Patent Publication Number: US-6912112-B2

Title: Reduced capacitance and capacitive imbalance in surge protection devices

Description:
FIELD OF THE INVENTION 
   This invention is related to surge protection devices, and more particularly, overvoltage protection devices, that employ metal oxide varistors (MOVs) in station protectors, central office overvoltage protection devices, on-line overvoltage protection devices, and remote terminal overvoltage protection devices. In particular, this invention is related to the use of MOVs in overvoltage protection devices designed for digital subscriber line (DSL) transmissions, generically referred to as xDSL, such as very high data rate digital subscriber line (VDSL) communications. 
   BACKGROUND OF THE INVENTION 
   Conventional overvoltage protection devices typically use a gas tube, or gas discharge, surge arrestor as a primary means for diverting voltage surges from a signal line to ground. Examples of such devices are shown in U.S. Pat. Nos. 5,388,023, 5,500,782 and 5,880,919. Gas tubes dissipate energy by producing electrical arcing to ground. A gas of known dielectric strength is ionized when subjected to an electrical surge. One drawback of gas tubes, however, is that they typically exhibit a relatively slow response time and thus, may not be able to safely suppress fast rise time voltage surges. Metal oxide varistors (MOVs) have therefore been used as secondary protectors in back-up and interacting overvoltage protection devices. For example, in a conventional hybrid station protector, an MOV is electrically connected in parallel with the gas tube between each signal line. Although the gas tube can repeatedly dissipate voltage surges without damage, the response time of the MOV is faster than that of the gas tube. Therefore, the MOV can be relied upon to shunt fast rise time voltage surges to ground, while the parallel gas tube is relied upon to shunt sustained voltage surges, which might otherwise damage the MOV. 
   Overvoltage protection devices utilizing MOVs as secondary protectors have been successfully employed to protect conventional twisted-pair (i.e., “tip” and “ring”) telecommunications lines. Broadband communications operate at transmission frequencies of at least 1 megahertz, which is substantially higher than the frequencies traditionally employed over twisted-pair telephone lines. Presently, frequencies of about 30 megahertz are typically utilized for xDSL communications transmitted over twisted-pair telephone lines. Existing twisted-pair telephone lines, also referred to as outside plant wire, are typically CAT-3 grade or less and were not intended for high frequency performance when originally manufactured or installed. Although xDSL communications are possible over existing twisted-pair telephone lines, in many instances conventional overvoltage protection devices are inadequate. This is especially the case when existing twisted-pair telephone signal lines are used for higher frequency digital transmissions, such as VDSL. Even if only a small number of overvoltage protection devices perform inadequately, the cost of identifying and replacing the overvoltage protection devices that may be adequate for lower frequency xDSL transmissions, but inadequate for higher frequency xDSL transmissions, is significant. 
   The inadequate performance of some conventional overvoltage protection devices, such as station protectors utilized at customer premises for higher frequency xDSL communications, has been traced to the greater capacitance and the variability of the capacitance of the MOVs that are employed in the station protector. At higher frequencies, the greater capacitance and the variability of the capacitance results in unacceptable insertion loss, return loss, and longitudinal imbalance. It is well known that the capacitance can be reduced by utilizing MOVs having the same thickness, but a smaller diameter. Many conventional station protectors employ 5 mm diameter MOVs with symmetrical 3.8 mm electrodes instead of smaller MOVs to absorb additional energy without causing permanent damage. MOVs of this size have a capacitance of about 60 picofarads with a tolerance of about 20 percent. This relatively large tolerance is believed to be due to variability in the varistor material and thickness, and/or to the relative placement and size of the electrodes on opposite sides of the varistor material. Electrodes, which are intended to be identical on both sides of the varistor material, can in practice be laterally displaced relative to each other. The concentricity of the two electrodes can also vary. Uneven placement and varying concentricity of the electrodes on opposite sides of the varistor material means that the overlapping surface area of the electrodes can vary significantly between MOVs that are intended to be identical, thereby generating dissimilar electric fields that result in relatively high capacitive tolerance, variability or spread. The difference in the capacitance of the MOV between the tip conductor and ground and between the ring conductor and ground results in significant capacitance mismatch, referred to herein as capacitive imbalance, which can cause excessive signal loss (e.g., insertion loss and return loss) and longitudinal imbalance at the higher frequencies utilized for xDSL communications transmitted over twisted-pair telephone lines. 
   As previously mentioned, it would be possible to reduce the capacitance between a signal line and ground in a station protector if an MOV having a smaller diameter was employed. Given the same thickness, because the smaller diameter MOV inherently has electrodes with smaller overlapping surface areas, the smaller diameter MOV also has less capacitance. However, a smaller diameter MOV is not able to withstand the same sustained current as a larger 5 mm diameter MOV. Furthermore, substitution of the smaller diameter MOV would result in significant engineering, re-tooling and testing expense. Even if the desired reduction in capacitance could be achieved by substituting a smaller diameter MOV for the 5 mm MOV presently in use, there could still be an excessive capacitive imbalance between the tip conductor and ground and the ring conductor and ground. Accordingly, it would be preferable if both a reduction in capacitance and a reduction in the capacitive imbalance could be achieved without the need for extensive modifications to conventional station protectors. 
   It has been determined that station protectors will perform satisfactorily for higher frequency xDSL communications over twisted-pair telephone lines if the capacitance across the MOV in parallel with the gas tube is reduced to about 30 picofarads with a capacitive tolerance among the MOVs of about ±0.25 picofarads. The number of existing station protectors and other overvoltage protection devices which are incapable of adequate performance is excessive, at least in the aggregate. In particular, when MOVs having relatively large capacitance and large capacitive tolerance are employed in twisted-pair telephone lines, an unacceptable capacitive imbalance between the tip conductor and ground and the ring conductor and ground will be present in an excessive number of station protectors. The capacitive imbalance for such station protectors has been found to be up to about 5 picofarads. For xDSL communications, including VDSL, a capacitive imbalance of less than about 1.3 picofarads is desired. Accordingly, what is needed is an MOV that reduces the capacitance and capacitive imbalance in an overvoltage protection device while sustaining the same current without permanent damage to the MOV. Such an MOV would provide adequate performance in a station protector or other overvoltage protection device utilized on twisted-pair telephone lines that transmit higher frequency xDSL communications, such as VDSL. 
   SUMMARY OF THE INVENTION 
   With this invention, the capacitance and the capacitive imbalance between signal lines in a surge protection device can be reduced when metal oxide varistors (MOVs) are employed in the surge protection device to protect personnel and telecommunications equipment against voltage surges. A surge protection device in accordance with this invention is used between at least one signal line and ground. The surge protection device includes one or more MOVs electrically connected between the signal line and ground. In one embodiment, two or more MOVs are stacked electrically in series between the signal line and ground so that the signal line is suitable for use in higher frequency transmissions, such as xDSL communications, including VDSL. Stacking two or more MOVs will not result in a significantly smaller capacitance relative to a single MOV having a diameter and thickness substantially the same as the diameter and thickness of the stacked MOVs. However, the variability of the capacitance will be less among the stacked MOVs than among the single MOVs. This smaller variability of the capacitance among stacked MOVs will result in less capacitive imbalance when stacked MOVs are used in a surge protection device for protecting two or more signal lines. Stacked MOVs can also be employed in parallel with a gas discharge tube to form a hybrid station protector suitable for use at the network interface between twisted-pair (i.e., “tip” and “ring”) telephone lines and the customer premises. 
   In another embodiment, the surge protection device includes an MOV with asymmetrical electrodes spaced apart on opposite sides of a varistor material. As used herein the term “asymmetrical” means that the electrodes have different surface areas. Misalignment of the electrodes is therefore less likely, and thus, the variability in capacitance among the asymmetrical MOVs is reduced. Because the overlapped surface area of the electrodes corresponds substantially to the surface area of the smaller electrode, the MOV has a smaller capacitance, as well as less variability, than an MOV of the same thickness having symmetrical electrodes. In addition, MOVs with asymmetrical electrodes can be stacked in the same manner as MOVs with symmetrical electrodes, which results in yet a further reduction in both capacitance and capacitive tolerance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view of a surge protection device, and in particular, a station protector of the type that can be configured to employ metal oxide varistors (MOVs) according to this invention. 
       FIG. 2  is an exploded perspective view of a protector assembly comprising an asymmetrical MOV that can be employed in the station protector shown in FIG.  1 . 
       FIG. 3  is an exploded perspective view of an alternate protector assembly comprising two MOVs stacked electrically in series that can be employed in the station protector shown in FIG.  1 . 
       FIG. 4  is a perspective view of a fully assembled station protector according to this invention. 
       FIG. 5  is an enlarged perspective view showing the smaller of the two electrodes on an asymmetrical MOV that can be used in the protector assembly of FIG.  2 . 
       FIG. 6  is an enlarged side view showing both the smaller electrode and the larger electrode of the asymmetrical MOV shown in FIG.  5 . 
       FIG. 7  is an enlarged perspective view showing the larger of the two electrodes on the asymmetrical MOV shown in  FIGS. 5 and 6 . 
       FIG. 8  is an enlarged plan view of an asymmetrical MOV in which the overlapped surface area of the two electrodes is represented by cross-hatching. 
       FIG. 9  is an enlarged side view of two symmetrical MOVs stacked electrically in series that can be employed in the protector assembly shown in FIG.  3 . 
       FIG. 10  is an enlarged side view of two asymmetrical MOVs stacked electrically in series that can likewise be employed in the protector assembly shown in FIG.  3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the invention shown and described herein comprise surge protection devices, and in particular overvoltage protection devices, that are used at the interface between a telecommunications network and customer premises on telephone signal lines comprising conventional twisted-pair tip and ring conductors. Overvoltage protection devices, commonly referred to as station protectors, protect personnel and telecommunications equipment from voltage surges and overvoltage transients by shunting the voltage surges and transients to ground. However, the surge protection devices of this invention are not limited to station protectors, which comprise only the representative embodiments. Other surge protection devices, such as central office overvoltage protection devices, on-line overvoltage protection devices, and remote terminal overvoltage protection devices can also benefit from the reduced capacitance and reduced capacitive imbalance achieved by this invention. Furthermore, MOVs configured according to this invention may be packaged in any manner, for example mounted on a printed circuit board, and need not be confined to the station protectors shown and described herein. 
   The present invention can be utilized in back-up and interacting surge protection devices on twisted-pair telephone lines that employ both gas discharge tubes and MOVs electrically connected in parallel between the tip and ring conductors and a common electrical ground. The gas discharge tubes and MOVs provide alternative electrical paths to ground. The gas discharge tubes are often considered to be the primary electrical path between the signal line (e.g., tip or ring conductor in the case of twisted-pair telephone lines) and ground, because the gas discharge tube is able to repeatedly withstand high current surges. The response time of an MOV, however, is faster than the relatively slow response time of a gas discharge tube because of the time required to ionize the gas in the gas tube. Sustained or repeated currents will, however, tend to damage the MOV. In hybrid surge protection devices employing interacting varistors, the parallel combination of the gas discharge tube and the MOV will permit the surge protection device to respond to fast rise time surges or transients because the MOV will react until the gas discharge tube fires. The gas discharge tube provides the primary electrical path to ground and protects the MOV because the MOV will not be subjected to sustained or repeated high currents. In other hybrid surge protection devices, MOVs having a much larger DC breakdown voltage than a gas discharge tube provide back-up surge protection in case of damage to the gas discharge tube (e.g., venting). This invention can be employed with either back-up or interacting hybrid surge protection devices. 
   An example of an overvoltage protection device in the form of a hybrid station protector comprising a protector assembly employing a gas discharge tube and at least one MOV is shown in FIG.  1 . The gas tube and the MOV are electrically connected in parallel between a signal line and ground. Two embodiments of a protector assembly that can employ one or more MOVs according to the invention in the station protector of  FIG. 1  are shown in FIG.  2  and  FIG. 3 , respectively. The protector assemblies shown in  FIGS. 2 and 3  are commonly referred to as a two-element gas tube protector assembly. Three-element gas tube protector assemblies can also employ MOVs according to this invention, but need not be separately discussed because the invention is employed in the same manner for a two-element gas tube protector assembly as a three-element gas tube protector assembly. The only significant difference between a three-element gas tube protector assembly and a two-element gas tube protector assembly is that the tip and ring conductors share a common gas chamber and a common ground terminal, as is well known in the art and therefore need not be described in greater detail. 
   Station protector  2  as shown in  FIG. 1  includes two protector assemblies  4  that are positioned within a generally hollow housing  12  in electrical contact with two corresponding conductive terminals  6 . Preferably, station protector  2  is of the type commercially available from Corning Cable Systems of Hickory, North Carolina, such as Model SPD-126. The terminals  6  provide means for separately connecting the protector assemblies  4  to the tip conductor (not shown) and the ring conductor (not shown) of a twisted-pair telephone line suitable for use in a telecommunications network. The tip conductor is electrically connected to one of the terminals  6  and the ring conductor is electrically connected to the other terminal  6 . A pair of ground springs  8  urge the protector assemblies  4  upwardly within the housing  12  into engagement with the corresponding lower ends of the terminals  6 . A cylindrical conductive pin  10  is in contact with and extends upwardly through the pair of ground springs  8 . The pin  10  can be riveted in place to form at least a portion of an electrical path between each protector assembly  4  and a ground terminal, such as ground terminal  24 , shown in FIG.  4 . 
   The two protector assemblies  4  shown in  FIGS. 2 and 3  are substantially the same except for the configuration of the MOV  20  and the MOVs  30 , respectively, employed in the station protector  2 . The protector assembly  4  shown in  FIG. 2  employs a single MOV with asymmetrical electrodes. Accordingly, the MOV  20  is also referred to herein as “asymmetrical MOV  20 .” The protector assembly  4  shown in  FIG. 3  employs two MOVs  30  stacked in series and electrically connected between one of the terminals  6  and the ground terminal  24 . Accordingly, the MOVs  30  are also referred to herein as “series stacked MOVs  30 .” The series stacked MOVs  30  is preferably limited to two MOVs  30  stacked electrically in series, however, the series stacked MOVs  30  may comprise any number of two or more MOVs  30  stacked electrically in series. Furthermore the series stacked MOVs  30  can comprise either MOVs with symmetrical electrodes, such as a conventional MOV with electrodes on opposite sides of a varistor material having the same surface area, or two asymmetrical MOVs  20  with electrodes on opposite sides of the varistor material having different surface areas, as will be described with reference to  FIGS. 9 and 10 . 
   The primary protector in the protector assemblies  4  shown in FIG.  2  and  FIG. 3  is a gas discharge tube  14 . Accordingly, in the exemplary station protector shown in  FIG. 1 , a first primary protector  14  is electrically connected between one of the terminals  6  and the ground terminal  24 , and a second primary protector  14  is electrically connected between the other terminal  6  and the ground terminal  24 . As is well known, inert gas within gas discharge tube  14  will ionize when subjected to a voltage surge of sufficient energy, thereby forming an electrical path between the signal line subjected to the voltage surge and ground. In the preferred embodiments of the invention shown herein, a conventional gas discharge tube, such as a Model N-80-C400X, manufactured by EPCOS of Berlin, Germany, is utilized. However, any other commercially available two-element or three-element gas discharge tube is suitable. In each protector assembly  4 , the gas discharge tube  14  is electrically connected in parallel with at least one MOV, such as the single asymmetrical MOV  20  shown in  FIG. 2 , or the two series stacked MOVs  30  shown in FIG.  3 . The varistor material forming the body of MOV  20  and MOVs  30  is substantially nonconductive below a predetermined energy, but rapidly becomes conductive when subjected to a voltage surge above the predetermined energy. Thus, the gas discharge tube  14  and the MOV  20  or MOVs  30  form two parallel paths between a signal line electrically connected to terminal  6  and ground terminal  24  when subjected to a voltage surge. In alternative embodiments of the invention not shown and described herein, other mechanical, electrical and solid state clamping devices may be substituted for the gas discharge tube  14 , such as an air gap, resistor, inductor, thyristor, diode, and the like. 
   An MOV spring  18  and a fail safe contact  22  formed from conductive materials retain the gas discharge tube  14  and the MOV  20  or MOVs  30 . The MOV  20  or the MOVs  30  are positioned between a top portion  19  of the MOV spring  18  and a central web  21  on the fail safe contact  22 . The gas discharge tube  14  is positioned between the central web  21  and the bottom portion  17  of the MOV spring  18 . The MOV spring  18  urges the asymmetrical MOV  20  or the series stacked MOVs  30  toward the central web  21  of the fail safe contact  22 . The electrodes on the opposite sides of the asymmetrical MOV  20  or the series stacked MOVs  30  engage the top portion  19  of the MOV spring  18  and the central web  21  of the fail safe contact  22 . However, the composition of the varistor material prevents conduction through the MOV until the signal line is subjected to a voltage surge of sufficient energy for the varistor material to become conductive and shunt the voltage surge to ground. A fusible member  16  is positioned between the central web  21  of the fail safe contact  22  and the gas discharge tube  14 . The fusible member  16  is normally formed of a eutectic solder so that it will rapidly soften and flow when subjected to a predetermined temperature. However, the fusible member  16  may be made of plastic or any other suitable material that softens and flows sufficiently to permit the gas discharge tube  14  to electrically contact the central web  21  of the fail safe contact  22 . Once the fusible element  16  softens and flows, the MOV spring  18  urges the failsafe contact  22  in the direction of the gas discharge tube  14  so that the failsafe contact  22  electrically contacts the terminal  6  and shorts the protector assembly  4  to the ground terminal  24  through ground springs  8  and pin  10 , thereby diverting the voltage surge to electrical ground. 
   Since the asymmetrical MOV  20  and the series stacked MOVs  30  are not conductive under normal circumstances, there will be a capacitance introduced by the MOV  20  or MOVs  30  between the tip conductor and ground and between the ring conductor and ground. If these capacitances differ, there will be a capacitive imbalance that exists between the electrical path from the tip conductor to ground and the electrical path from the ring conductor to ground. Capacitive imbalance is typically not problematic at lower frequencies, but results in excessive signal loss (e.g., insertion loss and return loss) and longitudinal imbalance at higher frequencies. As will be discussed hereinafter in greater detail, MOV  20  with asymmetrical electrodes will both reduce capacitance and will have less variability in capacitance (also referred to herein as capacitive tolerance), than conventional MOVs (i.e., MOVs having symmetrical electrodes). Similarly, series stacked MOVs  30  having symmetrical electrodes on opposite sides of the varistor material will likewise reduce the capacitance and the variability of capacitance among MOVs that can cause capacitive imbalance. Series stacked MOVs  30  having asymmetrical electrodes on opposite sides of the varistor material will even further reduce the capacitance and capacitive tolerance of the MOVs, and thus, the potential capacitive imbalance among MOVs, thereby statistically improving the performance of the station protector  2  at higher frequencies. 
     FIGS. 5-8  show one embodiment of an MOV  20  with asymmetrical electrodes  46  and  48  on opposite sides of body  40  formed of a varistor material, such as zinc oxide or other semi-conductive material used in conventional MOVs. The larger electrode  46  covers substantially the entire surface of the side  42  of the body  40 . The smaller electrode  48 , however, covers only a portion of the surface of opposite side  44  of the body  40 . Opposite sides  42 ,  44  and opposite electrodes  46 ,  48  are substantially parallel and, with the exception of the fact that the first electrode  46  is larger than the second electrode  48 , the MOV  20  is of substantially conventional construction. The electrodes  46  and  48  are positioned on the sides  42  and  44  of the body  40  using any one of a number of conventional fabrication techniques. For example, in the preferred embodiments of the invention shown and described herein, the electrodes  46  and  48  are vapor deposited onto the outwardly facing surfaces of sides  42  and  44 , respectively. In these representative embodiments, the electrodes  46 ,  48  are generally circular. However, other shapes could be employed. Furthermore, it is not necessary that the electrodes  46 ,  48  on opposite sides  42 ,  44  of the same asymmetrical MOV  20  even have the same shape. For example, at least one of the electrodes  46 ,  48  could have a square, rectangular, oval, diamond, star or annular (i.e., ring) shape. 
   In one preferred embodiment, the asymmetrical MOV  20  has a diameter of about 5 mm, and the larger electrode  46 , which covers substantially the entire surface of the side  42 , also has a diameter of about 5 mm. As previously mentioned, MOVs of the type used in conventional station protectors typically have a diameter of only about 3.8 mm. Since the larger electrode  46  covers substantially the entire side  42  of the varistor material, it will completely overlap the surface area of the smaller electrode  48 , regardless of the size, shape or lateral placement of the smaller electrode  48  on the side  44 .  FIG. 8  shows that the overlapped surface area of the two electrodes  46 ,  48 , which is represented by the cross-hatched area  47 , will always be equal to the surface area of the smaller electrode  48 , even if the smaller electrode is offset laterally relative to the center of the body  40  defined by the varistor material. For purposes of illustration only, this lateral offset is greatly exaggerated in FIG.  8 . In the embodiment shown in  FIGS. 5-8 , the diameter of the smaller electrode  48  is preferably about 1.9 mm, which is approximately the same size as the symmetrical electrodes of a conventional 3 mm MOV. Given the same thickness of varistor material, the asymmetrical MOV  20  will therefore have a capacitance that is less than the capacitance of a conventional 5 mm MOV, but will have a greater current handling capacity than smaller MOVs. It is not necessary that the larger electrode  46  cover the entire surface of the side  42  of the varistor material. In fact, manufacturing considerations may in some cases require that the electrode be at least slightly smaller than the side  42 . In a commercial embodiment, for example, a larger electrode  46  having a diameter of about 3.5 mm is used with a smaller electrode  48  having a diameter of about 1.9 mm to achieve a significant reduction in capacitance and capacitive tolerance. 
   FIG.  9  and  FIG. 10  show two different versions of series stacked MOVs  50 ,  60 , respectively. In the embodiment shown in  FIG. 9 , a series stacked MOV  50  having two substantially identical MOVs  32  stacked electrically in series is shown. Each MOV  32  is symmetrical with the electrodes  36 ,  38  on opposite sides of the varistor material having the same surface area. Sufficient normal forces are applied externally to the MOVs  32  so that electrical continuity is maintained between the adjacent electrodes of the MOVs  32 . Alternatively, a thin film of solder can be applied between the adjacent electrodes to bond the MOVs  32  together. Although the two MOVs  32  forming the series stacked MOV  50  shown herein are in physical contact, it should be understood that MOVs  32  can be stacked in series according to the invention even if the adjacent electrodes are not in direct physical contact. A reduction in the variability, tolerance or spread of the capacitance for an MOV subassembly can also be achieved if the MOVs  32  are physically separated and merely electrically connected in series. 
     FIG. 10  shows two MOVs  20  with asymmetrical electrodes stacked electrically in series according to the invention. In this embodiment, the smaller electrode  48  of the lower MOV  20  is in direct physical contact with the larger electrode  46  of the upper MOV  20 . Alternatively the two smaller electrodes  48  could be adjacent one another in opposed relationship and direct physical contact (or merely electrically connected in series), or the two larger electrodes  46  could be positioned adjacent one another in opposed relationship and direct physical contact (or merely electrically connected in series). In the embodiment shown in  FIG. 10 , the smaller electrodes  48  have a diameter of about 1.9 mm and the larger electrodes  46  have a diameter of about 3.5 mm. The diameter of the body  40  defined by the varistor material is about 5 mm. Thus, the larger electrode  46  does not cover the entire surface area of the varistor material, as previously described with reference to the embodiment shown in  FIGS. 5-8 . 
   Surge protection devices according to this invention are not limited to the station protectors shown in  FIGS. 1-4  or to the exemplary MOV subassemblies shown in  FIGS. 5-8 ,  9  and  10 . However, station protectors of the type shown and described herein facilitate the use of twisted-pair telecommunications lines for xDSL communications, including VDSL. This invention achieves that result by reducing the capacitance introduced by a surge protection device, such as a station protector, and by reducing any capacitive imbalance introduced between the tip conductor and ground and the ring conductor and ground. Although higher frequency broadband communications are possible for twisted-pair telecommunications lines utilizing conventional station protectors that do not incorporate this invention, the variability of conventional station protectors means that a number of conventional station protectors will not perform adequately for higher frequency transmissions. This inadequate high frequency performance of surge protection devices that perform satisfactorily in lower frequency transmissions requires replacement of at least some conventional station protectors before satisfactory broadband and xDSL communications can be achieved. 
   If the surge protection device is to be used to form separate paths to ground for two conductors, such as the tip and ring conductors in a twisted-pair telephone line, capacitive imbalance becomes an issue. In station protectors used to shunt voltage surges from the tip conductor to ground and/or from the ring conductor to ground at the network interface to the customer premises, separate MOVs are used between tip and ground and between ring and ground. If the capacitances of these two MOVs differ, capacitive imbalance is introduced between the tip and ring conductors. The difference in capacitance between MOVs used in the two shunt paths is reduced according to this invention by employing MOVs stacked electrically in series, by employing MOVs with asymmetrical electrodes, or by employing MOVs with asymmetrical electrodes stacked electrically in series. 
   It has been found that the capacitance introduced by the addition of an MOV to a surge protection device can be reduced by employing multiple MOVs stacked electrically in series. As used herein, the terms “stacked in series” or “electrically stacked in series” refer to two or more MOVs that are electrically connected in series, whether or not the electrodes of adjacent MOVs are in physical contact with one another. Preferably, the MOVs are pressed together or joined by a conductor, for example solder, such that the electrodes of adjacent MOVs are in direct physical contact. If two MOVs are stacked in series between a signal line and ground, the resultant capacitance will be less than the capacitance of a single MOV having substantially the same diameter and electrodes of substantially the same surface area as the stacked MOVs. The total capacitance of the stacked MOVs is determined in accordance with the following relationship: 
     C   total =( C   1   C   2 /( C   1   +C   2 )) 
   
       
       
         
           If C 1 =C 2 =C, then C total=1/2 C 
         
       
     
  
   It then follows that one way to reduce the capacitance introduced by an MOV of conventional construction would be to stack at least two MOVs having substantially the same diameter and electrodes of substantially the same overlapped surface area in series between a signal conductor and ground. In addition to reducing the total capacitance, the standard deviation due to the variability among supposedly identical MOVs will also be reduced. In particular, the standard deviation (σ) of the total capacitance of the stacked MOVs (i.e., C total=1/2 C) will be V 2  the standard deviation of C in accordance with the following relationship:
         σof ax=a(σx) where a is constant and x is variable; therefore
 
σ Ctotal   =aσ   c = 1/2 σ c .
       

   The standard deviation is a conventional measurement demonstrating the variability of the capacitance among supposedly identical MOVs. This relationship has been confirmed experimental by testing 230V, 5 mm MOVs with symmetrical 2.7 mm electrodes on opposite sides of a varistor material. The capacitance @ 1 MHz/0 Vdc Bias for two sets of twenty-five individual MOVs yielded the following results: 
   
     
       
         
             
             
             
           
             
                 
                 
             
             
                 
               Set 1 
               Set 2 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
                 
               Average 
               34.55 
               34.89 
             
             
                 
               Stdev 
               0.84 
               0.99 
             
             
                 
               Min 
               33.20 
               33.20 
             
             
                 
               Max 
               37.10 
               37.90 
             
             
                 
               +3σ 
               37.08 
               37.87 
             
             
                 
               −3σ 
               32.03 
               31.91 
             
             
                 
                 
             
          
         
       
     
   
   When one of the MOVs from set 1 was pressed together with the corresponding MOV from set 2 such that the two MOVs were stacked electrically in series, the total capacitance was determined and the following results were obtained: 
   
     
       
         
             
             
             
           
             
                 
                 
             
           
          
             
                 
               Average 
               17.99 
             
             
                 
               Stdev 
               0.42 
             
             
                 
               Min 
               17.17 
             
             
                 
               Max 
               18.80 
             
             
                 
               +3σ 
               19.24 
             
             
                 
               −3σ 
               16.74 
             
             
                 
                 
             
          
         
       
     
   
   The MOVs used for this test may or may not be the desired size for use in a particular surge protection device, such as a station protector for the tip and ring conductors of a twisted-pair telephone line. Nevertheless, these results confirm that the total capacitance and the variability of the capacitance can both be reduced by series stacking MOVs. Three or more series stacked MOVs will further reduce the total capacitance and the variability of the capacitance, but the reduction due to the addition of each MOV will not be as large. 
   Even though the capacitance will be reduced by stacking two or more MOVs in series, this reduction in capacitance will not necessarily be a means of effectively lowering the capacitance across a surge protection device, such as a station protector. Although the total capacitance will be lowered by incorporating additional MOVs in a series stack, the additional MOVs will also affect the electrical performance of the station protector. The voltage necessary to trigger the MOV will be greater for the stacked MOVs than for a single MOV of the same type used in the stack. A station protector comprising a stack of MOVs will therefore not respond to the same voltage as would a station protector comprising a single MOV. Of course, MOVs with proportionally smaller thicknesses can be stacked, and the electrical performance will be substantially the same as for a single MOV having the same thickness as the overall thickness of the stacked MOVs, at least insofar as response to a voltage surge. However, stacking MOVs will, relative to a single MOV having the same thickness as the overall thickness of the stacked MOVs, still reduce the variability of the capacitance, a desirable result when used with twisted-pair telecommunications lines for higher frequency transmissions. 
   MOVs can be stacked by placing two MOVs with the adjacent electrodes of the MOVs in electrical contact. A suitable electrical contact can be achieved by applying sufficient external force to the MOVs so that reliable electrical conduction can be maintained without the application of solder or other electrically conductive bonding means. Of course, adjoining electrodes could also be soldered together, especially if a stacked MOV subassembly is to be fabricated prior to subsequent assembly in a station protector or other surge protection device. If solder is to be applied, care should be taken to prevent the solder from extending laterally beyond the perimeter of the either electrode and to limit the occurrence of any voids in the solder joint. Excess solder could result in an effective enlargement of the surface area of the electrodes with a resultant increase in the capacitance and the variability of capacitance of the series stacked MOVs. 
   The variability of the capacitance among MOVs to be used in a surge protection device can also be reduced by modifying the structure of the MOV. One reason for relatively large capacitive tolerance for an MOV is misalignment of the electrodes on opposite sides of the varistor material. Misalignment can occur when the two electrodes are laterally displaced or when one or both of the electrodes is misshapen (e.g., not concentric. The electric field generated between the two electrodes will not be the same when the two electrodes are misaligned as when the two electrodes are properly aligned because the overlapped surface area of the electrodes will be different. Since capacitance is a function of the electric field generated in a nonconductive material between two spaced apart, conductive electrodes, the capacitance will also be a function of the relative alignment of the two electrodes. The varistor material will be nonconductive under normal conditions. Therefore, the capacitance introduced by the MOVs will be a function of the electric field generated between two electrodes that cannot be positioned relative to each other consistently with a level of precision that does not adversely affect higher frequency transmissions, such as xDSL communications, including VDSL. 
   Even if the exact placement of two electrodes on opposite sides of an MOV cannot be adequately controlled by conventional manufacturing techniques, it is nevertheless possible to position asymmetrical electrodes in a manner such that the electric field will not vary significantly for MOVs of the same overall size and shape. One way to accomplish this result is to fabricate one of the two electrodes so that its surface area will be substantially larger than the surface area of the other electrode. If one electrode is large enough, then regardless of the position of the smaller electrode, the larger electrode will completely overlap the smaller electrode. In other words, the perimeter of the larger electrode will extend laterally to or beyond the perimeter of the smaller electrode. Therefore, regardless of the placement of the smaller electrode, the electric field generated between the electrodes will have essentially the same size and shape, since the electric field will be primarily confined to the volume of the varistor material between the overlapped surface areas of the electrodes. If the larger electrode covers the entire surface of one side of the varistor material, then the lateral placement of the smaller electrode will not significantly affect the capacitance of the MOV. Since the surface area of the electrode can be more readily controlled then its precise placement, asymmetrical electrodes can be used to overcome capacitive variability due to imprecise placement. 
   Furthermore, the capacitance of the MOV can be varied by utilizing asymmetrical electrodes having different surface areas. For example, the capacitance of the MOV can also be reduced by reducing the surface area of the smaller electrode, since the size and shape of the electric field generated between the two electrodes will be dependent upon the surface area of the smaller electrode and will be relatively independent of the surface area of the larger electrode. Thus, use of an asymmetrical MOV with a smaller electrode overlapped by a larger electrode will reduce both the capacitance of each MOV and the capacitive imbalance due to the introduction of different MOVs electrically connected between the tip conductor and ground and between the ring conductor and ground. A further improvement can be achieved by stacking two or more asymmetrical MOVs electrically in series between the tip conductor and ground and/or between the ring conductor and ground in a station protector or other surge protection device. 
   The capacitance of twenty individual 230V, 5 mm MOVs having symmetrical 3.8 mm electrodes on opposite sides of a varistor material was measured and the average capacitance and standard deviation of the population determined. The capacitance of twenty individual 230V, 5 mm MOVs having asymmetrical electrodes on opposite sides of the same varistor material comprising a smaller 1.9 mm electrode overlapped by a larger 3.5 mm electrode was then measured and the average capacitance and the standard deviation of the population determined. The data reproduced below illustrates that the average capacitance of the asymmetrical MOVs was approximately one-half the average capacitance of the symmetrical MOVs and the standard deviation of the asymmetrical MOVs was approximately one-quarter the standard deviation of the symmetrical MOVs. Pairs of the symmetrical MOVs and pairs of the asymmetrical MOVs were then stacked electrically in series and the same characteristics determined. The data reproduced below illustrates that further reductions in both the total capacitance and the variability of the capacitance among asymmetrical MOVs can be achieved by stacking asymmetrical MOVs electrically in series. 
                                          230 V, 5 mm symmetrical   230 V, 5 mm           MOVs with   asymmetrical MOVs with           3.8 mm electrodes   1.9 mm and 3.5 mm electrodes                                             No   Single   Stack   No   Single   Stack                                                          1   49.5   25.2   1   23.9   11.7            2   49.5       2   24            3   49.6   25.2   3   24.2   11.9            4   49.5       4   24            5   54.4   26.1   5   24.4   12            6   49.2       6   24            7   49.6   25.1   7   23.1   11.7            8   48.8       8   24.2            9   49.5   25   9   23.7   11.8           10   48.8       10   23.6           11   49.4   25.1   11   23.9   12           12   49.6       12   24.1           13   49.3   25   13   23.4   11.7           14   49.3       14   24.1           15   49.1   24.9   15   24.3   12.1           16   49.3       16   24.1           17   49.6   25   17   23.9   11.9           18   49.2       18   23.8           19   48.9   24.6   19   24.4   12.1           20   48.2       20   24.1           Max   54.4   26.1   Max   24.4   12.1           Min   48.2   24.6   Min   23.1   11.7           Average   49.5   25.1   Average   24.0   11.9           Stdev   1.20   0.39   Stdev   0.32   0.16                        
Furthermore, based on the above data, it can be expected that a reduction in total capacitance and variability of capacitance among MOVs can be achieved by stacking a symmetrical MOV and an asymmetrical MOV electrically in series.
 
   As previously discussed, the invention is not intended to be limited to the representative embodiments depicted herein. The exemplary embodiments shown and described herein are illustrative of the particular benefits obtained when the invention is incorporated into a surge protection device, such as a station protector, utilized on twisted-pair telecommunications lines for higher frequency transmissions, such xDSL communications, including VDSL. However, it is anticipated that the invention can provide similar benefit when utilized in connection with any electrical device employing one or more MOVs wherein a reduction in capacitance or capacitive tolerance is desired. The invention is defined more broadly by the following claims, which include other structures and embodiments that would be apparent to one of ordinary skill in the art. Thus, the scope of the invention should not be limited to the above description, but instead, should be construed as broadly as possible in accordance with the appended claims.