Abstract:
A multipolar circuit breaker of great breaking capacity prevents arc gas from blowing out from the side at the time of cutting off a short-circuit current. A stationary contact and a movable contact generating arcs when a short-circuit current is cut off, and an arc extinguisher extinguishing the arc are stored in an arc extinguisher casing. By forming the ceiling, the bottom and the sidewalls connecting the ceiling and the bottom of the arc extinguisher casing in continuity so that no gap is formed by an outward internal pressure, blow out of arc gas from the sidewall portion is suppressed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a multipolar circuit breaker having a synthetic resin mold insulative casing.  
           [0003]    2. Description of the Background Art  
           [0004]    A circuit breaker has switch contacts, a switch mechanism operating the switch contact, a tripping mechanism operating when overcurrent flows to separate the switch contact via the switch mechanism, an arc extinguisher and the like disposed within a synthetic resin mold insulative casing.  
           [0005]    An example of a conventional circuit breaker is disclosed in Japanese Patent Laying-Open No. 63-119126. FIG. 23 is a sectional view of this circuit breaker, viewed from the side. In the casing formed of a synthetic resin mold insulative base  100  and a synthetic resin mold insulative cover  101  coupled to each other through a screw are provided a main circuit conductor including a stationary switch contact  102  also serving as a terminal base, a movable switch contact  103 , a flexible conductor  104 , a unit conductor  105  in a trip unit  109  that will be described afterwards, and a terminal base  106 . Also, a switch mechanism  107  operating movable switch contact  103 , a trip unit  109  operating when overcurrent flows to release the engagement of a latch mechanism  108  to separate movable switch contact  103  from stationary switch contact  102  via the switch mechanism, and an arc extinguisher  110  are provided in the casing.  
           [0006]    In such a conventional circuit breaker, there is a limit in the strength and tightening force of the screw that couples the base and cover together. Arc gas of high pressure generated when the circuit breaker cuts off a great short-circuit current will form a gap at the abutting face between the base and cover. The arc gas blows out through this gap at the side of the circuit breaker. There was a problem that the breaking capacity could not be increased.  
           [0007]    This conventional circuit breaker must have a robust overall casing since the arc gas will spread entirely within the casing. There was also the problem that the conventional circuit breaker was increased in size and cost since the trip unit must be accommodated in the casing.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a multipolar circuit breaker of large breaking capacity without arc gas blowing out from the sidewall.  
           [0009]    Another object of the present invention is to provide a compact and inexpensive multipolar circuit breaker.  
           [0010]    According to an aspect of the present invention, a multipolar circuit breaker includes a switch contact, an arc extinguisher, an input terminal, an output terminal, and an overcurrent tripping device for each pole. The multipolar circuit breaker includes a switch mechanism to operate the switch contact, and a trip mechanism to separate the switch contact through the switch mechanism in the operation of the overcurrent tripping device, common to each pole. The multipolar circuit breaker includes an arc extinguisher casing having a ceiling portion, a bottom portion and a sidewall joining the ceiling portion and the bottom portion and formed in continuity so that no gap is generated by an outward internal pressure. The switch contact and an arc extinguisher for each pole are stored in a separated manner from the counterpart of a neighbor pole by a partition wall separating respective poles, located parallel to the sidewall of the arc extinguisher casing.  
           [0011]    Since the ceiling portion, bottom portion and sidewall of the arc extinguisher casing are formed continuously, absent of an abutting portion, arc gas will not blow out from the side of the circuit breaker. The configuration of the above-mentioned portion of the arc extinguisher casing being formed so that no gap is generated by the internal pressure includes the case where the members are formed integrally in continuity by integral-molding as well as the case where members are overlapped on each other to avoid formation of an abutting portion. The latter case includes the layering of another sheet-like member from the inner side of the abutting portion to prevent gap generation. The separation by the partition wall will prevent the arc gas, when generated at one pole, from affecting the other poles. The arc extinguisher casing is preferably a synthetic resin mold insulator having a rectangular cross section.  
