Modular electrical assemblies with plastic film barriers

Abstract: An electrical device (50) includes a plurality of elec­ trical assemblies (52,54,56) formed as interchangeable modules, aligned in an end-to-end serial array and enclosed in a polymeric weathershed housing (58), the electrical assemblies (52,54,56) being electrically and mechanically coupled together. Each electrical assembly in turn has a plurality of electrical components (60,62) aligned in a row and in electrical connection with one another via their axially-directed ends and under an axially-­ directed compressive force via a non-conductive filament winding (64). A plastic film barrier (110) laterally surrounding the electrical components (60,62) is interposed between the electrical components (60,62) and the filament winding (64). A surge arrester with voltage ratings can be enlarged merely by serially coupling a plurality of separately wrapped electrical components (60,62) in the form of MOV blocks. Enclosing the electrical assemblies (52,54,56) is a weathershed housing (58).

Description: Background of the Invention A surge protector or arrester is commonly connected across a comparatively expensive piece of electrical equipment to shunt over-current surges. Such over-current surges occur, for example, when lightning strikes. When this happens, the surge arrester shunts the surge to ground, thereby protecting the piece of electrical equipment and the circuit from damage or destruction. Present day surge arresters commonly include an elongated, hollow cylindrical housing made of porcelain or the like, and a plurality of non-linear resistive blocks within the housing. Some of these structures also include spark gaps, the blocks and gaps being electrically interconnected to handle voltage and current surge conditions arising on a power line. The blocks commonly contain silicone carbide (SIC) or metal oxide varistors (MOV), and are usually in the shape of relatively short cylinders stacked within the arrester housing. The number of blocks employed is a function of the material (SIC or MOV) and the voltage and current ratings of the assembly. For a surge arrester to function properly, intimate contact must be maintained between the MOV or SIC blocks. This necessitates placing an axial load on the blocks within the housing. Prior art arresters utilize bulky contact springs within the housing to provide this axial load. Typically, these springs can provide only relatively small loads, for example, about sixty pounds. As a result, prior art surge arresters experience one or more problems such as poor heat transfer between the MOV or SIC blocks and arrester terminals; non-uniform current distribution; and high contact resistances at joints. Furthermore, units having low contact force sputter and the ionized metal which is produced can cause axial flashover at high currents. An additional problem with surge arresters of the prior art is that they, on rare occasions, fail in a dangerous fashion. When these arresters fail and experience high fault currents, the bursting unit may throw parts and cause property damage. In addition, some of the prior art devices are difficult to assemble, have poor dielectric design, are susceptible to water invasion, and require totally different devices to provide varied voltage ratings. Examples of prior art surge arresters are disclosed in the following U.S. patents: 2,587,587 to Bellezza et al; 2,947,903 to Westrom; 2,997,529 to Fink; 3,018,406 to Innis; 3,261,910 to Jacquier; 3,412,273 to Kennon et al; 3,524,107 to Reitz; 3,566,183 to Olsen; 3,567,541 to Kaczerginski; 3,586,934 to Nakata; 3,706,009 to Reitz; 3,725,745 to Zisa; 3,850,722 to Kreft; 3,973,172 to Yost; 3,987,343 to Cunningham et al; 4,029,380 to Yonkers; 4,092,694 to Stetson; 4,100,588 to Kresge; 4,107,567 to Cunningham et al; 4,161,012 to Cunningham; 4,218,721 to Stetson; 4,404,614 to Koch et al; 4,467,387 to Bergh et al; 4,491,687 to Kaczerginski et al; and U.S. Defensive Publication T102,103, as well as U.K. patents 730,710; 1,109,151; and 1,505,875. In the surge arresters of U.S. Patent No. 4,656,555 and U.S. patent application Serial No. 033,765, the resin soaked glass filaments directly engage and bond to the varistor blocks. This can damage the relatively fragile ceramic insulating collars on the varistor blocks during manufacture. Commonly assigned and concurrently filed U.S. patent application Serial No. 176,317 entitled Modular Electrical Assemblies with Pressure Relief of John D. Sakich discloses a surge arrester having a filament wrap with openings and a barrier between the electrical components and the filament wrap. Summary of the Invention Accordingly, an object of this invention is to provide electrical devices, particularly surge arresters, which are relatively simple and inexpensive to assemble without damaging the electrical components, have good dielectric design, resist water invasion, and have modular components and housings to simply vary voltage ratings. A further object of this invention is to provide electrical assemblies, such as surge arresters, having high axial loadings, thereby resulting in uniform current distribution, low contact resistances at joints, and excellent heat transfer to the arrester terminals. Another object of this invention is to provide an electrical assembly, such as a surge arrester, having a shatter-proof housing which has a high-impact strength and which does not fail in a dangerous fashion. Still another object of this invention is to provide a MOV block assembly with greatly improved tensile and cantilever strengths. Yet another object of this invention is to provide a surge arrester which is forgiving of dimensional variations in associated parts, thereby reducing the need for expensive close tolerances. The foregoing objects are basically attained by providing a modular electrical