Field Of The Invention

The present invention generally relates to circuit t makers, and more particularly, to a blade suspension assembly for a circuit breaker which provides improvements in terms of operation, ease of manufacturing and assembly, and reliability.

Background Of The Invention

Circuit breakers are commonly used for providing automatic circuit interruption upon detection of undesired overcurrent conditions on the circuit being monitored. These overcurrent conditions include, among others, overload conditions, ground faults and she t-circuit conditions.

Circuit breakers topically include an electrical contact on a movable arm which rotates away from a stationary contact in order to interrupt the current path. The type of overcurrent condition dictates how quickly the arm must rotate. For example, in response to overcurrent conditions at relatively low magnitudes but present for a long period of time, circuit breakers generally move the arm to break the current path by tripping a spring-biased latch mechanism which forces the contact on the arm away from the fixed contact. Spring-biased latch mechanisms are usually relatively slow. In response to overcurrent conditions at relatively high magnitudes, circuit breakers must break (or blow-open) the current path very quickly, reacting much faster than the reaction time for known spring-biased latch mechanisms. In either case, the contact arm must rotate to an open position as fast, as simply and as reliably as possible.

Circuit breaker designs attempting to achieve these objectives of quickness and reliability have failed. For

example, most circuit-breaker blade suspension mechanisms require complex manual assembly involving high part count, intricate positioning of one or more drive pins and one or more torsion springs for biasing movable arms, and their overall intricate assembly prohibits late point assembly adjustments, field adjustment and/or service. In addition, the complex design of most circuit-breaker blade suspension mechanisms is not conducive to straight-pull molding techniques during manufacturing. Many conventional circuit-breaker blade suspension mechanisms also exhibit problems in terms of their operation. These problems include slow contact arm rotation, the contact arm rebounding to the closed-contact position during interruption, breakage of the crossbar used to support the contact arm, and inconsistent contact force characteristics.

Generally, the speed and reliability at which the blade suspension mechanism breaks the current path is directly related to the complexity of the blade suspension mechanism, i.e., the faster the mechanism and the higher its reliability, the more complex the mechanism.

Accordingly, there is a need for a blade suspension assembly for a circuit breaker which overcomes the above- mentioned deficiencies of the prior art.

«-pmιη»τγ Of The Invention

The present invention provides a blade suspension assembly for a circuit breaker which affords improvements in terms of operation, ease of manufacturing and assembly, and reliability. In one particular embodiment, the blade suspension assembly comprises a pivot pin, a torsion spring, an elongated blade, and a blade carrier. The torsion spring includes a lateral middle section, a pair of end legs disposed on opposite sides of the middle section, and a lateral hole extending therethrough. The blade includes an electrical contact mounted thereto, a lower bearing

surface, and a lateral circular aperture. The blade carrier includes first and second pairs of bearing surfaces.

To assemble the blade suspension assembly, the torsion spring is placed over the blade with the lateral middle section abutting the lower bearing surface of the blade, with the end legs disposed on opposite sides of the blade, and with the lateral hole in the torsion spring disposed in line with the circular aperture in the blade. Next, the pivot pin is inserted through the lateral hole in the torsion spring and through the circular aperture in the blade. The combination of the blade, the torsion spring, and the pivot pin is then inserted into the blade carrier with the pair of end legs abutting respective ones of the first pair of bearing surfaces and opposite ends of the pivot pin abutting respective ones of the second pair of bearing surfaces. In accordance with the foregoing assembly, the torsion spring biases the blade toward a closed position with the electrical contact abutting an opposing stationary contact of the circuit breaker.

Brief Description Of The Drawinσa

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: FIG. 1 is a side view of a circuit breaker including a blade suspension assembly embodying the present invention;

FIG. 2 is a side view of a thermal trip unit of the circuit breaker in FIG. 1, shown in the untripped (or closed or ^M on ^M ) position; FIG. 3 is a side view of the thermal trip unit of the circuit breaker in FIG. 1, shown in the tripped position; FIG. 4 is a side view of a magnetic trip unit of the circuit breaker in FIG. 1, shown in the untripped position; FIG. 5 is a side view of the magnetic trip unit of the circuit breaker in FIG. 1, shown in the tripped position; FIG. 6 is a perspective view of the thermal and

magnetic trip units in FIGS. 2 through 5;

FIG. 7 is another perspective view of the thermal and magnetic trip units in FIGS. 2 through 5;

