Abstract:
A thermal trip unit within a multi-pole circuit breaker includes a bimetal element having an off-set end to provide cam function during the calibration of the bimetal element. A calibration screw is threadingly engaged within the bimetal element and is adjusted at various calibration levels by means of a step motor driver element.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to circuit breaker thermal trip units, and, more particular, to a method and arrangement for calibration of circuit breaker thermal trip units. 
     In circuit breakers thermal trip units the bimetal elements deform, upon reaching a predetermined temperature, into contact with the circuit breaker trip bar to interrupt circuit current. An early teaching of one such bimetal trip unit for residential type circuit breakers is found in U.S. Pat. No. 3,908,110 entitled Method of Calibrating an Electric Circuit Breaker, wherein lasers and the like are used to calibrate the bimetal during manufacture. U.S. Pat. No. 5,317,471 entitled Process and Device for Setting a Thermal Trip Device, describes the use of lasers for calibrating bimetal trip units within industrial rated circuit breakers in accordance with European Industry Standards. 
     Further, U.S. Pat. No. 4,616,198 entitled Contact Arrangement for a Current Limiting Circuit Breaker, describes the use of a first and second pair of circuit breaker contacts arranged in series to reduce the amount of current let-through upon the occurrence of an overcurrent condition. U.S. patent application Ser. No. 09/087,038, filed May 29, 1998, entitled Rotary Contact Assembly for High Ampere-Rated Circuit Breakers, describes the operation of the circuit breaker trip unit to release the circuit breaker operating mechanism and separate the circuit breaker contacts. 
     When rotary operating mechanisms are used to control the circuit breaker contacts, such as described in the aforementioned U.S. Pat. No. 4,616,198 it is imperative that the trip units within each pole of the multi-pole circuit breaker open simultaneously to insure current limiting function within each one of the separate poles, as well as to avoid the occurrence of so-called “single phasing” whereby one of the phases interrupts independently of the remaining phases causing increased current transport through the remaining phases. Certain U.S. Federal and state electric codes require individual calibration of the bimetal trip units within each separate pole to insure simultaneous circuit interruption within each of the poles upon occurrence of an overcurrent condition in any one of the individual poles. 
     With the use of automatic bimetal calibration devices, as described in the aforementioned U.S. Pat. No. 3,908,110, for example, care is taken to insure that the bimetal element, per se, is electrically-insulated from the screws or pins use in calibrating the position of the bimetal element from the operating mechanism trip bar. The imposition of an electrically-insulative sleeve or the like between the bimetal and calibration screw increases both the installation and calibration expense at the time of manufacture. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a thermal trip unit within a circuit breaker includes a bimetal element having an off-set end to provide cam function during the calibration of the bimetal element. The calibration screw is threadingly engaged within the bimetal and is adjusted at various calibration levels by means of a servo-drive or servo motor driver element. The circuit breaker operating mechanism trip bar configured to insure that the circuit breaker operating mechanism responds to simultaneous interruption of circuit current through each of the poles upon occurrence of an overcurrent condition through any one of the individual poles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top perspective view of a multi-pole circuit breaker containing the thermal trip unit in accordance with the invention; and 
     FIG. 2 is a side view of the circuit breaker of FIG. 1 in partial section depicting the thermal trip unit installed within one of the trip unit compartments; and, 
     FIG. 3 is a top perspective view of a circuit breaker employing a rotary contact assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an industrial-rated three pole circuit breaker is shown at  10  with a cover  50  (shown in FIG. 3) omitted from a case  11  to depict the interior thereof. A circuit breaker operating handle  12  manually rotates an operating mechanism  13  to move the circuit breaker contacts (not shown) between ON and OFF positions. Electrical connection with a protected electric circuit is made by means of load straps  14  on one end of case  11  and line straps (not shown) on the opposite side of case  11 . Operating mechanism  13  interacts with the circuit breaker contacts within the separate circuit breaker poles A-C by means of a cross bar  15  in the manner described within the aforementioned U.S. patent application Ser. No. 09/087,038, filed May 29, 1998, entitled Rotary Contact Assembly for High Ampere-Rated Circuit Breakers, which is incorporated herein by reference. As shown in FIG. 3, each pole of the circuit breaker  10  includes a rotary contact arm  52  holding a movable contact  54  on each end of the arm  52 . Each movable contact  54  is aligned for abutting or separating from a stationary or fixed contact  56  which is attached to either a load strap or a line strap. According to an exemplary embodiment of the present invention, a thermal trip unit  16  within each trip unit compartment  17  interacts with a top lever  20  on crossbar  15  by means of a bimetal  19  upon occurrence of an overcurrent condition of predetermined values. 
