Patent Publication Number: US-6222433-B1

Title: Circuit breaker thermal magnetic trip unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 09/501,425 filed Feb. 10, 2000, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to circuit breakers and more particularly to a circuit breaker employing a thermal-magnetic trip unit having an over centering mechanism for unlatching the circuit breaker operating mechanism and a trip flag system that discriminates between a short circuit trip and an overcurrent trip. 
     Circuit breakers typically provide protection against persistent overcurrent and against very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a thermal-magnetic trip unit having a thermal trip portion, which trips the circuit breaker on persistent overcurrent conditions, and a magnetic trip portion, which trips the circuit breaker on short-circuit conditions. 
     In order to trip the circuit breaker, the thermal magnetic trip unit must activate an operating mechanism. Once activated, the operating mechanism separates a pair of main contacts to stop the flow of current in the protected circuit. Conventional trip units act directly upon the operating mechanism to activate the operating mechanism. In current thermal-magnetic trip unit designs, the thermal trip portion includes a bimetallic strip (bimetal), which bends at a predetermined temperature. The magnetic trip portion includes an anvil disposed about a current carrying strap and a lever disposed near the anvil, which is drawn towards the anvil when high, short-circuit currents pass through the current carrying strap. The force created by the bimetal or lever, and the distance that they travel, may be insufficient to directly trip the operating mechanism. A conventional way to solve this problem is to use a latch system as a supplemental source of energy. However, the drawback of a latch system is the use of latching surfaces, which degenerate over repeated use. 
     Further, a circuit breaker having a thermal-magnetic trip unit can be tripped by three events, namely: overcurrent, short circuit and ground fault. It is important to know the cause due to which a breaker has tripped. Distinguishing the reasons for tripping allows the user to determine if the breaker can be reset immediately, as in the case of an overcurrent, or only after careful inspection of the circuitry, as in the case of a short circuit or ground fault. 
     Circuit breaker trip mechanisms of the prior art have solved this problem by the use of flags, which are visible through windows disposed in the case of the circuit breaker. In such trip mechanisms, a flag appears in one window upon the occurrence of an overcurrent condition, while another flag appears in another window upon the occurrence of a short-circuit condition. This solution works well for trip units having an inactive bimetal. That is, for trip units where the bimetal does not carry electrical current, but is attached to a current-carrying strap. However, this solution can provide indeterminate indications when it is used with a trip unit having an active bimetal. That is, when it is used with a trip unit where the bimetal carries electrical current. When such an active bimetal is used, it is possible during a short circuit event that, in addition to the magnetic trip portion, the bimetal also moves to expose the overcurrent flag, thereby leading to both the short-circuit and overcurrent flags being shown thus providing an indeterminate indication to the user. 
     SUMMARY OF INVENTION 
     In an exemplary embodiment of the present invention, a circuit breaker trip mechanism includes an over centering spring tripping linkage. The trip unit consists of a trip bar having a first leg and a second leg. The trip bar is rotatably mounted within the case about a first pivot where the first leg is adjacent to a bimetal mounted within the circuit breaker trip mechanism. A link, having a third leg and a fourth leg, is rotatably mounted within the case about a second pivot. The second leg is pivotally engaged to the third leg of the link by a moveable pin which slides in a slot in the trip bar. The fourth leg of the link is pivotally engaged to a slide by a moveable pin. A slide projection extending outward from the slide is disposed between the first end and the second end of the slide. Further, the link is biased in a first direction about second pivot when the trip unit is in a reset condition and biased in a second direction about pivot when the trip bar is rotated about first pivot thereby urging the slide to interact with the trip lever of the circuit breaker operating mechanism. 
     In a further exemplary embodiment of the present, an improved indication-of-trip system is employed comprising a two-piece trip bar mechanism. In this embodiment of the invention, visual confirmation of the cause of the trip is provided. This embodiment includes a second trip bar having a fifth and sixth leg. The second trip bar is rotatably mounted within the case about a third pivot. A second link, having a seventh leg and an eighth leg, is rotatably mounted within the case about a fourth pivot. The sixth leg is pivotally engaged to the seventh leg of the second link by a moveable pin. The eighth leg of the second link is pivotally engaged to a second slide by a moveable pin. A slide projection extending outward from the second slide is disposed between the third end and the fourth end of the second slide. Further, the second link is biased in a first direction about the fourth pivot when the trip unit is in a reset condition and biased in a second direction about the fourth pivot when the second trip bar is rotated about the third pivot thereby urging the second slide to interact with the trip lever of the circuit breaker operating mechanism. 
     The circuit breaker casein this embodiment of the invention includes a window disposed in the case in a location conducive to a user viewing a position indicator thus enabling the rapid determination of the type of trip that has occurred. To identify a trip caused by an overcurrent condition, an overcurrent indicator is employed with the first trip bar whereby the indicator senses the bimetallic force applied on the heat sensitive bimetal. To identify a trip caused by a short circuit condition, a short circuit indicator is employed with the second trip bar whereby the indicator senses the magnetic force applied to the improved indicator of trip bar system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a circuit breaker; 
     FIG. 2 is an exploded view of the circuit breaker of FIG. 1; 
     FIG. 3 is an illustration of the circuit breaker of FIG. 1 employing the spring trip unit; 
     FIG. 4 is an illustration of the indication of trip two-piece trip bar system; 
     FIG. 5 is an enlarged view of the second trip bar linkage of FIG. 4; and 
     FIG. 6 is an enlarged view of the position indicator and flag system of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an embodiment of a molded case circuit breaker  9  is generally shown. Circuit breakers of this type have an insulated case  11  and a mid-cover  12  that house the components of the circuit breaker  9 . A handle  20  extending through a cover  14  gives the operator the ability to turn the circuit breaker  9  “on” to energize a protected circuit (shown on FIG.  3 ), turn the circuit breaker “off” to disconnect the protected circuit (not shown), or “reset” the circuit breaker after a fault (not shown). When the circuit breaker is “on” a pair of electrical contacts  142  and  162  are closed thereby maintaining current flow through the circuit breaker  9 . A plurality of straps  156  and  35  also extend through the case  11  for connecting the circuit breaker  9  to the line and load conductors of the protected circuit. The circuit breaker  9  in FIG. 1 shows a typical three phase configuration, however, the present invention is not limited to this configuration but may be applied to other configurations, such as one, two or four phase circuit breakers. 
     Referring to FIG. 2, the handle  20  is attached to a circuit breaker operating mechanism  10 . The circuit breaker operating mechanism  10  is coupled with a center cassette  16 B and is connected with outer cassettes  16 A and  16 C by a drive pin  18 . The cassettes  16 A,  16 B, and  16 C along with the circuit breaker operating mechanism  10  are assembled into the base  2  and retained therein by the mid-cover  12 . The mid-cover  12  is connected to the base by any convenient means, such as screws  26 , snap-fit (not shown) or adhesive bonding (not shown). A cover  14  is attached to the mid-cover  12  by screws  28 . 
     A thermal-magnetic trip unit  22  enclosed within case  11  having straps  23 A,  23 B, and  23 C preferably attaching to the cassette straps  19 A,  19 B, and  19 C with screws  24 A,  24 B, and  24 C. Even though screws are shown herein for connecting the trip unit straps  23  to the cassette straps  19 , other methods commonly used in circuit breaker manufacture are contemplated, such as brazing. The trip unit  22  is assembled into the base  2  along with the cassettes  16 . Straps  23 A,  23 B, and  23 C conduct current from the power source to the protected circuit. 