FIG. 8 is a side view of a blade/cradle assembly of the circuit breaker in FIG. 1, shown in the untripped position;

FIG. 9 is a perspective view of the blade/cradle assembly in FIG. 8, shown in the untripped position;

FIG. 10 is a side view of the blade/cradle assembly of the circuit breaker in FIG. 1, shown in the tripped position;

FIG. 11 is a perspective view of the blade/cradle assembly in FIG. 10, shown in the tripped position;

FIG. 12 is a side view of the blade/cradle assembly of the circuit breaker in FIG. 1, shown in the reset position; FIG. 13 is a side view of the blade/cradle assembly of the circuit breaker in FIG. 1, shown in the "off" position;

FIG. 14 is a partially exploded perspective view of the blade suspension assembly embodying the present invention;

FIG. 15 is a side view of the blade suspension assembly in FIG. 14, shown in the untripped position;

FIG. 16 is a side view of the blade suspension assembly in FIG. 14, shown in the tripped position;

FIG. 17 is a side view of the blade suspension assembly in FIG. 14, shown in the blown open position; FIG. 18 is a partially exploded perspective view of a field barrier assembly for straddling the blade of the blade suspension assembly in FIG. 14;

FIG. 19 is a top plan view of a base of an enclosure for housing the components of the circuit breaker in FIG. l;

FIG. 20 is a section taken generally along line 20-20 in FIG. 19;

FIG. 21 is a section taken generally along line 21-21 in FIG. 19; FIG. 22 is a section taken generally along line 22-22 in FIG. 19;

FIG. 23 is a bottom plan view of a cover of the

enclosure for housing the components of the circuit breaϋcer in FIG. 1; and

FIG. 24 is a section taken generally along line 24-24 in FIG. 23. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the described embodiments are not intended to limit the invention to the particular form described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed Description Of The Preferred wmtM-M-Hm -fit Turning now to the drawings, the present invention is discussed in the context of an exemplary circuit breaker using a blade suspension assembly embodying the principles of the present invention. The particular circuit breaker illustrated and described (FIGS. 1 through 13) should not, however, be construed to limit the possible applications for the present invention, as these applications encompass a wide variety of circuit breaker types. To fully appreciate the utility of the present invention, however, the circuit breaker of FIGS. 1 through 13 will first be described, followed by a detailed description of a blade suspension assembly (in accordance with the present invention) generally depicted in the circuit breaker.

The circuit breaker includes a thermal trip unit (FIGS. 2, 3, 6, and 7), a magnetic trip unit (FIGS. 4 through 7), and a blade/cradle assembly (FIGS. 8 through 13) . The thermal trip unit and the magnetic trip unit include a common latching system shown in FIGS. 2 through 7, and the blade/cradle assembly includes the blade suspension assembly (FIGS. 14 through 17) embodying the present invention. While each of these portions of the circuit breaker are described below by reference to the

corresponding drawings, reference may be made to FIG. 1 to view the circuit breaker as a whole.

The latching system (FIGS. 2 through 7) includes a latch 10, a latch spring 12, and a trip crossbar 14. Under normal operating conditions (i.e., the circuit breaker is untripped/closed) , the latch 10 holds a cradle 16 in a stationary position such that a pair of parallel upper links 18 are disposed in line with a pair of parallel lower links 20. This is accomplished with the latch 10 being locked over the cradle 16 by a latch pin 22 mounted in the trip crossbar 14. A pair of parallel mechanism frame sides 24 house the latch 10, a cradle pivot pin 26, and the cradle 16.

The upper and lower links 18, 20 are identically constructed parts, which reduces production costs and eliminates the possibility of incorrectly assembling the links 18, 20. Moreover, the mechanism frame sides 24, the links 18, 20, the latch 10, and the cradle 16 are all flat stamped parts produced in a single stamping operation. This allows for automated assembly, thereby reducing production costs and increasing production rate.

In response to the occurrence of a fault condition causing a circuit interruption, the trip crossbar 14 is rotated counterclockwise (as viewed in FIGS. 1 through 5) which, in turn, rotates the latch pin 22 to a position where it is no longer in contact with the top of the latch 10. With the latch pin 22 moved, the force from the cradle 16 against the latch 10 causes the latch 10 to rotate counterclockwise, thereby releasing the cradle 16. The cradle 16 then rotates clockwise to collapse the upper and lower links 18, 20.

With respect to the thermal trip unit (FIGS. 2, 3, 6, and 7) , the thermal trip unit operates in response to the current reaching a predetermined percentage (e.g., 135 percent) of the rated current for a period of time to be determined by calibration of the unit. This elevated current level causes direct heating of a bimetal 28, which

results in the bending of the bimetal 28. The bimetal 28 is composed of two dissimilar thermostat materials which are laminated or bonded together and which expand at different rates due to temperature increases, thereby causing the bimetal 28 to bend.

The rated current for the circuit breaker is the maximum current which can be carried by the circuit breaker under normal (steady-state) operating cor.ditions. The rated current is the current the circuit breaker is designed to carry without tripping. In the preferred embodiment, the circuit breaker has a rated current of 250 amperes. In existing circuit breakers having a rated current of 250 amperes, a separate heater is used to heat the bimetal 28. An important feature of the thermal trip unit is that the bimetal 28 is directly heated. By directly heating the bimetal 28, the need for a separate heater is eliminated, thereby simplifying the design of the thermal trip unit and reducing the costs associated therewith. The bimetal 28 is directly heated by attaching a lower portion of the bimetal 28 to an L-shaped load terminal 30 and by attaching two flexible connectors 32 (e.g., pigtails) to a lower to middle portion of the bimetal 28 (FIG. l) . In the preferred embodiment, the bimetal 28 is approximately 2.75 inches in length, and the flexible connectors 32 are connected by single phase A/C resistance or capacitive discharge methods to the bimetal 28 at a location slightly less than one inch from the lower end of the bimetal 28. This creates a direct current path from the load terminal 30 through the bimetal 28 and into the flexible connectors 32, which, in turn, allows the maximum energy (heat) to be utilized to deflect the bimetal 28. Direct heating of the bimetal 28 makes the trip unit more efficient by eliminating the losses that occur between a separate heater and a bimetal. In addition, the employed bimetal 28 will have a lower resistance due to the low attachment on the bimetal 28 of the flexible connectors 32,

thereby reducing the power consumed by the bimetal 28 and allowing the product to operate at cooler temperatures. This, in turn, increases customer satisfaction.

The amount of power and heat generated in the circuit breaker lugs (not shown) is directly proportional to both the current carried by the circuit breaker and the resistance of the current path through the circuit breaker. The arrangement of the load terminal 30, the bimetal 28, and the flexible connectors 32 is designed to prevent overheating of the circuit breaker lugs and, at the same time, permit the circuit breaker to properly trip in response to an overcurrent condition. In particular, the flexible connectors 32 are connected to the lower middle portion of the bimetal 28 so that the current path through the bimetal 28 is relatively short compared to the length of the bimetal 28. This short current path through the bimetal 28, in turn, insures that the bimetal 28 adds a relatively small resistance to the current path through the circuit breaker. Since the amount of heat generated in the circuit breaker lugs is directly proportional to the resistance of the current path through the circuit breaker, the short current path through the bimetal 28 minimizes the amount of heat generated in the lugs. At the same time, the resistance of the bimetal along this short current path is sufficient to properly bend the bimetal 28 during an overcurrent condition.

As the bimetal 28 bends, it comes in contact with a trip screw 34 housed in the trip crossbar 14. The continued bending of the bimetal 28 forces the trip crossbar 14 to rotate in a counterclockwise motion (as viewed in FIGS. 2 and 3) . This rotation of the trip crossbar 14 causes the latch pin 22 to rotate above the latch 10. With the latch pin 22 no longer in contact with the latch 10, the cradle 16 forces the latch 10 to rotate counterclockwise, thereby releasing the cradle 16. The cradle 16 then rotates clockwise and causes the circuit breaker to trip (FIG. 3) .

With respect to the magnetic trip unit (FIGS. 4 through 7) , the magnetic trip unit operates in response to the current flowing through the circuit breaker reaching a specified level, causing the circuit breaker to clear the interruption. Tire elevated current level causes the magnetic field in a U-shaped magnetic yoke 36 to increase. When the magnetic field is large en gh such that the downward force caused by the magnetic attraction between the magnetic yoke 36 and an armature plate 38 is larger than the opposing force of a magnetic spring 40, the armature plate 38 is attracted to the magnetic yoke 36, thereby pulling an armature shaft 42 down. The armature shaft 42 is guided by an armature guide 44 having a slot for receiving the armature shaft 42. The movement of the armature shaft 42 causes the trip crossbar 14 to rotate in a counterclockwise motion (as viewed in FIGS. 4 and 5) . This movement of the trip crossbar 14 rotates the latch pin 22 above the latch 10. With the latch pin 22 no longer in contact with the latch 10, the force from the cradle 16 onto the latch 10 causes the latch 10 to rotate counterclockwise, thereby releasing the cradle 16. The cradle 16 then rotates clockwise and causes the circuit breeϋcer to trip (FIG. 5) .

Referring to FIGS. 6 and 7, to prevent an operator from entering the circuit breaker enclosure by the load terminal 30 and touching the trip unit components, the circuit breaϋcer is provided with a back barrier 46. The back barrier 46 and the armature guide 44 are preferably attached together using a spot weld. Alternatively, t e two parts may be attached together using a TOX joint, or the back barrier 46 may be integrally formed with the armature guide 44 using a progressive die. The back barrier 46 and the armature guide 44 are preferably composed of a nonferrous material, such as aluminum, so that they do not affect the magnetic field associated with the magnetic yoke 36, the armature plate 38, and the magnetic spring 40.

With respect to the blade/cradle assembly (FIGS. 8 through 13), when either the thermal trip unit or the magnetic trip unit cause the latch 10 to rotate counterclockwise and release the cradle 16, the force from a toggle spring 48, connected to a toggle pin 50 and a handle arm 52, causes the cradle 16 to rotate clockwise about a cradle pivot pin 54 (as viewed in FIGS. 8, 10, 12, and 13). The rotation of the cradle 16, in turn, causes the upper and lower links 18, 20 to collapse. More specifically, the toggle pin 50 connects the two upper links 18 to the two lower links 20. As the cradle 16 rotates, the upper links 18 rotate clockwise about an upper link pin 54, thereby pulling the toggle pin 50 back and upward. This movement of the toggle pin 50 forces the lower links 20 to rotate counterclockwise about a drive pin 56 and pull up on a blade carrier or crossbar 58. The movement of the blade crossbar 58 forces an elongated blade 60 to rotate counterclockwise, thereby separating the contacts 62, 64 (FIGS. 10 and 11). The stationary contact 64 is depicted in FIGS. 2 through 5 and is mounted to a line terminal 66.

After the circuit breaker has been tripped (FIGS. 10 and 11) , the latching system is reset by rotating the handle arm 52 counterclockwise. This movement of the handle arm 52 forces the cradle 16 to rotate counterclockwise until the cradle 16 has reached a reset position (FIG. 12) . The reset position is the farthest point the handle arm 52 is able to rotate counterclockwise because the mechanism frame sides 24 restrict any further rotation of the handle arm 52. With the cradle 16 in the reset position, the latch spring 12 forces both the latch 10 and the trip crossbar 14 to simultaneously rotate clockwise. This brings the latch pin 22 in contact with the latch 10 so as to lock the latch 10 over the cradle 16 and reset the latching system. In response to the latching system being reset, the handle arm 52 rotates clockwise to an "off" position (FIG. 13) .

The circuit breaker is placed in an "on" operating mode by rotating the handle arm 52 clockwise to an "on" position (FIG. 8) . The "on" position is the farthest point the handle arm 52 can be rotated clockwise. The mechanism frame sides 24 restrict further clockwise rotation of the handle arm 52 beyond the "on" position. As the handle arm 52 rotates clockwise, the toggle spring 48 pulls the toggle pin 50 forward to force the upper and lower links 18, 20 to rotate into alignment. This movement of the links 18, 20 forces the blade crossbar 58 to rotate clockwise, thereby allowing the blade 60 to close the contacts 62, 64. The cradle pivot pin 26 prevents the uppe and lower links 18, 20 from rotating beyond the aligned position.

Referring now to FIGS. 14 through 17, the blade suspension assembly 70 of the blade/cradle assembly includes the elongated blade 60, a blade pivot pin 72, a torsion spring 74, and the blade crossbar 58. The torsion spring 74 includes a U-shaped middle portion 76 and a pair of end legs 78 disposed on opposite sides of the middle portion. The U-shaped middle portion 76 includes a lateral section 77 disposed substantially perpendicular to the end legs 78. In addition, the torsion spring includes a lateral hole 80 extending therethrough. The blade 60 includes the electrical contact 62 mounted to one end thereof, a lower narrow bearing surface 82 for supporting the lateral section 77 of the torsion spring 74, and a lateral circular aperture 84 for laterally receiving the pivot pin. The aperture 84 is disposed near the non- contact end of the blade 60. Each pole of the blade crossbar 58 includes a pair of parallel opposing side walls 86, a front wall 88, and a back wall 90. A short linear portion of the respective junctions (corners) between the front wall 88 and the side walls 86 form a pair of bearing surfaces 90 for supporting the respective end legs 78 of the torsion spring 74. One of the bearing surfaces 90 supports one of the end legs 78, and the other of the bearing surfaces 90 supports the other

of the end legs 78. The side walls 86 have formed therein respective notches 94 for receiving and supporting respective ends of the cylindrical pivot pin 72.

To assemble the blade suspension assembly 70, the torsion spring 74 is placed over the blade 60 such that the lateral section 77 of the torsion spring 74 abuts the lower bearing surface 82 of the blade 60, the end legs 78 are arranged on opposite surfaces of the blade 60, and the lateral hole 80 in the torsion spring 74 is disposed in line with the circular aperture 84 in the blade 60. The lateral section 77 of the torsion spring 74 is sufficiently wide to permit the U-shaped middle portion 76 to fit over the blade 60. Next, the blade pivot pin 72 is inserted through both the lateral hole 80 in the torsion spring 74 and the circular aperture 84 in the blade 60. Finally, the combination of the blade 60, the torsion spring 74, and the pivot pin 72 is inserted into the blade crossbar 58 with the pair of end legs 78 of the torsion spring 74 abutting the respective bearing surfaces 92 of the blade crossbar 58 and with the two ends of the pivot pin 72 located in their respective notches 94 formed in the side walls 86 of the blade crossbar 58.

When the torsion spring 74 is unstressed, the lower bearing surface 82 of the blade 60 and the bearing surfaces 92 of the blade crossbar 58 are positioned apart by a distance less than the distance between the lateral section 77 of the torsion spring and the end legs 78. Therefore, a predetermined amount of stress must be applied to the torsion spring 74 prior to loading the combination of the blade 60, the torsion spring 74, and the pivot pin 72 into the blade crossbar 58. This preloading stress compresses the end legs 78 of the torsion spring 74 toward the U- shaped middle portion 76 by a sufficient amount that the torsion spring 74 can be loaded into the blade crossbar 58. After loading the combination of the blade 60, the torsion spring 74, and the pivot pin 72 into the blade crossbar 58, this preloading stress is released, thereby charging the

blade suspension assembly 70 with the contact force required for the circuit breaker application. That is, the torsion spring 74 exerts a force on the blade 60 so that its electrical contact 62 applies the required contact force to the opposing stationary contact 64 while the L ^* -ade 60 is disposed in an untripped/closed position.

The circuit breaker may include multiple poles. FIG. 14 illustrates the blade suspension assembly 70 used for a three-pole circuit breaker. The blade crossbar 58 is provided with three separate compartments each of which houses a respective combination of the blade 60, the torsion spring 74, and the pivot pin 72. FIG. 14 depicts the blade suspension assembly 70 in both its assembled form and its unassembled form. The blade suspension assembly 70 employs two methods of rotation to insure that the circuit breaker will clear any interruption within a specified interruption range. In the first method, the movable contact 62 is separated from the opposing stationary contact 64 by the rotation of the blade crossbar 58 and the blade 60 about a crossbar pivot 96 in response to a force applied to the drive pin 56 by the lower links 20 after the assembly 70 has opened due to the tripping of the thermal or magnetic trip unit. This first method is illustrated by the change from the closed position shown in FIG. 15 to the tripped position shown in FIG. 16.

The second method employs the blow-open characteristic designed into the blade suspension assembly 70. In particular, this method takes advantage of the repulsive electromagnetic force seen during a high level interruption to rotate the blade 60 about the pivot pin 72 away from a line terminal blow-off loop in opposition to the spring force created by the torsion spring 74. This second method is illustrated by the change from the closed position shown in FIG. 15 to the blown open position shown in FIG. 17. To increase the blow-off force of the blade 60 and thereby shorten the high level interruption times, the

circuit breaker is provided with a field barrier assembly 100 including a housing 102 and a pair of field enhancers 104. The housing 102 includes a pair of legs 106, 108 having respective elongated generally rectangular slots 110, 112 formed therein for receiving the field enhancers 104. A lateral section 109 bridges the pair of legs 106, 108. The field enhancers 104 are rectangular metal blocks, composed of a ferrous material such as steel, which are sized to fit within the rectangular slots 110, 112. An important feature of the field barrier assembly 100 is that the housing 102 is designed to firmly secure the field enhancers 104 within the respective slots 110, 112 without using an additional mechanism. A drawback of existing field barrier assemblies is that they require a separate mechanical means, such as adhesive, to hold the field enhancers within the housing. In contrast, the mechanics for securing the field enhancers 104 within the respective slots 110, 112 are built into the slots themselves. In particular, both side walls of each of the slots 110, 112 is provided with a plurality of elongated ribs 114 integrally formed therewith. To insure a tight fit of the field enhancers 104 from the top to the bottom of the slots 110, 112, the ribs 114 are designed to apply uniform pressure to the field enhancers 104 substantially between the top and bottom edges of the side walls.

To further secure the field enhancers 104 within the generally rectangular slots 110, 112, the side walls of each slot slightly bulge inward toward each other so that the slots 110, 112 have a slightly concave rectangular shape. In other words, the slots 110, 112 are narrower at their center portion than at their two ends. These contoured side walls, in conjunction with the ribs 114, compress and clamp the field enhancers 104 within the respective slots 110, 112. To construct the field barrier assembly 100, the housing 102 is manufactured using conventional injection compression molding techniques. The housing 102 is

composed of a thermoset material so that it remains dimensionally stable following its molding. The field enhancers 104 are then inserted into the respective slots 110, 112 as shown in FIG. 18. The various assemblies of the circuit breaϋcer are housed in an enclosure having a base 120 (FIGS. 19 through 22) and cover 122 (FIGS. 23 and 24) interlocked by means such as bolts or screws. The enclosure is designed to house a three-pole circuit breaker. In particular, the base 120 is partitioned into three pole sections 124, 126, and 128, and each of these sections houses one pole of the three-pole circuit breaϋcer. Similarly, the cover 122 is partitioned into three pole sections 130, 132, and 134 which, when the base 120 and the cover 122 are attached to each other, align with the respective sections 124, 126, and 128 of the base 120 so as to divide the enclosure into three parts.

The three sections 124, 126, and 128 of the base 120 house the various assemblies previously described. Some of these assemblies are housed in substantially identical fashion in each of these sections, while some of the assemblies are only disposed in the middle section 126. In particular, each of the sections includes the load terminal 30, the magnetic trip unit (FIGS. 2, 3, 6, and 7) , the thermal trip unit (FIGS. 4 through 7), the blade suspension assembly (FIGS. 14 through 17) , the field barrier assembly 100 (FIG. 18), an arc stack (not shown), and the line terminal 66. With respect to the trips units and the blade suspension assembly, the trip crossbar 14 and the blade crossbar 58 are common to all three sections 124, 126, and 128 so that only a single trip crossbar and a single blade crossbar are provided for the circuit breaker. These crossbars extend laterally across the three pole sections 124, 126, and 128 and are pivotally mounted to slots formed in the walls partitioning the sections from each other.

Only the middle section 126 is provided with the latch 10, the latch spring 12, the latch pin 22, and the blade/cradle

assembly (FIGS. 8 through 13) .

The arrangement of the foregoing circuit breaker components in the sections 124, 126, and 128 are described below for the middle section 126. Those components in the middle section 126 which are also employed in the adjacent sections 124 and 128 are arranged in substantially identical fashion. With respect to the middle section 126, the load terminal 30 extends between a lug chamber 136 and an adjacent compartment 138. The lug chamber 136 and the compartment 138 are separated by the back barrier 46, and the thermal and magnetic trip units are disposed in the compartment 138. The bimetal 28 is positioned between the compartment 138 and a compartment 140, and the latch 10, the blade/cradle assembly, and the blade suspension assembly are primarily disposed in the compartment 140.

The back end of the blade 60 is located in the compartment 140, while the contact end of the blade 60 is located in an arc chamber 142.

