Abstract:
A moving contact assembly for a high-current interruption electric power switching apparatus has an inner carrier for the contact fingers that is rigidly held in a withstand position in an outer carrier by a spring biased cam follower pin engaging a cam profile on the inner carrier and configured to positively seat an inner carrier stop against an outer carrier stop on the outer carrier and to rapidly drive the inner carrier to a blow open position in response to a fault. The cam profile has an extended width divided between spaced apart cam profile sections to absorb the high closing and withstand forces. An abutment on the outer carrier resists bowing of the cam follower pin between the cam profile sections. Complementary convex and concave partial cylindrical surfaces on the inner carrier and a gas shield on the moving assembly carrier body maintain an arc gas seal during blow open.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to electric power switching apparatus and in particular to blow open moving contact assemblies for such apparatus with very high current interruption ratings. 
   2. Background Information 
   Power circuit breakers typically are used as a main breaker in a power distribution system having additional downstream branch circuit breakers. They are also used as transfer switches for switching between alternative power sources, and as network protectors in larger distribution systems. In such systems, the power circuit breaker must have sufficient withstand capability to allow a downstream breaker to respond to a fault in order to minimize the extent of the outage. However, in the instance of a very large fault, such as a fault just downstream of the power breaker, it is desirable to have the power breaker respond promptly to limit the fault current. It is known to provide a power circuit breaker with a blow open contact structure for this current limiting purpose. This blow opening is driven by the electromagnetic repulsion force on the contacts and is very fast, limiting the actual current to less than the available fault current. Using this scheme in a power breaker requires a rugged, but compact, contact assembly with many individual contact fingers for a high continuous capacity and to withstand the higher closing energy and short time ratings compared to molded case circuit breakers. The contact fingers must be capable of opening collectively within the contact carrier assembly without movement of the operating mechanism. The entire contact carrier assembly is opened by the operating mechanism during normal nonfault operation (without the spontaneous contact opening), and also in the instant after the spontaneous opening of a high current interruption. It is desirable that the contact assembly with the blow open moving contact structure can be used in place of a standard power circuit breaker assembly with few changes to the breaker design so that a high interrupting version can be offered in the same product family. 
   The contact fingers of the spontaneously opening contact moving structure must have some individual motion with springs to apply contact pressure, supported rigidly until the current-induced force threshold is exceeded. The blow open portion of the assembly must have low inertia and be compact for rapid motion from closed to the widest achievable contact gap. When open, the carrier assembly should maintain good dielectric strength across the contact gap and direct the arc produced gases toward the arc chute. 
   Finally, the contact assembly must accurately control contact location, force and opening threshold and be tolerant of manufacturing variation while being cost-effective to manufacture. 
   SUMMARY OF THE INVENTION 
   Aspects of the invention are directed to a moving carrier assembly for an electric power switching apparatus for interrupting very high currents that is rigid and stable enough to maintain a rigid withstand position despite the high electromagnetic forces until the threshold current is reached and then to reliably blow open while maintaining a good gas seal to enhance arc extinguishment and to prevent flashover until the operating mechanism responds. Aspects that contribute to this performance include an arrangement that fixes the withstand position of the carrier components and ensures reliable response to threshold current that produces the spontaneous opening, a rigid cam structure and a mechanism for resisting bowing of the cam follower pin under the high forces developed with the carrier assembly in the closed position, and a configuration that provides an effective arc gas seal when the assembly blows open. 
   More particularly the invention includes aspects directed to a moving contact assembly for an electric power switching apparatus comprising: a carrier body, carrier legs for supporting the carrier body for pivotal movement between a closed position and an open position, an outer carrier secured to the carrier body and having an outer carrier stop, an inner carrier mounted on the outer carrier for pivotal movement between a withstand position and a blow open position and having a cam profile, an inner carrier stop, a plurality of contact fingers mounted on the inner carrier, a cam follower pin, and cam springs seated against the outer carrier and biasing the cam follower pin against the cam profile. The cam profile is configured so that for current through the contact fingers below a threshold current, the inner carrier is biased to the withstand position which is established by the inner carrier stop engaging the outer carrier stop, and for current through the contact fingers greater than the threshold current the inner carrier is rapidly pivoted to the blow open position. 
