Patent Publication Number: US-2004047100-A1

Title: Thermal switch containing preflight test feature and fault location detection

Description:
[0001] This application claims the benefit of U.S. Provisional Application Serial No. 60/237,847, filed in the names of George D. Davis and Byron G. Scott on Oct. 4, 2000, the complete disclosure of which is incorporated herein by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention is directed to temperature sensors and, more particularly, to snap-action thermal switches and resistance thermal sensors.  
       BACKGROUND OF THE INVENTION  
       [0003] Snap-action thermal switches are utilized in a number of applications, such as temperature control and overheat detection of mechanical devices such as motors and bearings. In some applications, multiple thermal switches are located at different positions around the equipment. For example, in some aircraft wing, fuselage, and cowling overheat detection applications, multiple thermal switches located just behind the loading edge flap, while other thermal switches are spaced along the length of each wing. Additional thermal switches are located in the engine pylon and where the wing attaches to the fuselage. In this example, the multiple thermal switches are connected electrically in parallel, such that just two wires are used to interface between all of the switches on each wing and an instrument that monitors the temperature of the aircraft&#39;s wing, fuselage, and cowling.  
       [0004] Current snap-action thermal switch designs typically provide open and closed functions only. Typically, all of the thermal switches in the aircraft wing, fuselage, and cowling overheat detection applications are operated in the normally open state. The thermal switches are thus all in the “open” state until an overheat condition is detected, at which time one or more of the switches change to the “closed” state, thereby completing the circuit causing a “right wing,” “left wing” or “fuselage” overheat indication to appear in the cockpit. The pilot then follows the appropriate procedure to reduce the overheat condition.  
       [0005] Current snap-action thermal switches used in parallel operation, multiple thermal switch overheat detection systems suffer from various drawbacks. The integrity of the wire harness between the cockpit and the wing tip cannot be assured because the circuit is always open under normal operating conditions. If a switch connector is not engaged or the wire harness contains a broken lead wire, a malfunction indication will not occur, but neither will the overheat detection system operate during an actual in-flight overheat condition. Furthermore, if an overheat condition does occur, current snap-action thermal switches are not equipped to provide information describing the exact location of the overheat. In both instances, flight safety is compromised, and later correction of the problem that caused the overheat condition is made more difficult because of the inability to pinpoint the overheat fault.  
       SUMMARY OF THE INVENTION  
       [0006] The present invention overcomes the limitations of the prior art by providing a device that provides a self-test function in combination with a thermal overheat detection function.  
       [0007] According to one embodiment of the invention, a snap-action thermal switch structured in a normally open configuration is combined with a resistance element integral with the snap-action thermal switch and coupled to an output thereof.  
       [0008] According to one embodiment of the invention, the resistance element and the snap-action thermal switch share one or more common terminals. For example, the snap-action thermal switch is structured having a pair of terminals being mutually electrically isolated when the snap-action thermal switch structured in the normally open configuration, and the integral resistance element is electrically coupled to provide an output on the pair of electrically isolated terminals. According to different embodiments of the invention, the resistance element is mounted either internally or externally to the snap-action thermal switch.  
       [0009] According to another embodiment, the invention is embodied as a three-terminal, snap-action thermal switch having first, second and third electrical terminals mounted in a header, the first, second and third terminal being mutually spaced apart and electrically isolated; a fixed electrical contact being positioned on the first terminal; a movable electrical contact being positioned on the second terminal and being biased into electrical contact with the fixed electrical contact; a bimetallic actuator being convertible as a function of temperature between a first state wherein an actuation portion is positioned to space the movable electrical contact away from the fixed electrical contact and a second state wherein the actuation portion is positioned to permit electrical contact between the movable electrical contact and the fixed electrical contact; and an electrically resistive element coupled between the third electrical terminal and one of the first and second electrical terminals.  
       [0010] The invention also provides methods of accomplishing the same. For example, the method of the invention includes structuring a pair of electrical contacts in a normally open configuration; electrically interconnecting an electrically resistive element with at least one of the pair of contacts; and detecting a minimum electrical resistance of the electrically resistive element. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
     [0012] FIGS.  1  is a top plan view of the present invention embodied as a single-pole, single-throw snap-action thermal switch having an interiorly mounted resistor;  
     [0013]FIG. 2 is a cross-sectional view of the snap-action thermal switch of the present invention embodied as shown in FIG. 1 with the contacts open and showing the interiorly mounted resistor;  
     [0014]FIG. 3 is a cross-sectional view of the snap-action thermal switch of the present invention embodied as shown in FIG. 1 with the contacts closed and showing the interiorly mounted resistor;  
     [0015]FIG. 4 is a schematic description of the single-pole, single-throw thermal switch shown in FIGS. 1 through 3;  
     [0016]FIG. 5 is a top plan view of one alternative embodiment of the present invention embodied as a snap-action thermal switch having an externally mounted resistor;  
     [0017]FIG. 6 is a side view of the snap-action thermal switch of the present invention embodied as shown in FIG. 5;  
     [0018]FIG. 7 is a top plan view of one alternative embodiment of the present invention embodied as a snap-action thermal switch having an externally mounted resistor, the thermal switch installed in an over-molded housing configured for mounting in an aircraft wing, fuselage, or cowling, as shown in FIG. 17;  
     [0019]FIG. 8 is a side view of the snap-action thermal switch of the present invention embodied as shown in FIG. 7 and shows the externally mounted resistor;  
     [0020]FIG. 9 is an illustration of the thermal switch of the invention implemented in an overheat detection system having one of the thermal switches coupled in parallel with a quantity of conventional snap-action thermal switches that do not include the resistor;  
     [0021]FIG. 10 illustrates the thermal switch of the invention implemented in an alternative overheat detection system having a quantity of thermal switches of the invention coupled together in parallel in a wiring harness, which is led to an indicator through a logic circuit;  
     [0022]FIG. 11 illustrates an alternative embodiment of the overheat detection system of the invention, wherein each of the multiple parallel-coupled thermal switches of the invention is embodied having respective resistor electrically coupled in parallel with the switch contacts and wherein each of the resistors has a resistance value different from that of the other resistors coupled to the other switches;  
     [0023]FIG. 12 illustrates an exemplary flow diagram of one optional embodiment of the logic circuit shown in FIG. 11;  
     [0024]FIGS. 13A and 13B together illustrates the logic circuit embodied according to an alternative exemplary flow diagram, wherein the logic circuit includes the structure of the embodiment illustrated in FIG. 11, but also includes a front-end portion that provides an initial state determination before attempting to isolate a fault;  
     [0025]FIG. 14 illustrates the thermal switch of the invention embodied as a three-terminal switch;  
     [0026]FIG. 15 is a cross-sectional view of the three-terminal thermal switch illustrated in FIG. 14;  
     [0027]FIG. 16 is a schematic description of the three-terminal thermal switch shown in FIGS. 14 and 15; and  
     [0028]FIG. 17 illustrates the overheat detection system of the invention having the thermal switch of the invention as installed in an aircraft for supplying overheat detection in the wing, fuselage, and cowling. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT  
     [0029] In the Figures, like numerals indicate like elements.  
     [0030] The present invention is a thermal protection device that provides a resistor in combination with a normally open, snap-action thermal switch until the switch changes state from open to closed. This resistor in combination with a normally open, snap-action thermal switch provides several advantages over typical thermal protection devices. For example, the resistor provides a means for determining if switch connector is not engaged, or the wire harness contains a broken lead wire. In these and like circumstances a malfunction indication will occur during preflight check or en route, if the failure occurs during flight. While the overheat detection system remains operational, a malfunction indication will occur during an actual in-flight overheat condition. Furthermore, if an overheat condition does occur, the thermal switch of the present invention is equipped with the serial connected resistor to provide information describing an exact location of the overheat. Flight safety is thereby enhanced, and later correction of the problem that caused the overheat condition is simplified because of the ability to pinpoint the location of the overheat fault.  
     [0031]FIG. 1 is a top plan view and FIG. 2 is a cross-sectional view of the present invention embodied as a snap-action thermal switch  10  having an internally mounted resistor  12 . The thermal switch  10  includes a pair of electrical contacts  14 ,  16  that are mounted on the ends of a pair of spaced-apart, electrically conductive terminal posts  20  and  22 . The electrical contacts  14 ,  16  are moveable relative to one another between an open and a closed state under the control of a thermally-responsive actuator  18 . According to one embodiment of the invention, the thermally-responsive actuator  18  is a well-known snap-action bimetallic disc that inverts with a snap-action as a function of a predetermined temperature between two bi-stable oppositely concave and convex states. In a first state, the bimetallic disc actuator  18  is convex relative to the relatively moveable electrical contacts  14 ,  16 , whereby the electrical contacts  14 ,  16  are moved apart such that they form an open circuit. In a second state, the bimetallic disc actuator  18  is concave relative to the relatively moveable electrical contacts  14 ,  16 , whereby the electrical contacts  14 ,  16  are moved together such that they form an closed circuit.  
     [0032] As illustrated in FIGS. 1 and 2, the thermal switch  10  includes the two terminal posts  20 ,  22  mounted in a header  24  such that they are electrically isolated from the header  24  and from one anther. For example, the terminal posts  20 ,  22  are mounted in the header  24  using an electrical isolator  26  (shown in FIG. 1) formed of an electrically isolating glass or epoxy material.  
     [0033] As shown in FIG. 2, the contact  14  is fixed on the lower end of one terminal post  20 . The contact  16  is moveable on the end of a carrier  28  in the form of an armature spring, which is fixed in a cantilever fashion to the lower end of the other terminal post  22 . The electrical contacts  14 ,  16  thus provide an electrically conductive path between the terminal posts  20 ,  22 . Upward pivoting of the armature spring  28  moves the movable contact  16  out of engagement with the fixed contact  14 , whereby an open circuit is created. Downward pivoting of the armature spring  28  moves the movable contact  16  into engagement with the fixed contact  14 , whereby the terminal posts  20 ,  22  are shorted and the circuit is closed.  
     [0034] The movable contact  16  is controlled by the disc actuator  18 , which is spaced away from the header  24  by a spacer ring  30  intermitted with a peripheral groove  32 . A cylindrical case  34  fits over the spacer ring  30 , thereby enclosing the terminal posts  20 ,  22 , the electrical contacts  14 ,  16 , and the disc actuator  18 . The case  34  includes a base  36  with a pair of annular steps or lands  38  and  40  around the interior thereof and spaced above the base. The lower edge of the spacer ring  30  abuts the upper case land  40 . The peripheral edge of the disc actuator  18  is captured within an annular groove created between the lower end of the spacer ring  30  and the lower case land  38 .  
     [0035] As shown in FIG. 2, while the thermal switch  10  is maintained below a predetermined overheat temperature, the disc actuator  18  is maintained concave relationship to the electrical contacts  14 ,  16 . The concave disc actuator  18  pivots the armature spring  28  upwardly to separate the contacts  14 ,  16  through the intermediary of a striker pin  42  fixed to the armature spring  28 . Separation of the contacts  14  and  16  creates normally open circuit condition.  
     [0036] The resistor  12  is mounted to the interior of the thermal switch  10  and electrically connected to the two terminal posts  20 ,  22 . For example, the resistor  12  is bonded to an inner surface of the header  24  using a bonding agent  44 , such as an epoxy. Lead wires  46 ,  48  attached to the resistor  12  are electrically coupled to each of the terminal posts  20 ,  22 . For example, the lead wires  46 ,  48  are spot welded to an outer surface of the corresponding terminal post  20 ,  22 . The output of the internally mounted resistor  12  is available on the terminal posts  20 ,  22  while the electrical contacts  14 ,  16  provide an open circuit.  
     [0037] The thermal switch  10  is sealed to provide protection from physical damage. The thermal switch  10  is optionally hermetically sealed with a dry Nitrogen gas atmosphere having trace Helium gas to provide leak detection, thereby providing the electrical contacts  14 ,  16  and the internal resistor  12  with a clean, safe operating environment.  
     [0038]FIG. 3 illustrates the thermal switch  10  as a closed circuit, wherein the contacts  14 ,  16  are shorted. In response to a increase in the sensed ambient temperature above a predetermined set point, the disc actuator  18  inverts in a snap-action into a concave relationship with the electrical contacts  14 ,  16 , the disc actuator  18  entering a space between the lower case land  38  and the case end  36 . The lower end  50  of the striker pin  42  is normally spaced a distance from the actuator disc  18  so that slight movement of the actuator disc  18  will not effect contact engagement. The armature spring  28  is pivoted downwardly, which moves the movable contact  16  into engagement with the fixed contact  14 , thereby creating a short and closing the circuit. The output of the internal resistor  12  is not available when the electrical contacts  14 ,  16  are shorted and the circuit is closed. As described in detail below, removal of the resistance of the internal resistor  12  identifies the particular switch that has responded to an overheat condition so that the location of the overheat event is identified.  
     [0039] Due to the nature of the snap-action disc actuator  18 , the output of the internal resistor  12  becomes available again when the sensed ambient temperature is reduced below the predetermined set point and the disc actuator  18  returns to its convex state relative to the electrical contacts  14 ,  16 , so that the resistance of the internal resistor  12  is again presented with an open circuit on the two terminal posts  20 ,  22 .  
     [0040]FIG. 4 is a schematic description of the single-pole, single-throw thermal switch  10  shown in FIGS. 1 through 3. As illustrated, the single-pole, single-throw thermal switch  10  is structured such that a resistance R 12  is by-passed when the switch contacts  14 ,  16  are closed.  
     [0041]FIGS. 5 and 6 illustrate an alternate embodiment of the invention wherein the resistor  12  is installed on an exterior surface  52  of the thermal switch  10  and the lead wires  46 ,  48  are attached to exterior surfaces of the terminal posts  20 ,  22  of the thermal switch  10 . The internal resistor  12  is, for example, bonded to the exterior surface  54  of the header  24 , as shown in FIGS. 4 and 5.  
     [0042]FIG. 7 is a top plan view of the thermal switch  10  of the present invention embodied as a snap-action thermal switch  10  having a resistor  12  coupled in parallel with the switch contacts  14 ,  16  (shown in FIGS. 2, 3) and installed in a housing  56  that is configured for mounting in an aircraft wing, fusclagc, or cowling, as shown in FIG. 17. FIG. 8 is a break-away side view of the snap-action thermal switch  10  of the present invention embodied as shown in FIG. 7. The housing  56  may include a threaded adapter member  58  for mounting, either in a threaded hole or through a clearance hole with a nut. An over-mold  60  is formed over and encases the thermal switch  10 , the resistor  12  (shown mounted externally), the terminal posts  20 ,  22 , and partially encases a pair of contact adapters  62 ,  64  that are electrically coupled to the terminal posts  20 ,  22 , respectively. The contact adapters  62 ,  64  are internally threaded to enable the thermal switch  10  to be electrically coupled into the overheat detection system. The over-mold  60  is formed of an electrically insulative material, such as one of the conventional high-temperature thermoplastic or thermo-set materials. The over-mold  60  may include an integral physical barrier portion  66  to protect against inadvertent contact between connectors (not shown) that are attached to the contact adapters  62 ,  64  for installing the switch  10  into the overheat detection system.  
     [0043]FIG. 9 illustrates the thermal switch  10  of the invention implemented in an overheat detection system  100  having one of the thermal switches  10  coupled in parallel with a quantity of conventional snap-action thermal switches  102  that do not include the resistor  12 . The single thermal switch  10  of the invention and the conventional thermal switches  102  are electrically coupled together in parallel by a wire harness  104 , which is led to an indicator  106 . In a conventional overheat detection system, the indicator  106  provides a visual and/or an aural indication of an overheat condition sensed by the overheat detection system. In other words, if one of the conventional thermal switches  102  responds to an overheat condition by closing its electrical contacts, whereby the circuit formed with the wire harness  104  is closed, the indicator  106  is connected to a voltage source V. The indicator  106  responds by either emitting an aural warning or displaying a visual warning of the overheat condition.  
     [0044] According to the embodiment of the overheat detection system  100  illustrated in FIG. 9, the wiring harness  104  exhibits a nominal resistance R N  resulting from the electrical wire in the harness  104 . The single thermal switch  10  is coupled into the overheat detection system  100  as the end switch. Thus, when the thermal switch  10  is on-line and in the intended normally-open state, the resistor  12  appears on the wiring harness  104  as a minimum resistance R T  in addition to the nominal resistance R N . Thus, the thermal switch  10  is detected as being on-line when a system resistance R S =R N +R T  is detected by a logic circuit  108 . Detection of the thermal switch  10  ensures that the wiring harness  104  is intact and operational, even though the connections of the conventional thermal switches  102  are not indicated.  
     [0045]FIG. 10 illustrates the thermal switch  10  of the invention implemented in an alternative overheat detection system  110  having a quantity of thermal switches  10   a ,  10   b  through  10   n  of the invention coupled together in parallel in the wiring harness  104 , which is led to the indicator  106  through a logic circuit  112 . The logic circuit  112  samples the total system resistance R S =R N +R Ta +R Tb  . . . +R Tn  of the detection system  110  at a predetermined sampling rate, wherein R N  is the nominal resistance of the wiring harness  104  and R Ta  through R Tn  are the resistances of the resistors  12  of the respective thermal switches  10   a  through  10   n.    
     [0046] As embodied in FIG. 10, the indicator  106 , as part of the overheat detection system  110  of the invention, additionally provides a fault indication when the resistance R S  of the system  110  detected by the logic circuit  112  fails to fall between a minimum and a maximum threshold resistance. The overheat detection system  110  employs a number of the thermal switches  10  of the invention, each including one of the resistors  12 , that provide at least a minimum resistance R S  that is below the maximum threshold resistance only when all of the resistors  12   a  through  12   n  are coupled together in parallel. If the resistor  12  of one of the normally-open thermal switches  10  is removed from the system circuit, then the overall resistance of the system  110  is increased above the maximum threshold, and the indicator  106  indicates a fault. Thus, the thermal switch  10  of the invention having the resistor  12  coupled in parallel with the electrical contacts  14 ,  16  provides a means for determining that all of the thermal switches  10  of the overheat detection system  110  are on-line. The thermal switch  10  of the invention farther provides a means for confirming the integrity of the wire harness  104  by indicating a fault unless the resistance provided by the resistor  12  portion of each of the switches  10  appears on-line. If the electrical contacts  14 ,  16  one of the thermal switches  10  are closed, instead of being in the normally-open state, the system circuit is CLOSED and the system resistance R S  is reduced to the actual resistance in the interconnecting wires of the wiring harness  104 , which is reduced below the minimum threshold resistance. Thus, in a self-test mode, a switch to that fails in the closed state results in a fault indication. Similarly, when a switch  10  of the invention closes in response to an overheat condition, a fault indication results on the indicator  106 .  
     [0047] According to one embodiment of the invention, a quantity of the thermal switches  10   a  through  10   n  of the invention, each including a respective resistor  12   a  through  12   n  coupled in parallel with the electrical contacts  14 ,  16 , are coupled to a pair of wire harnesses  104 . The thermal switches  10   a  through  10   n  and a respective wire harness  104  are deployed on one of the left and right sides of an aircraft to detect overheat conditions in the respective aircraft wing, fuselage, and cowling, as shown in FIG. 17.  
     [0048]FIG. 11 illustrates the overheat detection system embodied as an alternative overheat detection system  120 , wherein each of multiple parallel-coupled thermal switches  10   a ,  10   b , through  10   n  of the invention is embodied having respective resistor  12   a ,  12   b , through  12   n  electrically coupled in parallel with the switch contacts  14 ,  16 . Each of the resistors  12   a  through  1     2   n has a resistance value different from that of the other resistors  12   a  through  12   n . A logic circuit  122  is coupled in series with each of the parallel-coupled thermal switches  10   a  through  10   n  for detecting a resistance R S  that is the combined resistances of all of the resistors  12   a  through  12   n , plus the nominal resistance of the wiring harness  104 . The logic circuit  122  is structured to detect whether the total system resistance R S  of the system  120  is between the minimum and a maximum threshold resistance, as described above The logic circuit  122  is thus structured to detect whether the wiring harness  104  is intact and functional and whether all of the thermal switches  10   a  through  10   n  are on-line.  
     [0049] The logic circuit  122  is further structured, by means known to those of ordinary skill, to detect the actual resistance R S  of the overheat detection system  120  and, when a failure is detected, to determine from the actual resistance R S  which of the multiple thermal switches  10   a  through  10   n  is off-line or closed.  
     [0050]FIG. 12 illustrates the logic circuit  122  embodied in an exemplary flow diagram, wherein the logic circuit  122  includes a series of widow comparitor circuits  124   a  through  124   n  each being structured to determine whether the resistor  12   a  through  12   n  of the respective thermal switches  10   a  through  10   n  is on-line, or is missing from the circuit. In other words, failure to detect one specific resistance value indicates that a particular resistor  12   m  is no longer part of the circuit resistance R S , and that the respective switch  10   m  is off-line, i.e., disconnected. For example, the value of the resistance R S  of the overheat detection system  120  is between predetermined minimum and maximum resistance couples R a1  and R a2  through R an-1  and R an . Such a fault is optionally determined by applying a voltage V to the system  120  during a pre-flight self-test operation. If any of the thermal switches  10   a  through  10   n  is determined to be off-line, a respective fault signal  126   a  through  126   n  is generated and passed to the fault indicator  106 , which indicates the fault in the cockpit. Constant sampling at a predetermined sampling rate during operation causes the logic circuit  122  to continue to monitor the circuit resistance R S  for presence on-line of the multiple thermal switches  10   a  through  10   n.    
     [0051] Furthermore, the logic circuit  122  includes another series of widow comparitor circuits  128   a  through  128   n  each being structured to determine whether the resistors  12   a  through  12   n  of the respective thermal switches  10   a  through  10   n  are on-line, or whether one has been replaced by the minimal resistance of the closed switch contacts  14 ,  16  in series with the wire resistance of the parallel portion of the wiring harness  104 , which indicates that the respective switch  10  has closed in response to an overheat situation. If any of the thermal switches  10   a  through  10   n  is determined to be closed, a fault signal  130   a  through  130   n  is generated and passed to the fault indicator  106 , which indicates the fault in the cockpit. Constant sampling at a predetermined sampling rate during operation causes the logic circuit  122  to continue to monitor the circuit resistance R S  for presence on-line of the multiple thermal switches  10   a  through  10   n.    
     [0052]FIGS. 13A and 13B together illustrates the logic circuit  122  embodied according to an alternative exemplary flow diagram, wherein the logic circuit  122  includes the structure of the embodiment illustrated in FIG. 11, but also includes a front-end portion that provides an initial state determination before attempting to isolate a fault. For example, the logic circuit  122  includes a first state determination window comparitor  132  for determining whether all of the switches  10   a  through  10   n  are on-line by, for example, determining whether the overall resistance Rs of the overheat detection system  120  is between the predetermined minimum and maximum resistance thresholds. Such a fault is optionally determined by applying a voltage V to the system  120  during a pre-flight self-test operation. If the overall resistance R S  is outside the minimum and maximum limits, the signal is passed through the respective window comparitors  124   a  through  124   n  to determine which of the thermal switches  10   a  through  10   n  is off-line and to generate the fault signal  126   a  through  126   n  that corresponds to the switch  10   a  through  10   n  that is off-line. As described above, the fault indicator  106  indicates the fault in the cockpit in response to the respective fault signal  126   a  through  126   n  received.  
     [0053]FIG. 14 illustrates the thermal switch of the invention embodied as a three-terminal switch  140  having a third electrically conductive terminal post  142  using an electrical isolator  26 . The third terminal post  142  is a contact-less post that is physically spaced-apart from each of the first pair of terminal posts  20  and  22 . A second resistor  144  is mounted on the header and electrically coupled between the contact-less terminal post  142  and one of the first pair of terminal posts  20  and  22  (shown as coupled to post  22 ) by respective lead wires  146 ,  148 .  
     [0054]FIG. 15 is a cross-sectional view of the three-terminal thermal switch  140  shown in FIG. 14.  
     [0055]FIG. 16 is a schematic description of the three-terminal thermal switch  140  shown in FIGS. 14 and 15. As illustrated, the three-terminal thermal switch  140  is structured such that a resistance R 144  is remains when the switch contacts  14 ,  16  are closed. The switch  140  otherwise operates similarly to the above described thermal switch  10 .  
     [0056]FIG. 17 illustrates the overheat detection system  100 ,  110 ,  120  having the thermal switch  10 ,  140  of the invention as installed in an aircraft  150  for supplying overheat detection in the wing, fuselage, and cowling. The overheat detection system  100 ,  110 ,  120  includes the thermal switch  10 ,  140  installed in the wiring harness  104 . As described above, the thermal switch  10 ,  140  is either used throughout the overheat detection system  100 ,  110 ,  120  or coupled in parallel with a quantity of conventional snap-action thermal switches  102 . The overheat detection system  100 ,  110 ,  120  is operated as described above to perform a pre-flight self-test operation, to detect overheat situations, to generate and display an appropriate fault signal, and optionally to determine the specific thermal switch  10 ,  140  is responsible for the fault signal.  
     [0057] While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.