Abstract:
A molded case circuit breaker housing employs a tri-furcated design that imparts structural stability and manufacturing efficiency. The design provides a separate top cover, mid-cover and base wherein the mid-cover wall height to base wall height ratio is such that the stresses imparted upon the base sidewalls when pressures are exerted, for example, by gaseous discharges are withstood.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to molded case circuit breaker enclosures. More particularly the present invention relates to a base and mid-cover configuration for circuit breaker enclosures. 
     Circuit breaker enclosures are typically manufactured having a deep base for enclosing the components of a circuit breaker (i.e., a circuit breaker cassettes) and a cover having an opening for an operating handle (a bifurcated case). In existing circuit breaker enclosures, the base is constructed with sidewalls that extend to a height approaching or exceeding the height of the circuit breaker cassettes when upon the floor of the base. Other designs employ a divided cover, where in a mid-cover connects to the base and the top cover connects to the mid-cover and has an opening for the operating handle (a trifurcated case). The mid-cover may also comprise accessory recesses and/or trip actuator recesses positioned in mechanical cooperation with the operating handle of the circuit breaker. As with a bifurcated case, typical sidewalls of trifurcated case bases approach or exceed the height of the circuit breaker cassettes. 
     Typically, in trifurcated circuit breaker enclosures the height of the mid-cover wall is small in relation to the height of the base sidewalls. This is likely due to the limited purpose of existing mid-covers, i.e., to house accessories and trip actuators, which generally have a small height in comparison to the circuit breaker cassettes. The ratio of the mid-cover height H mc  to the base height H b  (H mc /H b ) in prior circuit breakers molded case is less than about 0.45. 
     However, with relatively small H mc /H b  ratios, manufacturing is difficult as the base must be relatively high for a very thin structure. This creates problems in the molding of the thermoplastic materials and increases the effort required to provide a finished product having thermoplastic resin and filler evenly distributed. 
     Further, a structural problem arises due to the high, thin sidewalls of the base. As gases are exerted when the circuit breaker cassettes trip, the stresses created cause the circuit breaker enclosure to deform and possibly crack at the base sidewalls. 
     Therefore, a need exist for a molded circuit breaker case that is easier to mold and that provides suitable structural integrity. 
     BRIEF SUMMARY OF THE INVENTION 
     A molded circuit breaker enclosure is provided having a base, a mid-cover, and a top-cover. The base includes generally parallel sidewalls, a bottom wall, a lined end dividing portion. The mid-cover comprises generally parallel sidewalls, and a load end dividing structure. Circuit breaker components, such as one or more circuit breaker cassettes, are positioned within the base of the circuit breaker. A mid-cover is removably affixed on top of the base sidewalls, and are secured there to, for example, on a pressed fit frictional engagement, interference engagement, fastener adhesive or any combination thereof. The top-cover has a slot to allow a handle to extend therethrough. 
     The circuit breaker enclosures must withstand mechanical stresses caused by the high gas pressure generated when a short circuit occurs. Accordingly, it is beneficial to transfer the stresses from the sidewalls of the base to the cover or the combined mid-cover and top-cover assembly. This may be accomplished by providing a step on the upper edge of the sidewalls of the base and a corresponding step on the bottom edge of the mid-cover sidewalls in a trifurcated case or cover sidewalls in a bifurcated case. 
     In an exemplary embodiment of the present invention, the ratio of the midcover sidewall height to the base sidewall height is at least 0.45. In a most preferred embodiment, the ratio is between 0.61 and 1.63. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top perspective view of an assembled molded case circuit breaker; 
     FIG. 2 is an exploded side perspective view of a molded case circuit breaker showing the enclosure structure and general internal components; 
     FIG. 3 is a top perspective view of a molded case circuit breaker with mid-cover and base heights indicated; 
     FIGS. 4 and 5 are cross-sectional views of a prior art circuit breaker case and a case of the present invention; respectively; 
     FIGS. 6 and 7 are cross-sectional views of a prior art circuit breaker enclosure base mold and a base mold of the present invention, respectively; 
     FIG. 8 is a cross-sectional view of a molded circuit breaker enclosure depicting internal forces imported upon the enclosure; and 
     FIG. 9 is an enlarged view of a lap joint that may be employed within preferred embodiments of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A molded circuit breaker case  10  is generally shown in FIG.  1 . The molded circuit breaker case  10  generally includes an electronic trip unit (not shown) for overcurrent protection and also may include at least one auxiliary electrical accessory, such as auxiliary switches, shunt trip elements and under-voltage sensing units. Molded circuit breaker case  10  generally includes an insulated base  12 , a mid-cover  14  and a top- cover  16 . Disposed within the molded case are plurality of breaker cassettes  40 ,  50  and  60  (not shown) including line terminal straps  44 ,  54  and  64  and load terminal straps  46 ,  56  and  66  (shown in phantom view). Load terminal straps  46 ,  56  and  66  are physically and electrically separated from each other by base load terminal dividing walls  28  and two mid-cover line terminal dividing walls  30 . When mid-cover  14  is attached to case  12 , the downwardly extending mid-cover load terminal dividing walls  30  align with corresponding upstanding base line terminal dividing walls  28  integrally formed within the case to electrically isolate the bad connections within a multi-phase electrical circuit. Likewise, line terminal straps  44 ,  54  and  64  are physically and electrically separated from each other by two base load terminal divider walls (not shown) and two mid-cover line divider walls (not shown). 
     Mid-cover  14  is disposed generally between base  12  and top cover  16 . Mid-cover  14  is secured to base  12  by a combination of fasteners and one or more frictional engagements such as a press fit engagement. Top-cover  16  is secured to mid-cover  14  by a plurality of fasteners. Further, circuit breaker case  10  may include load strap cover  18  and line strap cover  20  removably or permanently affixed to mid-cover  14  to prevent top access to the terminal straps  46 ,  56 ,  66  and  44 ,  54 ,  64  respectively. Load strap cover  18  and line strap cover  20  may be secured to mid-cover  14  in a press-fit manner. It is well known to one skilled in the art that alternative methods for securing the mid-cover to the base may be employed, for example, adhesives, interference engagements, fasteners, frictional engagements, or any combination thereof. It is also well know to one skilled in the art that top-cover  16 , load strap cover  18  and line strap cover  20  may be secured to mid-cover  14  by a variety of methods, for example, adhesives, interference engagements, fasteners, frictional engagements, or any combination thereof. 
     Top-cover  16  includes an operating handle slot  82  (see FIG. 2) to allow circuit breaker operating handle  80  to pass therethrough. Operating handle  80  controls the circuit breaker cassettes via a connection with an internal mechanism (not shown). 
     Referring to FIG. 2, an exploded side perspective view of the circuit breaker molded case of the present invention is depicted. Base  12  comprises opposed upstanding sidewalls  32  and  34 , base floor  36 , upstanding baseline terminal divider walls  28 , and upstanding load terminal divider walls  22 . Sidewalls  32  and  34  comprise a top lipped edge, the outside of which received inside of the bottom edge of the mid-cover sidewalls in a pressed fit manner (described in further detail herein with reference to FIG.  9 ). 
     Circuit breaker cassettes  40 ,  50  and  60  are disposed in an upstanding fashion within base  12  upon base floor  36 , such that cassette  40  is adjacent to sidewall  32 , cassette  60  is adjacent to sidewall  34 , and cassette  50  is disposed between cassette  40  and  60 . The handle  80  is attached to an internal mechanism  70 , which is coupled with the center cassette  50 . Center cassette  50  is coupled withcassettes  40  and  60  by the drive pin  72 . Cassettes  40 ,  50  and  60  engage and disengage simultaneously by drive pin  72 . The height of the circuit breaker cassettes is greater than the height of the base sidewalls, as apparent by a view of an inserted cassette  40 , shown in phantom at  40   a.    
     Cassettes  40 ,  50  and  60  along with mechanism  70  are assembled into the base  12  and retain therein by mid-cover  14 . Mid-cover  14  is connected to base  12  by any convenient means, such as screws  90 , snap fit or adhesive bonding. Specifically, in the embodiment depicted in FIG. 2, mid-cover  14  is secured to base  12  generally by press fit engagements about the middle portion of opposing sidewalls  92  and  94 , and via fasteners or screws  90  through mid-cover line terminal divider walls  30  and mid-cover load end divider walls  24 . Thus, side-walls  92  and  94  of mid-cover  14 , in conjunction with sidewalls  32  and  34  of base  12 , form protective walls for the outer side of circuit breaker cassettes  40  and  60 . Further, base divider walls  18  and  22  and mid-cover divider walls  30  and  24  structurally depend from support portions extending generally from sidewall  32  and  34  of the base and sidewall  92  and  94  of the mid-cover, respectively, generally perpendicular to the divider walls and sidewalls. 
     Mid-cover  14  further includes a central support portion  96  formed between sidewalls  92  and  94  to cover the top surfaces of cassettes  40  and  60  and to provide structural integrity to the mid-cover. Central support portion  96  generally includes opposing upstanding walls  98  between which internal mechanism  70  is disposed and operating handle  80  passes through. Optional accessories may be disposed within mid-cover  14  between support portion walls  98  and either of mid-cover sidewalls  92  or  94 . 
     The top portion end of circuit breaker cassettes  40 ,  50  and  60  are covered by mid-cover  14 . The operating handle  80  is accessible via opening  82  upon top-cover  16 . Top cover  16  is secured to mid-cover  14  by fasteners  92  which engage corresponding receptacles upon mid-cover  14 . It is known that top-cover  16  may further include openings or viewing windows to access and/or see visual displays upon various accessories. 
     Referring to FIG. 3, an assembled molded case circuit breaker of the present invention is depicted, wherein the heights of the mid-cover sidewalls and base sidewall are indicated as H b  and H mc . The height of base sidewalls  32  and  34 , H b , is depicted as 5.4 cm (2.125 inches). The height of the sidewalls  92  and  94  of mid-cover  14 , H mc,  is depicted as 3.3 cm (1.3 inches). Thus, the ratio of the height of the mid-cover to the height of the base, H mc / H b , is 0.6117 in the depicted embodiment. 
     Generally, prior art molded case circuit breaker the same overall size have a smaller ratio of the height of the mid-cover to the height of the base. That is, the height of the sidewall of mid-cover is typically 2.7 cm (1.063 inches) or less and the height of the base sidewall is typically 6 cm (2.362 inches) or more, providing a ratio of approximately 0.44-0.46 or less. 
     Referring now to FIG. 4 a cross-sectional view of a prior art circuit breaker case  10  is indicated. Upstanding circuit breaker cassettes  40 ,  50  and  60  are disposed within circuit breaker case  10 . Internal mechanism  70  (shown in phantom) is pivotally attached to operating handle  80  and envelopes the sidewalls of center cassette  50  and common movement throughout cassettes  40 ,  50  and  60  is effectuated via drive pin  72  (shown in phantom). Also shown are heights H b  and H mc . In prior art circuit breaker cases, upstanding cassettes  40 ,  50  and  60  extend to a height approaching H b . Thus, the mid-cover sidewalls generally protect any accessories provided. 
     Referring now to FIG. 5 a cross-sectional view of a circuit breaker case  10  of the present invention is indicated. Upstanding circuit breaker cassettes  40 ,  50  and  60  are disposed within circuit breaker case  10  generally upon base floor  36 , and internal mechanism  70  (depicted in phantom) is pivotally attached to operating handle  80  and envelopes the sidewalls of center cassette  50  whereby common movement throughout cassettes  40 ,  50  and  60  is effectuated via drive pin  72 . The heights of base sidewalls  32  and  34  and mid-cover sidewalls  92  and  94 , H b  and H mc  respectively, are indicated. In the configuration of the present invention, H b  is less than that of prior art circuit breakers and H mc  is greater than that of prior art circuit breakers. In the circuit breaker case of the present invention, upstanding cassettes  40 ,  50  and  60  extend to a height beyond H b . Base sidewalls  32  and  34  enclose the lower portion of circuit breaker cassettes  40 ,  50  and  60 . Mid-cover sidewalls  92  and  94  enclose the upper portion of circuit breaker cassettes  40 ,  50  and  60  via extended portions  93  and  95  having heights H e . Additionally, mid-cover sidewalls  92  and  94  protect any accessories provided upon the mid-cover support portion  96 . 
     A preferred embodiment of the circuit breaker case  10  of the present invention is fabricated by injection molding of a thermoplastic material. Thermoplastics include a polymeric resin and filler. For molded case circuit breakers, the preferred filler is glass particles, as they impart a high degree of hardness while maintaining the resiliency of the case, thereby minimizing breakage. However, for optimal injection molding manufacture, a consistent mixture of resin and filler is desired. The present invention facilitates such consistency by lessening the distance that the resin/filler distance must travel. Referring to FIG. 6 a cross-sectional view of a circuit breaker base mold of the prior art is indicated at  112 . Thermoplastic resin/filler mixture is injected through mold apertures  118  in the direction indicated by arrows  120 . 
     Referring to FIG. 7 a cross-sectional view of a circuit breaker base mold of the present invention is indicated at  113 . Thermoplastic resin/filler mixture is injected generally by the same process as with the prior art mold  112 , through mold apertures  118  in the direction indicated by arrows  120 . However, H b  of the prior art base mold  112  is greater than H b  in base mold  113  of the present invention. Therefore, during the injection molding process, the molten thermoplastic resin/filler mixture that is injected is required to travel a lesser distance in base mold  113  as compared to base mold  112  of the prior art. The tendency of the thermoplastic mixture to separate is due to the lower viscosity of resin compared to the higher viscosity of resin/filler mixture. Consequently, by decreasing the height of the base sidewalls as provided in the present invention, a more consistent filler/resin mixture may be attained while using less injection force, thereby increasing manufacturing efficiency and improving filler mixture throughout the mold. 
     Referring now to FIG. 8, a circuit breaker case  10  of the present invention is shown in cross section without the components therein whereby a stress analysis is shown. Circuit breaker case  10  includes a base  12  having a floor  36  and opposing sidewalls  32  and  34  (whereby H b  indicates the height of base sidewalls  32  and  34 ), and a mid-cover  14  including a support  96  and opposing sidewalls  92  and  94 . Sidewalls  92  and  94  further comprise opposing extended walls  93  and  95 , respectively, that extend below mid-cover support structure  96 . Base  12  including floor  36  and sidewalls  32  and  34 , and mid-cover  14  including support  96  and sidewalls  92  and  94  are acted upon by mechanical forces indicated by arrows  232 ,  234 ,  236 ,  293 ,  295  and  296 . The mechanical forces depicted are exerted by the discharge of gasses from the circuit breaker cassettes  40 ,  50  and  60  (not shown). The length of extended portions  93  and  95  is indicated by H e . Forces  296  and  236 , acting upon the mid-cover support structure  96  and the base floor  36  respectively, are generally constrained by screws  90  (see FIG. 2) disposed between mid-cover  14  and base  12 . Forces  293  and  295  act upon the extended portions  93  and  95  respectively, and forces  232  and  234  act upon base sidewalls  32  and  34  respectively. Forces  232  and  234  urge base sidewalls  32  and  34  respectively outward, shown in phantom, to a distance D b . As the distance D o  increases, the tendency of base sidewalls  32  and  34  to crack or break generally about base sidewall-floor juncture points  33  and/or  35  increases. As H b  is decreased to a shorter height, as provided for in the present invention, the maximum D b  also decreases thereby minimizing tendencies for the base sidewalls  32  and  34  to crack or break at points  33  and/or  35 . Forces  293  and  295  urge extended portions  93  and  95  respectively outward, shown in phantom, to a distance D mc . There is little tendency for D mc  to increase far enough as to crack or break extended portions  93  and  95 , as H e  is relatively small compared to the restraint from support structure  96 . 
     Additional support may be afforded at the junction of sidewalls  32  and  34  and extended portions  93  and  95 , respectively, by the provision of a lap joint engagement. Such a configuration is depicted in FIG. 9, where a cross-sectional enlarged view of the juncture between a sidewall  32  of a base  12  and an extended portion  93  of a mid-cover  14  is provided. The lap joint is formed via a step portion  99  along the inside of extended wall  93  of mid-cover  14  at the lower end thereof and a corresponding and complementary step portion  31  along the outside of sidewall  32  of base  12  at the upper end thereof. The dimensions of the molded case and mid-cover allow for a secure press fit or a frictional engagement therebetween. In a preferred embodiment shown in FIG. 9, the heights x of step portions  33  and  99  are equivalent to the wall thickness x. In a most preferred embodiment, the widths of step portions  33  and  99  are approximately equivalent to half of the wall thickness x, or 0.5x. 
     Although the molded circuit breaker enclosure of the present invention is described herein with reference to a multi-pole circuit breaker, is understood by one skilled in the art that the enclosure design may be adapted for more or less cassettes as needed. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.