Abstract:
An enclosed insulator assembly has a generally cylindrical polysiloxane body, a tubular Durethan® core with upper and lower terminals, Multilam™ contacts within the terminals, an arc-quenching tube, and a manually removable handle. Entrance holes in the body accommodate the coupling of external cables to the terminals. A threaded cylinder positioned on the handle&#39;s interior accommodates one of an interchangeable cutout fuse tube assembly and disconnect tube assembly and permits the assembly to be inserted into the core as the handle is positioned on the body. Both assemblies have top and bottom fittings with O-rings that prevent weather corrosion between the fittings and terminals. A grooved load interrupter is attached to the bottom fitting of the assemblies to suppress arcing when the assembly is pulled out from the body.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an improved enclosed insulator assembly of the type that functions as an electrical cutout fuse and/or as a disconnect switch and, more particularly, to a connect/disconnect switch for overhead and underground circuits in high voltage distribution systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    Electric utility companies have primary and secondary lines for distributing electric power. Primary lines are used to deliver electric power to customer service transformers, which provide electricity to designated geographic areas. Utilities protect these primary lines from overload with circuit breakers or other types of electric current interrupters which trip “off” when a fault condition occurs. (For the sake of brevity, all types of circuit interrupters will hereinafter be referred to as “circuit breakers”; however, it will be known and understood that the use of the term “circuit breaker” includes circuit breakers, cutouts and other types of circuit interrupters.) In the “off” state, the primary line sees an “open” circuit at the location of the “off” circuit breaker, and current will not flow in the circuit. When a fault condition occurs, all customers whose source of electric power is dependent upon the primary line can be without electric service until the cause of the fault is located and repaired. In order to confine power outage to a given area, circuit breakers are connected to the primary lines at or near the customer service transformers. Fuse links installed in the circuit breakers are designed to “blow” (or “open”) when a fault occurs; i.e., when the current passing through the fuse link exceeds a predetermined value. When the fuse link opens, it isolates a faulted customer service transformer from the primary line and prevents additional power failures to other areas served by the primary line. 
         [0003]    Utilities generally prefer to use enclosed circuit breakers rather than open-type circuit breakers because the enclosed type requires less spacing for the breaker&#39;s components. In congested areas, the height of a utility pole is typically shared by power lines and cables from utilities, communications, and television companies, and increased pole height results in increased operations costs. The use of enclosed circuit breakers is therefore one method for reducing operating costs, because pole heights do not need to be increased to accommodate all the material that is installed onto the poles. 
         [0004]    One such enclosed circuit breaker in use is the Positect® enclosed insulator assembly. There are other circuit breakers of this type, as well, sold under other brands. For the sake of simplicity, the term “Positect®-type enclosed insulator assembly” will be used to denote all of these. These enclosed insulator assemblies can be used as either a utilities distribution circuit breaker or as a disconnect switch, depending on whether a cutout fuse tube assembly or disconnect tube assembly is utilized within them. 
         [0005]    A Positect®-type enclosed insulator assembly comprises a hollow, generally cylindrical porcelain insulator body having a manually removable handle formed from the same material as the body. A generally central co-axial tube formed from a plastic material such as Delrin® resin is supported within the porcelain body, and encloses a manually insertable/removable, generally tubular cutout fuse holder (for operation as a circuit breaker) or manually insertable/removable disconnect switch holder (for operation as a disconnect switch) that is attached to the handle. 
         [0006]    Electrically conductive contact surfaces on the insertable holders couple to electrically conductive tulip-type contacts associated with the generally central co-axial tube to place the holder in circuit with the power lines. 
         [0007]    As a circuit breaker, the enclosed insulator assembly is typically used for four basic distribution utility applications: 1) protecting distribution transformers (single units or banks), 2) protecting underground risers, 3) protecting capacitor banks, and 4) sectionalizing branch circuits subjected to damage by tree branches. When used to protect transformers or underground risers, a universal fuse link is typically installed within the cutout fuse holder. When used to protect capacitors, an indicating fuse link is typically installed in the cutout fuse holder. 
         [0008]    As a disconnect switch, the enclosed insulator assembly is typically used to isolate a geographic area served by the line, and is required to interrupt primary circuits while energized at the required voltage. A gloved utility lineman can isolate an area from the circuit by manually pulling out the handle with attached disconnect switch holder (all referred to as disconnect bayonet) from the body without first de-energizing the entire circuit. When field conditions change (e.g., due to tree growth on private property), an enclosed disconnect switch used for sectionalizing the branch circuit can be converted to a circuit breaker by replacing the disconnect switch holder within the central tube with a cutout fuse holder. 
         [0009]    The enclosed design of these insulator assemblies is intended to offer a high degree of safety; their high interrupting capability and their shielded electrically-live parts keep the utility lineman safe and provide animal protection as well. The design is also intended to reduce and keep exhaust blast away from the utility lineman. When connected to these enclosed circuit breakers, transformers, underground risers, and capacitor banks energized at rated voltage and current can be de-energized by a utility lineman by manually pulling out the handle with attached cutout fuse tube assembly (all referred to as cutout bayonet) from the porcelain insulator body. When connected to these enclosed disconnect switches, an energized branch circuit at rated voltage and current can be sectionalized by a utility lineman by manually pulling out the disconnect bayonet from the porcelain insulator body. 
         [0010]    The design of the enclosed insulator assembly is also directed to suppressing the electric arc that can occur as either of the bayonets is pulled out from the insulating body. As the attached cutout fuse holder (or attached disconnect switch holder) is removed from the central tube by the pulling out of the handle, an arc can be created between the holder and the central tube from which it is being withdrawn. The arc must be extinguished as quickly as possible. A “load interrupter” is accordingly coupled to the bottom the cutout fuse holder (or disconnect switch holder) to extinguish the arc as the holder is withdrawn. The load interrupter is shaped to compress the arc and the dielectric gas given off as the arc contacts the central tube. As the load interrupter is pulled through the insulator body, it compresses the arc against the central tube. 
         [0011]    The Positect™-type enclosed insulator assembly is made from porcelain. Porcelain is heavy and brittle which may cause a long installation time or breakage during transport, handling, or installation. These cause operating costs to increase for a utility. The enclosed insulator assembly also uses lead to secure a threaded cylinder into the handle. Lead is heavy and toxic. Over time, the enclosed insulator assembly is subjected to a variety of weather conditions and air contamination, causing corrosion to occur between the insulator terminals and tube fittings which make it difficult to remove the bayonet. 
       SUMMARY OF THE INVENTION 
       [0012]    The invention herein in an improved Positect™-type enclosed insulator assembly. Briefly, the enclosed insulator assembly for high-voltage distribution systems comprises: 
         [0013]    a generally tubular axially-extending outer body formed from a non-brittle, electrically insulating thermoplastic material lighter in weight than porcelain and having a manually removable handle, the outer body having at least one entranceway for permitting cables electrically coupled to the distribution system to enter into the interior of the body; 
         [0014]    a threaded cylinder mounted generally coaxially onto the inside of the handle and affixed thereto with epoxy; 
         [0015]    an axially-extending threaded tube assembly threadably and securely engaging said cylinder so as to be removable with said handle as a detachably integral component thereof, said tube assembly being selected from the group comprising cutout fuse tube assemblies and disconnect tube assemblies; 
         [0016]    an axially extending load interrupter coupled to the tube assembly axially opposite the handle, the load interrupter having a generally tube-shaped body formed from epoxy material and having a plurality of axially extending grooves on its outer periphery, and 
         [0017]    a plastic tube mounted within the outer body and disposed between first and second terminal means that are adapted to be electrically coupled to the cables, the plastic tube having an inner dimension accommodating the insertion therein of the tube assembly, the tube assembly having first and second connection means for electrically coupling to the distribution system via the first and second terminal means, the internal dimension of the plastic tube and the external dimension of the load interrupter being such that any electric arc created as the tube assembly is withdrawn from the plastic tube is compressed and suppressed therebetween; and 
         [0018]    a pair of sealing rings respectively positioned with respect to the terminal means and tube assembly to inhibit air flow in the regions where the first and second terminal means are coupled to the first and second connection means. 
