Abstract:
A double break vacuum interrupter includes a first contact system with an annular stationary contact, which is engaged by a primary moving contact with the moving contact rod extending through the primary moving contact and through the opening of the annular stationary contact. A second contact system includes a secondary moving contact disposed on an end of the moving contact rod, which engages and operates a floating contact on the same axis. Both contact systems are enclosed in a sealed envelope. A mechanical adjustment system is provided for the floating contact, which controls its range of motion. The mechanical adjustment system allows the first and second contact systems to engage at approximately the same time. A system of capacitors and resistors is provided to balance the voltage between the first and second contact systems to provide more efficient interruption of the electric current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of high voltage vacuum switches and circuit interrupting devices and more particularly to a double break vacuum switch or vacuum interrupter with two contact breaks connected in series and driven by a single contact rod. 
     2. Discussion of the Prior Art 
     A number of vacuum prior art arrangements are directed to provide a vacuum interrupter with two or more contacts within the same envelope as illustrated in U.S. Pat. Nos. 3,250,880; 3,405,245; 4,107,496; 4,246,458 and 6,476,338 B2. The first three cited patents present devices in which two moving contact structures must be moved in opposite directs to achieve two series contact breaks, necessitating a complex and costly operating mechanism. The latter two patents represent devices that rely on the transfer of the electric arc to one or more sets of auxiliary contacts as the moving contact is drawn past them. This can result in longer arcing times as the moving contact is drawn through its stroke to establish the multiple breaks as well as severe erosion along the edges of the contacts at the arc transfer points. 
     A more common practice for creating switchgear with two or more contact breaks in series is to simply mount the required number of single break vacuum interrupters in series as shown by U.S. Pat. Nos. 2,859,309; 3,792,213; 3,813,506; 3,839,612; 4,027,123; 4,972,055; 6,242,708; 6,498,315 B1; 7,239,492 B2. This practice requires the use of complex and costly interconnecting mechanisms for the series interrupter modules and results in a bulky switchgear unit. 
     Other prior art vacuum interrupters utilize multiple contact systems in an axial configuration as illustrated in U.S. Pat. Nos. 6,255,615 B1, 6,720,515 B2 and patent application US 2008/0245772 A1. These vacuum interrupters engage one set of contacts by having the contact operating means move in one direction and engage a second set of contacts when the contact operating means moves in the opposite direction. This configuration is suitable for providing a means to ground the electric circuit in which the vacuum switch or interrupter is employed, but because the contact means is not capable of engaging both sets of contacts by moving in one direction, the device does not provide a double break mechanism. 
     Another prior art interrupter utilizes multiple contact systems where one set of contacts drives another as illustrated in U.S. Pat. No. 2,863,026. In this case, the operating spring for the driven contact is mounted inside the interrupter and is subject to annealing during the brazing together of the interrupter. While work hardening will result in the return of some of the spring force characteristics, its final force characteristics will be uncontrolled. Additionally, no means is provided to precisely position the driven contact, adjust out the tolerance accumulation between the multiple parts or to balance the voltage between the two contact gaps. 
     U.S. Pat. No. 3,283,101 and Patent application publication no. US 2007/0262054 A1 disclose a double break vacuum interrupter, which is operated by a single moving contact rod. The first cited patent shows an extremely complex method of assuring that the contacts make and break at the same time. However, U.S. Pat. No. 3,283,101 does not indicate the use of capacitance to balance the voltages between the two contact gaps. With the cited patent application, there is no indication of how tolerance accumulation of the components and contact wear are accounted for to assure that both contact breaks can continue to make over the life of the device. In addition, the fact that the contact structures are mounted in a parallel configuration results in a bulky vacuum module. 
     Patent application publication no.: US 2010/0108643 A1 also discloses a double break vacuum interrupter, which is operated by a single contact rod. This device contains an internally mounted bellows like spring, which would become annealed during the interrupter brazing cycle which would greatly affect its force characteristics. The spring would regain some of its spring force with work hardening; however its final force characteristics would be uncontrolled. When the contacts close, the contact rod drives one moving contact into the second moving contact and then the second contact into the stationary contact which provides for making on only one set of contacts instead of two, which can result in a longer pre-strike and possible welding of the contacts. In addition there is a further possibility of contact welding as the contact rod only drives one contact open, with the internal spring returning the other contact to the open position. 