           [0012]    In the multipolar circuit breaker of the present aspect, the portion of at least the ceiling portion, bottom portion and the sidewall of the arc extinguisher casing can be formed in one piece by integral-molding. Therefore, fabrication thereof is simple.  
           [0013]    In the multipolar circuit breaker of the present aspect, the arc extinguisher casing includes an upper member having the ceiling portion and the portion of the sidewall located at the ceiling side formed in one piece by integral-molding, and a lower member having the bottom portion and the portion of the sidewall located at the bottom side formed in one piece by integral-molding. Also, the portion of the sidewall of the upper member can be overlapped with the portion of the sidewall of the lower member. The sidewall formed by two sidewalls is absent of an abutting portion. Therefore, arc gas will not blow out at the side of the circuit breaker.  
           [0014]    In the multipolar circuit breaker of the present aspect, the switch mechanism can be provided on the outer surface of the arc extinguisher casing. Therefore, the robust arc extinguisher casing that must withstand arc gas can be limited to the size that can store the switch contact and the arc extinguisher.  
           [0015]    The multipolar circuit breaker of the present invention includes a switch unit with a switch contact, an arc extinguisher, an arc extinguisher casing, and a switch mechanism; a trip unit having an overcurrent tripping device stored in a synthetic resin mold insulative trip unit casing; and a synthetic resin mold insulative cover enclosing the switch unit and the trip unit.  
           [0016]    By such a structure, a circuit breaker can be fabricated easily by assembling a switch unit and an overcurrent trip unit individually and then coupling both units with each other. Therefore, in the case where there are compatible units of a plurality of types due to difference in the rated current or the like, exchange is allowed even after the two units have been coupled. Also, only the arc extinguisher casing requires strength to withstand the arc gas, and the trip unit casing and cover may be of lower strength. Furthermore, a molded component that can entirely accommodate the arc extinguisher casing, switch mechanism and the trip unit casing is not required, so that the entire dimension can be reduced.  
           [0017]    In the multipolar circuit breaker of the present invention, the synthetic resin mold insulative cover includes a ceiling portion that is substantially rectangular when viewed in plan, and a sidewall extending from the four sides of the ceiling portion and being in close contact with the arc extinguisher casing and trip unit casing in parallel. By virtue of this structure, the coupling of the two units is enhanced by the cover.  
           [0018]    In the multipolar circuit breaker of the present invention, any one of a convex portion and a concave portion that are both engageable with the other counterpart is formed at respective sidewalls of the arc extinguisher casing and trip unit casing, whereas the other engageable counterparts of the convex portion or the concave portion are formed at at least one pair of sidewalls opposite to each other of the synthetic resin mold insulative cover. By this structure, a screw to attach the cover is not required.  
           [0019]    In the multipolar circuit breaker of the present invention, the connection portion of the main circuit conductor included in the switch unit and the trip unit can include a screw that is screwed in from the back side of the circuit breaker. By this structure, the space to attach the screwing tool at the surface side of the coupling portion is dispensable. This provides a margin in the structure of the switch mechanism and the trip mechanism.  
           [0020]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a side sectional view of a circuit breaker according to an embodiment of the present invention.  
         [0022]    [0022]FIG. 2 is a front view of the appearance of the circuit breaker of FIG. 1.  
         [0023]    [0023]FIG. 3 is a side view of the appearance of the circuit breaker of FIG. 1.  
         [0024]    [0024]FIG. 4 is a partial sectional view of FIG. 2 taken along line IV-IV.  
         [0025]    [0025]FIG. 5 is an exploded perspective view of the switch mechanism portion of the circuit breaker of FIG. 1.  
         [0026]    [0026]FIG. 6 is an exploded perspective view of the arc extinguisher portion of the circuit breaker of FIG. 1.  
         [0027]    [0027]FIG. 7 is a diagram to describe the method of mounting a terminal base of the circuit breaker of FIG. 1.  