assembly or an electrical device formed of modular electrical assemblies, wherein each electrical assembly includes a plurality of electrical components aligned in a row or column and electrically connected through their axially directed ends, a plastic film barrier laterally surrounding the electrical components, and a non-conductive filament winding enclosing the plastic film and electrical components. The winding applies an axially directed compressive force on the electrical components to maintain their electrical connection. Other objects, advantages and salient 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. Brief Description of the Drawings Referring to the drawings which form a part of this original disclosure: Fig. 1 is a side elevational view of an electrical device in the form of a surge arrester, in accordance with the present invention, including a plurality of modular and interchangeable filament wrapped electrical assemblies enclosed in a weathershed housing; Fig. 2 is a side elevational view in partial longitudinal section of the device illustrated in Fig. 1 showing the electrical assemblies including the filament windings inside the weathershed housing; Fig. 3 is an enlarged side elevational view in longitudinal section of one modular electrical assembly shown in Fig. 2; and Fig. 4 is an enlarged end elevational view in section taken along line 4-4 of Fig. 3. Detailed Description of Preferred Embodiment Referring to Figs. 1-4, an electrical device 50, in the form of a surge arrester, according to the present invention is formed of a plurality of modular electrical assemblies 52, 54 and 56 enclosed in a polymeric, elastomeric weathershed housing 58. Each of the electrical assemblies is advantageously substantially identical to and interchangeable with the other electrical assemblies, and is in turn formed from a plurality of cylindrical electrical components 60 and 62. These components are aligned in a row, and are in electrical connection with one another through their axially-directed ends and under an axially-directed compressive force developed by a non-conductive filament winding 64, as disclosed in U.S. Patent No. 4,656,555 and Serial No. 033,765. The electrical components can be metal oxide varistors (e.g., zinc oxide varistor blocks), resistors, capacitors, or any combination thereof. While only two electrical components are shown in Fig. 3, more than two can be used. In the case of varistors used to form a surge arrester, voltage ratings can be enlarged merely by serially and selectively coupling the plurality of modular electrical assemblies together mechanically and electrically. The elastomeric weathershed housing 58 can be one section of polymeric material or a plurality of sections 66, 68 and 70 in an end-to-end aligned serial array as seen in Figs. 1 and 2. These sections receive the electrical assemblies therein via a slight interference fit and therefore facilitate construction and allow the practice of good dielectric design by reducing radial gaps. Each of the electrical assemblies 52, 54 and 56 are substantially the same, so only assembly 52 illustrated in Figs. 3 and 4 will be described in detail. Assembly 52 has a substantially cylindrical overall outer surface and comprises first end member, or terminal 72, spring washer 74, contact disc 76, electrical component 60, contact disc 78, electrical component 62, contact disc 80, spring washer 82, and second end member, or terminal 84. The non-conductive filament winding 64 is coupled to end members 72 and 84 via reception in peripheral grooves 73 and 85 respectively therein, encloses the electrical components, and maintains them under an axially-directed force, which is augmented by the spring washers. A plastic film barrier 110 laterally surrounding electrical components 60 and 62 is interposed coaxially between the electrical components and filament winding 64. Preferably, the plastic is polypropylene. The barrier is formed by wrapping a rectangular plastic sheet tightly about the electrical components and the adjacent portions of end members 72 and 84, i.e., those portions axially inside of grooves 73 and 85, in two layers 111 and 112 before filament winding 64 is added. The thickness of the plastic sheet and of each layer is about 0.0127mm (0.0005 inch). Since the plastic film barrier extends along the entire length of the electrical components and onto the end members, the plastic film barrier seals the electrical components from the epoxy or resin on the filament forming the winding. For surge arresters, this prevents the wet epoxy or resin on the filament from bonding to the fragile ceramic insulating collars on the metal oxide varistor blocks 60 and 62. All of the parts comprising electrical assembly 52 are conductive, except the film barrier and the winding, unless the components are insulators. Advantageously, end members 72 and 84 are formed from aluminum. The exposed outer surfaces of the filament winding 64 and the end members 72 and 84 are all substantially cylindrical and have substantially equal diameters. End members 72 and 84 form internal terminals, have cylindrical exposed outer surfaces, and have opposite, first and second axially-directed planar ends with internally threaded sockets or bores 86 and 88 formed respectively therein. Socket 86 threadedly receives threaded end stud 90 which can be connected to an electrical power source and is in the form of a metallic, conductive shoulder bolt. End plate 92 is received on end stud 90, tightly engages an end of the weathershed housing as seen in Figs. 1 and 2 and is held in place via rigid shoulder 94 on the stud. A second end plate 96 is similarly positioned at the other end of the housing and is received on end stud 98 which is connected to ground and maintained thereon via shoulder 99 on the stud. Studs 90 and 98 in essence form external terminals for the overall device 50. Weathershed housing section 66 has a through passageway in the form of a throughbore with an inwardly facing cylindrical surface 100 which tightly receives therein the outer cylindrical surface of the electrical assembly 52. The same is true regarding sections 68 and 70 and assemblies 54 and 56. The reception of the assemblies in the throughbore is preferably via an interference fit with the assemblies having outer surface diameters that are substantially equal to one another and from about 2% to about 9% greater than the throughbore diameter, which is substantially constant along its length. This reduces radial gaps and thus provides advantageous dielectric design. To mechanically and electrically connect the three electrical assemblies 52, 54 and 56 together in an aligned, straight end-to-end serial array, a pair of externally threaded, metallic, and conductive studs 104 are used, as seen in Fig. 2. These studs are advantageously substantially identical and interchangeable, as well as substantially rigid and formed of stainless steel. Stud 104 couples the adjacent ends of adjacent assemblies 54 and 56 by being threadedly received in the threaded sockets in each assembly's adjacent end member. The same is true regarding for the stud coupling adjacent assemblies lies 52 and 54. The adjacent ends of adjacent assemblies are screwed tightly together on the studs to provide a substantially gap-free engagement between the facing planar, axially-directed outer ends of the end members thereon. This provides an advantageous electrical and mechanical interface by reducing possible separation during bending of the device. If more assemblies are used, additional studs are added to connect them, with the addition of any necessary weathershed housing sections, or use of a larger, one-piece housing. To provide sealing against water invasion, preferably a neoprene gasket is interposed between each adjacent stud shoulder and end plate, and silicone grease is interposed between each adjacent end plate and end member, between adjacent end members, and between the outer surfaces of the electrical assemblies and the inwardly facing surfaces of the throughbore in each weathershed housing section. Use of grease between the weathershed housing section and the electrical assembly aids in construction and assembly by reducing friction and also reduces any radial gaps therebetween. Advantageously as seen in Figs. 1 and 2, the longitudinal axes of the studs 90, 104, and 98; the electrical components in each assembly 52, 54 and 56; and the weathershed housing 58 are coaxially aligned. Preferably, the planar ends of the end members are perpendicular to these aligned longitudinal axes. Preferably, with regard to the electrical device 50, the axial load on the electrical components before winding is about 5MPa (750 pounds per square inch), and the filament or stranded element of fibers is wet epoxy coated fiberglass which is wound through about 100 turns and is cured for about two hours at 150°C.

Claims: a modular electrical assembly comprising: a plurality of conductive electrical components (60,62) aligned in a row along an axis and electrically connected with one another at their axial ends (76,78,80); at each end of the assembly a conductive member (72,84); a non-conductive tubular filament winding (64) enclosing said electrical components and said end members so as to apply an axially directed compressive force to the components and to maintain the electrical connections therebetween; characterised in that a plastics film barrier (110) laterally surrounds the electrical components (60,62) and separates these components from the tubular filament enclosure (64). A modular electrical assembly as claimed in Claim 1, wherein means are provided at each end member (72,84) for connecting each said end member to an end member of an adjacent electrical assembly. A modular electrical assembly as claimed in Claim 2, wherein the end members (72,84) have an internally threaded socket (86, 88) wherein a threaded connector is receivable so as to connect adjacent electrical assemblies. A modular electrical assembly as claimed in Claim 3, wherein the threaded connector is a threaded stud. A modular electrical assembly as claimed in any preceding claim, wherein said plastics film (110) is formed of polypropylene. A modular electrical assembly as claimed in any preceding claim, wherein said plastics film barrier (110) is formed of two layers (111,112). A modular electrical assembly as claimed in Claim 6, wherein the thickness of each layer is about 0.0127 mm. A modular electrical assembly as claimed in any preceding claim, wherein the tubular filament enclosure (64) is a winding of wet epoxy resin coated glass fibres. A modular electrical assembly as claimed in any preceding claim , wherein said film barrier (110) extends over and surrounds a portion of each conductive end member (72,84). A modular electrical assembly as claimed in any preceding claim, wherein said end members (72,84) have peripheral grooves (73,85) wherein a filament winding is received so as to form the tubular filament enclosure and to apply said compressive force. A modular electrical assembly as claimed in any preceding claim, enclosed in the throughbore of an elastomeric weathershed housing (58). A modular electrical assembly as claimed in claim 11, wherein its outer diameter is from about 2% to about 9% greater than the diameter of said throughbore. A modular electrical assembly as claimed in any preceding claim, wherein said electrical components are varistors. A modular electrical assembly as claimed in Claim 13, which is a surge arrestor.

Inventors: RAUDABAUGH DONALD EUGINE, RAUDABAUGH, DONALD EUGINE
Applicants: HUBBELL INC, HUBBELL INCORPORATED
Application year: 1989
Main classification: H01C7
IPC codes: H01C7, H05K5, H01T4