     While the aforementioned prior art arrangements may be suitable for their intended use in accordance with their respective defined applications, as discussed hereinbefore, it would be desirable to provide an efficient and compact double break contact arrangement contained within a vacuum switch or interrupter module. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is the principal object of the present invention to provide a single vacuum interrupter module with two contact breaks connected in series and driven by a single contact rod. 
     In the practice of the invention, the primary contact system has an annular stationary contact, which is engaged by a disc shaped moving contact. Both contacts are preferably fabricated of copper-tungsten material, if the interrupter is designed for switching duty or chromium-copper, if the interrupter is designed to interrupt fault currents. The base of the stationary contact is supported between two tubular insulators, which are preferably made of ceramic and form the main portion of the interrupter housing. One of these insulators contains the first contact system. An end of the insulator is closed off by an end-cup preferably fabricated from stainless steel, which has an opening for the contact drive rod. The contact rod is preferably made of copper with a stainless steel reinforcing rod to prevent a reduction in length due to repeated impact. A flexible bellows preferably fabricated from stainless steel is used to allow motion of the drive rod and allow for sealing of the end-cup. The drive rod for the moving contact disc extends through the disc and annular stationary contact into the region of the second insulator. A second moving contact disc is mounted on the end of the drive rod and is engaged by a floating contact disc mounted on an independent contact rod. These contacts are also preferably fabricated from copper-tungsten or chromium-copper material and the independent contact rod is also preferably fabricated from copper with a stainless steel reinforcing rod. This contact rod is mounted on the other end of the second insulator using a bellows and end-cup arrangement to allow sealing and free motion of the floating contact. The floating contact is driven by the motion of the second moving contact, which is directly coupled to the first contact system. 
     A mechanism is mounted on the end-cup that supports the floating contact and allows the tolerance accumulation of the components to be adjusted out and the floating contact positioned so that the second moving contact and floating contact can be closed just before the primary contacts. The mechanism also has the capability of controlling the range of motion of the floating contact so that it may be contacted by the second moving contact at approximately the same time the primary contacts close. 
     The mechanism includes an annular housing with two long slots along the main axis placed 180 degrees apart. The length of these slots is the sum of the diameter of the holes in the adjuster described below plus the full range of tolerance accumulation of all parts that determine the spacing between the primary and secondary contacts. This allows the mechanism to have the capability of adjusting out the tolerance build-up in the system. The housing also has an internal thread to allow the insertion of the adjuster. The moving contact rod for the floating contact has a cross-hole placed in a position to allow the adjuster movement through its required range within the housing. A fixture pin placed in a through hole in the contact rod of the floating contact passes through both slots formed through the housing. In this manner, when the interrupter is processed through a brazing cycle, the relationship between the contact rod and housing is established and the housing can also be used as a bellows anti-twist device. After the interrupter is brazed, the fixture pin is removed and an annular adjuster with external thread is screwed into the housing. The adjuster has six holes spaced 60 degrees apart, perpendicular to the main axis and of a diameter that is calculated to provide a small amount of over travel (approximately 1/32 inch) to accommodate any erosion or compression of the primary contacts due to interruption duty and repeated impact upon closing. The adjuster also has a counter-bore into which a compression spring or series of Bellville washers may be inserted. With the primary contacts held together, the adjuster is rotated so that the lower edge of the holes are below the cross-hole in the moving contact rod by a planned contact wear allowance. The multiple holes in the adjuster allow for a finer adjustment in determining this setting. Once the adjustment is complete, a pin is inserted so that it passes through the housing, contact rod and adjuster and is secured with washers and retaining rings at both ends. A compression spring or a series of Bellville washers of appropriate design provide the required contact pressure for the secondary contacts and return force for the floating contact. The spring is placed in the counter-bore of the adjuster and is secured in place with a threaded cap. This forces the pin through the contact rod to the lower portion of the adjuster cross-holes and establishes the setting so the secondary contacts engage at approximately the same time as the moving contacts. 
     A portion of the moving contact rod extends through the cap that captures the compression springs to which a flexible lead or other current exchange method, such as garter springs or multi-lam current transfer devices may be attached. As the primary contact rod moves to the closed position, it can be seen that the secondary contacts will engage just before the primary moving contact engages the stationary contact. No current exchange is needed for the main contact rod as the electric current flows from the stationary contact of the primary contact set to the moving contact, up the contact drive rod and through the secondary contacts and out the top terminal of the interrupter. A system of capacitors and resistors connected to ground is provided in the insulated portion of the external contact drive rod to balance the voltage between the two contact systems to provide more efficient interruption of the electric current. The contacts may be of the butt style, transverse magnetic field or axial magnetic field designs as used in prior art. The invention described above is suitable for use in oil or SF6 switchgear. 