         [0028]    [0028]FIGS. 8 and 9 are side sectional views of the switch unit of the circuit breaker of FIG. 1 in a closed state and an open state, respectively.  
         [0029]    [0029]FIG. 10 is a side sectional view of the switch unit of the circuit breaker of FIG. 1 in a tripped state.  
         [0030]    [0030]FIG. 11 is a side sectional view of the switch unit of the circuit breaker of FIG. 1 during a reset operation.  
         [0031]    [0031]FIGS. 12 and 13 are partial side sectional views of the switch unit of the circuit breaker of FIG. 1 with different section line.  
         [0032]    [0032]FIG. 14 is a front view of the circuit breaker of FIG. 1.  
         [0033]    [0033]FIG. 15 is a perspective view of the appearance of the trip unit of the circuit breaker of FIG. 1.  
         [0034]    [0034]FIG. 16 is a plan view of the circuit breaker of FIG. 15.  
         [0035]    FIGS.  17  is a sectional view of FIG. 16 taken along line XVII-XVII and  
         [0036]    [0036]FIG. 18 are side sectional views of the circuit breaker of FIG. 15.  
         [0037]    [0037]FIG. 19 is a perspective view of the cover.  
         [0038]    [0038]FIG. 20A is a partial sectional view of FIG. 3 taken along line XXA-XXA.  
         [0039]    [0039]FIG. 20B is a diagram to describe a fixture of the cover.  
         [0040]    [0040]FIGS. 21A and 22 are diagrams to describe an arc extinguisher casing according to other embodiments of the present invention.  
         [0041]    [0041]FIG. 21B is a sectional view of FIG. 21A taken along line XXIB-XXIB.  
         [0042]    [0042]FIG. 23 is a side sectional view of a conventional multipolar circuit breaker. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    Embodiments of the present invention will be described hereinafter with reference to FIGS.  1 - 22 . The circuit breaker according to the embodiment shown in FIGS.  1 - 20 B is the  3 -pole type, and includes a switch unit, a trip unit, and a cover enclosing the two units. FIG. 2 is a front view of the entire circuit breaker of the present embodiment. FIG. 1 is a sectional view of the circuit breaker of FIG. 2 taken along line I-I. FIG. 3 is a side view of the appearance of the circuit breaker of the present embodiment. FIG. 4 is a sectional view of the circuit breaker of FIG. 2 taken along line IV-IV, wherein for the sake of clarifying the drawing, the left pole does seemingly not have an arc extinguisher  9  that is actually contained in the left pole and will be described afterwards.  
         [0044]    The switch unit is shown in FIGS.  1 - 14 , and the structure thereof will be described mainly based on FIGS. 1, 4,  5 ,  6 ,  7 ,  12  and  14 . The switch unit includes arc extinguisher casing  1  molded from insulative synthetic resin, a switch contact disposed in arc extinguisher casing  1 , an arc extinguisher stored in arc extinguisher casing  1 , and a switch mechanism provided at the outer side of arc extinguisher casing  1 .  
         [0045]    Arc extinguisher casing  1  has a bottom  1   a,  a ceiling  1   b,  sidewalls  1   c  and two partition walls id formed by integral-molding. Arc extinguisher casing  1 , when viewed from the side, has a configuration in which the left portion of the ceiling is low and the middle portion becomes gradually higher so that the right side portion is higher. The two partition walls  1   d  disposed vertically divide the middle portion and the right side portion into three rooms. In each room are provided a stationary contact  3  with a contact tip  2  and a terminal portion  3   a  at respective ends, a movable contact  6  with a contact tip  4  and a flexible lead  5  at respective ends, and an arc extinguisher  9  having a plurality of magnetic steel plates  8  held by an insulation plate  7 . The opening at the right side of each room is closed by a synthetic resin mold insulative terminal base  10  mounted on a plane extending from bottom  1   a  of arc extinguisher casing  1 . Terminal base  10  includes a block portion  10   a  supporting terminal portion  3   a  of stationary contact  3  and a wall portion  10   b.  A terminal through hole  10   c  and a vent hole  10   d  are formed in wall portion  10   b.    
