Patent Publication Number: US-5027096-A

Title: Key blocks for circuit breaker

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of application Ser. No. 256,878, filed on Oct. 12, 1988 now abandoned. 
     The invention disclosed herein relates to molded case circuit breakers. The following seven patent applications all relate to molded case circuit breakers and were filed on Aug. 1, 1988: Ser. No. 226,500, entitled RUBBER STOPS IN OUTSIDE POLES, William E. Beatty, Jr., Lawrence J. Kapples, Lance Gula and Joseph F. Changle, Westinghouse Case No. WE-54,532; Ser. No. 226,648, entitled CT QUICK CHANGE ASSEMBLY, by Jere L. McKee, William E. Beatty, Jr. and Glenn R. Thomas, Westinghouse Case No. WE-54,533; Ser. No. 226,503, entitled CROSS-BAR ASSEMBLY, by Jere L. McKee, Lance Gula, and Glenn R. Thomas, Westinghouse Case No. WE-54,579; Ser. No. 226,649, entitled LAMINATED COPPER ASSEMBLY, by Charles R. Paton, Westinghouse Case No. WE-54,580; Ser. No. 226,650, entitled CAM ROLL PIN ASSEMBLY, by Lance Gula and Jere L. McKee, Westinghouse Case No. WE-54,594; Ser. No. 226,655, entitled COMBINATION BARRIER AND AUXILIARY CT BOARD by Gregg Nissly, Allen B. Shimp and Lance Gula, Westinghouse Case No. WE-54,821; Ser. No. 226,654, entitled MODULAR OPTION DECK ASSEMBLY by Andrew J. Male, Westinghouse Case No. WE-54,822. 
     The following four commonly assigned U.S. patent applications were filed on Oct. 12, 1988 and all relate to molded case circuit breakers: Ser. No. 256,881 entitled SCREW ADJUSTABLE CLINCH JOINT WITH BOSSES, by James N. Altenhof, Ronald W. Crookston, Walter V. Bratkowski, and J. Warren Barkell, Westinghouse Case No. WE-54,694; Ser. No. 256,879 entitled TAPERED STATIONARY CONTACT LINE COPPER, by Ronald W. Crookston, Westinghouse Case No. WE-54,695; Ser. No. 256,880, entitled SIDE PLATE TAPERED TWIST-TAB FASTENING DEVICE FOR FASTENING SIDE PLATES TO THE BASE, by K. Livesey and Alfred E. Maier, Westinghouse Case No. WE-54,715; Ser. No. 256,878, entitled TWO-PIECE CRADLE LATCH FOR CIRCUIT BREAKER, by Alfred E. Maier and William G. Eberts, Westinghouse Case No. WE-54,870. 
     The following commonly assigned U.S. patent applications also relate to molded case circuit breakers: Ser. No. 260,848, filed on Oct. 21, 1988 entitled UNRIVETED UPPER LINK SECUREMENT, by Joseph Changle and Lance Gula, Westinghouse Case No. WE-54,713I; Ser. No. 331,769, filed on Apr. 3, 1989 entitled ARC RUNNER, CONTAINMENT SUPPORT ASSEMBLY by Charles Paton, Kurt Grunert and Glen Sisson, Westinghouse Case No. WE-55,102; Ser. No. 331,920, filed on Mar. 31, 1989 entitled EXTENDER SPRING FOR INCREASED MAGNETIC TRIP SETTINGS, by Kurt Grunert, Westinghouse Case No. WE-55,015. 
     Lastly, the following patent application is being filed on even date herewith: Ser. No. 343,037, entitled TWO PIECE CRADLE LATCH, KEY BLOCKS AND SLOT MOTOR FOR CIRCUIT BREAKER, by Alfred E. Maier, William G. Eberts and Richard E. White, Westinghouse Case No. WE-54,870-I-1. White, William G. Eberts and Alfred E. Maier, Westinghouse Case No. WE-54,870-I-2. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to molded case circuit breakers and more particularly to a two piece cradle latch having a non-heat-treated portion and a heat-treated portion defining latch and reset surfaces, securely fastened to the non-heat-treated cradle portion and to supports for bracing the sidewalls of the molded base against forces resulting from relatively high overcurrent conditions, such as a short circuit condition. In an alternate embodiment, slot motors are incorporated into the supports. 
     2. Description of the Prior Art 
     Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit condition. An overload condition is normally about 200-300 percent of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000 percent or more of the nominal current rating of the circuit breaker. 
     Molded case circuit breakers include at least one pair of separable main contacts which may be operated either manually by way of a handle, disposed on the outside of the case, or automatically in response to an overcurrent condition. In the automatic mode of operation, the main contacts may be opened by an operating mechanism, controlled by an electronic trip unit, or by magnetic repulsion forces generated between the stationary and movable contacts during relatively high levels of overcurrent. 
     In one automatic mode of operation, the contact assemblies for all poles are tripped together by an electronic trip unit and a mechanical operating mechanism. More particularly, the electronic trip unit is provided with current sensors to sense an overcurrent condition. When an overcurrent condition is sensed, the current transformers provide a signal to the electronic circuitry within the electronic trip unit to actuate the operating mechanism to cause the main contacts to be separated. 
