Patent Publication Number: US-8982526-B2

Title: Polymer surge arrester

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2012-098590, filed on Apr. 24, 2012; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a polymer surge arrester. 
     BACKGROUND 
     In a power system, a surge arrester is provided to protect facilities from abnormal voltage (surge) due to thunderbolt. The surge arrester has a nonlinear resistor, for example, containing zinc oxide as a main component. The nonlinear resistor is insulative when normal voltage acts, and becomes conductive by decreasing in resistance value when abnormal voltage acts. 
     Among surge arresters, a polymer surge arrester is configured such that an electrode is placed at each of an upper end and a lower end of a stack made by stacking a plurality of nonlinear resistors and a plurality of insulating rods are arranged side by side around the outer peripheral surface of the nonlinear resistors. Further, in the polymer surge arrester, an outer skin made of insulating resin covers the outer peripheral surface of the stack of the nonlinear resistors around which the insulating rods are arranged. The insulating rod is formed using, for example, FRP (Fiber Reinforced Plastics), and the outer skin is formed using, for example, silicone rubber. 
     Since the polymer surge arrester is lower in mechanical strength than an insulator surge arrester housing the nonlinear resistors in a porcelain container and therefore needs to be improved in mechanical strength. 
     The polymer surge arrester has the outer skin formed of an insulating resin with low mechanical strength. Therefore, the polymer surge arrester needs to secure the mechanical strength of the whole polymer surge arrester by the insulating rods and the nonlinear resistors higher in mechanical strength than the outer skin. 
     However, it is sometimes not easy to sufficiently improve the mechanical strength in the polymer surge arrester. For example, when fastening is realized by attaching a male screw part provided at the insulating rod to a female screw part of the electrode, the male screw part provided at the insulating rod may break when a bending stress is applied to the polymer surge arrester. Further, the fastening may be loosened because thermal processing is performed when forming the outer skin is formed by molding the insulating resin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view illustrating the whole polymer surge arrester according to an embodiment. 
         FIGS. 2A and 2B  are views illustrating a nonlinear resistor in the polymer surge arrester according to the embodiment. 
         FIGS. 3A and 3B  are views illustrating a metal plate in the polymer surge arrester according to the embodiment. 
         FIGS. 4A and 4B  are views illustrating an electrode in the polymer surge arrester according to the embodiment. 
         FIGS. 5A and 5B  are views illustrating a fixed plate in the polymer surge arrester according to the embodiment. 
         FIGS. 6A and 6B  are views illustrating a spacer in the polymer surge arrester according to the embodiment. 
         FIG. 7  is a view illustrating a fixing screw in the polymer surge arrester according to the embodiment. 
         FIG. 8  is a sectional view illustrating a manufacturing method of the polymer surge arrester according to an embodiment. 
         FIG. 9  is a sectional view illustrating the manufacturing method of the polymer surge arrester according to the embodiment. 
         FIG. 10  is a sectional view illustrating the manufacturing method of the polymer surge arrester according to the embodiment. 
         FIG. 11  is a chart presenting the result of a bending test in the polymer surge arrester according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A polymer surge arrester of this embodiment has metal plates placed at an upper end face and a lower end face of a nonlinear resistor. Electrodes are place on the upper end face and the lower end face of the nonlinear resistor via the metal plates. A plurality of insulating rods are placed at side surfaces of the nonlinear resistor and the metal plates, and an upper end portion and a lower end portion of each of the insulating rods are inserted into holes formed in the electrodes. A spacer is inserted between an inner peripheral surface of the hole and an outer peripheral surface of the insulating rod inside the hole of the electrode, and a fixing screw is attached to the hole of the electrode. A double-ended bolt couples the metal plate and the electrode together. The double-ended bolt has a first screw part and a second screw part opposite in a fastening direction to the first screw part which are provided on a same axis. The first screw part is attached to the metal plate, and the second screw part is attached to the electrode. 
     Embodiments will be described with reference to the drawings. 
     [A] Configuration 
       FIG. 1  is a sectional view illustrating the whole polymer surge arrester according to an embodiment. 
     As illustrated in  FIG. 1 , a polymer surge arrester  1  has nonlinear resistors  11 , metal plates  21 ,  22 , electrodes  31 ,  32 , double-ended bolts  41 ,  42 , a fixed plate  51 , insulating rods  61 , spacers  71 , fixing screws  81 , and an outer skin  201 . 
