In the arrangement described above, the fuse may be resiliently biased into engagement with a portion of the thermal link, so that should the primary overheat protector fail, the temperature of the link will rise to the extent that the fuse member will deform and move under the resilient biasing force, the movement being used to cause opening of contacts in the power supply to the element, advantageously the same contacts as opened by the primary overheat protector.

As mentioned above an arrangement in which a leaf spring extends between a cold lead of the element and a terminal pin of the appliance is particularly advantageous.

From a second aspect therefore the invention also provides an electric immersion heater comprising a metal sheathed heating element having cold leads extending from the respective ends thereof, resilient contact means mounted in contact with or to at least one of said cold leads and engaging with contact means provided on a fixed, vertically extending terminal pin of the heater, and a thermally responsive control, operable in the event of said element overheating, to move said resilient contact away from said terminal pin to interrupt the electrical supply to the element.

Preferably contacts are provided on both line and neutral terminal pins so that two pole protection may be provided. The thermally responsive control preferably comprises a primary overheat protector having preferred features as described hereabove. Furthermore, a back-up protector and/or a vapour sensitive control may also be provided, acting to open the contacts in an appropriate operating condition.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Fig. 1 is a perspective view of a heater/control unit in accordance with the invention;

Fig. 2 is a schematic rear view of the embodiment of Fig. 1, with the rear cover removed; and

Fig. 3 is a schematic vertical section taken though Fig. 2. With reference to Fig. 1, a combined element and control unit comprises an element 1 extending from a control 2. A thermally responsive, snap-acting bimetallic actuator 3 is mounted on the upper part of the control housing and acts upon a forwardly extending portion 4 of a lever arm extending into the control. With reference to Figs. 2 and 3 it will be seen that the cold tails 5 of the element 1 extend through

the front face of the control housing 6 through apertures 7. Furthermore a link 8 of a material having a high thermal conductivity such as copper or copper clad with stainless steel extends around a return portion 9 of the element through a further opening 10 in the front face of the housing 6 a seal member 11 seals not only the openings 7 and 10 but also seals the opening 12 in the vessel wall as shown when the control unit is mounted to the vessel. Such an arrangement is disclosed in our earlier mentioned International Patent Application No. PCT/GB91/01579.

A snap-acting, thermally responsive bimetallic actuator 13 of a type known in the art is mounted onto the end of the link member 8 by an end portion 14 of an earth pin 15.

As will 'be seen from Fig. 2, leaf springs 16 are mounted at one end to the cold tails 5 of the element and at their other end mount removable contacts 17. These contacts cooperate with fixed contact surfaces 18 formed on line and neutral pins 19 of the control.

The leaf springs 16 are provided beyond the contact 17 with extension portions 20 which are arranged to lie over forwardly projecting lugs 21 of a slidably mounted member 22. As will be seen from Fig. 3, the member 22 has a forwardly projecting portion 23 which overlies the free end of the snap-acting bimetallic actuator 13. Furthermore as mentioned earlier the upper end of the member 22 has a forwardly extending projection 4 which overlies the steam activated bimetallic actuator 3, extending through a slot 24 in an upward extension of the housing 6.

As can be seen from Fig. 2, the slidable member 22 is acted upon by an over-centre B spring 25 which is mounted in the housing 6.

The thermally responsive actuators 3,13 are of the well known type which reverse their curvature when they

exceed a predetermined operation temperature. Such actuators are described for example in GB 657434. Thus it will be seen from Fig. 3 that when the steam activated bimetallic actuator 3 is impinged upon by steam egressing from the vessel, via suitable conduit means provided in the vessel, it will change its curvature and so move the slide member 22 upwardly. This upward movement will cause the free ends 26 of the B spring 25 which engage the member 22 to pass through their neutral positions and so move the member 22 upwardly with a snap-action. The lugs 21 on the member 22 will in that case engage with the free ends 20 of the leaf springs 16 to part the contacts 17,18 on both the line and neutral sides of the supply to the element. Furthermore should, for example, the steam activated bimetallic actuator fail to operate and the temperature of the thermal link 8 rise to the point where the primary protector bimetallic actuator 13 operates it too will act upon the slide member 22 to cause its movement under the influence of the B spring 25.

In the embodiment shown a secondary back-up protector is provided in the form of a thermal fuse 30 which is resiliently biased by a coil spring 31 into contact with the thermal link 8. The thermal fuse has a bifurcated portion 32 terminating in lugs 33 which extend under the free end portions 20 of the leaf springs 16. As will be seen from Fig. 3 the forwardly projecting lugs 21 of the slide member 22 and the rearwardly extending lugs 33 of the thermal fuse 30 do not overlap so that their operation is independent of one another.

In the event that the primary overheat protector fails to operate, the temperature of the thermal link 8 will continue to rise to the point where the upper part of the thermal fuse 30 will deform thermally under the force of the spring 31, causing the lugs 33 to move upwardly and engage the free end portions of the leaf

springs 16, thereby opening the contacts 18,19. Should this happen, it will be clear that the contacts cannot be remade without replacing the thermal fuse member 30. As shown in Fig. 2, the upper end portion 40 of the slide 22 may be connected to a rod 41 which extends to an on-off knob of the appliance. It will be clear that once the slide member 22 operates, the spring mechanism 25 can be reset by .pressing downwardly on the rod 41 to recock the mechanism. In an alternative embodiment the portion 40 may act upon a member mounted pivotally on the control housing with a knob portion which in use extends from the appliance operation by a user.

The embodiment shown is a cordless appliance, the terminal pins 16,19 extending downwardly from the control for engagement with corresponding contact provided in a socket 50 in a base for the appliance. It will be appreciated that the invention may equally apply to corded appliances, in which case the terminal pin.'-. may be arranged to extend horizontally from the rear <-■ ' the control.

It will be seen from Fig. 3 that the rear of t_it- housing is closed by a cover plate 35. Apertures 60 are provided in the control housing and the cover plr, ' .- receiving mounting screws not shown which, in use, will mount the control and element as a unit to the wall of the appliance.