GB2206734A - Thermally responsive electrical device - Google Patents

Abstract

A thermally responsive protective electrical device 11 e.g. for a kettle comprises a primary switch 20 operable by a primary actuator 2, and a protective switch 21 operable by a protective actuator 3. The actuator 2 comprises a primary heat responsive member 4; and the actuator 3 comprises a protective heat responsive member 5 in tandem with a lock-out heat responsive member 6. The heat responsive members 4 and 5 change shape reversibly at respective first and second predetermined temperatures whereas the member 6 undergoes an irreversible change of shape when heated to a third temperature higher than the first and second predetermined temperatures if excessive thermal over-run arises after the switch has been opened by the member 5. The heat responsive members may be bimetallic and/or memory metal devices. <IMAGE>

Description

TIlERMALLY RESPONSIVE ELECTRICAL DEVICE This invention concerns thermally responsive electrical devices.

Protective devices such as thermostats, in which an actuator in the form of a bimetallic disc is arranged to displace one switch contact to open a two-contact switch, via a push rod extending through an insulating support between the disc and the switch, when the disc is heated to or beyond a predetermined temperature, are well known in the art; and from our British Patent Specification No. 2036541 it is known, firstly, to use such a thermostat in an electric kettle (FIGURES 1), and secondly, to incorporate such a device into control means (FIGURES 4 and 6) for an electric kettle (or like water heating vessel) and to displace the other contact of the switch, via a second push rod, by means of a mechanism (FIGURES 4) which actuable both manually and by a further device responsive to heating by steam generated when the water in the kettle is boiled.

In U.S.A. Patent Specification Nos. 3500277 there is disclosed a modified form of the protective device employing a second bimetallic disc actuator and a second push rod co-asual with the first push rod to open a second two-contact switch; and there is disclosed in our British Patent Application No. 8701406 control means which incorporates such a modified protective device.

In a device disclosed in U.S.A. Patent Specification No. 3943480 the switches are in series, and the first disc is reversible on cooling, whereas the second disc is irreversible, to lock-out the second switch, and is set to act at a higher temperature than the first disc. A functionally equivalent protective device, which employs non-disc-forms of bimetallic actuators, is shown in British Patent No. 211756813.

Such two actuator-two switch devices, and kettle control means incorporating such devices, are intended to provide protective secondary protection in the event of a malfunction of the first actuator or switch, eg to protect the kettle or a heater of the kettle.

Such devices providing secondary protection have the disadvantages that they are expensive to produce, need to be made with considerable precision and need accurate testing in order to operate satisfactorily; and can give rise to problems, such as inaccurate long term operation, malfunction due to being operated under problematical conditions, premature lock-out requiring otherwise unnecessaary attention to the kettle or belated lock-out resulting in excessive damage to the kettle.

An object of the invention is to enable such disadvantages and problems to the reduced or avoided.

According to the present invention there is provided a thermally responsive electrical device comprising first and second thermally responsive actuators; a first switch operable and closeable by the first actuator; and a second switch operable, independently of the state of the first switch, by the second actuator, the first actuator comprising a first device which changes shape upon being heated beyond a first predetermined temperature and reverts to its origianl shape upon being cooled to below said first temperature, and characterised in that the second actuator comprises a second device which changes shape reversibly upon a second predetermined temperature being traversed and a third device changes shape irreversibly when a third predetermined temperature is exceed; and in that the difference between said first and second temperature is less than the difference between the first and third temperatures.

Said first, second and third devices may be bimetallic or memory metal devices or any suitable combination thereof.

The second and third devices may be mechanically and/or thermally connected, and may act on the second switch via a common mechanical force transmitting member.

The difference beetween the first and second temperatures is preferably smaller than the difference between the second and third temperatures.

The second actuator is pereferably arranged to be heated via a conductor path which causes the rise in temperature of the second actuator to lag the rise in temperature of the first actuator; whereas the second actuator may be arranged to reset more rapidly than the first actuator when heating ceases.

The invention will be described further by way of example, with reference to the accompanying diagrammatic drawings, wherein: FIGURE 1 is a diagram to illustrate the operation of a device of the invention; FIGURE 2 shows a barrier of a switch body of a first embodiment of the device in control means of the invention; FIGURE 3 shows the switch body with the barrier removed to show switches mounted on a body moulding; FIGURES 4 and 5 show sections of the switch body and protective device corresponding to the lines III-III and IV-IV in FIGURE 3 but with the barrier in position; FIGURE 6 shows an enlarged detail of the protective device in a first overheated state; and FIGURE 7 is a cross-sectional diagram of a modified form of the device for use in the control means.

