Patent Publication Number: US-9412549-B2

Title: Electromagnetically enhanced contact separation in a circuit breaker

Description:
Embodiments presented herein relate generally to electrical circuit interruption using circuit breakers, such as to dual-break linear contact assemblies with magnetic cores in molded case circuit breakers (MCCB). 
     BACKGROUND 
     Circuit breakers are employed to interrupt AC or DC, single-phase or multi-phase electrical circuits when electrical fault conditions exist. The electrical fault conditions can include an instantaneous current in the electrical circuit that exceeds a predefined instantaneous current limit (i.e., an electrical short exists) or a long-term current that exceeds a predefined long-term current limit (i.e., an electrical overload exists). A single-break MCCB typically comprises one stationary contact mounted to a stationary current loop and a movable contact mounted to a rotatable contact arm for each phase. A dual-break MCCB comprises two stationary contacts and two movable contacts for each phase. In normal operation when electrical current flows through the device the movable contacts are in contact with the stationary contacts and the breaker is in the closed state. In the presence of an electrical fault the flow of electrical current is interrupted by separating the movable contacts from the stationary contacts, and the breaker is going from a closed to a tripped state. The contact arm is moved from a closed to a tripped position by means of a mechanism that acts on the contact arm. 
     The energy to move the contact arm may also come from electromagnetic fields that develop around the stationary current loop and the contact arm due to the electrical fault currents that flow through these components. The interactions between the electromagnetic fields and the fault current result in a repulsion force between the stationary current loop and the contact arm which, in conventional approaches, causes the contact arm to rotate and separate the movable contact from the stationary contact. As the movable contact begins to separate an arc may form between the stationary contact and the movable contact. Due to the electromagnetic fields there will be an arc force acting on the arc and affecting its path. The magnitudes of the repulsion force and the arc force diminish as the fault current and electromagnetic field intensity drop and as the alignment of the rotating contact arm with respect to the stationary current loop and with respect to the electromagnetic field becomes less favorable as the contact arm rotates. 
     The movement of the contact arm from the closed to the tripped position has to be fast to limit current and to minimize formation of arcs that may degrade the contacts. The circuit breaker mechanism also has to be of suitable size to fit in a predefined circuit breaker casing or electrical panel. Therefore, there is a need for maximizing the repulsion forces that accelerates the contact arm and for reducing the size of the mechanism. 
     BRIEF DESCRIPTION 
     In one embodiment, a circuit breaker is provided. The circuit breaker includes a latch assembly configured to release and to engage a cradle of the circuit breaker and a handle yoke configured to move between an open position and a closed position when the latch assembly is engaged to the cradle. The circuit breaker further includes a dual-break linear contact assembly that includes: a bridge connector configured to conductively connect stationary bus bars; a guide rod attached to the bridge connector and defining a linear movement path for the bridge connector; and a magnetic core disposed about the linear movement path and comprising at least a set of arc chute plates. The circuit breaker also includes a linkage assembly mechanically coupling the cradle, the handle yoke, and the guide rod, the linkage assembly having a spring which, at an operational current level, holds the bridge connector in place at the stationary bus bars when the latch assembly and cradle are engaged and when the handle yoke is in the closed position. 
     In an additional embodiment, a dual-break linear contact assembly is provided. The contact assembly includes a bridge connector configured to conductively connect contacts of two stationary bus bars. An electromagnetic repulsion force is generated and acts on the bridge connector when a current passes through the stationary bus bars and the bridge connector. The electromagnetic force pushes the bridge connector away from the stationary bus bars. The contact assembly also includes a biasing spring component that applies a force in opposition to the electromagnetic repulsion force to hold the bridge connector component in contact with the stationary bus bars when the current is below a current threshold. 
     In a further embodiment, a method for interrupting a circuit path is provided. In accordance with this method, stationary bus bars are connected with a bridge connector component. An electromagnetic repulsion force is generated acting on the bridge connector component when a current is passed through the stationary bus bars and the bridge connector component. The electromagnetic repulsion force pushes the bridge connector component away from the stationary bus bars. An opposing force is applied to the bridge connector component to overcome the electromagnetic repulsion force when the current is below a specified threshold. The bridge connector component is separated from the stationary bus bars when the current exceeds the specified threshold. The electromagnetic repulsion force exceeds the opposing force when the current exceeds the specified threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  depicts a schematic view of an embodiment of a circuit breaker with a dual-break linear contact assembly in a closed state, in accordance with aspects of the present disclosure; 
         FIG. 2  depicts a schematic view of an embodiment of a circuit breaker with a dual-break linear contact assembly in an open state, in accordance with aspects of the present disclosure; 
         FIG. 3  depicts a schematic view of an embodiment of a circuit breaker with a dual-break linear contact assembly in a tripped state, in accordance with aspects of the present disclosure; 
         FIG. 4  depicts a perspective close-up view of a first embodiment of a dual-break linear contact assembly, in accordance with aspects of the present disclosure; 
         FIG. 5  depicts a schematic close-up view of a second embodiment of a dual-break linear contact assembly, in accordance with aspects of the present disclosure; 
     
    
    
     DETAILED DESCRIPTION 
     A dual-break linear contact assembly, as discussed herein, may be suitable for MCCBs used in residential, commercial or industrial applications, such as applications with voltages up to 1,000 V DC or AC, operational currents up to 2,000 A, and fault currents of up to 100 kA. As discussed herein, various embodiments of a dual-break linear contact assembly are described where a movable bridge connector is employed as part of the electrically conductive path when the circuit breaker is in a closed configuration. While the bridge connector may have a variety of shapes, including straight, in certain embodiments the connector is non-straight, such as curved (e.g., U-shaped or horseshoe shaped) or angled (e.g., V-shaped). 
     In the event of an electrical fault, a mechanism of the circuit breaker that is connected to the dual-break linear contact assembly is tripped, and the bridge connector in the dual-break linear contact assembly is linearly displaced away from the two stationary bus bars otherwise electrically connected by the bridge connector, thus opening the electrical circuit. In one such embodiment, the dual-break linear contact assembly utilizes the fault currents to generate forces to propel the bridge connector linearly away from the stationary bus bars, opening the electrical circuit. In one embodiment, as the bridge connector is moved away from the stationary bus bars, the fields due to the fault current result in an arc force that deflects any arcs that are present into proximate arc chutes. In such an embodiment, the arc chutes act to enhance the magnetic field to increase the magnitudes of the repulsion force and the arc force. Further, in one embodiment, an electrically isolating medium may be provided that is interposed between the two stationary bus bars as the bridge connector is linearly displaced from the fixed contacts. In this manner, the electrically isolating medium may help to prevent arc jumping between the two stationary bus bars or between the two movable contacts when the bridge connector separates from the stationary bus bars. 
     As discussed in greater detail below, embodiments of a dual-break linear contact assembly in accordance with the present approach may employ one or more magnetic cores that enhance the electromagnetic fields and, thereby, the repulsion force and the arc force generated in the presence of an electrical fault current. The repulsion force acts to displace the bridge connector away from the stationary bus bars, and the arc force acts to deflect the arc towards the arc chute plates. In one embodiment, a dome structure may enhance the magnetic circuit and hence increase the repulsion force and arc force near and at the end of the separation event. 
     With the preceding in mind, and turning to  FIG. 1 , a schematic view of certain components of an embodiment of a circuit breaker  100  with dual-break linear contact assembly  220  connected to a mechanism  222  are depicted that are in accordance with the present disclosure. In this example, the mechanism includes a handle yoke  102  that may be moved by a user, such as between open and closed states, or by operation of the circuit breaker  100  itself, such as when the circuit breaker  100  is tripped. In the depicted example, the circuit breaker  100  is in a closed state, as can be seen by the contact between the bridge connector  160  (here depicted as a curved or horseshoe-shaped connector) and stationary bus bars (e.g., first stationary bus bar  162  and the second stationary bus bar  164 ). When the bridge connector  160  is in contact with the stationary bus bars (i.e., when the circuit breaker  100  is in a closed state), current may flow through the circuit breaker  100 . 
     In the depicted example, a latch assembly is depicted which, when engaged, acts to hold the bridge connector  160  in a closed position when the handle yoke  102  is in the corresponding closed position. The depicted latch assembly includes a primary latch  104  that can be engaged with a cradle  106  at one surface and with a secondary latch  108  at a second surface. As will be appreciated, the depicted latch assembly is a dual latch assembly (i.e., there are two engagement surfaces, the first between the primary latch  104  and the secondary latch  108  and the second engagement between the primary latch  104  and the cradle  106 ). In other embodiments, the latch assembly may differ, such as being a single latch assembly between a primary latch and cradle, or a triple or greater latch assembly, having more than two latches. 