     A ramification of the invention provides for the coaxial alignment of the primary and secondary contact systems. In this case the primary moving contact is cup shaped and the moving contact rod extends through the primary moving contact just far enough so the face of the primary and secondary moving contacts lie in the same plane. The moving contact rod for the floating contact is extended far enough so it passes through the primary stationary contact to the point that the floating contact and primary stationary contact lie in approximately the same plane. The adjustment mechanism described above would be utilized so that the floating and secondary moving contacts engage approximately 1/32 of an inch before the primary contacts. As stated above, this allows for any wear of the primary contacts due to interruption duty or yielding of the contact rod due to repeated impact upon closing. The contacts may be of the butt style, transverse magnetic field or axial magnetic field designs as used in prior art. Axial magnetic field contacts employed in this invention will actually produce a coaxial magnetic field and yield a more effective interruption due to the cancellation of magnetic fields outside the contact structure. The stationary contact may also be made cup shaped to stabilize the arc at the outside contact ring and eliminate the expulsion of plasma from the interruption into the contact shield. 
     A further ramification of the invention allows the double break vacuum interrupter to be encapsulated. This is facilitated by the addition of an added housing which prevents the encapsulation material from contacting the moving components of the adjuster mechanism. The housing includes a metallic cylinder with a top made from insulating material. The portions of the housing are preferably retained in place by screws, which engage insulators, which are secured to studs that are brazed to the end-cup of the interrupter. A flexible lead transfers current from the floating contact rod to a terminal, which extends out of the top of the housing. A terminal rod is extended out from the stationary contact. This configuration may be encapsulated using the various techniques known in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a double break vacuum switch including a vacuum envelope in accordance with the present invention. 
         FIG. 1   a  is an enlarged cross-sectional side view of a bellows anti-twist housing of a double break vacuum switch in accordance with the present invention. 
         FIG. 2  is a cross-sectional view of a double break vacuum switch prepared for encapsulation in accordance with the present invention. 
         FIG. 3  is a cross-sectional view of an operating rod of a double break vacuum switch in accordance with the present invention. 
         FIG. 4  is a cross-sectional view of a method of encapsulating a double break vacuum switch in accordance with the present invention. 
         FIG. 5  is a cross-sectional view of a first alternative embodiment of a double break vacuum switch in accordance with the present invention. 
         FIG. 6  is a cross-sectional view of a second alternative embodiment of a double break vacuum switch in accordance with the present invention. 
         FIG. 7  is a cross-sectional view of a third alternative embodiment of a double break vacuum switch in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  discloses a double break vacuum switch (vacuum switch)  1 . The vacuum switch  1  includes a vacuum envelope  2 . The major part of the vacuum envelope  2  includes a pair of insulating cylinders  4 A and  4 B preferably fabricated from alumina ceramic joined end-to-end by way of two stainless steel or monel triple point shields  6 A and  6 B and a stationary contact support ring  8 , preferably fabricated from copper. A threaded hole in the stationary contact support ring  8  allows the attachment of a terminal rod  10  to facilitate electrical connection to a source line. Opposite ends of the ceramic cylinders are enclosed by two end cups  12 A and  12 B, preferably fabricated from stainless steel or monel. A second set of triple point shields  14 A and  14 B preferably fabricated from stainless steel or monel are attached to the end cups. A generally tubular internal shield  16 A and  16 B is provided within each insulating cylinder  4 A and  4 B spaced from the interior wall and overlapping the triple point shields to prevent any vaporized material from contacting the interior wall. 
