Electrical Bushing Assembly

Abstract

A load break bushing assembly for receiving a plug-in cable connector or termination. The bushing assembly includes an insulating body member, a conductive sleeve member, a replaceable contact disposed within the conductive sleeve member, and an arc confining and extinguishing member. In one embodiment of the invention a protective tube is disposed to span the contact and arc confining and extinguishing member, to protect the conductive sleeve member from arcing by-products and maintain the replaceable characteristic of the contact. The conductive sleeve member has a chamber sealed at one end with a terminal stud, with this chamber functioning as a gas surge expansion chamber when the plug-in cable connector is operated to make or break load current, making it unnecessary to vent the load break bushing into the enclosure of its associated apparatus.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No. 771,707, filed Oct. 30, 1968, now abandoned which is assigned to the same assignee as the present application.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to electrical bushing assemblies, and more specifically to bushing assemblies having load bread, load make, and fault close-in capabilities, of the type adapted for coupling with a plug-in cable termination.

2. Description of the Prior Art

The large increase in underground distribution of electrical power for residential usage has resulted in the development of dead front, plug-in type cable connectors, including shielded cable terminations and bushing assemblies. These plug-in cable terminations and bushing assemblies enable the high voltage shielded cables of the electrical distribution system to be quickly connected to, or disconnected from, electrical apparatus such as distribution transformers and circuit interrupters or switches. The early plug-in cable connectors were non-load break devices, with load break capability being supplied by auxiliary load break switches. While load break switches are acceptable functionally, they add to the cost of the underground distribution system, and thus it would be desirable to provide a plug-in type bushing assembly which has load break capability. The term "load break capability", as used in this specification, also signifies load make and fault close-in capabilities.

To provide a plug-in type bushing assembly with load break capability, and also fault close-in capability, the arc drawn upon breaking the load must be quickly and effectively extinguished, and the arc and the gas pressures created when coupling the connector portions while a fault exists must be contained without catastrophic damage to the apparatus and without hazard to operating personnel. Since plug-in load break connectors of this type are used on systems with current limiting protection, such as fuses or breakers, between the cable termination and the high voltage supply feeder, the fault close-in requirements of the bushing assembly may be predetermined.

Since disconnecting the cable termination from the load break bushing assembly when a load is connected to the bushing assembly will draw an arc, the bushing contact should be easily replaceable, as repeated operations to make and break load current will wear the contacts and eventually destroy their usefulness. It is not sufficient, however, to construct the plug-in bushing assembly such that a new contact may be inserted and a new contact removed. It is critical that the contact be removable after pitting and eroding due to load interruption, or due to a fault close-in, and it must be removable in either event without impairing the bushin's ability to accept a new contact.

Plugging a cable termination into a load break bushing assembly which has a fault in the connected load creates arcing and hot ionized gases. Thus, it is also critical that the bushing be constructed of a strong, tough insulating material which will not crack or fly apart due to the expansion of these gases. Further, it is important that the hot ionized gases be retained by the bushing assembly, at least until they are cooled and partially condensed. If the ionized gases are allowed to escape from the bushing assembly, they may envelope a live part and ground, allowing a flashover to occur. Thus, the gases should not be allowed to escape from the bushing assembly into the associated electrical apparatus, as a flashover may occur, and the condensation products, including metallic vapors from the contacts, may reduce the insulating value of the fluid dielectric disposed in the casing or enclosure of the apparatus. Further, the gases should not escape to the atmosphere until they have been sufficiently cooled and deionized.

SUMMARY OF THE INVENTION

Briefly, the invention is a new and improved plug-in type bushing assembly which has load make and break and fault close-in capabilities. The bushing assembly includes an insulating body member formed of a rigid, solid resinous insulation system, which is strong, tough, and crack resistant. An electrically conductive sleeve member, having first and second ends and an aperture which extends between its ends, is disposed in the insulating body member. The conductive sleeve member is sealed at its second end by a terminal stud, and is internally threaded adjacent its first end. A replaceable contact member having a pressure terminal disposed at one end thereof is threadably inserted into the first end of the conductive sleeve member, and an arc confining and extinguishing member is also threadably secured to the first end of the conductive sleeve member, longitudinally spaced a predetermined dimension from the pressure terminal of the contact member. In one embodiment of the invention, a protective tube is disposed to span and encircle both the pressure terminal of the replaceable contact member, and a portion of the arc extinguishing member, to prevent contact splatter, due to arcing, from coming into contact with and fouling the threads, to thus maintain the replaceable characteristic of the contact member. In another embodiment, the protective tube is eliminated by extending the arc confining and extinguishing member until it is in contact with, or substantially in contact with, the pressure terminal. The aperture of the conductive sleeve member forms a surge expansion chamber for containing ionized gases generated during a fault close-in of a plug-in cable termination with the load break bushing assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description or exemplary embodiments thereof, taken with the accompanying drawings, in which:

FIG. 1 is an elevational view, partially in section of a plug-in type bushing assembly having a load bread and fault close-in capabilities, constructed according to the teachings of the invention;

FIG. 1A is a fragmentary view of the bushing assembly shown in FIG. 1, modified according to another embodiment of the invention; and

FIG. 2 is a plan view of the bushing assembly shown in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, FIGS. 1 and 2 illustrate elevational and plan views, respectively, of an electrical bushing assembly 10 constructed according to the teachings of the invention. FIG. 1 illustrates bushing assembly 10 partially in section, in order to more clearly illustrate its component parts and their cooperative assembly.

More specifically, bushing assembly 10 is of the plug-in type, having a projection 12 adapted to receive a cable termination. Since suitable plug-in cable terminations are well known in the art, the cable termination for connecting a high voltage circuit to bushing assembly 10 is not shown.

Bushing assembly 10 includes a substantially cylindrical, elongated body member 14 formed of a cast, rigid solid insulation system, a conductive sleeve or tubular member 16, a terminal stud 18, a replaceable contact member 20, and arc confining and extinguishing member 22, a tubular protective member 24, and a metallic mounting ring 26.

The body member 14 has first and second ends 28 and 30, respectively, with an aperture 32 extending between its ends. Body member 14 may be cast of any suitable resinous insulation system which possesses the following characteristics. It must be a good electrical insulator, it must be weather resistant, crack resistant, rigid but not brittle, it must possess a high physical strength at ambient and elevated temperatures, and it must have a coefficient of thermal expansion which closely matches the coefficient of the thermal expansion of the tubular conductive member 16. Body member 14 is preferably cast, instead of molded, because of the superior strength of cast resinous insulation systems over molded systems. In general, the filled epoxy cast resin systems will provide the desired characteristics, with the filler being selected to match the coefficient of thermal expansion of the filled resin system to that of the metallic conductor or insert. A finely divided filler formed of beryllium aluminum silicate has been found to be excellent in matching the coefficient of thermal expansion of the filled epoxy resin system to copper, but other fillers may be used, such as quartz or silica. For 7,200 volt applications where the encased end of a bushing assembly is disposed in oil, or other insulating dielectric fluid, fillers for providing arc and track resistance are not required. If the encased end is to be operated in air, finely divided alumina trihydrate (Al.sub.2 O.sub.3.sup.. 3H.sub.2 O) may be added to obtain the desired arc and track resistance.

Conductive sleeve member 16 is preferably formed of a thin wall tube, constructed of a good electrical conductor, such as copper, with the tube having first and second ends 34 and 36, respectively, and an aperture 38. The aperture 38 has a uniform diameter except for a shoulder or step 40 which reduces the diameter of the aperture for a short longitudinal dimension, at a predetermined location intermediate the first and second ends 34 and 36, respectively, of the conductive sleeve member 16.

The wall of the aperture 38 is threaded, starting at the first end 34 of the conductive sleeve member 16 and extending to the location of shoulder 40. As illustrated in FIG. 1, when using a thin wall tube, the threads 42 in the inner diameter of the conductive sleeve member 16 may be obtained by rolling threads 44 on the outside surface of the tube 16.

The second end 36 of conductive sleeve or tube member 16 is hermetically sealed with a terminal stud 18, which is also formed of copper, or other good electrical conductor, and is adapted for connection to encase electrical apparatus, such as the high voltage winding of a distribution transformer. Terminal stud 18 includes a portion 46 having a diameter selected to snugly fit the diameter of aperture 38 of the conductive sleeve member 16, with portion 46 being secured within the aperture 38 such as by silver solder bead 48, which electrically connects terminal stud 18 to conductive sleeve member 16, and also hermetically seals end 36 of conductive sleeve member 16. Portion 46 of terminal stud member 18 also includes an outwardly extending portion 50 which is adapted to receive an electrical lead and fastening means, such as a nut.