Referring to FIGURE 1, the thermally responsive protective electrical device 11 generally comprises a primary switch 20 operable by a primary actuator 2, and a protective switch 21 operable by a protective actuator 3. The actuator 2 comprises a primary heat responsive member 4; and the actuator 3 comprises a protective heat responsive member 5 in tandem with a lock-out heat responsive member 6.

Referring to FIGURES 2 to 6 the control means 1 may be generally similar to that described in our copending Application No. 8701406 and is constructed for fitting into a head 12 of a heater of a vessel having a plastics wall, and part of a heating element 16 is secured to part of the head 12 to provide a hot spot 17 in known manner. The control means 1 incorporates the protective device 11, and a switch body assembly consisting of a body moulding and barrier moulding 27.

The latter provides recesses 40 to receive end portions of the element 16 of the heater so that live and neutral cold tails 18A and 18B of the element (FIGURE 3) extend through the barrier into sockets 41 which extend through the body moulding 22; provides abutment 42 to abut the heater's head; and provides a hollow portion 43 which is spaced apart from said abutments 42 and, carries a retaining member in the form of a metal cap 44 to retain the actuators of the protective device 11 on the barrier. A first bimetal disc 23 serves as the primary heat responsive member 4; and a second bimetal disc 24 serves as the lock-out heat responsive member 6.A hollow actuating member 25 and a rod actuating member 26, slidably located in the member 25 and co-axial therewith, extend slidably through the barrier 27 from a socket recess 28 in the portion 43 in which the discs 23 and 24 are located in tendem, to a switch chamber 30. The barrier provides thermal and electrical insulation between the chamber 30 -and the recess 28. The discs 23 and 24 are dished and are spaced apart by ring spacer 29 of a material of high thermal conductivity whereby to allow each disc to change shape so as to reverse its curvature and thereby thrust its central portion towards the barrier 27, without being impeded by the other disc.

The central portion of the disc 24 abuts the member 25, is apertured to allow the member 26 to extend therethrough to abut a central portion of the disc 23, and confronts the hot spot 17 through an opening in the cap.

The body moulding 22 provides walling, which includes a peripheral wall 45 and interior walls 46, 27 which extend from the wall 45 to an abutment wall 48, and which abut one side of the barrier to define the switch chamber 30, and neutral and live connector chambers 50 and 51 from which the sockets 41 and recesses 40 extend.

The wall 48 provides apertures 52 and a seating 53 for an earth connector and a fastening device to extend through the switch body to the head for earthing the head and clamping the control means to the head.

The switch chamber 30 contains the primary switch 20 and the lock-out protective switch 21 in series with a live output conductor 35, a live input conductor 33 and an intermediate conductor 31.

The conductor 33 is secured at one end to the body moulding 22 by a live input terminal pin 19 which extends through and is partially embedded in the body moulding; and at its other end, on a limb of the conductor, there is a movable contact of the switch 20. The conductor 35 carries a fixed contact of the switch 21 at one end, extends through a confined space between the barrier and the walling (to which te conductor is secured) into the live connector chamber 51, and is secured to the live cold tail 18A. The conductor 31 is of reflex form to provide two limbs 54, 55 which extend from an elbow 56 secured to the body moulding 22.The limb 54 extends transversely of the limb of the conductor 33, carries a second movable contact of the switch 20, and extends beyond the contact to lie alongside one end of a movable member 14 which extends slidably through the moulding 22 for external actuation e.g. by a manually or steam actuable mechanism. The limb 55 passes, and is spaced from, the conductor 33; extends to support a movable contact for the switch 21; and has a central portion which is apertured to permit the second actuating member to extend therethrough towards the conductor 33 and which confronts one end of the first actuating member 25.

The neutral connector chamber 50 houses a neutral conductor 56 secured at its ends to a neutral pin 57, similar to the pin 19, and to the cold tail 18B.

In this embodiment the protective heat responsive member 5 comprises a further bimetal disc 60 which is interposed between the disc 24 and the member 25 so as to confront the barrier 27.

Under normal circumstances, the movable member 14 of the control means is actuated by a manually operable and/or steam responsive mechanism (not shown) of any suitable or known form. Such known mechanisms are bistable and movable between an "on" position and an 'off" position, e.g. by manual operation to switch on the vessel and by automatic operation to switch off the vessel when the liquid is boiled.

The primary switch can be opened by forwards movement (towards the head) of the member 14 and limb 54; and by reversal of shape (FIGURE 6), from forwardly convex to rearwardly convex, of the primary bimetal disc 23 to move the member 26 and conductor 33 rearwards. The disc 23 is st for reversal at about 1200C to 1400C and to revert automatically upon cooling to about 700C to 1000C.

The lock-out switch 21 is operable by reversal of shape of the lock-out disc 24 to move the member 25 and limb 55 rearwards. The disc 24 is set so that its change of shape is irreversible and takes place at about 1800C to 3000C to serve as a lock-out to hold open the switch 21.