     In the depicted example, the cradle  106  is operationally linked to the bridge connector  160  and to the handle yoke  102  via a linkage assembly that includes a first link  110 , a second link  112 , and a spring  114 , along with the respective pins and other components for this assembly. In other embodiments, the cradle  106 , handle yoke  102 , and bridge connector  160  may be linked and/or may be coupled by other structural features, such as, in one embodiment, a flexure having different stable states that translate to different configurations of the circuit breaker  100  (such as a closed state, an open state, and a tripped state). 
     The mechanical motion of the components described above is determined (or constrained) by various revolute joints provided as part of the assembly. For example, certain of the revolute joints may be fixed with respect to the frame or support structure of the circuit breaker  100 , such as revolute joints that are based on extensions or features of the frame wall or surface. For example, cradle pivot pin  120  forms a revolute joint about which the cradle  106  rotates with respect to the wall or frame of the circuit breaker  100 , primary latch pivot pin  122  forms a revolute joint about which the primary latch  104  rotates with respect to the wall or frame of the circuit breaker  100 , secondary latch pivot pin  124  forms a revolute joint about which the secondary latch  108  moves with respect to the wall or frame of the circuit breaker  100 , and handle yoke pivot pin  126  forms a revolute joint about which the handle yoke  102  rotates with respect to the wall or frame of the circuit breaker  100 . In the depicted example, the various fixed revolute joints, as well as revolute joints defined by the paired movable components, define the available motions and ranges for the depicted components in response to internally and externally applied forces. 
     In the depicted example, a central or median portion of the bridge connector  160  is connected to a guide rod  166 , such as by a bridge connector pin  168 . The guide rod  166 , as discussed in greater detail below, acts to move the bridge connector  160  away from and toward the contacts of the first bus bar  162  and the second bus bar  164  based on the state of the circuit breaker  100  (i.e., whether the circuit breaker  100  is open, closed, or tripped). 
     In the depicted example, movement of the bridge connector  160  away from the stationary bus bars may involve moving the bridge connector  160  through a magnetic core defined by a series of arc chute plates  172  and, in one embodiment, through or into a dome structure  170  which can be a laminated.  FIG. 4  depicts a perspective close-up view of an embodiment of the bridge connector  160 , arc chute plates  172 , and dome structure  170 . In one implementation, the spaced apart arc chute plates  172  may be positioned within an electrically isolating support frame  174 . In the depicted example, the guide rod  166  may travel through a non-conductive linear bushing  176  disposed in or adjacent to the dome structure  170 . As depicted in  FIGS. 1-3 , the guide rod  166  may be connected to the first link  110  of the linkage assembly via a connecting pin  178 . The first link  110 , in turn may be directly or indirectly linked to the handle yoke  102 , the cradle  106 , and the latching assembly. 
     Turning to  FIG. 5 , a schematic close-up view of an embodiment of a dual-break linear contact assembly is depicted. This embodiment includes a secondary biasing spring  210  (shown in cross-section) positioned between bridge connector  160  and dome structure  170 . The secondary biasing spring  210  is under compression and exerts a spring force on the bridge connector  160  such as to hold the bridge connector  160  in contact with the stationary bus bars  162 ,  164 . 
     As will be appreciated, in the depicted example, the current flowing through the horizontal portion of the first stationary bus bar  162  and the horizontal portion of the second stationary bus bar  164  is equal in magnitude and opposite in direction, thereby generating a repulsion force acting on the bridge connector  160  which acts to push the bridge connector  160  away from the stationary bus bars  162 ,  164 . However, in the depicted example, under normal operating conditions (i.e., no short circuit and no overcurrent), the secondary biasing spring  210  and the spring  114  of the linkage assembly may be calibrated or sized to overcome this repulsion force while the latching assembly remains engaged (i.e., in an un-triggered state) and the handle yoke  102  remains in the closed position. As will be appreciated, in other embodiments secondary biasing spring  210  and the spring  114  may be a type of biasing component capable of applying the specified bias force to the bridge connector  160  and overcome the repulsive force. However, in a short circuit situation, the repulsion force generated in the presence of short-circuit fault currents overcomes the bias force provided by the secondary biasing spring  210 , causing the bridge connector  160  to separate from the stationary bus bars  162 ,  164  and move away from the stationary bus bars  162 ,  164 . Subsequently, the electromagnetic flapper  190  trips the mechanism  222 . 
