GB2374730A

GB2374730A - Thermal control

Info

Publication number: GB2374730A
Application number: GB0109777A
Authority: GB
Inventors: Martin J Shepherd
Original assignee: Kettle Solutions Ltd
Current assignee: Kettle Solutions Ltd
Priority date: 2001-04-20
Filing date: 2001-04-20
Publication date: 2002-10-23
Anticipated expiration: 2021-04-20
Also published as: GB2374730B; CN2532780Y; CN1247140C; CN1382414A; GB0109777D0

Abstract

A thermal control 2 for use in an electrical appliance for boiling water of a type provided with an electrical heating element comprises a thermally responsive bimetal 12 which responds to boiling of water. Electrical conductors 40, 42; 50, 54 provide live and neutral conduction paths. A live switch contact pair 41, 44 in the live conduction path has a movable contact 41 and a neutral switch contact pair 58,59 in the neutral conducting path has a movable contact 59. An actuating lever 16 acts on the movable contacts in response to the bimetal to open the switches sequentially one after the other on boiling of water.

Description

CONTROL The present invention relates to a control for use in electric water heating appliances and in particular for electric kettles and jugs.

Electric kettles require a control to allow a user to heat water to a predetermined temperature, usually to boiling, at which point the kettle automatically switches off. A wide variety of controls exist to carry out this function, most commonly employing a heat-sensitive element such as a bimetallic element which acts indirectly on an electric contact to break the electrical supply to the heating element.

An example of such a type of control is that of British Patent GB 1470364 of Otter Controls Ltd in which the bimetallic element is disposed above an opening receiving steam from the kettle body. The bimetallic element acts on an over-centre lever which in turn acts to break a pair of contacts connected in series with the heating element.

Domestic kettles are also required for reasons of safety to include further controls which deactivate the kettle when operated without water inside, or when allowed to boil dry for example if water is boiled with the lid not properly

fitted or the over-centre lever is intentionally prevented from operating by the user. It is additionally required for reasons of safety that the kettle is safe if this"boil dry"or"dry switch on"protection fails or is rendered inoperative under laboratory conditions.

A variety of controls have been proposed to provide this "dry-boil"protection. For example, in British Patent 2204450 of Strix Ltd a collapsible thermal fuse is provided which melts or defuses at a predetermined temperature to release a spring mechanism which opens a pair of switch contacts in series with the heating element, permanently disabling the kettle.

In another type of control the subject of British Patent 2185857 of D H Haden Ltd a pair of bimetallic discs which have different switching temperatures are arranged to act on a pair of switch contacts, one disc switching reversibly at about 1300C to provide primary boil dry protection, the other having a higher switching temperature and providing irreversible switching as a back-up.

In yet another type of control, safety is improved by switching in both the live and neutral poles. For example in British Patent 2112209 of Strix Ltd"2-pole"switching under boiling and dry conditions is arranged through a

sliding link.

All of the above arrangements suffer particular drawbacks. Those devices employing thermal fuses or single secondary bimetals operate quite slowly allowing massive overshoot temperatures to develop inside the control, with unpredictable results for the electrically live internal parts and consequent damage to the kettle.

Switches operating in two poles simultaneously can also suffer problems of arcing when switching off. This can arise particularly with kettles employing a heating element comprising a long coil of relatively low-resistivity winding, which thereby has a relatively high inductance.

The switching of controls of the types described with such a high inductance heating element can lead to the build up of dangerously high potential differences.

The present invention is directed to providing a control which overcomes these drawbacks.

According to a first aspect of the invention there is provided a control for use in an electrical appliance for boiling water of a type provided with an electrical heating element, the control comprising a thermally-responsive member which responds to boiling of water, electrical

conductors providing live and neutral conduction paths, a live switch contact pair in said live conduction path having a movable contact, a neutral switch contact pair in said neutral conducting path having a movable contact, and first actuating means adapted to act on said movable contacts in response to said thermally responsive member to open said switches one sequentially after the other on boiling of water.

