US3210502A

US3210502A - Thermal device having rotatable heater and flexing actuator

Info

Publication number: US3210502A
Application number: US276053A
Authority: US
Inventors: John L Slonneger
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1963-04-26
Filing date: 1963-04-26
Publication date: 1965-10-05
Anticipated expiration: 1982-10-05

Description

- Oct. 5, 1965 J. L. SLONNEGER 3,210,502

THERMAL DEVICE HAVING ROTATABLE HEATER AND FLEXING ACTUATOR Filed April 26, 1963 [/7 1 6 2723'07": 39 53 Jo/mLS/anneger;

United States Patent 3,210,502 THERMAL DEVICE HAVING ROTATA-BLE HEATER AND FLEXING ACTUATOR John L. Slonneger, Morrison, 11]., assignor to General Electric Company, a corporation of New York Filed Apr. 26, 1963, Ser. No. 276,053 11 Claims. (Cl. 200122) My invention relates to thermally responsive devices, such as bimetallic switching devices, and more particularly to switching devices which employ an elongated bimetallic element in thermal association with a resistance type of heater.

.One well known bimetallic switching device of the prior art is of the ambient compensated type and employs two generally fiat elongated bimetallic elements one of which is encased in and thermally associated with a resistance heater of tubular configuration. In such a device, thermal lag between the heater and the bimetal can cause overheating of the bimetallic element whenever the heater is energized in excess of its designed cycling time under abnormal operating conditions. This will produce adverse efiects. For such a device, it has been found desirable to provide a simplified means for protecting the bimetallic element from overheating.

An important object of the present invention is to provide an improved bimetallic switch which includes a simplified means for protecting a bimetallic element thereof from overheating.

Another object of my invention is to provide an improved ambient compensated bimetallic switch which includes a simplified means for protecting a bimetallic element thereof from overheating under abnormal operating conditions but does not affect operation thereof under normal conditions.

A further object of my invention is to provide an improved thermally responsive device which includes a protective switch involving a minimum number of parts.

A still further object of my invention is to provide a bimetallic element and a tubular resistance heater cooperatively associated therewith in such a manner that the heater is rotated by curvature of the bimetallic element to provide thermal protection for the element.

In carrying out my invention, in one form thereof, I have provided a thermally responsive switching unit comprising a supporting means and at least one elongated bimetallic element mounted thereon. The bimetallic element extends outwardly in cantilever fashion from the supporting means and carries a movable contact on the free end thereof for cooperative engagement with another contact. An elongated resistance heater of tubular configuration is fitted loosely around the bimetallic element and is in cooperative engagement with the element between the supporting means and the movable contact. With such an arrangement, in accordance with the present invention, a movable contact of a protective switch is connected to and supported by a laterally extending first terminal of the heater, located near one end of the heater. A biasing spring is connected between the free end of the bimetallic element and an adjacent second terminal of the heater located near its other end, to impart a rotative biasing force to the heater which normally urges the movable contact of the protective switch into engagement with an associated fixed contact of the protective switch. When the elongated bimetallic element is heated to a predetermined critical temperature, curvature of the bimetallic element causes interference between it and the tubular heater, and the biasing force applied to the heater by the spring is thereupon overcome to rotate the tubular heater about its axis and open the protective switch contacts. By means of an additional pair of switch contacts and a spring, a simplified and elfective mechanism has 7 3,210,502 Patented Oct. 5, 1965 thus been provided for protecting a heater operated thermal switch from adverse effects due to abnormal operating conditions. It, therefore, will be apparent that such an arrangement is readily manufacturable, low in cost, and adaptable to numerous prior art thermally responsive devices.