     In the depicted example, a flexible conductor  180  (e.g., a flexible copper braid) is also shown which connects the second stationary bus bar  164  and a bimetallic strip  182  in contact with the latch assembly. In such an embodiment, the bimetallic strip  182 , as discussed in greater detail below, may form part of a trip mechanism that can cause the latch assembly to disengage the cradle  106 . In an overcurrent situation the bimetallic strip may undergo a conformational change and subsequent release of the cradle  106  by the latch assembly (i.e., tripping of the circuit breaker  100 ). In such a situation, disengagement of the cradle  106  from the latch assembly causes the spring  114  to move. This allows the repulsion force acting on the bridge connector  160  to separate the bridge connector  160  from the stationary bus bars  162 ,  164 , thus opening the circuit and tripping the circuit breaker  100 . 
     Turning to  FIGS. 1-3 , the circuit breaker  100  is shown in three differing operational configurations, closed in  FIG. 1 , open, in  FIG. 2 , and tripped in  FIG. 3 . For example,  FIG. 1  depicts a schematic view of the circuit breaker  100  in a closed configuration. In this example, the bridge connector  160  connects the contacts of the first stationary bus bar  162  and second stationary bus bar  164 . In particular, the latch assembly is engaged with the cradle  106  and the handle yoke  102  is in the closed position, resulting in the force of spring  114  overcoming the repulsion force during normal operation that acts on the bridge connector  160  to hold bridge connector  160  in the closed configuration. 
     In general, it may be desirable to move the circuit breaker  100  between the depicted closed position and an open position without tripping the circuit breaker  100 . That is, it may be desirable to allow a user to open the electrical circuit in a manner distinct from tripping the electrical circuit. With this in mind, and turning to  FIG. 2 , the circuit breaker  100  of  FIG. 1  is depicted in an open position, as shown by the bridge connector  160  being separated from the contacts of the first stationary bus bar  162  and second stationary bus bar  164 . 
     The circuit breaker  100  may be opened in this manner by a user moving the handle yoke  102  from the position shown in  FIG. 1  to the position shown in  FIG. 2 . In this open configuration, the latch assembly remains engaged with the cradle  106 , however, the movement of the handle yoke  102  translates and rotates the spring  114 , which results in a change of the force magnitude and force direction exerted by the spring  114  on the first link  110 . As a result the second link  112  rotates counterclockwise about the revolute joint  177  which also moves the first link  110 , the guide rod  166 , and the bridge connector  160 . In this manner, movement of the handle yoke  102  from the closed to the open position causes a linear translation of the bridge connector  160  away from the stationary contacts of the stationary bus bars  162 ,  164 . As will be noted in this example, the electrical circuit is opened without tripping the circuit breaker  100  (i.e., the primary latch  104 , secondary latch,  108 , and cradle  106  all remain engaged). That is, the circuit breaker  100  may be opened without tripping the circuit breaker  100 . 
     As noted above, in addition to the open and closed states, the circuit breaker  100  can also exist in a tripped state in which the circuit is opened. In general, once tripped, the circuit breaker  100  must be moved to the open configuration prior to being moved to the closed configuration. That is, because the latch assembly is disengaged when the circuit breaker  100  is tripped, the act of moving the handle yoke  102  to the open position shown in  FIG. 2  acts to re-engage the primary latch  104 , secondary latch,  108 , and cradle  106  (i.e., to reset the MCCB  100 ), allowing the handle yoke  102  to then be moved to the closed position of  FIG. 1  with the latch assembly engaged. 
     As noted above, with respect to the tripping of the circuit breaker  100  there may be at least two distinct mechanisms by which the circuit breaker  100  may be tripped, such as a mechanical tripping mechanism and electrical tripping mechanism. For example, the circuit breaker  100  may be tripped in the event of an electrical overload. In this circumstance, a bimetallic material, shown as bimetallic strip  182  may be heated and may deform (i.e., change shape or configuration) when a threshold temperature is reached (which is indicative of excessive current flow through the circuit breaker  100 ), causing release of the latch assembly and tripping of the circuit breaker  100  by mechanical force and displacement as discussed above. In other circumstances, an electromagnetic flapper  190  might be activated in the presence of a short circuit fault current. The flapper  190  may provide a force and displacement that disengages the latch assembly causing the circuit breaker  100  to trip. 
     In addition, as discussed herein, an electrical fault current may result in the repulsion force acting on the bridge connector  160  overcoming the force provided by the spring  114 , thereby causing the bridge connector  160  to separate from the stationary bus bars, opening the electrical circuit. As the bridge connector  160  separates from the stationary contacts on the bus bars  162 ,  164 , the electromagnetic forces resulting from the fault current may push any resulting arcs into the arc chute plates  172 , which serve the dual function of both absorbing the arc energy as well as enhancing the electromagnetic field, which in turn increases the electromagnetic force acting on the bridge connector  160  to drive the bridge connector  160  away from the stationary bus bars. Thus, in this embodiment, the arc chute plates  172  enhance the electromagnetic field, and thereby increase the arc forces acting on the arcs as well as the repulsion force acting on the bridge connector  160 . That is, the magnetic core formed by the arc chute plates  172  enhances the electromagnetic field and, thereby, the electromagnetic force generated during a short circuit that opens the circuit and extinguishes any arcs formed. Further, the dome structure  170  provides additional force enhancement at the end of the travel. 
     An example of a tripped circuit breaker  100  in accordance with present approaches is depicted in  FIG. 3 . In this example, the latch assembly has been disengaged (i.e., has been tripped), as shown by the separation or disengagement of the cradle  106  from the primary latch  104 . In addition, the handle yoke  102  may be moved, in response to the action of the assembly, to a tripped position, which is located between the open position and the closed position of the handle yoke  102 . While the depicted example employs rigid links to link the motions of the cradle  106 , the handle yoke  102 , the guide rod  166 , the second link  112 , and first link  110 , as noted above, in other embodiments coordination of the motion of these components may instead be mediated by the operation of a flexure configured to transition between stable states in response to the circuit breaker being closed, opened, or tripped. 
     In numerical experiments, assuming a constant fault current of 10 kA and using a magnetic core disposed about the bridge connector  160  (in the form of arc chute plates  170  and dome structure  170 ), the force found to initially act on the bridge connector  160  and on the arcs in the event of a short circuit was 60 N and 3 N, respectively. Further, in this setup the force found to act on the bridge connector  160  and on the arcs after the bridge connector had traveled 10 mm away from the fixed contacts after a short circuit was found to be 62 N and 22 N, respectively. That is, the repulsion force associated with a given fault current is largely independent of the position of the bridge connector  160 , with the forces acting on the bridge connector  160  being very similar both in the initial position (i.e., proximate to the fixed contacts) at the beginning of the fault as well as after the bridge connector  160  has traveled 10 mm away from the stationary contacts. Thus, the use of magnetic arc chute plates and a dome structure, as discussed herein, enhances the electromagnetic forces acting on the bridge connector  160  to cause separation in the event of an electrical fault and continues to provide a strong repulsion force even after the electrical circuit has been initially opened. Further, the increased arc forces seen when using a magnetic arc chute, as discussed herein, yield higher arc forces relative to implementations with no such arc chutes and, thereby, yield proportionately quicker arc extinction. 
     Technical effects of the invention include a circuit breaker having a dual-break linear contact assembly with a bridge connector that travels in a linear, non-rotating trajectory to separate from stationary bus bars and open the electrical circuit. Technical effects of the invention also include a circuit breaker and a dual-break linear contact assembly with magnetic cores, to enhance electromagnetic fields and the repulsion force applied to a bridge connector to separate the bridge connector from the stationary contacts of a first stationary bus bar and a second stationary bus bar when opening the circuit. Technical effects of the invention also include using the electromagnetic force generated by the flow of the fault current through a first stationary bus bar and a second stationary bus bar to repel a bridge connector connecting the first stationary bus bar and the second stationary bus bar in the event of an electrical fault condition. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.