Patent Publication Number: US-2023154715-A1

Title: Dual-element fuse with chemical trigger element and methods of manufacture

Description:
BACKGROUND OF THE DISCLOSURE 
     The field of the disclosure relates generally to electrical circuit protection fuses and, more specifically, to dual-element time-delay fuses. 
     Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Fuse terminals typically form an electrical connection between an electrical power source or power supply and an electrical component or a combination of components arranged in an electrical circuit. One or more fusible links or elements, or a fuse element assembly, is connected between the fuse terminals, so that when electrical current flowing through the fuse exceeds a predetermined limit, the fusible elements melt and open one or more circuits through the fuse to prevent electrical component damage. 
     So-called dual-element, time-delay fuses are known that include a high overcurrent fuse element and a low overcurrent fuse element inside a housing of the fuse and connected in series to one another inside the fuse. The low overcurrent fuse element includes a mechanical device, often referred to in the art as a fuse trigger, that will electrically open a circuit path through the low overcurrent fuse element during an overload condition after a specified amount of time. Such mechanical fuse triggers are effective to prevent electrical overload conditions from passing to upstream fuses in an electrical power system that would otherwise not cause the high overcurrent fuse element to open, and facilitate selective coordination of overcurrent protection devices to ensure reliability of electrical power systems supplying power to vital loads. 
     Conventional designs for mechanical fuse trigger devices in dual-element, time-delay fuses present a number of challenges from a manufacturing perspective, and improvements are desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments are described with reference to the following Figures, wherein like reference numerals refer to like parts throughout the various drawings unless otherwise specified. 
         FIG.  1 A  is a perspective view of an exemplary electrical fuse. 
         FIG.  1 B  is a perspective view of an exemplary fuse element assembly of the electrical fuse shown in  FIG.  1 A   
         FIG.  1 C  is a perspective view of another exemplary electrical fuse. 
         FIG.  1 D  is a perspective view of one more exemplary electrical fuse. 
         FIG.  1 E  is a perspective view of one more exemplary electrical fuse. 
         FIG.  2 A  is a perspective view of an exemplary elongated metal element of the electrical fuses shown in  FIGS.  1 A and  1 C- 1 E . 
         FIG.  2 B  is a top view of the metal element shown in  FIG.  2 A . 
         FIG.  3 A  is a perspective view of an exemplary trigger element of the electrical fuses shown in  FIGS.  1 A and  1 C- 1 E . 
         FIG.  3 B  is a cross-sectional view of the trigger element along cross-sectional line  3 B- 3 B in  FIG.  3 A . 
         FIG.  4    is a perspective view of another exemplary trigger element of the electrical fuses shown in  FIGS.  1 A and  1 C- 1 E . 
         FIG.  5    is a perspective view of one more exemplary trigger element of the electrical fuses shown in  FIGS.  1 A and  1 C- 1 E . 
         FIG.  6    is a flow chart illustrating an exemplary method of assembling the electrical fuse shown in  FIGS.  1 A- 5   . 
     
    
    
     DETAILED DESCRIPTION 
     Conventional designs for mechanical fuse trigger devices in dual-element, time-delay fuses present a number of challenges from a manufacturing perspective. Manufacturers are seeking fuse trigger devices with less components for manufacturing simplicity and cost reduction. Providing fuse trigger devices that can handle electrical overcurrents, while still providing acceptable interruption performance when subjected to electrical overcurrents for a specified amount of time and requiring fewer manufacturing components, is challenging. Improvements are needed to meet the longstanding and unfulfilled needs in the art. 
     In known dual-element fuses, the fuse includes a short circuit fusible element and a mechanical trigger. The short circuit fusible element opens a circuit path in the event of a high overcurrent condition such as a short circuit condition. The mechanical trigger electrically opens the circuit in the event of a low overcurrent condition after a period of time. The short circuit fusible element and the mechanical trigger are electrically connected in series with one another through a solder and a plunger, which is coupled with a spring. The mechanical trigger also includes a heater. When overload current flows through the fusible element for a period of time, heat generated by the overload current heats the heater, which in turn heats and melts the solder. Without the mechanical force from the solid solder to hold the plunger with the short circuit fusible element, the spring pulls the plunger away and disconnects the mechanical trigger from the short circuit fusible element. 
     Manufacture of the mechanical trigger is relatively complicated and expensive because the mechanical trigger includes a relatively high number of complicated mechanical parts and are typically manually assembled. While functionally effective to open a circuit in a low overcurrent condition, manual assembling of the components is relatively difficult, time consuming, and expensive. Manual assembling also tends to present undesirable performance variation and reliability issues. In addition, because the amount of heat generated by the current changes at different rated current, the heater needs to be modified for different current ratings. For example, the width and/or thickness of the heater is adjusted for each current rating. In other words, for each current rating, the design of the mechanical trigger would need to be adjusted for a known electrical fuse having a mechanical trigger. Accordingly, improved elements and methods of manufacture of dual-element fuses are desired. 
     Electrical fuse elements and methods of manufacture described herein advantageously reduce the number of components needed to manufacture the trigger element of the electrical fuse, while also reducing the need to change the designs of the trigger element for each current rating of the electrical fuses. While described below in reference to particular embodiments, such description is intended for illustration rather than limitation. The significant benefit of the inventive concepts will now be explained in reference to the exemplary embodiments illustrated in the Figures. Method aspects will be in part apparent and in part explicit in the following discussion. 
       FIGS.  1 A- 1 E  show exemplary electrical fuses  101  and an exemplary fusible element  100  of the fuse  101 .  FIG.  1 A  is a schematic diagram of an exemplary embodiment of the fuse  101 .  FIG.  1 B  is a perspective view of the fusible element  100 .  FIGS.  1 C- 1 E  show more exemplary embodiments of the fuse  101 . The fuse  101  is a dual-element fuse or a time delay fuse. 
     In the exemplary embodiment, the power fuse  101  includes a fuse body  103  and terminal assemblies  104 . The fuse  101  includes two terminal assemblies  104 , a first terminal assembly  104 - 1  and a second terminal assembly  104 - 2 . The terminal assemblies  104 - 1 ,  104 - 2  may be the same or different from one another. The fuse body  103  includes a housing  102  and the fuse element assembly  108 . The fuse element assembly  108  includes one fusible element  100  ( FIGS.  1 A and  1 C ). The fuse element assembly  108  is positioned inside the housing  102 . In some embodiments, the fuse element assembly  108  includes one or more fusible elements  100  extending through the fuse body  103  between terminal assemblies  104 - 1 ,  104 - 2  ( FIGS.  1 D and  1 E ). 
     In the exemplary embodiment, the terminal assembly  104  includes a terminal  111  and an end bell  112 . The terminal  111  is configured to connect the fuse  101  to a line- or load-side circuitry. The terminal  111 , sometimes referred to as a blade or a knife blade, extends outwardly from the end bell  112 . The end bell  112  is received in an end  113  of the fuse body  103 . The electrical connection of the fuse element assembly  108  is completed through the end bells  112  and the terminals  111  such that when electrical current flowing through the fuse  101  exceeds a predetermined limit, fusible elements  100  of the fuse element assembly  108  melt, disintegrate, or otherwise structurally fail and open the circuit path through the fuse  101  to prevent electrical component damage. The load-side circuitry is therefore electrically isolated from the line-side circuitry to protect load-side circuit components from damage when electrical fault conditions occur. 
     The fuse  101  may further include an arc extinguishing filler (not shown). The terminal assembly  104  may further include a filler hole (not shown) in the end bell  112  to facilitate the introduction of the arc extinguishing filler. The arc extinguishing filler surrounds at least part of the fuse element assembly  108  and is configured to block or mitigate arcing. The arc extinguishing filler may be fabricated from quartz silica sand and a sodium silicate binder. The quartz sand has a relatively high heat conduction and absorption capacity in its loose compacted state, but may be silicated to provide improved performance. For example, a liquid sodium silicate solution is added to the sand and then the free water is dried off. 