The compartment 140 and the arc chamber 142 are partitioned by the field barrier assembly 100, which acts to isolate the arc chaunber from the other components of the circuit breaker. As a result, the field barrier assembly 100 prevents any debris caused during an interruption from escaping the arc chamber 142 and interfering with these other internal components. To retain the field barrier assembly 100 within the section 124, the housing 102 includes a pair of lateral ears 116, 118 (FIG. 18) which engage with respective mating slots formed in the enclosure. More specifically, lower wide portions of the ears 116, 118 engage with respective slots 144, 146 formed in the base 120, and upper narrow portions of the ears 116, 118 engage with respective slots 148, 150 formed in the cover 122.

With the field barrier assembly 100 positioned between the compartment 140 and the arc chamber 142, the legs 106, 108 of the housing 102 straddle the blade 60 with the inner surfaces of the legs 106, 108 adjacent the opposite

surfaces of the blade 60. FIG. 18 shows the manner in which the field barrier assembly 100 is assembled to straddle the blade 60 within the base 120. The blade 60 extends between the legs 106, 108 with the contact end of the blade 60 located in the arc chamber 142 and the back end of the blade 60 located in the compartment 140.

The line terminal 66 extends between the arc chamber 142 and an adjacent lug chamber 152. The portion of the line terminal 66 having the stationary contact 64 mounted thereto is located within the arc chamber 142. Thus, the movable contact 62 on the blade 60 and the stationary contact 64 on the line terminal 66 are both located in the arc chamber 142. An electrical arc is created between these contacts as the blade 60 moves from a closed position to an open position during a fault condition. In order to suppress this electrical arc, a multi-piece arc stack is positioned in the arc chamber 142. This arc stack may be implemented as disclosed U.S. Patent Application Serial No. (CRC-34/SQUC131) , entitled "Arc Stack for a Circuit Breaker", filed concurrently herewith, assigned to the instant assignee, and incorporated herein by reference.

The base 120 and the cover 122 have formed therein respective keyways 149, 151 designed to secure the arc stack within the enclosure and to prevent improper installation of the multi-piece arc stack into the enclosure. Each piece/section of the arc stack is provided with respective notches or keys which mate with the keyways 149, 151. The keyways 149, 151 are designed such that each section of the arc stack must be properly oriented relative to the base 120 and cover 122 and properly positioned relative to the other arc stack sections in order for the arc stack section to fit properly into the keyways 149, 151. Thus, by dictating the orientation and relative position of the arc stack sections, the keyways 149, 151 secure and properly orient the arc stack within the circuit breaϋcer enclosure.

An important feature of the circuit breaker enclosure

is that it includes means for securing the mechanism frame sides 24 in the middle section 126 of the base 120. In particular, the middle section 126 includes a pair of bearing surfaces 150 for supporting protrusions 25 (FIGS. 8 and 9) integrally formed with the mechanism frame sides 24. Furthermore, four retaining clips (not shown) are connected by screws or bolts to the upper ledges of the walls separating the base sections 124, 126, and 128 at the positions 152. The cover 122 includes four recesses 154 arranged to accommodate the screws or bolts used for connecting the clips to the upper ledges of these walls. The retaining clips extend laterally into the middle section 126 and are constructed and arranged to engage respective horizontal sections 27 (FIG. 9) of the mechanism frame sides 24. While pressing the protrusions 25 of the mechanism frame sides 24 against the bearing surfaces 150, the retaining clips maintain the mechanism frame sides 24 in the compartment 140 of the middle section 126.

To provide added support and strength to the enclosure and alleviate the stresses applied thereto, the outer walls of both the base 120 and the cover 122 include a plurality of ribs 156. The ribs 156 of the base 120 are integrally formed with the outer walls and the bottom of the base 120 and are tapered toward the outer walls in a direction extending away from the bottom of the base 120. Thus, the ribs 156 are widest at the point where they meet the bottom of the base 120. Similarly, the ribs 156 of the cover 122 are integrally formed with the outer walls and the top of the cover 122 and are tapered toward the outer walls in a direction extending away from the top of the cover 122. To facilitate proper orientation and interlocking of the base 120 and the cover 122, the upper ledges of the base walls are provided with a configuration of raised portions 158 which mate with a configuration of grooves 160 formed in the ledges of the cover walls. Since the configurations are asymmetrical about a centrally located transverse axis (horizontal axis in FIGS. 19 and 23) , these

configurations will only mate with each other if the base 120 and the cover 122 are properly oriented relative to each other.

While the invention has been particularly shown and described with reference to certain embodiments, it will be recognized by those skilled in the art that modifications and changes may be made to the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims.