Abstract:
The invention relates to a re-settable, single-phase, thermo/electric circuit breaker utilizing a U-shape bimetallic element of substantial resistance properties in the circuit, so that when a predetermined overload current occurs the bimetallic element self heats and moves to trip a mechanism that opens the breaker circuit. The invention replaces the double-contact break configuration normally associated with a breaker of this type, with a single contact break configuration designed to produce a sliding action between contacts during the normal reset operation, providing a more reliable continuity at the moveable and stationary contact interface that is also less expensive to produce.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Use  
         [0002]     The invention comprises a resettable thermo/electric circuit breaker of a type utilizing a single-contact break supported by a U-shaped bimetallic thermal sensing element.  
         [0003]     2. Description of Related Art  
         [0004]     Circuit breakers utilizing a double-contact break supported by a U-shaped bimetallic thermal sensing unit are widely used in electric circuits to prevent current overload. Examples of this type of prior art circuit breaker, all of which are assigned to the assignee of this invention, include the Series 02 circuit breaker depicted in  FIG. 9  and inventions disclosed in U.S. Pat. Nos. 2,513,564; 2,514,545; and 2,689,895. Because of the relatively high contact resistance associated with the two pairs of contacts utilized in these designs, they are susceptible to intermittent loss of electrical continuity for applications involving low open circuit voltage. Each time the circuit breaker contacts are opened and reclosed, a new and unique contact interface is formed which can alter the contact resistance. Prior art designs use high contact force to elastically and plastically deform microscopic protrusions present on the contact surfaces. The resulting larger contacting area provides a lower contact resistance, but this is not always sufficient to preclude the occurrence of intermittent continuities within the circuit breaker.  
       SUMMARY OF THE INVENTION  
       [0005]     The invention relates to a novel U-shaped thermal sensing element to further alleviate causes of intermittent continuity during the application of low, open circuit voltage. The circuit breaker maintains the application of high contact force found in prior art designs, and incorporates a sliding contact motion applied during circuit breaker reset to mechanically break through poorly conducting oxide, sulfide and tungstate films normally associated with the contact surfaces. A further reduction in contact resistance is achieved by eliminating one of the contact pairs. This halves contact resistance within the circuit breaker and makes it more economical to produce.  
         [0006]     More specifically, the invention employs a single-contact break mechanism in place of the double-contact break mechanism associated with prior art U-shaped thermal sensing elements of this type. The elimination of one pair of contacts minimizes contact resistance, thereby reducing the occurrence of intermittent continuity conditions within the breaker. Additionally, upon normal reset of the breaker, the invention produces a sliding action between mating contacts, which also serves to minimize the occurrence of intermittent continuity conditions within the breaker. A further benefit of the invention is the cost savings realized by using a single pair of contacts in place of two.  
         [0007]     These and other objects and advantages residing in the construction, combination and arrangement of parts will be more fully understood from the following specifications and drawings. 
     
    
     DRAWINGS  
       [0008]      FIG. 1  is a perspective view of an electric circuit breaker in accordance with the preferred embodiment of the invention.  
         [0009]      FIG. 2  is a perspective view of the electric circuit breaker shown in  FIG. 1 , in the closed position, with one of the case halves removed.  
         [0010]      FIG. 3  is an exploded view of the parts comprising the circuit breaker latching mechanism of the invention.  
         [0011]      FIG. 4  is a partial, detail side elevation view of a cross section along the centerline of the breaker as indicted along lines  4 - 4  of  FIG. 2 .  
         [0012]      FIG. 5  is an elevation view of an electric circuit breaker in accordance with the invention, in the closed position, with one of the case halves removed.  
         [0013]      FIG. 6A  is a side elevation view of a cross-section along lines  6 - 6  through the breaker showing a latched breaker.  
         [0014]      FIG. 6B  is a side elevation view of a cross-section along lines  6 - 6  through the breaker showing a breaker just prior to tripping.  
         [0015]      FIG. 7  is an elevation view of an electric circuit breaker in accordance with the invention, in the open position, with one of the case halves removed.  
         [0016]      FIG. 8A  is an elevation view of an electric circuit breaker in accordance with the invention, with one of the case halves removed, depicting an interim reset position.  
         [0017]      FIG. 8B  is an elevation view of an electric circuit breaker in accordance with the invention, with one of the case halves removed, depicting an interim reset position.  
         [0018]      FIG. 8C  is an elevation view of an electric circuit breaker in accordance with the invention, in the closed position, with one of the case halves removed.  
         [0019]      FIG. 9  is an elevation view of a prior art electric circuit breaker in the closed position, with one of the case halves removed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]     The present invention applies to push-to-reset and switchable breaker configurations, but only the push-to-reset breaker configuration is illustrated herein. The illustrated form should be considered to be a typical application but is not meant to restrict or limit the teaching to just that kind of circuit breaker.  
         [0021]     In  FIG. 1 , the circuit breaker is shown composed of two similar case halves  20  and  21  made from molded insulating material. Rivets  22 ,  23 ,  24  and  25  permanently hold the assembled breaker together. The conductors  26  and  27  used to connect the breaker to an electric circuit extend outwardly from the bottom of the case halves. Extending outwardly from the top of the case halves is a mounting sleeve  28  which provides a means to restrain the breaker for use. Extending outwardly from the top of the mounting sleeve  28  is an actuator plunger  29 , which is used to reset a tripped breaker.  
         [0022]     The casing sections combine to form an enclosed separable contact chamber, half of which is indicated generally at  30  in  FIG. 2 . Mounted to the interior end of one conductor  27  is a fixed contact  31  and in which chamber there is also mounted a bimetallic thermal latch and movable contact unit, indicated generally at  32 , an actuator plunger  29  and a braided electrical conductor  33  that serves to electrically attach the conductor  26  to the bimetallic thermal latch and movable contact unit  32 . Conductors  26  and  27  are recessed in complementary channels in the case halves and serve to key the case halves together.  
         [0023]     Loosely mounted upon the actuator plunger  29  for both unitary and relative movement is the bimetallic thermal latch and movable contact unit  32 . This unit comprises two lateral arms, best seen in  FIG. 3 , one of which  34  carries a movable contact  35  in opposed relation to the fixed contact  31 , the other  36  is used to position the bimetallic thermal latch and movable contact unit  32  appropriately for open and closed circuit conditions; a three-tab set indicated generally as  37  serves to guide the bimetallic thermal latch and movable contact unit along the actuator plunger  29 , throughout the range of breaker motion. Spring-hook tabs  39  and  40  provide anchors for the upper end of springs  41  and  42 . Lateral ears  43  and  44  provide anchors for the lower ends of springs  41  and  42 .  
         [0024]      FIGS. 3 and 4  show bimetallic supports  45  and  46  of the bimetallic thermal latch and movable contact unit  32  projecting downwardly on opposite sides of the actuator plunger  29 . These supports have a radius on the bottom edge to allow for rotation of the bimetallic thermal latch and movable contact unit  32  during reset, and are normally stressed inwardly to provide a releasable latching engagement with the abutment ledge  47  of the catch  48 . The catch  48  is secured to the actuator plunger  29  by a rivet  49 . A tab  50  on the catch  48  extends into a plunger recess  51  where it rides on the conical tip of a calibration screw  52  threaded within the actuator plunger  29 . Calibration of the breaker is accomplished by adjusting the calibration screw  52  in or out, effectively raising or lowering the abutment ledge  47  of the catch  48  relative to the body of the actuator plunger  29 . The actuator plunger  29  provides a recess  53  for receiving the upper end of the spring  54 . The opposite end of the spring  54  rests on retainer  61 . Tabs on retainer  61  are initially open to allow access to the calibration screw  52 . After the circuit breaker is calibrated, tabs on retainer  61  are deformed to preclude access to the calibration screw  52 .  
         [0025]     In contrast, the bimetallic thermal latch and movable contact unit  55  depicted in the prior art, as seen in  FIG. 9 , varies significantly from the bimetallic thermal latch and movable contact unit  32  of the present invention in that it provides for a double rather than single contact-break and the radius on the downwardly projecting bimetallic supports is much larger, limiting the ability of the bimetallic thermal latch and movable contact unit  55  to self adjust for out of plane contact conditions.  
       OPERATION OF THE PREFERRED EMBODIMENT  
       [0026]     The operation of the invention is best understood by reference to  FIGS. 3 and 5  through  8 C. With the present invention breaker in the closed position, as seen in  FIG. 5 , electrical current passes through the conductor  26 , braided electrical conductor  33 , bimetallic thermal latch and movable contact unit  32 , movable contact  35 , fixed contact  31  and conductor  27 , all components connected in series. The bimetallic support  46 , of the bimetallic thermal latch and movable contact unit  32  is held stressed into engagement with the abutment ledge  47  of the catch  48  (best seen in  FIG. 6A ) by the action of a pair of springs  41  and  42  connected between the spring-hook tabs  39  and  40  and laterally protruding ears  43  and  44 . Stress between the movable contact  35  and fixed contact  31 , and lateral arm  36  and the fixed abutment surface  56  is provided by the action of spring  54 .  
         [0027]     Due to the substantial resistance properties of the bimetallic thermal latch and movable contact unit  32 , the bimetallic support  46  distorts laterally outwards from its position of contact with the abutment ledge  47  during the application of electrical current. The amount of lateral movement occurs at a predetermined rate dependant on the electrical current applied. When current greater than the ultimate trip value is applied distortion of the bimetallic support  46  will be sufficient to disengage it from the abutment ledge  47 .  FIG. 6B  depicts the distortion in bimetallic support  46  immediately prior to disengagement from abutment ledge  47 . As seen in  FIG. 7 , once disengaged, the bimetallic thermal latch and movable contact unit  32  immediately moves toward the inner end of the actuator plunger  29  due to pulling action of springs  41  and  42  causing the circuit to be broken suddenly at the contacts  31  and  35 . This movement of the bimetallic thermal latch and movable contact unit  32  will be arrested when lateral arms  34  and  36  come into contact with fixed abutment surfaces  57  and  58  formed within the split casing interior. With the opening of the contacts, the actuator plunger  29  will immediately snap outwards by the action of spring  54  and cause the trip indictor band  59  to be visibly positioned outside the mounting sleeve  28 .  
         [0028]     To reset the circuit breaker, the actuator plunger  29  is pushed inwards against the energy supplied by spring  54  to position the abutment ledge  47  with respect to bimetallic support  46 , it being appreciated that the bimetallic thermal latch and movable contact unit  32  is held positioned for this to take place due to the engagement of the lateral arms  34  and  36  with the abutment surfaces  57  and  58 .  FIG. 8A  depicts this interim position. Releasing the actuator plunger  29  allows the movable contact  35  to first, engage the fixed contact  31 , as seen in  FIG. 8B , then slide across the fixed contact  31  as the thermal unit  32  rotates until the lateral arm  36  is arrested by a fixed abutment surface  56  within the case, as seen in  FIG. 8C . The sliding of one contact over another during reset decreases contact resistance at the contact interface, making an intermittent continuity condition less likely to occur.  
         [0029]     In contrast, the prior art electrical circuit breaker shown in  FIG. 9  uses two sets of contacts, providing a double-contact break. Note the additional contact pair  60  used in the series circuit for the prior art circuit breaker, and the level, rather than canted, bimetallic thermal latch and movable contact unit  55 . The contact wiping movement associated with the present invention is not provided by the prior art design.  
         [0030]     The present invention has a number of advantages over the prior art. First, the lower contact resistance provided by the single contact pair configuration ensures it is less susceptible to intermittent loss of electrical continuity for applications involving low open circuit voltage than the double contact pair configuration employed by the prior art. Second, during normal reset of the circuit breaker, the invention creates a sliding action between mating contacts to further reduce contact resistance within the circuit breaker. Prior art designs have no such provision. Third, the invention eliminates one pair of contacts, making the circuit breaker more economical to produce.  
         [0031]     While the invention has been described with reference to a preferred embodiment, it will be appreciated by those of ordinary skill in the art that various modifications can be made to the structure and function of the invention and its parts without departing from the spirit and function of the invention as a whole.