         [0046]    Movable contact  6  is held on an arm  11   a  of a synthetic resin mold insulative cross bar  11  common to the 3 poles, in a rotatable manner by a shaft  12 , and urged clockwise by a spring  13  provided between the walls of cross bar  11  and movable contact  6 . The other end of flexible lead  5  is connected to a lead terminal  14 . A lead terminal base  15  accommodating lead terminal  14  of each pole is formed of a synthetic resin mold insulator. A hexagonal dent into which a nut  16  is fitted is formed at the region where lead terminal  14  is to be disposed. Cross bar  11  has a columnar support  11   b  at both ends. Support  11   b  is fitted between a U-shaped groove  1   e  formed in both sidewalls  1   c  of arc extinguisher casing  1  and the concave formed in lead terminal base  15  to support cross bar  11  in a rotatable manner.  
         [0047]    The inclination of the ceiling at the middle portion of arc extinguisher casing  1 , when viewed from the side, corresponds to the rotation angle of movable contact  6  and cross bar  11 . The height of the ceiling at the right side is set as low as possible while ensuring the current breaking performance (in the present embodiment, the lowest height allowing storage of arc extinguisher  9 ) to minimize the size of arc extinguisher casing  1 .  
         [0048]    The switch mechanism to rotate cross bar  11  is built on a pair of frames  17  provided along the two sidewalls of arc extinguisher casing  1  at the middle pole area. The switch mechanism employs the toggle link mechanism. Specifically, the switch mechanism includes a pair of links  19  and  20  connected to each other in a rotatable manner by a shaft  18 . The switch mechanism also includes a handle lever  21 , a trip lever  22 , a hook  23  to prevent the clockwise rotation of trip lever  22 , and a latch  24  preventing the counter clockwise rotation of hook  23 , all provided on frame  17  in a rotatable manner. The switch mechanism further includes a spring  25  provided between shaft  18  and handle lever  21 , and a synthetic resin mold insulative handle  26  inserted at the horn of handle lever  21 .  
         [0049]    Handle lever  21  is formed in a bent manner having an angulated U-shape, so that both arms move within a plane identical to that of frame  17 . An arcuated end  21   a  of the arm of handle lever  21  is urged towards a notch-like concave  17   a  formed in frame  17  by the force of spring  25 . Trip lever  22  is formed in a bent manner having an angulated U-shape so that both arms move along a plane adjacent to the inner side of frame  17 . A curl end  22   a  formed at the arm of trip lever  22  is fitted in a columnar projection  17   b  formed at the facing planes of the pair of frames  17 . The pair of links  19  is arranged to move within a plane identical to that of the arm of trip lever  22 . An arcuated end  19   a  of link  19  is urged against a notch-like concave  22   b  formed at both arms of trip lever  22  by the force of spring  25 . The pair of links  20  is connected to cross bar  11  in a rotatable manner by a shaft  27 .  
         [0050]    [0050]FIG. 12 shows a portion of this switch mechanism when viewed from outside the plane of frame  17 . Hook  23  has both legs  23   a  disposed in a triangular hole  17   c  formed in frame  17 . A latch  24  has both legs  24   a  disposed in a V-shaped notch  17   d  formed in frame  17 . Hook  23  and latch  24  are urged clockwise and counterclockwise, respectively, by the force of spring  28 .  
         [0051]    The switch unit is assembled as shown in FIGS. 5 and 6. Referring to FIG. 5, the above-described components of the switch mechanism are temporarily assembled on the pair of frames  17 . Specifically, the components of the switch mechanism are attached to arc extinguisher casing  1  so that support  11   b  of cross bar  11  fits in a U-shaped groove  1   e  of arc extinguisher casing  1  and frame  17  fits in a groove if formed at both sides of arc extinguisher casing  1  at the middle pole region. At this stage, lead terminal base  15  has its projection  15   a  engaged with notch  17   e  in frame  17  to be securely fastened, as shown in FIG. 12. Frame  17  on which these components are temporarily assembled is fastened by means of a screw  29  through partition wall  1   d  of arc extinguisher casing  1 .  
