Patent Publication Number: US-8993904-B2

Title: Tap changer with improved switch construction

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to tap changers and more particularly to switches for load tap changers. 
     As is well known, a transformer converts electricity at one voltage to electricity at another voltage, either of higher or lower value. A transformer achieves this voltage conversion using a primary winding and a secondary winding, each of which are wound on a ferromagnetic core and comprise a number of turns of an electrical conductor. The primary winding is connected to a source of voltage and the secondary winding is connected to a load. By changing the ratio of secondary turns to primary turns, the ratio of output to input voltage can be changed, thereby controlling or regulating the output voltage of the transformer. This ratio can be changed by effectively changing the number of turns in the primary winding and/or the number of turns in the secondary winding. This is accomplished by making connections between different connection points or “taps” within the winding(s). A device that can make such selective connections to the taps is referred to as a “tap changer”. 
     Generally, there are two types of tap changers: on-load tap changers and de-energized or “off-load” tap changers. An off-load tap changer uses a circuit breaker to isolate a transformer from a voltage source and then switches from one tap to another. An on-load tap changer (or simply “load tap changer”) switches the connection between taps while the transformer is connected to the voltage source. A load tap changer may include, for each phase winding, a selector switch assembly, a bypass switch assembly and a vacuum interrupter assembly. The selector switch assembly makes connections to taps of the transformer, while the bypass switch assembly connects the taps, through two branch circuits, to a main power circuit. The present invention is directed to an on-load tap changer having a bypass switch assembly with an improved switch construction. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an on-load tap changer is provided having a bypass switch assembly that includes a pair of bypass switches. Each bypass switch includes a fixed contact and a movable contact assembly having a contact carrier. A plurality of contacts is at least partially disposed in the contact carrier. Each of the contacts has a first end portion with a notch and a second end portion with a mounting opening extending therethrough. The mounting opening is defined by an interior surface of the contact. The interior surface includes a flat portion and an arcuate portion. The contacts are arranged in a stack such that the notches align to form a groove and the mounting openings align to form a mounting bore. A mounting post extends through the mounting bore such that the contacts are pivotable about the mounting post. An actuation assembly is connected to the bypass switches and is operable to pivot each bypass switch between a closed position, wherein the fixed contact engages the contacts and extends through the groove, and an open position, wherein the fixed contact does not contact the contacts. In each bypass switch, the flat portions of the contacts move over the mounting post during the pivoting between the open and closed positions, thereby causing the contacts to move longitudinally relative to the contact carrier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  shows a front elevational view of a tap changer of the present invention; 
         FIG. 2  shows a schematic view of the tap changer; 
         FIG. 3  shows circuit diagrams of the tap changer in linear, plus-minus and coarse-fine configurations; 
         FIG. 4  shows a schematic drawing of an electrical circuit of the tap changer; 
         FIG. 5  shows the electrical circuit progressing through a tap change; 
         FIG. 6  shows a front view of the interior of a tank of the tap changer; 
         FIG. 7  shows a rear view of a front support structure of the tap changer; 
         FIG. 8  shows a front perspective view of the support structure with a bypass switch assembly and a vacuum interrupter assembly mounted thereto; 
         FIG. 9  shows a plan view of a bypass cam of the bypass switch assembly; 
         FIG. 10  shows a perspective view of a bypass switch; 
         FIG. 11  shows a perspective view of a housing of the bypass switch; 
         FIG. 12  shows a perspective view of a base of the bypass switch; 
         FIG. 13  shows a side view of a contact of the bypass switch; 
         FIG. 14  shows a close-up view of an inner end of the contact having a mounting opening; 
         FIG. 15  shows a side sectional view of the bypass switch; 
         FIG. 16  shows a close-up view of an outer end of the contact initially touching a fixed contact post; and 
         FIG. 17  shows a close-up view of the outer end of the contact centered on the fixed contact post. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     It should be noted that in the detailed description that follows, identical components have the same reference numerals, regardless of whether they are shown in different embodiments of the present invention. It should also be noted that in order to clearly and concisely disclose the present invention, the drawings may not necessarily be to scale and certain features of the invention may be shown in somewhat schematic form. 
     Referring now to  FIGS. 1 and 2 , there is shown a load tap changer (LTC)  10  embodied in accordance with the present invention. The LTC  10  is adapted for on-tank mounting to a transformer. Generally, the LTC  10  comprises a tap changing assembly  12 , a drive system  14  and a monitoring system  16 . The tap changing assembly  12  is enclosed in a tank  18 , while the drive system  14  and the monitoring system  16  are enclosed in a housing  20 , which may be mounted below the tank  18 . The tank  18  defines an inner chamber within which the tap changing assembly  12  is mounted. The inner chamber holds a volume of dielectric fluid sufficient to immerse the tap changing assembly  12 . Access to the tap changing assembly  12  is provided through a door  24 , which is pivotable between open and closed positions. 
     The tap changing assembly  12  includes three circuits  30 , each of which is operable to change taps on a regulating winding  32  for one phase of the transformer. Each circuit  30  may be utilized in a linear configuration, a plus-minus configuration or a coarse-fine configuration, as shown in  FIGS. 3   a ,  3   b ,  3   c , respectively. In the linear configuration, the voltage across the regulating winding  32  is added to the voltage across a main (low voltage) winding  34 . In the plus-minus configuration, the regulating winding  32  is connected to the main winding  34  by a change-over switch  36 , which permits the voltage across the regulating winding  32  to be added or subtracted from the voltage across the main winding  34 . In the coarse-fine configuration, there is a coarse regulating winding  38  in addition to the (fine) regulating winding  32 . A change-over switch  40  connects the (fine) regulating winding  32  to the main winding  34 , either directly, or in series, with the coarse regulating winding  38 . 
     Referring now to  FIG. 4 , there is shown a schematic drawing of one of the electrical circuits  30  of the tap changing assembly  12  connected to the regulating winding  32  in a plus-minus configuration. The electrical circuit  30  is arranged into first and second branch circuits  44 ,  46  and generally includes a selector switch assembly  48 , a bypass switch assembly  50  and a vacuum interrupter assembly  52  comprising a vacuum interrupter  54 . 
     The selector switch assembly  48  comprises movable first and second contact arms  58 ,  60  and a plurality of stationary contacts  56  which are connected to the taps of the winding  32 , respectively. The first and second contact arms  58 ,  60  are connected to reactors  62 ,  64 , respectively, which reduce the amplitude of the circulating current when the selector switch assembly  48  is bridging two taps. The first contact arm  58  is located in the first branch circuit  44  and the second contact arm  60  is located in the second branch circuit  46 . The bypass switch assembly  50  comprises first and second bypass switches  66 ,  68 , with the first bypass switch  66  being located in the first branch circuit  44  and the second bypass switch  68  being located in the second branch circuit  46 . Each of the first and second bypass switches  66 ,  68  is connected between its associated reactor and the main power circuit. The vacuum interrupter  54  is connected between the first and second branch circuits  44 ,  46  and comprises a fixed contact  164  and a movable contact  166  enclosed in a bottle or housing  168  having a vacuum therein, as is best shown in  FIG. 10 . 
     The first and second contact arms  58 ,  60  of the selector switch assembly  48  can be positioned in a non-bridging position or a bridging position. In a non-bridging position, the first and second contact arms  58 ,  60  are connected to a single one of a plurality of taps on the winding  32  of the transformer. In a bridging position, the first contact arm  58  is connected to one of the taps and the second contact  60  is connected to another, adjacent one of the taps. 
     In  FIG. 4 , the first and second contact arms  58 ,  60  are both connected to tap  4  of the winding  32 , i.e., the first and second contact arms  58 ,  60  are in a non-bridging position. In a steady state condition, the contacts  164 ,  166  of the vacuum interrupter  54  are closed and the contacts in each of the first and second bypass switches  66 ,  68  are closed. The load current flows through the first and second contact arms  58 ,  60  and the first and second bypass switches  66 ,  68 . Substantially no current flows through the vacuum interrupter  54  and there is no circulating current in the reactor circuit. 
     A tap change in which the first and second contact arms  58 ,  60  are moved to a bridging position will now be described with reference to  FIGS. 5   a - 5   e.  The first bypass switch  66  is first opened (as shown in  FIG. 5   a ), which causes current to flow through the vacuum interrupter  54  from the first contact arm  58  and the reactor  62 . The vacuum interrupter  54  is then opened to isolate the first branch circuit  44  (as shown in  FIG. 5   b ). This allows the first contact arm  58  to next be moved to tap  5  without arcing (as shown in  FIG. 5   c ). After this move, the vacuum interrupter  54  is first closed (as shown in  FIG. 5   d ) and then the first bypass switch  66  is closed (as shown in  FIG. 5   e ). This completes the tap change. At this point, the first contact arm  58  is connected to tap  5  and the second contact arm  60  is connected to tap  4 , i.e., the first and second contact arms  58 ,  60  are in a bridging position. In a steady state condition, the contacts  164 ,  166  of the vacuum interrupter  54  are closed and the contacts in each of the first and second bypass switches  66 ,  68  are closed. The reactors  62 ,  64  are now connected in series and the voltage at their midpoint is one half of the voltage per tap selection. Circulating current now flows in the reactor circuit. 
     Another tap change may be made to move the second contact arm  60  to tap  5  so that the first and second contact arms  58 ,  60  are on the same tap (tap  5 ), i.e., to be in a non-bridging position. To do so, the above-described routine is performed for the second branch circuit  46 , i.e, the second bypass switch  68  is first opened, then the vacuum interrupter  54  is opened, the second contact arm  60  is moved to tap  5 , the vacuum interrupter  54  is first closed and then the second bypass switch  68  is closed. 
     In the tap changes described above, current flows continuously during the tap changes, while the first and second contact arms  58 ,  60  are moved in the absence of current. 
     As best shown in  FIG. 4 , the selector switch assembly  48  may have eight stationary contacts  56  connected to eight taps on the winding  32  and one stationary contact  56  connected to a neutral (mid-range) tap of the winding  32 . Thus, with the change-over switch  36  on the B terminal (as shown), the selector switch assembly  48  is movable among a neutral position and sixteen discreet raise (plus) positions (i.e., eight non-bridging positions and eight bridging positions). With the change-over switch  36  on the A terminal, the selector switch assembly  48  is movable among a neutral position and sixteen discreet lower (minus) positions (i.e., eight non-bridging positions and eight bridging positions). Accordingly, the selector switch assembly  48  is movable among a total of 33 positions (one neutral position, 16 raise (R) positions and 16 lower (L) positions). 
     Referring now to  FIG. 6 , three support structures  80  are mounted inside the tank  18 , one for each electrical circuit  30 . The support structures  80  are composed of a rigid, dielectric material, such as fiber-reinforced dielectric plastic. For each electrical circuit  30 , the bypass switch assembly  50  and the vacuum interrupter assembly  52  are mounted on a first (or front) side of a support structure  80 , while the selector switch assembly  48  is mounted behind the support structure  80 . 
     Referring now to  FIG. 7 , the bypass switch assembly  50  includes a bypass gear  82  connected by an insulated shaft  83  to a transmission system, which, in turn, is connected to an electric motor. The bypass gear  82  is fixed to a bypass shaft that extends through the support structure  80  and into the first side of the support structure  80 . The bypass gear  82  is connected by a chain  90  to a vacuum interrupter (VI) gear  92  secured on a VI shaft  94 . The VI shaft  94  also extends through the support structure  80  and into the first side of the support structure  80 . When the motor is activated to effect a tap change, the transmission system and the shaft  83  convey the rotation of a shaft of the motor to the bypass gear  82 , thereby causing the bypass gear  82  and the bypass shaft to rotate. The rotation of the bypass gear  82 , in turn, is conveyed by the chain  90  to the VI gear  92 , which causes the VI gear  92  and the VI shaft  94  to rotate. 
     On the first side of the support structure  80 , the bypass shaft is secured to a bypass cam  100 , while the VI shaft  94  is secured to a VI cam  102 . The bypass cam  100  rotates with the rotation of the bypass shaft and the VI cam  102  rotates with the rotation of the VI shaft  94 . As will be described in more detail below, the bypass and VI gears  82 ,  92  are sized and arranged to rotate the bypass cam  100  through 180 degrees for each tap change and to rotate the VI cam  102  through 360 degrees for each tap change. 
     Referring now to  FIG. 8 , the bypass switch assembly  50  includes the first and second bypass switches  66 ,  68 , the bypass shaft and the bypass cam  100 , as described above. Each of the first and second bypass switches  66 ,  68  comprises a plurality of contacts  104  arranged in a stack and held in a contact carrier  106 . The contacts  104  are composed of a conductive metal, such as copper. Each contact  104  has a first or inner end and a second or outer end. A tapered notch (with a gradual V-shape) is formed in each contact  104  at the outer end, while a mounting opening extends through each contact  104  at the inner end. In each of the first and second contact switches  66 ,  68 , when the contacts  104  are arranged in a stack, the tapered notches align to form a tapered groove. In addition, the mounting openings align to form a mounting bore extending through the switch. Each of the first and second bypass switches  66 ,  68  is pivotally mounted to the support structure  80  by a post  114  that extends through the mounting bore in the contacts  104 , as well as aligned holes in the contact carrier  106  and a major tie bar  116  that extends between the first and second bypass switches  66 ,  68 . The major tie bar  116  has been partially removed in  FIG. 8  to better show other features. The entire major tie bar  116  can be seen in  FIG. 6 . 
     Each of the first and second bypass switches  66 ,  68  is movable between a closed position and an open position. In the closed position, a fixed contact post  118  is disposed in the groove and is in firm contact with the contacts  104 . In the open position, the fixed contact post  118  is not disposed in the groove and the contacts  104  are spaced from the fixed contact post  118 . The fixed contact posts  118  are both electrically connected to the main power circuit and, more specifically, to a neutral terminal. Each of the first and second bypass switches  66 ,  68  is moved between the closed and open positions by an actuation assembly  120 . 
     The actuation assembly  120  is part of the bypass switch assembly  50  and comprises first and second bell cranks  122 ,  124 . Each of the first and second bell cranks  122 ,  124  has a main connection point, a linkage connection point and a follower connection point, which are arranged in the configuration of a right triangle, with the main connection point being located at the right angle vertex. The first and second bell cranks  122 ,  124  are pivotally connected at their main connection points to the support structure by posts  126 , respectively. The posts  126  extend through openings in the first and second bell cranks  122 ,  124  at the main connection points and through openings in the ends of a minor tie bar  130 . A first end of a pivotable first linkage  132  is connected to the linkage connection point of the first bell crank  122  and a second end of the pivotable first linkage  132  is connected to the contact carrier  106  of the first bypass switch  66 . Similarly, a first end of a pivotable second linkage  134  is connected to the linkage connection point of the second bell crank  124  and a second end of the pivotable second linkage  134  is connected to the contact carrier  106  of the second bypass switch  68 . A wheel-shaped first cam follower  136  is rotatably connected to the follower connection point of the first bell crank  122 , while a wheel-shaped second cam follower  138  is rotatably connected to the follower connection point of the second bell crank  124 . 
     Referring now also to  FIG. 9 , the bypass cam  100  is generally circular and has opposing first and second major surfaces. A pair of enlarged indentations  140  may be formed in a peripheral surface of the bypass cam  100 . The indentations  140  are located on opposing sides of the bypass cam  100  and have a nadir. The second major surface is flat and is disposed toward the support structure  80 . The first major surface is disposed toward the door  24  (when it is closed) and has an endless, irregular groove  142  formed therein. The groove  142  is partly defined by a central area  144  having arcuate major and minor portions  148 ,  150 . The major portion  148  has a greater radius than the minor portion  150 . The transitions between the major and minor portions are tapered. 
     The first and second cam followers  136 ,  138  are disposed in the groove  142  on opposite sides of the central area  144 . In a neutral or home position, the minor portion  150  of the bypass cam  100  is disposed toward the vacuum interrupter assembly  52 , while the major portion  148  of the bypass cam  100  is disposed away from the vacuum interrupter assembly  52 . In addition, the first and second cam followers  136 ,  138  are both in contact with the minor portion  150  at the junctures with the transitions to the major portion  148 , respectively. With the first and second cam followers  136 ,  138  in these positions, both of the first and second bypass switches  66 ,  68  are in the closed position. When the bypass cam  100  is in the home position, the first and second contact arms  58 ,  60  are in a non-bridging position. 
       FIG. 8  shows the bypass cam  100  after it has rotated clock-wise from its home, or neutral position in response to the initiation of a tap change. This rotation causes the first cam follower  136  to move (relatively speaking) through the transition and into contact with the major portion  148 , while the second cam follower  138  simply travels over the minor portion  150 . The movement of the first cam follower  136  through the transition increases the radius of the central area in contact with the first cam follower  136 , thereby moving the first cam follower  136  outward. This outward movement, in turn, causes the first bell crank  122  to pivot counter-clockwise about the main connection point. This pivoting movement causes the first linkage  132  to pull the first bypass switch  66  outward, away from the fixed contact post  118 , to the open position. As the first cam follower  136  moves over the major portion  148 , the first bypass switch  66  is maintained in the open position. As the bypass cam  100  continues to rotate, the first cam follower  136  moves over the transition to the minor portion  150 , thereby decreasing the radius of the central area  144  in contact with the first cam follower  136 , which allows the first cam follower  136  to move inward and the first bell crank  122  to pivot clockwise. This pivoting movement causes the first linkage  132  to push the first bypass switch  66  inward, toward the fixed contact post  118 , to the closed position. At this point, the tap change is complete and the bypass cam  100  has rotated 180 degrees to an intermediate position. The first and second cam followers  136 ,  138  are again both in contact with the minor portion  150  at the junctures with the transitions to the major portion  148 , respectively, but the major portion  148  of the bypass cam  100  is now disposed toward the vacuum interrupter assembly  52 , while the minor portion  150  of the bypass cam  100  is disposed away from the vacuum interrupter assembly  52 . With the bypass cam  100  in this, intermediate position, both of the first and second bypass switches  66 ,  68  are again in the closed position. In addition, the first and second contact arms  58 ,  60  are in a bridging position. 
     If another tap change is made so that the second contact arm  60  is moved to the same tap as the first contact arm  58 , i.e., a non-bridging position, the bypass cam  100  again rotates in the clock-wise direction, the second cam follower  138  moves through the transition and into contact with the major portion  148 , while the first cam follower  136  simply travels over the minor portion  150 . The movement of the second cam follower  138  through the transition increases the radius of the central area  144  in contact with the second cam follower  138 , thereby moving the second cam follower  138  outward. This outward movement, in turn, causes the second bell crank  124  to pivot clockwise about the main connection point. This pivoting movement causes the second linkage  134  to pull the second bypass switch  68  outward, away from the fixed contact post  118 , to the open position. As the second cam follower  138  moves over the major portion  148 , the second bypass switch  68  is maintained in the open position. As the bypass cam  100  continues to rotate, the second cam follower  138  moves over the transition to the minor portion  150 , thereby decreasing the radius of the central area  144  in contact with the second cam follower  138 , which allows the second cam follower  138  to move inward and the second bell crank  124  to pivot counter-clockwise. This pivoting movement causes the second linkage  134  to push the second bypass switch  68  inward, toward the fixed contact post  118 , to the closed position. At this point, the bypass cam  100  has rotated 360 degrees and the bypass cam  100  is back in the home position. 
     A pair of follower arms  152  may optionally be provided. The follower arms  152  are pivotally mounted to the support structure  80  and have rollers rotatably mounted to outer ends thereof, respectively. A spring  156  may be used to bias the outer ends of the follower arms  152  towards each other. This bias causes the rollers at the end of a tap change to move into the nadirs in the indentations  140 . In this manner, the follower arms  152  are operable to bias the bypass cam  100  toward the home position and the intermediate position at the end of a tap change. 
     The first and second bypass switches  66 ,  68  and their operation will be described in more detail so as to highlight another feature of the present invention. It should be understood that since the first and second bypass switches  66 ,  68  have substantially the same construction, only the first bypass switch  66  is shown. As set forth above, in each of the first and second bypass switches  66 ,  68 , the contacts  104  are held in a contact carrier  106  as shown in  FIG. 10 . The contact carrier  106  comprises a housing  160  secured to a base  162 . 
     As shown in  FIG. 11 , the housing  160  is generally channel-shaped and includes a top plate  164  joined between a pair of outward-extending side flanges  166 . Mounting rings  168  are joined to inward ends of the side flanges  166 , respectively. Outward ends of the side flanges  166  have holes  170  formed therein, respectively. The top plate  164  has a series of holes  172  formed therein. 
     Referring now to  FIG. 12 , the base  162  includes a plate  176  with a plurality of rods  178  extending outward therefrom. The plate  176  is joined to an I-shaped beam  180 . A bore  184  extends through the side of a body of the beam  180 . In the first bypass switch  66 , the base  162  is pivotally connected to the first linkage  132  by a pin that is journaled in the bore  184 , whereas in the second bypass switch  68 , the base  162  is pivotally connected to the second linkage  134  by a pin that is journaled in the bore  184 . 
     The base  162  is secured to the housing  160  such that the rods  178  extend through the holes  172  in the top plate  164  of the housing  160 . The base  162  is secured to the housing  160  by nuts and bolts or other fastening means. 
     As best shown in  FIG. 13 , each contact  104  has a first or inner end and a second or outer end. A tapered (gradual V-shaped) notch  186  is formed in each contact  104  at the outer end, while a mounting opening  188  extends through each contact  104  at the inner end. Between the inner and outer ends, an enlarged indentation  190  is formed in the contact  104 . A pair of posts  192  extend from a bottom surface of the indentation  190 . Toward the outer end, between the indentation  190  and the notch  186 , an oval guide opening  196  extends through the contact  104 . The surface of the contact  104  is raised around the guide opening  196  so as to form a rim  197 . An internal surface  195  defines the guide opening  196 . 
     Referring now to  FIG. 14 , an enlarged view of the mounting opening  188  in a contact  104  is shown. The surface of the contact  104  is raised around the mounting opening  188  so as to form a rim  198 . The mounting opening  188  is defined by an internal surface  200 . The internal surface  200  is circular, except for a rolling surface portion  202 , which is flat. As will be described below, the rolling surface portion  202  translates rotational movement of the contact  104  into longitudinal movement of the contact  104 . 
     The number of contacts  104  in each of the first and second bypass switches  66 ,  68  is determined by the amount of current being conducted. In the embodiment shown in  FIG. 10 , there are six contacts  104  in each switch. In each switch, the contacts  104  are arranged in the contact carrier  106  in a stack, as described above. The rims  197 ,  198  help separate middle portions and the outer ends of the contacts  104 . As described above, in each switch, the tapered notches  186  of the contacts  104  align to form a tapered groove and the mounting openings  188  align to form a mounting bore extending through the switch. In addition, in each switch, the guide openings  196  align to form a guide bore extending through the switch. In each switch, the mounting bore is aligned with the mounting rings  168  of the housing  160  of the contact carrier  106  and the guide bore is aligned with the holes  170  in the side flanges  166  of the housing  160 . 
     Referring now to  FIG. 15 , there is shown a sectional view of the first bypass switch  66 . In each of the first and second bypass switches  66 ,  68 , the post  114  extends through the mounting bore in the contacts  104  and through the mounting rings  168  of the contact carrier  106 . In addition, in each switch, a guide rod  204  extends through the guide bore in the contacts  104  and through the holes  170  of the contact carrier  106 . In this manner, the contacts  104  are retained in the contact carriers  106  of the first and second bypass switches  66 ,  68  and the first and second bypass switches  66 ,  68  are pivotally mounted to the support structure  80 . 
     In each of the first and second bypass switches  66 ,  68 , with the contacts  104  mounted in the contact carrier  106 , as described above, the posts  192  of each contact  104  are aligned with rods  178  of the base  162  of the contact carrier  106 . The ends of the posts  192  and the rods  178  are close together or even touching. Each aligned pair of post  192  and rod  178  extends through a helical spring  206  that is trapped between the top plate  164  of the contact carrier  106  and an inner edge  182  of the contact  104 . The springs  206  bias the contacts  104  away from the contact carrier  106 . 
     In each of the first and second bypass switches  66 ,  68 , one of the contacts  104  (hereinafter referred to as the arcing contact  104 ) is provided with springs  206  that exert more force than the springs  206  for the other contacts  104 . As a result, in each switch, the arcing contact  104  makes first and breaks last. This early making and late breaking causes any arcing to be confined to the arcing contact  104 . Since the arcing is confined to the arcing contacts  104 , the arcing contacts  104  have a more robust (arc-resistant) construction than the other contacts  104 . For example, the arcing contacts  104  may be composed of tungsten or an alloy of tungsten, whereas the other contacts  104  may simply be composed of copper. The arcing contact  104  in each of the first and second bypass switches  66 ,  68  is the contact  104  that is farthest from the support structure  80 . 
     As shown in  FIG. 10 , an arc block  208  may be fixed to an outside surface of the arcing contact  104 , proximate to the notch  186 , in each of the first and second bypass switches  66 ,  68 . Corresponding arc blocks  210  may be secured to the fixed contact posts  118 , respectively. The arc blocks  208 ,  210  are each comprised of a copper alloy, such as a copper-tungsten alloy, and help extend the lives of the arcing contacts  104  and the fixed contact posts  118 . For purposes of better showing the grooves formed by the stacks of contacts  104 , the arc blocks  208 ,  210  are not shown in  FIG. 8 . 
     The operation of the first bypass switch  66  will now be described, it being understood that the operation of the second bypass switch  68  is substantially the same, except for the direction of rotation of the switch and the bell crank. As described above, during the opening of the first bypass switch  66 , the first bell crank  122  pivots counter-clockwise about the main connection point, which causes the first linkage  132  to pull the base  162  of the contact carrier  106  outward. As a result, the contacts  104  start to rotate counter-clockwise about the post  114 . In each contact  104 , the flat rolling surface portion  202  of the internal surface  200  moves over the post  114 . This movement is translated into a longitudinal movement of the contact  104  outward in the direction of the arrow in  FIG. 15 . This outward movement is stopped by contact between the post  114  and a lower arcuate portion of the internal surface  200  and contact between a lower portion of the interior surface  195  and the guide rod  204 . At this point, the rolling surface portion  202  starts sliding over the post  114  as the first bypass switch  66  continues to open. 
     During the closing of the first bypass switch  66 , the first bell crank  122  pivots clockwise about the main connection point, which causes the first linkage  132  to push the base  162  of the contact carrier  106  inward. As a result, the contacts  104  start to rotate clockwise about the post  114 . In each contact  104 , the flat rolling surface portion  202  of the internal surface  200  moves over the post  114 . This movement is translated into a longitudinal movement of the contact  104  inward. This inward movement is stopped by contact between the post  114  and an upper arcuate portion of the internal surface  200  and contact between an upper portion of the interior surface  195  and the guide rod  204 . The contacts  104  initially contact the fixed contact post  118  as shown in  FIG. 16 . With regard to each of the contacts  104 , the fixed contact post  118  is not seated or centered, i.e., the fixed contact post  118  does not contact surfaces of the contact  104  in the notch  186  on opposing sides of the nadir of the notch  186 . Instead, the fixed contact post  118  only contacts a surface of the contact  104  on an outer side of the nadir. The springs  206 , however, cause all of the contacts  104 , except the arcing contact  104 , to become centered, i.e., to slide longitudinally outward and toward the fixed contact post  118 . This sliding or wiping of the contacts  104  helps keep the contacts  104  clean and free of carbon build-ups. A view of a contact  104  after being centered is shown in  FIG. 17 . 
     Due to the larger force exerted by the springs  206  associated with the arcing contact  104 , the arcing contact  104  does not center on the fixed contact post  118 . This larger spring force also causes the arcing contact  104  to be the last of the contacts  104  to separate from the fixed contact post  118  when the first bypass switch  66  moves toward the open position (breaks) and also causes the arcing contact  104  to be the first to contact the fixed contact post  118  when the first bypass switch  66  moves toward the closed position (makes). 
     Although the switch construction described above is particularly well suited for bypass switches (as embodied in the first and second bypass switches  66 ,  68 ), it should be appreciated that the switch construction may be utilized in other applications, such as in change-over (reversing) switches. 
     It is to be understood that the description of the foregoing exemplary embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.