Abstract:
A fuse includes a fuse state indicator on the exterior surface of the fuse body. The fuse state indicator includes an electrically conductive element electrically connected to and extending between the terminal elements. A temperature sensitive element proximate and in thermal contact with the electrically conductive element includes material capable of changing color when heated to a predetermined transition temperature to form a mark on the temperature sensitive element. The fuse state indicator forms a first mark on the temperature sensitive element when heat from the electrically conductive element transfers to the temperature sensitive element in response to a first overcurrent condition, and forms a second mark, distinct from the first mark, on the temperature sensitive element when heat from the electrically conductive element transfers to the temperature sensitive element in response to a second overcurrent condition distinct from the first overcurrent condition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. patent application Ser. No. 11/939,981, filed Nov. 14, 2007 and issued as U.S. Pat. No. 7,812,704, the entirety of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to fuses and, more particularly, to fuses with a fuse state indicator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and aspects of the invention will be best understood with reference to the following description of certain exemplary embodiments of the invention, when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a plan view of a fuse comprising a fuse state indicator that responds to temperature in accordance with an exemplary embodiment; 
         FIG. 2  is a cross sectional view of a fuse state indicator in accordance with an exemplary embodiment; 
         FIG. 3A  is a top view of a fuse comprising a fuse state indicator displaying a good fuse state in accordance with an exemplary embodiment; 
         FIG. 3B  is a top view of a fuse comprising a fuse state indicator displaying an overload open fuse state in accordance with an exemplary embodiment; 
         FIG. 3C  is a top view of a fuse comprising a fuse state indicator displaying a short circuit open fuse state in accordance with an exemplary embodiment; 
         FIG. 3D  is a top view of a fuse comprising a fuse state indicator displaying a thermal stress and/or open fuse state in accordance with an exemplary embodiment; 
         FIG. 4A  is a photographic view of a temperature sensitive element displaying a good fuse state in accordance with an exemplary embodiment; 
         FIG. 4B  is a photographic view of a temperature sensitive element displaying a 200% overload fuse state in accordance with an exemplary embodiment; 
         FIG. 4C  is a photographic view of a temperature sensitive element displaying a 1200 A overload fuse state in accordance with an exemplary embodiment; 
         FIG. 4D  is a photographic view of a temperature sensitive element displaying a 2400 A short circuit fuse state in accordance with an exemplary embodiment; 
         FIG. 4E  is a photographic view of a temperature sensitive element displaying a 10 KA short circuit fuse state in accordance with an exemplary embodiment; 
         FIG. 4F  is a photographic view of a temperature sensitive element displaying a 100 KA short circuit fuse state in accordance with an exemplary embodiment; 
         FIG. 5  is a plan view of a fuse comprising a fuse state indicator that responds to voltage in accordance with an exemplary embodiment; 
         FIG. 6  is a cross sectional view of a fuse state indicator in accordance with an exemplary embodiment; 
         FIG. 7A  is a perspective view of a smart window showing the orientation of a plurality of polymer dispersed liquid crystals when there is no voltage flowing across the smart window in accordance with an exemplary embodiment; 
         FIG. 7B  is a perspective view of a smart window showing the orientation of a plurality of polymer dispersed liquid crystals when there is voltage flowing across the smart window in accordance with an exemplary embodiment; 
         FIG. 8A  is a perspective view of a smart window showing the orientation of a plurality of suspended particle devices when there is no voltage flowing across the smart window in accordance with an exemplary embodiment; 
         FIG. 8B  is a perspective view of a smart window showing the orientation of a plurality of suspended particle devices when there is voltage flowing across the smart window in accordance with an exemplary embodiment; 
         FIG. 9  is a top view of a fuse comprising a fuse state indicator displaying an inoperable fuse state in accordance with an exemplary embodiment; 
         FIG. 10  is a top view of a fuse comprising a fuse state indicator displaying an operable fuse state in accordance with an exemplary embodiment; 
         FIG. 11A  is a perspective view of a smart window showing the positioning of a plurality of ions when there is no voltage flowing across the smart window in accordance with an exemplary embodiment; 
