Patent Publication Number: US-6667871-B2

Title: Arrester housing with weak section

Description:
BACKGROUND OF THE INVENTION 
     Conventional protective electrical devices, such as surge arresters, provide protection for equipment of power distribution systems during fault conditions caused by a system disturbance, such as a lighting strike. An overload of current resulting from a system disturbance can damage and/or destroy electrical equipment because the amount of current is much greater during the disturbance relative to during normal operating conditions. 
     Conventional surge arresters include an outer housing with two end terminals for connecting the arrester between a conductor device, such as a bushing insert, and ground. Held within the housing of a conventional arrester is a stack of arrester elements or metal oxide varistor (MOV) blocks. The MOV blocks allow the arrester to divert the overload current through the arrester to ground, thereby protecting the electrical equipment. In particular, as the voltage applied to the MOV blocks is increased, due to a system disturbance, the impedance of the MOV blocks decreases towards zero and the blocks become highly conductive thereby conducting the resulting current overload to ground. 
     Typically during fault conditions, conventional surge arresters rupture and separate from the bushing insert of the electrical equipment, to which it was connected. Arcing typically occurs within the arrester resulting in the generation of gas and heat as the internal arrester elements vaporize. During such a catastrophic failure, the arrester will rupture due to the generated gases that cannot be vented quickly enough from the arrester housing. Commonly, the housing ruptures in random areas, particularly near the connection of the bushing insert and the arrester, thereby forcing the arrester away from the bushing insert such that the arrester separates from the bushing insert. The conventional arresters fail to provide a mechanism for preventing separation of the arrester from the bushing insert during a fault event. 
     Examples of conventional arresters are disclosed in U.S. Pat. Nos. 6,014,306 to Berlovan et al.; 6,008,975 to Kester et al.; 5,633,620 to Doerrwaechter; 5,309,313 to Yaworski et al.; 5,088,001 to Yaworski et al.; 5,043,838 to Sakich; and 4,463,405 to Koch et al. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an electrical device for a power distribution system and a method of making same that provides protection for the system equipment during a fault condition. 
     Another object of the present invention is to provide an electrical device for a power distribution system and a method of making same that provides a mechanism for limiting separation of the electrical device from an electrical connector of the system. 
     Yet another object of the present invention is to provide an electrical device for a power distribution system and a method of making same that provides a weak section in the housing of the device that allows controlled venting of internal gases upon rupture of the housing. 
     The foregoing objects are basically attained by an electrical device, comprising a housing including first and second portions with each of the first and second portions having a first insulative layer and a second conductive layer. The first layer defines an inner cavity, and the second portion has opposing first and second lateral sides. The first layer defines a first thickness at the first lateral side and a second thickness at the second lateral side. An electrically conductive member is received within the inner cavity in the first portion. At least one electrical component is received within the inner cavity at the second portion. A weak section in the first lateral side of the second portion of the housing is defined by the first thickness at the first lateral side that is substantially less than the second thickness at the second lateral side diametrically opposite thereto at given points along a longitudinal axis of the second portion. 
     The foregoing objects are also basically attained by a method of making an electrical device, comprising the steps of forming an outer conductive layer, forming the inner cavity in first and second portions thereof and placing a mandrel in the inner cavity of the second portion of the conductive layer. The mandrel has a teardrop cross sectional shape. Molding an inner insulative layer by injecting a substantially resilient insulative material into the inner cavity at a second portion of the housing and around the mandrel, thereby forming an inner cavity in the insulative layer into teardrop cross-sectional shape that is substantially identical to the tear drop cross-sectional shape of the mandrel. 
     By fashioning the electrical device in this manner, a controlled venting of internal gases is provided through the weak section. Arranging the weak section rupture in a direction away from an electrical connector device to which the electrical device is connected to avoid disconnection. 
    
    
     Other objects, advantages and salient features of the invention will become apparent from the following detailed description, which, taken in conjunction with annexed drawings, discloses a preferred embodiment of the present invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings which form a part of this disclosure: 
     FIG. 1 is a side elevational view in section of a surge arrester in accordance with an embodiment of the present invention; 
     FIG. 2 is a top plan view in section of the surge arrester taken along line  2 — 2  of FIG. 1, showing a housing of the surge arrester with a weak section after insertion of a module of MOV blocks within the housing; 
     FIG. 3 is a top plan view in section of the surge arrester similar to FIG. 2, showing the housing of the surge arrester with the weak section, before insertion of the module of MOV blocks within the housing; 
     FIG. 4 is a side elevational view of the surge arrester illustrated in FIG. 1, showing the surge arrester mated with a bushing insert; and 
     FIG. 5 is a top plan view in section of the surge arrester similar to FIG. 3, showing the housing of the surge arrester with a teardrop mandrel inserted within the housing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1-4, a surge arrester  10  in accordance with the present invention generally includes housing  14  having a bushing interface portion  16  for connection with an electrical connector, such as a bushing insert  12 , and a shank portion  18  for connection to a ground. Bushing interface portion  16  and shank portion  18  form a substantially elbow shaped arrester, as is well known in the art. Shank portion  18  has a weak section  20  that provides a controlled rupture of the housing to vent or release internal gases that develop during a fault closure. The controlled rupture assists in preventing separation of arrester  10  and bushing insert  12 . 
     Housing  14  has the general shape of an elbow with bushing interface or first portion  16  extending along a first central longitudinal axis  22  and shank or second portion  18  extending along a second central longitudinal axis  24 , with the first axis being angularly disposed with respect to said second axis, preferably at generally ninety degrees. A conventional housing for a surge arrester is disclosed in U.S. Pat. No. 6,014,306 to Berlovan et al., the subject matter of which is hereby incorporated by reference. 
     A conductive jacket  26  forms the outer layer of housing  14  and an insulative layer  28  forms an inner lining, as is conventional in the art. The outer conductive jacket  26  is preferably made of conductive EPDM rubber, and the inner insulative layer  28  is preferably made of insulating EPDM rubber. Insulative layer  28  forms an inner cavity  30  at the shank portion  18  of housing  14  that receives an electrical component or module  34 . At the bushing interface portion  16  of the housing  14 , insulative layer  28  forms inner cavity  32  that includes a centrally disposed conductive member or probe  36  that mates with contacts of bushing insert  12 . 
     With respect to bushing interface portion  16  of housing  14 , a conductive insert  38 , formed of conductive EPDM rubber, sits within inner cavity  32  and provides an electrical connection between conductive probe  36  and electrical module  34 . A bushing port  40  for receiving the end of bushing insert  12  in a telescoping arrangement is defined between conductive insert  38  and an end opening  42  of inner cavity  32 . Conductive insert  38  includes a copper portion  39  that accepts a threaded end  44  of conductive probe  36  with its opposing end  46  extending through end opening  42 . An albative member  48  is included with opposing end  46  of probe  36 , as is known in the art. 
     As to shank portion  18 , electrical component  34  fits within inner cavity  30 . Electrical component forms a module that particularly includes first and second end terminals  50  and  52  with conventional metal oxide varistor (MOV) blocks  54  stacked and axially aligned between first and second end terminals  50  and  52 . Surrounding first and second end terminals  50  and  52  and MOV blocks  54  is a fiberglass weave casing  56  that tightly secures the blocks  54  and end terminals  50  and  52  together forming a generally tubular module having a right circular cylindrical shape. Springs  58  are applied on each of first and second end terminals  50  and  52 , respectively, to further compress the elements of electrical component  34 , thereby ensuring an electrical path through end terminals  50  and  52  and blocks  54 . 
     As with conductive probe  36 , conductive insert  38  is also electrically connected at copper portion  39  to electrical component or module  34  by a threaded connection  60  through first end terminal  50 . At the opposite or second end terminal  52 , a threaded fastener  62  engages terminal  52  and secures an end cap  64  to the end of shank portion  18 . As seen in FIG. 4, a grounding cable  63  can be connected to threaded fastener  62  at its bottom end  66  remote from terminal  52 , thereby providing an electrical connection between electrical module  34  and ground  67 . 
     Weak section  20  is located in the side of shank portion  18  of housing  14 , as best seen in FIGS. 1 and 2. Specifically, shank portion  18  has diametrically opposed first and second sides  68  and  70  laterally disposed from central longitudinal axis  24 . Inner insulative layer  28  defines a first thickness a in section transverse to central axis  24  at first lateral side  68  and similarly a second thickness b at second lateral side  70  of shank portion  18  with first thickness a being substantially less than second thickness b. Making housing  14  weaker at first lateral side  68  of shank portion  18  than at second lateral side  70  defines weak section  20 . Weak section  20  extends along and is substantially continuous along generally the entire length of shank portion  18 , as seen in FIG. 1, the length being generally defined between end cap  64  and the interface portion  16  of housing  14 . First thickness a being less than second thickness b laterally offsets electrical module  34  held in inner cavity  30  from central axis  24 , so that electrical module  34  is closer to first lateral side  68  than second lateral side  70 , and more of electrical module  34  is disposed on the side of central axis  24  that is near first lateral side  68 . 
     Assembly 
     Forming surge arrester  10  is generally a three step molding process of first molding outer conductive jacket  26 , then molding conductive insert  38 , and finally molding inner insulative layer  28 . Specifically, outer conductive jacket  26  is molded using a conventional mold including a solid generally L-shaped core mandrel. A conductive rubber is poured around the L-shaped core mandrel to form a one-piece unitary outer jacket  26  with a hollow interior. Jacket  26  can then be removed from the mold simply by removing it from the L-shaped core mandrel. Next, conductive insert  38  is separately formed in a conventional manner. 
     Once outer conductive jacket  26  and conductive insert  38  are each molded, both are placed in another mold for forming inner insulative layer  28 , with conductive insert  38  being placed within the hollow interior of jacket  26  at the junction point of the L-shaped jacket. First and second mandrels are then placed within the hollow interior of jacket  26  with conductive insert  38  being located between the mandrels. The first mandrel is placed in the interior at the part that will be the interface portion  16  of housing  14 . The second mandrel  80  is placed in the interior of the part that will be the shank portion of housing  14  as seen in FIG.  5 . Inner layer  28  is formed by injecting insulative material into jacket  26  and around the first and second mandrels, and conductive insert  38 , forming a one-piece unitary layer. 
     The first mandrel has a similar shape to the end portion  74  of bushing insert  12 , to thereby form bushing port  40  of housing  14 , which receives bushing insert  14 , as is known in the art. As seen in FIG. 5, the second mandrel  80  has a particular shape of a substantially teardrop cross-sectional shape to form weak section  20  in inner layer  28  of housing  14 . The material of inner layer  28  is injected through a funnel  72  formed in outer jacket  26 , into its interior, and around the first and second mandrels, thereby forming inner cavities  30  and  32  at shank portion  16  and interface portion  18 , respectively. The first and second mandrels can then be removed such that interface portion  16  of housing  14  is formed with inner layer  28  now defining inner bushing port  40 , and shank portion  18  of housing  14  is formed with inner layer  28  now defining inner cavity  30 . As seen in FIG. 3, inner cavity  30  has a substantially teardrop shape in section traverse to central axis  24  of shank portion  18  with the point  76  of the teardrop cross-section shape extending towards first lateral side  68  to create weak section  20 . 
     Finally, electrical module  34  is placed within inner cavity  30 . Upon insertion of module  34 , inner layer  28  at inner cavity  30  conforms to the shape of module  34  forming a friction or interference fit between module  34  and inner layer  28 , as best seen in FIG.  2 . Specifically, the cylindrical shape of module  34  forces the flexible and resilient material of inner layer  28  to conform to its shape, so that inner cavity  30  has a substantially right circular cylindrical shape defined by inner layer  28 . Since the point  76  is directed towards first lateral side  68 , in transforming from a substantially teardrop cross-sectional shape to a right circular cylindrical shape, first thickness a of inner layer  28  at first lateral side  68  is formed so that it is less than second thickness b at second lateral side  70 , thereby defining weak section  20  at first lateral side  68 . 
     The remaining assembly is conventional and therefore will not be described in detail. In general, module  34  and probe  36  are connected to conductive insert  38  by threaded connection  60  and threaded end  44 , respectively, so that an electrical path is created through probe  36 , insert  38 , and module  34 . End cap  64  is secured to the end of shank portion  18  by threaded fastener  62  which is connected to end terminal  52  of module  34 , and provides a ground connection. 
     Operation 
     Referring to FIGS. 1 and 4, surge arrester  10  connects to a bushing insert  12  of the electrical equipment for use with electrical equipment  82  of a power distribution system. During a fault event, weak section  20  of arrester  10  will provide a controlled venting of internal gases. The controlled venting will be directed away from bushing insert  12  and bushing interface portion  16  of arrester  10 , rather than in random directions or in a direction toward bushing insert  12 , thereby generally preventing separation of the arrester from the end portion  74  of bushing insert  12 . 
     In particular, as is known in the art, upon connection of arrester  10  and bushing  12 , end portion  74  of bushing insert  12  is received within bushing port  40  of arrester  10  in a telescoping manner. Probe  36  engages a female contact assembly  78  of bushing insert  12 , thereby forming an electrical connection between arrester  10  and bushing  12 . 
     During fault conditions, the overload of current results in the generation of gas and heat as the internal MOV blocks  54  of module  34  vaporize. This pressurized gas fills the inner cavities of arrester  10  until rupture occurs. The weak section  20  of shank portion  18  provides a controlled vent or rupture of the gases since the weak section will rupture first, thereby substantially preventing random ruptures in the arrester  10 . By disposing weak section  20  at first lateral side  68  of shank portion  18  opposite and remote from interface portion  16  and bushing insert  12 , arrester  10  is generally prevented from separating from bushing insert  12  because the force of the internal gases through weak section  20  tends to push arrester  10  toward bushing insert  12 , and the occurrence of ruptures near or towards bushing insert  10  are substantially eliminated since weak section  20  will always rupture first. 
     While a particular embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.