     The internal operating mechanism  160  of the trip unit  22  is shown in FIG.  3 . The trip unit  22  consists of a trip bar (first trip bar)  30  having a first leg  33  and a second leg  64 . The trip bar  30  is rotatably mounted within the case  11  about a first pivot  32 . Link (first link)  34  is rotatably mounted within the case  11  about a second pivot  86 . Link  34  includes a third leg  88  and a fourth leg  90 , both extending from second pivot  86 . The second leg  64  of the trip bar  30  is pivotally engaged to the third leg  88  of link  34 , for example by a moveable pin  36  which slides in a slot  31  in the trip bar  30 . A slide  38  has a first end  70  and a second end  67 . The fourth leg  90  of link  34  is pivotally engaged to the first end  70  of the slide (first slide)  38 , for example by a moveable pin  40 . A slide projection  39  extending outward from slide  38  is disposed between the first end  70  and the second end  67  of the slide  38 . 
     Further, link  34  is biased in a first direction about pivot  86  when the trip unit is in a reset condition and biased in a second direction about second pivot  86  when the trip bar  30  is rotated about first pivot  32  thereby urging the slide  38  to interact with the trip lever  92  of the circuit breaker operating mechanism  10 . A first spring  42  having moveable and fixed ends and preferably connecting between a moveable pin  36  and a fixed pin  76  attached to the case  11 . The moveable end of the first spring  42  is attached to the third leg  88 . First spring  42  as shown in FIG. 3 is arranged to bias the slide  38  away from the trip lever  92 . The ends of the first spring  42  are pivoted with respect to first pivot  32 , such that, it initially provides a counter-clockwise moment on the trip bar  30  to prevent nuisance tripping. 
     A heat sensitive strip, for example a bimetal,  84 , having a first end  60  and a second end  62 , is attached at the first end  60  to the strap  23 B by a screw  44 . While this attachment is shown as a screw, any process commonly used in circuit breaker manufacturing can be used, such as brazing or welding. The second end  62  of the bi-metal  84  is adjacent to the first leg  33  of the trip bar  30 . While only one bimetal is shown here for clarity, a corresponding bimetal would be attached to the adjoining straps  23 A and  23 C. 
     A lever  48  having a first end  68  and a second end  72  is mounted within the case  11  and pivots about a pin  49 . The lever  48  is made of a ferrous material. Preferably, a ferrous plate  50  is mounted on the first end  68  of the lever  48 . An anvil  46 , preferably U-shaped, is positioned around the strap  23 B adjacent to the first end  68  of the lever  48 . The anvil  46  generates a magnetic field in proportion to the current level. The second end  72  of the lever  48  is adjacent the slide projection  39 . A second spring  80  connects between a pin  74  connected to the case  11  and a pin  82  located on the lever  48 . Second spring  80  is arranged to bias the lever  48  away from the slide projection  39  as shown in FIG.  3 . 
     When an overcurrent condition occurs, the strap  23 B generates heat that increases the temperature of the bimetal  84 . If the temperature of the bimetal  84  increases sufficiently, due to the current draw exceeding a predefined current level, the second end  62  of the bimetal  84  deflects from an initial position thereby engaging the trip bar  30 . The trip bar  30  rotates in the clockwise direction in response to the bimetal force rotatably engaging link  34 . Link  34  rotates in a counter-clockwise direction about second point  86  pushing the slide  38  from the reset position as shown in FIG. 3 to the released position towards trip lever  92  (the released position is shown in phantom lines). Once the trip bar  30  rotates to a preset position, the first spring  42  changes with respect to first pivot  32 , providing a moment that rotates the trip bar  30  in the clockwise direction. Thus, after reaching a preset position, the first spring  42  takes over from the bimetal  84  and provides the required force and motion so that the slide  38  can engage the trip lever  92  thereby tripping the mechanism  10 . In link  34 , the ratio between the lengths of third and fourth legs  88  and  90  provides for the magnification of the linear motion of the slide  38  relative to the movement of the trip bar  30  due to the force applied by the bimetal  84 . Thus, the linear movement of the slide  38  will generally be greater than the movement of the trip bar  30 . 
