Patent Publication Number: US-2010118463-A1

Title: Surge arrester with mounting bracket

Description:
FIELD 
     The present invention relates to a surge arrester and more particularly to a surge arrester having a disconnector that disconnects the surge arrester from ground in the event of a failure of the surge arrester. 
     BACKGROUND 
     Surge arresters are devices arranged to protect other electrical equipment, usually in the distribution and supply of electric power, from an excessively high and usually damaging electric voltage caused, for example, by a lightning strike. The surge arrester is electrically connected between equipment at high voltage and earth potential (i.e., ground) and is electrically insulating in the absence of the overvoltage, such that electrical current travels normally through the distribution network. On occurrence of an overvoltage condition, the surge arrester becomes conductive so as to safely divert the consequent current to ground. 
     In some instances, a lightning strike may occur sufficiently close to a surge arrester such that the electrical power that flows through the arrester is so intense as to damage or destroy it. In less extreme cases, the damage may result in the formation of a continuous current path to ground, i.e., a fault. A surge arrester can also be subject to other fault conditions. 
     Disconnectors are sometimes used to isolate a failed arrester from the applied voltage or from ground. Upon detection of a predetermined fault condition, usually a flow of a minimum current for a minimum time, the disconnector is arranged to physically separate from the surge arrester. This separation interrupts the path to ground, and usually occurs by explosive separation. 
     During installation or repair of a surge arrester, a lineman may apply excessive torque to the disconnector when attaching it to the surge arrester. This can result in the disconnector breaking and/or its premature failure. 
     What is needed is a way to shield the disconnector in a surge arrester from inadvertent over-torquing by an installer without impeding the disconnector&#39;s ability to function in the event of an explosive separation of the disconnector from the arrester during an overvoltage condition. 
     SUMMARY 
     According to an exemplary embodiment of the invention, an arrester assembly is disclosed. The arrester assembly includes a surge arrester, a disconnector attached to the surge arrester and a mounting bracket disposed intermediate the surge arrester and the disconnector. The mounting bracket has a first end and a second end, the first end arranged and disposed for attaching the mounting bracket to a support. The mounting bracket further has a collar extending away from the second end of the surge arrester. The collar surrounds the disconnector and provides a mechanical interface to resist rotational movement of the disconnector with respect to the mounting bracket, such as in the presence of an external torsional force. 
     According to another exemplary embodiment of the invention, an arrester assembly includes a surge arrester, a disconnector and a mounting bracket. The disconnector has an upper fitting, a lower fitting having a ground stud extending away from the lower fitting, and a housing connecting the upper fitting to the lower fitting. The disconnector is attached to the surge arrester via the upper fitting. The mounting bracket is intermediate the surge arrester and the disconnector. The mounting bracket has a first end and a second end, the first end arranged and disposed for attaching the mounting bracket to a support. The mounting bracket also has a collar at the second end extending away from the surge arrester and surrounding the disconnector. The lower disconnector fitting has a head having between four to eight planar faces. The mounting bracket collar has an inner passage having a first width defined by a first set of inner collar walls and a second width defined by a second set of inner collar walls. The second set of inner collar walls are configured complementary to the lower fitting head. The collar provides a mechanical interface to resist rotational movement of the disconnector with respect to the mounting bracket but does not restrain axial travel of the disconnector. 
     An advantage of certain exemplary embodiments described herein is that the collar of the mounting bracket reduces the likelihood that a lineman or other individual installing a surge arrester will apply excessive torque to the disconnector and/or if applied, the excess torque is distributed to the mechanically more robust mounting bracket. 
     An advantage of certain other exemplary embodiments is that the collar surrounds, but does not restrain, the disconnector and thereby does not impair proper operation of the surge arrester in the event of an explosive separation of the disconnector from the arrester. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of exemplary embodiments, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a surge arrester mounted using a mounting bracket. 
         FIG. 2  illustrates a perspective view of a mounting bracket in accordance with an exemplary embodiment of the invention. 
         FIG. 3  illustrates a top view of the mounting bracket of  FIG. 2 . 
         FIG. 4  illustrates a partial cross section of a surge arrester mounted to the mounting bracket of  FIG. 2 . 
     
    
    
     Where like parts appear in more than one drawing, it has been attempted to use like reference numerals for clarity. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Referring to  FIG. 1 , an arrester assembly  5  is shown including a surge arrester  100 , a mounting bracket  200 , and a disconnector  300 . The surge arrester  100  includes one or more varistor elements (not shown) contained within a housing  115 . The housing  115  can include one or more sheds  117  for increasing the surface leakage current path. The housing  115 , including any sheds  117 , is made of a tracking resistant material, such as silicone or appropriately formulated polyolefin polymers and copolymers like ethylene-vinyl acetate copolymer (EVA), ethylene-propylene-diene monomer terpolymer (EPDM), and ethylene-propylene rubber (EPR) all by way of example only. 
     The arrester  100  also has conductive end caps  110 ,  120  at opposite ends of the arrester  100 . The end caps  110 ,  120  make electrical contact with the varistors within the arrester  100 , either directly or indirectly such as via conductive terminals within the housing  115 . The conductive terminals (also not shown) are positioned at opposing ends of the varistors. A line stud  105  is provided from the first end cap  110  for electrically connecting the arrester  100  to a distribution network. 
     The disconnector  300  includes an upper fitting  320  that attaches to the arrester  100  in any suitable manner, for example, via a connection stud (not shown) that permits the disconnector to be screwed to the arrester  100 . The disconnector  300  also includes a lower fitting  350  having a conductive ground stud  310  that provides a connection from the arrester  100  to ground. A housing  340  connects the upper and lower fittings  320 ,  350  and is positioned therebetween. The housing  340  is sized to protect functional components of the disconnector  300  that may include a resistor and one or more electrodes (not shown) that form a conductive pathway between the upper and lower fittings  320 ,  350  (and thus from the arrester  100  to the ground stud  310 ). The housing  340  is preferably constructed of a phenolic or other plastic and has a wall thickness such that upon the occurrence of a significant fault, the pressure of discharge gases formed within the arrester  100  causes the housing  340  to break apart. As a result of the breakage, the housing  340  no longer has sufficient strength to connect the upper fitting  320  to the lower fitting  350 . This allows the lower fitting  350  and ground stud  310  to fall away (typically by gravity), thereby separating the arrester  100  from the ground connection to remove the fault. The wall thickness of the housing  340  to achieve this desired result generally corresponds to a housing  340  that can resist up to about 20 foot-pounds of torque without cracking. In another embodiment the housing  340  may resist up to about 45 foot-pounds of torque without cracking. Other embodiments may be designed for more or less torque depending on the application. 
     Preferred surge arresters and disconnectors for use in exemplary embodiments of the invention are those which form the DAR and DAH IEEE Bowthorpe EMP arrester systems available from Tyco Electronics Corporation of Berwyn, Pa. 
     The mounting bracket  200  is constructed of an insulating material and is disposed intermediate the arrester  100  and the disconnector  300 . In one embodiment, the bracket  200  is constructed of a glass-filled polyester resin. The bracket has a first end  210  for mounting to a support, such as an electric pole, for example, and a second end  212  to support the arrester  100  which sits on an upper surface of the second end  212 . An aperture (not shown) in the second end  212  permits the upper fitting  320  to be connected to the surge arrester  100 . 
     Turning to  FIG. 2 , in one embodiment the mounting bracket  200  includes a collar  220  extending away from a lower surface of the second end  212  (i.e., the surface opposite that which supports the surge arrester  100 ). The collar  220  extends to surround the disconnector  300 , and more particularly to entirely surround the upper fitting  320  and disconnector housing  340  and to at least partially surround the lower fitting  350  (best seen in  FIG. 4 ). The collar  220  provides a mechanical interface to resist rotation of the disconnector  300  when disposed within the collar  220 . 
       FIG. 3  illustrates a bottom view of the bracket  200  looking into the collar  220 . The collar  220  has an inner passage  240  defined by inner collar walls  245  and which passes entirely through the bracket to permit the upper fitting  320  to be attached to the surge arrester  100 . In a preferred embodiment, the inner passage has a second width defined by second inner collar walls  235 , the second width being wider than that defined by the first set of inner collar walls  245 . The shape of the inner collar walls  235 ,  245  may be complementary to the disconnector  300 . Preferably, the second inner collar walls  235  are of a shape and size complementary to the lower disconnector fitting  350  in a nut and socket relationship. 
     When used in conjunction with a collar  220  having inner collar walls  235  complementary to the lower disconnector fitting  350 , the nut and socket relationship allows the mounting bracket  200  to be used as a tool to attach the disconnector  300  to the surge arrester  100 . The disconnector  300  can be disposed inside the collar  220  of the mounting bracket  200 . The mounting bracket  200  and disconnector  300  can then together be rotated in order to screw the disconnector  300  to the surge arrester  100 . In one embodiment, the lower disconnector fitting  350  ( FIG. 4 ) includes a head having between four to eight planar faces  330  and preferably has six planar faces (i.e., is a hex head). Using fewer than four or greater than eight planar faces may result in undesirable slipping of the mounting bracket  200  with respect to the disconnector  300  during assembly. 
     If excessive torque is applied to the lower disconnector fitting  350  during installation, the torsional strain and cantilever stress is transferred to the collar  220  and not the disconnector housing  340 , as rotation of the lower disconnector fitting  350  is opposed by the complementary inner collar walls  235 . The collar  220  has a higher resistance to breaking (typically  50  foot pounds of torque or greater) than the disconnector housing  340 . Thus, the risk that the disconnector housing  340  will be cracked during installation and that the disconnector  300  will fail prematurely is decreased. 
       FIG. 4  illustrates a completed arrester assembly  5  using the mounting bracket  200  of  FIGS. 2 and 3 , which includes the collar  220  surrounding the disconnector  300 . Here, the lower half of the arrester  100  is shown partially cut away, illustrating a lower terminal  170  and a varistor element  180  within the arrester housing  115 .  FIG. 4  further illustrates that a clamp  42  may be provided on the ground stud  310  that can be tightened, for example, using a nut  44  or other fastener to retain a cable and thereby complete the ground connection to the surge arrester  100 . Similarly, a clamp  42  may also be provided on the line stud  105  for attaching a power conductor to the surge arrester  100 . 
     Preferably, while the collar  220  surrounds the disconnector  300 , the collar  220  does not restrain axial travel of the disconnector  300 . That is, the mounting bracket  200  does not support the disconnector  300  such that but for the disconnector  300  being attached to the surge arrester  100 , the disconnector would be free to fall out of the collar  220 . 
     In many conventional surge arrester designs, the disconnector is a single unit containing an explosive shell and filled with epoxy. In the event of a lightning strike, the explosive shell is detonated and the disconnector is forced away from the arrester to disconnect or isolate the fault from the line. Conversely, disconnectors ordinarily used with exemplary embodiments of the invention do not employ an embedded explosive and achieve separation of the disconnector  300  primarily from the explosion of the fault itself, which causes the housing  340  to break as described above. To ensure the ground connection is removed, the disconnector  300  can more reliably separate from the arrester  100  by allowing the lower fitting  350  to fall away completely unrestrained by the mounting bracket collar  220 . 
     While the foregoing specification illustrates and describes exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.