     In the other automatic mode or operation, the contact arm assemblies are disengaged from the mechanical operating mechanism and are blown open by magnetic repulsion forces. More particularly, magnetic repulsion members or shunts are used to allow the contact arm, which carries the movable main contact, to pivot. Each magnetic repulsion member is generally V-shaped defining two legs. During relatively high level overcurrent conditions, magnetic repulsion forces are generated between the legs of the magnetic repulsion member as a result of current flowing through the legs in opposite directions. At a relatively high level overcurrent condition, these magnetic repulsion forces cause the contact arm carrying the movable main contact to be blown open. 
     During a blow open condition, each contact arm is operated independently of the mechanical operating mechanism. For example, for a three phase circuit breaker having a high level overcurrent on the A phase; only the A phase contact arm will be blown open by its respective repulsion member. The contact arms for the B and C phases would remain closed and thus are unaffected by the operation of the A phase. The contact arms for the B and C phases are tripped by the electronic trip unit and the operating mechanism. This is done to prevent a condition known as single phasing, which can occur for circuit breakers connected to rotational loads, such as motors. In such a situation, unless all phases are tripped, the motor may act as a generator and contribute to the overcurrent condition. 
     The circuit breaker includes a cradle having latch and reset surfaces for latching and resetting the operating mechanism. Due to the wear on the latch and reset surfaces, these surfaces are often heat-treated. However, due to the complicated shape of the cradle having bends in many different directions, heat-treating can cause the cradle to become brittle and distort. 
     In a multi-phase molded case circuit breaker, each pole is compartmentalized in the base by way of sidewalls. These sidewalls are used to segregate the poles from one another and also for carrying the operating handle and the operating mechanism. In particular, in a three phase circuit breaker, two internal sidewalls are provided. The internal sidewalls along with the exterior walls of the base are used to form three compartments; two outside compartments and a center compartment. The outside compartments are formed from an exterior wall of the base and one interior sidewall. The center compartment is formed from the two interior sidewalls. 
     During a relatively high overcurrent condition, such as a short circuit condition, relatively large magnetic repulsion forces are developed. These magnetic repulsion forces are developed between various electrical current carrying components of the circuit breaker. Since such current carrying components are carried by the base of the circuit breaker and the sidewalls, the force between the current carrying components is transmitted to the sidewalls and the cover, attached to the base. Such forces can damage the sidewalls. 
     In known molded case circuit breakers, separate components are generally provided for each of the various design objectives which can make the cost of manufacturing the circuit breaker relatively more expensive. For example, each compartment in the base is generally provided with a barrier for preventing arc products resulting from the separation of the main contacts from attacking other components in the circuit breaker. In some known molded case circuit breakers, these barriers are generally integrally molded with the base. The barriers are disposed on opposing sidewalls of each compartment and spaced apart to allow free movement of the pivotally mounted contact arm. 
     The pivotally mounted contact arm which carries the movable main contact is generally insulated. The insulation keeps the arc resulting from the separation of the main contacts concentrated at the contacts, as opposed to the contact arm. Generally, the insulator is attached with an adhesive to the contact arm. However, this requires an extra step in the assembly of the circuit breaker, thus increasing the cost. 
     An arc chute is disposed adjacent the separable main contacts for dispersing the arc resulting from contact separation. One arc chute is provided for each pole and is generally captured in the compartment between opposing sidewalls. In some known applications, the sidewalls are formed with barriers for locating the arc chute to prevent the arc from moving along the longitudinal axis of the circuit breaker. However, molding of these barriers may interfere with placement of the line conductor in the base. 
     Some circuit breakers are provided with slot motors. The slot motors assist the separable main contacts in blowing open. Known slot motors consist either of a series of generally U-shaped steel laminations encased in electrical insulation or of a generally U-shaped, electrically insulated solid bar which may be disposed adjacent the separable main contacts. Slot motors concentrate the magnetic field generated during a relatively high level overcurrent condition to increase the magnetic repulsion forces and assist the contact arms in blowing open. Such slot motors can be relatively expensive. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a cradle with heat-treated latch and reset surfaces which overcomes the problems associated with the prior art. 
     It is a further object of the present invention to provide a cradle with heat-treated latch and reset surfaces that is not brittle or distorted. 
     It is another object of the present invention to provide a support for bracing the sidewalls of the molded case against forces resulting from relatively high overcurrent conditions, such as a short circuit condition. 
     It is a further object of the present invention to provide a barrier adjacent the area of contact separation of the main contacts to prevent the arc products from attacking various components of the circuit breaker. 
     It is a further object of the present invention to provide means for capturing the arc chutes to prevent their movement along the longitudinal axis of the circuit breaker. 
     It is yet a further object of the present invention to provide means for holding an insulation barrier in place on the lower contact arm without the use of adhesives. 
     It is another object of the present invention to provide a slot motor which resolves the problems associated with known slot motors. 