If the vessel is operated whilst it contains insufficient water -J to immerse the heater 16, the latter will overheat thus raising the temperature of the hot-spot 17 and the disc 23 until the latter reverses its shape to open the switch 20.

however, in the absence of the member 5 or disc 60, if the primary actuator 2 or switch 20 should fail to operate, such heating would continue until the disc 24 had reached the much higher temperature at which it reverses its shape, by which time the heater, and parts of the vessel adjacent thereto, could have reached destructive temperatures; and reducing the temperature setting of the lock-out disc 24 can give rise to premature lock-out under conditions in which the disc 23 cycles between its reversed and original shapes, due to progressive build up of heat in the control means, or under conditions in which the disc 23 is slightly slow to respond or the disc 24 responds slightly too soon e.g. due to slight mis-calibration of the temperature settings thereof. Said temperature settings are thus critical for satisfactory or safe operation of the vessel.

Furthermore, even of the settings are precise, response times and heater temperatures may vary unacceptably due to variation in installation of the device or heater, or variation in composition or form of components thereof, so that all such components have to be made and installed with costly precision; and there are further problems which can arise due to "thermal-overrun" in which the surface temperature of the heater and the hot spot continues to rise for a while after the current has been switched off.

The provision of the protective member 5 or disc 60 enables such costs and problems to be reduced, and enables advantage to be taken of the thermal overrun.

The disc 60 is set to reverse its shape at a temperature which is fairly close to the setting of the disc 23, e.g. about 1300C to 1800C, so that when the disc 23 operate and cycles (at times when the vessel is operated with an insufficient water level) the disc 60 is brought, at intervals, to a temperature close to its nominal setting. The settings of these discs 23 and 60 need not be too precise, if the difference therebeetween is slightly less than optimum, the disc 60 may cycle intermittently during cycling of the disc 23 thus increasing the off-periods to improve cooling of the heater.

Furthermore, in the event of failure of the switch 20 to open or be opened, the disc 60 will open the switch 21 only momentarily after the switch 20 should have opened, thus minimising further increase of the temperature reached by the heater.

The setting of the lock-out disc 24 is also less critical. This setting is preferably such that in the event of the switch 20 failing, the thermal overrun, after first operation of the disc 60, either actuates the disc 24, or brings out the disc 24 to the threshold of actuation, so that any slight further increase in temperature, e.g.

due to cycling of the disc 60, causes the disc 24 to reverse.

Thus, instead of the thermal overrun continuing after lock-out, the overrun continues only from the much lower temperature at which the disc 60 operates and this lesser overrun is utilised to provide lock-out, with a consequential large reduction in damage risks.

The geometry of the device 11 is such that once the disc 24 has reversed, the switch 21 is held open irrespective of the state of the disc 60.

The invention not confirmed to details if the foregoing example, and many variations are possible within the scope of the invention.

For example, cooling means, such as air vents 61 (FIGURE 6) may be provided for the disc 60. The members 4, 5 and 6 may be of any suitable form and disposition, e.g. may, as indicated in FIGURE 7, comprise a memory-metal coil 62 interposed, instead of the ring 29, between discs 23 and 24.

Claims (7)

1. A thermally responsive electrical device comprising first and second thermally responsive actuators; a first switch operable and closeable by the first actuator; and a second switch operable, independently of the state of the first switch, by the second actuator, the first actuator comprising a first device which changes shape upon being heated beyond a first predetermined temperature and reverts to its original shape upon being cooled to below said first temperature, and characterised in that the second actuator comprises a second device which changes shape reversibly upon a second predetermined temperature being traversed and a third device changes shape irreversibly when a third predetermined temperature is exceeded; and in that the difference between said first and second temperature is less than the difference between the first and third temperatures.

2. A device as claimed in Claim 1 wherein the second and third devices are mechanically and/or thermally connected.

3. A device as claimed in Claim 1 or 2 wherein the second and third devices act on the second switch via a common force transmitting member.

4. A device as claimed in Claim 1, 2 or 3 wherein the difference between the first and second temperatures is smaller than the difference between the second and third temperatures.

5. A device as claimed in Claim 1, 2, 3 or 4 wherein the second actuator is arranged to be heated via a conductor path which causes the rise in temperature of the second actuator to lag the rise in temperature of the first actuator.

6. A device as claimed in Claim 1, 2, 3, 4 or 5 wherein the second actuator is arranged to reset more rapidly than the first actuator when heating ceases.

7. A device substantially as hereinbefore described with reference to FIGURE 1; or FIGURE 1 as modified by FIGURES 2 to 6 or FIGURE 7; or FIGURES 2 to 6, or FIGURE 7.