WO1998001874A1

WO1998001874A1 - Improvements relating to thermally responsive electric switches

Info

Publication number: WO1998001874A1
Application number: PCT/GB1997/001856
Authority: WO
Inventors: Robert Andrew O'neill; Antonio Martin Gaeta
Original assignee: Otter Controls Limited
Priority date: 1996-07-10
Filing date: 1997-07-10
Publication date: 1998-01-15
Also published as: GB2315161B; GB9614437D0; AU3454297A; GB2315161A

Abstract

Steam sensor controls in electrical water heating appliances, such as jugs and kettles, operate to switch off the power supply to the heating element of the appliance when water boils in the appliance. The control operates in a wet environment on account of steam condensing on relatively cold parts of the control and this can lead to electrical arcing across wet surfaces of the control. The control is commonly manufactured using moulded plastics material parts (20, 21, 22) and these can be degraded by arcing and give rise to carbon tracks which progressively degrade the electrical insulation properties of the plastics mouldings and can result in failure of the control. To resolve this problem without need for expensive sealing the invention proposes to continue to utilize moulded plastics material for the major component parts (20, 21, 22) of the control, but to insulate electrical terminal parts (23, 24) of the control from each other by means of ceramics material parts (25) incorporated into the control, preferably as push-fit components which engage with appropriately formed apertures in plastics material components of the control. Ceramics materials are generally not susceptible to carbon tracking when wet.

Description

IMPROVEMENTS RELATING TO THERMALLY RESPONSIVE ELECTRIC SWITCHES

Field of the Invention;

This invention concerns improvements relating to thermally-responsive electric switches, and has particular though not exclusive application to the field of thermally-responsive electric switches for use in electrically heated vessels such as kettles, jugs, urns, pans, laboratory equipment and the like for switching off or reducing the supply of power to an electric heating element of the vessel when water or another liquid boils in the vessel and the hot vapour that is generated impinges upon a thermally-responsive switch-actuating element.

Background of the Invention:

In our British Patent Specification No. GB-A-2 212 664 there is described a steam sensor for a water boiling vessel, such as a kettle or a hot water jug, which has been marketed by us as our J-type steam sensor. One exemplary form of the J-type steam sensor is shown in Figures 3A-3M of the drawings of GB-A-2 212 664 and is adapted to be plugged into the top of an X-type element protector control, also marketed by us, either directly or via a flying lead. The X-type element protector control is the subject of our British Patent Specification No. GB-A-2 194 099 and an exemplary form of such control is shown in Figures 4A and 4B of GB-A-2 212 664. As is well known, the function of an element protector control such as our X-type control is to protect the heating element of an electrically powered water boiling vessel such as a kettle or hot water jug against overheating caused for example by switching the vessel on without first filling it with water, the element protector control responding rapidly to the element temperature rising above its normal operating level and switching off the electrical supply to the element. An alternative exemplary form of the J-type steam sensor is shown in Figures 9A and 9B of

GB-A-2 212 664. The disclosures of GB-A-2 212 664 and

GB-A-2 194 099 are incorporated herein by reference.

Steam controls and element protector controls for water boiling vessels are well known and commonly make use of bimetal technology, as in the case of the J-type and X-type controls abovementioned. The thermally responsive element of the control, be it a steam control or an element protector control, is commonly constituted by a bimetal, often a snap-acting bimetal, which is arranged to change condition upon exposure to a predetermined temperature change, for example the change that occurs when it is exposed to steam generated when water boils in the respective vessel. However, whilst bimetal technology is most commonly employed in thermal controls for water boiling vessels, other temperature sensing technologies such as shape memory effect technology has been proposed and has been utilized. See in this connection our British Patent Application No. 9507386.2 and British Patent Specification No. GB-A-2 150 759 which both describe steam sensors for water boiling vessels which make use of coils of shape memory metal which elongate upon exposure to steam. The present invention is not restricted to any particular form of thermally responsive actuator.

The J-type control that is described in GB-A-2 212 664 with reference, for example, to Figures 9A and 9B of the drawings thereof, comprises a main synthetic plastics moulding which defines spaced-apart abutments for an in-line combination consisting of a so-called trip lever, to which an ON-OFF operating knob of the vessel can be attached, and a creep acting bimetallic blade, the trip lever/bimetallic blade combination being arranged to move over-centre in response to impingement of steam onto the bimetallic blade. The main moulding has a chamber which accommodates a switch sub-assembly consisting of two contact-carrying leaf springs, and the trip lever has a projection which co-operates with one of the leaf springs so that when the trip lever/bimetallic blade combustion moves to its hot condition the projection bears upon the respective leaf spring and moves it out of electrical contact with the other leaf spring. Having responded in this manner to the generation of steam within an associated vessel, the arrangement remains in this contacts-open condition until such time as it is manually reset. In the J-type control illustrated in Figures 9A and 9B of GB-A-2 212 664, the main plastics material moulding is designated 44, the trip lever is designated 45, the bimetallic blade is designated 46, and the switch sub-assembly consists of leaf springs designated 41 and 42 which are affixed to a moulded plastics carrier 43.

Within its operating environment when incorporated into an electric kettle or jug as described and illustrated in GB-A-2 212 664, the J-type control unavoidably gets wet as a result of steam condensing on the relatively cold control. The control carries the electrical supply voltage of the associated vessel since the powering of the vessel heating element is arranged to be dependent upon the status of the steam control. The moulded plastics parts of the control are thus subject to the normal operating voltages arising between the electrical parts of the control, namely between the leaf springs of the switch assembly, and, given that the control may get wet in operation, this can lead to arcing across wet moulded plastics surfaces of the control. This in turn can degrade the plastics material of the control and give rise to carbon tracks which progressively degrade the electrical insulation properties of the plastics mouldings. Failure of the control can result.

In the steam sensor arrangement described in our British Patent Application No. 9507386.2, the disclosure whereof is incorporated herein by reference, the approach that is taken towards a solution to the aforementioned problem is to employ a thermally-responsive actuator in the form of a coil of Titanium-Nickel-Copper alloy shape memory effect material which has the distinction of developing a high force and substantial movement as compared to a bimetal, and to employ a sealed system wherein the switching contacts of the control are sealed within a body part of the control which is isolated from the SME sensor by a diaphragm seal formed of silicone rubber which has an integrally-formed push-rod transferring the temperature responsive movement of the SME sensor to the electrical contacts within the control. The proposal of our British Patent Application No. 9507386.2 thus is to isolate the electrical components of the control from those parts of the control, the "front end" of the control, which are wetted by steam in use of the control.

Objects and Summary of the Invention:

The principal object of the present invention is to provide a solution to the problem aforementioned of carbon tracking on the moulded plastics parts of steam sensors for water boiling vessels, without need for relatively expensive sealing arrangements incorporating silicone rubber diaphragm seals.

According to the present invention the above object is at least substantially attained by an arrangement wherein moulded plastics materials continue to be utilized for the major components of the control, despite their susceptibility as hydrocarbon materials to degradation and the formation of carbon tracks under the influence of electrical arcing over wet surfaces, but the electrical terminal parts of the control are insulated from each other by means of ceramics material parts that are incorporated into the control, preferably as push-fit components which engage with appropriately formed apertures in the control. Ceramics materials, which generally speaking contain little or no carbon, are not susceptible to carbon tracking when wet.

In an exemplary embodiment of the present invention which will hereinafter be described in detail, the embodiment being configured as a form of the J-type steam sensor of GB-A-2 212 664, a main chassis moulding, a trip lever and a switch carrier member are all formed of synthetic plastics material as has heretofore been customary in J-type controls. The switch carrier member has a portion defining a rectangular frame, and first and second contact carrying leaf springs are retained in the frame by means of non-conductive ceramics material spacers which assemble together and with the leaf springs to form a generally rectangular block which fits closely into the frame. As an alternative to the use of plural ceramics material spacers which fit together to define the leaf spring retainer block, a single preformed block may be provided having recesses formed therein for receiving the ends of the leaf springs.

In yet another embodiment of the present invention which is described hereinafter, the main chassis moulding is replaced by a pressed metal or die-cast metal housing. A plastics material switch housing affixes to the metal housing in push-fit manner and contact carrying leaf springs are retained in the plastics material switch housing by means of ceramics material spacers. The metal chassis can serve as a mounting for an in-line bimetal and trip lever overcentre combination as in the J-type control, or can mount a different kind of thermally-responsive switch actuator such as a creep-acting or snap-acting bimetal or a shape memory effect actuator.

The abovementioned and further features of the present invention are set forth with particularity in the appended claims and will become clear from consideration of the following description of exemplary embodiments. It will be seen that the inclusion of a relatively short non-conductive ceramics material break in a plastics material tracking path provides an advantageous solution to insulation problems arising in steam sensor switches and the like which are to be used in damp, steamy atmospheres.

Description of the Drawings:

Figures 1A and IB are perspective views from different directions of a J-type steam sensor embodying the present invention, but with the bimetal and trip lever omitted. Figure 1C is a perspective view showing first and second leaf springs and associated ceramics material spacers, Figure ID is a perspective view showing how the assembled leaf springs and ceramics material spacers are retained in a switch carrier, and Figure IE shows how the switch carrier with its retained leaf springs and ceramics material spacers is retained in the chassis moulding of the embodiment;

Figure 2A shows an alternative J-type steam sensor embodying the present invention with its component parts disassembled and Figure 2B is a perspective view of the assembled steam sensor; and

Figure 3 shows yet another steam sensor embodying the present invention.

Detailed Description of the Embodiments: Referring first to Figures 1A to IE, it is recommended that reference be made simultaneously to

GB-A-2 212 664 and particularly to Figures 3A to 3M or

Figures 9A and 9B of the drawings there of which show a similar control. The steam sensor control of Figures 1A to IE comprises a plastics material chassis or main body moulding 1 which defines spaced-apart abutments 2 and

3 for an in-line creep acting bimetal blade and trip lever combination (not shown) . The moulding 1 has a chamber 4 formed therein for receiving a switch sub-assembly 5 which is shown in Figure IE in assembled condition and in progressively disassembled condition in Figures ID and lC. The switch sub-assembly 5 consists of a moulded plastics material carrier 6 which has a first part 7 which serves as a side wall closure for the chamber 4 of the main chassis moulding 1 and a second part 8 which defines a rectangular open frame, first and second contact-carrying leaf springs 9 and 10, and first and second non-conductive ceramics material spacers 11 and 12.

The leaf springs 9 and 10 are formed with tab receptacles 13 at their rear ends and the spacers 11 and 12 are formed with recesses 14 substantially complementary to the tab receptacles 13. Formations 15 are provided on the tab receptacles 13 and project from opposite sides thereof in the manner of ears, and complementary recesses 16 are provided in the spacers 11 and 12 for accommodating the ears 15. By virtue of these provisions, the receptacles 13 at the ends of the leaf springs nest in and are retained by the recesses 14 of the ceramics material spacers 11 and 12 in the manner shown in Figure ID. The spacer 12 has an extended end portion which, in the assembled arrangement, increases the electrical creepage distance between the tab receptacles and reduces the risk of tab terminations received in the receptacles accidentally contacting each other.

As shown in Figure ID, the assembled leaf springs 9 and 10 and ceramics material spacers 11 and 12 define a generally rectangular block structure 17 which is dimensioned to fit tightly into the frame defined by the part 8 of the carrier member 6 so as to constitute the switch sub-assembly 5 shown in Figure IE. Co-operating dovetail slide features 18 are provided on respective edges of the frame part 8 of the leaf spring carrier 6 and on the chassis moulding 1, and click features 19 are provided on these parts to retain the switch sub-assembly 5 in the chassis moulding 1. Although the bimetal actuator and the trip lever of the embodiment of Figures 1A to IE are not shown in the drawings, the operation of the described steam sensor will be well understood from consideration of GB-A-2 212 664. The trip lever has a finger portion which extends into the chamber 4 of the main chassis moulding where, depending on the state of the bimetal, it can abut the free end of the leaf spring 10 and depress it out of electrical contact with the other leaf spring 9. In the illustrated arrangement, the leaf springs 9 and 10 are electrically isolated from each other by the non-conductive ceramics material spacers 11 and 12. The receptacle 13 at the end of leaf spring 10 is in fact surrounded by ceramics material on all sides, whereas the receptacle 13 at the end of leaf spring 9 is isolated on three sides by ceramics material and on the fourth side by the plastics material of the carrier member 6. This arrangement ensures that the maximum possible amount of ceramics material surrounds the leaf spring 10 and isolates it from leaf spring 9 thereby ensuring optimum electrical separation between the two leaf springs. Since the leaf springs 9 and 10 are isolated from each other by non-conductive ceramics material which does not suffer from the tracking problems associated with synthetic plastics materials as described hereinbefore, an improved electrical performance of the embodiment as compared to conventional all plastics J-type controls will be achieved.

Figures 2A and 2B show an alternative embodiment of the present invention which has much in common with the embodiment of Figures 1A to IE. A main body moulding or chassis formed of synthetic plastics materials is designated 20 and defines spaced-apart abutments as in the above-described embodiment for the in-line combination of a trip lever 21 and a creep acting bimetal (not shown) . A moulded plastics switch enclosure 22 is adapted to be welded or otherwise affixed to the main body moulding 20 as shown in Figure 2B and serves to accommodate a switch assembly comprising contact-carrying leaf springs 23 and 24 and one-piece ceramics material support block 25. The block 25 has recesses 26 formed therein for receiving the ends of the leaf springs 23 and 24 and is sized to make a tight interference fit into the open end of enclosure 22. A finger 27 on the trip lever 21 extends into the enclosure 22 through an aperture 28 in its upper wall and, within the enclosure 22, abuts the lowermost leaf spring 23 for urging the same into a contacts-open condition of the switch assembly when the bimetal responds to the impingement of steam thereupon. In other respects the embodiment of Figures 2A and 2B is functionally the same as the embodiment of Figures 1A to IE.

Yet another embodiment of the invention is illustrated in Figure 3 where a metal chassis 30, formed by pressing or by die casting for example, is adapted to carry a moulded plastics material switch housing 31 which accommodates a switch sub-assembly 32 formed of contact-carrying leaf springs 33 and 34 and ceramics material spacer elements 35 and 36. As with the previously described embodiments, the metal chassis 30 of the Figure 3 embodiment provides spaced-apart abutments 37 and 38 for an overcentre movement type thermally-responsive actuator such as the in-line bimetal and trip lever combination previously described herein and described more fully in GB-A-2 212 664 with reference for example to Figures 9A and 9B of the drawings thereof. The moulded plastics switch housing 31 has formations 39 which engage with complementary formations 40 in the metal chassis 30 to retain the two parts together, and arcuate slots 41 and 42 that are provided respectively in contiguous walls of the switch housing 31 and the chassis 30 register with each other to permit a limb of the trip lever to extend into the switch housing for operating the switch sub-assembly 32. A further formation 43 provided on the switch housing 31 extends through an opening 44 in the chassis 30 and provides a mounting for a rotary cam (not shown) which constitutes an adjustable abutment against which the bimetal of the bimetal/trip lever combination works when switching the overcentre arrangement from its cold to its hot condition (see in this connection the fixed abutment 62 in Figure 3J of GB-A-2 212 664 and the corresponding description at lines 19-22 of page 22) . The ceramics material spacer elements 35 and 36 operate to secure the leaf springs 33 and 34 in the switch housing 31 substantially as in the first embodiment. Note however that the two elements 35 and 36 are identical and are formed to stack positively one upon the other with projections 45 formed on the under sides of the spacers engaging in recesses 46 formed in their upper sides. Note also the rearwardly extended portions 47 of the spacers which protect against accidental short-circuiting of electrical tab terminals and attached conductors which are engaged with the tab receptacles of the leaf springs 33 and 34 in use.

The metal chassis 30 as shown in Figure 3 is adapted for use with a creep acting bimetal and trip lever combination as taught in GB-A-2 212 664 and it is to be appreciated that alternative thermally responsive switch actuators could be employed, the chassis then being redesigned as appropriate. For example, a snap-acting bimetal or a shape memory effect actuator could be utilized in place of the creep acting bimetal and trip lever combination.

It will be appreciated from the foregoing that the mounting of the electrical switch components of the described steam sensors by use of non-conductive ceramics spacers introduces a break into the tracking path that otherwise would potentially exist if the switch components were secured wholly in plastics material mouldings. Only a short ceramics material break is necessary in order to provide a solution to the tracking problem previously mentioned herein caused by the steam sensor being exposed inevitably to a damp operating environment. The arrangement of Figures 1 and 3 whereby one of the spring conductors of the switch is supported between two ceramics material spacers and the other is supported between one of the spacers and the plastics material switch carrier moulding is considered advantageous as compared to the Figure 2 arrangement. The reason for this is because the supporting of one switch blade at a ceramics/plastics interface enables a greater electrical separation to be achieved, within the ceramics components of the switch, between the two switch terminals.

It is to be well appreciated that the present invention is not restricted to the described embodiments and that modifications and variations are possible without departure from the spirit and scope of the invention as set forth in the appended claims.

Claims

CLAIMS :

1. An electrical switch for use in high moisture environments, said switch comprising a housing part formed of synthetic plastics material, electrically conductive switch components retained in said housing part, and a non-conductive ceramics material insert in the electrical current path that would otherwise exist between said switch components across the surface of said housing part when such surface is wet, the presence of said insert serving to inhibit the formation of a conductive tracking path between said switch components.

2. A electrical switch as claimed in claim 1 wherein at least one of said electrically conductive switch components is mounted in said housing part by means of non-conductive ceramics material.

3. An electrical switch as claimed in claim 2 wherein all of said electrically conductive switch components are mounted in said housing part by means of non-conductive ceramics material.

4. An electrical switch as claimed in claim 3 wherein the non-conductive ceramics material defines a block which is received in an opening of said plastics material switch housing part, said electrically conductive switch components being supported by means of said block.

5. An electrical switch as claimed in claim 4 wherein said block supports at least one of the electrically conductive switch components at an interface between the block and the plastics material of the housing part, another of the electrically conductive switch components being supported within the block.

6. An electrical switch as claimed in claim 4 or 5 wherein said block is composed of plural ceramics material parts, at least one of said electrically conductive switch components being supported between two of said ceramics material parts.

7. An electrical switch as claimed in claim 6 wherein some at least of said ceramics material parts have formations complementary to the electrically conductive switch component(s) supported thereby.

8. An electrical switch as claimed in any of the preceding claims wherein said electrically conductive switch components comprise cantilevered leaf springs.

9. An electrical switch as claimed in claim 8 wherein first and second leaf springs are cantilevered from the same end of the switch.

10. An electrical switch as claimed in any of the preceding claims further comprising a thermally-responsive actuator for said switch.

11. An electrical switch as claimed in claim 10 wherein said actuator comprises a bimetallic element.

12. An electrical switch as claimed in claim 11 wherein said bimetallic element is a creep-acting bimetallic element.

13. An electrical switch as claimed in claim 12 wherein said creep-acting bimetallic element is arranged in line with a switch-operating element between two opposed abutments, the arrangement being movable overcentre in dependence upon the temperature of the bimetallic element.

14. An electrical switch as claimed in claim 13 wherein said abutments are defined in a chassis member of the switch.

15. An electrical switch as claimed in claim 14 wherein said chassis member is formed of moulded synthetic plastics material.

16. An electrical switch as claimed in claim 14 wherein said chassis member is formed of metal.

17. An electrical switch as claimed in claim 11 wherein said bimetallic element comprises a snap-acting bimetal.

18. An electrical switch as claimed in claim 10 wherein said thermally responsive actuator comprises a shape memory effect material.

19. An electrical switch as claimed in claim 10 wherein said thermally responsive actuator is accommodated separately from said synthetic plastics housing part of the switch.

20. An electrical switch as claimed in claim 19 wherein a moulded plastics material switch component defines a first part accommodating said thermally responsive actuator and a second part constituting said housing part.

21. An electrical switch as claimed in claim 19 wherein the thermally responsive actuator is accommodated in a metal part which is affixed to the moulded plastics switch housing part.

22. An electrical switch as claimed in any of claims 10 to 21 and configured as a steam sensor switch for an electrically powered water boiling appliance, the thermally-responsive actuator of the switch being arranged for the impingement thereupon of steam and the housing part of the switch substantially isolating the switch components from exposure to steam.

23. An electrical switch substantially as herein described with reference to any of the accompanying drawings.

24. An electrically powered water boiling appliance incorporating an electrical switch as claimed in claim 22 or 23.