Abstract:
An electrical switch which includes an insulative housing having a wall defining an axial bore therein, a first electrical contact disposed in the housing bore and a second electrical contact movably disposed in the housing bore between an open position and a closed position. When the contacts are in their open position, the second electrical contact is spaced apart from the first electrical contact and when the contacts are in their closed position, the second electrical contact is in electrical contact with the first electrical contact. The switch includes features to enhance safety and operation by reducing the possibility of arcing or flashover before and during the switching operation and to provide of visual indication of the state of the switch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of U.S. application Ser. No. 11/141,571, filed on May 31, 2005 now U.S. Pat. No. 7,397,012. 

   FIELD OF THE INVENTION 
   The present invention relates generally to high current switches used in electric power distribution systems and, more particularly, to an electrically insulated, deadfront, single operation, medium voltage, high current closing device. 
   BACKGROUND OF THE INVENTION 
   An urban utility experiences approximately 1,500 failures on its network feeders each year. Each feeder outage duration is directly proportional to the risk of customer interruption and the stress experienced by other feeders and transformers in the network. The defective component must remain out of service during repair and/or replacement. This means that the whole feeder remains out of service or a live end cap must be installed to separate the main feeder from the spur containing the defect. If a live end cap is installed, the feeder must be de-energized a second time to reconnect the required spur. This second outage is usually scheduled as soon as possible to restore the system to normal full capability. However, the perceived risk of scheduling the entire feeder out of service to pick-up a small spur is very large, especially during the summer or other high load periods. 
   Encapsulated switch assemblies with sub-atmospheric or vacuum type circuit interrupters for electric power circuits and systems are well known in the art, such as is shown in U.S. Pat. Nos. 4,568,804; 3,955,167; 3,471,669; 3,812,314; and 2,870,298. In some prior art switch assemblies and circuit breakers, a pair of coacting contacts, one fixed and the other movable, are provided for controlling and interrupting current flow. The contacts are provided in a controlled atmosphere contact assembly which may include a relatively fragile glass or ceramic housing, commonly referred to as a “bottle” for housing the contacts. A metal bellows may be provided on one end of the bottle, and the movable contact is linked to the inside of the bellows. An operating rod attached to the outside of the bellows can be moved so as to move the movable contact inside the bottle. The interior of the bottle is maintained under a controlled atmosphere, such as air or another gas under a low subatmospheric pressure, to protect the contacts from damage caused by arcing when the contacts are opened and closed. The glass or ceramic wall of the bottle provides a permeation-resistant enclosure which maintains the controlled atmosphere for the life of the device. 
   More recently, elastomer-insulated switch housings using a controlled atmosphere contact assembly have been introduced for underground power distribution systems and other, similar applications. Switches for use in such applications must meet several demanding requirements. Those parts of the switch assembly connected to line voltage during use, including the contact assembly and operating rod, must be encased in a solid insulating housing having dielectric strength sufficient to withstand the maximum voltage which may be imposed on the system, which may be tens of thousands of volts for a distribution-level system. For safety, the insulating housing should be covered with a conductive layer that can be grounded. The switch should be operable from outside of the dielectric housing, without opening the housing and should be capable of withstanding many years of exposure to temperature extremes, water and environmental contaminants. 
   Elastomers such as EPDM (ethylene propylene diene monomer) combine high dielectric strength with excellent resistance to the effects of ozone and corona discharge. These elastomers can also provide good physical properties such as abrasion resistance, and can be molded at reasonable cost. Additionally, these elastomers can be compounded with conductive additives and molded to provide an electrically conductive grounding layer integral with the dielectric housing. For these and other reasons, elastomers molded and vulcanized under heat and pressure, such as EPDM, have been almost universally adopted as materials of construction for the housings used in many underground electrical distribution systems. 
   An important feature in such switch assemblies and circuit breakers is the ability to visually determine the switched condition of the contacts. This is obviously important for safety reasons in that power must be disconnected before accessing or repairing a switch branch. U.S. Pat. No. 4,568,804 discloses a high voltage vacuum type circuit interrupter having a one-piece ceramic insulating housing connected to a two-part metallic base. The base encloses a solenoid operated toggle mechanism that controls and operates movement of a switch contact to open and close the switch. The base further includes a sight glass or lens secured to the bottom of the base, through which a switch position indicator is visually discernible. 
   One drawback with the circuit interrupter disclosed in the &#39;804 patent is its size and complexity in manufacture. Another drawback relates to the fact that the position indicator is located at the toggle mechanism away from the switch contacts. In other words, while the position indicator of the &#39;804 patent may show the condition of the toggle mechanism, there is no provision for visually confirming whether the switch contacts are indeed in contact or separated. 