Such sequential switching, in reducing the electrical potential of both live and neutral parts in sequence greatly reduces the risk of arcing on the contact pair that operates second which can then be used as the primary dry protection. Moreover, the control by switching both live and neutral poles can operate near-identically even if electrical connection to the control is reversed.

A variety of arrangements can be used to provide said sequential switching. The first actuating means may comprise an actuating lever one end which is disposed to be acted on by said thermally-responsive member and the opposite end of which bears against a first pair of actuating rods associated respectively with the live and neutral contact pairs, and wherein said opposite end of the lever is constructed so as to engage one of the actuating rods before the other to provide said sequential opening.

Alternatively, the actuating means may comprise an actuating lever one end which is disposed to be acted on by said thermally-responsive member and the opposite end of which bears against a first pair of actuating rods associated respectively with the live and neutral contact pairs, and wherein the contact pairs are disposed relative to the rods with one pair closer to the rods than the other pair to provide said sequential opening. Alternatively, the actuating means may comprise an actuating lever one end of which is disposed to be acted on by said thermallyresponsive member and the opposite end of which bears against a first pair of actuating rods associated respectively with the live and neutral contact pairs, and wherein the rods are of different length to ensure said sequential opening.

The thermally-responsive member may be a bimetallic member which changes shape at a predetermined temperature. The actuating lever may be a bi-stable spring-biased overcentre lever.

The control may further provide overheat protection in which case it comprises first and second members thermallyresponsive to an overheat condition, and actuating means disposed between said first and second members including first and second pivotally mounted lever arms each having

one end associated with the said thermally-responsive members and opposite ends of which respectively act on the movable contacts to open the switches to respond to an overheat condition.

In a further aspect the invention provides a control providing overheat protection for use in an electrical appliance for boiling water of a type provided with an electrical heating element, the control comprising electrical conductors providing live and neutral conduction paths, a live switch contact pair having a movable contact in said live conduction path, a neutral switch contact pair having a movable contact in said neutral conduction path, first and second thermally-responsive members responsive to an overheat condition, actuating means disposed between said first and second thermally-responsive members including first and second pivotally mounted lever arms each having one end associated with the thermallyresponsive members and opposite ends which respectively act on said movable contacts to open said switches in response to an overheat condition.

The lever arms may be pivotally mounted so that their pivot points lie closer to the lever ends associated with the thermally-responsive means than to the said opposite ends, whereby displacement of the said opposite ends is amplified

to the movement of the ends of the lever arms associated with the thermally-responsive means, for example the levers may be pivoted about one third of the way along their length.

This structure allows switching of both live and neutral sides in an overheat condition through a structure which is extremely compact and yet simple and reliable. The amplification of movement afforded by the levers arranged in this way allows a structure of sensors elements to be used which have only a small movement.

More particularly, the first and second thermallyresponsive members may each comprise bimetallic discs of dished form which invert when their temperatures exceed predetermined values, and wherein the actuating means comprise respective actuator rods which extend between the bimetallic discs and the ends of the levers. The discs may be disposed substantially parallel to each other, a second disc which lies closest to said lever ends having a central opening with the actuator rod therefor comprising a hollow sleeve, whilst said first disc has an actuator rod which extends to said lever ends through said central opening in the second disc and through said sleeve. Such affords a particularly compact arrangement.

The control may be adapted to open said switch contacts in a sequential manner in an overheat condition. The first and second bimetallic discs may be supported spaced by a metallic ring at their periphery, whereby in use there is a thermal delay between heat reaching said second ring and that reaching said first ring, whilst the discs may have substantially the same switching temperature, whereby in an overheat condition the first bimetallic disc switches before the second, thereby opening through its respective lever and actuator its switch contact before the switch contact associated with the second bimetallic disc.

The movable switch contacts may be provided on respective leaf springs. A third bimetallic disc may be provided having a switching temperature considerably above the switching temperatures of said first and second discs, and which acts on the sleeve actuator to open both switch contacts. This may switch irreversibly on an overheat condition so as to effectively disable the appliance.

An embodiment of the invention is now described with reference to the following drawings in which: Figure 1 is a perspective view of the control in accordance with an embodiment of the invention ;

Figure 2a is a side view of the control of Figure 1 in an OFF position; Figure 2b is a side view of the control in an ON position; Figure 3 is a view of the rear of the control switch; Figure 4 is a rear perspective view; Figure 5 shows the front of the control with a cover removed plate; Figure 6 shows bimetallic elements of a"dry boil"switch part; Figure 7 is an exploded view of the operative components of the control; and Figure 8 is an exploded view of the electrically conducting parts of the control.

Turning to the drawings and firstly to Figures 1 and 2, these show the control 2 according to an embodiment of the invention. The control is of a general type which is used in an electrical water heating appliance, such as a kettle or jug, which includes an immersed heating element. When

fitted such an element extends laterally into the water carrying body of the kettle from a head plate which is fitted into an opening in the side wall near the base of the water carrying body. Extending through the head plate are the cold tails of the heating element to which the control is electrically connected as discussed further below.

The control provides both the automatic switch-off of the kettle when water has reached the required temperature, typically boiling, as well as providing overheat protection in the event of failure of automatic switch off or in the event of operation of the kettle with no water inside, ie so-called"dry-boil"protection.

The control has a lower generally cylindrical cup-like hollow housing 4 formed of moulded plastics from which upstands a plate-like support 6. The housing 4 contains or supports the majority of the switching components and is closed by a circular cover 8 secured by two screws 9.

On the front of the cover 8 is arranged part of the switching means for providing the dry boil protection, namely, a number of bimetallic elements discussed as further below which are held beneath a metallic cap 10 having a circular opening through which the foremost

bimetallic element 62 is visible. In use, the control is disposed so that the cap 10 lies adjacent or in close thermal contact with a hot spot on the heating element head, as is well-known in the art.

Disposed on the front of the control switch 2 at an upper region of the support 6 is a bimetallic switch element 12. In the assembled kettle the element 12 is disposed in a steam passageway in communication with the interior of the kettle, whereby steam passes over the element 12. The element 12 is of a well-known construction comprising on thin bimetallic disc which is slightly dished so as to be rearwardly concave when cold having a generally U-shaped opening cut therein defining a central tongue 14. At a predefined temperature the element snaps to an inverted position in which it is dished in the opposite direction.

The element 12 is held at its periphery, whereby the central tongue 14 moves rearwardly with respect to the support 6 at the defined temperature.

On the rear of the support 6 is a bi-stable lever 16 most clearly seen in Figure 7 arranged to pivot about an upwardly directed knife edge 18 which engages in a shallow slot in rearwardly protruding fingers 20 on the support 6, and more particularly, to snap between a first ("on") position (Figure 2b) where the upper end of the lever lies

closest to the support 6 and a second ("off") position (Figure 2a) where a lower end of the lever lies against formations on the housing 4. A C-spring 22 is constrained in compression between a shoulder 24 on the support 6 and beneath a claw 26 at an upper end of the lever 16. The upper end of the lever 16"is also provided with a nose 28 extending towards the element 12 so that the tongue 14 of the element 12 engages the nose 28 when snaps to its hot position, causing the over-centre lever to trip from the ON position shown in Figure 2b to the OFF position shown in Figure 2a. The lower end of the lever is provided with a bridge portion 30 having actuating surfaces 31 which bear on the ends of a pair of actuating rods 32,33 which extend through respective bores provided in the rear of the housing 4. The function of the rods 32,33 is further described below.

The control 2 provides conducting electrical paths between rearwardly extending live, neutral and earth pins 34,36, 38 which when the control is fitted in a kettle extend into a socket cavity and to which in use, a co-operating plug or power cord is connected for connection to a mains supply (not shown). As best seen in Figures 7 and 8 the pins 34, 36 and 38 have T-shaped ends.

A switch is provided in the live side of the control by

means of movable live contact spring strip 40 having contact piece 41 at its upper end and fixed live strip 42 having contact piece 44. The opposed contacts pieces 41, 44 may comprise discs of silver or other electrical contact alloy. The fixed strip 42 extends between a flat portion adjacent the contact piece'44 at which the strip is located in the housing 4 and has an upstanding leg which terminates in a bent over region 46 at which the strip 42 is in use secured to the live cold tail of the heating element (not shown).

The movable live contact spring strip 40 has a lower rectangular aperture through which the live pin 34 extends and at which the contact spring strip 40 is supported as a cantilever. An enlarged actuating surface 48 is provided about two thirds of the distance up the contact spring strip from the pin 34.

A switch is also provided in the neutral side of the control by means a neutral fixed contact strip 50 of similar construction to the contact strip 42 having a bent over end portion 52 at which the contact strip is secured to the opposite neutral cold tail of the heating element extending through the element head (not shown). Movable neutral contact spring strip 54 is of similar construction to contact strip 40 having an enlarged actuating surface 56

with opposed ends of the neutral contacts strips 50,54 being provided with silvered contact pieces 58,59. The movable spring contact strip 54 is secured at its lower end to neutral pin 36 which extends through a rectangular opening therein.

The earth pin 38 is connected through conductor 60 which is connected to the element head (not shown).

The dry boil switching elements comprise, as best seen in Figure 6, a series of bimetallic disc elements held within cap 10 of known type which when cold are dished with concave surfaces facing to the switch rear, that is to the right in Figure 6, and which snap to an inverted configuration at predetermined temperatures. Such discs have a relatively small distance of movement (typically about imam) but provide a large force (typically 500 gf).

More particularly, there is provided a first bimetallic disc 62 selected to snap (switch) at about 1400C and to reset automatically when it has cooled to a lower temperature typically below about 700C to 100 C. A second bimetallic disc 64 is separated from the first disc by a metal heatconductive spacer ring 66, this disc switching at a similar temperature to the first disc 62, and likewise being reversible on cooling. Third disc 68 interposed between first and second discs 62,64 is selected to switch at a

rather higher temperature, typically about 2600C serving as a back-up switch, and being of a type which does not automatically snap back on cooling, thereby essentially disabling the kettle. The bimetal discs are operatively connected to the switch contact pairs through actuating means as now described. First bimetal disc 62 acts on a push rod 70, which extends through central openings in the second and third discs, and through an axial opening in a push rod sleeve 72 for the second and third discs. Both push rod 70 and sleeve, 72 extend through an opening in the cover 8 to extend into the interior of the housing 4.

Figure 7 shows the location of the push rod and sleeve 70, 72 in relation to the other operative switch components. Live and neutral dry boil switch levers 74,76 are provided between the push rod and sleeve 70,72 and the switch contacts. Both levers 74,76 are pivotably mounted on shaft 78, with the pivot arranged closer to the lower ends thereof associated with the push rod 70 and sleeve 72, typically about one third of the way along the length of the levers. The live switch lever 74 has at its lower end a flat surface at 80 against which an end of the push rod 70 engages and at its upper end 82 a ring-like formation which bears against the rear side of the actuating surface 48 of the contact 40 and through which the actuator rod 32 passes in an unobstructed manner so that this can likewise engage the rear of surface 48. Neutral lever 76 has at its lower

end a ring-like formation 83 dimensioned to allow push rod 70 to pass unobstructed therethrough to reach the end 80 of the live lever 74, but obstructing the push sleeve 72. A small gap exists between this ring-like formation 83 and surface 80 for the purpose of allowing independent movement of the lever 76 in the event of contact pair 41,44 being unable to open. An upper end 85 of the lever 74 engages beneath region 56 of the neutral contact, likewise having an upper ring formation through which the actuator rod 33 extends unobstructed to bear against the neutral movable contact spring strip 54. The pivoting of the levers close to the lower ends has the effect that the small distance of movement of the push rod and sleeve is amplified to a larger displacement of upper lever ends where these engage the contact spring strips 40,54, by a ratio of approximately 2 : 1. The additional movement is used to provide lost motion between the levers at the lower end if movement is restricted in the case of a welded live contact pair 41,44.