   Additional aspects of the invention are directed to a moving carrier assembly for an electric power switching apparatus comprising: a carrier body, carrier legs supporting the carrier body for movement between the closed position and an open position, an outer carrier secured to the carrier body and having a pair of spaced outer carrier sidewalls with confronting elongated slots and a base section between the outer carrier sidewalls, the base section having a medial abutment surface, an inner carrier mounted in the outer carrier for pivotal movement between a withstand position and a blow open position and having a cam profile with a pair of axially spaced apart cam profile sections, a plurality of contact fingers mounted on the inner carrier, a cam follower pin having ends received in the elongated slots, and cam springs bearing against the outer carrier and biasing the cam follower pin against the axially spaced apart cam profile sections. The cam profile is configured so that with current through the contact fingers below a threshold current the inner carrier is biased to the withstand position, and for current through the contact fingers above the threshold current, the inner carrier is rapidly pivoted to the blow open position. The medial abutment on the outer carrier is positioned to engage the cam follower pin intermediate the spaced apart cam profile sections with the inner carrier in the withstand position to resist bending of the cam follower pin. 
   Other aspects of the invention are directed to a moving carrier assembly for an electric power switching apparatus comprising: a carrier body, carrier legs supporting the carrier body for pivotal movement between a closed position and an open position, an outer carrier secured to the carrier body, an inner carrier having inner carrier sidewalls mounted on the outer carrier for pivotal movement between a withstand position and a blow open position, an end wall having a cam profile and a cross wall each between the inner carrier sidewalls, contact springs seated on the inner carrier and bearing against the plurality of contact fingers, a cam follower pin, and cam springs seated against the outer carrier biasing the cam follower pin against the cam profile. The cam profile is configured so that for current through the contact fingers below a threshold current the inner carrier is biased to the withstand position and for current through the contact fingers above the threshold current the inner carrier is rapidly driven to the blow open position, and a gas shield associated with the carrier body and having a concave inner surface facing the cross wall. The cross wall has a convex outer wall complimentary and in close proximity to the concave inner surface on the gas seal to maintain a gas shield as the inner carrier pivots from the withstand position to the blow open position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
       FIG. 1  is an exploded isometric view of a moving contact assembly in accordance with the invention. 
       FIG. 2  is an exploded isometric view of pertinent parts of the moving contact assembly of  FIG. 1  as viewed from opposite the side shown in  FIG. 1 . 
       FIG. 3  is an isometric view of the outer carrier of the moving contact assembly rotated to show interior features. 
       FIG. 4  is an isometric view of the inner carrier of the moving contact assembly showing the opposite side from that shown in  FIG. 2 . 
       FIG. 5  is a fractional enlarged view showing the cam profile on the inner carrier. 
       FIG. 6  is an enlarged sectional view through the inner and outer carriers with the inner carrier in the withstand position. 
       FIG. 6A  is similar to  FIG. 6  but showing the inner carrier in the blow open position. 
       FIG. 7  is a vertical section through the pertinent portion of one pole of a current limiting power circuit breaker incorporating the moving contact assembly of  FIGS. 1 through 6  shown in the closed position. 
       FIG. 8  is similar to  FIG. 7  but showing the current limiting power circuit breaker in the open position. 
       FIG. 9  is similar to  FIGS. 7 and 8  but showing the current limiting power circuit breaker in the blow open position. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 through 6 , the exemplary embodiment of the moving contact assembly  1  incorporating aspects of the invention includes a carrier body  3  molded of an electrically insulative resin. A pair of carrier legs  5  are locked to the carrier body  3  by a number of molded protrusions  7  that seat in complementary openings  9  in the legs, and are held in place by fasteners  11  so that the connection between the legs and the carrier body is rigid. A sub-assembly  13  is received in a cavity  15  in the carrier body  3 . 
   The sub-assembly  13 , which is shown exploded in  FIG. 2 , includes an outer carrier  17  that is firmly secured in the carrier body  3 . An inner carrier  19  is pivotally mounted to the outer carrier  17  by pivot pins  21  that pass through holes  23  in inner carrier sidewalls  25  and seat in holes  27  in outer carrier sidewalls  29 . A plurality of contact fingers  31  are pivotally mounted on the inner carrier sidewalls  25  by a contact finger pin  33  that passes through holes  35  in the contact fingers  31  and engage holes  37  in the inner carrier sidewalls  25 . Two of the contact fingers  31   a  extend beyond the other contact fingers and bend inward to form arc fingers that, as will be seen, direct arcs formed during current interruption into an arc chute of the circuit breaker. Moving contacts  39  are affixed to each of the contact fingers  31 . 