     It is a further object of the present invention to provide a slot motor, relatively less expensive than known slot motors. 
     It is yet a further object to provide means for accomplishing multiple design objectives. 
     Briefly, the present invention relates to a two piece cradle assembly having a cradle portion and a heat-treated portion. The heat-treated portion forms latch and reset surfaces. The cradle portion is integrally formed from a pair of spaced apart cradle-shaped arms joined together by a connecting portion. The heat treated portion is securely fastened to the connecting portion to form the two piece cradle assembly. Key blocks or elongated molded blocks are used to support the sidewalls. These key blocks are formed with a key shaped projection along one side, adapted to be received in key shaped grooves molded in the sidewalls of a circuit breaker base. The key shaped projections and the key shaped grooves may be formed as a dovetail connection. These key blocks may be secured to either a line conductor or the base and extend substantially to the tops of the sidewalls. The key blocks provide additional support for the sidewalls from forces resulting from relatively high overcurrent conditions, such as a short circuit condition. In addition to supporting the sidewalls, the key blocks form a barrier between the area of contact separation of the main contacts and other components to reduce the amount of arc products allowed to interact with other components in the circuit breaker. The key blocks also form a stop surface for capturing the arc chutes to prevent movement of the arc chutes along the longitudinal axis of the circuit breaker. Lastly, the key blocks hold the insulation barrier on the line conductor in place, thus obviating the need for adhesives. In an alternate embodiment of the invention, a slot motor is incorporated into the key blocks. More particularly, the key blocks are provided with a longitudinal bore. This bore is adapted to receive a metal screw, used to secure the key blocks to a metal plate molded in the base of the circuit breaker. The metal screws and the metal plate are insulated from the line conductor. The metal screws as well as the embedded metal plate form the slot motor. Counterbores may be provided in the key blocks for providing additional metallic material if necessary. 
    
    
     DESCRIPTION OF THE DRAWING 
     These and other objects and advantages of the present invention will become readily apparent upon consideration of the following detailed description and attached drawing wherein: 
     FIG. 1 is a top elevational view of the circuit breaker in accordance with the present invention; 
     FIG. 2 is a cross-sectional view taken substantially along line 2--2 of FIG. 1; 
     FIG. 3 is a plan sectional view taken along line 3--3 of FIG. 2; 
     FIG. 4 is an enlarged sectional view taken along line 4--4 of FIG. 2; 
     FIG. 5 is an exploded perspective view of some of the components of the circuit breaker in accordance with the present invention; 
     FIG. 6 is a plan elevation view of a line conductor in accordance with the present invention; 
     FIG. 7 is an enlarged cross-sectional view taken along line 7--7 of FIG. 6 with the contact arms shown in dot-dash lines; 
     FIG. 8 is a partial cross-sectional view taken along line 8--8 of FIG. 3; 
     FIG. 9 is an exploded perspective view of the side plates accordance with the present invention and some of the components associated therewith; 
     FIG. 10 is an enlarged cross-sectional view taken along line 10--10 of FIG. 9 showing the spin plate in accordance with the present invention; 
     FIG. 11 is a bottom elevation view taken along line 11--11 of FIG. 8; 
     FIG. 12 is an enlarged view of FIG. 8; 
     FIG. 13 is a cross-sectional view taken substantially along line 13--13 of FIG. 12; 
     FIG. 14 is similar to FIG. 12 but illustrates twist tabs in accordance with the present invention before twisting; 
     FIG. 15 is a perspective view of the cradle assembly in accordance with the present invention; 
     FIG. 16 is a cross-sectional view taken along line 16--16 of FIG. 15; 
     FIG. 17 is an exploded perspective view of the components of the cradle assembly; 
     FIG. 18 is a partial plan sectional view, similar to FIG. 3 illustrating the key blocks in accordance with the present invention; 
     FIG. 19 is a cross-sectional view taken along the line 19--19 of FIG. 18 extending the width of the entire circuit breaker; 
     FIG. 20 is a perspective view of a pair of key blocks in accordance with the present invention; 
     FIG. 21 is a perspective view of an alternate embodiment of the key blocks in accordance with the present invention; 
     FIG. 22 is a partial sectional view similar to FIG. 19, illustrating the slot motor in accordance with the present invention; and 
     FIG. 23 is similar to FIG. 22, illustrating an alternate embodiment of the slot motor in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     A molded case circuit breaker, generally indicated by the reference numeral 20, comprises an electrically insulated housing 22 having a molded base 24 and a molded coextensive cover 26, assembled at a parting line 28. The internal cavity of the molded base 24 is formed as a frame 30 for carrying the various components of the circuit breaker. As illustrated and described herein, a Westinghouse Series C, L-frame molded case circuit breaker will be described. However, it should be understood that the principles of the present invention are applicable to various types of molded case circuit breakers. 
     At least one pair of separable main contacts 32 are carried by the frame 30. More specifically, the pair of main contacts 32 include a rigidly mounted main contact 34 and a movably mounted main contact 36. The rigidly mounted main contact 34 is mounted to a line side conductor 37 having a line side terminal portion 38 at one end. The line side terminal portion 38 extends outwardly from the housing 22 to permit connection with an external electrical circuit. The line side conductor 37 is attached to the frame 30 with a plurality of fasteners 40. 
