GB2173045A - Electric heating elements - Google Patents

Abstract

An electric heating element for a kettle or other water boiling vessel has a tubular housing 7 brazed to the element head 1. The housing 7 contains a spring-loaded plunger 10 retained with the spring 11 under compression by a plug 12 of fusible material. The plunger 10 extends through the element head 1 and, when the fusible material 12 melts in response to the element overheating, it moves under the action of the spring 11 so as to operate a switch or eject the power supply plug of the vessel thereby disabling the element. The fusible material 12 e.g. aluminium is selected to have a melting temperature well above the element temperature at which a conventional dry boil protection control would operate, so that when the element is used with such a control the arrangement of the spring-loaded plunger 10 provides a secondary protection against extreme overheating of the element in the event of failure of the primary control. The plunger 10 is arranged to be non-resettable. <IMAGE>

Description

SPECIFICATION Improvements relating to heating elements This invention concerns improvements in heating elements and particularly electrically powered heating elements for water heating vessels such as electric kettles, jugs, urns etc.

Element protection devices are well known which are adapted to interrupt the power supply to the heating element of a kettle or the like in the event of the occurrence of an overtemperature condition such as might occur for example if the kettle were to be switched on without there being any water in it (a so-called "dry switch-on" situation) or if the kettle were to be allowed to boil dry (a so-called "dry boil" situation).

Disclosed in our British Patent Specification No. GB2 117 568 is an element protection unit adapted to provide two separate levels of protection against dry switch-on or dry boil situations, the unit comprising two independent thermal sensors each controlling a separate pair of switching contacts and each in thermally conductive relationship with the heating element, one of such sensors and its associated switching contacts providing primary protection and the other providing secondary protection against the possibility of failure of the primary protection. The arrangement of GB2 117 568 has been developed for particular application to water boiling vessels formed of synthetic plastics materials where a failure of the element protection can lead to melt down of the plastics material of the vessel and attendant fire risk.

It may be considered desirable to ensure that, in such a protector system provided with primary and also with secondary or back-up protection, the protector unit cannot be reset after operation of the secondary protection if the secondary protection operated following failure of the primary protection. The aim of this arrangement is to ensure that the protector unit cannot be reset into a potentially hazardous situation. In other words, the secondary protection should latch out permanently if it ever operated without the primary protection having operated correctly first.

The present invention concerns yet a further arrangement for protecting against the fire risk which accompanies the use of water boiling vessels formed of plastics materials in the event, however unlikely, that a primary protection unit fails to respond to an overtemperature condition. The invention stems from the realisation that there is a very considerable difference between the temperature, typically of the order of 350 to 400"C which the heating element sheath reaches under a "normal" dry switch-on situation where the element is quickly switched off within about 15 to 20 seconds by operation of the element protector unit, and the temperature, typically of the order of 700 to 750"C after about 35 seconds, which the element sheath reaches under the extended power-on situation which occurs when the element protector unit fails.The invention aims to exploit this considerable temperature difference in a non-resettable form of protection employing a fusible component having an appropriately high melting point in contact with the element sheath and arranged so that when it melts it releases a spring-loaded plunger or the like which opens switch contacts in an associated element protector unit and/or ejects the power supply connector plug from the vessel socket.

In an exemplary arrangement in accordance with the present invention the fusible component might be formed of aluminium, which has a melting point of 660 C. An additional tubular housing, formed of copper or nickel silver for example, might extend from the element head towards and into close thermal contact with the hottest part or parts of the element away from the coolrod/element connection. The housing would be closed at its end remote from the element head and might be crimped onto a plug of the fusible material positioned within the tubular housing near its closed end, the plug retaining a spring-loaded plunger under tension. Alternatively, the plunger itself could be formed of the fusible material.The plunger would extend through the element head and on the dry side of the element head would be arranged to cooperate with a switch such that when the plunger was released the switch was opened to disconnect the power supply to the element. For ensuring that the plunger cannot be reset by the user of the appliance in which the heating element is provided, it may be sufficient merely to ensure that the plunger is not accessible, or at least not readily accessible to the user, or alternatively some means, such as a ratchet and pawl arrangement, might be provided for ensuring that the plunger can move in one direction only.

It is envisaged that a non-resettable fusibleelement protection arrangement according to the present invention may be used in combination with an element protector unit typically comprising a thermally-responsive snap-acting bimetallic blade held in close thermal contact with the heating element head and arranged to open a pair of switch contacts via a push-rod arrangement when the temperature of the element head rises above its normal water-boiling temperature; one such element protector unit is the Otter Controls Z30 unit as described in British Patent Specification GB2 045 588. A steam-sensing unit could also be provided for switching off the power supply to the element when water boils in the vessel, and one such steam-sensing unit is the Otter Controls Z40 unit which is also described in GB2 045 588.