Further aspects of my invention will become apparent hereinafter, and the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which I regard as my invention. The invention, however, as to organization and method of operation, together with other objects and advantages thereof, may best be understood by reference to the following description, when taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a partially broken away perspective view of a thermally responsive timing device embodying my invention;

FIG. 2 is a side elevational view of the device of FIG. 1, with the heater operated bimetallic element under normal ambient conditions;

FIG. 2a is a sectional view taken along the line 2a-2a of FIG. 2;

FIG. 2b is a right end view of the heater operated bimetallic assembly of FIG. 2, with the movable contact screw removed;

FIG. 3 is a view similar to the heater operated bimetallic element and heater arrangement of FIG. 2, but showing the bimetallic element after the heater has been energized for an extended period of time;

FIG. 3a is a sectional view taken along the line 3a-3a of FIG. 3;

FIG. 3b is a right end view of the heater operated bimetallic assembly of FIG. 3, with the movable contact screw removed;

FIG. 4 is a view similar to FIG. 3, but showing the bimetallic element after the heater has been energized for a prolonged period;

FIG. 4a is a sectional view taken along the line 4a4a of FIG. 4;

FIG. 4b is a right end view of the heater operated bimetallic assembly of FIG. 4, with the movable contact screw removed; and

FIG. 5 is a circuit diagram showing schematically the employment of the thermally responsive timing device of FIG. 1.

Referring to the drawing, and initially to FIGS. 1 and 2 thereof, there is illustrated a thermal timing device 1 which, briefly stated, comprises a supporting plate 3 of thin insulating material, a pair of matched upper and lower

bimetallic elements

5 and 7 mounted in spaced parallel relationship on plate 3, a pair of mating contacts 9 and 11 mounted respectively on the free ends of

elements

5 and 7, and a resistance type heater 13 surrounding and in thermal association with the upper bimetallic element 5.

For mounting the

bimetallic elements

5 and 7 in insulated relationship upon plate 3 and securing associated

bimetal terminals

15 and 17 respectively thereto, as further shown in FIGS. 1 and 2, a pair of screws 19 are provided. The screws 19 are suitably extended through aligned apertures formed respectively in stacked upper clamping plate 21, insulating spacer 23, the supported end 25 of bimetallic element 5, terminal 15, insulating spacer 27, a terminal 29, insulating spacer 31, terminal 17, the supported end 33 of bimetallic element 7, insulating spacer 35, an end of supporting plate 3, the threaded lower clamping plate 37.

As further shown in FIG. 2, the

bimetallic elements

5 and 7 are of fiat and elongated construction, and they are supported in cantilever fashion from their

ends

25, 33.

Elements

5, 7 each have their high expansion side facing upwardly and their low expansion side facing downward- 1y to provide ambient compensation for the contacts 9, 11. Thus more particularly, the contacts 9, 11 are normally in an open relationship (FIG. 2) when both bimetallic elements are at the same temperature, and the tip gap or spacing between these contacts when they are in this relationship will be maintained substantially constant (e.g., 0.005 inch) over a wide range of ambient conditions (e.g., -30 F. to 120 F.). The contacts 9, 11 are arranged to close when the temperature of the heated bimetallic element 5 reaches a predetermined condition (e.g., F.) warmer than that of bimetallic element 7. It will thus be seen that the electrically heated bimetallic element 5 serves as the main thermally operable means for actuating the contacts 9, 11, and element 7 serves as an ambient compensating means for the switch contacts 9, 11.

Tuming now to an important aspect of the present invention which concerns an improved means for preventing overheating of an electrically heated bimetal under abnormal operating conditions without aifecting the eflicient operation or timing under normal conditions, attention is directed to FIGS. 2, 2a and 2b. The resistance heater 13 is elongated, relatively fiat and of generally oblong cross section (FIG. 2a), having an elongated tunnel 39 formed therethrough (as also shown in FIG. 4) of similar configuration.

Terminal tabs

41 and 43 of heater 13 extend laterally outwardly from the longitudinal axis thereof, as best shown in FIG. 1, and are located respectively near opposite ends of heater 13. Each of the

tabs

41, 43 is thus located on the same side of the heater (FIG. 1) and they are generally parallel to the top and

bottom walls

45, 47 of the heater (FIGS. 2 and 2a). Tab 43 is located near the free end of bimetallic element 5, and includes an aperture 49 formed therein (FIG. 1), the purpose of which shall become apparent hereinafter. Tab 41 is located near the supported end 25 of element 5 and has a movable contact supporting arm 44 secured thereto, such as by welding. The movable arm 44 and its mode of operation forms an important aspect of my invention, as shall be set forth in detail hereinafter.

The elongated bimetallic element 5 is extended through tunnel 39 of the heater 13, and fits loosely therein, as shown in FIG. 2a so that the heater 13 has freedom for limited rotation on element 5. Heater 13 thus loosely surrounds and is supported by element 5, being rotatable thereon between slotted free end 51 and the supported end 25 of element 5. The loose relationship between heater 13 and element 5 prevents the heater 13 from affecting the tip gap or spacing between contacts 9, 11 under normal conditions. To enable the rotative move ment of heater 13 to be effectively utilized for preventing overheating of 'bimetal 5, as further shown in FIGS. 1 and 2b, a piece of spring wire 53 is connected between the slotted free end 51 of the bimetallic element 5 and the terminal tab 43 of heater 13, to normally rotatively bias the heater 13 relative to its supporting element 5 into the position shown in FIG. 2a.

More particularly, one end 55 of the spring wire 53 is extended upwardly from underneath the element 5 and threaded through the slotted end 51 thereof (viewing FIG. 2b). The wire 53 is then bent over to the left of the underside of element end 51 (viewing FIG. 2b), and doubled back around the left sideand top surface of end 51 (i.e., toward the right side of end 51) to form a loop 57 for securing the wire 53 to element 5. The wire is then bent around the other side of the element 5 (i.e., the right side, viewing FIG. 2b) and back underneath toward the general direction of terminal tab 43. A looped over end 59 of the wire 53 is then formed and engaged with aperture 49 of the terminal tab 43, as shown in FIGS. 1 and 2. The spring wire 53 is stressed by arranging it in such a manner as described, to apply a torsional moment upon the heater 13 at terminal tab 43. This torsional moment biases the heater in a counterclockwise direction of rotation (viewing FIG. 2a) and causes the heater to normally assume a position relative to element 5 as shown in FIG. 2a. When the bimetallic element 5 is in this condition, it is relatively flat and coplanar along its entire length and the upper surface 5a of element 5 is spaced from the upper Wall or ceiling of tunnel 39 of the heater.

As previously mentioned, the terminal tab 41 of heater 13 has a movable contact supporting arm 44 secured thereto (FIG. 1). The arm 44 is attached at one of its ends to an underside of tab 41, and it has a contact 61 formed on the bottom of its other end (FIG. 2). Arm 44 extends in a direction generally parallel to the longitudinal axis of heater 1,3, and toward the left of tab 41 (viewing FIG. 2) so that the contact 61 overlies a mating contact 63 facing upwardly from a tab-like terminal section 65 of terminal 29. Thus, as shown in FIGS. 1 and 2, the

contacts

61, 63 form a switch operable by rotative movement of heater 13. This switch has been provided for effectively preventing overheating of the electrically heated bimetallic element 5, as shall now be described.

Turning now to a description of the operation of my improved thermally responsive device, attention is initially directed to FIGS. 2, 2a, 2b and 5. As shown in FIG. 5, the thermal timing device may be connected into a circuit for eifectively controlling the timed energization of an electric light L. For this purpose, an energizing circuit for the lamp L is provided upon closure of timer contacts 9, 11, from line terminal L1 through terminal 15, bimetallic contact carrying element 5, bimetallic contact carrying element 7, terminal 17, and lamp L to line terminal L2. A control circuit for energizing the heater 13 of bimetallic element 5 is also connected across the line terminals L1 and L2 and includes terminal 15, bimetallic contact carrying element 5, spring 53, heater 13,

protective contacts

61, 63, and a normally open pushbutton switch S.