     The movable main contact 36 is carried by a contact arm 42. As will be discussed in more detail below, the contact arm 42 is pivotally connected to a load conductor assembly 44. The load conductor assembly 44 includes a pivot bracket 46, rigidly connected to a load conductor base 48. The load conductor base 48 is rigidly mounted to the frame 30 and electrically connected to a U-shaped load conductor 50. The U-shaped load conductor 50 forms a portion of an electronic trip unit 51. One end of the U-shaped conductor 50 is secured to the frame 30 and the load conductor base 48. The other end of the U-shaped conductor 50 is electrically connected to a load side terminal 53 to allow connection to an external electrical circuit. 
     The electronic trip unit 51 contains one or more internal current sensors for detecting current flowing through the main contacts 32. The electronic trip unit 51 also includes a latch mechanism 54. The latch mechanism 54 is interlocked with an operating mechanism 55 of the circuit breaker 20. Upon detection of an overcurrent condition, the electronic trip unit 51 operates the latch mechanism 54 to unlatch the circuit breaker operating mechanism 55 to allow the main contacts 32 to be separated. The electronic trip unit 51 also contains a pushbutton (not shown) which allows the circuit breaker 20 to be tripped by depressing the button. The electronic trip unit 51 does not form a part of the present invention. 
     OPERATING MECHANISM 
     An operating mechanism 55 is provided for opening and closing the main contacts 32. The operating mechanism includes a toggle assembly 56, which includes a pair of upper toggle links 58 and a pair of lower toggle links 60. Each upper toggle link 58 is pivotally connected at one end to a lower toggle link 60 about a pivot axis 62. The other end of the lower toggle links 60 is pivotally connected about a pivot axis 63 to a U-shaped bracket 61, having depending operating arms 64. More specifically, apertures 70, provided in the operating arms 64, receive a pin 72 forming a pivotal connection between the lower toggle links 60 and the operating arms 64 about the pivot axis 63. The U-shaped bracket 61 is rigidly connected to a crossbar 65. The operating arms 64 are disposed adjacent each side of the contact arms 42 and are pivotally connected to a pair of side plates 75, disposed adjacent each side of the center pole, about a pivot axis 74. The side plates 75, as will be discussed in detail below, are rigidly connected to the molded base 24. Thus, rotation of the crossbar 65 about the pivot axis 74 will cause the lower toggle links 60 to pivot about the pivot axis 63. 
     The operating arms 64 are provided with cam surfaces 76. These cam surfaces 76 allow for the mechanical coupling of the contact arms 42 to the operating mechanism 55. More specifically, each of the contact arms 42 are provided with a slot 78 for receiving a cam roller pin 80. The cam roller pin 80 extends outwardly from the sides of the contact arm 42. Cam rollers 82 are received on each end of the cam roller pin 80. The cam rollers 82 cooperate with the cam surfaces 76 to mechanically couple the contact arms 42 to the operating mechanism 55. In all conditions except a blown open condition, the cam rollers 82 are captured in a pocket 83 formed in the cam surfaces 76. In a blown open condition, the cam rollers 82 are displaced out of the pockets 83 by the magnetic repulsion forces to uncouple the operating mechanism 55 from the contact arm assembly 42. This allows the contact arms 42 to open independently of the operating mechanism 55 as a result of magnetic repulsion forces. Biasing springs 84, coupled between the cam roller pin 80 and the pivot axis 74, provide contact pressure which must be overcome by the magnetic repulsion forces in order to allow the contact arm 42 to be blown open. More specifically, in the closed condition, since the cam rollers 82 are not quite seated in the pockets 83, but rather, are located slightly adjacent and upward of the pocket 83, the contact arm 42 is urged in a counterclockwise direction (FIG. 2) by the biasing springs 84, which produces a contact pressure between the main contacts 32. 
     The upper toggle links 58 are pivotally connected to a cradle assembly 86 about a pivot axis 88. More specifically, the upper toggle links 58 are provided with a U-shaped notch 89 at one end. A pivot pin 90, is supported by the cradle assembly 86. The pivot pin 90 is captured by the U-shaped notch 89 to define a pivotal connection about the pivot axis 88. The cradle assembly 86 is pivotally connected to the side plates 75 about a pivot axis 97. 
     The cradle assembly 86, which will be discussed in more detail below, is provided with a latch surface 92. The latch surface 92 cooperates with the latch mechanism 54 on the electronic trip unit 51. More particularly, when the latch surface 92 is latched, operating springs 93, connected between the pivot axis 62 and operating handle arm 94, bias the operating mechanism 55 to cause the upper toggle links 58 and the lower toggle links 60 to be disposed colinearly with respect to each other when the main contacts 32 are closed. In response to an overcurrent condition, the latch mechanism 54 on the electronic trip unit 51 releases the latch surface 92 provided on the cradle assembly 86. The operating springs 93 then cause the cradle assembly 86 to rotate in a counterclockwise direction (FIG. 2) about the pivot axis 97 which causes the toggle assembly 56 to collapse. This causes the operating arms 64 and the attached crossbar 65 to rotate in a clockwise direction, thereby rotating the contact arms 42 and separating the main contacts 32, if the cam rollers 82 are captured in the pockets 83 in the cam surface 76. 