In normal use of a kettle incorporating such a control 2, from an OFF position (as illustrated) the user manually moves a button on the kettle exterior which is operatively connected to the lever 16 to cause it to snap over-centre to an ON position where the top of the lever lies adjacent the tongue 14 of the bimetal. On boiling, the bimetal 12

senses the steam and snaps to its hot position, the tongue 14 causing the lever 16 to snap over-centre to its OFF position in which the lower end of the lever urges actuating rods 32,33 to bear against surfaces 48,56 and opening the contact pairs. In accordance with an aspect of the invention it is arranged that the contact pairs be opened in a sequential manner, that is that the neutral contact pair opens momentarily before or after the live contact pair, thereby isolating the kettle element on both neutral and live sides in a sequential manner, reducing the changes in potential which are occurring at any instant of switching and thereby reducing the risk of arcing on the second contact pair. This can be achieved by constructing the lower region of the lever 30 differently on opposite sides with one actuating surface 31 disposed slightly forward of the other so that the end of the rod 32 is engaged before or after rod 33. Alternatively, one could make one of the actuating rods 32,33 longer than the other. As a further alternative, the actuating rods 32,33 can be the same length but the live contact spring strip 40 disposed a different distance from the end of the rod 32 than the neutral spring strip 54 as compared to the rod 33.

A combination of these approaches or other simple adaptions could also be adopted to ensure this sequential opening.

In the event of a dry situation, in the absence of steam

the steam-sensing bimetal 12 will not switch. In this case the temperature of the heating element will rise above its normal operating temperature. The first bimetal 62 of the dry boil switch elements which is closest to the element head will switch first, causing the central region of the bimetal to move rearwardly, pushing push rod 70 rearwardly and causing lever 74 to pivot, opening the live contact pair 41,44 thereby switching off the power to the element at the live side. Very shortly thereafter bimetal 64 will switch, moving push-sleeve 72 to the rear, causing lever 76 to pivot opening neutral contact pair 58,59. The time lag is dictated by the precise switching temperature of the second bimetal 42 relative to the first and the thermal conductivity of the thermal path between disc 62 and 64 via ring 66 and outer retaining cap 10 and third bimetal disc 68.

On cooling, the bimetals 62,64 revert to their cold position, the resilience of movable contact spring strips 40,54 closing the contact pairs.

In the unlikely event of failure of bimetal 62 or pushrod 70 or lever 74 or welding of contact pair 41,44 either of the bimetal discs 64 or 68 will reverse shape at their respective temperature moving lever 76 and opening contact pair 58,59 switching off the kettle.

The control thus provides increased safety and in a control which is of very compact overall design.

Claims

CLAIMS : 1. A control for use in an electrical appliance for boiling water of a type provided with an electrical heating element, the control comprising a thermally- responsive member which responds to boiling of water, electrical conductors providing live and neutral conduction paths, a live switch contact pair in said live conduction path having a movable contact, a neutral switch contact pair in said neutral conducting path having a movable contact, and first actuating means adapted to act on said movable contacts in response to said thermally responsive member to open said switches one sequentially after the other on boiling of water.

2. A control according to claim 1 wherein the first actuating means comprises an actuating lever one end which is disposed to be acted on by said thermally- responsive member and the opposite end of which bears against a first pair of actuating rods associated respectively with the live and neutral contact pairs, and wherein said opposite end of the lever is constructed so as to engage one of the actuating rods before the other to provide said sequential opening.

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3. A control according to claim 1 wherein the first actuating means comprises an actuating lever one end which is disposed to be acted on by said thermally- responsive member and the opposite end of which bears against a first pair of actuating rods associated respectively with the-live and neutral contact pairs, and wherein the contact pairs are disposed relative to the rods with one pair closer to the rods, than the other pair to provide said sequential opening.