   As can be seen best in  FIGS. 2 and 4 , the inner carrier  19  has a cross wall  41  extending between the inner carrier sidewalls  25 . This cross wall  41  has two rows of contact spring pockets  43  on an inner surface in which are seated contact springs  45  that bias the contact fingers  31  against a contact finger stop pin  47  extending between holes  48  the inner carrier side walls  25 . The contact springs  45  provide contact pressure and adjustment for contact wear as is well known. 
   The inner carrier  19  also has an end wall  49  extending between the inner carrier side walls  25 . This end wall  49  can be integral with or separate from the cross wall  41 . On the end wall  49  is a cam profile  51  which is made up of two spaced apart cam profile sections  53  at the ends of the end wall  49 . This leaves a recess  55  in the end wall between the cam profile sections  53 . It will be noted from  FIG. 4  that the cam profile sections  53  extend axially along the end wall  49  a greater distance than the thickness, t, of the inner carrier sidewalls  25 . 
   The outer carrier  17 , as best viewed in  FIGS. 2 and 3 , has a pair of confronting elongated slots  59  in the outer carrier sidewalls  29 . A cam follower pin  61  that may have bushings  63  on the ends  65  slides in the elongated slots  59 . The outer carrier  17  has a base section  67  extending between the outer carrier sidewalls  29  that has a row of cam spring pockets  69  in which are seated a number of cam springs  71 . A cam spring holder  73 , see  FIGS. 2 and 6 , has a number of posts  75  on which the opposite ends of the cam springs  71  seat. Opposite the posts  75  is a partial cylindrical surface  77  that bears against the cam follower pin  61 . A flange  79  on the base section  67  has a medial rib  81  that has a surface  83  forming a medial abutment while the end of the central rib  81  forms an outer carrier stop  85 . The end wall  49  on the inner carrier  19  forms an inner carrier stop  87  adjacent the cam sections profile  53 , that as will be seen engages the outer carrier stop  85  to accurately fix the withstand position of the inner carrier  19 . 
   The blow open action of the breaker is created by the cam profile  51  (through the cam sections  53 ) and the cam follower pin  61  guided by the elongated slots  59  in the outer carrier sidewalls  29 . The cam follower pin  61  is pressed against cam follower profile sections  53  by the cam springs  71 . The cam spring holder  73 , fit securely to the spring ends by the posts  75 , creates a stable seat for the cam springs  71  against the cam follower pin  61 . A plurality of small springs  71  is used to achieve a compact package and to allow the cam-off force of the assembly to be adjusted by leaving a variable number of spring locations vacant. The cam profile  51  is designed to hold the inner carrier  19  stiffly in place in the withstand position shown in  FIG. 6  up to the peak force generated by a selected threshold current through the contact fingers  31  and then to rotate abruptly to the blow-open position shown in  FIG. 6A . As best seen in  FIG. 5 , the steeply-rising portion of the cam profile  51  that creates the high withstand force may include a withstand segment  89  of constant slope (radius rise relative to angular position) to accommodate manufacturing variation without substantial change in peak force. After the peak force in the opening direction, the cam profile  51  falls gently to a lower radius at the open end of travel  91 . This portion is a rising radius when the inner carrier  19  is resetting and is optimized to minimize dynamic rebound of the inner carrier  19  (and possible re-ignition of an arc) during high current interruption, but allow resetting when the operating mechanism trips as will be discussed. When the inner carrier  19  is under the peak force before blow-off, the cam follower pin  61  is bearing against the edges of the elongated slots  59  in the outer carrier sidewalls  29 . Friction is reduced by the rolling bushings  63  on the ends  65  of the cam follower pin  61 . 
   The one-piece inner and outer carriers  19 ,  17  with integral spring pockets  43 ,  69  increase overall strength and reduce the number of parts, assembly costs and manufacturing variation in the moving contact assembly  1 . The carriers  17 ,  19  can be cast, metal-injection molded, or otherwise produced from various magnetic or non-magnetic grades of stainless steel, and hardened as required. The one-piece carriers  17 ,  19  also provide the design flexibility to reinforce areas like the cam profile sections  53  and the outer carrier side walls  29  at the elongated slots  59  with extra width where needed. The width of the cam profile sections  53  can be selected with the remainder of the width relieved by the recess  55  for clearance with the cam follower pin  61 . The selective cam profile section width allows reduction of the contact stress, optimization of manufacturing methods and other desired characteristics of the cam function. It also resists the tendency of a bowing cam pin  61  to “walk out” of a full-length cam profile or out of a cam with excess straightness error relative to its width. To minimize the natural bending of the cam follower pin  61 , one or more intermediate bearing ribs such as the rib  81  on the outer carrier  17  can be located with the medial abutment surface  83  in line with the edges with the elongated slots  59  at any location across the outer carrier  17 . The end of this central rib  81  forms the outer carrier stop  85  against which the inner carrier stop  87  is biased as shown in  FIG. 6  to fix the withstand position of the inner carrier  19  with accuracy. 