     The circuit breaker 20 can also be manually turned off by rotating an insulated operating handle 95, mechanically coupled to the handle arm 94, in a clockwise direction to the open position. This causes the toggle assembly 56 to collapse, which allows the contact arm 42 to rotate upwardly under the influence of the operating springs 93. 
     The handle arm 94 is formed as a U-shaped member having two depending arms 98. The free ends 102 of the depending arms 98 are provided with notches 104 for capturing a pivot pin 106. The pivot pin 106 is carried by V-shaped notches 107 provided in the side plates 75. In the closed and tripped positions of the circuit breaker 20, the pivot pin 106 is captured in a pocket 109 defined by the V-shaped notch 107. In the open position, the pivot pin 106 is disposed adjacent the pocket 109. In this condition the toggle assembly 56 is collapsed. More specifically, the lower toggle links 60 are disposed clockwise relative to their position in a closed or an open position. Similarly, the upper toggle links 58 are disposed counterclockwise relative to their position in closed or on position. 
     Once the latch surface 92 on the cradle assembly 86 has been disengaged from the latch mechanism 54 on the electronic trip unit 51, it is necessary to reset the operating mechanism 55. This is accomplished by rotating the operating handle 95 in a clockwise direction until the latch surface 92 on the cradle assembly 86 engages the latch mechanism 54 on the electronic trip unit 51. 
     A reset pin 108, carried by the operating handle 95, is captured in notches 110, provided in the upper portion of the depending arms 98 of the U-shaped handle arm 94 when the insulated handle 95 is rotated clockwise. The reset pin 108, in turn, engages a reset surface 114 provided on the cradle assembly 86. Further rotation of the operating handle 95 causes the cradle assembly 86 to rotate clockwise until the latch surface 92 on the cradle assembly 86 engages and latches the latch mechanism 54 on the electronic trip unit 51. 
     SCREW ADJUSTABLE CLINCH JOINT WITH BOSSES 
     An important aspect of the invention relates to the pivotally mounted contact arm 42 formed as a clinch joint. The clinch joint defines the pivotal connection between the contact arm 42 and the load conductor assembly 44. The pivotal connection eliminates the need for woven copper wire or laminated shunt assemblies used in known circuit breakers. 
     A critical aspect of the invention relates to the ability to control the contacting surfaces between the contact arm 42 and the pivot bracket 46 in order to control the friction and the electrical resistance of these surfaces. These two factors need to be controlled because of their effect on the performance of the circuit breaker 20. More specifically, the electrical resistance has to be controlled to control the current flow through the assembly. Also, the friction between the contacting surfaces has to be controlled since an excessive amount of friction could slow down the opening of the main contacts 32. 
     The contact arm 42 is a bifurcated assembly formed from two coextensive irregular shaped arms 115, joined together at one end 116. The other end 118 of the arms 115 is bent outwardly forming spaced apart arm portions 119. The spaced apart arm portions 119 receive the pivot bracket 46. Aligned apertures 122 in the arms 115 are aligned with an aperture 124 in the pivot bracket 46. A pivot pin 125, received in the apertures 122 and 124, provides a pivotal connection between the contact arm 42 and the pivot bracket 46 about the pivot axis 74. The pivot bracket 46 is electrically connected to the load conductor base 48. 
     In order to control the contact surfaces between inner surfaces 128 of the contact arm 42 and the pivot bracket 46, bosses 130 are provided on the pivot bracket 46, concentric with the aperture 124. These bosses 130 are provided on each side of the pivot bracket 46 and extend outwardly therefrom. The bosses 130 may be coated with silver to provide a relatively smooth contacting surface. These bosses 130 provide a relatively uniform contact surface between the pivot bracket 46 and the inner surfaces 128 of the contact arm 42 in order to allow the friction and the electrical resistance of the joint to be controlled. 
     Aligned apertures 132, provided in the spaced apart arm portions 119, receive a clinch screw 134. Wave washers 136 are disposed about a shank portion of the clinch screw 134 at one end. The clinch screw 134 is secured at the end opposite a head portion by a nut or other fastener causing the wave washers 136 to be captured between the head portion of the clinch screw 134 and an outer surface 137 of the contact arm 42. The clinch screw 134 and the wave washers 136 allow the friction between the inner surfaces 128 of the contact arm 42 and the bosses 130 to be controlled. 
     Slots 78 are provided in the spaced apart arm portions 119 of the contact arm 42 to receive the cam roller pin 80 as discussed above. The biasing springs 84, connected between the cam roller pin 80 and the pivot pin 74, bias the cam roller pin 80 within the slot 78. 
     The above assembly allows the current from the contact arm 42 to be transferred from the contact arm 42 to the bosses 130 and into the load side conductor base 48 by way of the pivot bracket 46 without the use of laminated or woven copper wire shunts. 