As illustrated in FIG. 1, conductive sleeve member 16 is sealingly disposed in the aperture 32 of body member 12, with its first end 34 starting within the aperture 32 a predetermined dimension from the first end 28 of body member 14, and with its second end 36 being substantially aligned with the second end 30 of body member 14.

As will be hereinafter explained, the conductive sleeve member 16 is disposed within the casting mold and then the body member 14 is cast, to provide a seal between aperture 32 of body member 12 and the outer surface of conductive sleeve member 16.

The metallic mounting ring member 26 includes a flange portion 52 which is embedded within the cast body portion 14, an outwardly extending disc or ring portion 54, and a plurality of spaced extensions 56, 58, 60 and 62, which, along with the flange portion 52, extend toward the first end 28 of body member 14, and which have openings for receiving clips disposed on the cable termination, to mechanically secure the cable termination in assembled relation with bushing assembly 10.

The embedded flange 52 of mounting ring member 26 extends upwardly from the disc or ring portion 54, toward the first end 28 of body member 14, forming a smooth cylindrical surface coaxial with the axis of the conductive sleeve member 16. The flange portion 52 may be of any suitable longitudinal length, and in addition to providing a strong mechanical bond with the body member 14, it also functions as a ground shield, as it provides a smooth equipotential surface which is connected to the metallic casing or enclosure of its associated apparatus.

Mounting ring member may be formed of any suitable material, such as steel, and it may be welded to the casing of the associated electrical apparatus. For example, bushing assembly 10 may be inserted into an opening 64 in a metallic casing 66, with the ring portion 54 of the mounting ring member 26 resting against casing 66. The mounting ring member 26 may then be welded to the casing 66, as illustrated by the welding bead 67. Or, if it is not desirable to permanently mount the bushing within an opening of its associated apparatus, a gasket member (not shown) may be disposed between portion 54 of mounting member 26, and the casing 66, and the bushing assembly 10 secured in place by a conventional spring and flange assembly (not shown), disposed on the encased end of the bushing assembly 10. The circumferential groove 68 in body member 12 receives the spring member of the spring and flange type mounting means.

Thus, if forming body member 14 of bushing assembly 10, it is necessary to properly position the conductive sleeve member 16 and mounting ring member 26 within the casting mold, prior to the introduction of the casting resin system. In order to preclude an air leak between the inside of casing 66 and the atmosphere, about the embedded portion of the mounting ring member 26, due to non-adhesion of the cast resin system to the embedded portion of the mounting ring member 26 which may develop due to the welding heat if the ring member is welded to the casing 66, or due to differences in the coefficients of thermal expansion of mounting ring member 26 and the cast body member 12, a coating 70 of a resilient, elastomeric temperature resistant material may be disposed on the flange 52. For example, a single brush coat of a silicon type elastomer may be disposed on the flange 52 prior to introducing the casting resin into the mold. This material will adhere to the flange 52 and also to the cast resin system, providing a hermetic seal between the mounting ring member 26 and cast body portion 14.

A coating 72 of material similar to the material of coating 70, may be disposed about the conductive sleeve member 16 for a predetermined longitudinal dimension, prior to its being embedded in the body member 14, in order to insure that an oil seal has been obtained between the conductive sleeve member 16 and body member 12. The external threads 44, if provided, on the conductive tube member 16 will aid in obtaining a good air tight bond, but it has been found that when the encased end is disposed in oil, that oil may be forced between the conductive tube member 16 and body member 14 due to capillary action, even when air cannot be forced through the same path. Thus, coating 72 is additional protection against this occurrence. An elastomer ring disposed about conductive sleeve member 16 and embedded in the body member 12 could also be used to provide the seal, instead of coating 72. An elastomer ring could also be used in place of coating 70 on mounting ring member 26, by disposing the elastomer ring about the upwardingly extending portion of flange 52.