     To insure operation of bimetal  19  within thermal trip unit  16 , the bimetal is calibrated in the manner best seen by referring now to FIG.  2 . Although only one trip unit  16  is shown within a single compartment  17  in case  11 , the trip units within all three poles are calibrated and tested in a similar manner. A heater  22  connects with load strap  14  and is attached to bimetal  19  to insure good thermal relation between the heater and the bimetal for test and operational purposes. As indicated, the bottom of the bimetal and heater are separated from the case by means of an insulative plate  21 . The crossbar  15  is pivotally arranged within the case by means of a crossbar pivot  31  to provide rotation in the clockwise indicated direction upon contact between the top of bimetal  19  and a calibration screw  30  extending within a threaded aperture  29  formed within the top lever  20  of crossbar  15 . Bottom levers  23 ,  24  on the crossbar, interact with the operating mechanism to release the circuit breaker operating springs (not shown) and separate the contacts (shown in FIG. 3) in the manner described in the aforementioned U.S. patent application Ser. No. 09/087,038, entitled Rotary Contact Assembly for High Ampere-Rated Circuit Breakers. 
     Precise calibration of the bimetal for so-called “long time” circuit interruption is achieved in the following manner. In the latter stages of the manufacturing process, when the circuit breaker operating components are fixed within the circuit breaker case  11 , the bimetal  19  can be calibrated with high tolerance. A servo-motor  25 , e.g., a type GRMTR servo-motor obtained from Pitman Co., is operatively connected with the slotted end  28  of the calibration screw  30  by means of the driver arm  26 . The servo-motor is programmed in feed-back relation with bimetal  19  by virtue of the electrical signal transfer between the bimetal and the calibration screw for extremely fast servo-motor response. 
     In the first stage of calibration, heater  22  is supplied with 200 to 400 percent rated circuit breaker current for a predetermined period of time causing the bimetal to deflect to the position indicated in phantom. After the time period has expired, the current is reduced to 100 percent rated circuit breaker current to allow bimetal  19  to remain in the deflected position. At the same time, the servo-motor driver arm  26  begins rotation of the calibration screw and continues such rotation until the end of the calibration screw  30  contacts the bimetal, at which time an electric circuit with the servo-motor controller is completed causing the servo-motor to immediately cease rotation of the driver arm allowing the calibration screw to remain fixed within the threaded aperture  29  formed within the top lever  20  of the crossbar  15 . The first stage of calibration is performed within the remaining poles to insure exact positioning of the calibration screw relative to the bimetal within each pole. 
     In the second stage of calibration, the a test current of 300 percent circuit breaker rated current is applied to heater  22  in each separate pole and the time for bimetal  19  to deflect into contact with the calibration screw  30  is electrically determined. If the time taken for contact between the calibration screw  30  and bimetal  19  is less than the predetermined time, the servo-motor rotates the driver arm and the calibration screw in the counterclockwise direction to compensate for the required time increment. It is believed that the use of electric circuit between the bimetal, calibration screw and the servo-motor controller represents a substantial improvement over earlier techniques that include the combination of lasers, pyrometers and the like to determine temperature in the calibration process. 
     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.