IMPROVEMENTS RELATING TO THERMAL CONTROLS FOR ELECTRIC HEATING ELEMENTS

Abstract: A contactstat thermal sensor comprises a snap acting disk bimetal housed under a thermally conductive end cap fitted to a sensor body and operating a set of primary switch contacts via a push rod. For providing secondary protection, operable in the event that the primary protection afforded by the bimetal fails to operate, the moving contact of the primary switch contacts is provided at one end of a spring metal beam and the other end of the beam is held down, against its spring bias, in electrical contact with a terminal part of the sensor by means of a fusible rod arranged to melt and release the spring metal beam, thereby disrupting the electrical path through the sensor, in the event that the fusible rod is subjected to a temperature above the normal operating temperature of the bimetal.

Description: IMPROVEMENTS RELATING TO THERMAL CONTROLS FOR ELECTRIC HEATING ELEMENTS Field of the Invention: This invention concerns improvements relating to thermal controls for electric heating elements and particularly, though not exclusively, concerns the arrangements that are disclosed in our British Patent Applications Nos. 9717144.1 filed 12 August 1997 and 9724382.8 filed 18 November 1997. Background of the Invention: In our British Patent Application No. 9717144.1 abovementioned there are described various different chassis or carrier members adapted for attachment to a planar heating element, especially a thick film heating element, and having provision for the assembly therewith of a plurality of thermal sensors at selectably variable positions. The described embodiments comprise an appliance inlet connector based upon the Otter Controls CP7 inlet connector as presently manufactured, this being an inlet connector for a cordless appliance which enables the appliance to be set down upon its base in any rotational orientation of the two, but with a plurality of limbs extending outwardly from the connector in different directions all in the same plane, each limb having provision for the mounting of one or more thermal sensors on the limb at a number of different positions. The invention of British Patent Application No. 9717144.1 has as its rationale the provision of flexibility as regards the positioning of sensors on planar heating elements inter alia to accommodate the different requirements of different appliance manufacturers. It is proposed in British Patent application No. 9717144.1 to employ simple contactstat type thermal sensors as the sensors to be mounted on the limbs, and an exemplary contactstat thermal sensor well suited to this purpose is described in our British Patent Application No. 9724382.8. As therein described, the contactstat thermal sensor comprises a dished disk bimetal which moves with a snap-action between oppositely dished configurations when the sensed temperature rises above a predetermined level, the bimetal movement being transferred by means of a push-rod to a pair of switch contacts. The contactstat thermal sensor described in our British Patent Application No. 9724382.8 provides only a primary level of thermal protection, namely it has no facility for provision of a secondary or back-up level of protection to be operative in the event, however unlikely, of failure of the primary protection, for example due to failure of the bimetal or welding together of the switch contacts. Objects and Summary of the Invention: Accordingly, it is the principal object of the present invention to provide a contactstat type thermal sensor which can be used with the arrangements described in our British Patent Application No. 9717144.1 and provides primary and secondary levels of protection. According to the present invention there is provided a contactstat thermal sensor comprising a bimetallic actuator, a push rod and a set of switch contacts providing a primary protection function, and a member comprising fusible material arranged to disrupt an electrical path through the sensor in the event that the primary protection fails to operate and the fusible material is subjected to a temperature above the normal operating temperature of the bimetal. In accordance with an exemplary embodiment of the present invention, as described in detail hereinafter, the primary switch contacts are comprised by a fixed contact and a movable contact at one end of a spring metal beam and the other end of the beam is held in electrical contact with an electrical