         FIG. 11B  is a perspective view of a smart window showing the positioning of a plurality of ions when there is voltage flowing across the smart window in accordance with an exemplary embodiment; 
         FIG. 12  is a top view of a fuse comprising a fuse state indicator displaying an inoperable fuse state in accordance with an exemplary embodiment; and 
         FIG. 13  is a top view of a fuse comprising a fuse state indicator displaying an operable fuse state in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a plan view of a fuse  10  comprising a fuse state indicator  12  that responds to temperature in accordance with an exemplary embodiment. The fuse  10  includes an insulative (i.e., nonconductive) fuse body  14  and conductive ferrules  16  attached thereto on either end thereof. The fuse state indicator  12  extends on an outer surface  18  of the fuse body  14  between the ferrules  16  and is electrically connected to the ferrules  16  by a wire  17 . The fuse body  14  is elongated in the direction of a longitudinal axis  19  and is generally cylindrical in the illustrated embodiment. It is appreciated that the benefits of the instant invention may also apply to non-cylindrical fuses, including but not limited to rectangular fuses, in alternative embodiments. Further, it is understood that the Invention is applicable to a wide variety of fuses intended for a wide variety of applications and having a wide variety of fuse ratings. Therefore, the embodiments of the invention shown and described herein are for illustrative purposes only, and the invention is not intended to be restricted to a particular fuse type, class, or rating. 
     In an exemplary embodiment, the ferrules  16  are generally cylindrical and complementary in shape to the fuse body  14 . It is, however, appreciated that the benefits of the instant invention may also apply to non-cylindrical ferrules, including but not limited to rectangular ferrules, in alternative embodiments. 
     The fuse state indicator  12  comprises at least one temperature sensitive element  20  capable of undergoing a visible change upon being subjected to a threshold temperature. The temperature sensitive element  20  is adapted to visibly indicate the state of fuse  10 . The state of fuse  10  may be indicated as a good fuse, an overload open fuse, a short circuit open fuse, and/or thermal stress and/or open fuse. Other fuse states and other descriptions for the fuse states may be used in alternative embodiments without departing from the scope and spirit of the exemplary embodiment. The temperature sensitive element  20  may be employed as part of the fuse state indicator  12  coupled to the outer surface  18  of the fuse  10  or the temperature sensitive element  20  may be employed independently. The temperature sensitive element  20  is coupled to the outer surface  18  of the fuse body  14  between the ferrules  16 , but is not electrically connected to the ferrules  16  by the wire  17 . The temperature sensitive element  20  is positioned on or in close proximity to the wire  17  so that it may detect the heat emanating from the wire  17 . The heat is generated by the current passing through the wire  17  and is dependent upon the resistance of that wire  17 . The wire  17  is designed so that it melts or becomes disconnected once a short circuit or a harmful overload condition occurs. In one embodiment, the wire  17  comprises a NiChrome alloy that melts above 1000° C. It should be understood that the wire may be constructed of other materials capable of melting, when exposed to a harmful overload condition, without departing from the scope and spirit of the exemplary embodiment. 
       FIG. 2  is a cross sectional view of a fuse state indicator  12  in accordance with an exemplary embodiment. In this embodiment, the fuse state indicator  12  comprises a clear laminate  21 , a print ink  22 , a wire  17 , at least one foil  24  and a temperature sensitive element  20 . The print ink  22  comprises an adhesive (not shown) positioned adjacent to the clear laminate  21 , wherein the print ink  22  has a first window  23  defined therewithin. The wire  17  is electrically coupled to the ferrules  16  ( FIG. 1 ) and passes longitudinally across the first window  23 . The wire  17  may be positioned adjacent to the print ink  22 . The at least one foil  24  may be coupled to the portion of the wire  17  that is not passing longitudinally across the first window  23 . The temperature sensitive element  20  may be coupled to at least the portion of the wire  17  that passes longitudinally across the first window  23 . The fuse state indicator  12  may further comprise a label  25  positioned adjacent to the temperature sensitive element  20 , wherein the label  25  has a second window  26  defined therewithin. The second window  26  may be positioned above the temperature sensitive element  20  such that the temperature sensitive element  20  may be visible while viewing through the second window  26 . In this exemplary embodiment, the temperature sensitive element  20  comprises thermographic paper. 