In casting body member 12, it is very important that the projection 12 of body member 14 be formed without a parting line. A parting line from the mold may score the inner wall of the elastomeric housing of the plug-in cable termination which snugly encompasses projection 12, and also a sharp parting line would cause electrical stress concentrations which may approach or exceed the corona point. The parting line may be eliminated between the first end 28 of body member 14 and shoulder 80, with shoulder 80 being the stop for the plug-in cable termination, by using an auxiliary mold member. The auxiliary mold member is shaped to form the length of the inside diameter of aperture 32 which starts at the first end 28 of body member 14 and extends to the first end 34 of conductive sleeve member 16, and it also forms the outside diameter of the projection 12 of body member 14. Thus, the mounting ring 26 and conductive sleeve member 16 are disposed within the mold, and the auxiliary mold member is disposed on the first end 34 of the conductive sleeve member 16. The fluid casting insulation system forms the projection 12 by flowing upwardly to fill the cavity between the circumferentially continuous auxiliary mold member and the conductive sleeve member 16. The fluid casting insulation system is then gelled and cured to a high strength solid.

Bushing assembly 10 thus includes a body member 14, a mounting ring 26, a conductive sleeve member 16, and a terminal stud member 18, which components are permanently assembled. The remaining components, i.e., contact member 20, arc confining and extinguishing member 22, and tubular member 24 are all replaceable.

Contact member 20 is formed of a tubular conductor, such as copper, having an externally threaded portion 82 sized to cooperate with the threads 42 on the inside diameter of conductive sleeve member 16, and a pressure terminal portion 84 which has an outside diameter which is slightly smaller than the inside diameter of the conductive sleeve member 16, to provide a predetermined space between the pressure terminal portion 84 and conductive sleeve member 16 when the threaded portion 82 is threadably engaged with the conductive sleeve member 16. Contact member 20 is inserted into the conductive sleeve member and turned, using a tool designed for this purpose, until the end of the threaded portion is turned tightly against shoulder 40, to provide good electrical contact between the contact member 20 and the conductive sleeve member 16. A sealant and lubricant, such as silicon grease, may be disposed on contact member 20 prior to assembly with the conductive sleeve member 16, to enhance the assembly of the components. Advancing contact member 20 into firm contact with shoulder 40 enables an excellent electrical contact to be established, forcing the sealant and lubricant from between the contact points.

The pressure terminal portion 84 of contact member 20 may be longitudinally slotted to provide a plurality of upwardly extending finger portions, such as fingers 83 and 85, which extend toward the first end 34 of conductive sleeve member 16 when assembled therewith, with the outside diameter of the pressure terminal portion 84 being reduced near its extreme end to receive a spring member 86 which is circumferentially disposed about the finger portions to maintain the desired inside diameter of the opening in the contact member 20, and provide a good tight electrical connection between the fingers and the electrical contact of the cable termination, when the electrical contact of the cable termination extends into the opening defined by the inside surfaces of the contact fingers.

After contact member 20 has been snugly turned into place within conductive sleeve member 16, with its extreme leading end disposed against shoulder 40, the protective tube member 24 is disposed within aperture 38 of conductive sleeve member 16, and telescoped over the pressure terminal portion 84 of the contact member 20. The outside diameter of tubular member 24 should be selected to enable it to be easily slipped into aperture 38 of conductive sleeve member 16, and its inside diameter should be selected to allow it to slip over the largest outer diameter of the pressure terminal portion 84 of contact member 20. The wall thickness of the tubular member 24 is also important, as will be hereinafter explained. Thus, the space between the pressure terminal 84 and inside diameter of conductive sleeve member 16 should be selected to provide space for the required wall thickness of the insulating tube member 24.

The longitudinal location of the tube member 24 is also critical. It should be longitudinally located such that it surrounds at least a portion of pressure terminal 84, and it should extend upwardly from pressure terminal 84 towards the first end 34 of conductive sleeve member 16, past the ends of the contact fingers of the pressure terminal 84, by a predetermined dimension. The main function of tubular member 24 is to protect the threads 42 on the inside diameter of conductive sleeve member 16 from contact splatter when an arc is drawn between the contact fingers of the pressure terminal portion 84 of contact member 20 and the conductor of the cable termination. Thus, the material of which the protective tube 24 is formed must have special characteristics. It must be able to withstand the temperature of the arc and also molten metal from the contact member 20, without being destroyed. A secondary function, but one which is also important, is to cool and absorb energy from the arc. A material which has been found to be excellent is polytetrafluoroethylene.

