LIQUID HEATING APPARATUS

Abstract: A liquid heating apparatus comprises a vessel (4) for receiving liquid to be heated and a thermally sensitive control (2) having a thermally sensitive actuator (14, 16) operable in the event that the vessel overheats to open a set of contacts (36, 44) in the electrical supply to the element (8) of the vessel, said control (2) comprising means (54) for holding said contacts (36) open after operation of the actuator (14), said means (54) further permitting said contacts (36) to re-close upon lifting of the vessel from a support surface, filling the vessel with liquid or replacing said vessel on said support surface.

Description: Liquid Heating Apparatus This invention relates to liquid heating apparatus comprising a liquid heating vessel having an electrical heating element and in particular to such apparatus having thermally sensitive overheat protection means arranged in thermal contact with the element, and which are operable to interrupt or reduce the supply of electrical energy to the element in the event of the element overheating, as might happen if the vessel boils dry or if it is switched on without any liquid in it. Typically, the overheat protection means comprises a thermally sensitive actuator, such as a bimetallic actuator, arranged in good thermal contact with the element and which operates to open a set of electrical contacts in the electrical supply to the element. If such a liquid heating vessel is accidentally left to boil dry or is switched on with no liquid in it, for example, the temperature of the element will rise to the point where the thermally sensitive actuator operates to interrupt the electrical supply to the element, thereby allowing the element to cool. Eventually the element will cool sufficiently that the actuator will reset. If the user of the vessel has not noticed what has happened by this stage and has not therefore taken action either to fill the vessel or switch it off, e.g. at the wall socket, then the electrical supply will be reconnected to the element, which will then start to heat up again. Thus the element will cycle on and off in the manner described until remedial action is taken. This means that the vessel becomes very hot and in the event that the primary thermal protection fails, it would cause the thermal fuse to operate (assuming that the vessel had one) , causing permanent damage to the vessel. Such cycling is also wasteful of energy, particularly if the vessel is left unattended for a long period after the control begins to cycle. In order to overcome this problem, latch mechanisms have been proposed which latch the electrical contacts open and which are reset by the user operating a reset button or switch. Unfortunately, some users do not realise that the latch can be re-set and therefore assume the kettle to be faulty. The present invention seeks to overcome these problems, and from a first aspect provides a liquid heating apparatus comprising a vessel for receiving liquid to be heated and including a thermally sensitive control having a thermally sensitive actuator operable in the event that the vessel overheats to open a set of contacts in the electrical supply to the element of the vessel, said control comprising means for holding said contacts open after operation of the actuator, said means further permitting said contacts to re-close automatically upon lifting of the vessel from a support surface filling the vessel with liquid, or replacing said vessel on said support surface. Thus in apparatus in accordance with the invention the potential cycling problem encountered with known liquid heating vessels is prevented, by virtue of means which hold a pair of electrical contacts open, thereby interrupting the electrical supply to the element. The heating element will remain disconnected from the electrical supply until the user uses the vessel again. In order to use the vessel again the user must lift it up, fill it at least partially with liquid and then replace it on a supporting surface . The means may permit the contacts to re-close upon any one or a combination of these operations, although preferably it does so when the user lifts the vessel. Thus, operation of the thermally sensitive control is effectively made invisible to the user, since following a boil-dry or dry-switch-on situation the user must lift the vessel and fill and replace it in order to be able to use the vessel again. Although the various components of the control may be mounted separately with respect to the vessel, preferably, the control is an assembly which can be mounted as a unit to a liquid heating vessel. From a second aspect therefore, the invention provides a thermally sensitive control unit for a liquid heating vessel having a thermally sensitive actuator operable in the event that the vessel overheats to open a set of contacts in the electrical supply to the element of the vessel, said control comprising means for holding said contacts open after operation of the actuator, said means being arranged and adapted so as to permit said contacts to re-close automatically upon lifting of the vessel from a support surface, filling the vessel with liquid or replacing said vessel on said support surface. The invention is particularly but not exclusively applicable to liquid heating vessels in which an electric heating element is provided on the underside of the vessel base. In such circumstances, the control may be mounted under the base of the vessel with its thermally sensitive actuator in thermal contact with the base of the vessel. The thermally responsive actuator may comprise any suitable actuator, for example a shape memory metal, but preferably it comprises a bimetallic actuator and most preferably a snap-action bimetallic actuator to ensure rapid and positive disconnection of the electrical supply. The actuating motion of the actuator may be transmitted to a movable contact by suitable