Abstract:
A surge suppressor includes a surge suppression device. A novel current limiting fuse design prevents current flow when the surge suppression device fails. A thermal fusing device is configured to prevent current flow when excessive heat is generated in the surge suppression device. A novel enclosure is used for containing the surge suppression device, over current fuse and thermal fuse. 
     The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.

Description:
BACKGROUND 
   Transient Voltage Surge Suppression (TVSS) devices use Metal Oxide Varistors (MOVs). A MOV is connected between an A.C. power line and a neutral line. The MOV becomes conductive during a voltage transient. In the conductive state, the MOV temporarily discharges the voltage transient to the power line. When MOVs fail they can create a short circuit that draws excessive A.C. current from the power line. This excessive current is drawn though the MOV circuit until the fuse in the electrical circuit clears or until a circuit breaker opens. Until the fuse or circuit breaker opens, a large current is conducted through the circuit possibly causing catastrophic results such as extensive smoke, mechanical damage and fire. 
   A small fuse could be used in the surge suppression system to limit the over current condition and prevent this type of damage. However smaller fuses severely limit the ability of the MOV to temporarily short transient voltage surges. 
   Multiple MOVs are often connected together in parallel to share transient voltage spikes. The multiple MOVs are typically located in the same enclosure. If a voltage spike or power surge is in excess of the combined energy handling capability of the multiple MOVs, one or more of the fused MOVs blow. Other MOVs and electrical equipment in the enclosure may be damaged by an explosion or fire that happens during the power surge condition. This may prevent some or all of the MOVs in the enclosure from providing protection during subsequent power surges. Further, the MOVs are typically contained inside a plastic enclosure. If the explosion is severe enough, the smoke and explosion from the power surge event may destroy the plastic enclosure and other electrical equipment located next to the enclosure. For example, the electrical surge event may damage electrical equipment in a load center or transfer switch containing the surge suppressor, resulting in large monetary losses. 
   The present invention addresses this and other problems associated with the prior art. 
   SUMMARY OF THE INVENTION 
   A surge suppressor includes a surge suppression device. A novel current limiting fuse design prevents current flow when the surge suppression device fails. A thermal fusing device is configured to prevent current flow when excessive heat is generated in the surge suppression device. A novel enclosure is used for containing the surge suppression device, over current fuse and thermal fuse. 
   The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded view of a surge suppressor. 
       FIG. 2  is a partial cutaway view of the surge suppressor shown in FIG.  1 . 
       FIG. 3  is a circuit diagram for the surge suppressor shown in FIG.  1 . 
       FIG. 4  is a isolated view of a thermal fuse used in the surge suppressor shown in FIG.  1 . 
       FIG. 5  is an isolated view of an over current fuse used in the surge suppressor shown in FIG.  1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a surge suppressor  12  that includes a Metal Oxide Varister (MOV)  24 , a current limiting fuse  20 , and a thermal fuse  44  all contained inside a metal enclosure  14 . The surge suppressor  12  is a module that can be incorporated into other power protection equipment. The MOV  24  in one embodiment is an off the shelf device that is known to those skilled in the art. 
   The enclosure  14  includes an upper piece  14 A and a lower piece  14 B. The two enclosure pieces  14 A and  14 B form an internal cavity  32  that retains the MOV  24 . In one example, the enclosure  14  is around 2¾ inches wide, 2¾ inches in height and around 1½ inches in depth. The walls  15  of the enclosure  14  in one example are around one quarter inch thick and are made from aluminum or some other conductive metal material. 
   Encasing the MOV  24  in metallic enclosure  14  isolates arcing and other destructive events caused by an electrical surge condition of MOV failure. For example, the strength of enclosure  14  isolates explosions, arching and fires inside cavity  32 . Because only one, or a few, MOVs  24  are located in the enclosure  14 , an explosion inside the enclosure  14  will not damage other surge suppressors or electrical equipment located outside of enclosure  14 . 
   A wire  18  is coupled to a first terminal  26  of the MOV  24  and extends out from the enclosure  14  through a hole  42 . Opposite halves of hole  42  are formed in the upper piece  14 A and lower piece  14 B of enclosure  14 . This allows the wire  18  to be laid in the lower half of the hole  42  when the MOV  24  is being installed in cavity  32 . When the upper half  14 A of the enclosure  14  is seated over the lower half  14 B of the enclosure  14 , the wire  18  is then snugly encased inside hole  42 . 
