Abstract:
A housing encapsulating first and second terminals of a surge arrester disconnector becomes structurally weakened before activation of a disconnect device in the disconnector when the disconnector is exposed to heat, thereby preventing the disconnector from producing a projectile with a force sufficient to classify the disconnector as a hazardous material under Department of Transportation regulations.

Description:
FIELD OF THE INVENTION 
   The invention relates generally to high voltage electrical power generation and transmission systems, and more specifically to the safe transportation and storage of surge arresters having a heat activated disconnector. 
   BACKGROUND 
   Electrical power transmission and distribution equipment is subject to voltages within a fairly narrow range under normal operating conditions, and the equipment may operate at high voltages of, for example, 1000V or greater. However, system disturbances, such as lightning strikes and switching surges, may produce momentary or extended voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. These voltage variations often are referred to as over-voltage conditions. If not protected from over-voltage conditions, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed by such over-voltage conditions and associated current surges. Accordingly, it is routine practice for system designers to use surge arresters to protect system components from dangerous over-voltage conditions. 
   A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment to divert over-voltage-induced current safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. The surge arrester normally operates in a high impedance mode that provides a low current path to ground having a relatively high impedance. In this mode, normal current at the system frequency is directed to the electrical equipment and is prevented from following the surge current to ground along the current path through the surge arrester. When exposed to an over-voltage condition, the surge arrester operates in a low impedance mode that provides a high current path to electrical ground having relatively low impedance. When the surge arrester is operating in the low-impedance mode, the impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester. In this mode, current from the over-voltage condition is directed to ground and not to the electrical equipment. Upon completion of the over-voltage condition, the surge arrester returns to operation in the high impedance mode. The surge arrester also includes a disconnector that disconnects the surge arrester from ground if the over-voltage condition is too extreme or continues too long. 
     FIG. 1  is a partial cross-sectional view of a conventional high voltage surge arrester  90 . As illustrated in  FIG. 1 , the high voltage surge arrester  90  typically includes an elongated outer enclosure or housing  100  made of an electrically insulating material, a pair of electrical terminals  102 ,  104  at opposite ends of the enclosure  100  for connecting the arrester between a line-potential conductor (not shown) and electrical ground (not shown), respectively, and a stack or array of other electrical components  106  that form a series electrical path between the terminals  102  and  104 . Terminal studs  108 ,  110  connect to the line and ground terminals  102  and  104 , respectively. An insulated mounting bracket or hanger  114  also may be provided for mounting of the arrester  90  to, for example, another piece of equipment or to a utility pole. 
   To prevent short circuiting of line potential conductors connected to the surge arrester  90 , a disconnector  112  is provided on the ground terminal stud  110 . The disconnector  112  may include an internal resistor or other electrical element connected in parallel with a spark gap assembly and a charged black powder in an unprimed .22 caliber cartridge that is heat activated. Thus, in the event of a sustained over-voltage current flow through the terminal stud  110 , a spark is generated by the spark gap assembly of the disconnector  112 . Heat from the spark detonates the charged powder cartridge to mechanically sever electrical connection between the terminal stud  110  and the lower terminal  104  in the housing  100 , thereby isolating the terminal stud  110  from the line connection. The force created by the activation of the charged powder cartridge typically causes the terminal stud  110  to separate from the surge arrester  90 , thereby effectively isolating the failed arrester from the power system. 
   Undesirably, portions of the heat sensitive disconnector  112 , including the terminal stud  110 , can become a projectile when the cartridge is inadvertently exposed to heat during shipping, transit, or storage. During transport and storage, if an accident or other occurrence results in a fire near one or more arresters, activation of the charged powder cartridges of the disconnectors in the arresters can be hazardous to first responders at the scene of the fire. Projectiles attributable to detonation of the charged powder cartridges of the disconnectors in such circumstances are of particular concern, particularly when a large number of arresters with such disconnectors are shipped and stored. A variety of different types of conventional surge arresters with disconnectors are vulnerable to the hazards noted above. Additionally, similar problems may be experienced by all disconnector devices. The problems noted above are therefore not considered unique to any particular disconnector or to any particular surge arrester. 
   In light of the hazards posed by arresters when subjected to a fire during shipping, transit, or storage, the United States Department of Transportation (DOT) has classified conventional surge arresters as hazardous materials that must be transported in accordance with DOT hazardous material transportation regulations. Transporting arresters under those guidelines increases the cost of such transportation. Alternatively, the DOT safety regulations can be met by fitting arresters with restraints that prevent the terminal stud and portions of the disconnectors from becoming projectiles when the disconnector cartridge is inadvertently exposed to heat during shipping, transit, or storage. However, adding such restraints increases the cost of arresters. Another option is to package the arresters in sturdy metallic cases during shipping, transit, or storage to meet the DOT requirements, though such packaging may be prohibitively expensive. 
   Accordingly, a need in the art exists for a surge arrester disconnector that is not classified as a hazardous material under DOT regulations. 
   SUMMARY OF THE INVENTION 
   The invention provides an inexpensive and practical way to prevent the terminal studs and other portions of a heat-sensitive arrester disconnector from becoming forceful projectiles when the disconnector is exposed to excessive heat during shipping, transit, or storage. In accordance with one aspect of the invention, the terminal studs of the disconnector are encapsulated in a housing comprising materials that melt or burn at a temperature that is lower than the activation temperature of the disconnector&#39;s disconnect cartridge. The activation temperature of the cartridge is the temperature at which the propellant in the cartridge ignites. The activation temperature is sometimes referred to as the auto-ignition temperature. When the arrester encounters rising temperature caused by fire during shipping, transit, or storage, the disconnector housing materials melt or burn away before the increasing temperature causes activation of the cartridge. As the housing melts or burns away, the terminal studs of the disconnector are released. Thus, when the cartridge does activate, the terminal studs, or other portions of the disconnector, are not projected by the explosion created by the cartridge. 
   According to another aspect of the invention, the terminal studs of the disconnector are encapsulated in a housing comprising materials that become sufficiently weakened at a temperature that is lower than the activation temperature of the disconnector&#39;s disconnect cartridge. When the arrester encounters rising temperature caused by fire during shipping, transit, or storage, the temperature sufficiently weakens the walls of the disconnector housing or the adhesive holding the disconnector housing together before the increasing temperature causes activation of the cartridge. Thus, when the cartridge does activate, the explosive force from the activated cartridge does not produce a large pressure increase within the disconnector because the weakened walls do not contain the expanding gases from the cartridge. In this case, the explosive force for the activated cartridge is not sufficient to produce projectiles of the magnitude required for classification as a hazardous material by the DOT. 
   According to yet another aspect of the invention, the terminal studs of the disconnector are encapsulated in a housing comprising materials that melt or burn, or are sufficiently weakened, during a fire prior to activation of the disconnector&#39;s disconnect cartridge. Because the cartridge is encapsulated in the disconnector&#39;s housing, the cartridge will experience a slower rise in temperature than the housing during a fire. Accordingly, the housing can comprise materials that melt or burn, or are sufficiently weakened, at a temperature that is above the activation temperature of the cartridge, as long as the housing materials melt, burn, or become weakened before the temperature inside the disconnector increases to the activation temperature of the cartridge. 
   These and other aspects, objects, and features of the invention will become apparent from the following detailed description of the exemplary embodiments, read in conjunction with, and reference to, the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial cross-sectional view of a conventional high voltage surge arrester with a heat-activated disconnector. 
       FIG. 2  is a cross-sectional view of an arrester disconnector according to an exemplary embodiment. 
       FIG. 3  is a cross-sectional view of the arrester disconnector of  FIG. 2  illustrating a current path during a transient over-voltage condition according to an exemplary embodiment of the invention. 
       FIG. 4  is a cross-sectional view of the arrester disconnector of  FIG. 2  illustrating a current path in a low-impedance, over-voltage fault current mode according to an exemplary embodiment. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   The invention allows for safe detonation of a disconnect cartridge in a disconnector of a surge arrester in the event of fire during transportation and storage. The disconnector&#39;s housing comprises materials that melt, burn, or otherwise become weakened during a fire before the fire causes activation of the cartridge, thereby allowing the disconnector&#39;s terminal studs safely to detach from the disconnector before activation of the cartridge or preventing a more forceful explosion. Such action prevents the terminal studs or other portions of the disconnector from being discharged as forceful projectiles when the cartridge is activated. When the cartridge eventually discharges, parts of the disconnector are not projected at speeds or distances that qualify as a DOT classified hazardous material. 
   The following description of exemplary embodiments refers to the attached drawings, in which like numerals indicate like elements throughout the figures. 
     FIG. 2  is a cross-sectional view of an arrester disconnector  200  according to an exemplary embodiment of the invention. The disconnector  200  comprises two terminal studs  202 ,  204  separated by an electrical element  206 . In exemplary embodiments, the electrical element  206  can comprise a resistor, a capacitor, a varistor, an insulator, or combinations of two or more of these items. A housing  208  encapsulates the terminal studs  202 ,  204  and the electrical element  206 , thereby creating a sealed chamber  210  between the terminal studs  202 ,  204 . A disconnect cartridge  212  is disposed within a recess of the terminal stud  202  and is positioned with an end adjacent to a projection  204   a  of the terminal stud  204  such that an air gap  214  is created between the projection  204   a  and the cartridge  212 . An o-ring  216  is compressed between the terminal stud  204  and the cartridge  212 . 
   In an exemplary embodiment, when used in conjunction with a surge arrester, such as the surge arrester  100  illustrated in  FIG. 1 , the terminal stud  204  can be the ground terminal stud  110  of the surge arrester. Additionally, the terminal stud  202  can be coupled to the electrical terminal  104  in the housing of the surge arrester. 
   The terminal studs  202 ,  204  are formed of conductive materials, such as stainless steel. The electrical element  206  is designed to resist current flow during normal voltage conditions in which a particular disconnector  200  is operated. 
   The disconnect cartridge  212  can comprise a .22 caliber cartridge having a black powder actuated charge. For example, the powder charge can comprise a Q2065 propellant sold under the WINCHESTER trademark. 
   In a surge arrester operating under normal voltage conditions, the surge arrester operates in a high-impedance mode that provides a low current path to ground having relatively high impedance. Because of the high-impedance of the current path caused by the electrical components  106  of the surge arrester, relatively little, if any, current is directed to ground. Accordingly, in this mode, current is directed to the electrical equipment to which the surge arrester is connected. 
     FIG. 3  is a cross-sectional view of the arrester disconnector  200  of  FIG. 2  illustrating a current path  302  during a transient over-voltage condition according to an exemplary embodiment of the invention. If a transient over-voltage condition occurs in a surge arrester comprising the disconnector  200 , the electrical components  106  of the surge arrester operate in a low-impedance mode to direct the over-voltage through the disconnector  200  to ground. In this operation, current through the disconnector  200  follows the current path  302  through terminal stud  202 , electrical element  206 , and terminal stud  204  to ground via a ground wire (not shown). When the over-voltage conditions ends, the electrical components  106  again operate in the high-impedance mode to direct current to the electrical equipment to which the arrester having the disconnector  200  is connected. 
   If the electrical components  106  of the surge arrester fail, the surge arrester  200  operates in a low-impedance mode that provides a high current path to electrical ground having relatively low impedance. When the surge arrester is operating in the low-impedance mode, the impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester in which the disconnector  200  is disposed. 
   When the surge arrester in which the disconnector  200  is disposed fails, then a fault current can be directed through the current path  302 . Initially, the fault current is directed to ground via the current path  302  as described previously for the transient over-voltage condition. However, a continuous fault current will follow a current path  402  illustrated in  FIG. 4  by arcing across the air gap  214  in the disconnector  200  to bypass the electrical element  206 .  FIG. 4  is a cross-sectional view of the arrester disconnector  200  of  FIG. 2  illustrating the current path  402  in a low-impedance, over-voltage fault current mode according to an exemplary embodiment of the invention. 
   As illustrated in  FIG. 4 , when the disconnector  200  encounters the continuous fault current, the available fault current follows the current path  402  through the terminal stud  202 , the cartridge  212 , and the terminal stud  204  (via the protrusion  204   a ) to ground via a ground wire (not shown). This current path  402  sparks a detonating arc  404  in the air gap  214  between the projection  204   a  of the terminal stud  204  and the end of the cartridge  212 . 
   The detonating arc  404  supplies heat energy sufficient to detonate the propellant in the cartridge  212 . The detonation of the cartridge  212  initially is contained in the sealed chamber  210 . However, the pressure within the sealed chamber  210  increases until the force created by the detonation causes the housing  208  to fracture, which can project pieces of the housing  208  into the surrounding area. Additionally, the force created by the detonation projects the terminal stud  204  away from the disconnector  200 , thereby severing the current path  402 . The failed arrester in which the disconnector  200  is disposed is thereby effectively isolated from ground. 
   Because activation of the cartridge  212  in the disconnector  200  produces a projectile (the terminal stud  204  and/or fragments from the housing  208 ) with a certain force, the DOT could classify the disconnector  200  as a hazardous material because of the potential for the cartridge  212  to activate in the event of a fire during shipping, transit, or storage. However, the disconnector  200  comprises safeguards to prevent the forceful projection of the terminal stud  204  and housing  208  fragments. 
   In an exemplary embodiment, the housing  208  comprises at least one material having a lower melting point and/or ignition point than the activation temperature of the cartridge  212 . The activation temperature of the cartridge  212  is the temperature at which the propellant in the cartridge  212  ignites. The activation temperature is sometimes referred to as the auto-ignition temperature. In this embodiment, the terminal studs  202 ,  204  are disposed in the housing  208  comprising at least one material that melts or ignites at a relatively lower temperature with respect to the activation temperature of the cartridge  212 . At least a portion of the housing  208  melts or burns during a fire before heat from the fire causes the cartridge to activate, thereby allowing the terminal studs  202 ,  204  and housing  208  materials safely to detach from the disconnector  200  before activation of the cartridge  212 . Such action prevents the terminal studs  202 ,  204 , the housing  208 , or other portions of the disconnector  200  from being discharged as forceful projectiles when the cartridge  212  activates. Thus, when the cartridge  212  eventually discharges, parts of the disconnector  200  are not projected at speeds or distances that qualify as a DOT classified hazardous material. 
   In an exemplary embodiment, the entirety of the housing  208  comprises a material having a lower melting point and/or ignition point than the activation temperature of the cartridge  212 . Alternatively, only joints in the housing  208  or that seal the housing  208  to the terminal studs  202 ,  204  and the electrical element  206  comprise the material having a lower melting point and/or ignition point than the activation temperature of the cartridge  212 . In this embodiment, as the material in the joints burns or melts, the housing  208  opens and/or falls away from the disconnector  200 . In another alternative embodiment, the housing  208  comprises a material having a lower melting point and/or ignition point than the activation temperature of the cartridge  212  while the joints comprise a different material. 
   In another exemplary embodiment, the housing  208  comprises at least one material that becomes sufficiently weakened at a temperature that is lower than the activation temperature of the cartridge  212 . In this embodiment, the terminal studs  202 ,  204  of the disconnector  200  are encapsulated in a housing  208  comprising at least one material that becomes sufficiently weakened at a temperature that is lower than the activation temperature of the cartridge  212 . When the disconnector  200  encounters rising temperature caused by fire during shipping, transit, or storage, the temperature sufficiently weakens the walls of the housing  208  or its joints before the increasing temperature causes activation of the cartridge  212 . Thus, when the cartridge  212  does activate, the explosive force from the activated cartridge  212  does not produce a large pressure increase within the sealed chamber  210  of the disconnector  200  because the weakened walls or joints of the housing  208  do not contain the expanding gases from the cartridge  212 . In this case, the explosive force for the activated cartridge  212  is not sufficient to produce projectiles of the magnitude required for classification of the disconnector  200  as a hazardous material by the DOT. 
   In an exemplary embodiment, the entirety of the housing  208  comprises a material that becomes sufficiently weakened at a temperature that is lower than the activation temperature of the cartridge  212 . Alternatively, only joints in the housing  208  or that seal the housing  208  to the terminal studs  202 ,  204  and the electrical element  206  comprise the material that becomes sufficiently weakened at a temperature that is lower than the activation temperature of the cartridge  212 . In this embodiment, the material in the joints provides the weakened structure that prevents the housing  208  from containing the expanding gases from the cartridge  212 . In another alternative embodiment, the housing  208  comprises material that becomes sufficiently weakened at a temperature that is lower than the activation temperature of the cartridge  212  while the joints comprise a different material. 
   In another exemplary embodiment, the housing  208  comprises at least one material that melts or burns, or is sufficiently weakened, during a fire prior to activation of the disconnector&#39;s cartridge  212 . In this embodiment, the terminal studs  202 ,  204  of the disconnector  200  are encapsulated in a housing  208  comprising at least one material that melts or burns, or is sufficiently weakened, during a fire prior to activation of the disconnector&#39;s cartridge  212 . Because the cartridge  212  is encapsulated in the housing  208 , the cartridge  212  will experience a slower rise in temperature than the housing  208  during a fire. Accordingly, the housing  208  (and/or its joints) can comprise materials that melt or burn, or are sufficiently weakened, at a temperature that is above the activation temperature of the cartridge  212 , but the housing  208  materials melt, burn, or become weakened before the temperature inside the disconnector  200  increases to the activation temperature of the cartridge  212  to detonate the cartridge  212 . 
   Exemplary materials suitable for the housing  208 , including the joints, include epoxy, PVC, other thermo-plastic materials, or any suitable material having the melting, burning, or weakening characteristics described herein. 
   In an exemplary embodiment, the auto-ignition point of the propellant in the cartridge  212  is approximately 190 degrees centigrade. Accordingly, the housing  208  materials will melt, ignite, or become sufficiently weakened at a temperature that is less than 190 degrees centigrade. Alternatively, the housing  208  materials will melt, ignite, or become sufficiently weakened at a temperature that is above 190 degrees centigrade but before the temperature of the cartridge  212  in the disconnector  200  reaches 190 degrees centigrade during a fire. 
   The disconnector  200  according to the exemplary embodiments described herein can be used with any surge arrester employing such isolation functions. Additionally, the housing  208  described herein can be used with any disconnector to provide a fire safe function for such disconnector. 
   The foregoing exemplary embodiments enable a fire safe arrester disconnector. Many other modifications, features, and embodiments will become evident to a person of ordinary skill in the art having the benefit of the present disclosure. It should be appreciated, therefore, that many aspects of the invention were described above by way of example only and are not intended as required or essential elements of the invention unless explicitly stated otherwise. It should also be understood that the invention is not restricted to the illustrated embodiments and that various modifications can be made within the spirit and scope of the following claims.