     TAPERED STATIONARY CONTACT LINE COPPER 
     Another important aspect of the invention relates to a line side conductor 37 which carries the rigidly mounted main contact 34. More specifically, the line side conductor 37 is provided as a generally rectangular shaped member having a generally U-shaped slot 138 defining two conducting leg portions 144 and 146 and a peninsula portion 148 having two oppositely disposed edges 149 and 150. The edges 149 and 150 of the peninsula portion 148 are tapered outwardly toward the base 151 of the peninsula portion 148 to provide for a larger cross-sectional area of the conductor to provide better current density and heat dissipation. The tapered edges 149 and 150 also allow the cross-sectional area of the peninsula portion 148 to be made substantially equivalent to the cross-sectional area of the conducting leg portions 144 and 146. 
     The U-shaped slot 138 in the line side conductor 37 is for receiving a slot motor (not shown) and also to form a portion of the magnetic repulsion loop to allow the main contacts 32 to be blown open during relatively high level overcurrent conditions. In known devices, the opposing edges of the peninsula portion are not tapered. This can result in undesirable temperature increase of line side conductor because of the decrease in the overall cross-sectional area. This undesirable heat must be dissipated by other means, such as by providing a larger size conductor. By utilizing a line side conductor configuration as in the present invention, the overall cross-sectional area of the conductor is increased which results in better current density and heat dissipation without utilizing a relatively larger size line side conductor. 
     As discussed above, one of the functions of the U-shaped slot 138 is to form a magnetic repulsion loop. This is accomplished by causing the current in the line conductor 37 to flow in a direction opposite to the direction of current flow in the contact arm 42. More specifically, the line side conductor 37 contains an electrical terminal portion 38 to allow connection between an external electrical circuit and the rigidly mounted main contact 34. The current applied to the line side terminal portion 38 flows in the direction of the arrows shown in FIG. 6. This current is divided up between conducting leg portions 144 and 146 as shown in FIG. 6. This current in the leg portions 144 and 146 flows together in the peninsula portion 148 in a direction opposite that in the conducting leg portions 144 and 146. As best shown in FIG. 2, the current which flows through the movable main contact 36 in the contact arm 42 is in an opposite direction relative to the direction of current flow in the peninsula portion 148. Thus, during relatively high level overcurrent conditions, the opposing currents develop magnetic repulsion forces which cause the main contacts 32 to be blown open by causing the contact arm 42 to be rotated in a clockwise direction. 
     The other function of the U-shaped slot 138 is to receive a slot motor. The slot motor assists the contacts 32 blowing open. More particularly, the slot motor, consisting either of a series of generally U-shaped steel laminations encased in electrical insulation or of a generally U-shaped, electrically insulated solid bar, is received in the U-shaped slot 138, adjacent the main contacts 32. The slot motor concentrates the magnetic field generated during a relatively high level overcurrent condition to increase the magnetic repulsion forces between the peninsula portion 148 and the contact arm 42. This rapidly accelerates the separation of the main contacts 32 which results in a relatively high arc resistance which limits the magnitude of the fault current. 
     The rigidly mounted main contact 34 is securely fastened to the peninsula portion 148. An arc runner 158 is disposed adjacent the main contact 34 to allow the arc to travel into arc chutes 160. The arc chutes 160 are used to divide a single electrical arc, formed as a result of the separating main contacts 32, into a series of electrical arcs thereby increasing the total arc voltage which results in a limiting of the magnitude of the fault current. 
     Another important aspect of the line side conductor 37 relates to the means for providing adequate electrical separation between the line side conductor 37 and the contact arm 42 when the main contacts 32 are separated. More specifically, one side 162 of the line side conductor 37 is tapered downwardly. This is done to provide more separation between the line side conductor 37 and the contact arm 42 when the main contacts 32 are separated since these two points are at different potentials. 
     SIDE PLATE TAPERED TWIST TAB FASTENING DEVICE FOR FASTENING SIDE PLATES TO THE BASE 
     Another important aspect of the invention relates to the means for fastening the side plates 75 to the molded base 24. The side plates 75 are used to support a portion of the operating assembly 55 of the circuit breaker 20. More specifically, these side plates 75 are disposed adjacent the center pole and are used to provide various functions. For example, aligned apertures 164 in the side plates 75 define the pivot axis 74 for the crossbar 65. Another pair of aligned apertures 166 define the pivot axis 97 for the cradle assembly 86. Another set of aligned apertures 168 receive a stop pin 170 to limit counterclockwise rotation of the cradle assembly 86 during tripping of the contacts. A V-shaped notch 107 in the side plates 75 captures the pivot pin 106 for the handle arm 94. Lastly, an irregular slot 172 allows the crossbar 65 to rotate about the pivot axis 74. 
     In known circuit breakers, the side plates 75 are connected to the molded base 24 by various means, such as tabs extending downwardly from the bottom edge with threaded ends, spun over ends or staked ends, received in apertures or load bearing plates in the molded base 24. 