         [0052]    Referring to FIG. 6, stationary contact  3  and arc extinguisher  9  of each pole are inserted from the right side of arc extinguisher casing  1 . Terminal base  10  is mounted on an extending plane of bottom  1   a  of arc extinguisher casing  1 . Specifically, terminal base  10  is mounted as set forth in the following. Referring to FIG. 7, terminal base  10  is slightly tilted and inserted so that projection  10   e  formed at the bottom of terminal base  10  fits into the two round holes  1   g  formed at the extending plane of arc extinguisher casing  1 . Concurrently, terminal portion  3   a  of stationary contact  3  is inserted into terminal through hole  10   c.  Terminal base  10  is arranged vertically as indicated by the chain-dotted line and inserted into arc extinguisher casing  1 . A terminal nut  30  is disposed between block portion  10   a  and terminal portion  3   a  to receive terminal screw  31 . Projection  10   e  of terminal base  10  has a hole communicating upwards. The screw used to mount the circuit breaker of the present embodiment is screwed into this hole, whereby a cover  48  that will be described afterwards, terminal base  10  and arc extinguisher casing  1  are secured together.  
         [0053]    The switch unit of the above-described configuration independently enables itself to make the operation of opening, closing, tripping, and resetting, as will be described hereinafter. FIGS. 8, 9,  10 , and  11  are side sectional views of the switch unit corresponding to a closed state, an open state, a tripped state and a reset state, respectively. FIGS. 12 and 13 show sectional views of a portion of the switch unit, viewed from the outer side of the other front-side frame  17 , corresponding to a closed state and a tripped state, respectively. FIG. 14 is a front view of the switch unit in an open state, absent of the illustration of handle  26 .  
         [0054]    In the closed state shown in FIG. 8, the rotation of handle lever  21  urged clockwise by the force of spring  25  about arcuated end portion  21   a  is blocked by the contact at the edge of frame  17 . Trip lever  22  is urged clockwise about projection  17   b  of frame  17  through the force of spring  25  via shaft  18  and link  19 . Trip lever  22  urges hook  23  counterclockwise against the force of spring  28  by the engagement of latch portion  22   c  and a latch receiving portion  23   b  of hook  23 . This urge causes latch  24  clockwise against the force of spring  28  via latch portion  23   c  of hook  23  and latch receiving portion  24  of latch  24 . Since the rotation of latch  24  is blocked by the V side of notch  17   d  of frame  17 , the rotation of hook  23  and trip lever  23  are also blocked. Here, shaft  18  is urged rightwards by the force of spring  25 . The lower end portion of link  19  abuts against curl end  22   a  of trip lever  22  to stop with link  20  and link  19  in a substantially straight stretching state. Contact is established between contacts  2  and  4  with cross bar  11  rotated clockwise.  
         [0055]    The opening operation is effected by rotating handle  26  of the circuit breaker counterclockwise. This rotation causes the line of action of the force of spring  25  to be shifted from the right to left of arcuated end portion  19   a  of link  19 , whereby shaft  18  moves leftward so that link  20  and link  19  take the arrangement of crossing in a bent manner. As a result, cross bar  11  rotates counterclockwise to open the contacts, and then comes into contact with a portion not shown of arc extinguisher casing  1  to be suppressed in rotation. Handle lever  21  has its bent portion  21   b  brought into contact with trip lever  2  to be blocked of rotation, attaining the open state of FIG. 9.  
