Patent Publication Number: US-2003231439-A1

Title: Bullet assembly for a vacuum arc interrupter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application is related to commonly assigned, concurrently filed:  
     [0002] U.S. patent application Ser. No. ______, filed ______, 2002, entitled “Shorting Switch And System To Eliminate Arcing Faults In Power Distribution Equipment” (Attorney Docket No. 01-EDP-213);  
     [0003] U.S. patent application Ser. No. ______, filed ______, 2002, entitled “Shorting Switch And System To Eliminate Arcing Faults In Power Distribution Equipment” (Attorney Docket No. 01-EDP-326);  
     [0004] U.S. patent application Ser. No. ______, filed ______, 2002, entitled “Shorting Switch And System To Eliminate Arcing Faults In Low Voltage Power Distribution Equipment” (Attorney Docket No. 01-EDP-385);  
     [0005] U.S. patent application Ser. No. ______, filed ______, 2002, entitled “Shorting Switch And System To Eliminate Arcing Faults In Power Distribution Equipment” (Attorney Docket No. 01-EDP-386);  
     [0006] U.S. patent application Ser. No. ______, filed ______, 2002, entitled “Vacuum Arc Interrupter Having A Tapered Conducting Bullet Assembly” (Attorney Docket No. 01-EDP-428);  
     [0007] U.S. patent application Ser. No. ______, filed ______, 2002, entitled “Vacuum Arc Interrupter Actuated By A Gas Generated Driving Force” (Attorney Docket No. 01-EDP-429);  
     [0008] U.S. patent application Ser. No. ______, filed ______, 2002, entitled “Blade Tip For Puncturing Cupro-Nickel Seal Cup” (Attorney Docket No. 01-EDP-471); and  
     [0009] U.S. patent application Ser. No. ______, filed ______, 2002, entitled “Vacuum Arc Eliminator Having A Bullet Assembly Actuated By A Gas Generating Device” (Attorney Docket No. 01-EDP-472).  
    
    
     
       BACKGROUND OF THE INVENTION  
       [0010] 1. Field of the Invention  
       [0011] This invention relates to a vacuum arc interrupter and, more specifically, to a bullet assembly for a vacuum arc interrupter having a pointed lance coupled to a piston having a conical pressure surface.  
       [0012] 2. Background Information  
       [0013] There is the potential for an arcing fault to occur across the power bus of a motor control center (MCC), another low voltage (LV) enclosure (e.g., an LV circuit breaker panel), other industrial enclosures containing LV power distribution components, as well as medium voltage (MV) enclosures. This is especially true when maintenance is performed on or about live power circuits. Frequently, a worker inadvertently shorts out the power bus, thereby creating an arcing fault inside the enclosure. The resulting arc blast creates an extreme hazard and could cause injury or even death. This problem is exacerbated by the fact that the enclosure doors are typically open for maintenance.  
       [0014] It is known to employ a spring device and piston to rapidly couple a live conductor to a grounded conductor in a vacuum arc interrupter in order to short the circuit upstream of the LV components. A vacuum arc interrupter utilizes two contacts in a vacuum chamber. One contact is fixed and the other contact is movable. The movable contact includes a stem, which is coupled to a bellows, that extends outside of the vacuum chamber. The spring is coupled to the stem and to a release device. The release device is coupled to an arc sensor in the LV or MV enclosure. The stem, and therefore the movable contact, moves from a first position at one end of the chamber to a second position at the opposite end of the chamber. One contact is coupled to the LV or MV circuit and the other contact is grounded. In operation the first position of the piston corresponds to the open position of the contacts. When an arc occurs in the LV or MV equipment, the arc sensor actuates the spring release device, thereby allowing the contacts to move into the second position and short the circuit.  
       [0015] Another device, that is, a device which is not a vacuum arc interrupter, for shorting a circuit included a tapered slug which is propelled by high pressure gas into a tapered set of openings extending through two bus bars and a layer of insulation. The slug is maintained in a pressure chamber coupled to a gas-generating device. When gas is rapidly introduced to the pressure chamber, the slug is propelled into the tapered opening, contacting both bus bars. Typically, one bus is coupled to a live circuit and the other bus is grounded. Thus, when the slug contacts both buses, the circuit is shorted.  