       FIG. 2A  to  FIG. 7  are views illustrating parts constituting the polymer surge arrester according to this embodiment.  FIGS. 2A and 2B  illustrate the nonlinear resistor  11 ,  FIGS. 3A and 3B  illustrate the metal plate  21 ,  FIGS. 4A and 4B  illustrate the electrode  31 ,  FIGS. 5A and 5B  illustrate the fixed plate  51 ,  FIGS. 6A and 6B  illustrate the spacer  71 , and  FIG. 7  illustrates the fixing screw  81 .  FIG. 2A  to  FIG. 6A  illustrate enlarged upper surfaces, and  FIG. 2B  to  FIG. 6B  illustrate enlarged lateral cross-sections. Further,  FIG. 7  illustrates an enlarged side surface of the fixing screw  81 . 
     The parts constituting the polymer surge arrester  1  will be described below in order using  FIG. 2A  to  FIG. 7  together with  FIG. 1 . 
     [A-1] Regarding the Nonlinear Resistor  11   
     A plurality of the nonlinear resistors  11  are stacked as illustrated in  FIG. 1 . One nonlinear resistor  11  is a disc-shaped sintered compact part containing zinc oxide as a main component as illustrated in  FIG. 2A ,  FIG. 2B , and electrode parts  12  made of metal such as aluminum are provided on upper and lower flat surfaces of the nonlinear resistor  11  and an insulating layer  13  is provided on the side surface (cylindrical surface) thereof. The nonlinear resistor  11  is insulative when normal voltage acts, and becomes conductive by decreasing in resistance value when abnormal voltage higher than the normal voltage acts. 
     [A-2] Regarding the Metal Plates  21 ,  22   
     The metal plates  21 ,  22  are placed respectively on the upper end face and the lower end face of a stack made by stacking the plurality of nonlinear resistors  11  as illustrated in  FIG. 1 . The metal plate  21 ,  22  has the same outer diameter as that of the nonlinear resistor  11 . 
     The one metal plate  21  of the pair of metal plates  21 ,  22  which is placed on the upper end face is cylindrical and provided with an opening  21 K at its center as illustrated in  FIG. 3A ,  FIG. 3B . 
     As illustrated in  FIG. 1 , the opening  21 K of the metal plate  21  passes in the stacking direction of the nonlinear resistors  11 . In addition, the opening  21 K of the metal plate  21  is formed with a female screw, on the inner peripheral surface, to which a male screw of a left screw part  411  of the later-described double-ended bolt  41  is screwed. 
     Though the enlarged view is omitted, the other metal plate  22  placed on the lower end face is formed similarly to the one metal plate  21  placed on the upper end face. More specifically, the other metal plate  22  placed on the lower end face is cylindrical and provided with an opening  22 K at its center. The opening  22 K of the metal plate  22  is formed with a female screw, on the inner peripheral surface, to which a male screw of a left screw part  421  of the double-ended bolt  42  is screwed as illustrated in  FIG. 1 . 
     [A-3] Regarding the Electrodes  31 ,  32   
     The electrodes  31 ,  32  are placed on the upper end face and the lower end face of the stack made by stacking the plurality of nonlinear resistors  11  via the metal plates  21 ,  22  respectively as illustrated in  FIG. 1 . The electrode  31 ,  32  has an outer diameter larger than that of the nonlinear resistor  11 , and has a recessed part  31 TR,  32 TR formed in the other surface located on the opposite side to one surface in contact with the metal plate  21 ,  22 . 
     The one electrode  31  of the pair of electrodes  31 ,  32  which is placed on the upper end face is cylindrical as illustrated in  FIG. 4A ,  FIG. 4B . The electrode  31  has an opening  31 K at the center of the recessed part  31 TR. In addition, a plurality of holes  31 H are arranged at regular intervals around the opening  31 K provided at the center in the recessed part  31 TR of the electrode  31 . 
     As illustrated in  FIG. 1 , the opening  31 K provided at the center of the electrode  31  passes in the stacking direction of the nonlinear resistors  11 . In addition, to the opening  31 K, a right screw part  412  of the double-ended bolt  41  is attached. More specifically, the opening  31 K is formed with a female screw on the inner peripheral surface on the side of the surface in which the recessed part  31 TR is formed in the electrode  31  as illustrated in  FIG. 4B , and a male screw of the right screw part  412  of the later-described double-ended bolt  41  is screwed to the female screw. 