As hereinbefore stated, the wall thickness of tubular insulating member 24 is important. It is important in that it must not be too thin. For example, when using a tubular member formed of polytetrafluoroethylene, a tube having a wall thickness of 14 mils was damaged extensively by the arc drawn between the conductor of the cable terminator and the ends of the contact fingers, while a tube formed of the same material having a wall thickness of 25 mils performed successfully. Other insulating materials which may be used for tubular member 24 are the polyamide-imide materials, such as an aromatic polyimide, or an amide-modified polyimide. Examples of these materials are disclosed in U.S. Pat. Nos. 3,179,630 through 3,179,635, with the last patent of this series being assigned to the same assignee as the present application. The protective tube 24 may also be formed of electrically conductive materials which have a high melting temperature and high temperature of vaporization, such as molybdenum or tungsten.

The arc confining and extinguishing member 22 is a tubular member formed of a material which evolves a volume of gas when subject to the heat of an arc, with the gas having a deionizing effect on the arc, which, along with gas-blast effect, will promptly extinguish an arc drawn between the contact fingers of the pressure terminal 84 and the conductor of a cable termination. Arc confining and extinguishing member 22 has first and second ends 90 and 92, respectively, with the surface 94 adjacent its second end 92 having an outside diameter sized to extend into the tubular member 24, and terminate a predetermined small dimension from the ends of the contact fingers on the pressure terminal 84. In other words, the tubular member 24 spans or bridges the adjacent ends of the arc confining and extinguishing member 22 and the contact member 20, to prevent arc by-products, such as molten metal contact splatter, from coming into contact with the internal threads 42 of conductive sleeve member 16.

The arc confining and extinguishing member 22 then steps outwardly from surface 94, to a surface 96 which is threaded to cooperate with the internal threads 42 of conductive sleeve member 16. After clearing the first end 34 of conductive sleeve member 16, the arc confining and extinguishing member 22 again steps outwardly to a surface 98 which snugly fits the aperture 32 of insulating body member 14. The arc confining and extinguishing member 22 again steps outwardly at the first end 28 of body member 14, providing a shoulder 100 which rests against the first end 28 of body member 14, to limit the travel of member 22 and properly locate its inner end 92 within insulating tube 24, and adjacent the pressure terminal portion 84 of contact member 20. The outer surface of member 22 may then flare smoothly outward from shoulder 100, and provide a smooth radius into its outer end 90, for receiving and cooperating with a plug-in cable termination.

The arc confining and arc extinguishing member 22 may be formed of a high molecular weight polyoxylmethylene. The arc interrupting characteristics of this material are disclosed in U.S. Pat. No. 3,059,081, which is assigned to the same assignee as the present application. U.S. Pat. No. 3,027,352 discloses other materials related structurally to polyoxylmethylene, which may also be used.

A sealant and lubricant, such as a silicon grease, should be used to insert member 22 into cooperative engagement with conductive sleeve member 16, to seal the small clearance between member 22 and the adjacent inner wall of the insulating body member 14 to prevent an arc from following this path to the outside surface of the bushing member 14, where it may proceed over the outer surface of the bushing to ground.

FIG. 1A is a fragmentary view of bushing 10 shown in FIG. 1, constructed according to another embodiment of the invention. Like reference numerals in FIGS. 1 and 1A indicate like components. In this embodiment, the protective tube 24 is eliminated, with the internal threads of conductive sleeve member 16 being protected by reducing the longitudinal spacing 91 between the adjacent ends of the arc confining and extinguishing member 22 and contact member 20. It has been found that by reducing the spacing 91 between the adjacent ends of these members to about 0.063 inches or less, with the minumum being no space, that the arc and its by products are confined within the apertures of members 22 and 20, protecting the inner wall of conductive sleeve member 16 from damage. Thus, the removable characteristics of terminal 20 are preserved.