     The foil  24  is designed to protect an operator from exposure to excessive temperatures from the wire  17  while handling fuse  10 . The foil  24  may comprise any material capable of insulating the heat, including, but not limited to, copper foil or any thick film, without departing from the scope and spirit of the exemplary embodiment. 
       FIGS. 3A-3D  illustrate a top view of a fuse  10  comprising a fuse state indicator  12  displaying various fuse states in accordance with an exemplary embodiment.  FIG. 3A  is a top view of a fuse  10  comprising a fuse state indicator  12  displaying a good fuse state  30 . The good fuse state  30  is indicated on the thermographic paper  20  as being entirely clear or having a faint wire line barely noticeable by an operator. 
       FIG. 3B  is a top view of a fuse  10  comprising a fuse state indicator  12  displaying an overload open fuse state  32 . The overload open fuse state  32  is indicated on the thermographic paper  20  as being a thin wire line. 
       FIG. 3C  is a top view cia fuse  10  comprising a fuse state indicator  12  displaying a short circuit open fuse state  34 . The short circuit open fuse state  34  is indicated on the thermographic paper  20  as a thicker wire line comprising intersecting wavy lines. 
       FIG. 3D  is a top view of a fuse  10  comprising a fuse state indicator  12  displaying a thermal stress and/or open fuse state. The thermal stress and/or open fuse state  36  is indicated on the thermographic paper  20  as a black mark. 
     Although the exemplary embodiment described above has illustrated that certain markings have corresponding fuse state meanings, the same or similar marking may be given a different fuse state meaning in alternative embodiments without departing from the scope and spirit of the exemplary embodiment. 
     In an exemplary embodiment, the 80% current fuse tube temperatures may range from about 35° C. to about 65° C. depending upon the location of the measurement. Additionally, the 500% overload fuse tube temperatures may range from about 45° C. to about 90° C. depending upon the location of the measurement. However, at a particular location, the temperatures may be more consistent. It should be understood that these ranges may differ among different fuse types, classes and ratings without departing from the scope and spirit of the exemplary embodiment. 
       FIGS. 4A-4F  illustrate photographic views of a temperature sensitive element displaying various fuse states in accordance with an exemplary embodiment. These photographic views were taken during an experiment on a GT3-FRS-R-30 fuse, which is manufactured by Cooper Bussmann, Inc.  FIG. 4A  is a photographic view of a temperature sensitive element displaying a good fuse state  40 .  FIG. 4B  is a photographic view of a temperature sensitive element displaying a 200% overload fuse state  42 .  FIG. 4C  is a photographic view of a temperature sensitive element displaying a 1200 A overload fuse state  44 .  FIG. 4D  is a photographic view of a temperature sensitive element displaying a 2400 A short circuit fuse state  46 .  FIG. 4E  is a photographic view of a temperature sensitive element displaying a 10 KA short circuit fuse state  48 .  FIG. 4F  is a photographic view of a temperature sensitive element displaying a 100 KA short circuit fuse state  49 .  FIGS. 4A-4F  illustrate the various responses of the temperature sensitive element  20  to the heat generated by the wire  17 , which is positioned underneath the temperature sensitive element  20 . As the amperage flowing across the wire  17  increases, the heat generated from the wire  17  also increases. Consequently, the increased heat causes the markings on the temperature sensitive element  20  to become more pronounced. Although this embodiment has the wire located underneath the temperature sensitive element, alternative embodiments may have the wire located on top of the temperature sensitive element without departing from the scope and spirit of the exemplary embodiment. 
     In an alternative embodiment, the temperature sensitive element  20  of the fuse state indicator  12  may comprise at least one material selected from a group consisting of thermochromic ink, thermochromic paint, thermal paper, liquid crystal polymers, thermal calibrated wax, nitrocellulose, and any substance that may be consumed and or out gas upon exposure to high temperatures, which are all capable of indicating a fuse state upon exposure to a particular temperature range. 