     The primary contact system  11  includes an annular stationary contact support  18  preferably fabricated from copper and is attached to the aforementioned stationary contact support ring  8 . An annular stationary contact  20  preferably made of copper tungsten is attached to a lower end of the stationary contact support  18 . The stationary contact  20  is engaged by an annular moving contact  22  also preferably fabricated from copper tungsten. The annular moving contact  22  is attached to a disc shaped moving contact support  24  preferably fabricated from copper. The moving contact support  24  is reinforced by a moving contact reinforcement cone  26  preferably fabricated from stainless steel. Both the moving contact support  24  and moving contact reinforcement cone  26  are retained on a moving contact rod  28 , preferably fabricated from copper. The moving contact rod  28  is reinforced by a reinforcing rod  30 , preferably fabricated from stainless steel and is sealingly passed through the end cup  12 A and the triple point shield  12 B by a bellows  32 . The bellows  32  is preferably fabricated from stainless steel. The end of the reinforcing rod  30  is preferably threaded and extends beyond the lower end of the moving contact rod  28  to facilitate the attachment of a drive rod from an external drive mechanism. The bellows  32  is preferably protected from damage by vaporized material by a bellows shield  34 . The bellows shield is preferably fabricated from stainless steel. A bellows anti-twist housing  36  is attached to the opposite side of end cup  12 A and is centered by the circular depression formed in the end cup. The bellows anti-twist housing  36  is preferably fabricated from stainless steel. With reference to  FIG. 1   a , the bellows anti-twist housing  36  is indexed to the moving contact rod  28  by a hardened pin  38 , preferably fabricated from steel and nickel plated. The hardened pin  38  passes through a cross-hole  40  in the moving contact rod  28  and slides in a slot  42  in the bellows anti-twist housing  36 . Two threaded holes  39  are formed in the bellows anti-twist housing  36  to facilitate attachment of a current exchange housing  126 . 
     A second contact system  13  includes the extension of the moving contact rod  28 , which passes through the moving contact support  18 . A moving contact support  44  preferably fabricated from copper is attached to an end of the moving contact rod. A moving contact disc  46  preferably fabricated from copper tungsten is attached to the moving contact support  44 . The second contact system  13  further includes a floating contact  48  preferably fabricated from copper tungsten, which is attached to an end of a disc-shaped floating contact support  50 , preferably fabricated from copper. The floating contact support  50  is attached to a floating contact rod  52  preferably fabricated from copper, which is reinforced by a reinforcing rod  54  preferably fabricated from stainless steel and sealingly passed through end cup  12 B and triple point shield  14 B by a bellows  56 . The bellows  56  is protected from damage by vaporized material by a bellows shield  58 . The bellows  56  and the bellows shield  58  are preferably fabricated from stainless steel. A mechanism housing  60  preferably fabricated from stainless steel is attached to the opposite side of end cup  12 B and is centered by the circular depression formed in the end cup. The mechanism housing  60  is indexed to the floating contact rod  52  by a hardened pin  62  preferably fabricated from a nickel plated steel, which passes through a cross-hole  64  in the floating contact rod  52  and slides in a slot  66  in the mechanism housing  60 . During the brazing cycle for the vacuum switch  1 , a pin  62  is replaced by a fixture pin to assure the alignment of these parts. 
     An operating mechanism for a floating contact  15  includes an adjuster  68  preferably fabricated from brass, which is threaded into the mechanism housing  60 . The mechanism housing  60  includes two slots  66  located at opposite sides around its circumference. The adjuster  68  has six holes  70  equally spaced around its perimeter, so that the pin  62  can be inserted into any opposite facing pair of holes  70  during the adjustment process. When threading the adjuster  68  into the mechanism housing  60 , the pin  62  is withdrawn from the mechanism housing  60 . The adjuster  68  is positioned so that the center of one pair of holes  70  line up with the center of the cross hole  64  in the floating contact rod  52  and the top of hole  70  is preferably 0.031 inch above cross-hole  64 , but other dimensions may also be used. During this adjustment, both the first and second set of contacts must be closed. The pin  62  is then inserted back through the mechanism housing  60 , adjuster  68  and the floating contact rod  52 . Pin  62  is held in place by a pair of retaining rings  61 A and  61 B and a pair of washers  63 A and  63 B. The pair of retaining rings  61 A and  61 B and the pair of washers  63 A and  63 B are both preferably fabricated from steel. A compression spring  72  preferably made of music wire is inserted into the counter-bore in the adjuster  68  and a threaded spring retainer  74  is tightened. The threaded spring retainer  74  is preferably fabricated from a nickel plated steel. The compression spring  72  forces the pin  62  to the bottom of the hole  70 . The diameter of the holes  70  in the adjuster  68  are preferably 0.062 larger than the diameter of the cross hole in the floating contact rod  52  to provide for an allowance for contact wear. The slots  66  in the mechanism housing  60  have a minimum length equal to the tolerance build-up between the location of the cross hole  64  in the floating contact rod  52  and the end of the second moving contact  46  plus the diameter of the holes  70  in the adjuster  68 . This allows the adjuster  68  to be able to be adjusted through the full range of possible locations of the cross hole  64 . 