In the operation of bushing assembly 10, the plug-in cable termination should be coupled with bushing assembly 10 with a positive action which will bring the conductor of the cable termination into rapid, positive contact with the pressure terminal 84. If there is a fault in the apparatus of which bushing 10 is associated, or in its connected load, ionized gases produced by the resulting arc between the conductors of the plug-in cable termination and the bushing will expand into the chamber 102. Chamber 102, which is defined by the inside wall of conductive sleeve member 16, it thus very important, as it provides space for ionized gases to expand, cool and condense. Without this expansion space, the expanding gases may force the plug-in cable termination out of engagement with the bushing assembly 10, with possible hazard to the operator. The surge or expansion chamber 102 also makes it unnecessary to vent the ionized gases to the inside of casing 66 through a pressure release seal. Thus, the desired insulating level of the fluid dielectric disposed within casing 66 is maintained, and possible flashover within the casing from a live part to ground is also precluded, since ionized gas is not released to the inside of casing 66. The surge chamber 102 contains the ionized gases until they partially condense and cool, to reduce their pressure. Any elevated pressure within chamber 102 which remains following a close-in in which ionized gases are produced is inconsequential, as it will slowly equalize to atmospheric pressure, through the small clearance between the plug-in cable termination and the body member 14, and through the insulating body portion of the plug-in cable termination.

It has been found that the longitudinal length dimension 103 of surge chamber 102 is critical for proper operation of bushing 10, and must be selected to be within a range of about 1 inch to 4 inches. The criticality of the length dimension of surge chamber 102 was observed while testing bushings for cover mounting, and bushings for sidewall mounting, on distribution transformers. The bushings for cover mounting are necessarily longer than the bushings for wall mounting, as the former must extend through the air space between the transformer oil level and cover resulting in surge chamber length dimensions of about 6.5 inches, and 2.25 inches for the cover and sidewall bushings, respectively. During quick make-break tests, the bushing with the 2.25 inch surge chamber extinguished the arc drawn between the conductors of the plug-in connector and bushing without re-ignition. During the quick make-break tests on the bushings with the 6.5 inch surge chamber, the arc was extinguished, but it re-ignited after about a 30 to 50 millisecond delay. Reducing the length of the 6.5 inch surge chamber to 2.25 inches, by plugging the blind end of the 6.5 inch surge chamber with a wood dowel, resulted in the bushing for cover mounting passing the quick make-break test without re-ignition of the arc.

Attempts to analyze this phenomenon, in order to develop an optimum length dimension for the surge chamber, are complicated by the fact that decoupling a plug-in cable connector and bushing produces two different reactions. When the contacts of the plug-in connector and bushing separate, a pressure wave is produced by the hot arc and decomposing surfaces of the arc extinguishing members of the plug-in connector and bushing. Also, as the close-fitting plug-in connector is removed from the bushing, a vacuum is created, lowering the pressure within the plug-in connector and bushing, followed by a violent inrush of air as the vacuum is broken. These two reactions make it difficult to determine if the criticality of the surge chamber length is merely a matter of volume and pressure, with the smaller surge chamber volume of the wall mounted bushing building up a higher pressure, which higher pressure is required to prevent re-ignition of the arc; or, whether the wavelength of the shockwave generated by the arc and length of the surge chamber are related such that with the longer surge chamber of the cover mounted bushing an acoustic resonance or reflection is created which lowers the pressure in the region where the arc may restrike.

Regardless of the theory behind the phenomenon, if the surge chamber 102 has a length dimension 103 which exceeds about 4 inches, the excess length should be filled with filler means 104, as shown in FIG, 1. The filler means 104 should be added to provide a longitudinal dimension 103 in the range of 1 inch to 4 inches, and preferably about 2 inches to 2.5 inches. Filler means 104 may be conductive, such as tightly packed aluminum or steel wool, or non-conductive, such as an insulating dowel, or a resin system, such as an epoxy resin.

If the fault in the load upon closing or coupling a cable termination with bushing assembly 10 is of sufficient magnitude, i.e., a low impedance fault, the protective current limiting means, such as fuses or breakers, in the high voltage cable feeder will clear the circuit and limit the maximum current magnitude. Also, if a fault occurs after the plug-in cable termination is coupled with bushing assembly 10, the protective current limiting means will clear the circuit and thus there will be no danger to an operator when the cable termination is removed from the bushing assembly 10. If the plug-in cable termination is decoupled from the bushing assembly 10 during normal load conditions, for example up to 200 amperes in a 7,200 volt circuit, an arc will be drawn between the ends of the contact fingers of the pressure terminal 84 and the terminal of the plug-in termination, which draws the arc into the arc confining and extinguishing member 22. The arc heat will liberate deionizing gases from member 22, with the gases deionizing and blasting the arc to effect an early extinction thereof. Tests have shown that the arc drawn in a 7,200 volt circuit in which a load current of 200 amperes is flowing is extinguished within one-half to one cycle.

