Abstract:
A seal is provided around a vacuum interrupter and an air-filled cavity, and a tube is provided within the seal. The tube has a first end open to the air-filled cavity and a second end open to an exterior of the seal. The seal, the vacuum interrupter, and the air-filled cavity are encapsulated.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional (and claims the benefit of priority under 35 U.S.C. §120) of U.S. application Ser. No. 10/802,409, filed on Mar. 16, 2004, now allowed, and titled VACUUM ENCAPSULATION HAVING AN EMPTY CHAMBER, which claims priority from U.S. Provisional Application Ser. No. 60/465,269, filed on Apr. 25, 2003, both of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This description relates to electrical switchgear, and, more particularly, to a vacuum interrupter encapsulation. 
     BACKGROUND 
     Conventional vacuum switchgear exists for the purpose of providing high voltage fault interruption. Examples of such vacuum switchgear include vacuum fault interrupters (also referred to as “vacuum interrupters” or “interrupters”), which generally include a stationary electrode assembly having an electrical contact, and a movable electrode assembly on a common longitudinal axis with respect to the stationary electrode assembly and having its own electrical contact. The movable electrode assembly generally moves along the common longitudinal axis such that the electrical contacts come into and out of contact with one another. In this way, vacuum interrupters placed in a current path can be used to interrupt extremely high current, and thereby prevent damage to an external circuit. 
     Such a vacuum interrupter may be encapsulated in a rigid or semi-rigid structure that is designed to provide insulation to the interrupter. The rigid structure may be designed to encapsulate one or more air cavities, in addition to the vacuum interrupter and related components. The air cavities may be used to facilitate construction and/or operation of the vacuum interrupter and its encapsulating structure. For example, such an air cavity may provide space for movement of various components, or may allow thermal expansion of one or more materials associated with making or using the vacuum interrupter. 
     SUMMARY 
     In one general aspect, a vacuum switching device includes a vacuum interrupter, a current exchange housing adjacent to the vacuum interrupter, a seal provided around the vacuum interrupter and the current exchange housing so as to define a cavity within the current exchange housing and adjacent to the vacuum interrupter, and a tube provided within the seal, the tube disposed such that a first end of the tube accesses the cavity and a second end of the tube accesses an exterior of the seal. 
     Implementations may include one or more of the following features. For example, the tube may include a syringe needle inserted through the seal. The tube may be integrally formed into the seal during formation of the seal. 
     The second end of the tube may be open to an encapsulation material provided around the vacuum interrupter, the current exchange housing, and the seal. In this case, the encapsulation material may include a pre-filled, hot-curing, two-component epoxy resin. 
     Also, a diameter of the tube may be selected such that air within the cavity is permitted to escape from the cavity to the exterior of the seal during a molding process that involves injection of the encapsulation material in liquid form into a reduced-pressure space surrounding the vacuum interrupter, the current exchange housing, and the seal. In this case, the diameter of the tube may be selected such that the encapsulation material in liquid form will not travel from the exterior of the seal to the cavity during the injection. 
     The vacuum switching device may include an operating rod that extends through the seal into the cavity, and is operable to actuate the vacuum interrupter. 
     In another general aspect, a seal is provided around a vacuum interrupter and an air-filled cavity. A tube provided within the seal has a first end that accesses the air-filled cavity and a second end that accesses an exterior of the seal. The seal, the vacuum interrupter, and the air-filled cavity are encapsulated. 
     Implementations may include one or more of the following features. For example, in encapsulating the seal, the vacuum interrupter, and the air-filled cavity, an air pressure in an area of the exterior of the seal may be reduced, such that air from within the air-filled cavity is removed from the air-filled cavity through the tube. 
     During encapsulation, the seal, the vacuum interrupter, and the air-filled cavity may be placed into a mold that contains a space that is in contact with the exterior of the seal. Air may be removed from the space that is in contact with the exterior of the seal, epoxy may be injected into the space in liquid form, and the mold may be removed after the epoxy is cured. 
     To remove air from the space, a pressure differential between the air-filled cavity and the space may be reduced by allowing a transfer of air from the air-filled cavity through the tube. 