       [0016] These interrupters suffer from several disadvantages. For example, the prior art vacuum arc interrupters require multiple components to be maintained in the vacuum chamber. Certain components, such as the bellows, are difficult and expensive to construct. Construction of the vacuum arc interrupter could be simplified if more components could be maintained outside of the vacuum chamber. Prior art vacuum arc interrupters utilizing springs, because of their nature, do not have a means for stopping the upward motion of the movable contact. That is, the spring mechanism is structured to absorb the reactive forces caused by the contacts colliding. Thus, the prior art vacuum arc interrupters do not have a mechanism for stopping the advance of the moving component.  
       [0017] Furthermore, with regard to the prior art utilizing a slug, the slug relied on the application of gas pressure on the piston to ensure that the piston remained in the second position. Or, if the slug moved in a downward direction and the slug was heavy, gravity provided a sufficient force to hold the slug in place. That is, this system did not include a mechanical lock to maintain the slug in the second position. Additionally, the prior art slugs have a generally flat pressure surface. Because the gas is typically introduced through a small opening, the pressure distribution on the slug pressure surface is uneven. The uneven pressure distribution prevents the slug from moving as fast as a slug where the pressure distribution is even. Another disadvantage of this device is that, where the slug is received in a conductor having a small cross-sectional area, the electromagnetic field created by the contact may by very strong.  
       [0018] There is, therefore, a need for a vacuum arc interrupter that utilizes a bullet assembly that is disposed external to the vacuum chamber.  
       [0019] There is a further need for a vacuum arc interrupter bullet assembly that is structured to securely engage two conductors thereby creating a path of electrical communication.  
       [0020] There is a further need for a vacuum arc interrupter bullet assembly shaped to allow the rapid and generally even dispersal of a gas so that the gas creates a generally even pressure distribution on the bullet assembly pressure side.  
       SUMMARY OF THE INVENTION  
       [0021] These needs, and others, are met by the disclosed invention which provides a bullet assembly for a vacuum arc interrupter having a tapered and flared lance coupled to a piston assembly having a pressure surface that is not flat. The vacuum arc interrupter includes a first conductor that is disposed within, or forms part of, a vacuum chamber. The second conductor is disposed outside of the vacuum chamber. The vacuum arc interrupter bullet assembly is disposed in a pressure assembly that includes a pressure chamber and a gas generating source. The gas generating source is structured to rapidly fill the pressure chamber with a gas, thereby increasing the pressure within the chamber. The pressure chamber has a barrel with two open ends, a narrow, first sized portion, a tapered transition portion and a wide, second sized portion. The gas generating source is coupled to the first sized portion. The bullet assembly is slidably disposed within the second sized portion, with the lance extending away from the first sized portion. Thus, the piston has a pressure surface that is exposed to the first sized portion and the gas generating source.  
       [0022] The piston assembly has a first side and a second side. Hereinafter, the first side will be the side exposed to the gas generating source and therefore may also be referred to as the pressure surface. The piston assembly pressure surface is not flat. As such, gas from the gas generating source is dispersed across the surface of the pressure surface thereby reducing areas of localized pressure. The pressure surface may be either concave or convex. Preferably, the pressure surface is convex, and, where the piston is circular, conical. The conical surface, preferably, has a more obtuse angle than the angle of the taper of the pressure chamber transition portion. As such, there is a gap between the pressure surface and the sidewall of the transition portion. In use, when the gas generation source is activated, the gas entering the chamber first sized portion and the chamber transition portion contacts the conical surface and is dispersed in the gap. The dispersal of the gas creates an even pressure distribution on the pressure surface and causes the piston to move from a first position adjacent to the transition portion to a second position away from the transition portion.  
       [0023] The bullet assembly lance is made from a conductive material and includes an elongated body having a tapered tip and a flared base. The tapered tip is structured to engage a cup on a first conductor, where the cup has a cavity corresponding to the shape of the lance tip. Alternatively, the cup may have a cavity that partially corresponds to the shape of the lance tip, thereby having an interference fit. The flared base is structured to correspond to the shape of a tapered passage in a conductor. The lance is sized so that as the flared base engages the tapered opening in one conductor, the tip firmly engages the cup disposed on the other conductor. Thus, the lance acts to electrically couple the two conductors. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0024] 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:  
     [0025]FIG. 1 is a cross-sectional view of the present invention with the piston in the first position.  
     [0026]FIG. 2 is an exploded isometric view of the present invention.  
     [0027]FIG. 3 is a cross-sectional view of the present invention with the piston in the second position.  
     [0028]FIG. 4A is an isometric view of the bullet assembly wherein the lance has a circular medial portion and a conical tip. FIG. 4B is an isometric view of the bullet assembly wherein the lance has a circular medial portion and a knife edge tip. FIG. 4C is an isometric view of the bullet assembly wherein the lance has a square medial portion and a pyramidal tip. FIG. 4D is a cross-sectional view of a piston body having a concave first side. FIG. 4E is an isometric view of the bullet assembly wherein the lance has a circular medial portion and a blade tip.  
     [0029]FIG. 5 is a schematic view of a vacuum arc interrupter utilizing the piston of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0030] As shown in FIGS.  1 - 3 , a vacuum arc interrupter  10  includes a vacuum chamber assembly  12  and a pressure chamber assembly  14 . The vacuum chamber assembly  12  includes a first conductor  16 , a non-conductive housing  18 , and a seal cup  20 . The first conductor  16  is made from a conductive material and, preferably, is shaped as a circular disk. The first conductor  16  may include a radial extension  22  having an attachment opening  24  therethrough. The attachment opening  24  is structured to allow a power line to be coupled to the first conductor  16 . The first conductor  16  also includes an electrode  23  having a stem  25  and a receiving cup  26 . The cup  26  is disposed at the distal end of the stem  25  and extends into the vacuum chamber  36  described hereinafter. The cup  26  is made from a conductive material and includes a continuous sidewall  28  having an open end  29 , thereby defining a cavity  30 . The cup  26  is supported by the stem  25  so that the cup  26  is spaced from the first conductor  16 . The open end  29  has a cross-sectional area that is smaller than the widest portion of the lance tip  118 , described hereinafter. The stem  25  may have a smaller cross-sectional area than the cup  26 , thus, the strength of the electromagnetic field around the cup  26  will be reduced compared to a cup  26  with a smaller cross-sectional area.  
     [0031] The non-conductive housing  18  is made from a non-conductive material, preferably a ceramic. The non-conductive housing  18  has a shape that corresponds to the shape of the first conductor  16 . Thus, when the first conductor  16  has a disk shape, the non-conductive housing  18  is a hollow cylinder. One axial end of the non-conductive housing  18  is sealingly coupled to the first conductor  16 .  
     [0032] The seal cup  20  includes a generally planar base member  32  and a sidewall  34  generally perpendicular thereto. The seal cup  20  is made from a rigid, non-brittle material such as a cupro-nickel alloy. The alloy material preferably has between about 50 to 95% copper, and more preferably about 70% copper, and between about 5 to 50% nickel, and more preferably about 30% nickel. The alloy may also have lesser amounts of other elements or impurities. Generally, the seal cup  20  material may be torn without a substantial amount of fragmentation. The seal cup sidewall  34  is sealingly coupled to the axial end of the non-conductive housing  18  opposite the first conductor  16 . Thus, the combination of the first conductor  16 , the non-conductive housing  18 , and the seal cup  20  define a vacuum chamber  36 . As will described hereinafter, the seal cup  20  contacts the second conductor  70 . To prevent an arc from forming within the vacuum chamber  36 , the first conductor  16 , or the electrode  23  if present, and the seal cup  20  are separated by a distance sufficient to lower the magnitude of the electric field to below that which would lead to an electrical breakdown within the vacuum. This distance is, generally, about 0.4 inch to 2.0 inches and varies depending upon the voltage in the system. For example, for a voltage of about 125 kilovolts, the distance is preferably about 0.6 inch.  
     [0033] To reduce the magnetic field at the point where the seal cup  20  is attached to the non-conductive housing  18 , a ring shaped metal shield  35  may extend into the vacuum chamber  36  from the seal cup  20 . The shield  20  extends adjacent to the seal cup side wall  34  and has a height sufficient so that the shield  35  is disposed between the point where the seal cup  20  is attached to the non-conductive housing  18  and the electrode  23 . Additionally, there may be an upper seal cup  21 , similar to the seal cup  20  described in detail above, disposed between the first conductor  16  and the ceramic housing  18 . The upper seal cup  21  includes an opening to allow the stem  25  to pass therethrough.  
     [0034] The pressure chamber assembly  14  includes a gas generation device  40 , a pressure chamber body  42 , a second conductor  70 , and a bullet assembly  46 . The gas generation device  40  may be any gas generation device such as those manufactured by TRW Airbag Systems GmbH &amp; Co. KG, Wernher-Von-Braun-STR. 1, D-84544 Asehan am Inn, Germany.  
     [0035] The pressure chamber body  42  is preferably cylindrical and includes a barrel  50  and a mounting flange  51 . The barrel  50  has a first end  52  and a second end  54 . The barrel  50  has an inlet port opening  56  on the first end  52  and a bullet assembly opening  58  at the second end  54 . The inlet port opening  56  is smaller than the bullet assembly opening  58 . The inlet port opening  56  is in fluid communication with the bullet assembly opening  58 . Thus, the barrel  50  defines a pressure chamber  60 . The pressure chamber  60  includes a first sized portion  62 , a transition portion  64 , and a second sized portion  66 . The first sized portion  62  has a smaller cross-sectional area than the second sized portion  66 . The first sized portion  62  is in fluid communication with the inlet port opening  56 . The second sized portion  66  is in fluid communication with the bullet assembly opening  58 . The transition portion  64  is disposed between, and in fluid communication with, the first sized portion  62  and the second sized portion  66 . The transition portion  64  has a cross-sectional area that tapers from the cross-sectional area of the first sized portion  62  to the cross-sectional area of the second sized portion  66 . The pressure chamber  60  preferably has a generally circular cross-sectional area. The flange  51  extends radially from the barrel second end  54  and includes a plurality of fastener openings  53 .  
     [0036] The second conductor  70  is made from a conductive material and, preferably, is shaped as a circular disk. The second conductor  70  may include a radial extension  72  having an attachment opening  74  therethrough. The attachment opening  74  is structured to allow a ground line to be coupled to the second conductor  70 . The second conductor  70  has a first side  76  and a second side  78 . The second conductor  70  also includes a tapered passage  80 , preferably medially disposed on the disk. The tapered passage  80  has a first sized opening  82  on the second conductor first side  76  and a second sized opening  84  on the second conductor second side  78 . The first sized opening  82  is larger than the second sized opening  84 . Thus, the tapered passage  80  has a tapered sidewall  86  extending between the openings  82 ,  84 . The tapered passage  80  is tapered at an angle corresponding to the angle of the flare of the lance base portion  120 , described below. As described hereinafter, typically a power line is coupled to the first conductor  16  and a ground line is connected to the second conductor  70 .  
     [0037] The bullet assembly  46  includes a piston assembly  90  and a lance  110 . The piston assembly  90  includes a piston body  92 , and may include a piston ring  94 . The piston body  92  is a solid body which is generally planar having a first side  96 , a second side  98 , and a sidewall  100 . The piston body  92  has the same general cross-sectional shape and size as the pressure chamber second portion  66  and is structured to be slidably disposed therein. The sidewall  100  includes a groove  101  wherein the piston ring  94  may be seated. The piston first side  96  is not flat having either a concave surface, see FIG. 4D, or, preferably, a convex surface, See FIGS.  1 - 3 . Where the piston body  92  is a disk, i.e., when the pressure chamber  60  is circular, the first side  96  is conical having an angle, Ø, between about 30 to 90 degrees, and preferably about 80 degrees as measured from a line passing through the axis of the piston body  92 . The first side  96 , preferably, has a more obtuse angle than the angle of the taper of the pressure chamber transition portion  64 . As is described hereinafter, the piston body first side  96  is exposed to the pressure created by the gas-generating device  40  and may be referred to as the “pressure surface.” The piston body second side  98  is generally flat and includes an attachment device  102 , for example, a threaded opening  103 .  