     In other known circuit breakers, downwardly extending twist tabs are provided having straight shank portions and enlarged head portions. These twist tabs are received by slots disposed in spin plates carried in the underside of the base. The twist tabs are twisted to secure the side plates to the base. In this design, it is necessary to control the length of the shank portions of the twist tabs relatively closely in order to avoid play in the side plates 75 after the twist tabs are twisted, which may affect the operation of the operating mechanism. 
     The twist tabs 174, provided in accordance with the present invention, extend downwardly from the bottom edge of the side plate 75 and are formed with shank portions 176, a tapered portion defining a sloped surface 178 and a head portion 180. The twist tabs 174 are received in slots 182, provided in a generally rectangular spin plate 184, carried in a cavity 185 formed in the underside of the molded base 24. Once the twist tabs 174 are twisted, the spin plate 184 is captured in the molded base 24. 
     The sloped surfaces 178 contact the slots 182 in the spin plates 184. As the twist tab 174 is twisted, the shank portion 176 becomes shorter thereby drawing a wider portion of the sloped surface 178 into engagement with the slot 182 to provide a secure connection between the side plates 75 and the molded base 24. 
     Since the spin plates 184 are stamped, they are configured to be received in the cavity 185 in the underside of the molded base 24 such that any rough edges on the break side resulting from the stamping process are not in engagement with the sloped surfaces 178. More particularly, as a result of the stamping process one side of the spin plate 184 is relatively smooth while the break side of the spin plate 184 may contain burrs. In order to prevent improper orientation of the break side with respect to the molded base 24, the spin plate 184 is keyed so that it can only be received such that the break side contacts the underside of the molded base 24. This is accomplished by providing means for indexing the spin plate 184. The indexing means include extending finger portions 186 disposed generally parallel to each other on diametrically opposite corners 188 of the spin plate 184. 
     TWO PIECE CRADLE LATCH FOR CIRCUIT BREAKER 
     Another important aspect of the present invention relates to the two piece cradle assembly 86 comprising a U-shaped cradle portion 190 and an L-shaped heat treated portion 192. The heat treated portion 192 includes a latch surface 92 and a reset surface 114. Because of the wear on these parts, they are generally heat treated. However, due to the complicated shape of cradle portion 190 having bends in many different directions, heat treating these portions can cause the cradle to become brittle and distort. Accordingly, the cradle assembly 86, provided in accordance with the present invention, is formed from a two piece assembly wherein only the wear surfaces, such as the latch surface 92 and the reset surface 114 are heat treated. The cradle portion 190 and the heat treated portion 192 may be fastened together with rivets 194 or other suitable fasteners to form the cradle assembly 86. 
     The cradle portion 190 is integrally formed from two spaced apart, parallel cradle shaped arms 196 joined together at one end by a connecting portion 198 disposed substantially perpendicular to the cradle-shaped arms 196. A first pair of aligned apertures 200 is provided in the cradle shaped arms 190 which define the pivot axis 90 for the cradle assembly 86 with respect to the side plates 75. A second pair of aligned apertures 202, provided in the cradle shaped arms 196, define the pivot axis 97 between the upper toggle links 58 and the side plates 75. 
     The connecting portion 198 joins the cradle shaped arms 196 together. Apertures 203 are provided in the connecting portion 198 for receiving the rivets 194 to allow the heat treated portion 192 to be fastened thereto. The attachment of the heat treated portion 192 to the connecting portion 198 also serves to reinforce the connecting portion 198. 
     The heat treated portion is an integrally formed piece which defines the latch surfaces 92 and the reset surface 114. Because the heat treated portion is not as complicated as the cradle portion 190 and does not contain as many bends in different directions, it is less likely to distort as a result of the heat treating. 
     Another important aspect of this invention is that the heat treated portion 192 is formed such that the engaging portions of the latch surface 92 and the reset surface 114 are flat, smooth surfaces to distribute the load. The use of the flat, smooth surfaces also reduces the friction between the components. 
     KEY BLOCKS 
     Another important aspect of the invention relates to means for bracing the sidewalls of the base 24 against forces resulting from a relatively high overcurrent condition, such as short circuit condition. More particularly, key blocks or elongated support blocks 300 are disposed adjacent each interior sidewall 301 such that the longitudinal axis 299 of the key block 300 is generally parallel to the plane of the sidewalls 301. The key blocks 300 are formed with key shaped projections 302 along one side 303, adapted to be received in key shaped slots 304, disposed generally perpendicular to the base 24, integrally formed in the sidewalls 301. As shown, both the key shaped projections 302 and the key shaped slots 304 extend substantially the entire length of the key block 300. However, the key blocks 300 may also be formed with key shaped projections 302 having lengths more or less than the length of the key blocks 300. Similarly, the key shaped slots 304 may be longer or shorter than the length of the key blocks 300. Also, various combinations of key shaped projections 302 and key shaped slots 304 are contemplated to be within the scope of the present invention. Thus, although a dovetail connection is shown, the scope of the invention is not intended to be limited to a particular type of connection. 