terminal part of the contactstat by means of a rod formed of fusible thermoplastics material, the beam being stressed over an abutment when the primary switch contacts are closed at one end of the beam and the other end contacts the electrical terminal part. When the bimetallic actuator operates, the push rod allows the said one end of the beam to resile from the fixed contact of the device and, if this fails to happen and the contacts at the said one end of the beam remain closed, the increasing temperature will cause the thermoplastics rod to soften thereby allowing the said other end of the beam to resile from the electrical terminal part. The above and other features of the present invention will best be appreciated from consideration of the following detailed description given with reference to the accompanying drawings. Description of the Drawings: Figure 1 shows an exploded perspective view of a contactstat embodying the present invention; and Figure 2 is a cross-sectional view of the contactstat of Figure 1 shown in assembled condition. Detailed Description of the Embodiment: Referring to Figure 1, the contactstat thermal sensor shown therein comprises an auto-resetting dished disk bimetal 1, a push-rod 2 and a pair of switch contacts 3 and 4, the contact 3 constituting the moving contact of the switch contacts and being affixed to one end of a spring-metal beam 5, and the contact 4 constituting the fixed contact of the switch and being affixed to a pressed metal terminal part 6 of the device. A body part 7 of the contactstat is formed of a temperature resistant plastics or ceramics material and has a bore 8 which serves to accommodate the push-rod 2. The disk bimetal 1 is accommodated on top of the body part 7 within a pressed metal cap 9, formed of copper or aluminium for example, which is adapted to be clenched onto the upper end of the body part 7. As shown in Figure 2, the bimetal 1 sits under the cap 9 with its edge resting on the upper end of the push-rod 2 on one side of the bimetal disk and on a small projection 10 on the diametrically-opposite side of the disk. A central depression 11 in the centre of the cap 9 provides an abutment with the bimetallic disk 1 and can be adjusted by deformation of the cap to adjust the bimetal operation. A second body part 12 of the contactstat is adapted to fit together with the first-mentioned body part 7 so as to capture the beam 5, the terminal part 6 and a further terminal part 13 between the two body parts. The two terminal parts 6 and 13 have portions 14 and 15 respectively which, when the two body parts 7 and 12 are fitted together, extend outside of the contactstat body for making spring contact with terminal pads provided on a thick film heating element the temperature of which is to be controlled by the contactstat. The cap 9 has a radial extension 16 which is designed to accommodate the upper end of a rod 17 of fusible material, a thermoplastics material for example, and an opening 18 is formed in the body part 7 for admitting the lower end of the rod 17 to the internal chamber that is defined when the body parts 7 and 12 are fitted together. As shown best in Figure 2, the lower end of the rod 17 abuts the end of spring-metal beam 5 opposite to the end which carries the moving switch contact 3, the beam 5 being upturned at the respective end 19 for ensuring positive location of the rod 17 with the respective beam end 19 and ensuring that only axial forces are exerted on the rod 17. Locating lugs 20 are provided on the elongate edges of the beam 5 and co-operate with formations 21 provided in the body parts 7 and 12 to locate the beam appropriately when the contactstat is being assembled. An upstand 22 on the body part 12 is provided for applying an upwards thrust at an appropriate time (as described hereinafter) to the beam 5 at a location closer to the end 19 thereof than to the other end which carries the moving contact 3 of the switch. As shown in Figure 2, which shows the contactstat in its normal cold condition, assembly of the contactstat causes the beam 5, which is flat in its unstressed condition as shown in Figure 1, to be bowed upwardly between its moving contact carrying end, where it is held down in contacts-closed condition by the action of the bimetal 1 and the push-rod 2, and its other end 19, where it is held down by the fusible material rod 17 with its upper end abutting the cap 9 and extends over and in electrical contact with an upstanding formation 23 on the pressed metal terminal part 13. In this condition of the contactstat, current can flow between the two terminal parts 14 and 15. In an overtemperature situation such as to cause the bimetal disk 1 to snap into its oppositely dished configuration, the movement of the bimetal would normally allow the moving contact 3 to spring away from the fixed contact 4 without disrupting the contact established between the other end 19 of the beam 5 and the formation 23 on terminal part 13. However, if this does not happen, for example because the bimetal 1 cracks or the switch contacts 3, 4 weld shut, power will continue to flow through the contactstat to an associated heating element and the heating element temperature will continue to rise. At some point, the fusible rod 17 will soften or melt and the end 19 of the beam 5 will resile out of electrical contact with the formation 23 provided on terminal part 13. The fusible rod 17 thus provides a secondary or back-up protection level which is operative in the event that the primary protection provided by the bimetal 1 fails. A formation 24 is provided on the lower body part 12 to enable the contactstat to be assembled with a carrier as described in our British Patent Application No. 9717144.1 abovementioned. The upstand 22 may or may not, but need not, contact the beam 5 in the cold condition of the contactstat as shown in Figure 2, since in this condition the action of the fusible rod 17 on the end 19 of the beam 5 and the abutment of the beam 5 with the formation 23 upstanding from terminal part 13 will serve to stress the beam sufficiently to ensure proper operation of the primary contacts 3, 4. However, when the rod 17 softens or melts in the secondary protection mode of the contactstat and the beam begins to resile towards its normal flat condition, thereby breaking electrical contact between the end 19 of the beam and the formation 23, only the contact of the beam with the upstand 22 will give rise to a continuing force such that the beam 5 continues to resile and opens a safe electrical gap between the beam and the terminal part 13. If the upstand 22 does not contact the beam 5 in the cold condition of the contactstat, the forces between the end 19 of the beam and the formation 23 on the terminal part 13 will be optimized so that silver plating of these parts for better electrical contact may not be necessary. The geometry of the described and illustrated arrangement is such that, in the cold condition of the contactstat, a high force is applied to the fusible rod 17 and a lower force is applied to the push-rod 2, reflecting the lower actuating force available from the bimetallic disk. The advantage of this arrangement is that a very high force is available from a short, stiff spring, namely the part of beam 5 between formation 23 and beam end 19, to initiate deformation of the fusible rod 17 at the appropriate temperature. Once deformation has started, the upstand 22 provides a lower force from a longer, more resilient spring, namely the part of beam 5 between formation 22 and beam end 19, to provide sufficient deformation of rod 17 to open a safe spacing between the beam 5 and the formation 23 on terminal part 13. While the invention has been described in the foregoing by reference to a particular embodiment, it is to be well understood that the described embodiment is in all respects exemplary and that modifications and variations thereto will occur to those possessed of appropriate skills without departure from the spirit and scope of the invention as set forth in the following claims. For example, whereas the fusible rod 17 is preferably formed of an electrically insulating thermoplastics material, since it is in contact with the current carrying beam 5, it could be formed of a suitable eutectic metal alloy material provided with an electrically-insulating cap. Furthermore, the fusible material rod 17 could be arranged such that, in use of the contactstat, the upper end of the rod 17 directly abuts the heating element rather than being captured within the can extension 16. With this arrangement, the beam 5 would not be stressed in the free-standing condition of the contactstat, namely before it was fitted to a heating element, and would only be stressed when the contactstat was assembled with the heating element, such assembly causing the rod 17 to be driven axially inwardly of the contactstat so as to bend the end 19 of the beam 5 over the upstand 23 on terminal part 13.

Claims: CLAIMS: 1. A contact thermal sensor comprising a bimetallic actuator arranged to operate a set of switch contacts in normal operation of the sensor, and wherein a member comprising fusible material is arranged to disrupt an electrical path through the sensor in the event that the primary protection afforded by the bimetal and the switch contacts fails to operate and the fusible material is subjected to a temperature above the normal operating temperature of the bimetal. 2. A contact thermal sensor as claimed in claim 1 wherein the bimetallic actuator comprises a dished disk bimetal which is movable with a snap action between oppositely dished configurations. 3. A contact thermal sensor as claimed in claim 2 wherein the set of switch contacts are arranged to be operated by means of a push rod responsive to movement of the periphery of the bimetal as it snaps between its oppositely dished configurations. 4. A contact thermal sensor as claimed in claim 3 wherein the sensor comprises a sensor body provided with a thermally conductive metal cap, and the bimetal is captured between the cap and the sensor body, the cap being formed to define an abutment defining the position of the centre of the bimetal. 5. A contact thermal sensor as claimed in claim 4 wherein the cap is deformable for adjusting the position of said abutment. 6. A contact thermal sensor as claimed in any of the preceding claims wherein the member comprising fusible material comprises a push rod which serves to maintain electrical contact between a spring member and a conductor of the sensor. 7. A contact thermal sensor as claimed in claim 6 wherein the fusible material push rod operates to bend the spring member over an upstanding part of the said conductor. 8. A contact thermal sensor as claimed in claim 7 wherein said spring member is elongate and at one end thereof serves as the moving contact of the set of switch contacts associated with the bimetal and at the opposite end thereof serves in co-operation with said fusible material rod. 9. A contact thermal sensor as claimed in claim 8 wherein the upstanding part of said conductor is closer to said opposite end of said spring member than to said one end thereof. 10. A contact thermal sensor as claimed in claim 7 or 8 or 9 wherein an upstand is provided in a body part of the sensor, said upstand serving as an abutment for interaction with said spring member in the disruption of said electrical path. 11. A contact thermal sensor comprising a sprung electrical conductor serving at opposite ends thereof as the moving contacts of first and second sets of switch contacts which are normally held in closed condition against the bias of said sprung electrical conductor, the first set of switch contacts being arranged to be held in closed condition by a bimetallic switch actuator which is adapted to allow the first contacts to open at a predetermined temperature, and the second set of switch contacts being arranged to be held in closed condition by a member of fusible material which is adapted to allow the second contacts to open at a temperature above the operating temperature of the bimetallic switch actuator, the arrangement being such that, in use of the sensor, in the event of failure of the bimetallic switch actuator to open the first set of switch contacts the second set of switch contacts can be opened in response to melting of said member of fusible material. 12. A contact thermal sensor as claimed in claim 11 wherein the electrical conductor is formed of spring metal and, in the cold condition of the sensor wherein both sets of contacts are closed, is bent over an abutment by the action of the bimetallic switch actuator at one end and the action of the member of fusible material at an opposite end, the bending of the spring metal conductor providing the necessary spring force for operation of the two sets of switch contacts. 13. A contact thermal sensor comprising a body part capturing therein first and second spaced-apart electrical terminal parts and an electrically conductive spring metal bridge member interconnecting said spaced-apart electrical terminal parts, said spring metal bridge member being held against its spring bias at one end by means of a bimetallic switch actuator acting through a push-rod and at its other end by means of a member of fusible material, the aπangement being such that operation of the bimetallic switch actuator at a predetermined temperature can release said one end of the bridge member from electrical contact with said first electrical terminal part and, in the event of failure of such mechanism, the melting of said fusible material member at a temperature above said predetermined temperature can release the other end of the bridge member from electrical contact with said second electrical terminal part. 14. A contact thermal sensor as claimed in claim 13 wherein the push-rod operating at one end of the bridge member and the fusible material member operating at the other end of the bridge member bend the bridge member over an abutment against its own resilience. 15. A contact thermal sensor as claimed in claim 14 wherein said abutment is formed as part of the second electrical terminal part. 16. A contact thermal sensor as claimed in claim 14 or 15 wherein said abutment is not evenly spaced between the opposite ends of the bridge member so that a greater spring force is exerted on one of the push-rod and fusible material member than on the other.

Inventors: HOWARTH JOHN ANTHONY, O'NEILL ROBERT ANDREW, HOWARTH, JOHN, ANTHONY, O'NEILL, ROBERT, ANDREW
Applicants: HOWARTH JOHN ANTHONY, ONEILL ROBERT ANDREW, OTTER CONTROLS LTD, HOWARTH, JOHN, ANTHONY, O'NEILL, ROBERT, ANDREW, OTTER CONTROLS LIMITED
Application year: 1999
Main classification: H01H37
IPC codes: H01H37