If the plug-in cable termination is of the type which terminates the cable shield, with the ground return conductors of the cable being twisted together and connected to a suitable terminal on the casing 66, and corona extinction voltage within system requirements is obtained, the bushing assembly 10 will not require means for continuing the cable shield to the casing. If the plug-in cable termination is of he type which requires the bushing to continue the cable shield to the casing 66, a metallic coating 110, such as sprayed aluminum, may be disposed about the body member 14, starting between the shoulder 80 and mounting ring member 26, and continuing to the groove 68 in the body member 14.

In summary, there has been disclosed a new and improved load break bushing assembly adapted for coupling with a plug-in cable termination, with the parts which are subject to deterioration through repeated circuit interruptions, such as the contacts and arc extinguishing member, being easily replaceable. Further, the replaceable characteristic of these components is not impaired by contact splatter from the contacts, as in one embodiment of the invention a protective tube protects the threads form the contact splatter, and also absorbs energy from the arc which aids in cooling, deionizing and extinguishing the arc, and in another embodiment the threads are protected by the pressure terminal and arc extinguishing member. Still further, the load break bushing assembly may be safely coupled to an energizer cable termination when the bushing assembly is connected to a load, and the load break bushing assembly may be connected to an energized cable termination when a fault of predetermined maximum magnitude exists in the load connected to the bushing assembly 10. The magnitude of the fault current is limited to a predetermined maximum magnitude by current limiting means associated with the primary supply circuit. The hot ionized gases produced when closing or coupling bushing assembly 10 with a cable termination due to a fault in the load connected to the bushing assembly, expand into a surge chamber formed in the conductive sleeve member, with the gases cooling and condensing within this chamber. Thus, it is unnecessary to vent these hot ionized gases into the enclosure of the associated electrical apparatus through a pressure release seal. Therefore, bushing assembly 10 will continue to provide service without requiring the whole bushing assembly to be replaced, as the arc quenching tube and contacts of a bushing assembly are easily replaceable, and there are no pressure release seals which tend to deteriorate with time. Further, the bushing assembly 10 is of simple, rugged construction which will provide the required load break and fault close-in functions without cracking or flying apart, and the bushing assembly 10 may be manufactured for a relatively low cost.

Claims

We Claim As Our Invention:

1. A load break bushing assembly adapted to receive a plug-in cable connector, comprising:

an insulating body member,

a conductive sleeve member disposed in said body member, said conductive sleeve member having first and second ends, and an aperture which extends between its ends,

a contact member removably disposed in said conductive sleeve member, said contact member having a terminal disposed near the first end of the conductive sleeve member which is adapted to contact the conductor of the plug-in connector,

a second terminal disposed at the second end of the conductive sleeve member, said second terminal sealing the second end of the conductive sleeve member to provide a surge chamber within the conductive sleeve member for the expansion of gases when the plug-in connector is assembled with the load break bushing assembly,

an arc confining and extinguishing member disposed to surround an arc formed between said contact member and the conductor of the plug-in connector,

and a tubular protective member removably disposed about the terminal of said contact member and a portion of said arc confining and extinguishing member, said tubular protective member maintaining the removable characteristic of said contact member by absorbing energy from an arc, and protecting said conductive sleeve member from arc produced contact splatter.

2. The load break bushing assembly of claim 1 wherein the surge chamber has a longitudinal dimension in the range from about 1 inch to 4 inches.

3. A load break bushing assembly adapted to receive a plug-in cable connector, comprising:

an insulating body member,

a conductive sleeve member disposed in said body member,

a contact member removably disposed in said conductive sleeve member, said contact member having a terminal adapted to contact the conductor of the plug-in connector,

an arc confining and extinguishing member disposed to surround an arc formed between said contact member and the conductor of the plug-in connector,

and a tubular protective member removably disposed about the terminal of said contact member and a portion of said arc confining and extinguishing member, said tubular protective member maintaining the removable characteristic of said contact member by absorbing energy from an arc, and protecting said conductive sleeve member from arc produced contact splatter,

said insulating body member being cast of a resinous insulation system which includes finely divided inorganic filler means selected to substantially match the coefficient of thermal expansion of the cast resinous insulation system with that of the conductive sleeve member.