   As mentioned above, another concern with such switch assemblies is flashover or arcing of the electric current between switch contacts. Aside from safety concerns, such arcing causes damage to the contacts and the surrounding housing. While efforts to reduce arcing by enclosing the contacts in an evacuated chamber or by insulating the contacts with an arc quenching gas or oil have proven somewhat successful, arcing still occasionally occurs in the field. Additionally, vacuum chambers typically require a housing made from ceramic. Air insulation chambers are generally very large. Chambers filled with SF 6  arc quenching gas must be hermetically sealed and maintained to ensure no leakage and insulating oils have been found to fail catastrophically resulting in injury to people and damage to equipment. 
   Yet another problem with high current switches described above is related to electromagnetic fields which generate undesirable bending forces. In particular, the feeder contact is arranged generally at a 90° angle to the switches current carrying contact pin. These electromagnetic forces are produced on the current carrying members causing a cantilever bending movement at the connection interface. 
   Accordingly, it is desirable to provide a simply constructed, electrically insulated, switch assembly having direct visible verification of open or closed contacts. It is further desirable to provide such a switch assembly that minimizes the possibility of arcing between electrical contacts and provides good electrical continuity through the switch assembly. 
   SUMMARY OF THE INVENTION 
   The present invention is an electrical switch, which generally includes an insulative housing having a wall defining an axial bore therein, a first electrical contact disposed in the housing bore and a second electrical contact movably disposed in the housing bore between an open position and a closed position. When the contacts are in their open position, the second electrical contact is spaced apart from the first electrical contact and when the contacts are in their closed position, the second electrical contact is in electrical contact with the first electrical contact. 
   In a preferred embodiment, the switch further includes a viewing port disposed in the insulative housing wall adjacent the first electrical contact to permit viewing of the first electrical contact within the housing bore. The viewing port preferably includes a transparent element made from a clear insulative plastic material fixed within the housing wall. The transparent element may further be provided with a magnification feature to enhance viewing and the housing wall may include a protruding boss portion having a hole for receiving the transparent element. 
   The switch may further include a frangible insulative plate disposed in the housing bore between the first and second electrical contacts when the second electrical contact is in its open position, The frangible insulative plate is adapted to be broken by the second electrical contact as the second electrical contact is moved to its closed position. The frangible insulative plate is preferably made from a high dielectric strength glass material. 
   Another feature of the present invention is a high current electrical connector system that includes a male pin having a first end and a second end formed by a pair of resilient legs that define a slot; a female socket having a substantially cylindrical side wall and a bottom surface which define a cavity, an open end and a post that extends from the bottom surface. The female socket is configured to receive and electrically contact the second end of the male pin and the slot receives and electrically contacts the post. The male pin can be tapered from the first end to the second end and the post can have a base and a knurled end. Preferably, the slot in the male pin is configured to receive the knurled end. The cylindrical side wall of the female socket can include one or more apertures that are adapted to vent the cavity. 
   The high current electrical connector system can also include an electrically insulated rod and an actuating mechanism. The electrically insulated rod connects the actuating mechanism to the first end of the male pin. 
   Another feature of the present invention is an insulating seal ring for electrically insulating a movable energized contact in a housing for a high-current electrical switch. The insulating seal ring includes a generally annular body having an outer wall with an outside diameter that defines an outer sealing surface, an inner wall with an inside diameter that defines an aperture with an inner sealing surface. The outer sealing surface is adapted to be sealably received by the housing and the inner sealing surface is adapted to sealably receive an actuating rod. The insulating seal ring also includes a generally annular core inside the annular body. The body has a first durometer (or hardness) and the core has a second durometer and the materials that form the body and core are selected so that the second durometer is greater than the first durometer. Preferably, the body is formed from an elastomeric polymeric material, such as natural rubbers, synthetic rubbers or fluoropolymers. The body can also be formed from a thermoplastic material, most preferably one that includes a polyethylene, a polypropylene or a polybutylene. The core is preferably formed from a thermoplastic material, an elastic synthetic polyamide material (Nylon), a polycarbonate, an acrylonitrile-butadiene styrene, a polyester terephthalate or a styrene-acrylonitrile. 
   The insulating seal ring preferably has an outside diameter that is greater than or equal to two times the inside diameter. The body can have a first substantially flat surface and a second substantially flat surface, wherein the distance between the first and second surfaces defines a thickness, and wherein the thickness is greater than or equal to the outside diameter. 
   In another embodiment, the insulating seal ring includes a generally annular body having an inner concentric layer and an outer concentric layer, wherein the inner concentric layer is formed from a first elastomer material having a first durometer and the outer concentric layer is formed from a second elastomer material having a second durometer, wherein the second durometer is greater than the first durometer. The insulating seal ring can also include a first substantially flat surface and a second substantially flat surface, an outer wall with an outside diameter that defines an outer sealing surface and an inner wall with an inside diameter that defines an aperture having an inner sealing surface, preferably the outside diameter is greater than or equal to two times the inside diameter. The distance between the first and second surfaces defines a thickness which is preferably greater than or equal to the outside diameter. The outer sealing surface is adapted to be sealably received by the housing and the inner sealing surface is adapted to sealably receive the actuating rod. 
   The inner concentric layer and the outer concentric layer are formed from different elastomeric materials selected from the group consisting of natural rubbers, synthetic rubbers, and fluoropolymers. The outer concentric layer material is selected so that its durometer is greater than the durometer of the inner concentric layer material. The outer concentric layer material can also be an elastic synthetic polyamide material (Nylon), polycarbonate, acrylonitrile-butadiene styrene, or styrene-acrylonitrile. 
   The switch assembly may further include method and apparatus for reducing bending forces on an electrical connection point. More specifically, the feeder contact and current carrying male pin are electrically coupled and include longitudinal axes which are substantially non-parallel. The feeder contact preferably includes a mechanically weakened portion adjacent the electrical connection. Upon closing of the switch, high current flows through the male pine and feeder contact generating electromagnetic bending forces. These bending forces tend to act on the electrical connection thereby loosening or damaging the connection. The mechanically weakened portion directs the bending forces away from the electrical connection to reduce undesirable bending forces on the connection point. 
   The preferred embodiments of the switch of the present invention, as well as other objects, features and advantages of this invention, will be apparent from the following detailed description, which is to be read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of the switch according to the present invention. 
       FIG. 2  is a detailed cross-sectional view of the viewing port of the present invention. 
       FIG. 3  is a cross-sectional view of the housing central bore showing the space between the open contacts separated by glass insulating plates. 
       FIG. 3   a  is a cross-sectional view of the housing central bore shown in  FIG. 3  with the contacts in a closed position and the glass plates broken. 
       FIG. 4  is a cross-sectional view of the insulating seal ring and actuating rod. 
       FIG. 5  is a cross-sectional view of the feeder post contact and male pin electrical connection. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring first to  FIG. 1 , in a preferred embodiment, the switch  10  according to the preferred embodiment of the present invention is a medium-voltage, one-operation switch. As used in this disclosure with reference to apparatus, the term “medium voltage” means apparatus which is adapted to operate in electric utility power systems, such as in systems operating at nominal voltages of about 5 kv to about 35 kv, commonly referred to as “distribution” systems, as well as equipment for use in “transmission” systems. A high current switch of this type is disclosed in commonly owned U.S. Pat. No. 5,808,258, the disclosure of which is incorporated herein by reference in its entirety. 
   The term “one-operation” generally means a device used to temporarily interrupt power between a “feeder” or “source” circuit and a “spur” circuit in order to safely access or effect repairs on the spur circuit. Upon successful repairs of the spur circuit, the switch is closed to restore power to the spur circuit and is replaced by a permanent connection at a later low load planned outage. 
   The switch  10  includes a housing  12  formed from a dielectric elastomer which is vulcanized under heat and pressure, such as ethylene propylene diene monomer (EPDM) elastomer. The housing  12  defines an elongated bore  14  extending in endwise directions parallel to an axis  16 . The housing has a terminal end  18  and a second, opposite end  20 , referred to herein as the operating end. For reasons discussed below, the direction parallel to axis  16  toward terminal end  18  is referred to herein as the closing endwise direction, whereas the opposite endwise direction, towards operating end  20  is referred to as the opening endwise direction. 
   The housing defines a tapered bushing  22  at the fixed end and a further tapered bushing  24  extending perpendicular to the endwise axis. Bushing  24  has a cylindrical metallic current-carrying element  25  extending therein to the bore  14  in a direction perpendicular to axis  16 . This current-carrying element  25  of the bushing  24  is generally adapted for electrical connection to the “spur” circuit of the power distribution system, as described above. 
   The portion of the housing  12  disposed between the tapered bushing  22  and the operating end  20  has a generally cylindrical exterior surface, so that the wall of the housing in this region is generally in the form of a cylindrical tube. The housing is provided with an electrically conductive insert  26  formed from a mixture of the same elastomer used for the remainder of the housing and an electrically conductive material such as carbon black. Insert  26  covers the interior wall of bore  14  from the operating end  20  to a point adjacent the bushing  24 . 
   Overlying the majority of the exterior surface of the housing  12  is a conductive jacket  28 . The bushing  24  extends from the housing through a hole in the conductive jacket  28 . The conductive jacket  28  may also be formed from a mixture of the same elastomer used for the remainder of the housing and an electrically conductive material such as carbon black. The exterior conductive jacket  28  is in intimate, void-free contact with the outside of the housing  12 , and is securely bonded thereto. Likewise, the semiconducting lining  26  is intimately bonded to the dielectric elastomer of the housing  12 . These components may be fabricated by insert molding, as described in U.S. Pat. No. 5,808,258, which was previously incorporated by reference. 
   Fixed at the terminal end  18  of the housing  12  is a metallic terminal end closure  30 , which seals the central bore  14  at the terminal end. A fixed contact  32  is mounted to the terminal end closure  30  and projects into the central bore  14  of the housing  12 . The fixed contact  32  includes an engagement end  33  and further includes a terminal end stub contact  34  formed integrally with the fixed contact, which projects outwardly from the central bore  14  beyond the terminal end closure  30 . 
   The switch  10  further includes an actuating device  38  mounted to the operating end  20  of the housing  12 . The actuating device  38  is connected to a moveable or operating-end male contact pin  40  extending into the central bore  14  of the housing  12 . The contact pin  40  is in electrical contact with the first cylindrical metallic current-carrying element  25  disposed in the second bushing  24 . More specifically, the first current carrying element  25  includes a threaded end  29  which is received in a threaded bore  31  of a donut contact  27 . The donut contact  27  includes an axial bore to slidingly electronically communicate with the contact pin  40 . The first current carrying device or post contact  25  includes a central axial bore therein to receive the post of the high voltage connector, such as an elbow connector (not shown). The contact pin  40  is driven by the actuating device  38  in the closing endwise direction from an open position, as shown in  FIG. 1 , to a closed position, wherein the contact pin engages the fixed contact  32 . The actuating device  38  moves the contact pin  40  rapidly between opened and closed positions so as to minimize arcing. 
   The actuating device  38  is preferably extremely compact and accommodated in a tubular housing  39  of essentially the same diameter as the switch housing  12 . An O-ring or other conventional seals (not shown) can be provided between the actuator housing  39  and the switch housing  12  so as to provide a weather-tight seal protecting the elements of the actuating mechanism  38 . Any of the numerous drive mechanisms known in the art for moving switch contacts can be used in the switch  10 . For example, pneumatically-operated devices, solenoid-actuated devices, spring-operated devices and other known mechanisms can be used. Moreover, these can be either manually activated or automatically activated by a control system or by a sensor associated with the switch for detecting a condition in the circuit. 
   The interior central bore  14  surrounding the fixed contact  32  and the contact pin  40  is preferably at atmospheric pressure and filled with air. Alternatively, the central bore  14  may include a controlled atmosphere therein. As used in this disclosure, the term “controlled atmosphere” means an atmosphere other than air at normal atmospheric pressure. When using a controlled atmosphere, it is preferred that the central bore  14  is maintained at a subatmospheric pressure. The composition of the controlled atmosphere may also differ from normal air. For example, arc-suppressing gases such as SF 6  may be present within the bore. 
   The switch  10  further includes a terminal end cover  42  formed from a dielectric elastomer similar to the housing  12 . The cover  42  may include a terminal end electrical stress relief element  44 , formed from a semiconducting elastomer, disposed therein. The terminal end cover  42  is positioned on the housing  12  so that an internal taper in the cover is firmly engaged with the conical seat  22  at the terminal end  18  of the housing and so that the electrical stress release element  44  surrounds the contact stub  34  extending out of the terminal end of the housing. The terminal end cover has a second cylindrical metallic current carrying element  46  mounted therein, which is electrically coupled to the contact stub  34 . This second current-carrying element  46  of the end cover  42  is generally adapted for electrical connection to the “feeder” or “source” circuit of the power distribution system, as described above. 
   In operation, the switch  10  is connected in the circuit through current-carrying elements  25  and  46 , and hence through terminals  40  and  34 . In the position illustrated in  FIG. 1 , the switch is open. To close the switch, the actuating device  38  is activated to axially translate the movable contact pin  40  in the closing direction toward the fixed contact  32  until the two are mechanically and electrically engaged. As mentioned above, this movement occurs suddenly, thereby minimizing any possibility of arcing between the contacts. 