means such as a push rod. The actuator may be of the type which will operate upon overheating of the element but which will not reset automatically during normal use, but which must be reset by the application of an external force. Such a force could be provided, for example, by a spring loaded member which acts on the actuator when the vessel is lifted, filled or replaced so a s to reset it and allow the contacts to reclose. Preferably however the means for holding the contacts open comprises latch means, said latch means being automatically released upon lifting of the vessel from a support surface, filling the vessel with liquid or replacing said vessel on said support surface. This is particularly applicable when a thermally sensitive actuator is used which will reset automatically upon cooling, to below say 95°C, i.e. during normal use. The latching means may hold the contacts open in any suitable manner. For example it may latch the actuator itself, so that it cannot reset. Preferably however, the latch means cooperates with the set of electrical contacts to hold them open even if the thermally responsive actuator resets . Such an arrangement is advantageous since the electrical contacts will typically require a much smaller force to hold them open than would be required by a latch means to prevent the thermally responsive actuator returning to its original condition. Preferably, therefore, the latch means engages a movable contact member of the contact set. Preferably the latch means is arranged to be resiliently urged into its latching position. In a particularly preferred embodiment therefore, the latch means is resiliently urged into latching engagement with a movable contact member. The latch means may itself be resilient and in a preferred embodiment is formed as a spring arm. The latch means is preferably released by release means arranged to urge the latch means out of its latched position as the vessel is lifted. The release means may, for example, comprise a cam surface arranged on a release member which moves when the vessel is lifted. Preferably the release member is resilient or resiliently mounted, so as to produce the desired movement when the vessel is lifted. However, a weighted member would also be suitable, moving under its own weight when the vessel is lifted. Preferably the release member is operatively coupled to or integral with a member which projects from the base of the vessel or control unit and which is deflected as the vessel is placed on its supporting surface so as, in the preferred embodiment, to preload the release member. Most preferably, the preloading member projects vertically from the bottom of the vessel . Of course other arrangements could be envisaged, for example a projection or the like being provided on the support surface cooperating with the release member. In the preferred arrangement, the latching means provides a generally radial latching movement, and the release means provides a general axial release movement relative to the vessel. The invention may be applied to so called corded, fixed cord or 'cordless' type vessels, in which case the apparatus may comprise a base unit to receive the vessel. In such constructions, mating electrical connectors are provided in the base of the vessel and on a base unit designed to receive the vessel . In the preferred embodiment the release mechanism may protrude from the vessel connector part for engagement with a part of the base unit connector. A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a liquid heating vessel and control embodying the invention, placed on a cordless base; Fig. 2 is a plan view of the control of Fig. 1 with the actuator mounting plate removed for clarity; Fig. 3 is a cross-sectional view of the control after the vessel has overheated; Fig. 4 is a cross-sectional view of the control in a latched-off state following subsequent cooling; and Fig. 5 is a cross-sectional view of the control removed from the cordless base and the latch released. Referring to the Figures and particularly to Figs . 1 and 2 a thermally sensitive control, denoted generally by the reference numeral 2, is mounted (by means not shown) to the base of a liquid heating vessel 4, which rests on a base unit 6. Access to the control is prevented by a base cover 7. A conventional sheathed heating element 8 is mounted in a known manner to the base of the vessel via a thermal diffuser plate 10. The control 2 is of the type generally as described in WO 95/34187, modified to include latch means. It includes a metallic plate 12 which is mounted to a plastics control moulding 13 and which mounts two snap- acting bimetallic actuators 14,16 of the type generally described in GB 1542252 which, as shown, are arranged in good thermal contact with the base of the vessel . The actuators 14,16 are preferably chosen so as to have the same operating temperature. Integrally formed with the control 2 is a cordless electrical connector 18 (best seen in Fig. 5) , which mates with a corresponding connector 20 in the base unit 6 to supply power to the element 8. The connectors 18, 20 are of the type described in WO 95/08024 which will allow electrical connection to be made between them irrespective of their relative rotational orientations. The base unit connector 20 has line and neutral sockets to receive a corresponding line terminal pin 22 and an annular neutral terminal 24 of the control connector 18 (see Figs 2 and 5) , and a radially projecting earth contact which engages an annular earth terminal 26 on the control connector 18. This terminal 26 is in turn electrically connected to the actuator mounting plate 12 so as to earth the base of the vessel 4 in use. The line pin 20 is staked to a leaf spring 28 at one end of which is mounted a movable contact 30. This cooperates with a fixed contact 32 to form a set of switch contacts 36. The fixed contact 32 is electrically connected to a terminal