         [0056]    The close operation is effected by rotating handle  26  of the circuit breaker clockwise. This rotation causes the line of action of spring  25  to move from the left to right of arcuated end portion  19   a  of link  19 , whereby shaft  18  moves rightward while links  19  and links  20  are arranged in a straight stretching manner. Therefore, cross bar  11  rotates clockwise to attain the closed state of FIG. 8.  
         [0057]    The trip operation is realized by rotating latch  24  attaining a closed state counterclockwise. This rotation cancels the engagement between latch  24  and hook  23  and the engagement between hook  23  and trip lever  22 . As a result, trip lever  22  rotates clockwise while pushing bent portion  21   b  of handle lever  21  until contact is established with bent portion  17   f  of frame  17  shown in FIG. 14. At this stage, links  19  and links  20  both move upwards while cross bar  11  rotates counterclockwise, whereby the contacts open. Then, cross bar  11  comes into contact with arc extinguisher casing  1  to attain the tripping state of FIGS. 10 and 13 where the rotation is suppressed.  
         [0058]    The reset operation is effected by rotating handle  26  of the circuit breaker attaining a tripping state counterclockwise. This rotation causes trip lever  22  to be rotated counterclockwise in response to the push of bent portion  21   b  of handle lever  21 . Following the travel of a shoulder portion  22   d  of trip lever  22 , hook  23  rotates clockwise and latch  24  rotates counterclockwise. As a result, the engageable state between trip lever  22  and hook  23  is established, as shown in FIG. 11. By freeing handle  26  under this state, engagement is established between trip lever  22  and hook  23  and between hook  23  and latch  24 , attaining the open state of FIG. 9.  
         [0059]    As shown mainly in FIG. 1 and FIGS.  15 - 18 , the trip unit is provided in a synthetic resin mold insulative trip unit casing  32 . This trip unit includes a heater  33  and a terminal  34  which are the main circuitry conductor portion, a terminal screw  35 , a bimetal tripping device and an instant tripping device that are provided for each pole. This trip unit also includes a rotatable trip shaft  36  and a latch shaft  37  provided common to the three poles.  
         [0060]    The bimetal tripping device includes a bimetal  38  attached at the root to the reverse U-shaped flexion of heater  33  as shown in FIG. 1. This bimetal trip device has bimetal  38  gradually curved leftwards by the Joule heat generated at heater  33  when overcurrent flows. At an elapse of a predetermined time, the leading end of bimetal  38  presses a pin  39  attached at an arm  36   a  of trip shaft  36 , whereby trip shaft  36  urged clockwise by a spring not shown is rotated counterclockwise.  
         [0061]    The instant tripping device includes a fixed core  40  attached to heater  33 , a movable core  42  provided rotatably to shaft  41  and bent in an angulated U-shaped manner, and a spring  43  that urges movable core  42  counterclockwise, as shown in FIG. 1. When a current generating an electromagnetic attraction exceeding the urging force of spring  43  flows to heater  33 , movable core  42  instantly rotates clockwise. The leading end of movable core  42  pushes a lower arm  36   b  of trip shaft  36 , whereby trip shaft  36  rotates counterclockwise.  
         [0062]    Latch shaft  37  includes a latch arm  37   a,  a switch mechanism trip arm  37   b,  two trip set arms  37   c  and a tripping device reset arm  37   d.  Latch shaft  37  is urged counterclockwise in FIG. 1 by spring  44 . Latch arm  37   a  engages latch reception  36   c  provided at trip shaft  36 , whereby the rotation of latch arm  37   a  is blocked. A reset lever  45  is provided in a rotatable manner by a shaft  46  at a front wall  32   a  of trip unit casing  32 . The notch portion of reset lever  45  is arranged to engage with a pin provided at latch shaft  37  in an eccentric manner. Therefore, reset lever  45  and latch shaft  37  are arranged so that, when one thereof rotates, the other will also rotate.  
         [0063]    The operation of the trip unit will be described hereinafter. FIG. 17 is a sectional view of the trip unit taken along line XVII-XVII of FIG. 16 showing an untripping state. FIG. 18 shows the trip unit in a tripping state.  