4. The load break bushing assembly of claim 3 wherein the cast resinous insulation system includes an epoxy resin.

5. A load break bushing assembly adapted to receive a plug-in cable connector, comprising:

an elongated body member formed of a cast resinous insulation system, said body member having first and second ends and an aperture which extends between its ends, said cast resinous insulation system including an epoxy resin and filler means, said filler means being selected to substantially match the coefficient of thermal expansion of the cast resinous insulation system to that of the tubular conductive member,

a tubular conductive member having first and second ends and an aperture which extends between its ends, said tubular conductive member being disposed in the aperture of said body member with its first end spaced from the first end of said body member, and its second end located substantially adjacent the second end of said body member,

a contact member having first and second ends, and a pressure terminal disposed at its first end, said contact member being removably disposed in the aperture of said tubular conductive member, with its pressure terminal being spaced form the wall of the aperture,

an arc confining and extinguishing member removably disposed in the aperture of said insulating body member at its first end, having a cylindrical projection disposed within and spaced from the wall of the aperture, which extends toward the terminal of said contact member, providing a maximum distance between the arc confining and extinguishing member and contact member of 0.063 inches.

6. A load break bushing assembly adapted to receive a plug-in cable connector, comprising:

an elongated body member formed of a cast resinous insulation system, said body member having first and second ends and an aperture which extends between its ends,

a tubular conductive member having first and second ends and an aperture which extends between its ends, said tubular conductive member being disposed in the aperture of said body member with its first end spaced from the first end of said body member, and its second end located substantially adjacent the second end of said body member,

a contact member having a pressure terminal disposed at one end thereof, said contact member being removably disposed in the aperture of said tubular conductive member, with its pressure terminal being spaced from the wall of the aperture,

an arc confining and extinguishing member removably disposed in the aperture of said insulating body member at its first end, having a cylindrical projection disposed within and spaced from the wall of the aperture, which extends toward the pressure terminal of said contact member,

and a terminal stud disposed in the aperture of said tubular conductive member at its second end, sealing the second end of the aperture to provide a gas expansion chamber within said tubular conductive member for containing gas generated when a plug-in cable connector is assembled with the load break bushing assembly, said gas expansion chamber having a maximum longitudinal dimension of 4 inches,

and a tubular protective member removably disposed in the aperture of the tubular conductive member, about at least a portion of the cylindrical projection of the arc confining and extinguishing member and at least a portion of the pressure terminal of the contact member, to protect the tubular conductive member from arcing by-products produced by an arc between the pressure terminal and the conductor of a plug-in cable connector.

7. The load break bushing assembly of claim 6 wherein the tubular protective member is formed of an electrical insulating material.

8. The load break bushing assembly of claim 6 wherein the tubular protective member is metallic.

9. A load break bushing assembly adapted to receive a plug-in cable connector, comprising:

an elongated body member formed of a cast resinous insulation system, said body member having first and second ends and an aperture which extends between its ends,

a tubular conductive member having first and second ends and an aperture which extends between its ends, said tubular conductive member being disposed in the aperture of said body member with its first end spaced from the first end of said body member, and its second end located substantially adjacent the second end of said body member,

a contact member having a pressure terminal disposed at one end thereof, said contact member being removably disposed in the aperture of said tubular conductive member, with its pressure terminal being spaced from the wall of the aperture,

an arc confining and extinguishing member removably disposed in the aperture of said insulating body member at its first end, having a cylindrical projection disposed within and spaced from the wall of the aperture, which extends toward the pressure terminal of said contact member,

and a terminal stud disposed in the aperture of said tubular conductive member at its second end, sealing the second end of the aperture to provide a gas expansion chamber within said tubular conductive member for containing gas generated when a plug-in cable connector is assembled with the load break bushing assembly, said gas expansion chamber having a longitudinal dimension in the range of 1 inch to 4 inches.