     [0038] The lance  110  includes an elongated body  112  having a first end  114  and a second end  116 . The lance body  112  includes a tip  118  disposed at the first end  114  and a base  120  disposed at the second end  116 . Between the tip  118  and the base  120  is a medial portion  122 . The tip  118  tapers to an edge or a point. The end of the tip  118  acts as a blade portion  124  to assist in cutting the seal cup  20  as described below. The angle of the tip taper, α, is between about 90 and 150 degrees and preferably about 120 degrees as measured from a line parallel to the outer surface of the surface of the medial portion  122 . The medial portion  122  preferably has a constant cross-sectional area. The medial portion  122  preferably has a circular or square cross-section. As shown in FIG. 4A, when the medial portion  122  is circular, the tip  118  and the blade portion  124  are, preferably, conical. However, as shown in FIG. 4B, the medial portion  122  may be circular and the tip  118  and blade portion  124  may be a knife edge  124 B. As shown in FIG. 4C, when the medial portion  122  is square, the tip  118  and blade portion  124 C are pyramidal. Alternatively, as shown in FIG. 4E, the medial portion  122  may be circular and have a tapered blade  124 D. The base portion  120  is flared relative to the medial portion  122 . The base portion  120  flare is at an angle, θ, between about 90 and 150 degrees, or, preferably about 94 degrees as measured from a plane passing radially through the lance medial portion  122 . The lance second end  116  includes an attachment device  125 , for example, a threaded rod  126  structured to engage the piston attachment device  102 .  
     [0039] The bullet assembly  46  is formed when the lance  110  is coupled to the piston assembly  90  by coupling the lance attachment device  125  to the piston attachment device  102 . Thus, the lance  110  extends from the piston second side  98 . The lance  110  has a length sufficient to span the gap between the second conductor  70  and the cup  26 . The lance  110  is, however, sized so that the flared base  120  contacts the second contact tapered opening as the tip  118  contacts the cup  26 .  
     [0040] The pressure chamber assembly  14  is formed by inserting the bullet assembly  46  into the chamber second size portion  66  with the lance  110  extending toward the bullet assembly opening  58 . The bullet assembly  46  is disposed in a first position where the piston body  92  is in the pressure chamber second sized portion  66  and adjacent to the chamber transition portion  64 , with the lance  110  extending into the second sized portion  66 . The lance  110  does not, however, extend beyond the bullet assembly opening  58 . Because the piston body first side  96  has a taper angle that is more obtuse that the taper angle of the pressure chamber transition portion  64 , a gap exists between the piston body first side  96  and the pressure chamber transition portion  64 . The piston ring  94  engages the sidewall of the chamber second sized portion  66 . The second conductor  70  is coupled to the pressure chamber mounting flange  51  by fastener  53  with the second conductor first side  76  disposed toward the pressure chamber  60 . Thus, the larger, first sized opening  82  of the tapered passage  80  is adjacent to the bullet assembly  46 . The gas generation device is coupled to, and in fluid communication with, the inlet port opening  56 .  
     [0041] In this configuration, the bullet assembly  46  is structured to move from the first position, described hereinbefore, to a second position, shown in FIG. 3, where the piston body  92  is moved adjacent to the second conductor  70 . In the second position, the flared base  120  of the lance  110  engages the second conductor tapered passage  80 , and the lance  110  extends beyond the second conductor  70 .  
     [0042] Accordingly, to assemble the vacuum arc interrupter  10 , the vacuum assembly  12  is coupled to the pressure chamber assembly  14  with the seal cup  20  contacting, or immediately adjacent to, the second conductor  70 . In this configuration, translation of the bullet assembly  46  from the first position to the second position will result in the lance blade portion  124  piercing the seal cup  20  and the lance  110  contacting the first conductor cup  26 . As stated hereinbefore, the lance  110  is sized such that the tip  118  engages the cup  26  at the same time the flared base  120  engages the second contact tapered passage  80 . Thus, when the bullet assembly  46  is in the second position, the first and second conductors  16 ,  70  are in electrical communication.  
     [0043] In operation, the bullet assembly  46  is moved from the first position to the second position by the gas-generating device  40 . That is, the gas generating device  40  delivers gas at a pressure between about 180 and 375 psi, and preferably about 180 psi, through the inlet port opening  56  in to the chamber first size portion  62 . This increase of pressure occurs in about 0.50 msec and causes the bullet assembly  46  to move from the first position to the second position in less than 2.0 msec. Because the inlet port opening  56  is on the piston first side  96 , gas from the gas generating device will flow into the chamber first sized portion  62  and transition portion  64  and contact the angled piston first side  96 . The angle of the piston first side  96  assists the gas in dispersing through the chamber transition portion  64  and thus creates a more even pressure distribution on the piston first side  96 . As the bullet assembly  46  moves from the first position to the second position, the lance tip  118  and medial portion  122  pass through the tapered passage  80  causing the blade portion  124  to puncture the seal cup planar member  32 . Because the seal cup  20  is made of a cupro-nickel material, the seal cup  20  is torn as opposed to fragmenting.  
     [0044] As stated hereinbefore, the lance tip  118  engages the cup  26 . If the lance tip  118  is conical, the taper of the tip  118  and the taper of the cup  26  sidewall is, preferably, similar. Thus, the lance  110  and the cup  26  cooperatively engage each other. If, however, the lance tip  118  is pyramidal, the lance  110  and cup  26  will engage in a mechanical connection as the square lance  110  collides with the circular cup  26 . This collision will form a mechanical connection that may be enhanced if an arc forms between the lance  110  and the cup  26  thereby partially melting either the lance  110  or the cup  26 . Additionally, after the downstream arc is interrupted and electricity is flowing through the vacuum arc interrupter  10 , heat generated in the flared base  120  and the second contact tapered passage  80  will partially melt the metal components and form a weld. As such, the bullet assembly  46  is mechanically locked by a weld to the second conductor  70 .  
     [0045] As shown in FIG. 1, to prevent arcing in a LV or MV device  1 , the vacuum arc interrupter  10  must be electrically coupled to the circuit, between the power source  2  and the LV or MV device  1  by a power line  3 . Typically, the power line  3  connected to the circuit is coupled to the first conductor  16  and a ground line  4  is connected to the second conductor  70 . An arc detection device  5 , which may be any common arc detector or a device such as the one described in co-pending application Ser. No. ______(01-EDP-385), incorporated by reference, is used to detect an arc within the LV or MV device  1  and to activate the gas generation device  40 . Thus, when an arc in the LV or MV device  1  is detected, the vacuum arc interrupter  10  is activated thereby grounding the circuit upstream of the LV or MV  1  device and interrupting the arc. The circuit with the bolted fault created by the vacuum arc interrupter  10  is broken by a circuit breaker (not shown) upstream of the vacuum arc interrupter  10 .  
     [0046] Aspects of this invention may also be used in conjunction with an alternate embodiment of the vacuum arc interrupter  210  having two contacts in a vacuum chamber assembly  200 . That is, as shown in FIG. 5, a second embodiment of the vacuum arc interrupter  210  includes the vacuum chamber assembly  200  having two contacts  212 ,  214  disposed in a vacuum chamber  216 , as well as a first bus  213  and a second bus  215 . The vacuum chamber  216  includes a non-conductive housing  218 . A first contact  212  is fixed, and the other, second contact  214  is movable. The fixed contact  212  is sealingly coupled to the non-conductive housing  218  and is in electrical communication with a first bus  213  that is external to the vacuum chamber  216 . The movable contact  214  is coupled to a rod  220  having a first end  222 , a medial portion  224  and a second end  226 . The movable contact  214  is disposed at the rod first end  222 . A bellows  228  is coupled to the rod medial portion  224  and to the non-conductive housing  218 . The rod  220  is structured to move between a first position wherein the contacts are spaced from each other, to a second position wherein the contacts contact each other. A second bus  215  is coupled to the rod  220  and is in electrical communication with the second contact  214 . The vacuum arc interrupter  210  further includes a pressure chamber assembly  14 . The pressure chamber assembly  14  is substantially similar to the pressure chamber assembly  14  described hereinabove. The second end of the rod  220  is coupled to a piston assembly  90  disposed in a pressure chamber assembly  14 . The piston assembly  90  is substantially similar to the piston assembly  90  described hereinabove. That is, a piston assembly  90  has a concave or convex first, pressure surface  96 , that is exposed to the gas created by a gas generation device  40 . In this embodiment of the vacuum arc interrupter  210 , however, the piston assembly  90  is coupled to the rod  220 . As such, when the gas generation device  40  is activated, the piston assembly  90  moves the rod  220  between the first position and the second position, thereby moving the contacts  212 ,  214  from the open position to the closed position. The closing of the contacts  212 ,  214  occurs in less than 2.0 msec. Typically the first bus  213  is coupled to, and in electrical communication with, the circuit having the MV or LV device and the second bus  215  is in electrical communication with a ground. Additionally, the rod  220  may include one or more impact absorbing devices  221 , such as springs, disposed between the piston assembly  90  and the second movable contact  214 .  
     [0047] 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 invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.