         [0064]    When the instant tripping device or bimetal tripping device operates to cause trip shaft  36  to rotate counterclockwise, the engagement between latch arm  37   a  and latch reception  36   c  is canceled. Therefore, latch shaft  37  rotates counterclockwise. The leading end of trip arm  37   b  protrudes out from front wall  32   a  of trip unit casing  32  and reset lever  45  rotates clockwise.  
         [0065]    The reset operation of the trip unit is realized by rotating reset lever  45  counterclockwise manually against the force of spring  44 . This rotation causes latch shaft  37  to rotate clockwise. When reset lever  45  is made free after latch arm  37   a  and latch reception  36   c  attain an engageable state, latch shaft  37  rotates counterclockwise. As a result, a reset state where latch arm  37   a  and latch reception  36   c  engage is established.  
         [0066]    As described above, the circuit breaker independently enables itself to make the operation of the instant tripping device or bimetal tripping device adjusted and confirmed without connecting with the switch unit.  
         [0067]    The coupling of the switch unit and the trip unit configured as described above will be set forth hereinafter with reference to FIGS.  1 - 4 ,  19 ,  20 A, and  20 B. Trip unit casing  32  is formed with a plane portion  32   b  having an inclined leading end, and a U-shaped block portion  32   c  provided at plane portion  32   b  for each pole. An end portion  33   a  of heater  33  is located at the upper end portion of block portion  32   c.  A hexagon socket screw  47  is inserted between the legs of block portion  32   c  and between end portion  33   a  of heater  33  and plane portion  32   b.  A small through hole  32   d  for the passage of a tool to turn screw  47  is formed in plane portion  32   b.    
         [0068]    The switch unit and the trip unit are coupled so that end portion  33   a  of heater  33  and lead terminal  14  overlap, and the inclining surface of plane portion  32   b  and the inclining surface at the left side of the bottom of arc extinguisher casing  1  overlap. Then, screw  47  is turned around from the bottom to securely fasten lead terminal  14  and end portion  33   a  with nut  16 .  
         [0069]    The units coupled as described above are covered with a mold insulative cover  48  shown in FIG. 19. The coupling between the units is strengthened by the wall extending vertically from the four sides of the front portion of the cover. Specifically, as shown in FIG. 1, a vertical wall  48   a  located at the right side of cover  48  extends in the direction of terminal portion  3   a  along wall  10   b  of terminal base  10  in close contact, and vertical wall  48   b  at the left side of cover  48  extends in the direction of terminal  34  along the edge of the sidewall of trip unit casing  32  in close contact. Thus, the coupling in the left and right directions is enhanced. Furthermore, as shown in FIGS. 1, 3,  5 ,  19 ,  20 A, and  20 B, a sidewall  48   c  extends from both side ends of the front portion of cover  48  along the sidewalls of trip unit casing  32  and arc extinguisher casing  1  in close contact. Cover  48  is also secured by engaging a plurality of rectangular holes  48   d  formed at the lower edge portion of sidewall  48   c  with a plurality of projections  1   h  of arc extinguisher casing  1  and a projection  32   e  of trip unit casing  32 . Thus, the circuit breaker shown in FIG. 3 is enhanced in the width direction of the circuit breaker as well as the horizontal and vertical directions. Since sidewall  48   c  of cover  48  is formed relatively thin and has elasticity, the above engaging process can be carried out easily and will not be readily disengaged. However, for the sake of ensuring the fixation, a fixture  49  is provided, as shown in FIG. 3 and FIG. 20A which is a cross section taken along line XXA-XXA of FIG. 3. Fixture  49  has an H-shaped cross section shown in the perspective view of FIG. 20B. This fixing bracket  49  is attached so as to sandwich the slit portion formed at the lower edge of sidewall  48   c  of cover  48  and the outer walls of a groove  32   f  where a slit is formed at the outer wall of trip unit casing  32 . A similar fixing bracket is provided between cover  48  and arc extinguisher casing  1 .  