The arrangement according to the present invention could even be utilised in combination with a primary and secondary protection system as disclosed in GB2 117 568 to achieve a high degree of protection.

Many different switching arrangements could be employed in the practice of the present invention for converting the movement of the spring-loaded member released by melting of the fusible component into a corresponding switch-opening and/or connector ejecting movement. The arrangement could be similar to those described in GB2 117 568 for example but with the secondary pair of switching contacts arranged either additionally or alternatively to be operated by the release of the spring-loaded member.

The advantage of the present invention stems from its simplicity. The amount of modification required of a conventional kettle or hot water jug element in order to practice the invention is only relatively small, the manufacturing techniques involved are well known and the additional components required are simple and inexpensive. Furthermore, it is not envisaged that any difficulties whatsoever would arise in regard to modifying an otherwise conventional element protector unit for operation in combination with the present invention.

The use of fusible components in association with electric heating elements is per se known, but not to our knowledge in the manner of the present invention. It has previously been proposed that a section of the resistance heating wire within the heating element itself could be configured as a fusible element for protecting plastics bodied vessels from fire hazard, but this proposal has not proved universally acceptable on account of difficulties experienced in ensuring for the element an adequate life in normal use and yet the ability to rupture within say 50 to 60 seconds in the event of failure of the primary element protection.Another arrangement utilising fusible material is described in British Patent Specification GB1 151 083 and comprises a plug ejector in a kettle element having a spring-loaded ejector plunger retained by a ratchet wheel which is restrained against rotation by a fusible material; in this arrangement, the plug ejector constitutes the primary element protector arrangement and the fusible material is selected to have a melting point of around 200"C which is considerably lower than that of the fusible component of the present invention, and furthermore the arrangement is designed expressly with the aim of being resettable which is contrary to the teachings of the present invention.

An exemplary embodiment of the invention is schematically illustrated in the accompanying drawings wherein Figure 1 shows a plan view of the element partly cut away to show internal construction and Figure 2 is a part sectional side elevational view on the line II ...

II in Figure 1.

The illustrated element is of substantially conventional design apart from the features provided in accordance with the present invention. The element as shown comprises a head portion 1 with element cold tails 2 and 3 and looped element portions 4 and 5 including a hot return portion 6 brazed to the element head. A tubular housing 7, shown open in the drawings but sealed in actuality, is brazed to the element head and extends forwardly of the head alongside portions of the heating element proper. The housing 7 is brazed to the heating element proper at portions 8 and 9 so as to be in effective heat transfer relationship therewith.Within the housing 7 there is provided a plunger 10 which is retained against the action of a strong compression spring 11 by means of a plug 12 of fusible material such as aluminium for example which is held within the housing 7 by virtue of the housing being crimped down onto the plug for example.

In the event of the element overheating to such an extent that the plug 12 melts, the plunger 10 is released and moves in the direction of the arrow under the action of the spring 11. Those skilled in the art will readily appreciate that this movement of the plunger can be utilised to open an electrical switch or to eject an appliance power plug.

Schematically illustrated in the Figures is also an exemplary means for inhibiting the plunger, once released, from being reset. A spring member 13 is affixed to the end of the plunger 10 and has limbs which are received within the end of the tubular housing 7 when the plunger is in its retained condition as shown. When the plunger 10 is released, the limbs of the spring member 13 move out of the end of the tubular housing 7, spring apart slightly, and prevent the plunger from being reset.

In the illustrated arrangement, the spring 11 is located at the forward and hottest end of the tubular housing 7 and would have to be able to withstand such temperatures without deterioration. It will be appreciated however that the spring could readily be relocated to a cooler part of the housing 7 or could even be positioned on the dry side of the element head portion 1 where it would be in a relatively cold environment.

Having thus described an embodiment of the invention in outline terms it is believed that no further description is necessary to enable one skilled in the art to put the invention into practical effect. Various modifications and variations will occur readily to those possessed of the appropriate skills, particularly as regards the nature of the fusible material, the nature of the cooperation between the springloaded member which is released by the melting of the fusible material and the disruption of the power supply to the element, the form and operation of the spring-loaded member itself, and the nature of any means provided to ensure that the arrangement is non-resettable, and all such modifications are intended to be embraced within the scope of the invention.

In addition to the advantages hereinbefore described, the present invention obtains yet a further advantage in an arrangement wherein an element has a first sensor provided at the rear of the element, such as for example a conventional element protector comprising a bimetal or other thermally responsive element in thermal contact with the element head and arranged to disable the element in a sensed overtemperature situation, and a second sensor provided at a forward position on the element and likewise arranged to disable the element in a sensed overtemperature situation.If a water boiling vessel, such as a kettle for example, fitted with such an element is switched on with only a relatively small amount of water in it and when it is standing on an inclined surface such as the draining board of a sink, then it could happen that the forward part of the element could be exposed above the water level even when the element head is relatively submerged. Under such conditions, an element provided with forward protection in accordance with the present invention would respond to overheating of the forward part of the element and would be switched off even if the rearward protection did not operate on account of the rear of the element being relatively cool; a conventional element provided only with rearward overtemperature protection would not necessarily respond to such conditions which could prove hazardous particularly if the vessel were a plastics bodied vessel.

Claims (18)

1. An electric heating element for a water boiling vessel wherein secondary dry boil protection is provided responsive only to the super-elevated element temperatures experienced in an extended dry boil situation following failure of primary dry boil protection.

2. An electric heating element for a water boiling vessel wherein an extended dry boil situation is sensed as a function of the superelevated element temperature experienced when the element is powered in a dry condition for longer than a predetermined time period.

3. An electric heating element for a water boiling vessel wherein a fusible component is provided in heat transfer relationship with the heating element and dry boil protection is provided responsive to melting of said fusible component, the fusible component having a melting point which is so high that the heating element temperature will rise to a level such as to cause said fusible component to melt only in response to the heating element being powered in a dry condition for an extended time period greater than a predetermined time period.

4. An electric heating element as claimed in claim 3 wherein the dry boil protection afforded by said fusible component is a secondary protection and means are adapted to be associated with the element for providing primary dry boil protection to disable the heating element at an element overtemperature below the melting temperature of said fusible component in response to the heating element being powered in a dry condition for a time period no greater than said predetermined time period.

5. An electric heating element as claimed in claim 4 wherein the heating element comprises an element portion proper and a head portion whereby the element may be mounted to a wall of a water boiling vessel, and the primary protection means comprises a thermal sensor adapted to be located in heat transfer relationship with the element head on the dry side thereof and switch means arranged to be controlled by said thermal sensor for determining the powering of the element.

6. An electric heating element as claimed in claim 5 wherein the thermal sensor of the primary protection means comprises a bimetallic element.

7. An electric heating element as claimed in claim 6 wherein said bimetallic element is snap-acting.

8. An electric heating element as claimed in any of claims 3 to 7 wherein said fusible component is arranged so that when it melts a spring-loaded plunger is released so as to effect a disconnection of the power supply to the element.

9. An electric heating element as claimed in claim 8 wherein the spring-loaded plunger is arranged to operate a pair of switching contacts provided in an element protector unit adapted to be associated with the heating element.

10. An electric heating element as claimed in claim 8 or 9 wherein the spring-loaded plunger serves when released to eject a power supply connector plug from a socket of the element.

11. An electric heating element as claimed in any of claims 8 to 10 wherein the plunger is arranged so as not to be resettable once released.

12. An electric heating element as claimed in claim 11 wherein means are associated with the plunger for ensuring that the plunger can move in one direction only.

13. An electric heating element as claimed in any of claims 3 to 12 which comprises an element head portion, element cold tails secured in said head portion, and looped element portions extending from each of said cold tails and looping round to a hot return portion of the element in contact with the element head, and wherein said fusible component is housed in a tubular housing extending from the element head towards and into close thermal contact with parts of said looped element portions remote from the element cold tails.

14. An electric heating element as claimed in any of claims 3 to 13 wherein the fusible component has a melting temperature significantly in excess of 400"C.

15. An electric heating element as claimed in claim 14 wherein the fusible component comprises aluminium with a melting temperature of 660"C.

16. An electric heating element substantially as herein described with reference to the accompanying drawing.

17. Each and every novel feature herein disclosed, either singly or in combination with any other feature.

18. An electric heating element for a water boiling vessel, said element having a rearward part enabling the element to be mounted, for example in a wall of the vessel, and a forwardly-extending part for immersion in water in the vessel and wherein first thermally sensitive means are associated with the rearward part of the element and are arranged to cooperate with switch means provided at the rearward part of the element for interrupting the supply of electric current to the element in response to a sensed overtemperature condition at the rearward part of the element and second thermally sensitive means are associated with the forwardly-extending part of the element at a location remote from the rearward part and are arranged to cooperate either with the same said switch means or with a separate switch means provided at the rearward part of the element for interrupting the supply of electric current to the element in response to a sensed overtemperature condition at the forward part of the element.