When the thermal timing device 1 is subjected to normal ambient conditions, as shown in FIG. 2, bimetallic element 5 is substantially flat and generally parallel to the plane of plate 3. The main controlling contacts 9, 11 of the

bimetallic elements

5 and 7 are then in their normally open position and there is a relatively small gap which spaces these contacts apart. No current thus flows between the

terminals

15 and 17 of the

bimetallic elements

5 and 7, these terminals being suitably connected in series with an electrically energizable device, such as the electric light L, which is to be controlled. The torsional moment of biasing force which is applied to the heater 13 by spring 53, causes the heater to assume the rotational position relative to element 5, wherein it is shown in FIG. 2a, and the

overload contacts

61, 63 are spring biased into a closed position (FIG. 2). Under this condition, there is relatively little, if any, interference, such as binding between the bimetallic element 5 and tunnel 39 of heater 13, since the upper and

lower surfaces

5a and 5b of the element 5 are in loose engagement with or spaced from adjacent surfaces of the: tunnel. The spring biasing force applied to heater 13 by spring 53 and the contact pressure applied thereby to con.

tacts

61, 63 has no appreciable effect upon the operative: movement of the free end of the bimetallic element 5 be-- cause the biasing force is a torsional force (and not a linear force), and also because the contact pressure for

contacts

61, 63 is applied in vertical alignment with the supported end 25 of element 5 (viewing FIGS. 2, 3 and 4). Thus more particularly, since the contact pressure for

protective switch contacts

61, 63 is exerted upon the fixed contact terminal 29 to the left of the cantilevered part of bimetallic element 5 and in general vertical alignment with supported end 25 of element 5 (FIG. 2), the bimetallic element is allowed to operate with substantially the same thermal warpage response under normal Operating conditions as if the spring 53 and

contacts

61, 63 were not being utilized.

In FIG. '2 near the free end of bimetallic element 5 there is shown a reference dimension A which represents the travel of the free end of element 5 under normal operating conditions. Such a travel as this may be alfected by depressing the actuator of pushbutton switch S and holding in this actuator for approximately 8 seconds. This causes current to flow through the heater circuit, and the free end of bimetallic element 5 warps downwardly to close the contacts 9, 11 for energizing lamp L. Under temperature conditions producing such a travel as this, no interference such as binding occurs between the bimetallic element 5 and heater 13 (FIG. 2a), and the

protective contacts

61, 63 will remain in their closed position.

When the heater 13 is energized for an extended period of time (i.e., a period of time longer in duration than the desirable normal operating range for the bimetallic element 5), the temperature of bimetallic element 5 in creases to beyond a predetermined limit, and the free end of the element 5 will move from its position in FIG. 2 to the position shown in FIG. 3. In FIG. 3 reference dimension B has been shown to indicate a travel of the free end of element 5 from its normal horizontal position, which is greater in magnitude than travel dimension A of FIG. 2. As the element 5 is heated to the condition illustrated in FIG. 3, it warps by curving between its ends, and as a result, engagement in the form of binding occurs between the element 5 and heater 13 (see also FIGS. 3a and 3b).

Thus, more particularly, for example, the left side and edge of upper surface 5a of element 5 (FIG. 3a) near the middle of the heater (FIG. 3) may engage the upper wall of tunnel 39 of heater 13 (FIG. 3a), and the right side and edge of lower surface 5b (FIG. 3a) of section In (near the free end 51 as shown in FIG. 3) of the element may engage the lower wall of tunnel 39 at one of its ends. In addition, the right side and edge of lower surface 5b (FIG. 3a) of section It (near the supported end of the element) may also engage the lower wall of the tunnel at its opposite end. Such engagement at this, which is caused by the thermal curvature of element 5 coacting with heater 13, applies a generally clockwise torsional moment to the heater 13 (viewing FIG. 3a) which overcomes the spring biasing force and opens the

protective contacts

61, 63. As a result of rotary movement of the heater 13 with respect to its supporting bimetallic element 5, the control circuit to the heater 13 (which includes bimetal 5, spring 53, heater 13,

contacts

61, 63 and switch S) is thus interrupted, to prevent overheating of the bimetallic element 5. The circuit to light L remains closed.