     In order to facilitate encapsulation of the double break vacuum switch  1 ; a housing  101  is placed over the mechanism as shown in  FIG. 2 . The housing includes a cover housing  102  preferably fabricated from aluminum and a cover plate  104  preferably fabricated from an insulating material such as GP01 or GP03 fiberglass or G10 epoxy glass. A pair of studs  106 A and  106 B preferably fabricated from stainless steel are attached to an outside surface of the end cup  12 B. An insulating stringer  108 A and  108 B preferably fabricated from filament wound epoxy glass is threaded onto each stud  106 A and  106 B. A screw  110 A and  110 B preferably fabricated from stainless steel is threaded into the opposite end of each stringer  108 A and  108 B to retain the cover plate  104  and the cover housing  102 . A split-clamp connector  112  preferably fabricated from copper and is tightened onto an end of floating contact rod  52  using a bolt  114  and nut  116 . A pair of highly flexible multi-stranded conductors  118 A and  118 B preferably fabricated from copper are conductively secured to the split clamp connector  112  on one end and to a terminal connector  120  preferably fabricated from copper on the other end thereof. The terminal connector is preferably threaded onto a lower portion of a source terminal  122  and secured with a jam nut  124 ; creating a current exchange between the floating contact rod  52  and the source terminal  122 . The opposite end of the vacuum switch  1  is prepared for encapsulation by installation of the current exchange housing  126  over the bellows anti-twist housing  36  and securing it with a pair of bolts  128 A and  128 B preferably fabricated from stainless steel. The current exchange housing  126  is preferably fabricated from a thermoset plastic. 
     The double break vacuum switch  1  requires a capacitor-resistor voltage divider to distribute the voltage equally between the two contact gaps during interruption. As shown in  FIG. 3 , this is provided by an operating rod  202 . The operating rod  202  includes an insulating tube  204  preferably made from a filament wound epoxy glass and of a sufficient diameter to allow the insertion of a capacitor-resistor network  205 , which includes a plurality of capacitors  206  and a plurality of resistors  208 . The capacitors  206  are preferably 500 pf 30 kV disc capacitors and the resistors are preferably 20 Meg-ohm 2 watt resistors. Each capacitor  206  is connected in parallel with a single resistor  208 . Fifteen of these capacitor-resistor units are connected in series on the inside of the insulating tube  204  and the insulating tube  204  is filled with an epoxy  210  or the like to improve dielectric characteristics. The operating rod  202  also requires a minimum length of 29 inches between live parts to allow operation at line voltages up to 72 kV. The end of the contact rod  28  connected to the double break vacuum switch  1  includes a contact pressure device. The contact pressure device includes an adapter  212  preferably fabricated from steel, a pin  214  preferably fabricated from steel, a spring  216  preferably fabricated from music wire and an outer shell  218  preferably fabricated from brass. The pin  214  allows the adapter  212  to ride up and down the slot  220  in outer shell  218  with the force of the spring  216  biasing the adapter  212  toward the upper end of the slot. The outer shell  218  is pinned to the insulating tube  204  with roll pins or groove pins  222  both preferably fabricated from steel and having a terminal  224 A preferably fabricated from a tin plated brass and attached with a screw  225 A preferably fabricated from a tin plated steel to allow connection of one end of the capacitor-resistor network. The other end of the insulating tube  204  includes an adapter  226  preferably fabricated from steel to allow the operating rod  202  to be connected to an operating mechanism. The adapter  226  is pinned to the insulating tube  204  with roll pins or groove pins  222 B both preferably fabricated from steel and a terminal  224 B preferably fabricated from a tin plated brass and attached with a screw  225 B preferably fabricated from a tin plated steel to allow connection of the other end of the capacitor-resistor network  205 . 