   When the surge suppressor  12  is installed, the wire  18  is usually coupled to either a hot power line or a neutral line. In one embodiment, the wire  18  is a #10 Average Wire Gauge (awg). However, in different applications, the wire  18  may be a different awg sizes. For example, the wire  18  could be smaller or larger depending on the voltage and/or current requirements of the surge suppressor  12 . 
   The surge suppressor  12 , among other things, reduces the short circuit problems that arise during a MOV failure. This is achieved in one way by providing current limiting fuse  20 . The current limiting fuse  20  is coupled between the wire  18  and the terminal  26  of the MOV  24 . In one embodiment, the current limiting fuse  20  is a #16 awg wire having a substantially smaller diameter than the wire  18 . Other wire sizes can be used for fuse  20  depending on current limiting requirements of the electrical system connected to the surge suppressor  12 . For example, a smaller #18 awg wire could be used for quicker over current fusing. In one embodiment the fuse  20  is copper with a tin outside coating. Of course other types of conductive materials could also be used for fuse  20 . 
   The current limiting fuse  20  is shown in more detail in FIG.  5  and includes optional crimps  22 . The crimps  22  have a reduced diameter (thinner cross-sectional shape), than the general round circumference of the fuse wire  20 . The flatten shape of crimps  22  concentrate current density and promote arcing. Arcs could be generated at multiple crimps  22  at the same time. This speeds of the process of the arcs opening up portions of fuse  20  enhancing the current limiting ability of fuse  20 . In one embodiment the crimps  22  are located about % inch apart and there may be two or three crimps for around a two inch long fuse  20 . 
   Alternatively, if space permits a commercially available fuse could also be used in place of the current limiting fuse  20 . For example, any commercially available off-the-shelf fuse with the required current fusing properties could be used. 
   Referring back to  FIG. 1 , a second terminal  28  of MOV  24  is coupled to a divider portion  34  of the enclosure  14  through a thermal fuse  44 . The thermal fuse  44  is shown in further detail in FIG.  4  and in one embodiment is made from a piece of spring steel. One end of thermal fuse  44  is coupled by solder  74  to terminal  28  of the MOV  24 . The opposite end of thermal fuse  44  is attached with a screw  38  to divider  34 . The screw  38  is threadingly engaged with a hole  36  in the divider  34 . 
   Divider  34  is formed in the lower piece  14 B of the enclosure  14  and creates an elevation between the two ends of thermal fuse  44 . The thermal fuse  44  is springingly bent from the divider  34  down to the MOV terminal  28 . 
   The temperature of the terminal  28  will increase during abnormally high voltage conditions. This causes the solder  74  to melt allowing the thermal fuse  44  to free itself from, terminal  28 . A spring effect moves the thermal fuse  44  from position  70  to position  72  in FIG.  4 . This opens the circuit preventing current from flowing from the MOV  24  to enclosure  14 . Referring back to  FIG. 1 , a paper sheet  30  is located inside the bottom of cavity  32  underneath the MOV  24 . The sheet  30  can be any fibrous material such as paper or cloth that provides an insulating and condensation absorbing layer between the MOV  24  and the enclosure  14 . A heat shrink  46  is slid over the terminal  26  and a portion of wire  52  for insulating purposes. 
   Two wires  50  and  52  are coupled to terminals  28  and  26  of MOV  24  respectively. The wires are coupled to a monitoring circuit  60  shown in FIG.  3 . The monitoring circuit  60  measures the voltage across the terminals  28  and  26  to determine the operational status of MOV  24 . 
   A large voltage across terminals  28  and  26  indicates the MOV  24  is not conducting current and is operational. Little or no voltage across the terminals  28  and  26  may indicate the MOV  24  has shorted and thus nonoperational. Alternatively, either one of the fuses  20  or  44  may blow. This would also reduce the voltage across the terminals  26  and  28 . 
   The monitoring circuit  60  upon detecting any of these low voltage conditions across the MOV  24  generates an annunciation signal that notifies an operator of the failure condition. This allows an operator to identify and replace the failed surge suppressor  12 . 
   A hole  40  on the side of enclosure  14  is used for inserting sand into cavity  32  after the enclosure  14  has been bolted together. A rivet (not shown) is then inserted into hole  40  to prevent the sand from escaping. 
   The enclosure  14  includes screw holes  16 . Screws  17  are threadingly engaged in the holes  16 . One or more of the screws  17  may have an extended length for extending through both the holes  16  in the upper enclosure piece  14 A and the lower enclosure piece  14 B and then threadingly engaging with a hole in a bus bar (not shown). Other screws may only be long enough to hold the upper piece  14 A and lower piece  14 B together. 