After the

protective contacts

61, 63 of the overload switch have been opened, continued heating of the bimetallic element 5 as a result of the thermal lag between it and the heater 13, may cause the

protector switch contacts

61, 63 to open further from their FIG. 3a position (where there is a relatively small contact spacing) to an amount shown in FIG. 4. This is as a result of further thermal warpage of element 5 and increased rotation of the heater 13 caused by the binding between the bimetallic element 5 and heater 13, as shown in FIGS. 4a and 4b. In FIG. 4, the reference dimension C has been shown to indicate the travel of the free end of element 5 from its normal horizontal position (FIG. 2).

After the control circuit energizing heater 13 through bimetallic element 5 has been deenergized by opening

contacts

61, 63 of the protective switch, if the switch S in series with

contacts

61, 63 is in an open position, the bimetallic element 5 will then cool down and return to its relatively flat position, as shown in FIG. 2. The contacts 9, 11 which are operated by the thermal timer 1 are thereupon reopened to deenergize the circuit to the light L. As the bimetallic element 5 returns from the position illustrated in FIG. 4 to the position of FIG. 2, the binding force exerted upon the heater 13 by element 5 diminishes to a point where it allows the force of biasing spring 53 to return the heater to the rotational position wherein it is shown in FIG. 2a. Spring 53 thus causes the heater 13 to rotate in a counterclockwise direction relative to element 5 (viewing FIG. 2a) until

protector contacts

61, 63 close to their normal position. The control circuit to heater 13 may thus again be energized across the line by actuating switch S to its closed position. In the event that the normally open switch S which controls energization of the heater 13 fails to reopen after a predetermined interval under normal conditions, upon reclosure of

contacts

61, 63 the control circuit to heater 13 is again energized across the line.

If the

protective contacts

61, 63 open when switch S is closed, it will be further understood from viewing FIG. 5 that these contacts will thereupon cycle from one position to another to protect the bimetallic element 5 from overheating.

By way of example, the thermally responsive device 1 illustrated and described hereinbefore may be effectively utilized for closing a circuit to the light L within 2-4 seconds after heater 13 is energized, and to maintain the light circuit for approximately 1 /2 minutes whenever the heater 13 is energized for approximately 8 seconds by the actuation of switch S. It will thus be seen by those skilled in the art that the device 1 can provide a relatively rapidly acting thermal response together with a relatively long time delay.

It will now therefore be seen that I have provided a new and improved thermally responsive device wherein a pair of protective contacts and a spring prevent overheating of an electrically heated bimetal. It will also be realized that such an approach is simplified in construction and readily lends itself to economy of manufacture. In accordance with my invention, it will be further understood that a biasing spring has been utilized in a thermally responsive device in such a manner that it not only provides contact pressure, but also provides an electrical connection between a resistance heater and a bimetallic element thermally associated therewith.

While in accordance with the patent statutes, I have described what at present is considered to be the preferred embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from my invention, and I therefore aim in the following claims to cover all such equivalent variations as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure Patent of the United States is:

1. A thermally responsive device comprising a supporting means, an elongated bimetallic element mounted on and extending from said supporting means for actuating a first switch, an electric heater coil having an elongated tunnel therethrough, said tunnel being of greater cross sectional dimension than said bimetallic element, a portion of said bimetallic element extending from said supporting means, said coil supported on said bimetallic element for limited movement thereon and in thermal association therewith, a heater switch having a movable contact and a mating contact, said movable contact being carried by said heater for movement therewith, said heater switch connected in circuit with said heater, biasing means urging said heater into a first position about said element, said element engaging with the walls of said tunnel and effecting a movement of said heater against the bias of said biasing means and away from said first position when said bimetallic elements warpage exceeds a predetermined amount, said heater abutting said movable switch contact against said mating contact of said heater switch when said heater is in its first position and separating said movable contact from the mating contact when said heater is moved away from said first position to protect said bimetallic element from damage.

by Letters 2. A thermally responsive device comprising a supporting means; an elongated bimetallic element mounted on said supporting means and extending along its longitudinal axis in cantilever fashion therefrom; an elongated heater supported on said bimetallic element and in thermal association therewith, said heater being supported on said element for limited rotational movement about said longitudinal axis of said element; biasing means urging said heater to a first rotated position; said heater being rotated about said longitudinal axis of said element from said first position in response to a predetermined thermal warpage of said element; and a switch connected to said heater and actuated by said heater as said heater rotates.

3. A thermally responsive device comprising a supporting means; an elongated bimetallic element mounted on and extending along its longitudinal axis in cantilever fashion from said supporting means; an elongated heater mounted on and extending along said longitudinal axis of said bimetallic element, said heater being in thermal association with said element and being capable of rotating about said longitudinal axis; a movable contact carried by said heater; and means engaging said heater to normally bias said heater in one rotational direction about said longitudinal axis and thereby normally maintain said contact in one position; said elongated bimetallic element being thermally responsive to a predetermined temperature to engage said heater and apply a force that rotates said heater about said longitudinal axis in opposition to said one direction and thereby move said contact to a second position.

4. A thermally responsive device comprising a supporting means, first and second bimetallic elements mounted in cantilever fashion on said supporting means in spaced apart parallel relationship, said bimetallic elements having mating contacts of a first switch on the free ends thereof, said bimetallic elements being mounted on said support with their high expansion sides facing in the same direction to provide ambient compensation for the first switch, an elongated heater surrounding one of said bimetallic elements and being mounted on said one element for limited movement thereabout, said heater being in loosely fitted thermal association, said heater moving about said one element in response to said one elements warpage, and a second switch connected to said heater and actuated by said heater in response to the movement thereof for protecting said one bimetallic element from being overheated by said heater.

5. The device of claim 4 wherein the heater has an elongated tunnel of oblong cross-section formed therein, said one bimetallic element having an oblong cross section smaller than that of said tunnel and being extended through said tunnel in such a manner that said heater is loosely supported upon said one element and arranged for limited rotative movement with respect thereto.

6. The device of claim 4 wherein said second switch is in series circuit with said heater.

7. The device of claim 4 wherein the second switch is located adjacent the supporting means.

8. A thermally responsive device comprising a supporting means, first and second bimetallic elements mounted in cantilever fashion on said supporting means and extending from said supporting means in spaced apart parallel relationship, said bimetallic elements having mating contacts of a first switch on the free ends thereof, said bimetallic elements being mounted in said device with their high expansion sides facing in the same direction to provide ambient compensation for the first switch, an elongated heater surrounding one of said bimetallic elements and supported therearound, said heater having limited movement relative to said one bimetallic element in response to a predetermined thermal warpage of said element, and a second switch mounted in said device adjacent said supporting means, said second switch being engaged with said heater for actuation by said heater in response to the movement thereof for protecting said one bimetallic element from being overheated by said heater while not affecting the normal operation of said first switch.

9. A thermally responsive device comprising a supporting means, first and second bimetallic elements mounted on and extending in cantilever fashion from said supporting means, said elements being mounted in spaced apart parallel relationship and having contacts of a first switch on the free ends thereof, said bimetallic elements mounted on said support with their low expansion sides facing in the same direction to provide ambient compensation for the first switch, an elongated heater mounted on one of said bimetallic elements, said heater being in thermal association with said one element and having limited rotative movement, a second switch mounted in said device adjacent said supporting means and actuated by said heater as said heater totates, and a spring located near the free end of said one bimetallic element and engaged between said one element and said heater and normally biasing said heater in one direction of rotation thereby to normally maintain said heater in one position, said one bimetallic element warping in response to a predetermined temperature to bind against said heater and rotate said heater in opposition to the bias of said spring thereby to rotate said heater in an opposite direction of rotation, said heater actuating said second switch between open and closed positions as said heater rotates said second switch being in series circuit relation with said heater, whereby said second switch is operable by said heater to protect said one bimetallic element from overheating without affecting the normal operation of said first switch.

10. The device of claim 9 wherein the biasing spring is also utilized for connecting the heater into series circuit relationship with said second switch.

11. A thermal timing system for an electrically energizable device comprising a circuit controlled by said device; a thermal timing switch in said circuit for controlling said circuit, said timing switch having at least one bimetallic contact carrying element having a longitudinal axis; a heater surrounding said bimetallic element and extending along said longitudinal axis in thermal association therewith, said heater being rotated about said longitudinal axis when said element impinges against said heater in response to a predetermined thermal warp-' age of said element; an energizing circuit connected with said heater; and a protective switch in said ener-' gizing circuit, said protective switch being actuated by said heater in response to a rotational movement thereof to energize said heater, said energizing circuit including said bimetallic element, said heater, and said protective switch.

References Cited by the Examiner UNITED STATES PATENTS 1,402,417 1/22 Hamilton 200113 2,192,631 3/40 Beam 200122 X. 2,371,018 3/45 AshWorth et al.. a 200122 2,589,614 3/52 Ireland 2l9511 X- 2,643,311 6/53 Giuffrida et a1. 200122 3,031,551 4/62 White et al ZOO-138.

BERNARD A. GILHEANY, Primary Examiner.

Claims

1. A THERMALLY RESPONSIVE DEVICE COMPRISING A SUPPORTING MEANS, AN ELONGATED BIMETALLIC ELEMENT MOUNTED ON AND EXTENDING FROM SAID SUPPORTING MEANS FOR ACTUATING A FIRST SWITCH, AND ELECTRIC HEATER COIL HAVING AN ELONGATED TUNNEL THERETHROUGH, SAID TUNNEL BEING OF GREATER CROSS SECTIONAL DIMENSION THAN SAID BIMETALLIC ELEMENT, A PORTION OF SAID BIMETALLIC ELEMENT EXTENDING FROM SAID SUPPORTING MEANS, SAID COIL SUPPORTED ON SAID BIMETALLIC ELEMENT FOR LIMITED MOVEMENT THEREON AND IN THERMAL ASSOCIATION THEREWITH, A HEATER SWITCH HAVING A MOVABLE CONTACT AND A MATING CONTACT, SAID MOVABLE CONTACT BEING CARRIED BY SAID HEATER FOR MOVEMENT THEREWITH, SAID HEATER SWITCH CONNECTED IN CIRCUIT WITH SAID HEATER, BIASING MEANS URGING SAID HEATER INTO A FIRST POSITION ABOUT SAID ELEMENT, SAID ELEMENT ENGAGING WITH THE WALLS OF SAID TUNNEL AND EFFECTING A MOVEMENT OF SAID HEATER AGAINST THE BIAS OF SAID BIASING MEANS AND AWAY FROM SAID FIRST POSITION WHEN SAID BIMETALLIC ELEMENT''S WARPAGE EXCEEDS A PREDETERMINED AMOUNT, SAID HEATER ABUTTING SAID MOVABLE SWITCH CONTACT AGAINST SAID MATING CONTACT OF SAID HEATER SWITCH WHEN SAID HEATER IS IN ITS FIRST POSITION AND SEPARATING SAID MOVABLE CONTACT FROM THE MATING CONTACT WHEN SAID HEATER IS MOVED AWAY FROM SAID FIRST POSITION TO PROTECT SAID BIMETALLIC ELEMENT FROM DAMAGE.