     The plurality of holes  31  provided at the periphery in the electrode  31  pass in the stacking direction of the nonlinear resistors  11  as illustrated in  FIG. 1 . After the insulating rod  61  is inserted into the hole  31 H and the spacer  71  is inserted to the outer periphery of the insulating rod  61  therein, the fixing screw  81  is attached to the outer periphery of the insulating rod  61 . 
     Specifically, the hole  31 H provided at the periphery in the electrode  31  has a first cylindrical part  311 , a second cylindrical part  312  (cylindrical part), and a tapered part  313  as illustrated in  FIG. 4B . 
     The first cylindrical part  311  is formed on the side of the surface opposite to the surface in which the recessed part  31 TR is formed in the electrode  31  as illustrated in  FIG. 4B . 
     The second cylindrical part  312  has an inner diameter larger than that of the first cylindrical part  311 , on the side of the surface in which the recessed part  31 TR is formed in the electrode  31  as illustrated in  FIG. 4B . The second cylindrical part  312  is formed with a female screw, on the inner peripheral surface, to which a male screw of the fixing screw  81  is screwed in a state that the insulating rod  61  is inserted therein as illustrated in  FIG. 1 . 
     The tapered part  313  is formed between the first cylindrical part  311  and the second cylindrical part  312  as illustrated in  FIG. 4B . The tapered part  313  is conical and formed to have an inner diameter increasing from the side of the first cylindrical part  311  to the side of the second cylindrical part  312 . Specifically, in the tapered part  313 , an inner diameter on the first cylindrical part  311  side is substantially the same as that of the first cylindrical part  311  and an inner diameter on the second cylindrical part  312  side is smaller than that of the second cylindrical part  312 . The tapered part  313  is formed such that, for example, a height H is 15 mm or more. Further, as illustrated in  FIG. 1 , the spacer  71  is fitted into the tapered part  313  with the insulating rod  61  being inserted therein. 
     Though the enlarged view is omitted, the other electrode  32  placed on the lower end face is formed similarly to the one electrode  31  placed on the upper end face. In other words, the electrode  32  has the opening  32 K at the center of the recessed part  32 TR. In addition, a plurality of holes  32 H are arranged at regular intervals around the opening  32 K provided at the center in the recessed part  32 TR of the electrode  32 . Further, each of the plurality holes  32 H has a first cylindrical part  321 , a second cylindrical part  322 , and a tapered part  323 . 
     [A-4] Regarding the Double-Ended Bolts  41 ,  42   
     The double-ended bolts  41 ,  42  fasten both the metal plates  21 ,  22  and the electrodes  31 ,  32  as illustrated in  FIG. 1 . 
     The double-ended bolts  41 ,  42  have left screw parts  411 ,  421  (first screw parts), right screw parts  412 ,  422  (second screw parts), and middle parts  413 ,  423 . In the double-ended bolt  41 ,  42 , the left screw part  411 ,  421  and the right screw part  412 ,  422  are arranged on the same axis via the middle part  413 ,  423  along the stacking direction of the nonlinear resistors  11 . 
     The left screw parts  411 ,  421  are attached inside the openings  21 K,  22 K provided at the centers of the metal plates  21 ,  22 . The left screwparts  411 ,  421  are rotated in the counter-clockwise direction to move to the back (in the direction of the nonlinear resistor  11  in  FIG. 1 ) inside the openings  21 K,  22 K. 
     The right screw parts  412 ,  422  are attached inside the openings  31 K,  32 K provided at the centers of the electrodes  31 ,  32 . The right screw parts  412 ,  422  are rotated in a direction opposite to that of the left screw parts  411 ,  421 , that is, the clockwise direction to move to the back (in the direction of the nonlinear resistor  11  in  FIG. 1 ) inside the openings  31 K,  32 K. 
     Other than the above, the double-ended bolts  41 ,  42  are provided with fasten holes  414 ,  424  in top surfaces on the sides on which the right screw parts  412 ,  422  are provided. The fasten holes  414 ,  424  are, for example, hexagon holes into which a tool such as a hexagonal wrench is inserted when the double-ended bolts  41 ,  42  are rotated to adjust the fastening between the metal plates  21 ,  22  and the electrodes  31 ,  32 . 
     [A-5] Regarding the Fixed Plate  51   
     The fixed plate  51  is interposed at a predetermined position of the stack of the nonlinear resistors  11  as illustrated in  FIG. 1 . The fixed plate  51  here is placed near the center in the stacking direction of the stack of the nonlinear resistors  11  as an example. 
     As illustrated in  FIG. 5A ,  FIG. 5B , the fixed plate  51  has an insulating part  511  and a conductive part  512 . 
     The insulating part  511  is in a ring shape as illustrated in  FIG. 5A ,  FIG. 5B . The conductive part  512  is in a disc shape and provided at an inner peripheral portion of the insulating part  511 . 
     As illustrates in  FIG. 1 , the insulating part  511  has an outer diameter larger than that of the nonlinear resistor  11 , and the conductive part  512  has an outer diameter substantially the same as that of the nonlinear resistor  11 . The conductive part  512  is sandwiched between the nonlinear resistors  11  to electrically connect the plurality of nonlinear resistors  11 . 
     As illustrated in  FIG. 5A ,  FIG. 5B , in the insulating part  511 , a plurality of holes  51 H are arranged at regular intervals around the conductive part  512 . The plurality of holes  51 H pass in the stacking direction of the nonlinear resistors  11  as illustrated in  FIG. 1 , into which the insulating rods  61  are inserted. Each of the plurality of holes  51 H has an outer diameter substantially the same as that of the insulating rod  61 . 
     [A-6] Regarding the Insulating Rod  61   
     The insulating rod  61  is in a rod-shaped body and is disposed along the stacking direction of the nonlinear resistors  11  as illustrated in  FIG. 1 . The insulating rod  61  has a diameter of, for example, 10 mm or more, and is formed of FRP. 
     The insulating rod  61  is placed on the side surfaces (outer peripheral surfaces) of the nonlinear resistors  11  and the metal plates  21 ,  22 . The insulating rod  61  has an upper end portion and a lower end portion inserted into the holes  31 H,  32 H provided in the electrodes  31 ,  32 . In addition, the insulating rod  61  is inserted into the hole  51 H provided at the periphery of the fixed plate  51 . As is clear from  FIG. 1 , a predetermined number of insulating rods  61  are arranged at regular intervals around the outer peripheral surfaces of the stack of the nonlinear resistors  11  and the metal plates  21 ,  22 . 
     [A-7] Regarding the Spacer  71   
     The spacers  71  are placed inside the holes  31 H,  32 H provided at the periphery of the electrodes  31 ,  32  as illustrated in  FIG. 1 . The spacer  71  here intervenes between the inner peripheral surface of the tapered part  313 ,  323  and the outer peripheral surface of the insulating rod  61  inside the hole  31 H,  32 H. 
     As illustrated in  FIG. 6A ,  FIG. 6B , the spacer  71  has a first spacer part  711  and a second spacer part  712 . When the first spacer part  711  and the second spacer part  712  are combined together, a tubular body is formed. The tubular body made by combining the first spacer part  711  and the second spacer part  712  together is provided with a tapered part  71 T on one end of a cylindrical part  71 S. The tapered part  71 T is conical and has an outer diameter on the cylindrical part  71 S side that is the same as that of the cylindrical part  71 S and becomes smaller as it is separated more from the cylindrical part  71 S. 
     In other words, each of the first spacer part  711  and the second spacer part  712  has a cross-section of the tapered part  71 T in a wedge shape, and has a thickness on the cylindrical part  71 S side that is the same as that of the cylindrical part  71 S and becomes smaller as it is separated more from the cylindrical part  71 S. 
     [A-8] Regarding the Fixing Screw  81   
     The fixing screw  81  is placed inside the hole  31 H,  32 H provided at the periphery of the opening  31 K,  32 K in the electrode  31 ,  32  as illustrated in  FIG. 1 . The fixing screw  81  has a through hole  81 H formed therein, and the insulating rod  61  is inserted into the through hole  81 H therein. 
     As illustrated in  FIG. 7 , the fixing screw  81  has a head part  811  and a screw part  812 . In the fixing screw  81 , the head part  811  is, for example, in a regular hexagonal column shape (bolt shape) and fastened by a fastening tool placed thereon. The screw part  812  has a male screw formed on the outer peripheral surface and attached to the second cylindrical part  312  of the hole  31 H provided in the electrode  31 . 
     Though details will be described later, the fixing screw  81  pushes the spacer  71  with a predetermined tightening torque inside the hole  31 H,  32 H of the electrode  31 ,  32  to fix the insulating rod  61  to the electrode  31 ,  32 . 
     [A-9] Regarding the Outer Skin  201   
     The outer skin  201  covers the outer peripheral surface of the stack of the nonlinear resistors  11  for which the insulating rods  61  are disposed as illustrated in  FIG. 1 . The outer skin  201  is formed by molding an insulating resin such as a silicone rubber. 
     [B] Manufacturing Method 
       FIG. 8  to  FIG. 10  are sectional views illustrating a manufacturing method of the polymer surge arrester according to an embodiment. 
     When manufacturing the above-described polymer surge arrester  1 , both of the metal plate  21  and the electrode  31  are combined together first with the double-ended bolt  41  as illustrated in  FIG. 8 . 
     Specifically, the right screw part  412  of the double-ended bolt  41  is screwed into the opening  31 K provided at the center of the electrode  31 , whereby the double-ended bolt  41  is attached to the electrode  31 . Then, the left screw part  411  of the double-ended bolt  41  is screwed into the opening  21 K of the metal plate  21 , whereby the double-ended bolt  41  is attached to the metal plate  21 . In this manner, a combined body of the metal plate  21  and the electrode  31  is formed. 
     Though the combined body of the metal plate  21  and the electrode  31  which is placed on the upper end side in the polymer surge arrester  1  as illustrated in  FIG. 1  is illustrated in  FIG. 8 , a combined body of the metal plate  22  and the electrode  32  which is placed on the lower end side is assembled similarly to the above. 
     Then, the plurality of insulating rods  61  are attached to the combined body of the metal plate  22  and the electrode  32  which is placed on the lower end side as illustrated in  FIG. 1 . Then, in a space surrounded by the plurality of insulating rods  61 , the plurality of the nonlinear resistors  11  are stacked. In this event, the fixed plate  51  is appropriately interposed between the plurality of nonlinear resistors  11 . Then, the combined body of the metal plate  21  and the electrode  31  which is to be placed on the upper end side is attached to the plurality of insulating rods  61 . 
     As illustrated in  FIG. 9 , when attaching the combined body of the metal plate  21  and the electrode  31  to the plurality of insulating rods  61 , the spacers  71  and the fixing screws  81  are used. 
     Specifically, the spacer  71  is inserted into the hole  31 H of the electrode  31  into which the insulating rod  61  has been inserted as illustrated in  FIG. 9 . Here, the spacer  71  is inserted from the side of the surface in which the second cylindrical part  312  is provided in the hole  31 H of the electrode  31 , whereby the tapered part  71 T of the spacer  71  is interposed between the inner peripheral surface of the tapered part  313  of the hole  31 H and the outer peripheral surface of the insulating rod  61 . 
     Thereafter, as illustrated in  FIG. 9 , the fixing screw  81  is attached inside the hole  31 H of the electrode  31 . Here, the fixing screw  81  is screwed from the side of the surface in which the second cylindrical part  312  is provided in the hole  31 H of the electrode  31  to attach the fixing screw  81  to the electrode  31 . 
     When attaching, the fixing screw  81  advances to the side (a black arrow in  FIG. 9 ) of the tapered part  313  of the hole  31 H formed in the electrode  31  in the state that the insulating rod  61  is inserted in the through hole  81 H formed therein. Then, the fixing screw  81  pushes the spacer  71  placed at the tapered part  313  of the hole  31 H with a predetermined tightening torque. Thus the tapered part  71 T of the spacer  71  is pushed into the tapered part  313  of the hole  31 H, so that the spacer  71  compresses and tightens the insulating rod  61  from the periphery. Along with this, a tensile load is applied on the insulating rod  61  in its axial direction. Therefore, the electrode  31  and the insulating rod  61  are strongly fixed by the frictional force with respect to the spacer  71 . 
     Though illustration is omitted, the plurality of insulating rods  61  are attached to the combined body of the metal plate  22  and the electrode  32  which is placed on the lower end side by the method similar to the above. 
     Then, the double-ended bolt  41  is tightened (a downward arrow in  FIG. 10 ) as illustrated in  FIG. 10 . 
     As described above, both of the metal plate  21  and the electrode  31  are coupled together by the double-ended bolt  41 . The left screw part  411  of the double-ended bolt  41  is attached to the metal plate  21 . In contrast to this, the right screw part  412  of the double-ended bolt  41  is attached to the electrode  31 . 
     Therefore, by inserting the fastening tool into the fasten hole  414  provided in the double-ended bolt  41  and tightening the double-ended bolt  41  (rotating the right screw part  412  in the clockwise direction), the metal plate  21  and the electrode  31  move in directions (both arrows in  FIG. 10 ) in which they are separated from each other in the axial direction of the double-ended bolt  41 . As a result of this, a compressive load is applied on the plurality of nonlinear resistors  11  in the axial direction of the double-ended bolt  41  (the stacking direction), and a tensile load is applied on the insulating rods  61  in the axial direction. Therefore, the stack of the electrode  31 , the metal plate  21 , the insulating rods  61  and the plurality of nonlinear resistors  11  is strongly fixed in the stacking direction (the axial direction of the polymer surge arrester). 
     Though illustration is omitted, for the combined body of the metal plate  22  and the electrode  32  which is placed on the lower end side, the double-ended bolt  42  is also tightened by the method similar to the above. 
     Then, as illustrated in  FIG. 1 , the outer peripheral surface of the stack of the nonlinear resistors  11  in which the insulating rods  61  are disposed is covered with the outer skin  201 . Here, the outer skin  201  is provided by molding an insulating resin such as a silicone rubber. 
     By providing the parts as described above, the polymer surge arrester  1  is completed. 
     [C] Bending Test Result 
       FIG. 11  is a chart presenting the result of the bending test in the polymer surge arrester according to the embodiment.  FIG. 11  presents the result of a bending fracture value of an internal element provided inside the outer skin  201  in the polymer surge arrester  1 . Further, a case of a polymer surge arrester in which a male part provided in the insulating rod is attached and fastened to a female part of the electrode is taken as a comparative example. 
     The bending test was carried out by the following test method. First, (the internal element of) the surge arrester being a device under test was horizontally placed and its one end was strongly supported. Thereafter, force was applied to the other end at a certain rate in its vertical direction. Concurrently therewith, the internal element of the surge arrester was observed, and the force when abnormality such as crack or the like was recognized in any portion thereof was regarded as the fracture value. 
     As illustrated in  FIG. 11 , this embodiment is larger in the bending fracture value than the comparative example. Specifically, the bending fracture value in this embodiment when the tightening torque of the fixing screw  81  is 45 N·m is four times that of the comparative example, and the bending fracture value in this embodiment when the tightening torque of the fixing screw  81  is 110 N·m is much larger than that of comparative example. 
     In observation of the appearance of the fracture, when the tightening torque of the fixing screw  81  was 45 N·m, slippage occurred in the insulating rod  61  due to the bending load. On the other hand, when the tightening torque of the fixing screw  81  was 110 N·m, the insulating rod  61  was in the state that fibers of FRP were separated from each other. It was found from the result that the fibers of FRP were hard to fracture because the male part as in the comparative example was not formed in the insulating rod  61  in this embodiment, and that sufficient bending strength was able to be secured by the tensile force of the insulating rod  61 . 
     Further, since the plurality of insulating rods  61  are strongly fixed to the electrode  31  in this embodiment, the tensile force or the compression force is applied on the plurality of insulating rods  61  when the bending load is applied. Therefore, the mechanical strength can be improved as the whole polymer surge arrester. 
     In addition to the above, the bending test was carried out for the case where the size of the double-ended bolt  41 ,  42  was M12 and M20. As a result, the displacement amount at application of the bending load in the case of M20 was ⅓ of that in the case of M12. Along with this, the strength of the internal element in the case of M20 was 1.5 times that in the case of the M12. 
     [D] Conclusion 
     As described above, the tapered parts  313 ,  323  are formed between the first cylindrical parts  311 ,  321  and the second cylindrical parts  312 ,  322  in the holes  31 H,  32 H of the electrode  31 ,  32  in this embodiment. The tapered part  313 ,  323  has an inner diameter smaller on the side of the nonlinear resistor  11  than on the side of the second cylindrical part  312 ,  322 . The spacer  71  includes the tapered part  71 T having an outer diameter smaller on the side of the nonlinear resistor  11  than on the second cylindrical part  312 ,  322 , and the tapered part  71 T of the spacer  71  is fitted into the tapered part  313 ,  323  of the hole  31 H,  32 H of the electrode  31 ,  32 . The fixing screw  81  is attached to the cylindrical part  312 ,  322  of the hole  31 H,  32 H of the electrode  31 ,  32 , and pushes the spacer  71  to the side of the nonlinear resistor  11  with the predetermined tightening torque inside the hole  31 H,  32 H of the electrode  31 ,  32 . This causes the fixing screw  81  to fix the insulating rod  61  to the electrode  31 ,  32 . 
     Therefore, the tapered part  71 T of the spacer  71  is pushed into the tapered part  313  of the hole  31 H in this embodiment, so that the spacer  71  can compress and tighten the insulating rod  61  from its periphery. Accordingly, the plurality of insulating rods  61  can be strongly fixed to the electrodes  31 ,  32  and therefore can improve the mechanical strength of the polymer surge arrester  1  in this embodiment. 
     Further, in this embodiment, the insulating rod  61  is not subjected to screw processing on the outer peripheral surface and does not have the male screw part as in the comparative example. Accordingly, fracture of the male screw part never occurs in the insulating rod  61  in this embodiment, and therefore the mechanical strength of the polymer surge arrester  1  can be improved by the tensile strength of the insulating rod  61 . 
     In this embodiment, the metal plate  21 ,  22  and the electrode  31 ,  32  are coupled together by the double-ended bolt  41 ,  42 . In the double-ended bolt  41 ,  42 , the left screw part  411 ,  421  (first screw part) and the right screw part (second screw part) opposite in the fastening direction to the left screw part  411 ,  421  are arranged on the same axis. The metal plate  21 ,  22  is provided with the opening  21 K,  22 K to which the left screw part  411 ,  421  is attached. In contrast, the electrode  31 ,  32  is provided with the opening  31 K,  32 K to which the right screw part  412 ,  422  is attached. Further, the double-ended bolt  41 ,  42  is provided with the fasten hole  414 ,  424  on the top surface on the side on which the right screw part  412 ,  422  is provided. 
     Therefore, the stack of the plurality of nonlinear resistors  11  can be strongly fixed in the stacking direction by the simple work of rotating the double-ended bolts  41 ,  42  as described above in this embodiment. In addition, the metal plates  21 ,  22  are coupled to the left screw parts  411 ,  421  of the double-ended bolts  41 ,  42  in this embodiment, so that when tightening the double-ended bolts  41 ,  42  after assembly, the rotation of the metal plates  21 ,  22  is suppressed by the friction with respect to the nonlinear resistors  11 . As a result, it is possible to suppress twist of the internal elements provided inside the outer skin  201  to improve the mechanical strength of the polymer surge arrester  1 . Further, by appropriately managing the tightening torque of the double-ended bolts  41 ,  42 , it is possible to ensure sufficient conduction of the nonlinear resistors  11  and prevent poor contact so as to improve the reliability of the polymer surge arrester  1 . 
     In this embodiment, the fixing screw  81  has the through hole  81 H formed therein into which the insulating rod  61  is inserted. Therefore, the fixing screw  81  can uniformly push the spacer  71  to the side of the nonlinear resistor  11  in this embodiment. Accordingly, this embodiment can uniformly and strongly fix the insulating rods  61  to the electrodes  31 ,  32  and therefore can improve the mechanical strength of the polymer surge arrester  1 . 
     [E] Modification Example 
     For the spacer  71  in the above embodiment, an asperity may be formed on the inner peripheral surface in contact with the outer peripheral surface of the insulating rod  61 . It is preferable to form the asperity on the inner peripheral surface of the spacer  71 , for example, by surface treatment such as the knurling or the sandblasting. By forming the asperity on the inner peripheral surface of the spacer  71 , the frictional force with respect to the outer peripheral surface of the insulating rod  61  can be improved. As a result, the bending fracture value can be improved and the occurrence of displacement at application of the bending load can be suppressed, thus leading to further improvement in mechanical strength. 
     Note that though the case where the plurality of nonlinear resistors  11  are stacked has been described in this embodiment, the structure is not limited to this. For example, when one nonlinear resistor  11  is provided, the parts may be the structured as described above. 
     Though the holes  31 H,  32 H of the electrodes  31 ,  32  have the tapered parts  313 ,  323  formed between the first cylindrical parts  311 ,  321  and the second cylindrical parts  312 ,  322  in the above embodiment, the structure is not limited to the above. The first cylindrical parts  311 ,  321  do not always need to be formed. 
     Though the spacer  71  is composed of two parts that are the first spacer part  711  and the second spacer part  712  in the above embodiment, the structure is not limited to the above. The spacer  71  may be composed of three or more parts. Further, the spacer  71  is not composed of a plurality of parts but may be composed of one part. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of the forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.