     Thermochromic inks or dyes are temperature sensitive compounds that temporarily change color with exposure to heat. When using the thermochromic inks or dyes, the color of the ink may change when exposed to the heat generated from the fuse  10  and/or the wire  17  while the fuse  10  is operating. The wire  17  is designed to disintegrate when the fuse  10  experiences a short circuit or a harmful overload condition and may then stop generating heat. Therefore, when the fuse  10  is not operating, either due to an open fuse, a fuse that has been installed improperly or an open circuit, the color of the ink may be its original color. This color change may be reversible and may allow an operator to easily diagnose the state of the fuse  10 . 
     Thermochromic paints are temperature sensitive pigments that temporarily change color with exposure to heat—After absorbing a certain amount of light or heat, the crystallic or molecular structure of the pigment reversibly changes in such a way that it absorbs and emits light at a different wavelength than at lower temperatures. When using the thermochromic paints, the color of the paint may change when exposed to the heat generated from the fuse  10  and/or the wire  17  while the fuse  10  is operating. The wire  17  is designed to disintegrate when the fuse  10  experiences a short circuit or a harmful overload condition and may then stop generating heat. Therefore, when the fuse  10  is not operating, either due to an open fuse, a fuse that has been installed improperly or an open circuit, the color of the paint may be its original color. This color change may be reversible and may allow an operator to easily diagnose the state of the fuse  10 . 
     Thermal papers comprise one or more temperature sensitive chemicals that change color with exposure to heat. One example of a thermal paper includes paper impregnated with a solid mixture of a fluoran dye with octadecylphosphonic acid. This mixture is stable in solid phase. However, when the octadecylphosphonic acid is melted, the dye undergoes chemical reaction in the liquid phase, and assumes the protonated colored form. Since this color change may not be reversible, the thermal paper may be used to indicate a short circuit or an overload. There may be some color change during normal operation, but the intensity of the color change may increase as the temperature rises into the temperature range associated with a short circuit or an overload. In one embodiment, the thermal paper has a transition temperature between about 100° C. to about 120° C. It should be understood, however, that alternative thermal papers may be used having different transition temperatures without departing from the scope and spirit of the exemplary embodiment. 
       FIG. 5  is a plan view of a fuse  50  comprising a fuse state indicator  52  that responds to voltage in accordance with an exemplary embodiment The fuse  50  includes an insulative (i.e., nonconductive) fuse body  54  and conductive ferrules  56  attached thereto on either end thereof. The fuse state indicator  52  extends on an outer surface  58  of the fuse body  54  between the ferrules  56  and is electrically connected to the ferrules  56  by a wire  57 . The fuse body  54  is elongated in the direction of a longitudinal axis  59  and is generally cylindrical in the illustrated embodiment. It is appreciated that the benefits of the instant invention may also apply to non-cylindrical fuses, including but not limited to rectangular fuses, in alternative embodiments. Further, it is understood that the invention is applicable to a wide variety of fuses intended for a wide variety of applications and having a wide variety of fuse ratings. Therefore, the embodiments of the invention shown and described herein are for illustrative purposes only, and the invention is not intended to be restricted to a particular fuse type, class or rating. 
     In an exemplary embodiment, the ferrules  56  are generally cylindrical and complementary in shape to the fuse body  54 . It is, however, appreciated that the benefits of the instant invention may also apply to non-cylindrical ferrules, including but not limited to rectangular ferrules, in alternative embodiments. 