     Once the key shaped projections 302 are received in the key shaped slots 304, the bottom surface 305 of each key block 300 rests upon an insulation barrier 320, disposed on top of the line conductor 37. The top surface 306 of the key blocks 300 is relatively flush with the top surface 307 of the sidewalls 301. However, it is also contemplated that the top surface 306 of the key blocks 300 may also extend above the top surface 307 to provide support for the cover 26. 
     The key block 300 is provided with a longitudinal bore 308. This bore 308 allows the key block 300 to be fastened to the line conductor 37 by way of a fastener 309, received into threaded apertures 311 in the line conductor 37. The key block 300 may also be fastened to the base 24. It is also understood that key blocks 300 can be fastened from either the top or the bottom. 
     In multi-phase molded case circuit breakers 20, internal sidewalls 301 are utilized to compartmentalize each pole. Thus, for a three phase molded case circuit breaker, the molded base 24 will be divided into three longitudinal phase compartments 310. The line side portion 312 of the phase compartment 310 contains the separable main contacts 32 and an arc chute 160. For outside poles, the compartments 310 are from an exterior wall 313 of the molded base 24 and an internal sidewall 301. For the center pole, the compartment 310 is formed with two interior sidewalls 301. 
     The key blocks 300 may also be used to hold the arc chutes 160 in place. This is done by locating the key blocks 300 such that the arc chutes 160 are firmly held against a front wall 316 of the phase compartment 310. The front side 318 of the key blocks 300 act as a stop surface for the arc chute 160 to prevent the arc chute 160 from moving with respect to the longitudinal axis 314 of the circuit breaker 20. 
     The key blocks 300 are disposed adjacent walls in each phase compartment 310. The key blocks 300 are generally disposed between the separable main contacts 32 and the operating mechanisms 55. The gap defined between the key blocks 300 in each phase compartment is sufficient to allow free movement of the pivotally mounted contact arm 42. 
     An insulation barrier 320 is normally provided on the top side of the line conductor 37. The insulation barrier 320 is provided to contain the arcing resulting from the separation of the main contacts 34, 36. The insulation barrier 320 is held in place by the key block 300 without the use of an adhesive since the key blocks 300 are either secured to the line conductor 37 or the base 24. 
     The key blocks 300 also act as barriers to reduce the amount of arc interruption products entering a load portion 322 of the phase compartment 310. The key blocks 300 thus obviate the need for separate barriers. 
     In an alternate embodiment, illustrated in FIG. 21, a support bridge 324 is provided for applications where the sidewalls 301 and exterior walls 313 are subject to relatively large forces resulting from an overcurrent condition of considerable magnitude. In this embodiment, the support bridge 324 is attached between two spaced apart key block portions 326, substantially similar to the key blocks 300. The key block portions 326 are joined together at the top by a bridge portion 330 to form a C-shaped member. As such, additional support against transverse movement of the sidewalls is provided. The support bridge 324 may also act as a stop surface for the pivotally mounted contact arm 42. 
     The key block portions 326 and the bridge portion 330 may be either integrally molded or formed from individual members and secured together with various fasteners, adhesives or the like, or they may be formed with interlocking surfaces to form, for example, a dovetail connection. 
     SLOT MOTORS 
     In another alternate embodiment of the invention, a slot motor 360 is incorporated into the key blocks 300. In this embodiment, the slot motor 360 is different than the one heretofore described. A metal plate 350, such as a steel plate, is embedded in the molded base 24. This metal plate 350 is disposed in a pocket 352 formed in the base 24 adjacent the sidewalls 301. Since each of the phases operate independently, it should be understood that the metal plates 350 for each phase are electrically insulated from each other. Apertures 354 are provided in the metal plate 350 for receiving metallic fasteners 356, inserted into the longitudinal bore 308 in the key blocks 300. The apertures 354 in the metallic plate 350 are aligned with apertures 355 in the line conductor 37 to receive the metallic fasteners 356. In order to insulate the line conductor 37 from the metallic plate 350 and the metallic fasteners 356, the apertures 355 in the line conductors 37 are provided with insulation 357, such as micarta. The end of the metallic fasteners 356 are provided with threads to receive nuts 358 such that the entire assembly 360 can be secured together. Such assembly thus secures the line conductors 37 to the base 24 obviating the need for fasteners 40 and the associated tapped holes in the line conductor 37. 
     The assembly 360 consisting of the metallic fasteners 356 and the metallic plate 350 form the slot motor. Such a slot motor 360 may be used in assisting the main contacts 32 in blowing open. More particularly, the slot motor 360 is used to concentrate the magnetic field generated during a relatively high level overcurrent condition to increase the magnetic repulsion forces between the main contacts 32. This rapidly accelerates the separation of the main contacts 32 which results in a relatively high arc resistance which limits the magnitude of the fault current. 
     In an alternate embodiment, counterbores 362 may be provided concentric with the longitudinal bores 308. These counterbores 362 may be used to define a pocket 364 for receiving additional metallic material 366 to enhance the characteristics of the slot motor assembly 360. 
     Obviously many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described hereinabove.