         [0019]    As explained in greater detail below, the outer insulator body preferably has an inner tubular core formed from a thermoplastic having a sufficiently high mechanical strength, good electrical insulation properties, good resistance to heat and chemicals in the context of the intended application. One excellent thermoplastic is offered under the Durethan® trademark. The core is preferably provided with an exterior polysiloxane covering. The preferred removable handle also has a Durethan® core with an exterior polysiloxane covering. The plastic tube is preferably made from Delrin®. Upper and lower terminals are preferably inserted at opposite end regions of the outer insulator body. Multilam™ contacts are preferably lined on the terminals for contact with upper and lower fittings on either a cutout fuse tube assembly or disconnect tube assembly. The fittings on cutout fuse tube or disconnect tube assemblies preferably have a respective O-rings mounted on them for inhibiting the air flow in the region of the terminals. In accordance with the preferred embodiment, the bottom fittings of the cutout and disconnect tube assemblies include a load interrupter attachment. 
         [0020]    By making an enclosed insulator assembly in this fashion, a circuit breaker can be converted to a disconnect switch, and vice-versa, by using the disclosed interchangeable cutout fuse tube assembly or disconnect tube assembly, respectively. In addition, the energized parts are shielded, thus protecting personnel and animals from accidental contact. The O-rings on the fittings inhibit air flow over the contact points, thus preventing corrosion that could weld the contacts together, and impede or prevent safe pulling out of the handle and tube assembly. Furthermore, the load interrupter attached to the bottom of the cutout fuse tube or disconnect tube assemblies provides load break capability. 
         [0021]    Other objects, advantages and significant features of the invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the invention. 
         [0022]    It will be understood that orientations described in this specification, such as “up”, “down”, “top”, “side” and the like, are relative and are used for the purpose of describing the invention with respect to the drawings. Those of ordinary skill in the art will recognize that the orientation of the disclosed device can be varied in practice, and that the orientation used herein has been chosen for explanatory purposes only. Similarly, it will be recognized by those skilled in the art that the materials referred to herein, and particularly those identified by trademark, are examples of materials that meet the requirements and specifications mandated by safety concerns and by the use of the invention with electric power lines. Accordingly, other acceptable materials are within the scope of the invention whether known by generic names and/or other trademarks, or comprising other functionally equivalent material. 
     
    
     
       DESCRIPTION OF THE DRAWING 
         [0023]    In the drawing, 
           [0024]      FIG. 1  is a front elevation view of an enclosed bayonet-type silicone disconnect switch assembly, without clamp strap, with load interrupter installed onto the disconnect tube assembly according to the present invention. 
           [0025]      FIG. 2  is a front elevation view of an enclosed bayonet-type silicone cutout assembly, without clamp strap, with load interrupter installed onto the cutout fuse tube assembly according to the present invention. 
           [0026]      FIG. 3  is a rear elevation view of an enclosed bayonet-type silicone disconnect switch assembly with clamp strap. 
           [0027]      FIG. 4  is a rear elevation view of an enclosed bayonet-type silicone cutout assembly with clamp strap. 
           [0028]      FIG. 5  is an isometric view of the clamp strap, bolt and nut shown in  FIGS. 3 and 4 . 
           [0029]      FIG. 6  is a front elevation view, in explosion, of a disconnect switch bayonet assembly constructed in accordance with the invention and showing the handle, disconnect tube assembly with weather-resistant upper and lower O-rings, and grooved load interrupter attachment. 
           [0030]      FIG. 7  is a front elevation view, in explosion, of a cutout bayonet assembly constructed in accordance with the invention and showing the handle, cutout fuse tube assembly with weather-resistant upper and lower O-rings, and grooved load interrupter attachment. 
           [0031]      FIG. 8  is a bottom isometric view of a grooved load interrupter attachment shown in  FIGS. 1-4  and  6 - 7 . 
           [0032]      FIG. 9  is a side elevation view in section of the enclosed insulator assembly, without load interrupter attachment, taken along line  2 - 2  of  FIGS. 1-4 . 
           [0033]      FIG. 10  is an isometric view of the upper terminal shown in  FIG. 9 . 
           [0034]      FIG. 11  is an isometric view of the lower terminal shown in  FIG. 9 . 
           [0035]      FIG. 12  is a detailed view of the Multilam™ contacts shown in  FIG. 9 . 
           [0036]      FIG. 13  is a side cross-sectional view taken along line  3 - 3  of the upper terminal shown in  FIG. 10 . 
           [0037]      FIG. 14  is a side cross-sectional view taken along line  4 - 4  of the lower terminal shown in  FIG. 11 . 
           [0038]      FIG. 15  is a view of the Delrin® tube shown in  FIG. 9 . 