     When a short circuit condition occurs, a magnetic field in the anvil  46  is generated proportional to the current passing through strap  23 B. When the magnetic force attracting the ferrous plate  50  of the lever  48  is greater than a predetermined level, the first end  68  of the lever  48  is attracted to the anvil  46  causing the second end  72  to engage the slide projection  39  thereby moving the slide  38  to the released position towards trip lever  92  (the released position is shown in phantom lines). Once the trip bar  30  rotates to a preset position, the first spring  42  changes with respect to first pivot  32 , providing a moment that rotates the trip bar  30  in the clockwise direction. 
     It is noted that when an active bimetal is used, it is very possible during a short circuit event that in addition to the lever  48  engaging the slide projection  39  in response to the magnetic force generated by the anvil  46 , the bimetal  84  also engages the trip bar  30 . 
     In a further exemplary embodiment of the present invention, an improved indication-of-trip system is employed comprising a two piece trip bar mechanism. In this embodiment of the invention, visual confirmation of the cause of the trip is provided. This system is shown in FIGS. 4,  5  and  6 . The first trip bar mechanism includes the trip bar  30 , the link  34 , and the slide  38  as described hereinabove. The second trip bar mechanism includes a second trip bar  94 , a second link  100  and a second slide  104 . The first trip bar mechanism senses the bimetallic force and the second trip bar senses the magnetic force. 
     The internal operating mechanism  160  of the improved indication-of-trip system used in trip unit  22  is shown in FIG.  4 . The trip unit  22  consists of a trip bar  30  having a first leg  33  and a second leg  64 . The trip bar  30  is rotatably mounted within the case  11  about a first pivot  32 . Link  34  is rotatably mounted within the case  11  about a second pivot  86 . Link  34  includes a third leg  88  and a fourth leg  90 , both extending from second pivot  86 . The second leg  64  of the trip bar  30  is pivotally engaged to the third leg  88  of link  34 , for example by a moveable pin  36  which slides in a slot  31  in the trip bar  30 . A slide  38  has a first end  70  and a second end  67 . The fourth leg  90  of link  34  is pivotally engaged to the first end  70  of the slide  38 , for example by a moveable pin  40 . 
     Further, link  34  is biased in a first direction about pivot  86  when the trip unit is in a reset condition and biased in a second direction about pivot  86  when the trip bar  30  is rotated about first pivot  32  thereby urging the slide  38  to interact with the trip lever  92  of the circuit breaker operating mechanism  10 . The first spring  42 , having moveable and fixed ends and preferably connecting between a moveable pin  36  and a fixed pin  76  attached to the case  11 . The moveable end of the first spring  42  is attached to the third leg  88 . First spring  42  as shown in FIG. 3 is arranged to bias the slide  38  away from the trip lever  92 . The ends of the first spring  42  are pivoted with respect to first pivot  32 , such that, it initially provides a counter-clockwise moment on the trip bar  30  to prevent nuisance tripping. 
     In the second trip bar mechanism, the trip unit  22  also consists of a second trip bar  94  having a fifth leg  96  and a sixth leg  98 . The second trip bar  94  is rotatably mounted within the case  11  about a third pivot  144 . Second link  100  is rotatably mounted within the case  11  about a fourth pivot  148 . It is within the scope of this embodiment of the present invention and apparent to those skilled in the art that both trip bar  30  and second trip bar  94  could be modified to rotate about first pivot  32 , independent of each other. Second link  100  includes a seventh leg  128  and an eighth leg  130 , both extending from fourth pivot  148 . It is within the scope of this embodiment of the present invention and apparent to those skilled in the art that both link  34  and second link  100  could be modified to rotate about second pivot point  86 , independent of each other. The sixth leg  98  of the trip bar  94  is pivotally engaged to the seventh leg  128  of second link  100 , for example by a moveable pin  136  which slides in a slot  152  of the second trip bar  94 . Second slide  104  has a third end  102  and a fourth end  106 . The eighth leg  130  of second link  100  is pivotally engaged to the third end  102  of the second slide  104 , for example by a moveable pin  150 . A slide projection  140  extending outward from second slide  104  is disposed between the third end  102  and the fourth end  106  of the second slide  104 . 
     Further, second link  100  is biased in a first direction about fourth pivot  148  when the trip unit is in a reset condition and biased in a second direction about fourth pivot  148  when the trip bar  94  is rotated about third pivot  144  thereby urging the second slide  104  to interact with the trip lever  92  of the circuit breaker operating mechanism  10 . A third spring  138  having moveable and fixed ends and preferable connecting between the moveable pin  136  and a fixed pin  158  attached to the case  11 . The moveable end of the third spring  138  is attached to the seventh leg  128 . The third spring  138  as shown in FIG. 4 is arranged to bias the second slide  104  away from the trip lever  92 . The ends of the spring are pivoted with respect to third pivot  144 , such that, it initially provides a counter-clockwise moment on the second trip bar  94  to prevent nuisance tripping. 
     A heat sensitive strip, for example a bimetal,  84 , having a first end  60  and a second end  62 , is attached at the first end  60  to the strap  23 B by a screw  44 . While this attachment is shown as a screw, any process commonly used in circuit breaker manufacturing can be used, such as brazing or welding. The second end  62  of the bimetal  84  is adjacent to the first leg  33  of the trip bar  30 . While only one bimetal is shown here for clarity, a corresponding bimetal would be attached to the adjoining straps  23 A and  23 C. 
     A lever  48  having a first end  68  and a second end  72  is mounted within the case  11  and pivots about a pin  49 . The lever  48  is made of a ferrous material. Preferably, a ferrous plate  50  is mounted on the first end  68  of the lever  48 . An anvil  46 , preferably U-shaped, is positioned around the strap  23 B adjacent to the first end  68  of the lever  48 . The anvil  46  generates a magnetic field in proportion to the current level. The second end  72  of the lever  48  is adjacent the slide projection  140 . A second spring  80  connects between a pin  74  connected to the case  11  and a pin  82  located on the lever  48 . Second spring  80  is arranged to bias the lever  48  away from the slide projection  140 . Although the magnetic portion of the trip unit, as described hereinabove, engages a slide projection  140  on the second slide  104 , it is apparent to one skilled in the art that the magnetic portion can be modified to engage the third leg  96  of the second trip bar  94 . 
     When an overcurrent condition occurs, the strap  23 B generates heat that increases the temperature of the bimetal  84 . If the temperature of the bimetal  84  increases sufficiently due to the current draw exceeding a predefined current level, the second end  62  of the bimetal  84  deflects from an initial position thereby engaging the trip bar  30 . The deflection is proportional to the current level. The trip bar  30  rotates in the clockwise direction in response to the bimetal force rotatably engaging link  34 . Link  34  rotates in a counter-clockwise direction about point  86  pushing the slide  38  to the released position towards trip lever  92  (the released position is shown in phantom lines). Once the trip bar  30  rotates to a preset position, the first spring  42  changes with respect to first pivot  32 , providing a moment that rotates the trip bar  30  in the clockwise direction. Thus, after reaching a preset position, the first spring  42  takes over from the bimetal  84  and provides the required force and motion so that the slide  38  can engage the trip lever  92  thereby tripping the mechanism  10 . In link  34 , the ratio between the lengths of third and fourth legs  88  and  90  provides for the magnification of the linear motion of the slide  38  relative to the movement of the trip bar  30  due to the force applied by the bimetal  84 . Thus, the linear movement of the slide  38  will generally be greater than the movement of the trip bar  30 . 
     When a short circuit condition occurs, a magnetic field in the anvil  46  is generated proportional the current passing through strap  23 B. When the magnetic force attracting the ferrous plate  50  of the lever  48  is greater than a predetermined level, the first end  68  of the lever  48  is attracted to the anvil  46  causing the second end  72  to engage the slide projection  140  thereby moving the second slide  104  to the released position towards trip lever  92  (the released position is shown in phantom lines). Once the trip bar  94  rotates to a preset position, a third spring  138  changes with respect to third pivot  144 , providing a moment that rotates the trip bar  94  in the clockwise direction. Thus, after reaching a preset position, third spring  138  takes over from the lever  48  and moves the second slide  104  engaging the trip lever  92  and thereby tripping the mechanism  10 . In the second link  100 , the ratio between the lengths of the seventh and eighth legs  128  and  130  provides for the magnification of the linear motion of the slide  38  relative to the movement of the trip bar  94  due to the force applied by the lever  48 . Thus, the linear movement of the slide  38  will generally be greater than the movement of the trip bar  94 . 
     The case  11  in this embodiment of the invention includes a window  124  disposed therein in a location conducive to a user viewing an identification flag on the end of a position indicator thus enabling the rapid determination of the type of trip that has occurred. To identify a trip caused by an overcurrent condition, a position indicator (overcurrent indicator)  120  is employed. The overcurrent indicator  120  carries the first flag (overcurrent flag)  132  and senses the bimetallic force applied on the bimetal which is heat sensitive. To identify a trip caused by a short circuit condition, a position indicator (short circuit indicator)  122  is employed. The short circuit indicator  122  caries the second flag (short circuit flag)  134  and senses the magnetic force applied to the improved indicator of trip bar system. The overcurrent indicator  120  and flag  132  are viewable through the window  124  for indicating a tripped position which occurs when the current path is interrupted in response to a trip event caused by overheating. The overcurrent indicator  120  is located some distance between the first end  70  and second end  67  of the first slide  38 . The short circuit indicator  122  and second flag  134  are viewable through the window  124  for indicating a tripped position which occurs when the current path is interrupted in response to a short circuit. The short circuit indicator  122  is located some distance between the third end  102  and fourth end  106  of the second slide  104 . 
     If an overcurrent event occurs, then the first slide  38  moves to expose the first flag  132  through the window  124  of the case  11 . If a short circuit event occurs, only the second slide  104  moves to expose the second flag  134  through the window  124  of the case  11 . 
     When an active bimetal is used, it is very possible during a short circuit event that in addition to the lever  104  engaging the slide projection  128  in response to the magnetic force generated by the anvil, the bimetal  84  also engages the trip bar  30 . In this instance the first flag  132  would be exposed thereby leading to a false indication as to the cause of the trip. In order to address this situation, in this embodiment of the invention, the second flag  134  is located at a plane higher that the first flag  132 . Therefore, as shown in FIG. 5, the overcurrent indicator  120  is shorter in length than the short circuit indicator  122 . Also, the second flag  134  has an extended top surface which completely overlaps the first flag  132 . Therefore, during a short circuit event, only the second flag  134  is seen from the window  124  thereby preventing a false indication of what caused the trip event. 
     It is also within the scope of the present invention and apparent to one skilled in the art that a position indicator  120  and  122  may also be utilized on the slide  38  to indicate a trip caused by overheating or a short circuit. 
     The advantage of the over centering spring tripping mechanism is that it eliminates the requirement for latching surfaces which degenerate with repeated use. In addition, the mechanism provides the additional force and motion required to trip a circuit breaker. 
     Further, the two-piece trip bar and position indicator flag system discriminates between a trip caused by over heating and a trip caused by a short circuit. In addition, the position indicator and flag system does not mislead the user when a short circuit event has occurred. When a short circuit event has occurred, only the second flag  134 , and not the first flag  132 , is visible from the window  124  of the case  11 . 
     While this 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 the 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 rather that the invention will include all embodiments falling within the scope of the appended claims.