   Referring additionally to  FIG. 2 , to visually confirm the condition of the internal contacts  32  and  40  with respect to each other (i.e., open or closed), the switch housing  12  is provided with a viewing port  48  positioned directly adjacent the engagement end  33  of the fixed contact  32 . The viewing port  48  is preferably in the form of a transparent element  50  fixed within the insulative material of the housing  12  so as to provide visual access into the interior bore  14  of the housing at a point  51  directly adjacent the engagement end  33  of the fixed contact  32 . The transparent element  50  is preferably made of a clear insulative plastic material and may be provided with a magnifying feature to enhance viewing. However, any insulating material having a sufficient level of transparency can be used for the transparent element  50 . 
   The transparent element  50  may be press-fit or bonded within a hole of the housing  12  formed during molding of the housing. In this regard, it is preferred to form the housing  12  with a protruding boss portion  52  having a hole for receiving the transparent element  50 . By providing the boss portion  52 , the depth of the hole can be increased, thereby increasing the contact surfaces between the hole and the transparent element  50  to enhance the hold therebetween. An electrical stress grading coating can also be applied between the hole surface and the transparent element  50  to ensure adequate electrical interface therebetween. 
   The conductive jacket  28  of the switch housing  12  preferably extends upwardly to cover the side walls of the boss portion  52  and defines an opening for the end face  54  of the boss portion. Thus, the transparent element  50  penetrates the insulation wall of the housing  12  while maintaining the insulative layer between the energized contacts  32  and  40  and the external grounded shield  28  of the housing. A cap (not shown) is provided to cover the viewing port to keep it free from debris. 
   Referring now additionally to  FIG. 3 , to further minimize arcing between the movable contact pin  40  and the fixed contact pin  32 , the housing  12  of the present invention further preferably includes at least one frangible insulative plate  56  fixed in the housing central bore  14  between the contacts. The plate  56  is preferably made from a high dielectric strength glass, about ⅛″ thick, which can be fixed in the central bore  14  during molding of the housing  12 . In the preferred embodiment, the housing  12  includes two glass plates  56  disposed adjacent respective contacts  32  and  40 . 
   As a result, the contacts  32  and  40  can be separated by air without the need for a large volume. Moreover, the contacts  32  and  40  can be placed closer together since the glass plates  56  serve to increase the static dielectric strength between the contacts to control the arcing during closure. The glass plates  56  provide the limited arc time needed for a successful metal to metal connection to extinguish any arc. 
   In operation, the normally open switch  10  can be installed between a faulted spur circuit and a source circuit after shutting off the voltage source. The spur circuit is grounded via the first current carrying element  25  of the switch while the source circuit is connected to the second current carrying element  46 . Grounding of the spur circuit and disconnection of the source circuit is easily confirmed by viewing the open position of the contacts  32  and  40  within the housing bore  14  through the viewing port  48 . The faulted spur circuit can now be safely repaired. 
   Once repaired, the actuating mechanism  38  can be activated to translate the movable contact pin  40  forward toward the fixed contact  32 . As the contact pin  40  travels, it breaks the nearest glass plate  56  but is still insulated by arcing by the far glass plate situated directly in front of the fixed contact  32 . Only when the second glass plate  56  is broken will an arc strike, but by this point, the pin  40  is already into engagement with the engagement end  33  of the fixed contact  32 , as shown in  FIG. 3   b . Engagement of the contacts  32  and  40  is also easily confirmed with the viewing port  48 . 
   Thus, power is restored to the spur circuit without interruption of power in the source circuit. The advantage of the switch  10  is that service is maintained to the majority of power customers on other spur circuits during the repair of the faulted spur circuit and a second interruption is prevented to restore power to the faulted spur circuit during a high load period. The switch  10  can be subsequently removed and replaced with a permanent connection during a low load planned outage. 
     FIG. 3  shows an electrical contact system that includes a movable male pin  40  and a stationary female socket contact  32 . In  FIG. 3 , the male pin  40  and the female socket  32  are in the open position and in  FIG. 3   a  the contacts  32 ,  40  are in the closed position. The pin contact  40  is segmented into sections  41  and a portion of its longitudinal axis is bored out to form a slot  43  which accepts a post  35  provided in the center of the female contact  32 . Preferably, the segmented section of the pin contact  40  is tapered and the segmented sections or fingers provide some resilient spring when engaging the female socket. The female contact  32  is cylindrically-shaped with a bottom surface  45  and an inner side wall  47  extending from the bottom surface. In addition, the internal post  35  inside the female socket  32  extends from the bottom surface  45  and is configured to be received by the axial bore or slot  43  in the male pin  40  so that the pin  40  is trapped between the inner wall  47  of the female socket and the outer wall of the internal post  35  to prevent any movement upon coupling of the pin and socket. The inside wall  47  of the socket  32  and outer surface of the post  35  preferably include a roughened surface, such as being serrated or knurled. Multiple contact surfaces and the scraping action of the serrated surfaces provide good high current transfer and prevent broken shards of glass from interfering with the connection. 