           [0039]      FIG. 16  is a side elevation in section view of the manual removable handle taken along line  2 - 2  of  FIG. 1 ,  FIG. 2 ,  FIG. 3 , and  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0040]    Referring to  FIGS. 1-4 , an enclosed insulator assembly  10  is illustrated that can be used as a circuit breaker ( FIGS. 1 and 3 ) or a disconnect switch ( FIGS. 2 and 4 ) by simply installing an interchangeable cutout fuse tube assembly  50  ( FIG. 7 ) or disconnect tube assembly  80  ( FIG. 6 ). 
         [0041]    The enclosed insulator assembly preferably comprises a removable handle  20  and tubular body  30 . As best illustrated in  FIG. 9 , the tubular body  30  and handle  20  comprise an inner tubular thermoplastic core  32  having sufficiently high mechanical strength, good electrical insulation properties and good resistance to heat and chemicals in the context of the intended application. One excellent thermoplastic is offered under the Durethan® trademark. The outer surface of the thermoplastic core of the body and handle is covered with a polymer coating  31 , preferably a silicone coating such as polysiloxane. One acceptable material is Dow Coming&#39;s SE90166UL material. The resulting body  30  and handle  20  are non-brittle, electrically insulating and lighter in weight than porcelain. Returning to  FIGS. 1 and 3 , the preferred assembly is shown to further comprise a first pair of jumper cable entries  42  near the top of the assembly, and a second pair of jumper cable entries  44  near the bottom of the assembly. 
         [0042]    As best shown in section in  FIG. 16 , the removable handle  20  of  FIGS. 1 and 2  is preferably made of a Durethan® core  24  with a polysiloxane covering  22  molded and bonded to the core. The inside center of the handle provides a rod-shaped protrusion  27  that extends downwardly into the body  30  when assembled. A threaded cylinder  26 , preferably made of brass, is fitted onto the rod-shaped protrusion  27 , and preferably affixed firmly to the handle with a suitable epoxy, glue or adhesive material (hereinafter collectively referred to as “epoxy”). One suitable epoxy is Epoxy Resin AW106MP mixed with Hardener HV953UMP, both of which are available from Golden Gate Chemical Co., Ltd. of Taipei Hsien, Taiwan. Either a cutout fuse tube assembly  50  ( FIG. 7 ) or disconnect tube assembly  80  ( FIG. 6 ) can be removably coupled to the handle  20  via the threaded cylinder  26 , enabling the enclosed insulator assembly  10  ( FIG. 1 ) to be respectively used as a circuit breaker or a disconnect switch. Accordingly, the top end of the tube assemblies  50 ,  80  are preferably threaded externally so as to mate with internal threads in the bottom portion of the threaded cylinder  26 . 
         [0043]    As further illustrated in  FIGS. 6 and 7 , the bottom end of the tube assemblies  50 ,  80  are adapted to be coupled onto a grooved load interrupter  90 , best illustrated in  FIG. 8 . The grooved load interrupter attachment  90  has a threaded brass insert  92  embedded at one end of a tube-shaped body  91  formed from epoxy material. Axially-extending grooves are formed on the exterior surface of the loaded interrupter. As shown in  FIGS. 6 and 7 , the tube assemblies  80 ,  50  are preferably coupled to the load interrupter  90  via the bottom fitting  88 ,  57  to form a “bayonet” that is inserted into body  30  to form a bayonet-type enclosed circuit breaker and a bayonet-type enclosed disconnect switch, respectively. In either case, the load interrupter  90  is thereby attached to the removable handle  20  as part of the bayonet thus formed. The load interrupter is used for arc suppression when the bayonet is withdrawn from the enclosure. 
         [0044]    As illustrated in  FIGS. 3 and 4 , a clamp strap  11  is fastened onto the mid-section of the body  30  with a bolt  12 , lock washer  13 , and nut  14 . As shown in greater detail on  FIG. 5 , the clamp strap  11  is shaped to generally encircle the body  30 , and has a pair of opposing legs which have opposing inner holes  18  and opposing outer holes  19 . The strap is fastened onto the body  30  by inserting a bolt  12  through the opposing inner holes  18 , and tightening a lock washer  13  and nut  14  onto the bolt in the conventional manner. A swivel bracket  15  is fastened between the clamp strap legs by placing its hole  16  between and in alignment with the outer holes  19  of the clamp strap  11 , passing a second bolt  12  through the three holes  19 ,  16  and tightening a second lock washer  13  and second nut  14  onto said second bolt in the conventional manner. The swivel plate can be adjustably pivoted with respect to the body  30  by loosening the second bolt, positioning the swivel bracket and body with respect to each other as desired, and tightening the second lock washer and nut onto the second bolt. 