     The fuse state indicator  52  comprises at least one voltage sensitive element  60  capable of undergoing a visible change upon being subjected to a voltage. The voltage sensitive element  60  is adapted to visibly indicate the state of fuse  50 . The state of fuse  50  may be indicated as operable or inoperable. The voltage sensitive element  60  may be employed as part of the fuse state indicator  52  coupled to the outer surface  58  of the fuse  50  or the voltage sensitive element  60  may be employed independently. The voltage sensitive element  60  is coupled to the outer surface  58  of the fuse body  54  between the ferrules  56  and is electrically connected to the ferrules  56  by the wire  57 . The voltage sensitive element  60  may indicate a change in the state of the fuse  50  upon exposure to voltage. The voltage sensitive element  60  may also indicate a change in the state of the fuse  50  upon exposure to heat which may or may not be caused by resistive heating of the wire  17 . The wire  57  is designed so that it melts or becomes disconnected once a short circuit or a harmful overload condition occurs. In one embodiment, the wire  57  comprises a NiChrome alloy that melts above 1000° C. It should be understood that the wire may be constructed of other materials capable of melting, when exposed to a harmful overload condition, without departing from the scope and spirit of the exemplary embodiment. 
       FIG. 6  is a cross sectional view of a fuse state indicator  52  in accordance with an exemplary embodiment. In this embodiment, the fuse state indicator  52  comprises a voltage sensitive element  60 , a wire  57  electrically coupling the ferrules  56  ( FIG. 5 ) to the voltage sensitive element  60 , and at least one foil  64  coupled to the wire  57 . The fuse state indicator  52  may further comprise a label  65  positioned adjacent to the voltage sensitive element  60 , wherein the label  65  has a window  66  defined therewithin. The window  66  may be positioned above the voltage sensitive element  60  such that the voltage sensitive element  60  is visible through the window  66 . 
     The foil  64  is designed to protect an operator from exposure to excessive temperatures from the wire  57  while handling fuse  50 . The foil  64  is designed to insulate the temperature from being too hot when an operator handles the fuse  50 . The foil  64  may comprise any material capable of insulating the heat, including, but not limited to, copper foil or any thick film, without departing from the scope and spirit of the exemplary embodiment. 
     Referring now to  FIGS. 7A and 7B , the voltage sensitive element  60  is illustrated and its operation is described hereinbelow in accordance with an exemplary embodiment. In this embodiment, the voltage sensitive element  60  comprises a smart window  70 .  FIG. 7A  is a perspective view of a smart window  70  showing the orientation of a plurality of polymer dispersed liquid crystals  71  when there is no voltage flowing across the smart window  70  in accordance with an exemplary embodiment.  FIG. 7B  is a perspective view of a smart window  70  showing the orientation of a plurality of polymer dispersed liquid crystals  71  when there is voltage flowing across the smart window  70  in accordance with an exemplary embodiment. 
     As illustrated in these figures, the smart window  70  comprises a transparent lens  72 , a first interlayer film  73  adjacent to the transparent lens  72 , a first liquid crystal film  74  adjacent to the first interlayer film  73 , a first conductive coating  75  adjacent to the first liquid crystal film  74 , a plurality of polymer dispersed liquid crystals  71  adjacent to the first conductive coating  75 , a second conductive coating  76  adjacent to the plurality of polymer dispersed liquid crystals  71 , a second liquid crystal film  77  adjacent to the second conductive coating  76 , a second interlayer film  78  adjacent to the second liquid crystal film  77  and a backing layer  79  adjacent to the second interlayer film  78 . 
     These polymer dispersed liquid crystals  71  are liquid crystals capable of changing its orientation from a first orientation  68 , wherein a substantial portion of the light does not pass through the layer of polymer dispersed liquid crystals  71 , to a second orientation  69 , wherein a substantial portion of the light passes through the layer of polymer dispersed liquid crystals  71 . The polymer dispersed liquid crystals  71  are positioned in the first orientation  68  when an electrical charge is absent, which results when the fuse is in an inoperable state. Thus, when the fuse is in an inoperable state, the polymer dispersed liquid crystals  71  may be opaque thereby preventing the operator from viewing the backing layer  79 . However, the polymer dispersed liquid crystals  71  are positioned in the second orientation  69 , positioned parallel to one another, when an electrical charge is present, which results when the fuse is in an operable state. Thus, when the fuse is in an operable state, the polymer dispersed liquid crystals  71  may be translucent thereby allowing the operator to view the backing layer  79 . Hence, when using polymer dispersed liquid crystals  71 , the polymer dispersed liquid crystals  71  are either opaque (fuse is in an inoperable state) or translucent (fuse is in an operable state). 