   In a preferred embodiment, the stationary female contact  32  is connected to one 600 A separable connector rod contact  46  and the movable male pin  40  is physically connected to, but electrically insulated from, the actuating mechanism  38 . The pin  40  passes through and is slidingly electrically coupled, preferably by means of a spring contact, to the donut contact  27 . As earlier described, the donut contact  27  includes a threaded bore  31  to receive the threaded end  29  of the first current carrying contact  25 . 
   In the open position of the preferred embodiment, the electrical contact system has approximately 3.5 inches separating the male pin contact  40  and the female socket contact  32 . However, in other embodiments, the separation distance can vary from about 2 to 6 inches or more. The insulation medium between the contacts is air and glass. The two ⅛-inch thick glass plates  56  provide a dual function of maintaining dielectric strength across the open contacts, and controlling the arc distance and time between the closing contacts. One ⅛-in thick glass plate  56  provides sufficient dielectric strength to prevent an arc strike until the glass plate  56  is broken by the closing pin contact  40 . Considering the contact chamber is a closed vessel, and the current can be a maximum of 40 kA symmetrical, it is critical to limit the arc energy for a successful close. Excess arc energy will cause a rapid increase of pressure and excess erosion of the contacts. This will result in a housing rupture and fault to ground. With the ⅛-in thick glass plate and a contact closing speed of 387 in/sec, the arcing time is limited to approximately 0.32 milliseconds. Fault-close tests at 40 kA have demonstrated successful closure with minimal damage to the contacts. 
   The male pin  40  is electrically isolated from the actuating mechanism  38  by a non-conductive coupling (or actuating) rod  80 , preferably made of fiberglass. The first end  82  of the rod  80  is connected to the actuating mechanism  38  and the second end  84  is connected to the pin contact  40 . When the contacts  32 ,  40  are open, the pin contact  40  side is connected to the feeder, which is grounded, and voltage withstand need not be considered. When the contacts  32 ,  40  are closed and energized, the pin contact  40  is insulated from the grounded actuating mechanism  38  by the insulated coupling rod  80 . 
   Another feature of the present invention, is an insulating seal ring  70  as shown in  FIG. 4 . Any medium or high voltage switch having an electrically grounded mechanism that is mechanically connected to and operates an energized contact must have an insulating barrier between the two to prevent flashover or creep. The insulating barrier must maintain a continuous seal when the switch is actuated without interfering with the travel of the actuating mechanism of the switch. By controlling the frictional interference level between the sealing surface of the ring and the rod, the seal can be maintained over the entire travel of the rod. This concept can be used in most types solid dielectric switches. 
     FIG. 4  shows an insulating seal ring  70  for electrically insulating the movable energized contact  40  in the housing  12  of the high current switch  10 . The insulating seal ring  70  is generally donut shaped with sealing surfaces  71 ,  73  on the respective outer and inner circumferences of its annular body. The insulating seal ring  70  has a ring-shaped core  72  that is covered with an insulating layer  74 . The core  72  is formed from material that is harder than the insulating layer  74  material so that the core  72  has a stiffening effect on the insulating layer  74 . In another embodiment, the insulating seal ring  70  is formed from two concentric rings of different materials, wherein the material that forms the outer ring is harder than the material that forms the inner ring. This allows the inner sealing surface  73  to be less stiff and have different sealing properties from the sealing surface on the outside surface. 
   The actuating rod  80  that connects the energized contact pin  40  and the actuating mechanism  38  of the switch  10  shown in  FIG. 1  preferably has an insulating barrier between the contact pin  40  and the actuating mechanism  38 , which allows about 4 inches of movement. The insulating seal ring  70  provides an electrically insulated barrier that permits the rod  80  substantially unrestricted travel over most of its length. 
   The inner diameter of the insulating seal ring  70  has an inner sealing surface  73  which is sized based on the diameter of the rod  80  that connects the pin contact  40  and the actuating mechanism  38 . The rod  80  is formed from an insulating material and has a diameter configured so that the inner sealing surface  73  does not sealably engage the rod  80  until it has substantially reached the end of its travel. The frictional interferences of the outer sealing surface  71  and the inner sealing surface  73  provide an electrically insulating seal between the switch contacts  32 ,  40  and the actuating mechanism  38 . The stiff core  72  of the insulating seal ring  70  allows the inner and outer sealing surfaces  71 ,  73  to operate independently, without a significant transfer of the forces from one surface to the other surface. Thus, tracking on the surface of the rod  80  is prevented by the inner sealing surface  73  of the insulating seal ring  70  which provides electrical insulation around the rod  80 . Similarly, the outer sealing surface  73  of the insulating seal ring  70  provides electrical insulation with the inner surface of the housing chamber  12 . The insulating seal ring  70  provides the required AC, DC and BIL withstand levels between the open contacts and between the contacts and case ground. 
   In a preferred embodiment, the insulating seal ring  70  is formed from a plastic ring-shaped core  72  that is overmolded with an insulating layer  74  of an elastomer material, preferably rubber. The outer diameter (“OD”) of the insulating seal ring  70  defines an outer sealing surface  71  that is configured to sealably contact the generally cylindrical, inside wall of the switch housing  12 . The aperture  75  in the insulating seal ring  70  has an inner diameter (“ID”) which is configured to sealably receive the rod  80  and provide electrical isolation between the actuating mechanism  38  and the high current pin  40  and socket  32  electrical contact system. 
   More specifically, the rod  80  is formed from an insulating material, such as fiberglass, a thermoplastic material or other non-conductive material with sufficient hardness to maintain structural integrity during the operation of the switch  10 . The rod  80  has a first end  82  that is connected to the electrical pin connector  40  and a second end  84  that is connected to an actuating mechanism  38 . Actuation of the switch  10  moves the rod  80  through the aperture  75  in the insulating seal ring  70 . The rod  80  is shaped so that the outside diameter of the rod  80  at the second end  84  allows it to pass through the aperture  75  in the insulating seal ring  80  without sealably contacting the inner sealing surface  73  of the ring  70  over most of its travel. At the point where the rod  80  nears the end of its travel, the diameter near the first end  82  of the rod  80  passing through the aperture  75  in the ring  70  increases so that the rod  80  sealably contacts the inner sealing surface  73  of the ring  70  that prevents arcing from one side of the ring  70  to the other. 
   Preferably, the rod  80  has at least a first diameter, which allows it to unobstructively pass through the aperture  75  in the ring  70 , and a second diameter which sealably contacts the inner sealing surface  73  of the ring  70 . However, other configurations of the rod  80  such as a tapered construction or more than two different diameters are also contemplated by the invention. 
   The outer and inner sealing surfaces  71 ,  73  of the insulating seal ring  70  provide electrically insulating seals between the ring  70  and the housing  12  and the ring  70  and the rod  80 . The insulating seal ring  70  can withstand the voltage gradient that occurs when the switch  10  closes and isolates the switch contacts  32 ,  40  inside the housing  12 . The rigid core  72  allows independent frictional interference levels at the sealing surfaces  71 ,  73  and prevents the force applied on one sealing surface from being transferred to the other sealing surface. In addition to minimizing the transfer of forces between the two sealing surfaces  71 ,  73 , the ring-shaped core  72  evenly distributes any force that is transferred. 
   The configuration and dimensions of the core  72 , as well as the thickness of the insulating layer  74  on either side of the core  72 , provides adjustable levels of friction at the sealing surfaces  71 ,  73 . The harder material of the core  72  acts as a stiffener for the insulating layer  74  on either side of the ring  70 . The closer the core  72  is to the sealing surfaces  71 ,  73 , the greater the stiffening effect on the insulating layer  74 . A thicker core  72  results a less flexible insulating layer  74  and hence more friction at the sealing surfaces  71 ,  73 . While a smaller core  72  results in an insulating layer  74  with more flexibility and movement and hence less friction on the rod  80 . 
   The insulating seal ring  70  engages the rod  80  at its inner sealing surface  73  and the switch housing  12  at its outer sealing surface  71 . The frictional interference level required to properly seal these two surfaces is different. The rigid plastic core  72  allows the stiffness of each sealing surface  71 ,  73  to be designed for the specific application and controlled independently. Preferably, the core  72  is designed to provide an insulating seal ring  70  having a higher frictional interference level with greater stiffness at the substantially stationary outer sealing surface  71  and a lower frictional interference level with less stiffness at the inner sealing surface  73 . The lower frictional interference level of the inner sealing surface  73  allows substantially unrestricted movement of the rod  80  through the aperture  75  in the insulating seal ring  70 . Without the plastic core  72 , forces on one of the sealing surfaces would be transferred to the other sealing surface. 
   Alternatively, as will be understood by those skilled in the art, the insulating seal ring  70  can be formed from an elastomer without a core, preferably a rubber, which sealably contacts the switch housing at the outer sealing surface and sealably contacts the rod at the inner sealing surface. In one embodiment, the insulating seal ring can be made from two concentric rings formed from elastomer materials having different durometers (hardness). The elastomer that forms the outer ring preferably has a higher durometer and is stiffer, while the inner ring is formed from a lower durometer elastomer which is less stiff and facilitates the travel of the rod through the insulating seal ring. The two elastomer rings are bonded together using methods well known to those skilled in the art. 
   Yet another feature of the present invention is the provision of a mechanical weak point on the spur side first current carrying contact  25  to accommodate electromagnetic forces generated upon electrical connection. As shown in  FIG. 5 , the switch  10  includes a contact pin  40  located within a central bore  14  thereof. As previously discussed, the contact pin  40  is provided to be axially movable within the bore  14  and makes electrical contact with a contact donut  27 . The contact donut  27  includes a threaded bore  31  to receive the threaded end  29  of first current carrying contact  25  to provide a current path from the first current carrying contact to the contact pin  40 . The first current carrying contact  25  extends at approximately a 90° angle with respect to the contact pin  40  and provides a current path through the switch. The first current carrying contact  25  is housed within the bushing  24  and includes a central axial bore  87  therein adapted to receive an electrical contact from a spur side separable connector (not shown). The separable connector may preferably take the form of a high voltage elbow connector such as an Elastimold® K655 LR, rated 25 kv, 600 A available from Thomas &amp; Betts Corporation, Memphis, Tenn. 
   The mechanical weak point of the present invention is provided on the first current carrying contact  25  near the threaded end  29  thereof. The mechanical weak point is preferably in the form of a recessed portion  85  of the contact  25 . The purpose of the mechanical weak point is to permit some degree of bending to accommodate electromagnetic forces from distorting and/or loosening the connection between the threaded end of the contact  25  and the donut contact  27 . More specifically, during high current flow as illustrated by arrows I 1  and I 2 , electromagnetic forces illustrated by arrows F 1  and F 2  are produced on the current carrying members. It has been found that such forces applied to an unsupported electrical contact point, such as a rigid threaded contact  25  not including the recessed portion tended to distort and/or loosen the threaded connection between the contact  25  and donut contact  27 . This distortion or loosening of the connection has been found to weaken the electrical connection and lead to possible failure of the device. 
   The electromagnetic forces generate a bending force because the current flows through the first current carrying contact into the switch device and makes a right angle turn to the contact pin  40  and socket contacts  32 . Accordingly, the electromagnetic force generated by the current flowing through the contact  25  is in a direction different from the electromagnetic forces generated by current flowing through the contact pin  40  and socket contacts  32  as shown by arrows F 1  and F 2 . These electromagnetic forces act in different directions and tend to try to straighten the current flow path creating undesirable bending forces on the electrical system assembly components and especially at the juncture between the first current carrying contact  25  and contact pin  40 . 
   The present invention provides a solution to accommodate these electromagnetic forces and maintain a good electrical connection during high current operation. The first current carrying contact is provided with a recessed portion  85  such that the bending forces are directed to the mechanical weak point of the contact relieving the stress on the threaded connection. Stated differently, the bending forces will tend to bend the contact  25  in the recessed portion thereby reducing the stress on the electrical connection point. In a preferred embodiment, the first current carrying contact may be formed of a conductive material having increased maleability so that forces generated on the post contact tend to bend the contact at the mechanical weak point or undercut, not tend to loosen or distort the electrical connection point. 
   The mechanical weak point or recessed portion of a contact to permit some bending in the region can be applied to any high current application where limiting bending forces is desirable. Such an electrical contact system is particularly useful in reducing electromagnetic bending forces to prevent damage or failure of a connection point wherein the longitudinal axes of the contacts is substantially non-parallel. The provision of the recessed portion on the contact can be used with a variety of different connections, such as threaded, welded, soldered, sliding, crimp or any other known electrical connection method to direct bending forces away from the connection point. 
   Although preferred embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various other changes and modifications may be affected herein by one skilled in the art without departing from the scope or spirit of the invention, and that it is intended to claim all such changes and modifications that fall within the scope of the invention. 
   For example, while the switch of the present invention has been primarily described herein as a medium-voltage, one-operation switch, those skilled in the art will appreciate that the switch of the present invention may also be employed in any high-current application, wherein a switching operation under load is required. Such other devices are intended to come within the scope of the invention. In particular, the switch of the present invention may be designed for multiple and/or continuous operation and may further be additionally rated for low and/or high voltages.