This invention relates to a thermally-sensitive electrical protector which may find particular application in household electrical appliances such as cast iron hobs for electric cookers, tumble dryers and the like as a protection against the appliance overheating.

It is well known to provide electric immersion heaters for containers such as kettles and jugs with controls incorporating a thermally-sensitive switch operated by a snap-acting bimetallic actuator. These controls are used as safety devices to ^" interrupt the power supply if the container boils dry or is switched on during or, in automatic kettles or jugs, when boiling occurs. A common form for such an actuator comprises a stressed sheet of bimetal having an aperture defining a tongue which is free at one end, a margin or bridge portion of the bimetal sheet extending transversely to and being spaced from the free end of the tongue. Such an actuator has a generally dished configuration and can be made to snap between two oppositely dished stable configurations with a change in temperature and is termed hereinafter "an actuator of the kind described". Examples of actuators of the kind described are disclosed in British Patent specifications Nos. 657,434 and 1542252.

The snap-action of the actuator of the kind described is used in thermally-sensitive switches in kettles and jugs to push electrical contacts out of engagement as it snaps between its two opposite configurations with changes in temperature. This is often done through the intermediary of a push rod of insulating material which rests against the actuator and which is urged by the actuator to separate the contacts

with the appropriate change in temperature, either directly or through the intermediary of a trip mechanism.

The object of the present invention is to fulfil a need for a robust, simple and hence inexpensive thermally-sensitive electrical protector for use as an overheat protector or temperature limiter.

Thus the present invention provides a thermally- sensitive electrical protector including an actuator of the kind described mounted within an electrically insulating housing, a pair of electrical contacts and a leaf spring disposed within said housing, the leaf spring having one of the contacts mounted thereon so that the contacts are sprung to the closed position, a lost motion connection being provided between the tongue of the actuator and the leaf spring such that upon operation of the actuator with a sufficient rise in temperature the tongue acts to pull said leaf spring to open said contacts, there being prior to such operation no force transmitting connection between the tongue and leaf spring.

Such a construction allows the contacts to rest together, allows for their erosion in use and avoids mutual stressing of the spring and the actuator which could affect the calibration of the actuator. The provision of such a lost motion connection also has the added benefit of making it easier to construct such a switch. Furthermore use of a lost motion connection enables a more positive action as the contacts are opened abruptly due to the momentum gained by the moving actuator before it acts on the contacts.

In one preferred arrangement the tongue of the actuator is freely received in a tongue receiving portion of the leaf spring so as with snap action of the actuator to engage said portion to pull the free end of the spring and thereby open the electrical contacts.

Preferably, a thermally-sensitive protector

according to the invention comprises a leaf spring having said free end turned at right angles with respect to the rest of the spring. The tongue receiving portion of the spring may be a generally rectangular opening defined, in part, by a web at said free end. Clearance is provided by making the opening oversized with respect to the dimensions of the tongue.

A housing having cooperating upper and lower portions may be provided. Using a ceramic material for the housing would not only provide electrical insulation but would also allow the switch to be used at elevated temperatures where other insulating materials would either melt or distort. However, depending on the intended application, synthetic plastic materials may be used for the housing e.g. nylon.

According to a further preferred feature of the invention the thermally-sensitive electrical protector may include a second thermally-sensitive actuator arranged to cause said pair of contacts or a second pair of contacts to open at a predetermined operating temperature higher than that at which said first mentioned actuator of the kind referred to operates, the arrangement being such that once said contacts are open after operation of said second actuator, they are not, in use, automatically reclosed with falling temperature.

The second actuator may be of the same construction as the first mentioned thermally-sensitive actuator of the kind referred to but with an operating characteristic such that the temperature fall necessary to bring about an automatic resetting of the second actuator is far lower than would be met under intended operating conditions of the protector. Another option is to provide a thermal fuse as the second actuator. The provision of a non-resetting actuator provides a protector according to the invention with a fail safe function.

Preferably the second pair of contacts is connected

in series with the first pair of contacts and a particularly simple and convenient construction is the provision of two pairs of electrical contacts in series, a contact of each pair being mounted adjacent opposite ends of the leaf spring which is mounted centrally between opposed portions of said housing in a double cantilever manner. Fixed contacts are respectively associated with said contacts carried by said leaf spring and have associated terminals extending outside said housing. If the second actuator is of the same kind as the first mentioned actuator it may act on the associated cantilevered part of the leaf spring in the same manner as the first actuator. If, however, a thermal fuse is provided as said second actuator it may serve normally to hold the leaf spring mounted contact in the closed position. Operation of the fuse can then permit the movable contact to move away under a biasing force preferably provided by the leaf spring itself.

Alternatively, there may be provided a pair of electrical contacts connected in parallel. The second pair of contacts may be arranged in combination with an actuator of the kind referred to or with a thermal fuse.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which:

Figure 1 is a plan view of a first embodiment of a thermally-sensitive protector according to the invention;

Figure 2 is a part sectional view taken along II-II of Figure 1;

Figure 3 is a slightly enlarged perspective view of the free end of the leaf spring used in the first embodiment- showing the contact and tongue receiving portion;

Figure 4 is a plan view with an upper housing part removed of a second embodiment of the invention including a non resetting thermally-sensitive actuator;

Figure 5 is a part sectional view taken along V-V of Figure 4 and including said upper housing part;

Figure 6 is a plan view with an upper housing part removed of a third embodiment of the invention including a non-resetting thermal fuse; and

Figure 7 is a part sectional side view taken along VII-VII of Figure 6 and including said upper housing part;

Figure 8 is an end view to an enlarged scale of the thermal fuse as positioned in use.

The same reference numbers have been used throughout to indicate equivalent parts.

Referring firstly to Figures 1 and 2, a one-piece housing 1 contains an actuator 2 of the kind referred to, a leaf spring 3 and contacts 4, 5. Electrical terminals 6 enter the housing 1 from opposite ends and are sprung into place in slots in the housing in a conventional manner.

The actuator 2 is a disc of bimetal from which a generally C-shaped portion has been removed to leave a tongue 7 and a bridge portion 8. The actuator 2 is dished such that the tongue 7 lies out of the plane defined by the disc's outer margin, as can be clearly seen in Figure 2.

The leaf spring 3 is connected to one of the terminals 6. The contact 4 is located towards the free end of the spring 3, has a generally domed surface and is biased by the spring 3 against the other fixed contact 5. In this embodiment, as can be more clearly seen in Figure 3, the free end of the spring 3 is wider than the rest of the spring and includes a generally rectangular opening 9 defined by webs 10. There is an appreciable clearance between the webs 10 and the tongue of the actuator.

Figures 4 and 5 shows a second embodiment of a protector according to the invention which includes a non-resetting thermally-sensitive actuator 2 ' . The

housing 1 comprises upper and lower portions la, lb which together define two recesses 12, 12' . It should be noted here that numerals marked with a prime are concerned with the non-resetting actuator 2• . Each recess 12, 12' contains an actuator 2, 2' and a free end of a leaf spring 3. Each end of the leaf spring 3 is turned downwardly and includes an opening 9, 9 ' . The leaf spring 3 is centrally supported in a double cantilever manner between the housing portions. The contacts 4, 4• on the leaf spring are sprung against respective fixed contacts 5,5', both of which are carried by spade terminals 13, 13' . The actuator 2' is constructed such that the temperature at which it snaps to a position having the tongue 7' in a downwardly flexed position (not shown) is higher than the

>* temperature at which the actuator 2 snaps to this position. Also the temperature at which the actuator 2 ¹ would snap back to its normal position is much lower than normal operating temperatures for the switch.

Figures 6, 7 and 8 show a third embodiment of a protector according to the invention which includes an alternative non-resetting device. As in the previous embodiment, the housing 1 comprises upper and lower portions la, lb which together define a recess 14 and a chamber 15. The recess 14 houses an actuator 2 having its tongue 7 located in the opening 9 in the leaf spring 3. The leaf spring passes from the recess 14 into the chamber 15 and is mounted in a cantilever manner between parts 16 of housing portions la, lb. The first part of the spring 3 in the recess 14 acts to urge contact 4 against contact 5, as in the previous embodiments, whilst the second apart of the spring 3 in the chamber 15 acts to urge contact 4' away from contact 5' . Part of a hairpin shaped thermal fuse 17 clips into a slot 18 in lower portion lb and extends into the chamber 15, supporting, the end 19 of the leaf spring to bias the leaf spring so that contact 4', 5' are closed. Spade

terminals 13, 13' carry the fixed contacts 5, 5'. The thermal fuse 17 is of thermoplastic material and at the required temperature yields to allow contact 4' to move away from contact 5' under the action of the spring 3.

The operation of the protectors described above will now be discussed.

The first embodiment shown in Figures 1 to 3 may be used as a protector for a cast iron hob on an electric cooker. The protector can be located in the hob and have an actuator 2 which snaps to the other position at a temperature around 450°C, the tongue 7 pulling the contact 4 away from contact 5, the actuator 2 reacting against the housing which supports it. The protector would thereby act as a circuit breaker if the hob temperature were to exceed 450"C. It would come into play for example if the hob was turned full on but there was no pan on the hob to absorb the heat. At a predetermined temperature below 450"C e.g. 400°C, the tongue actuator will reverse to its "cold" position and the electrical contact between contacts 4 and 5 will be restored under the action of the leaf spring 3.

The second embodiment shown in Figures 4 and 5 may be used in a tumble dryer or the like and for this purpose the housing may be formed of nylon. The actuator 2 is constructed such that it flips to its other position where the circuit is broken at a temperature of about 100°C and automatically resets at a temperature of about 50°C. In series with the contacts broken and made by the actuator 2 is a second set of contacts controlled by a non-resetting actuator 2' . This second actuator acts as a safety device if the automatically resetting protector becomes defective e.g. if the contacts 4, 5 should weld together. The actuator 2 ¹ snaps to its other position at about 180°C and requires a very low temperature before it will reset so that, in normal operating conditions, it is non- resetting.

If the first part of the protector fails and the temperature of the appliance continues to rise, the non- resetting actuator will snap into its other flexed position and thereby break the circuit. On cooling the actuator 2* will not return to its normal position and the electrical contact will not be restored.

In the third embodiment illustrated in Figures 6, 7 and 8 the non-resetting device comprises a thermal fuse 17. The melt or yield temperature of the fuse 17 is higher than the temperature at which the actuator 2 snaps to the contact breaking position. If, for any reason, the first part of the protector fails and the temperature of the appliance continues to rise, part of the thermal fuse melts or yields thereby permitting the contact 4 to move away from contact 5' under the action of the leaf spring.

It will be readily apparent to those skilled in the art that modifications can be made without departing from the scope of the invention. For example, rather than have the tongue received in an opening in the leaf spring, the tongue may be bifurcated and each part of the tongue may be received in a respective recess in the margin of the leaf spring or a flexible tension member might connect the tongue and leaf spring. Also, other forms of thermal fuse could be provided to hold the contact closed in the last embodiment.