     The electrical charge does not flow through the wire  57 , which is electrically connected to the smart window  70 , when the fuse is inoperable, which may result from an improperly installed fuse, an off circuit, or a fuse wherein the wire  57  may be melted or broken off due to a short circuit or an overcurrent. The electrical charge flows through the wire  57 , which is electrically connected to the smart window  70 , when the fuse is operable. 
     Although the embodiment described above illustrates that an electrical charge does not flow through the wire  57  when the fuse is inoperable, while an electrical charge flows through the wire  57  when the fuse is operable, the fuse and wire  57  may be designed such that the reverse occurs without departing from the scope and spirit of the exemplary embodiment. Specifically, the fuse and wire  57  may be designed so that an electrical charge flows through the wire  57  when the fuse is inoperable, while an electrical charge does not flow through the wire  57  when the fuse is operable. 
     Referring now to  FIGS. 8A and 8B , another embodiment of the voltage sensitive element  60  is illustrated and its operation is described hereinbelow. In this embodiment, the voltage sensitive element  60  comprises a smart window  80 .  FIG. 8A  is a perspective view of a smart window  80  showing the orientation of a plurality of suspended particle devices  81  when there is no voltage flowing across the smart window  80  in accordance with an exemplary embodiment.  FIG. 8B  is a perspective view of a smart window  80  showing the orientation of a plurality of suspended particle devices  81  when there is voltage flowing across the smart window  80  in accordance with an exemplary embodiment. 
     As illustrated in these figures, the smart window  80  comprises a transparent lens  82 , a first conductive coating  84  adjacent to the transparent lens  82 , a plurality of suspended particle devices  81  adjacent to the first conductive coating  84 , a second conductive coating  86  adjacent to the plurality of suspended particle devices  81 , and a backing layer  88  adjacent to the second conductive coating  86 . 
     These suspended particle devices  81  are capable of changing orientation from a first orientation  87 , wherein a substantial portion of the light does not pass through the layer of suspended particle devices  81 , to a second orientation  89 , wherein a substantial portion of the light passes through the layer of suspended particle devices  81 . The suspended particle devices  81  are positioned in the first orientation  87  when an electrical charge is absent, which results when the fuse is in an inoperable state. Thus, when the fuse is in an inoperable state, the suspended particle devices  81  may be opaque thereby preventing the operator from viewing the backing layer  88 . However, the suspended particle devices  81  are positioned in the second orientation  89 , positioned in alignment with one another, when an electrical charge is present, which results when the fuse is in an operable state. Thus, when the fuse is in an operable state, the suspended particle device  81  may be translucent thereby allowing the operator to view the backing layer  88 . Hence, when using suspended particle devices  81 , the suspended particle devices  81  are either opaque (fuse is in an inoperable state) or translucent (fuse is in an operable state). 
     The electrical charge does not flow through the wire  57 , which is electrically connected to the smart window  80 , when the fuse is inoperable, which may result from an improperly installed fuse, an off circuit, or a fuse wherein the wire  57  may be melted or broken off due to a short circuit or an overcurrent. The electrical charge flows through the wire  57 , which is electrically connected to the smart window  80 , when the fuse is operable. 
     Although the embodiment described above illustrates that an electrical charge does not flow through the wire  57  when the fuse is inoperable, while an electrical charge flows through the wire  57  when the fuse is operable, the fuse and wire  57  may be designed such that the reverse occurs without departing from the scope and spirit of the exemplary embodiment. Specifically, the fuse and wire  57  may be designed so that an electrical charge flows through the wire  57  when the fuse is inoperable, while an electrical charge does not flow through the wire  57  when the fuse is operable. 
     Referring now to  FIGS. 9 and 10 , the various states of the fuse  50  are illustrated. In the embodiment shown in  FIGS. 9 and 10 , a fuse state indicator  52  comprising at least one smart window  70 ,  80  is illustrated. 
     In this embodiment, the smart window  70 ,  80  may further comprise a first marking  100  coupled to the backing layer  79 ,  68 , wherein the first marking  100  indicates that the fuse  50  is operable. Although this embodiment uses the word “on” as the first marking  100 , any marking may be used, including a particular color, e.g. green dot or square, or any other marking associated with an operable status, without departing from the scope and spirit of the exemplary embodiment. The first marking  100  may be marked on the surface of the backing layer  79 ,  88  or may be marked on a material directly or indirectly coupled to the backing layer  79 ,  88 . 
       FIG. 9  is a top view of a fuse  50  comprising a fuse state indicator  52  displaying an inoperable fuse state  90  in accordance with an exemplary embodiment. When the smart window  70  has no voltage passing through it, the polymer dispersed liquid crystals  71  orient to the first position, which is when the molecules point in a random manner and prevent the operator from viewing the first marking  100 . Similarly, in the alternative embodiment, when the smart window  80  has no voltage passing through it, the suspended particle devices  81  orient to the first position, which is when the molecules point in a random manner and prevent the operator from viewing the first marking  100 . When the fuse  50  is in the inoperable fuse state  90 , the polymer dispersed liquid crystals  71  and the suspended particle devices  81  both become opaque, 
       FIG. 10  is a top view of a fuse  50  comprising a fuse state indicator  52  displaying an operable fuse state  105  in accordance with an exemplary embodiment. When the smart window  70  has voltage passing through it, the polymer dispersed liquid crystals  71  orient to the second position, which is when the molecules point in mostly the same direction and allow the operator to view the first marking  100 . Similarly, in the alternative embodiment, when the smart window  80  has voltage passing through it, the suspended particle devices  81  orient to the second position, which is when the molecules line up and allow the operator to view the first marking  100 . When the fuse  50  is in the operable fuse state  105 , the polymer dispersed liquid crystals  71  and the suspended particle devices  81  both become translucent. 
     Although the embodiment described above illustrates that an electrical charge does not flow through the wire  57  when the fuse is inoperable, while an electrical charge flows through the wire  57  when the fuse is operable, the fuse and wire  57  may be designed such that the reverse occurs without departing from the scope and spirit of the exemplary embodiment. Specifically, the fuse and wire  57  may be designed so that an electrical charge flows through the wire  57  when the fuse is inoperable, while an electrical charge does not flow through the wire  57  when the fuse is operable. 
     Referring now to  FIGS. 11A and 11B , another embodiment of the voltage sensitive element  60  is illustrated and its operation is described hereinbelow. In this embodiment, the voltage sensitive element  60  comprises a smart window  110 .  FIG. 11A  is a perspective view of a smart window  110  showing the positioning of a plurality of ions  111  when there is no voltage flowing across the smart window  110  in accordance with an exemplary embodiment.  FIG. 11B  is a perspective view of a smart window  110  showing the positioning of a plurality of ions  111  when there is voltage flowing across the smart window  110  in accordance with an exemplary embodiment. 
     As illustrated in these figures, the smart window  110  comprises a transparent lens  112 , a first conductor  113  adjacent to the transparent lens  112 , an ion storage layer  114  adjacent to the first conductor  113 , an ion conductor/electrolyte layer  115  adjacent to the ion storage layer  114 , an electrochromic layer  116  adjacent to the ion conductor/electrolyte layer  115 , a second conductor  117  adjacent to the electrochromic layer  116 , a backing layer  118  adjacent to the second conductor  117 , and a plurality of ions  111  capable of moving between the ion storage layer  114  and the second conductor  117 . 
     These smart windows  110  center around special materials that have electrochromic properties. “Electrochromic” describes materials that can change color when energized by an electrical current. Essentially, electricity initiates a chemical reaction in this sort of material. The reaction changes the way the material reflects and absorbs light. In this embodiment, the changes between color comprise opaque and translucent, wherein the opaque color corresponds to a first fuse state (operable fuse state) and a translucent color corresponds to a second fuse state (inoperable fuse state). Although this embodiment utilizes opaque and translucent colors, other colors maybe used without departing from the scope and spirit of the exemplary embodiment. Additionally, there may be a color gradient from opaque and translucent without departing from the scope and spirit of the exemplary embodiment. 