     In removing the mold, a mold core may be removed along with the mold, and an operating rod for activation of the vacuum interrupter may be inserted into a cavity left by removal of the mold core. In providing the seal, the air-filled cavity may be sealed against the mold core while epoxy is injected into the space that is in contact with the exterior of the seal. 
     The tube may be selected to have a diameter that allows air from the air-filled cavity to escape into the space that is in contact with the exterior of the seal, and that prevents the liquid-form epoxy from traveling between the space that is in contact with the exterior of the seal and the air-filled cavity. 
     To provide the seal, a compliant material may be provided around the vacuum interrupter and the air-filled cavity, and a plug may be provided adjacent to the compliant material, with the plug positioned to seal the air-filled cavity. To provide the tube within the seal, the tube may be provided through the plug. 
     In another general aspect, a vacuum switching device includes a vacuum interrupter, a hollow housing adjacent to the vacuum interrupter, a seal provided around the vacuum interrupter and the hollow housing to define an air-filled cavity within the hollow housing, and means for reducing a pressure differential between the air-filled cavity and a space exterior to the seal during a vacuum gelation process in which air pressure in the space is reduced for injection of a liquefied encapsulation material into the space, such that the integrity of the seal is maintained during the vacuum gelation process. 
     Implementations may include one or more of the following features. For example, the means for reducing a pressure differential may include an air passageway from the air-filled cavity to the space exterior to the seal, or may include a tube inserted through the seal between the air-filled cavity and the exterior space. In the latter case, the tube may have a diameter large enough to reduce the pressure differential by transferring air from the air-filled cavity to the space exterior to the seal during the vacuum gelation process, and small enough to prevent transmission of the liquefied encapsulation material from the space into the air-filled cavity. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cutaway side view of a vacuum switching device. 
         FIG. 2  is a magnified view of a vacuum assembly of the device of  FIG. 1 . 
         FIG. 3  is a cross-section of a mold used to form an epoxy encapsulation around the vacuum assembly of  FIG. 2 . 
         FIG. 4  is a cross section of the encapsulated vacuum assembly. 
         FIG. 5  is a flowchart illustrating a process for forming the vacuum switching device of FIG  1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a vacuum switching device  100  that includes a vacuum fault interrupter  102  that may be used to protect an external circuit (not shown) from excessively high current. The vacuum interrupter  102  includes a stationary terminal rod  104  that is connected to an upper contact terminal  106 . The upper contact terminal  106  allows a connection of the vacuum interrupter  102  to the external circuit. 
     The vacuum interrupter  102  is affixed to an operating rod  108  that is contained within a dielectric-filled cavity  110  (the dielectric, not shown in  FIG. 1 , may be gaseous or liquid) and extends through an opening  112 . The opening  112  is sealed around the operating rod  108  by way of a sealing diaphragm  114 . The operating rod  108  is wrapped within a silicone rubber sleeve or skirt  116 . As shown, circumferential ribs are included along the length of silicone rubber skirt  116  in order to increase the “creep distance” (length of insulating surface) so as to help prevent debilitating short circuits and to improve dielectric properties of the operating rod  108  and associated elements. 
     The operating rod  108  is connected at an end extending through the opening  112  to an external device (not shown) operable to cause axial movement thereof. At its other end, the operating rod  108  is connected to a movable electrical contact within the vacuum interrupter  102 . As a result, the movable electrical contact may be moved into or out of contact with a stationary electrical contact within the vacuum interrupter  102  (interior of vacuum interrupter not shown). In this way, a flow of current within the vacuum interrupter  102  may be interrupted when necessary to protect the external circuit. 
     A current exchange is housed within a current exchange housing  118 , and permits current flow between the vacuum interrupter  102  and a conductor  120 . In general, such an assembly facilitates current flow between two points and may include, for example, a roller contact, a sliding contact, or a flexible connector Although not explicitly shown in  FIG. 1 , the actuation end of the vacuum interrupter  102  also includes a bellows that permits motion of the moving contact while still maintaining a vacuum seal. 
     A compliant material  122 , which may be, for example, a silicone rubber sleeve, encases the vacuum interrupter  102 . In one implementation, the compliant material  122  is adhered to the vacuum interrupter  102  by, for example, a silane-based adhesive such as SILQUEST A-1100 silane (that is, gamma-aminopropyl triethoxysilane). In addition to encasing the vacuum interrupter  102 , the compliant material  122 , in conjunction with at least one rubber plug  124 , defines an air cavity  126  within the current exchange housing  118 . This cavity  126  is used to allow motion of the operating rod  108  during operation of the vacuum interrupter. 
     A rigid encapsulation material  128 , which may be, for example, an epoxy encapsulation material, is used to enclose the whole of the vacuum switching device  100  of  FIG. 1 . In one implementation, the epoxy encapsulation  128  is cast from a cycloaliphatic, pre-filled, hot-curing, two-component epoxy resin. 
     The compliant material  122  also is used to cushion the different coefficients of linear thermal expansion between the vacuum interrupter  102  and the encapsulation epoxy  128 . In order to perform this function effectively, the compliant material  122  requires a mechanical escape (i.e., a region where the compliant material  122  comes into contact with air, e.g., in the cavity  126 ). 
       FIG. 2  is a magnified view of a vacuum assembly  200  of  FIG. 1 . The vacuum assembly  200  generally refers to portions of the vacuum switching device  100  of  FIG. 1  placed within a mold during a formation of the epoxy encapsulation  128 . The vacuum assembly  200  includes the vacuum interrupter  102 , the stationary terminal rod  104 , the upper contact terminal  106 , the current exchange housing  118 , the compliant material  122  and the rubber plug  124 . 
     During formation of the epoxy encapsulation  128 , as explained in more detail below, a vacuum is formed between the vacuum assembly  200  and a mold into which epoxy will be injected for forming the epoxy encapsulation  128 . During this process, the compliant material  122 , along with, e.g., the rubber plug  124 , may form at least part of a seal that will prevent epoxy from filling the cavity  126  within the current exchange housing  118 . In this way, the current exchange and bellows are protected from the injected epoxy. 
     However, as a result of this sealing, air cannot be pumped out of the region that will form the cavity  126 . As a result, a pressure differential between the vacuum within the mold (i.e., external to the vacuum assembly  200 ) and the air in the sealed-off cavity  126  may cause various difficulties. For example, the pressure differential may cause the compliant material  122  to inflate away from the vacuum interrupter  102  and the current exchange housing  118  like a balloon, or may blow out some of the rubber plug  124 . Such problems may cause difficulties with the encapsulation process, and may result in, for example, poor insulation of the vacuum interrupter, cracks or voids in the epoxy encapsulation, or epoxy leaking into the current exchange area (which may prevent operation of the vacuum interrupter). 
     To avoid these difficulties, including, for example, inflation or seal blow-out, one or more small needles or capillary tubes  202  are pushed through or molded into a portion of the rubber plug  124  that helps seal the vacuum interrupter assembly  200 . In one implementation, an inside diameter of the needle  202  or tube is such that air can be removed from the sealed cavity  126 , so as to prevent the air pressure differentials, while being small enough that epoxy can not flow through the needle or tube  202  without curing (thus sealing the tube off and preventing epoxy from filling the cavity  126  and other portions of the assembly that are to be kept free of epoxy). For example, a diameter of the needle or tube  202  may be approximately 0.010 inches, or needles may be used having a gauge in the range of 23-26, so that an inner diameter of such needles ranges from approximately 0.25-0.35 mm. 
     Although  FIG. 2  illustrates only one needle  202  within the rubber plug, it should be understood that a number and placement of needles may be optimally selected for the vacuum assembly at hand. For example, in one implementation, four needles may be symmetrically placed around a center axis of the vacuum assembly. In another implementation, the compliant material may be extended to serve the function of the rubber plug  124 ; in this implementation, the rubber plug  124  may not be necessary, and the needle(s)  202  may be placed directly into the compliant material  122 . 
       FIG. 3  is a cross-section of a mold used to form an epoxy encapsulation around the vacuum assembly  200  of  FIG. 2 . Initially, the vacuum assembly  200  is placed within the mold  300 . For example,  FIG. 3  may represent one symmetrical half of the mold  300 , so that, by separating the mold into its two symmetrical halves, the vacuum assembly  200  may easily be placed within the mold  300 . 
     The mold  300  includes a space  302  that is to be filled with the epoxy encapsulation  128 . A mold core  304  extends upward into the space  302 , in order to define the cavity  110  into which the operating rod  108  is inserted. The mold core  304 , in one implementation, seals against the bottom of the current exchange housing  118 . In this way, epoxy is prevented from filling the bellows and the cavity  126  within the current exchange housing  118 , thus allowing these components to continue to be free to move in the epoxy encapsulation. 
     Prior to molding, a vacuum port  306  removes air from the space  302 , which is sealed by vacuum seals  308 . Then, a fill port  310  is used to inject epoxy, at high heat and in liquid form, into the space  302 . Subsequently, the epoxy is allowed to cure into the epoxy encapsulation  128 , and the mold  300  is removed. This molding process is generally known as vacuum gelation. 
     As referred to above, removal of air from the space  302  through the vacuum port  306  may create a pressure differential between the air within the air cavity  126  and the vacuum created within the space  302 , so that the compliant material  122  and rubber plug  124  may be detrimentally affected. The presence of the needle  202  prevents such a pressure differential, while ensuring that epoxy does not get into the air cavity  126 . 
       FIG. 4  is a cross section of the encapsulated vacuum assembly  200  and illustrates the vacuum assembly after the molding process is complete, the epoxy encapsulation  128  has cured, and the mold  300  and mold core  304  have been removed. 
     As shown in  FIG. 4 , the space  110  is created by removal of the mold core  304 , so that the operating rod  108  and associated portions may be inserted in their place for completion of the vacuum switching device of  FIG. 1 , including placement of the seal  114 . It should be understood with respect to  FIGS. 2 and 4  that a relative size of the needle  202  is exaggerated with respect to remaining portions of the vacuum assembly. Thus, with respect to  FIG. 1 , it should be understood that the needle  202  is included within the rubber plug  124 , but is not visible in  FIG. 1  due to its relative size. 
       FIG. 5  is a flowchart  500  illustrating a process for forming the vacuum switching device of  FIG. 1 . In  FIG. 5 , the process begins with the sealing of the vacuum interrupter  102  and the current exchange housing  118  using the compliant material  122  ( 502 ). Then, the needle(s)  202  are inserted into this seal ( 504 ). Of course, the needle(s)  202  also maybe formed into the seal as part of, or prior to, the sealing process. 
     Then, the contact portions  104 ,  106 , and  120  are attached to the sealed vacuum interrupter  102  and current exchange housing  118  to complete the vacuum assembly  200  ( 506 ). The vacuum assembly  200  is placed into the mold  300  ( 508 ), and the air is removed from the space  302  within the mold  300  ( 510 ) to create a vacuum. Then, epoxy is injected into the mold  300  ( 512 ). 
     As already explained, the presence of needles  202  prevent any pressure differential from being created between the space  302  and the cavity  126  so that the seal around the vacuum interrupter  102  and the current exchange housing  118  is not disturbed. At the same time, diameters of needles  202  are small enough that any epoxy incidentally entering the needles  202  is cured before the epoxy can reach the cavity  126 . As a result, the needles  202  prevent a pressure differential from forming as the vacuum is pulled on the mold  300 , with the number of the needles  202  being directly proportional to the rate at which air is removed from the cavity  126 , and inversely proportional to the pressure differential. By a time that epoxy  128  has been fully injected into the mold  300 , any air within the cavity  126  has been substantially removed, and the needles  202  are plugged with cured epoxy, so as to prevent the epoxy from filling the cavity  126 . 
     Once the epoxy is cured and the mold  300  and the mold core  304  are removed ( 514 ), assembly of the vacuum switching device  100  may be completed by placing the operating rod  108  and associated components into the space  110  created by the mold core  304  ( 516 ). 
     As explained above, a vacuum assembly including a vacuum interrupter may be sealed with a compliant material and/or rubber plugs, so that a cavity is created and maintained within the assembly for use with a current exchange housing and/or bellows during operation of the vacuum interrupter. During vacuum molding of the vacuum assembly to encase the vacuum assembly in epoxy, a resulting pressure differential caused by the vacuum molding is prevented from disturbing the seal around the vacuum assembly, by way of a needle or tube included in the seal. In this way, air from within the cavity is allowed to escape, while the epoxy is prevented from entering the cavity. The vacuum assembly than can be joined with an operating rod and other components to complete a vacuum switching device. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other implementations are within the scope of the following claims.