Abstract:
A monitor can sense the difference in temperature between the inside and the outside of an enclosure containing electrical power equipment. The monitor has a case adapted for mounting at the enclosure. Also included is a first and a second sensor for producing a first signal and a second signal, respectively. The first sensor is mounted at the case and is adapted for insertion through an opening in the enclosure. The second sensor is adapted to sense temperature at a location remote from the first sensor. Specifically, the case is mounted so that one of the sensors is inside the enclosure and the other is outside. The monitor also includes an alarm system mounted at the case and coupled to the first and the second sensor for producing a warning signal in response to the first and the second signals from the first and the second sensor signifying a temperature difference exceeding a predetermined threshold.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to apparatus and techniques for detecting the temperature differential existing around various electrical power equipment and enclosures. 
   2. Description of Related Art 
   Residential, commercial, and industrial buildings typically have electrical panels in the form of a box or covered recess that may contain circuit breakers, power contactors, relays, fuses or other equipment designed to deliver or route primary utility current to locations in the building. Failure of such electrical equipment is typically preceded by a temperature increase. The temperature increase can be caused by excessive current, unbalanced load, oxidation or corrosion at contact surfaces, lossy contacts that generate heat, arcing, etc. 
   Measuring a temperature increase within an electrical panel is complicated by the fact that the temperature rise can be caused by equipment defects, or simply by a rise in the ambient temperature. For this reason equipment for detecting or anticipating equipment failure will measure a temperature difference, that is, the temperature at a piece of monitored equipment relative to ambient. 
   A disadvantage with measuring these temperature differences is the complexity associated with the equipment capable of performing such measurements. For example, monitoring the temperature inside a power panel will often require a skilled electrician who is able to remove the panel cover while energized and safely install a temperature sensor as well as wiring that leads to the outside of the power panel. Also, an external temperature sensor must be mounted outside the panel at a position appropriate for measuring ambient temperature. This external sensor must then be wired to a monitoring circuit that can perform the differential analysis and provide an appropriate warning signal. Being relatively complex, such systems often consume a fair amount of power and are therefore often connected to utility power lines, which adds to the complexity of the installation. 
   In U.S. Pat. No. 4,901,060 a temperature sensitive thyristor 34 energizes warning element 20 when high temperature is sensed at device 18, which is illustrated as a standard socket for house current. Warning element 20 can be a light or a flasher. A temperature reference can be provided to thyristor 34 by diode 64 of device 62, which is spaced a distance d from the thyristor 34. 
   In U.S. Pat. No. 6,470,735 the temperature of lubricant in an axle housing is measured by sensor 26 and compared to the ambient temperature measured by sensor 30. An excessive temperature difference indicates a probable need for service and can illuminate light 36. 
   In U.S. Pat. No. 5,541,803 a temperature difference is sensed by a sensor conductor and a reference conductor. In the embodiment of FIG. 17 a sensor conductor 174 is routed inside an appliance to compare the temperature inside the appliance to the temperature on the outside of the appliance. Power to the appliance is interrupted in response to excessive internal temperature. LEDs 32 and 33 indicate the status of the system as either “tripped” or “on.” 
   In U.S. Pat. No. 6,707,652 a terminal of a circuit breaker (or the like) may glow hot if it has a poor, high resistance connection. Temperature sensing diodes 8 and 10 can sense the different temperatures and the magnitude of the difference determines whether the comparator will trip a circuit breaker coil. 
   In U.S. Pat. No. 5,982,849 a device adhesively attached to an x-ray tube can operate a blinker 11 when resistance sensor 7 detects a high temperature. 
   In U.S. Pat. Nos. 5,847,653 and 6,060,990 a heat alarm is attached to the face of an electrical panel by magnets or otherwise. A bimetallic switch senses temperature reaching 135° F. to illuminate an LED and operate an audible alarm. 
   In U.S. Pat. No. 5,461,367 a bimetallic temperature sensor is mounted inside an electrical panel to produce an audible alarm when the panel temperature exceeds 135° F. 
   In FIG. 4 of U.S. Pat. No. 4,331,888 temperature sensitive transistors Tr1 and Tr2 have different thermal time constants. Accordingly, a differential voltage will be produced in response to a rapid temperature increase. This differential voltage can trigger a temperature sensitive thyristor that can also trigger in response to high temperatures, without regard to the differential voltage. Once triggered, the thyristor operates an alarm 7. 
   In U.S. Pat. No. 4,406,550 temperature sensors 10 and 11 are applied to differential comparator 30 to display a temperature difference on display 38. The device is described as useful for monitoring temperature differences at different positions on a diesel engine, a furnace, a solar collector, or at different positions around a building (including the inside and outside of the building). If the temperature difference exceeds a certain positive threshold the system activates an alarm 55. If the temperature difference exceeds a certain negative threshold, alarm 56 is activated instead. 
   In U.S. Pat. No. 6,359,565 the temperatures at various locations on several electronic cards are monitored by sensors mounted on those cards. The measured temperatures are compared electronically to the ambient temperature measured at a fan. Components having a high temperature differential over the ambient temperature are deemed to be malfunctioning and the operator is given an alarm signal and is offered information in the form of a thermal map of the system. 
   In U.S. Pat. No. 5,081,359 a differential thermal sensor using thermopiles can detect temperature differences along a patient&#39;s spine, or in various industrial processes. 
   In U.S. Pat. No. 4,608,565 an indoor/outdoor thermometer uses a radio frequency connection to avoid cutting a hole through a building. Temperature is displayed by a numeric display. 
   In U.S. Pat. No. 3,688,295 a thermocouple for measuring brake temperature is compensated so that changes in ambient temperature due to cold or hot weather do not affect the temperature measurement at the hot junction of the thermocouple. As temperature increases the system illuminates a warning light and then an overheat light. 
   In U.S. Pat. No. 4,188,623 the hot junction of the thermocouple is placed near an automobile&#39;s catalytic converter. If the thermocouple measures a high temperature, reed switch 20 closes to illuminate light 26. The cold junction is responsive to the “circumferential temperature” but a temperature sensitive diode or thermistor is used to compensate for or cancel out the effects of the circumferential temperature. 
   In U.S. Pat. No. 5,229,612 a thermopile is coupled with a thermocouple so that a measured temperature is referenced to a remote temperature sensed by the thermocouple. In some embodiments separate thermopiles measure temperatures at spaced positions to develop a differential temperature measurement. 
   In U.S. Pat. No. 6,429,777 thermistors are mounted in junction boxes throughout a building. The measured temperatures can be displayed on a panel at a central station. 
   In U.S. Pat. No. 4,922,230 a reference temperature signal is initially set at startup and is allowed to vary slowly if temperature increases. The difference between this reference temperature and the actual temperature as sensed by the sensor 10 is used to trigger an alarm, basically when the sensed temperature is rising so fast as to indicate the outbreak of the fire. 
   In U.S. Pat. No. 3,753,194 a pair of thermostats produces a signal when the temperature falls between an upper and lower limit. 
   Accordingly, there is a need for an improved device for measuring temperature differences associated with electrical equipment that avoids the shortcomings and complexities of the prior art. 
   SUMMARY OF THE INVENTION 
   In accordance with the illustrative embodiments demonstrating features and advantages of the present invention, there is provided a monitor for sensing the difference in temperature between an inside and an outside of an enclosure containing electrical power equipment. The monitor has a case adapted for mounting at the enclosure. Also included is a first and a second sensor for producing a first signal and a second signal, respectively. The first sensor is mounted at the case and adapted for insertion through an opening in the enclosure. The second sensor is adapted to sense temperature at a location remote from the first sensor. The monitor also includes an alarm system mounted at the case and coupled to the first and the second sensor for producing a warning signal in response to the first and the second signals from the first and the second sensor signifying a temperature difference exceeding a predetermined threshold. 
   In accordance with another aspect of the invention a method is provided for sensing the difference in temperature between an inside and an outside of an enclosure containing electrical power equipment. The method employs a case having a pair of sensors for producing temperature signals. The method includes the step of mounting the case at the enclosure with one of the sensors inside the enclosure and the other outside. Another step is producing a warning signal in response to the signals from the first and the second sensor signifying a temperature difference exceeding a predetermined threshold. 
   By employing apparatus and techniques of the foregoing type an improved equipment monitor is achieved. In one embodiment a plastic case is fitted with a thermistor that projects from the front and another thermistor that projects from the back. The back thermistor is designed to fit through a hole in an enclosure. This hole can either pre-exist or can be drilled in preparation for installation of the case. The case may be mounted by integral magnets, adhesives, or other simple fastening means. 
   This case will include appropriate circuitry for sensing the temperature difference sensed by the inside and outside thermistors. The disclosed embodiment will have a trio of LEDs: green for normal, yellow for exceeding a first threshold, and red for exceeding a second higher threshold. To conserve the battery powering the system the lights will blink, with the green blinking once every 20 seconds, yellow blinking twice as fast and the red blinking three times as fast. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above brief description as well as other objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a front view of a monitor in accordance with principles of the present invention; 
       FIG. 2  is a vertical, cross-sectional view of the monitor of  FIG. 1  taken along line  2 - 2   FIG. 1 ; 
       FIG. 3  is a perspective view of the monitor of  FIG. 1  about to be mounted on an enclosure containing electrical power equipment; 
       FIG. 4  is a cross-sectional view of the first sensor of the monitor of  FIG. 2 ; 
       FIG. 5  is a fragmentary, perspective view of the second sensor of the monitor of  FIG. 2 ; and 
       FIG. 6  is a schematic view of the circuit inside the monitor of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 and 2 , a monitor is shown as a case  10  containing a printed circuit board  12  with a number of integrated circuits  14  and a battery  16 , all arranged to act as an alarm system. An access door  18  on the back of case  10  allows installation or replacement of battery  16 . Mounted behind integral windows W 1 , W 2 , and W 3  on circuit board  12  are a number of lights, one such light being shown herein as light emitting diode (LED) L 3  mounted behind circular window W 3  in the front of case  10 . 
   Glued into openings in the back of case  10  are a pair of bar magnets  20  for magnetically securing case  10  to an enclosure made a steel or other ferromagnetic materials. 
   Referring to  FIG. 3 , an exemplary enclosure is shown as a power panel in the form of a steel box  22  into which are routed power lines  24  and  26 . Door  28  can be opened to gain access to electrical power equipment inside box  22 . Such equipment can be circuit breakers, contactors, relays, transformers, or other equipment that may be useful in routing and delivering current from utility lines. In some embodiments the enclosure may encompass a relatively large volume containing electromagnetic motors, solenoids, etc. Instead of a discrete box, some embodiments may work with a recess built into the structure of a building, which is then covered by a metal plate or the like. 
   Referring to  FIGS. 2 ,  4 , and  5 , a first sensor  30  is shown as an encapsulated thermisistor  32  mounted on a pair of insulated wires  34 , which are soldered into printed circuit board  12  for delivering a first signal thereto. The leads of thermistor  32  are routed through a thermally insulating grommet  36  mounted in a hole in the back of case  10 . A plastic sleeve  38  having an inside flange is mounted in grommet  36  to protect thermisistor  32 . In  FIG. 5  a second sensor  40  identical to the one shown in  FIG. 4  is mounted inside a plastic sleeve  42  and grommet  44 , which are identical to previously mentioned sleeve  38  and grommet  36 . Such grommets can be obtained from Mueller Die Cut Solutions of Charlotte, N.C. Sensor  40  has a thermisistor  48  that issues a second signal along wires  46 . 
   Referring to  FIG. 6 , resistor R 1  has one terminal connected to positive potential and its other terminal shunted to ground through previously mentioned thermistor  32  (first sensor). Resistor R 2  has one terminal connected to positive potential and its other terminal shunted to ground through previously mentioned thermistor  48  (second sensor). The junction of elements  32  and R 1  provide a first signal that is connected to the non-inverting terminal of differential amplifier Z 1 , whose inverting terminal connects to the junction of elements R 2  and  48 , which provides a second signal. 
   The output of differential amplifier Z 1  commonly connects to the inverting terminals of comparators Z 2  and Z 3 . The non-inverting terminal of comparator Z 2  connects to the junction of serially connected variable resistor R 4  and resistor R 5 , which connect between positive potential and ground, in that order. The non-inverting terminal of comparator Z 3  connects to the junction of serially connected variable resistor R 6  and resistor R 7 , which connect between positive potential and ground, in that order. 
   The output of comparator Z 2  connects to one input of OR gate G 2  whose other input connects to output  3 X of clock circuit CK, whose other outputs are identified as outputs  1 X and  2 X. Outputs  1 X,  2 X and  3 X produce square waves with a frequency of 3, 6, and 9 Hz, respectively (i.e., once, twice, and thrice every 20 seconds). Clock circuit CK may include a free running multivibrator with a divider, or three independent oscillators whose outputs are clipped. 
   Output  2 X connects to one input of OR gate G 1  whose other input connects to the output of OR gate G 4  whose inverting and non-inverting inputs connect to the outputs of comparators Z 2  and Z 3 , respectively. Output  1 X connects to one input of OR gate G 3  whose other input connects to the output of NAND gate G 5 . The output of gate G 4  connects to one input of NAND gate G 5 , whose other input connects to the output of comparator Z 3 . 
   The outputs of gates G 1 , G 2 , and G 3  connect to the cathodes of yellow LED L 1 , red LED L 2 , and green LED L 3 , whose anodes connect to positive potential. The square waves of clock CK may operate with a duty cycle of about 10% to reduce the amount of time the LEDs remain on. 
   To facilitate an understanding of the principles associated with the foregoing apparatus, its operation will be briefly described. The circuit of  FIG. 6  is initially calibrated by raising the temperature of thermistor 12 F.° (6.7 C.°) relative to thermistor  48 . The resulting increased resistance of thermistor  32  increases the potential at the non-inverting input of differential amplifier Z 1  relative to its inverting terminal. Consequently, the output of amplifier Z 1  increases. Next, variable resistor R 6  is adjusted by increasing its resistance from a minimum value until the output of comparator Z 3  changes from a high to a low value, i.e., from approximately the supply potential to 0 V. 
   After that, the temperature of thermistor  32  is increased to a temperature of 27 F.° (15 C.°) relative to thermistor  48 . Again, the further increased resistance of thermistor  32  further increases the output of differential amplifier Z 1 . Then, variable resistor R 4  is adjusted by increasing its resistance from a minimum value until the output of comparator Z 2  changes from a high to a low value. 
   Once adjusted, comparators Z 3  and Z 2  provide a first and a second threshold, respectively, in the nature of a warning signal indicating that the temperature difference has exceeded predetermined limits. The specific temperature differences defining the first and second threshold may be established based upon the users&#39; preferences. In conservative designs relatively small temperature differences will cross the thresholds. Also, the temperature difference corresponding to the thresholds will vary depending on the environment and the device being protected. As an example, the device may often be used to protect an enclosure that is 2 feet (61 cm) tall, 1.5 feet (46 cm) wide and 10 inches (25 cm) deep. For such an enclosure, an exemplary embodiment set the temperature difference for the first threshold at 5 F.° (2.8 C.°), while the temperature difference for the second threshold was set at a value in the range of 10 to 15 F.° (5.6 to 8.3 C.°). It will be appreciated that the foregoing temperature thresholds are by no means the only thresholds that may be selected and the actual thresholds employed will depend on the equipment being monitored, the expected temperature variations, the type of ventilation, the criticality of equipment failure, etc. 
   Also, comparators Z 2  and Z 3  can be designed with hysteresis so that once a comparator changes state it will not revert back to the earlier state until a significant temperature reversion is sensed (e.g., 0.5 C.°). 
   In the embodiment of  FIG. 3  enclosure  22  is prepared by drilling a hole  50  in the face of the enclosure above door  28  (although in the hole may preexist in certain types of enclosures). It is desirable to install device  10  high on enclosure  22  since heat inside the enclosure will tend to rise and device  10  will then be monitoring what is normally the hottest part of the enclosure. Placement of device  10  on the front of the enclosure  22  is also desirable so that device  10  and its LED indicators are prominent and easily visible. On the other hand, the device can be mounted on other locations on enclosure  22 . In some instances, device  10  may be mounted on the top surface of enclosure  22 , in which case device  10  can be modified so that its LED indicators (visible through windows W 1 -W 3 ) are located on the edge of the device to enhance visibility. 
   In the illustrated embodiment case  10  is placed on the front of enclosure  22  with the first sensor  30  inserted through hole  50 . The length of sensor  30  is chosen to allow thermistor  32  to project inside enclosure  22  approximately 2.5 cm. Since enclosure  22  is in this case made of steel, magnets  20  will immediately attach case  10  to the enclosure without the need for further fastening means. For embodiments where the enclosure is not ferromagnetic, case  10  can be secured with glue, double sided tape, etc. Also, in some embodiments case  10  may be formed with screw holes (or flanges with screw holes) that allow the case to be fastened to an enclosure with screws or other fastening devices. 
   Once case  10  is installed, the interior of enclosure  22  will reach an equilibrium that under normal circumstances is no more than 3 C.° warmer than the ambient temperature outside the enclosure. Accordingly, the output of differential amplifier Z 1  ( FIG. 6 ) will be relatively small so that the output of comparators Z 2  and Z 3  will be high. These high outputs produce high outputs on OR gates G 1 , G 2 , and G 4 , which produces a zero potential across LEDs L 1  and L 2 , keeping them off. 
   The high outputs from comparator Z 3  and gate G 4  produces a low signal from gate G 5 , which is applied to one input of gate G 3 . The other input of gate G 3  receives from output  1 X of clock CK square waves with a period of 20 seconds. Consequently, gate G 3  applies the same square waves to the cathode of green LED L 3 , which then blinks at the rate of once every 20 seconds. 
   If however there is a failure, an imminent failure, or some other thermal problem inside enclosure  22 , the temperature inside enclosure  22  will increase. When the temperature differential exceeds a threshold of approximately 6.7 C.°. Output of comparator Z 3  becomes low. Since the output of comparator Z 2  remains high, OR gate G 4  applies a low output signal to one input of OR gate G 1 . Since the other input of gate G 1  is receiving the square waves from output  2 X of clock CK, gate G 1  applies the same square waves to the cathode of yellow LED L 1 , which blinks at the same rate, i.e., twice every 20 seconds. Also, because comparator Z 3  applies a low signal to one input of NAND gate G 5 , this gate produces a high signal that is conveyed through gate G 3  to the cathode of green LED L 3 , keeping it off. 
   If the temperature differential increases beyond the threshold of approximately 15 C.°, comparator Z 2  now produces a low signal indicating passage through the second threshold. The low output from comparator Z 2  produces a high output on gates G 4  and G 1 , turning off yellow LED L 1 , but without further affect on green LED L 3 , which remains off. The low output of comparator Z 2  is applied to one input of OR gate G 2  whose other input receives the square wave from the output  3 X of clock CK. Consequently, gate G 2  applies the same square wave to the cathode of red LED L 2 , which then blinks at the rate of three times every 20 seconds. 
   Service or maintenance personnel can easily determine whether the temperature in the interior of enclosure  22  is suspiciously high relative to the ambient temperature outside the enclosure. Personnel familiar with the legend  52  ( FIG. 1 ) will know that a cool condition, graphically indicated by a single green circle on the first line of legend  52 , is indicated by the green LED slowly blinking through window W 3 . A warm condition, indicated on the second line of legend  52  by two yellow circles, corresponds to the yellow LED blinking twice as fast through window W 1 . A hot condition, indicated on the third line of legend  52  by 3 red circles, corresponds to the red LED blinking three times as fast through window W 1 . Since a blinking light attracts attention, personnel can determine the condition of the protected equipment at a glance. Also, the fact that during normal conditions the green LED L 3  ( FIG. 6 ) only blinks once every 20 seconds, conserves power, so that under normal conditions the life of battery  16  ( FIG. 2 ) can be one or more years. Long battery life may be promoted by using durable batteries such as those provided by Micropower Battery Company of Miami Fla. 
   In some cases the three foregoing conditions as indicated by the three LEDs L 1 , L 2 , and L 3 , may be sent electronically to a building management system or a building alarm system. For this purpose, connector  54  is provided for conveying the signals from the outputs of elements Z 2 , G 4 , and G 5 . In more complicated systems, connector  54  may include a digital processor for multiplexing the signals, producing an RS-232 output, or other signals conditioned as appropriate for interfacing with another system. 
   It is appreciated that various modifications may be implemented with respect to the above described, preferred embodiment. While the foregoing system employs simple combinational logic, other embodiments may employ a microprocessor programmed to control system operations. Alternatively, an EEPROM can be programmed to produce the foregoing logical events. Furthermore, all of the foregoing components may be integrated into a single integrated circuit (an ASIC), with the possible exception of the temperature sensors and battery. In some embodiments the alarm system may have a delay circuit, counter or filter to prevent production of a warning signal in case of an intermittent event. Also, instead of a differential amplifier, the thermistors can be connected in a bridge whose output connects to the rest of the circuit either directly or through an amplifier. Instead of thermistors, some embodiments can employ thermocouples, temperature sensitive semiconductors, bimetallic components, etc. Moreover, some embodiments may employ an audible alarm, instead of, or in addition to, a warning light. The shape, size, configuration, and material composition of the foregoing case can be altered depending upon the size of the circuit components, available space, desired case strength, etc. 
   Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.