     In this design, the chemical reaction involved comprises an oxidation reaction, wherein molecules of a compound lose an electron. As shown in  FIG. 11A , when there is no applied voltage to the smart window  110 , the plurality of ions  111  are positioned within the ion storage layer  114 , which results when the fuse is in an inoperable state. This positioning of the plurality of ions  111  allows light to pass through to the backing layer  118 . Thus, the smart window  110  becomes translucent and allows the operator to view the backing layer  118 . When voltage is applied to the smart window  110 , as illustrated in  FIG. 118 , the voltage drives the plurality of ions  111  from the ion storage layer  114  through the ion conductor/electrolyte layer  115  and into the electrochromic layer  116 , which results when the fuse is in an operable state. This positioning of the plurality of ions  111  prevents light from passing through to the backing layer  118 . Thus, the smart window  110  becomes opaque and prevents the operator from viewing the backing layer  118 . 
     The electrical charge does not flow through the wire  57 , which is electrically connected to the smart window  110 , when the fuse is inoperable, which may result from an improperly installed fuse, an off circuit, or a fuse wherein the wire  57  may be melted or broken off due to a short circuit or an overcurrent. The electrical charge flows through the wire  57 , which is electrically connected to the smart window  110 , when the fuse is operable. 
     Although the embodiment described above illustrates that an electrical charge does not flow through the wire  57  when the fuse is inoperable, while an electrical charge flows through the wire  57  when the fuse is operable, the fuse and wire  57  may be designed such that the reverse occurs without departing from the scope and spirit of the exemplary embodiment. Specifically, the fuse and wire  57  may be designed so that an electrical charge flows through the wire  57  when the fuse is inoperable, while an electrical charge does not flow through the wire  57  when the fuse is operable. 
     Referring now to  FIGS. 12 and 13 , the various states of the fuse  50  are illustrated. In the embodiment shown in  FIGS. 12 and 13 , a fuse state indicator  52  comprising at least one smart window  110  is illustrated. 
     In this embodiment, the smart window  110  may further comprises an alternative marking  120  coupled to the backing layer  118 , wherein the alternative marking  120  indicates that the fuse  50  is inoperable. Although this embodiment uses the word “off” as the alternative marking  120 , any marking may be used, including a particular color, e.g. black dot or square, or any other marking associated with an inoperable status, without departing from the scope and spirit of the exemplary embodiment. The alternative marking  120  may be marked on the surface of the backing layer  118  or may be marked on a material directly or indirectly coupled to the backing layer  118 . 
       FIG. 12  is a top view of a fuse  50  comprising a fuse state indicator  52  displaying an inoperable fuse state  122  in accordance with an exemplary embodiment. When the smart window  110  has no voltage passing through it, the plurality of ions  111  are positioned within the ion storage layer  114 , which allows the operator to view the alternative marking  120 . 
       FIG. 13  is a top view of a fuse  50  comprising a fuse state indicator  52  displaying an operable fuse state  130  in accordance with an exemplary embodiment. When the smart window  110  has voltage passing through it, the plurality of ions  111  become mostly positioned within the electrochromic layer  116 , which prevents the operator from viewing the alternative marking  120 . 
     With respect to all the embodiments described, the fuse state indicator  52  may comprise lettering to describe the fuse  50  and the fuse states. The fuse state indicator  52  may also comprise a color chart for assisting a user in identifying the meaning of the color change. To further assist operators in analyzing the status of the fuse  50 , pocket cards comprising color charts may be provided to the operators. 
     Additionally, although the exemplary embodiments described above illustrate the fuse  50  comprising one voltage or temperature sensitive element, multiple voltage or temperature sensitive elements may be utilized without departing from the scope and spirit of the exemplary embodiment. 
     Furthermore, although some exemplary embodiments have been described above, it is envisioned that the various voltage and temperature sensitive elements that have been described may be used alternatively in lieu of one another or in combination with each other without departing from the scope and spirit of the invention. 
     Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the scope of the invention.