         [0045]    Returning to  FIG. 9 , the tubular body  30  preferably has a Durethan® inner core  32  with a polysiloxane covering  31  molded and bonded to the core. Within the mid-section of the body  30  is a generally co-axial tube  37  formed from a synthetic resinous plastic material such as Delrin®, having a large flared opening at its top.  FIG. 15  is an isometric view of the tube  37 . The tube  37  is aligned with, and held in place within, the core  32  by an upper terminal  33  and a lower terminal  38 . The alignment permits a cutout fuse tube assembly  50  or disconnect tube assembly  80  to be inserted smoothly. 
         [0046]    The upper and lower terminals  33 ,  38  are best illustrated in  FIGS. 10 and 11 , respectively, and comprise respective generally tubular bodies, within which (as shown in  FIGS. 13 and 14  respectively), internally disposed bands of torsion or leaf spring contact elements  36 ,  41  are captured. Contact elements of this type are sold, for example, under the Multilam trademark, and are best shown in  FIG. 12 . The tubular opening of lower terminal  38  is preferably smaller than the tubular opening of upper terminal  33 . 
         [0047]    The contact element  36  ensures good physical and electrical contact with an electrically conductive upper fitting  81  of disconnect tube assembly  80  ( FIG. 6 ) and with an electrically conductive upper fitting  51  for cutout fuse tube assembly  50  ( FIG. 7 ), depending on which of the two assemblies are inserted into the tube  37  ( FIG. 9 ). Similarly, contact element  41  ensures good electrical contact with an electrically conductive lower fitting  88  for disconnect tube assembly  80  ( FIG. 6 ) and an electrically conductive lower fitting  57  for cutout fuse tube assembly  50  ( FIG. 7 ) depending on which of the two assemblies are inserted into the tube  37  ( FIG. 9 ). 
         [0048]    The disconnect tube assembly  80  is generally of the type available from S&amp;C Electric Company (Chicago, Ill.). Referring to  FIG. 6 , the disconnect tube assembly  80  preferably comprises of a metal tube  86  having an upper fitting  81  and a lower fitting  88 . The upper fitting  81  and lower fitting  88  of the disconnect tube assembly  80  have respective grooves formed therein to accommodate respective O-rings  55  as shown in  FIG. 6 . Suitable O-rings are made from polychloroprene rubber (CR), have a hardness of 70 (JIS A), a tensile strength of approximately 21 MPa and a low temperature resistance (TR-10) of approximately −40° C. Examples are Mitsubishi&#39;s Neoprene JBP-25 and JBP-28 O-rings. Similarly, and as illustrated in  FIG. 7 , upper fitting  51  and lower fitting  57  of the cutout fuse tube assembly  50  have respective grooves formed therein to accommodate respective O-rings  55 . The cutout fuse tube assembly  50  preferably comprises of a vulcanized fiber tube with a phenol paper wrapping. The lower fitting  57  is partially cut away at its bottom portion below the bottom O-ring  55  to permit the venting of gas when the fuse within the tube blows, and to provide a path for a fuse link to exit the tube when an indicator fuse is used. When the disconnect tube assembly  80  or cutout fuse tube assembly  50  is inserted into place within body  30 , the O-rings  55  inhibit air flow where the contacts elements  36  and  41  are in contact with upper fittings  81  and  51  and lower fittings  88  and  57 , respectively. 
         [0049]    As shown in  FIG. 8 , a grooved load interrupter attachment  90  has a threaded brass insert embedded at one end of a tube-shaped epoxy material  91 . The load interrupter  90  is attached to either lower fitting  88  or  57  and subsequently to handle  20  to form a bayonet. As the bayonet is withdrawn from the resinous plastic tube  37  as the handle is pulled out, an electric arc may be generated between the lower fitting  57 ,  88  and the interior of the resinous plastic tube  37 . The size and shape of the load interrupter causes the arc, as well as any gas generated from the interior wall of the tube by the arc, to be compressed between the load interrupter and tube wall in a manner that promptly extinguishes the arc. The diameter of the load interrupter must accordingly be of sufficient size to minimize the gap between it and with the interior wall of tube  37  ( FIG. 9 ) so as to force the arc and gas into the grooves. Moreover, it is desirable for the load interrupter to be firmly affixed to the bayonet, even if manually removable when desired, so that it&#39;s axially directed travel and rate of travel coincides with that of the bayonet&#39;s fitting. 