   As can be seen in  FIGS. 7 through 9 , the moving contact assembly  1  is pivotally mounted in the housing  93  of a power circuit breaker  95  for rotation about bosses  97  on legs  5  between a closed position shown in  FIG. 7  and an open position shown in  FIG. 8 . In the closed position of  FIG. 7 , the inner carrier  19  is in the withstand position with regard to the outer carrier  17  as seen more clearly in  FIG. 6 . The moving contacts  39  on the contact fingers  31  are pressed by the contact springs  45  against fixed contacts  99  on the monolithic stationary conductor  101 , which has a terminal section  103  forming the line terminal of the power circuit breaker  95 . The lower ends of the contact fingers  31  are connected by flexible shunts, not shown for clarity, that are connected to a load terminal (not shown) located below the line terminal  103 . With the power circuit thus completed through the circuit breaker  95  current flows in the directions of the arrows  107 . 
   The moving contact assembly  1  is connected through a drive link  109  and crank  111  to a pole shaft  113  connecting the moving contact assembly  1  of each of the poles of circuit breaker  95  to an operating mechanism (not shown). Rotation of the pole shaft  113  in a clockwise direction, either manually or through an operation of a trip unit (not shown) in response to selected amplitude/time characteristics of current, causes the moving contact assembly  1  to be rotated to the open position shown in  FIG. 2 . As the moving contacts  39  and fixed contacts  99  separate, an arc is struck, which due to electromagnetic forces is driven up the arc runner section  115  of the monolithic stationary contact  101  and into arc plates  117  of an arc chute  119  where the arc is cooled and extinguished in a known manner. Arc gasses generated through vaporization of contact material and gas evolving materials expand up into the arc chute  119  and are exhausted through a vent  121  in the top of housing  93 . In order to prevent these arc gasses from expanding downward to the load terminal, the carrier body  3  has an associated gas shield  123  which can be molded as part of the carrier body  3  or can be attached thereto. This gas shield  123  has an outer arcuate surface  125  that is complementary and slides relative to an arcuate surface  127  on the housing  93 , as best seen in  FIG. 8 . Thus, the gas shield  123  blocks the passage of arc gasses downward for all positions of the moving contact assembly  1 . 
   Returning to  FIG. 7 , it will be seen that the current path represented by the arrows  107  forms a reverse current loop. As is known, such a reverse current loop generates very high electromagnetic forces at fault current levels. When this current reaches a threshold level, the forces generated are sufficient to overcome the bias force applied by the cam springs  71  through the cam follower pin  61  to the cam profile sections  53  and the inner carrier  19  is rapidly rotated (“blown open”) to the blow open position shown in  FIG. 9 . This occurs before the operating mechanism has time to respond to the fault current so as can be seen in  FIG. 9 , the carrier body  3  remains in the closed position. An elastomeric bumper  129  decelerates the rapidly moving contact fingers  31  and prevents them from rebounding to the withstand position. It will be noticed in  FIG. 9  that the gas shield  123  also has a concave partial cylindrical interior surface  131  and that the cross wall  41  on the outer carrier  19 , which incorporates the contact spring pockets  43  has an outer convex partial cylindrical surface  133  that is complementary to and in close proximity to the concave surface  131 . This arrangement maintains the seal formed by the gas shield  123  even as the inner carrier  19  rotates from the withstand to the blow open position. At the same time, the gas shield  123  is also electrically insulative and along with the insulative member  135  on the front face of the monolithic stationary conductor  101  prevents flashover between the moving contact assembly  1  and the stationary conductor as the inner carrier  19  rotates to the blow open position. 
   When the operating mechanism (not shown) responds to the fault current, the pole shaft  113  is rotated to rotate the moving contact assembly  1  to the open position shown in  FIG. 8 . The contact fingers  31  then pivot about the bumper  129  until the inner carrier  19  resets with the cam follower pin  61  engaging the constant sloped portion  89  of the cam profile  51 . 
   While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.