4. A control according to claim 1 wherein the first actuating means comprises an actuating lever one end of which is disposed to be acted on by said thermally- responsive member and the opposite end of which bears against a first pair of actuating rods associated respectively with the live and neutral contact pairs, and wherein the rods are of different length to ensure said sequential opening.

5. A control according to any preceding claim wherein the thermally-responsive member is a bimetallic member which changes shape at a predetermined temperature.

6. A control according to any preceding claim wherein the actuating lever is a bi-stable spring-biased over-

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centre lever.

7. A control according to any preceding claim further comprising first and second members thermally- responsive to an overheat condition, and actuating means disposed between said first and second members including first and second pivotally mounted lever arms each having one end associated with the said thermally-responsive members and opposite ends of which respectively act on the movable contacts to open the switches in response to an overheat condition.

8. A control providing overheat protection for use in an electrical appliance for boiling water of a type provided with an electrical heating element, the control comprising electrical conductors providing live and neutral conduction paths, a live switch contact pair having a movable contact in said live conduction path, a neutral switch contact pair having a movable contact in said neutral conduction path, first and second thermally-responsive members responsive to an overheat condition, actuating means disposed between said first and second thermally- responsive members including first and second pivotally mounted lever arms each having one end associated with the thermally-responsive members and

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opposite ends which respectively act on said movable contacts to open said switches in response to an overheat condition.

9. A control according to claim 8 wherein the lever arms are pivotally mounted'so that their pivot points lie closer to the lever ends associated with the thermally-responsive means than to the said opposite ends, whereby displacement of the said opposite ends is amplified to the movement of the ends of the lever arms associated with the thermally-responsive means.

10. A control according to claim 9 wherein the levers are pivoted about one third of the distance along their length.

11. A control according to claim 8,9 or 10, wherein the first and second thermally-responsive members each comprise bimetallic discs of dished form which invert when their temperatures exceed predetermined values, and wherein the actuating means comprise respective actuator rods which extend between the bimetallic discs and the ends of the levers.

12. A control according to claim 11 wherein the bimetallic discs are disposed substantially parallel to each

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other, a second disc which lies closest to said lever ends having a central opening with the actuator rod therefor comprising a hollow sleeve, whilst said first disc has an actuator rod which extends to said lever ends through said central opening in the second disc and through said sleeve.

13. A control according to claim 12 wherein the lever ends associated with the bimetallic disc overlap each other, that lever end overlying the other having an opening through which the actuator rod for the first disc extends and can move uninterrupted, but through which said actuator sleeve cannot pass- 14. A control according to any one of claims 8 to 13 adapted to open said switch contacts in a sequential manner in an overheat condition.

15. A control according to any one of claims 12 to 14 wherein the first and second bimetallic discs are supported spaced by a metallic ring at their periphery, whereby in use there is a thermal delay between heat reaching said second ring and that reaching said first ring.

16. A control according to any one of claims 11 to 15

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wherein the bimetallic discs have substantially the same switching temperature, whereby in an overheat condition the first bimetallic disc switches before the second, thereby opening through its respective lever and actuator its switch contact before the switch contact associated with the second bimetallic disc.

17. A control according to any one of claims 8 to 16 wherein the movable switch contacts are provided on respective leaf springs.

18. A control according to any one of claims 11 to 17 further comprising a third bimetallic disc having a switching temperature considerably above the switching temperatures of said first and second discs, and which acts on the sleeve actuator to open both switch contacts.

19. A control according to claim 18 wherein the third bimetallic disc is switched irreversibly on an overheat condition so as to disable the appliance.

20. A control according to any one of claims 8 to 19 further comprising a further thermally responsive member which responds to boiling of water and

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actuating means adapted to act on said movable contacts in response to said thermally responsive member to open said switches one sequentially after the other on boiling of water.

1. A control substantially as hereinbefore described and illustrated with reference to the accompanying drawings.