Patent Publication Number: US-6671155-B2

Title: Surge protector with thermally activated failsafe mechanism

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to surge protectors, and more particularly, to a surge protector provided with a thermally activated failsafe mechanism for use with, for example, telephone equipment. 
     BACKGROUND OF THE INVENTION 
     Surge protectors are widely used for the protection of equipment from overvoltage conditions that may be caused, for example, by lighting or high voltage line contact. For example, telecommunication lines employ various types of surge protectors, which at a minimum, provide overvoltage protection. This is typically done with at least one protection element that is inserted between a conductive tip element of a surge protector and ground. Likewise, typically at least one protection element is inserted between a conductive ring element of the surge protector and ground. When a hazardous overvoltage is present on a line, the overvoltage protection element, for example a gas tube, changes from a high impedance to a low impedance state. This change of impedance effectively shorts the hazardous overvoltage and its associated overcurrent to ground and away from equipment and/or personnel. 
     A sustained overvoltage is an overvoltage event that which causes excessive heat when the overvoltage, along with the associated overcurrent, flows through the surge protector and is shorted to ground. For example, a sustained overvoltage can occur where a power line has come in continued contact with a protected telephone line, thereby producing a continuous ionization of the gas tube and the resultant passage of overcurrent through the gas tube to ground. Such overcurrent will in many cases destroy equipment and/or the surge protector. 
     A failsafe mechanism will remain unaffected when subjected to short and/or less severe overvoltage conditions that the surge protector is intended to handle; however, the failsafe mechanism is intended to permanently short this sustained overvoltage to ground. 
     One known method of providing a failsafe mechanism in a surge protector is the use of a metal fusible element such as a solder joint. The metal fusible element is designed to melt at a predetermined temperature and short the sustained overvoltage to ground. The use of a metal fusible element as a failsafe mechanism is reliable; however, the metal fusible element method requires multiple components, which makes the metal fusible element relatively expensive. 
     Another known method of providing a failsafe mechanism is the plastic compressive displacement method. This method requires an electrically conductive spring and a plastic member. The plastic member physically and directly contacts both a portion of a ring side, and/or a portion of a tip side and a ground element of a surge protector to insulate the electrical contact path therebetween. For example, the spring is electrically connected with the tip side and biased towards the plastic member, but cannot make electrical contact to short the tip side to the ground element because the plastic member prevents electrical contact. In other words, the plastic member displaces the spring while physically and directly contacting both the electrical contact point of the spring and the electrical contact point of the ground element. The electrical contact point of the spring is intended to come into electrical contact with the electrical contact point of the ground element if the failsafe mechanism is activated. In operation, as the temperature of the ground element of the surge protector increases due to a sustained overvoltage the plastic member melts allowing the spring to push its way through the plastic member to electrically contact and short the tip side and/or ring side to the ground element. Although, the plastic compressive displacement method is relatively inexpensive, the method is inherently unreliable. The plastic compressive displacement method is inherently unreliable because residual plastic from the melted plastic member can remain between the spring and the intended electrical contact point during the sustained overvoltage condition, thereby interfering with the path to ground. Consequently, telephone equipment and/or personnel can be exposed to hazardous voltages and/or currents because the spring did not properly short to ground. 
     SUMMARY OF THE INVENTION 
     The present invention is directed towards a surge protector having a failsafe mechanism including at least one overvoltage protection element, at least one arm assembly, at least one ground element, at least one resilient member, wherein the at least one resilient member is electrically connected to the at least one ground element, at least one protrusion operably positioned between the at least one resilient member and the at least one arm assembly, wherein the at least one protrusion is in thermal contact with the at least one resilient member, the at least one protrusion prevents the at least one resilient member from electrically contacting the at least one arm assembly during normal operation, and wherein as a result of a sustained overvoltage condition the temperature of the at least one resilient member increases to soften the at least one protrusion and allow the at least one resilient member to electrically contact the at least one arm assembly and thereby short the at least one arm assembly to the ground element. 
     The present invention is further directed to a surge protector having a failsafe mechanism including a base, at least one overvoltage protection element, at least one ground element, at least one arm assembly, at least one resilient member, wherein the at least one resilient member is electrically connected to the at least one ground element, at least one protrusion extending from the base, wherein the at least one protrusion is in thermal contact with the at least one resilient member and prevents the at least one resilient member from electrically contacting the at least one arm assembly during normal operation, and wherein as a result of a sustained overvoltage condition the temperature of the at least one resilient member increases thereby softening the at least one protrusion and allowing the at least one resilient member to electrically contact the arm assembly to short the arm assembly to ground. 
     The present invention is further directed to a surge protector having a failsafe mechanism including a base, the base having a generally planar surface, at least one overvoltage protection element, a ground element, the ground element comprising a ground pin, the ground pin having a collar, at least one arm assembly, a torsional spring, the torsional spring having at least one arm and a coil with an aperture therethrough, wherein the torsional spring is in electrical contact with the ground pin, and the coil of the torsional spring is disposed between the collar of the ground pin and the planar surface of the base, at least one protrusion extending from the planar surface of the base, wherein the at least one protrusion is in thermal contact with the at least one torsional spring and prevents the at least one torsional spring from electrically contacting the at least one arm assembly during normal operation, and wherein as a result of a sustained overvoltage condition the temperature of the at least one arm of the torsional spring increases thereby softening the at least one protrusion and allowing the at least one arm of the torsional spring to electrically contact the arm assembly to short the arm assembly to the ground pin. 
    
    
     BRIEF DESCRIPTION OF THE FIGS. 
     FIG. 1 is an exploded perspective view of a surge protector of one embodiment according to the present invention. 
     FIG. 2 is a perspective view of the surge protector of FIG. 1 as assembled shown with the cover removed. 
     FIG. 3 is a sectional view of the surge protector of FIG. 1 as assembled and taken through the ground element. 
     FIG. 4 is a perspective view of the base of FIG.  1 . 
     FIG. 5 is a sectional view of the surge protector of FIG. 1 with the cover removed taken through a transverse plane depicting the failsafe mechanism in an open circuit condition. 
     FIG. 6 is a sectional view of the surge protector of FIG. 1 with the cover removed taken through a transverse plane depicting the failsafe mechanism in a short circuit condition. 
     FIG. 7 is a perspective view of the ring arm of the surge protector of FIG.  1 . 
     FIG. 7 a  is a sectional view of the ring arm of FIG. 7 taken through line a—a. 
     FIG. 8 is an exemplary graph illustrating the interaction of a varistor and a gas tube in responding to a voltage surge over time. 
     FIG. 9 is a perspective view of the ground element and the resilient member assembly according to another embodiment of the present invention. 
     FIG. 10 is a plan view of a cover according to another embodiment of the present invention. 
     FIG. 10 a  is a sectional view of the cover of FIG. 10 taken through line a—a. 
     FIG. 10 b  is a sectional view of the cover of FIG. 10 taken through line b—b. 
     FIG. 11 is a sectional view of a surge protector of another embodiment as assembled and taken through the ground element. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in FIGS. 1-3 is a surge protector  10  having a failsafe mechanism according to the present invention. Surge protector  10  is commonly referred to as a central office protector and is typically inserted into a connector block at a telephone central office to protect central office personnel and equipment from being damaged by surges caused, for example, by lightening or power crosses. However, the concepts of the present invention are applicable to other devices that employ failsafe mechanisms. 
     In one embodiment, surge protector  10  includes a dielectric base  12 , tip arm assembly  34 , a ring arm assembly  36 , a pair of gas tubes  40 , a pair of varistors  48 , a ground element  50 , a resilient member  60 , and a cover  70 . However, the concepts of the present invention may be used with other types of surge protectors such as station surge protectors, surge protectors having additional components such as sneak current protection components and/or fewer component(s), for example, no varistors. Additionally, instead of using gas tubes  40  and varistors  48  as an overvoltage protection element, other suitable overvoltage protection elements may be used, for example, only gas tubes, gas tubes having an air backup, gas tubes with interacting varistors and/or solid state devices. 
     As shown in FIGS. 4 and 5, base  12  includes a pair of protrusions  12   a  for preventing resilient member  60  from shorting tip arm assembly  34  and/or ring arm assembly  36  to ground element  50  during normal operation. Protrusions  12   a  disposed on base  12  are operable to soften and/or melt as a result of a sustained overvoltage condition that increases the temperature of ground element  50  and resilient member  60 . As a result of a sustained overvoltage condition, the contact pressure of the compressed resilient member  60  against protrusions  12   a  causes resilient member  60  to displace, by deflecting and/or slicing a portion thereof, the softened and/or melted protrusions  12   a.  When protrusions  12   a  are so displaced by resilient member  60 , arm assemblies  34  and/or  36  short to ground element  50 , through resilient member  60 , without protrusions  12   a  interfering with the electrical path between resilient member  60  and arm assemblies  34  and/or  36 . In other words, in one embodiment of the present invention protrusions  12   a  are advantageously spaced apart from a portion of arm assemblies  34  and/or  36  that are aligned to electrically contact resilient member  60  (See FIGS.  3  and  5 ). As used herein, spaced apart means protrusions  12   a  may contact arm assemblies  34  and/or  36 ; however, protrusions  12   a  are disposed so they are not located physically and directly between the point of electrical contact of resilient member  60  and arm assemblies  34  and/or  36 . For example, as shown in FIG. 3 protrusion  12   a  is located so that it can be displaced and not remain between a spring arm  60   a  of resilient member  60  and a stop tab  16   e  of arm assembly  36 . 
     Base  12  also includes a plurality of apertures  8  formed therethrough for inserting electrical inputs and outputs therein. More specifically, each particular pin, a ground pin  13 , an outside plant tip pin  24   a,  a central office tip pin  24   b,  an outside plant ring pin  26   a,  and a central office ring pin  26   b  are inserted into a corresponding aperture  8  of base  12 . Tip pins  24   a  and  24   b  are attached and electrically connected to a tip arm  14  forming a tip arm assembly  34 . Attaching pins  24   a  and  24   b  to tip arm  14  simplifies the manufacture and assembly of surge protector  10 . Likewise, ring pins  26   a  and  26   b  are attached and electrically connected to a ring arm  16  forming a ring arm assembly  36 . However, arm assemblies  34  and  36  could include only one component. 
     In one embodiment of the present invention, protrusions  12   a  of base  12  are integrally molded with base  12  and extend therefrom. However, as shown protrusions  12   a  may be removably attached to base  12 . When protrusions  12   a  are integrally molded with base  12 , the manufacture and assembly of surge protector  10  is simplified. On the other hand, removably attaching protrusions  12   a  to base  12  permits the use of two materials having different properties for base  12  and protrusions  12   a.  Additionally, protrusions  12   a  may be integrally molded with or removably attached to other suitable components and/or portions of surge protector  10 . For example, protrusions  12   a  may be molded into cover  70 . Molding protrusions  12   a  with cover  70  advantageously allows replacement of damaged protrusions  12   a  by simply removing and replacing cover  70 . 
     Suitable materials for protrusions  12   a  will have melt and heat deflection temperatures in the range corresponding to thermal conditions at the sustained overvoltage condition of surge protector  10 . Suitable materials for protrusions  12   a  include thermoplastics, thermosets, metals such as solder posts, or other suitable materials having desirable characteristics. Suitable materials should be free of embrittlement due to heat aging, be non-flammable under the overvoltage conditions, have acceptable mechanical properties and be inert to corrosives and weather. For example, base  12  and protrusions  12   a  can be formed from a polybutylene teraphthalate such as Valox® available from General Electric Plastics of Pittsfield, Mass. Other suitable materials may include polycarbonates such as Lexan®, or blends of polyphenylene ether and styrene butadiene, such as Noryl®, both materials being available from General Electric Plastics; however, other suitable thermoplastics may be used. 
     In one embodiment, base  12  is formed from Valox® DR48 and has protrusions  12   a  integrally molded therewith. Protrusions  12   a  have a width w (FIG. 4) of about 0.05 inches; however, other suitable widths and/or materials may be used. Valox® DR48 has a melt temperature of about 250° C. and a heat deflection temperature of about 180° C. A heat deflection temperature is the temperature at which the material of  12   a  softens allowing resilient member  60  to displace protrusion  12   a ; however, the heat deflection temperature may also be a function of the restoring force of resilient member  60 . Other materials having different melt and/or heat deflection temperatures may be used; however, a minimum heat deflection temperature, for example, about 100° C. may be desired to reduce the distortion of base  12  during, for example, high current testing of surge protector  10 . 
     As best shown in FIGS. 1 and 2, tip arm assembly  34  and ring arm assembly  36  are similar, but arm assemblies  34  and  36  may have different configurations and/or different components. Arm assemblies  34  and  36  include an electrically conductive arm, more specifically a tip arm  14  and a ring arm  16 , respectively. The details of tip arm  14  will be explained with the understanding that in the embodiment depicted ring arm  16  is similar. Tip arm  14  includes a first end portion  14   a,  a medial portion  14   b,  and a second end portion  14   c.  Tip pins  24   a  and  24   b  are electrically connected to tip contact  14  at medial portion  14   b.  Likewise, ring arm  16  includes a first end portion  16   a,  a medial portion  16   b,  and a second end portion  16   c.  Ring pins  26   a  and  26   b  are electrically connected to tip arm  16  at medial portion  16   b.  Tip arm  14  is generally shaped to provide resiliency between first end  14   a  and second end  14   c  for securely positioning gas tube  40 , a portion of a ground plate  52 , and varistor  48  therebetween when assembled. 
     Gas tube  40  is a 2-element gas tube, for example, a N80-C400X gas tube available from Epcos, Inc. of Chicago, Ill. Gas tube  40  includes a pair of lead electrodes  40   a  disposed on distal ends of gas tube  40 . However, other suitable gas tubes may be used. Moreover, other configurations of surge protector  10  may employ a three-element gas tube, rather than the pair of two-element gas tubes. For example, a T-60-C350XS three-element gas tube available from Epcos, Inc. 
     When assembled as shown in FIG. 2, first end  14   a  of tip arm  14  is electrically connected to one of the pair of lead electrodes  40   a  of gas tube  40 . The other lead electrode  40   a  of the same gas tube  40  is electrically connected to ground plate  52 . Varistor  48  (not visible in FIG. 2) is disposed and electrically connected between second end  14   c  of tip arm  14  and ground plate  52 . First end  14   a  of tip arm  14  may include a surface that generally complements the profile of a lead electrode  40   a  of gas tube  40  for securing gas tube  40  in position, or the surface may be generally planar. Likewise, second end  14   c  of tip arm  14  may include a surface having a profile for securing varistor  48  in position, or the surface may be generally planar. 
     Ring arm  16  is shown in FIGS. 7 and 7 a  to clearly illustrate relevant portions thereof. Ring arm  16  includes a dimple  16   d,  stop tab  16   e,  and a cutout  16   f.  Although not shown, tip arm  14  likewise includes a dimple, a stop tab, and a cutout. Dimple  16   d  is disposed between medial portion  16   b  and second end portion  16   c  of ring arm  16  for inhibiting gas tube  40  from being inserted past its desired position (FIG.  3 ). Stop tab  16   e  is disposed generally on medial portion  16   b  of ring arm  16  and is aligned to provide a stop surface and electrical contact point for one of the spring arms  60   a  of resilient member  60  if protrusion  12   a  is displaced (FIG.  6 ). Cutout  16   f  keys ring arm assembly  36  so that pins  26   a  and  26   b  of ring arm assembly  36  can only be inserted into the correct apertures  8  of base  12 . Moreover, cutout  16   f  allows for a more compact packaging of the components of surge protector  10 . 
     As shown, cutout  16   f  is positioned behind, and out of the way of, stop tab  16   e.  This allows protrusions  12   a  to be spaced away from stop tab  16   e  when assembled. Thus, in operation if protrusions  12   a  soften and/or melt they will not remain in a path between the resilient member  60  and arm assemblies  34  and/or  36 , thereby allowing resilient member  60  to make clean electrical contact therewith shorting a sustained overvoltage to ground element  50 . 
     Ground element  50  includes ground plate  52  and ground pin  13 . Ground plate  52  includes a first end portion  52   a  and a second end portion  52   b.  First end portion  52   a  of ground plate  52  is electrically connected to ground pin  13 . More specifically, ground pin  13  includes a first end  13   a,  a collar  13   b  of a predetermined size, and a second end  13   c.  Collar  13   b  of ground pin  13  is disposed between first end  13   a  and second end  13   c  of ground pin  13 , but is generally closer to second end  13   c.  Second end  13   c  of ground pin  13  is electrically attached to first end portion  52   a  of ground plate  52 . Second end portion  52   b  of ground plate  52  may include a surface that complements the profile of lead electrode  40   a  of gas tube  40  for securing gas tube  40  in position, or it may be planar. 
     Resilient member  60  is electrically connected to ground element  50  and is in thermal contact therewith. In order to be operable, ground element  50  must effectively transfer heat to resilient member  60  to soften and/or melt protrusions  12   a  as a result of a sustained overvoltage. The heat transfer rate from ground element  50  to resilient member  60  may be influenced by, among other things, the contact surface area between the two components. Likewise, in order to be operable resilient member  60  requires a predetermined contact pressure to displace protrusions  12   a  and make suitable electrical contact with arm assemblies  34  and/or  36 . 
     In one embodiment, resilient member  60  is a torsional spring having a pair of spring arms  60   a  with a coil  60   b  therebetween. However, resilient member  60  may be, for example, a helical spring, a leaf spring, or other suitable resilient member. When assembled, a first end  13   a  of ground pin  13  passes through an aperture (not shown) of coil  60   b  before first end  13   a  of ground pin  13  is received in the corresponding aperture  8  formed through base  12 . Coil  60   b  is disposed between collar  13   b  of ground pin  13  and a surface  12   c  (FIG. 4) of base  12 . Collar  13   b  is larger than the aperture of coil  60   b  to maintain resilient member  60  in a predetermined position between collar  13   b  and surface  12   c  of base  12 . Additionally, collar  13   b  of ground pin  13  thermally contacts resilient member  60  facilitating heat transfer therebetween. Protrusions  12   a  of base  12  generally have an elevation above surface  12   c  about equal to, or higher, than collar  13   b.  However, in alternative embodiments other suitable configurations may be employed. For example, collar  13   b  of ground pin  13  may be eliminated so that resilient member  60  is disposed between ground plate  50  and surface  12   c  of base  12  as long as suitable heat transfer requirements are satisfied between ground plate  50  and resilient member  60 . 
     As shown in FIG. 5, spring arms  60   a  of resilient member  60  are held in a compressed position by protrusions  12   a  of base  12  and are in thermal contact therewith. In this position, protrusions  12   a  prevent spring arms  60   a  from electrically contacting tip arm assembly  34  and ring arm assembly  36 , thereby creating an open circuit between assemblies  34  and  36  and ground element  50 . Moreover, protrusions  12   a  are positioned in such a manner so as to not interfere with the portions of spring arms  60   a  that are operable to short arm assemblies  34  and/or  36  to ground element  50 . However, as shown in FIG. 6, when spring arms  60   a  are not biased by protrusions  12   a  they should be able to physically touch and electrically contact tip arm assembly  34  and ring arm assembly  36 , thereby causing arm assemblies  34  and/or  36  to short to ground element  50  through resilient member  60 . In one embodiment, resilient member  60  has a contact pressure of about 140 ksi against protrusions  12   a  during the open circuit condition, and a contact pressure of about 86 ksi against arm assemblies  34  and/or  36  during a short circuit condition. However, other suitable contact pressures may be used during open and short circuit conditions. 
     Cover  70  attaches to base  12  protecting internal components of surge protector  10  from adverse environmental effects and to provide personnel safety. Cover  70  is formed from a dielectric material, for example, a thermoplastic material. Cover  70  can be attached to base  12  by any suitable means, for example, tabs  12   b  on base  12  that correspond to apertures  70   b  on cover  70  may be used to secure cover  70 . 
     During normal operation electrical current flow is from outside plant tip pin  24   a,  through electrically conductive tip arm  14 , and to central office tip pin  24   b.  Likewise, during normal operation electrical current flow is from outside plant ring pin  26   a,  through electrically conductive ring arm  16 , and to central office ring pin  26   b.    
     If a sustained overvoltage event occurs, for example, where a high voltage line permanently contacts a line, gas tube  40  shorts the associated overcurrent to ground element  50 , thereby increasing the temperature of ground element  50 . Consequently, ground element  50  transfers heat to resilient member  60  increasing the temperature of resilient member  60 . When resilient member  60  reaches a predetermined temperature range, spring arms  60   a  of resilient member  60  soften and/or melt the material of protrusions  12   a . Consequently, spring arms  60   a  of resilient member  60  displace protrusion(s)  12   a  electrically contacting tip arm  14  of tip arm assembly  34  and/or ring arm  16  of ring arm assembly  36  shorting arm assemblies  34  and/or  36  to ground element  50  through resilient member  60 . Thus, sustained overvoltages are permanently shorted to ground preventing damage to equipment and/or other injury to personnel. 
     Additionally, the present invention may combine the surge protection characteristics of gas tube  40  and varistors  48  achieving a surge protector wherein varistors  48  interact with gas tube  40  within a range of DC breakdown voltages to divert surges to the ground element. For example, varistor  48  may be a metal oxide varistor (MOV) having predetermined protection characteristics. With gas tube  40  and varistors  48  interacting, better surge response is achieved. However, depending on its configuration with respect to gas tube  40 , varistors  48  may act merely as a back up device instead of interacting with gas tube  40 . 
     Gas tube  40  by its nature is difficult to repeatedly manufacture with a precise DC breakdown voltage. Consequently, for a given population of gas tubes  40 , the DC breakdown voltage varies across a range that is wider than the ranges of the other components. Accordingly, for a particular gas tube and manufacturing type, an acceptable DC breakdown voltage range is determined by selecting a minimum and a maximum DC breakdown voltage. Each gas tube is tested, and only those gas tubes that fall within predetermined minimum and maximum breakdown voltages are passed, thereby creating a population of gas tubes that fall within a preselected range of DC breakdown voltages. If the DC breakdown voltage range is too small, then too large of a percentage of gas tubes that are manufactured are not used, and thus wasted. If the DC breakdown voltage range is too large, then the ability to properly combine varistors with any gas tube in the range becomes more difficult. 
     The DC breakdown voltage is the voltage at which a gas tube breaks down and diverts electricity to the ground element when the rate of rise of the voltage is sufficiently low such that the ionization time of the gas tube is not exceeded. When the rate of rise of voltage reaches surge levels, the gas tube breaks down at an impulse breakdown voltage that is higher than the DC breakdown voltage. The impulse breakdown voltage is higher than the DC breakdown voltage because the ionization time of the gas tube allowed the voltage to rise above the DC breakdown voltage level before the gas tube could divert the surge. The impulse breakdown voltage of the gas tube varies as a function of the rate of rise of the voltage and the time it takes for a particular gas tube to direct the voltage surge to the ground element is commonly termed its “operate time”. 
     On the other hand, varistors clamp voltages and thereby prevent voltages from getting too high. Varistors are immediate and are not rate of rise dependent like the gas tube. Instead, the clamping voltage of a varistor is a function of current. As current increases, the clamping voltage of the varistor increases. 
     In one embodiment, a varistor is combined with a gas tube so that the varistor acts as a replacement for an air gap back-up, and the clamping voltage of the varistor is sufficiently higher than the DC breakdown voltage of the gas tube. Consequently, the impulse breakdown voltage of the gas tube is not appreciably affected. However, in another embodiment the clamping voltage of the varistor relative to the DC breakdown voltage of the gas tube is predetermined so that the varistor will clamp voltage surges during the ionization time of the gas tube, thereby lowering the impulse breakdown voltage of the gas tube. FIG. 8 illustrates an exemplary voltage response of the present invention whereby the interacting varistor acts to lower the impulse breakdown voltage by clamping the voltage surge until the gas tube responds. 
     However, even gas tubes made on the same manufacturing line have a wide range of DC breakdown voltages. The present invention takes into account the range of DC breakdown voltages of gas tubes by setting the varistor clamping voltage at a point to achieve optimal coordination between the varistor and any gas tube in the range of DC breakdown voltages as described below. Doing so balances two competing objectives, namely: 1) lowering the impulse breakdown voltage below that of a gas tube alone for any gas tube in the population; yet 2) allowing the gas tube to protect the varistor from being burned out for any gas tube in the population. 
     If the clamping voltage of the varistor is set too high, there may be some gas tubes at the low end of the range where the impulse breakdown voltage will not be lowered and the varistor operates merely as a back-up device. If the clamping voltage of the varistor is set too low, the varistor could be burned out before the gas tube can divert the surge to the ground element when the varistor is matched with a gas tube at the high end of the range of DC breakdown voltages. 
     In one embodiment, the difference between the minimum and the maximum DC breakdown voltage of gas tube  40  is between about 115 volts and about 155 volts, and more preferably is about 135 volts. Preferably the minimum DC breakdown voltage is about 265 volts and the maximum DC breakdown voltage is about 400 volts. The operate time of gas tube  40  is preferably between about 1 to about 20 microseconds. 
     In one embodiment, the clamping voltage of the varistor at 1 mA is set in the middle 60% of the range of the DC breakdown voltages, and more preferably, is set at about the middle of the range of the DC breakdown voltages. In the preferred range of DC breakdown voltages of 265 to 400 volts, the clamping voltage of the varistor is preferably between about 300 volts and about 400 volts or more. In these preferred ranges, the varistor can be selected to have a clamping voltage that will lower the impulse breakdown voltage of a gas tube with a DC breakdown voltage at 265 volts, and yet will not burn out when matched with a gas tube with a DC breakdown voltage of 400 volts. By way of example, a T67 gas tube may be used with two 5 mm metal oxide varistors both available from Epcos, Inc. of Chicago, Ill. 
     In other embodiments of the present invention, protrusions  12   a  may be integrally molded or attached to other suitable components of surge protector  10 , rather than base  12 . For example, as shown in FIGS. 10 a  and  10   b,  a pair of protrusions  12   a ′ are integrally molded with a cover  70 ′ suitable for use with a surge protector  10 ′. Surge protector  10 ′ is similar to surge protector  10  in both concept and operation and the general differences between the two embodiments will be described herein. 
     As shown in FIG. 11, surge protector  10 ′ includes a base  12 ′ for inserting a tip arm assembly (not shown), a ring arm assembly  36 ′, and a ground element  50 ′ therein. Ground element  50 ′ includes a ground pin  13 ′ and ground plate  52 ′. Ground plate  52 ′ is generally longer than ground plate  52  of surge protector  10  because a first end  52 ′ of ground plate  52 ′ generally extends to base  12 ′. Additionally, ground pin  13 ′ is generally shorter than ground pin  13  of surge protector  10  because ground pin  13 ′ does not require collar  13   b.    
     Instead, as shown in FIG. 9, a resilient member assembly  65 ′ is electrically attached to ground element  50 . Resilient member assembly  65 ′ includes a stud  62 ′ and resilient member  60 ′. Resilient member  60 ′ is thermally and electrically connected at coil  60   b ′ to a stud  62 ′, which is thermally and electrically connected to a second end portion  52   b ′ of ground plate  52 ′. When surge protector  10 ′ is assembled with the cover removed, respective spring arms  60   a ′ of resilient member  60 ′ contact a portion of tip arm assembly and a portion of ring arm assembly  36 ′. More specifically, the respective spring arm  60   a ′ of resilient member  60 ′ electrically contacts ring arm assembly  36 ′ at a stop tab (not shown) disposed on ring arm  16 ′ that is generally aligned with spring arm  60   a ′. Likewise, the respective spring arm  60   a ′ of resilient member  60 ′ electrically contacts tip arm assembly at a stop tab on tip arm (not shown). However, when cover  70 ′ is inserted over and attached to base  12 ′, knife edges  72 ′ of cover  70 ′ slide between respective spring arms  60   a ′ of resilient member  60 ′ 0  and a portion of tip arm assembly and ring arm assembly  36 ′ allowing protrusions  12   a ′ to be disposed therebetween. In other words, when cover  70 ′ is attached to base  12 ′, protrusions  12   a ′ on cover  70 ′ bias spring arms  60   a ′ of resilient member  60 ′ towards each other preventing electrical contact between the spring arms  60   a ′ and a portion of the respective tip arm and/or ring arm assemblies. Thus, unless, and until, a sustained overvoltage condition occurs that will soften and/or melt protrusions  12   a ′, the tip arm and/or ring arm assemblies remain in an open circuit condition with respect to ground element  50 ′. 
     Other suitable configurations of the present inventive concepts may also be practiced. For example, surge protector  10  and/or  10 ′ may be configured as a 1-pin, a 4-pin, or other suitable configuration of a surge protector. In the 1-pin configuration, the single pin is electrically connected the ground element and the ring and tip arm assemblies are configured for inserting pins therein. In other embodiments, a 4-pin configuration includes two pins located on each of the tip arm and ring arm assemblies and a ground element suitably configured for inserting a pin therein. 
     Many modifications and other embodiments of the present invention, within the scope of the appended claims, will become apparent to a skilled artisan. For example, the pair of two-element gas tubes may be replaced with a single three-element gas tube. Additionally, electrical components may be plated for environmental protection. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments may be made within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. The invention has been described with reference to central office protectors but the inventive concepts of the present invention are applicable to other surge protectors and other suitable devices having failsafe mechanisms.