     There are several examples of prior art, which show the encapsulation of vacuum modules.  FIG. 4  indicates one possible way of encapsulating the aforementioned vacuum switch as demonstrated by U.S. Pat. No. 5,917,167. A module  302  includes the vacuum envelope  2  and the vacuum housing  101 . The module  302  is encased in a silicone rubber tube  304  and cast in an encapsulation  306  preferably of epoxy. The result is a two terminal encapsulation with a source terminal  308  and a load terminal  310 . Within the vacuum interrupter module  302  both the primary and second sets of contacts are electrically connected in series via the extended portion of contact rod  28  and no current is conducted through the lower portion of the moving contact rod  28 , which eliminates the need for a current exchange system at that point. 
     In operation, the aforementioned encapsulated double break vacuum switch  1  would be coupled by the operating rod  202  to an operating mechanism  228 . A closing stroke of the operating mechanism  228  and the operating rod  202  would drive the moving contact rod  28  upward. Because of the aforementioned adjustment of the mechanism adjuster  68 , when the spring  72  is installed, the pin  62  is forced to the bottom of the hole  70 , which causes the floating contact rod  52  to be pushed downward 0.031 inch. This causes the second set of contacts  46  and  48  to engage slightly in advance of the first set of contacts  20 ,  22 . As the moving contact rod  28  continues its closing stroke, the floating contact rod  52  is driven upward resulting in the pin  62  moving upward in the hole  70  and compressing spring  72 . The closing stroke is completed when moving contact rod  28  is driven to a point that the first set of contacts  20 ,  22  make contact, which results in the pin  62  being centered in the hole  70 . At this point, electric current flows from the source terminal  308  through the first set and second of contacts and directly out the load terminal  310 . 
     Upon initiation of the opening stroke, the moving contact rod  28  moves downward causing the first set of contacts  20 ,  22  to immediately part and initiate an arc. The energy stored in the spring  72  forces the floating contact rod  52  downward maintaining contact through the second set of contacts  46 ,  48  for the first 0.031 inch of contact travel until the pin  62  is driven to the bottom of hole  70 . At this point, floating contact rod  52  is no longer able to follow moving contact rod  28  downward and the second set of contacts  46 ,  48  begin to part initiating a second arc. The capacitor-resistor network  205  contained in the operating rod  202  acts to distribute the voltage evenly across the two contact gaps resulting in an efficient interruption of the arc as the moving contact rod  28  completes its opening stroke and provides the full open gap for both set of contacts. Because both sets of contacts are electrically connected in series, this results in a double break of the arc when the contacts open allowing the vacuum interrupter to be utilized at elevated voltages. The fact that the hole  70  is preferably 0.062 larger than the pin  62 , allows +/−0.031 for wear of the contacts, which may be unequally distributed between either set of contacts. 
     A first alternative embodiment of the double break vacuum switch  1 ′ is shown in  FIG. 5 . In this case, the length of the moving contact rod  28 ′ is reduced and the length of floating contact rod  52 ′ is increased so both the first set and second set of contacts part in the same plane. This embodiment eliminates the passage of the moving contact rod  28 ′ through the arc zone of the first set of contacts. 
       FIG. 6  shows a second alternative embodiment of the double break vacuum switch  1 ″. The annular stationary contact  20 ″, the annular moving contact  22 ″, the moving contact disc  46 ″ and the floating contact  48 ″ are preferably fabricated from copper chromium instead of copper tungsten utilizing any of the transverse or axial magnetic field contact structures shown in prior art.  FIG. 6  shows one possible axial magnetic field contact structure as demonstrated by U.S. Pat. Nos. 4,871,888 and 6,867,385, and US Pat App No. 2006/0016787, which are hereby incorporated into this application by reference in their entirety. The double break vacuum switch  1 ″ includes contact rods  28 ″,  52 ″. The revised contact structures convert the contacts  20 ″,  22 ″,  46 ″ and  48 ″ from switching duty to fault interrupting duty and results in a double break vacuum interrupter. 
       FIG. 7  illustrates a third alternative embodiment of the double break vacuum switch  1 ′ with coplanar axial magnetic field contacts. In this case, the length of the moving contact rod  28 ″′ is reduced and the length of the floating contact rod  52 ″′ is increased, so both sets of axial magnetic field contacts  20 ″′,  22 ″′,  46 ″′ and  48 ″′ are in the same plane. In this embodiment the fields are coaxial and the interruption would benefit from the fact that in a coaxial electrical system, the fields of the two conductors cancel outside the enclosing conductor so that the effect outside magnetic fields is shielded from the central conductor. 
     While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.