         [0070]    An opening  48   e  is formed at the front of cover  48  at the middle pole portion through which handle  26  protrudes outwards. Also, a dent  48   f  is formed at the side pole portion to accommodate internal accessories. An auxiliary cover  50  covering dent  48   f  that can be opened/closed by a hinge as shown by the chain dotted line in FIG. 3 is provided at cover  48 .  
         [0071]    The manipulation and operation of the circuit breaker coupled as described above are set forth below. The opening operation and closing operation are similar to those carried out by the switch unit alone described above. Therefore, description thereof will not be repeated.  
         [0072]    When the trip unit is operated by the bimetal tripping device or instant tripping device, the switching mechanism trip arm  37   b  of latch shaft  37  protrudes from front wall  32   a  of trip unit casing  32 , as shown in FIG. 15. Latch  24  that is the member of the switch unit in direct relation with the trip unit is indicated in FIGS. 17 and 18 in a chain dotted line. A trip arm  24   c  of latch  24  is urged by switch mechanism trip arm  37   b,  whereby latch  24  rotates counterclockwise to trip the switch unit.  
         [0073]    The reset operation is effected by rotating handle  26  in a trip state counterclockwise, as indicated by the chain dotted line in FIG. 18. In response to this operation, left end  26   a  of handle  26  pushes reset edge  45   a  of reset lever  45 , whereby reset lever  45  is rotated counterclockwise to attain a resettable state. At this stage, handle  26  is set free, whereby the trip unit is reset together with the switch unit described previously.  
         [0074]    With regards to the circuit breaker of the present invention, a plurality of types of trip units interchangeable according to the current capacity and a switch unit common to each trip unit are kept in stock separately. The circuit breaker of the present embodiment can be completed by coupling the trip unit and the switch unit according to a customer&#39;s order. The exchange of a trip unit in a circuit breaker completed as a product can be readily carried out. Specifically, screw  47  is loosened, and fixture  49  of the trip unit is removed. Engagement between rectangular hole  48   d  of cover  48  and projection  32   e  of trip unit casing  32  is canceled. The trip unit is detached from the switch unit. Then, a new trip unit is to be mounted in an order opposite to that described above.  
         [0075]    In contrast to the arc extinguisher casing having the bottom, the ceiling, both sidewalls and the partition walls formed integrally as in the above-described embodiment, the object of the present invention can be achieved even with an arc extinguisher casing including an upper side member and a lower side member that can be divided into upper and lower parts, as shown in FIGS. 21A, 21B, and  22 . FIG. 21A is a front view with the upper side member and the lower side member coupled, and FIG. 21B is a sectional view taken along line XXIB-XXIB of FIG. 21A. FIG. 22 is an exploded perspective view of this combination. The arc extinguisher casing is formed of a synthetic resin mold insulative lower member  60  and upper member  61 . Lower member  60  includes a bottom  60   a,  sidewalls  60   b,  and an auxiliary partition wall  60   d  upright from bottom  60   a,  at a portion corresponding to the partition wall between the poles. Upper member  61  includes a ceiling  61   a,  sidewalls  61   b,  a wall  60   c  with a vent hole, a partition wall  61   d,  and a groove  61   e  formed in partition wall  61   d.  This arc extinguisher casing is arranged so that sidewall  61   b  of upper member  61  is in close contact at the inner side of sidewall  61   b  of lower member  60 , and that groove  61   e  of sidewall  6   b  of upper member  61  engages auxiliary partition wall  60   d  of lower member  60 . The overlap of the sidewalls of the upper member and the lower member prevents arc gas from blowing out from the side in such an arc extinguisher casing. Furthermore, the overlap between the partition wall between the poles and the auxiliary partition wall prevents arc gas from flowing into the adjacent pole. Likewise the previous embodiment, a synthetic resin mold insulative cover  62  can be mounted to the arc extinguisher casing arranged as described above.  
         [0076]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention.