Abstract:
A temperature sensor for use in a circuit breaker is disclosed. The temperature sensor is included in the circuit breaker and detects temperature of a target within the current path of the breaker. If the temperature of the target falls within a preselected critical range, a trip signal is triggered in a trip processor board which actuates a solenoid to trip the circuit breaker. A bridge is provided in the current path to continue normal function of the circuit breaker outside the critical range of temperatures.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to circuit breakers, and more particularly relates to over-current protection for circuit breakers. 
     Over current protection is provided by circuit breakers, fuses, and their associated switches, relays and other devices. Each type of equipment has variations in classes and ratings. Most types of over-current protection are designed to prevent damage to conductors and insulation from small or large excesses of current. 
     Circuit breakers, including molded case circuit breakers, protect against overheating of the conductor. The current path within a typical breaker is through a bimetallic strip; the resistance of the bimetal develops heat which causes the bimetal to bend until it moves far enough to unlatch the mechanism that allows the breaker to trip open. This is considered thermal tripping for overload currents. 
     The thermal action provides inverse time response. That is, a small overload takes a long time to heat the bimetal and trip the breaker. As the overload increases, the heating and tripping time is reduced. The larger the current, the less the tripping time, until the current reaches the setting of the magnetic trip. Magnetic response is instantaneous. Magnetic tripping is utilized for faster tripping at higher fault currents. Electronic circuit breakers may operate by sensing current level and trip when preset current is reached. 
     One known circuit breaker uses a temperature sensor to calibrate the bimetallic strip during the manufacturing of the product. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a trip processing unit for a circuit breaker comprises a target for temperature measurement preferably located on the current path of the circuit breaker. The unit further includes a temperature sensor for reading the temperature of the target. A trip processor board within the circuit breaker receives temperature measurements from the temperature sensor. The temperature sensor triggers a trip signal in the trip processor board when a temperature of the target falls within a preselected critical range where further use of the circuit breaker could become detrimental to the conductors and insulation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an internal plan view of a circuit breaker according to an embodiment of the present invention; and, 
     FIGS. 2A-2C are temperature vs. time tables demonstrating the variance in tripping temperatures of prior art circuit breakers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As can be seen in FIG. 1, an exemplary embodiment of the present invention involves a residential circuit breaker  10  having a plastic case  12 . Although a residential circuit breaker is shown, the use of commercial or industrial circuit breakers, as well as other types of residential circuit breakers, are within the scope of this invention. 
     Extending within the circuit breaker  10  is a control handle  14  which may be used for manually operating the circuit breaker  10 . The handle  14  is connected to a movable arm  16 , which carries a movable contact  18 . As is demonstrated by the curved arrow, the movable arm  16  can move the movable contact  18  into abutting relation with the stationary contact  20  attached to the line stab  22 . Further included in the circuit breaker  10  is a connector lug  50  where a customer/end user connects load (e.g. a lamp or particular circuit) to the circuit breaker  10 . 
     The present invention preferably includes a standard mechanical trip mechanism  15  composed of a cradle  24  biased by a cradle spring  26  in a counterclockwise direction about point  25  where the cradle  24  is attached within the case  12 . The spring  26  interconnects the cradle  24  to the movable arm  16 . A latch  28  is biased in a clockwise direction about a control spring  30 . Cradle  24  includes a cradle tip  36 , which extends through a latch window  38  formed in the latch  28 . The latch  28  prevents the cradle  24  from rotating, thus maintaining the movable contact  18  of the movable arm  16  into contact (not shown) with the stationary contact  20 . Sleeve  32  is shown adjacent latch  28 . When movable contact  18  is in contact with stationary contact  20  a current path  48  through circuit breaker  10  is created. The current path  48  is formed through stab  22 , stationary contact  20 , movable contact  18 , contact arm  16 , flexible braid  49 , bridge  41 , and lug  50 . 
     When a short circuit condition occurs at a high level of current within the protected circuit (e.g., seven times the rated current of the breaker), the magnetic force of the sleeve  32  will draw the latch  28  towards sleeve  32  allowing the cradle  24  to rotate in the counter clockwise direction. Such a movement of the cradle  24  will activate the cradle spring  26  in a manner that will pull the movable arm  16  and movable contact  18  away from the stationary contact  20 . Thus, magnetic tripping is made possible. 
     The circuit breaker of the present invention is also designed to prevent damage to the protected circuit caused by overheating. A thermal trip processing unit  43  includes a temperature sensor  40 , a target region  42 , and a trip processor circuit board  44 . The present invention utilizes the temperature sensor  40  to detect the temperature of a target region  42  on bridge  41 . The temperature sensor  40  provides a signal indicative of this temperature to a trip processor circuit board  44  (which includes electronic circuit  52 ). If the temperature indicated by this signal is above a predetermined threshold, trip processor circuit board  44  provides a trip signal to a trip solenoid  46 . In response to this trip signal, a plunger  54  in trip solenoid  46  extends, moving the latch  28  in a counter clockwise direction. Movement of latch  28  in the counter clockwise direction causes the trip mechanism  15  to separate contacts  18  and  20  in the manner previously described. 
     Advantageously, the present invention does not require a bimetallic member, previously located at location  34 , to provide the motion required to unlatch the mechanism for thermal tripping. Because a bimetallic member is not required, the engagement of the cradle tip  36  into the latch window  38  and the tolerances and variations encountered in this area will not be a significant and tedious factor in the actuation or calibration of this breaker. 
     The molded case circuit breaker  10  of the present invention utilizes temperature sensor  40 , as the means to detect a range of temperature variations within the current path  48  of the device. One suitable non-contact temperature sensor is that made by Exergen Corporation, product number IRt/c. 1X. Of course, improvements in size and accuracy of temperature sensors will dictate the most appropriate temperature sensor to employ in the present invention. That is, the employed temperature sensor  40  is preferably very accurate and small enough to fit within the confines of trip processor circuit board  44 . Infrared technology allows very small devices to measure a large range of temperatures. Such an infrared device could be used as sensor  40  in a “non-contact” embodiment of the present invention. Alternatively, in a “contact” embodiment, a thermocouple could be used which is in contact with target  42 . The temperature sensor  40  is shielded or insulated such that the effect of ambient temperature or heat emitted from components other than the target  42  is reduced. 
     The temperature sensor  40  monitors the target  42  at all times when the breaker is under power to provide a signal indicative of the target temperature to the trip processor circuit board  44 . Certain temperature sensor devices known in the art utilize minimum amounts of radiant energy to self power their circuitry. Some of these devices can self-power at below ambient temperature. Utilizing such a device as the temperature sensor  40  would provide the opportunity of having a signal available for the trip board  44  even when the breaker  10  is not under power. In an alternative embodiment, utilizing such a self-powered device as the temperature sensor  40  would allow the sensor  40  to monitor the target  42  only when target  42  reaches a certain threshold temperature. In the latter embodiment, the reduction in the amount of time that sensor  40  monitors target  42  would increase the life of the sensor  40 . 
     The target  42  is formed on bridge  41 , and is included as part of the current path  48 . For example, target  42  may be a reduced thickness section of the bridge  41  or a section of the bridge  41  between two apertures formed in the bridge  41  so as to create a “hot-spot” (a region of high temperature relative to the temperatures at other regions on bridge  41 ) at the target  42 . In alternative embodiments, target  42  is a thermally-conductive device in intimate contact with bridge  41 , such as a strip of metal, to conduct heat from bridge  41 , or a device arranged proximate to bridge  41 , such as a metal collar disposed about bridge  41 , to receive heat emitted from bridge  41 . Target  42  may preferably be black in color, to increase the emissivity of the target  42 . If the temperature sensor device  40  is a contact device, such as a thermocouple, the device would be attached directly to bridge  41  or to another section of the current path  48 . 
     The trip processor circuit board  44  provides a suitable location to mount temperature sensor  40 . This location would be permanent and, once the board  44  is assembled into the molded case  12  of the breaker  10 , a positive location for the sensor  40  is provided. Because of the importance of the focus distance in an infrared sensor, (distance in between the target  42  and the sensor  40 ) it is important to provide the sensor  40  with a constant focus point to the target  42 . This will be achieved by the accurate positioning of the board  44  with respect to the bridge  41  in the molded case  12 . 
     The solenoid  46  is mounted on trip processor circuit board  44 . Solenoid  46  receives operating power, as well as a trip signal, from trip processing circuit board  44 . Plunger  54  extends from solenoid  46  in response to the trip signal, thereby activating trip mechanism  15  to separate contacts  18  and  20 . The trip processing circuit board  44  also contains the other components necessary within electronic circuit  52  to process the temperature sensor  40  signal and provide the trip signal to the solenoid  46  if the temperature sensor signal exceeds a predetermined threshold. This threshold temperature may be fixed or programmable by the installer or end user. 
     Because the present invention does not require the use of a bimetallic element to provide thermal protection, the present invention offers improvements and options over the prior art. For example, the present invention will allow the electronic calibration of the breaker rather than a mechanical one. By utilizing the present invention, all mechanical variations produced by the mechanism, such as latching forces, or engagement, will have no affect on calibration. Calibration of the breaker will be done at board level. This is an advantage over prior art designs, which use a mechanically linked bimetal to activate a tripping mechanism. Such prior art designs require the precise alignment of the bimetal and other components, as well as mechanical calibration to account for variations in the components. 
     Another advantage of the present invention is that the tripping of the breaker  10  of the present invention will be substantially instantaneous once the threshold temperature is sensed, allowing little or no difference in the performance between two or more breakers employing the present invention. In contrast, the prior art design, employing a bimetallic strip, results in performance differences from one breaker to another. FIGS. 2A,  2 B, and  2 C provide a comparison of the performance of three similar, calibrated, prior-art circuit breakers. As can be seen in FIGS. 2A,  2 B, and  2 C, the difference in trip times between the circuit breakers is about 4.5 sec. The breaker in FIG. 2A trips at about 88 degrees, the breaker in FIG. 2B trips at about 99 degrees, and the breaker in FIG. 2C trips at about 93 degrees. While these prior art breakers are all properly calibrated, they all trip at a different temperatures. Thus, even when properly calibrated, the tripping temperature can vary by as much as 11 degrees in prior art breakers. However, a circuit breaker employing a temperature sensor, as described in the present invention, will result in no more than 3 to 4 degrees difference in the tripping temperature between two or more properly calibrated circuit breakers. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.