     FIG. 2  shows the MOV  24  inserted inside the cavity  32  of enclosure  14 . The thermal fuse  44  is shown coupled between the MOV terminal  28  and the divider section  34 . A first portion  66  of cavity  32  contains the terminal  28  and thermal fuse  44 . A second portion  68  of cavity  32  contains MOV terminal  26  and a portion of current limiting fuse  20 . 
   In one embodiment, cavity portion  66  is filled with a paraffin wax material  64 . The divider  34  in one instance operates as a dyke to retain wax  64  within cavity section  66  when the liquid wax is initially poured over thermal fuse  44 . The other portions of cavity  32  are encased with sand  62 . The sand  62  more quickly extinguishes arcing across sections of current limiting fuse  20  that are opened during over current conditions. 
     FIG. 3  shows an electrical diagram of the surge suppressor  12 . The wire  18  couples a first end of the current limiting fuse  20  to a power line  70 . A second end of the current limiting fuse  20  is coupled to terminal  26  of MOV  24 . The second terminal  28  of MOV  24  is coupled through thermal fuse  44  to a neutral wire or bus bar  72 . In the embodiment shown in  FIG. 1 , the thermal fuse  44  is coupled to enclosure  14  and the enclosure  14  is then electrically and mechanically connected to the power (line or neutral) connection  72 . The monitoring circuit  60  is coupled across MOV terminals  26  and  28  by wires  52  and  50 , respectively. 
   In an alternative embodiment, wire  18  is coupled to the neutral connection  72  and the enclosure  14  is connected to the power line  70 . 
   Referring to  FIGS. 1-5 , an energy transient  76  may occur on power line  70 . For example, lighting may strike power line  70 . Under normal operating conditions, the transient  76  is directed from the power line  18 , through the MOV  24  and enclosure  14  to the neutral connection  72 . This redirects the transient  76  away from any electrical equipment  74 . 
   The MOV  24  may fail due to “old age,” excessive transient current or abnormally high AC voltage. In the failed condition, the MOV  24  may essentially become a short circuit. This allows current from power line  70  to go directly to neutral  72 . The current limiting fuse  20  opens in this short circuit condition at one or more of the crimps  22 . This prevents damage to any equipment outside the enclosure  14 . 
   Under certain situations the suppressor  12  may be subjected to high AC voltage with limited current. This may happen for instance when an electrical system loses the neutral connection  72  and causes power line  70  to have more voltage than normal. This higher voltage condition can cause the MOV  24  to draw some current, and if the voltage is high enough, the MOV  24  may fail. 
   While current is flowing, the MOV  24  will heat. The heat travels down terminal  28  on the MOV  24  to the thermal fuse  44 . If the temperature is high enough, the solder  74  ( FIG. 4 ) connecting the thermal fuse  44  to terminal  28  melts. This allows the thermal fuse  44  to spring up as shown in  FIG. 4  breaking the connection between MOV  24  and enclosure  14 . This opens the connection between power line and neutral  72 . 
   The thermal fuse  44  is encased in wax  64  (FIG.  2 ). The wax  64  will be liquid when the solder  74  melts. The liquefied wax allows the fuse  44  to freely spring up to position  72  ( FIG. 4 ) during a high temperature condition. The wax  64  in the solidified state prevents sand  62  from packing too tightly around the thermal fuse  44  and restricting the fuse  44  from freely springing upwards from contact  28  when the solder  74  melts. 
   The wax  64  could be any kind as long as it liquefies when the solder  74  melts. The melting point of the solder  74  can be varied according to the electrical requirements of surge suppressor  12 . This is done by using different alloy ratios in solder  74 . For example, different ratios of materials such as tin, lead, bismuth, indium, etc. can be used to vary the melting point of solder  74 . 
   The enclosure  14  in one embodiment is a conductive metallic material such as aluminum. However, any material can be used that is strong enough to contain the explosive energy that may exist during a power surge. For example, the enclosure  12  could be a non-metallic material, such as plastic. If a non-conductive material is used, in addition to wire  18 , a second wire would be extended out of the enclosure  14  from the end of thermal fuse  44  to the neutral connection  72 . The enclosure  14  can be any size necessary to contain the MOV  24  and the fuses  20  and  44 . 
   Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. I claim all modifications and variation coming within the spirit and scope of the following claims.