or cold tail of the element 8 via a spade terminal 38.. The neutral terminal 24 is electrically connected to a second leaf spring 40 on which is mounted a movable contact 42, which forms a second set of contacts 44 with a second fixed contact 46. This fixed contact 46 is electrically connected to the other terminal of the element 8, via a second spade terminal 48, thereby completing the electrical circuit to the element 8. The respective leaf springs 28,40 are acted upon by respective push-rods 50,52 in response to operation of the bimetallic actuator 14,16 when the element 8 overheats, so as to open the respective sets of contacts 36,44. Referring to Fig 2, a latch member 54 is formed as an elongate arm of a torsion spring 56 mounted on a pillar 58 formed in the control moulding 13. The limit positions of the latch member 54 are shown by respective solid and phantom lines 54,54' in Fig.2. In the position shown in Figs 1 and 2, the latch member 54 abuts against a downward extension 60 of the leaf spring 28, which may be seen more clearly from Fig 3. Also disposed in the control unit 12 is a resilient cantilevered release member 62 which is fixedly mounted in the control moulding 13 at one end 64. Its other end is formed as a downwardly extending angled cam 66 which is arranged to bear upon the latch member 54 as will be described further below. The released position of the release member 62 is shown in Fig 5, but in the states shown in Figs 1-4 it is preloaded in an upper position by a preloading member or pin 68 which extends downwardly out of the upper inner surface of the control connector 18 for engagement with an upwardly facing surface of the base connector 20. Clearly, the pin 68 could be arranged to bear upon any suitable surface upon which the vessel 4 is supported. Operation of the thermally sensitive control unit will now be described with reference to Figs 1 through 5. Figs 1 and 2 show the control unit in its normal operating condition. As may be seen, the two sets of contacts 36,44 are closed and therefore electrical current may flow from the cordless base 18 to the element 8. In the event that, for example, the vessel is accidentally switched on with no liquid inside, then the temperature of the element 8 will rise rapidly and, if left unchecked, would soon become dangerous. However, when the element reaches a given threshold temperature, the two bimetallic actuators 14,16 will operate so as to push down the two push-rods 50,52 respectively. The push-rods 50,52 then in turn deflect the respective leaf springs 28,40, to open both sets of contacts 36,44 as shown in Fig 3. As the leaf spring 28 is pushed down, its downward extension 60 moves below the latching member 54, allowing the latching member 54 to move across above the upper surface of the leaf spring 26, to the position shown by the dashed line 54' in Fig. 2 and as shown in Fig. 3. With both sets of contacts 36,44 open, power to the element 8 is disconnected and it will begin to cool . When it has cooled below a certain lower threshold, the bimetallic actuators 14,16 will return to their original positions, as shown in Fig 4. The second set of contacts 44 will then close once again. The first set of contacts 36 is, however, prevented from re-closing by the latch member 54, which prevents the first leaf spring 28 from deflecting upwardly again. Thus, the first set of contacts 36 is latched open and the electrical supply to the element 8 remains disconnected. The first set of contacts 36 will remain latched open until the vessel 4 is lifted from its supporting surface, in this case the cordless base unit 6. As shown in Fig. 5, when the vessel is lifted, the pin 68 is unrestrained by the base unit 6 and allows the release member 60 to deflect downwardly under its own resilience. As it does so, the release cam surface 66 urges the latch member 54 out of engagement with the leaf spring 28, so allowing the leaf spring 28 to return to its rest position and close the first set of contacts 36. When the vessel 4 has been refilled and placed back onto the base unit 6, the pin 68 will be urged upwardly again, to preload the release member 60. As it does so, the latch member 54 will once again move into contact with the downward extension 60 of the leaf spring 28 and the control unit 2 is thus returned to the state shown in Fig 1, ready to be used again to heat liquid in the vessel 4. It will be understood by those skilled in the art that a number of modifications may be made to the above described embodiment within the scope of the invention. For example, the latch means could be released other than when the vessel is lifted, e.g. when the lid is opened in order to refill the vessel or an arrangement similar to that used for retractable ball-point pens could be used to release the latch when the vessel is replaced on its supporting surface. Also, the pin 68 could extend out horizontally from the base unit for engagement with another surface, if appropriate. Also, the control may be applied to a 'traditional' corded kettle rather than a cordless one, with for example the pin 68 extending out of the vessel base to engage an underlying surface . In a yet further variation the latch need not be released by a spring mechanism, but by other means such as a weight . Furthermore, the invention may be applied to vessels having other forms of heating element to that disclosed. For example, rather than being a sheathed heating element as shown, the underfloor element may be a 'thick film' element described for example in WO 96/18331. The invention may also extend to more traditional vessels in which a sheathed heating element extends into the vessel through an opening in a side wall or base wall thereof. Also, while the contacts in the preferred embodiment have been shown to be in the 'vessel' part, in an alternative arrangement some or all of the contacts, the latch and so on being provided in the base unit .

Claims: Claims 1. Liquid heating apparatus comprising a vessel for receiving liquid to be heated and a thermally sensitive control having a thermally sensitive actuator operable in the event that the vessel overheats to open a set of contacts in the electrical supply to the element of the vessel, said control comprising means for holding said contacts open after operation of the actuator, said means further permitting said contacts to re-close automatically upon lifting of the vessel from a support surface, filling the vessel with liquid or replacing said vessel on said support surface. 2. A thermally sensitive control unit for liquid heating apparatus comprising a vessel for receiving liquid to be heated, said control having a thermally sensitive actuator operable in the event that the vessel overheats to open a set of contacts in the electrical supply to the element of the vessel, said control comprising means for holding said contacts open after operation of the actuator, said means being arranged and adapted so as to permit said contacts to re-close automatically upon lifting of the vessel from a support surface, filling the vessel with liquid or replacing said vessel on said support surface . 3. A liquid heating apparatus or control unit as claimed in claim 1 or 2, wherein said means permits said contacts to re-close upon lifting of the vessel. 4. A liquid heating apparatus or control unit as claimed in claim 1, 2 or 3 , wherein the means for holding said contacts open comprises latch means, said latch means being automatically released to permit the contacts to re-close. 5. A liquid heating apparatus or control unit as claimed in claim 4, wherein said latch means cooperates with a movable electrical contact of the contact set. 6. A liquid heating apparatus or control unit as claimed in claim 4 or 5, wherein said latch means is resiliently biased into its latching position. 7. A liquid heating apparatus or control unit as claimed in claim 6, wherein said latch means is formed as a spring arm. 8. A liquid heating apparatus or control unit as claimed in any of claims 4 to 7 , wherein the latch means is released by release means arranged to urge the latch means out of its latched position as the vessel is lifted, filled or replaced. 9. A liquid heating apparatus or control unit as claimed in claim 8, wherein said release means comprises a cam surface. 10. A liquid heating apparatus or control unit as claimed in claim 8 or 9, wherein said release means is resilient or resiliently mounted, so as to produce a releasing movement. 11. A liquid heating apparatus or control unit as claimed in claim 10, wherein said release means is operatively coupled to a preloading member which loads the release means as the vessel is placed on its supporting surface. 12. A liquid heating apparatus or control unit as claimed in claim 11 wherein said preloading member projects from the base of the vessel or control unit so as to be deflected as the vessel is placed on its supporting surface. 13. A liquid heating apparatus or control unit as claimed in any preceding claim, further comprising an electrical connector. 14. A liquid heating apparatus or control unit as claimed in claim 12 and 13 wherein said preloading member projects from a portion of said electrical connector. 15. A liquid heating apparatus as claimed in any of claims 1 or 3 to 14 further comprising a base unit to receive the vessel . 16. A liquid heating apparatus or control unit substantially as hereinbefore described with reference to the accompanying drawings .

Inventors: TAYLOR JOHN CRAWSHAW, TAYLOR, JOHN, CRAWSHAW
Applicants: STRIX LTD, TAYLOR JOHN C, STRIX LIMITED, TAYLOR, JOHN, CRAWSHAW
Application year: 1997
Main classification: H05B1
IPC codes: A47J27, H05B1