       Assembly and Disassembly 
       [0050]    As shown in  FIG. 10 , upper terminal  33  is machined to shape with jumper cable hole  46 . Pressure equalizer  35  is inserted into hole  46 . Bolts  34  are then screwed into the upper terminal and into the pressure equalizer. The securing bolts within the pressure equalizer are then tightened to hold the bolts and equalizer together. 
         [0051]    As shown in  FIG. 11 , lower terminal  38  is machined to shape with jumper cable hole  47 . Pressure equalizer  40  is inserted into hole  47 . Bolts  39  are then screwed into the lower terminal and into the pressure equalizer. The securing bolts within the pressure equalizer are then tightened to hold the bolts and equalizer together. 
         [0052]    As shown in  FIGS. 13 and 14 , an inner channel is machined in the upper and lower terminals so that contacts  36 ,  41  can be installed into upper terminal  33  and lower terminal  38 , respectively. 
         [0053]    The upper terminal  33 , Delrin® tube  37 , and lower terminal  38  are then aligned with a rod. During this step, the jumper cable holes in the upper and lower terminals are also aligned to one another. Once the alignments are completed, a Durethan® core  32  is molded around the tube  37  and portions of the terminals  33 ,  38 , as shown in  FIG. 9 . The ends are flared out with large open ends. Along the walls are holes  42  and  43  which are aligned with jumper cable holes  46  and  47 , respectively. 
         [0054]    Once the core molding process is complete and the alignment rod removed, a polysiloxane insulator  31  is molded and bonded on the core to form insulator body  30 . Multiple sheds  45  are made during this process. 
         [0055]    As shown in  FIGS. 3 and 4 , clamp strap  11  is fastened onto the mid-section of body  30  with bolt  12 , lock washer  13 , and nut  14  through the inner clamp strap holes  18 . A swivel bracket  15  is fastened between the clamp strap ends through the outer clamp strap holes  19  and swivel bracket hole  16  with bolt  12 , lock washer  13 , and nut  14 . Swivel bracket hole  17  receives a fastener to secure body  30  to a utility pole or other suitable support. This completes the insulator body. 
         [0056]    As shown in  FIG. 16 , handle  20  is made of a Durethan® core  24  with a polysiloxane insulator coating  22  molded and bonded onto the core. At the inside center of the handle, there is a rod-shaped protrusion extending outward. A threaded brass cylinder  26  is screwed onto the bar and glued into place. A preferred glue is an AB glue CIBA having a rated strength of greater than 50 kg. This eliminates the use of lead in securing the tube assemblies to the handle, as in prior art devices. 
         [0057]    As shown in  FIGS. 6 and 7 , the upper and lower end fittings for the disconnect tube assembly  80  and cutout fuse tube assembly  50  are machined to accommodate respective O-rings  55  onto each fitting. As shown in  FIG. 8 , a grooved load interrupter attachment  90  is made to fit onto the lower end fitting. The attachment is made of molded epoxy  91  with a threaded nut insert  92  at the top. 
         [0058]    A bayonet is formed when a disconnect tube assembly  80  with grooved load interrupter  90  is installed into threaded cylinder  26  on the handle  20 . Insertion of this bayonet into body  30  creates a bayonet-type disconnect switch. 
         [0059]    A different bayonet is formed when a cutout fuse tube assembly  50  with grooved load interrupter  90  is installed into threaded cylinder  26  on the handle  20 . Insertion of this bayonet into body  30  creates a bayonet-type cutout. 
         [0060]    Disconnect tube assembly  80  and cutout fuse tube assembly  50  are interchangeable. A disconnect switch can thereby be converted into a circuit breaker by replacing the disconnect tube assembly with a cutout fuse tube assembly. Similarly, a circuit breaker can thereby be converted into a disconnect switch by replacing the cutout fuse tube assembly with a disconnect tube assembly.