     In the exemplary embodiment, the first terminal assembly  104 - 1  has grooves  132  for accommodating a short circuit fusible element  109  of the fusible element  100 , and the second terminal assembly  104 - 2  has grooves  132  in a different pattern for coupling to a trigger element  105  of the fusible element  100 . Alternatively, the terminal assemblies  104  have the same number and patterns of grooves  132 , 
     In the exemplary embodiment, the fusible element  100  includes the short circuit fusible element  109  and the trigger element  105 . The short circuit fusible element  109  is connected in series with the trigger element  105 . The connection in series facilitates overcurrent protection of electrical devices connected to the power system. The trigger element  105  activates chemically to interrupt a predefined overload condition with a predetermined time delay, where under a predefined overload condition, the trigger element  105  interrupts the circuit through the fuse  101  after a predetermined time delay, and the interruption is caused by chemical reaction of components in the trigger element  105 . 
     In the exemplary embodiments, the short circuit fusible element  109  includes a first perforated wall  125 - 1  extending between a first end  126  and a second end  128  of the short circuit fusible element  109  ( FIG.  1 B ). The short circuit fusible element  109  may further include a second perforated wall  125 - 2  extending in spaced apart but generally parallel planes to the first perforated wall  125 . The short circuit fusible element  109  may further include a third perforated wall  125 - 3  extending perpendicularly to and between the first perforated wall  125  and the second perforated wall  125 - 2 . The perforated wall  125  is fabricated from a metal or metal alloy such as copper and copper alloy. The perforated wall  125  defines a plurality of linear arrangements  134  of perforations  136 . Each linear arrangement  134  extends across the widthwise dimension  138  of the perforated wall. The linear arrangements  134  are spaced apart from one another in the lengthwise dimension  140 . 
     The perforations defines weak spots, which has thin sections than other areas of the short circuit fusible element  109 . When subject to high overcurrent conditions, including but not necessarily limited to a short circuit condition, the short circuit fusible element  109  opens at the weak spots before the trigger element  105  responds. Opening of the short circuit fusible element  109  protects circuitry connected to the fuse from an otherwise damaging high overcurrent condition. 
     In the exemplary embodiment, the trigger element  105  extends parallel to the first perforated wall  125  and the second perforated wall  125 . The trigger element  105  includes an elongated metal element  115 . The elongated metal element  115  is freestanding, where the elongated metal element  115  is not extended from the short circuit fusible element  109  or the terminal assembly  104 , and is formed separately from the short circuit fusible element  109 . The metal element may be a planar sheet metal element  115 . The elongated metal element  115  is fabricated from copper or copper alloy. The trigger element  105  further includes a Metcalf-effect (M-effect) element  142 , where a portion of the elongated metal element  115  is overlaid with a low melting-point metal material  110 . The low melting-point metal material  110  has a lower melting point than the elongated metal element  115 . An M-effect occurs when a low-melting point metal (e.g., tin) is disposed on a high-melting point metal (e.g., copper) and under an overload condition, melts and diffuses into the high-melting point metal, reducing the melting point of the high-melting point metal. Tin and copper are used herein as examples only. Other low-melting point metal or metal alloy or other high-melting point metal or metal alloy may be used in M-effect element  142 . The low melting-point metal material  110  is fabricated from tin or tin with a small percentage, such as less than 3%, of silver to further reduce the melting temperature of the metal. 
     In the exemplary embodiment, the trigger element  105  further includes an arc barrier material  120 . The arc barrier material  120  may be positioned between the metal material  110  and the short circuit fusible element  109 . The arc barrier material  120  may also be positioned at opposite sides of the metal material  110 . The arc barrier material  120  may include a silicone material. In some embodiments, the arc barrier material  120  further includes room temperature vulcanizing silicones or UV curing silicones. The arc barrier material  120  blocks or mitigate arc burn. 
     In operation, under an overload condition, the low melting-point metal material  110  melts and diffuses into the elongated metal element  115  in an attempt to form a eutectic material. The result is a lower melting temperature somewhere between that of copper and tin. That is, the elongated metal element  115  at the M-effect element  142  melts at a temperature lower than the melting temperature at which the elongated metal element  115  by itself melts. As a result, the trigger element  105  responds to an overload condition by interrupting the circuit at the M-effect element  142 . Electric arc may form after the trigger element  105  opens at the M-effect element  142 . The arc barrier material  120  provides barrier to the arc. 
     The trigger element  105  may be assembled separately from the short circuit fusible element  109 . That is, during assembling of the fuses  101 , the trigger elements  105  are preassembled and may be assembled with various short circuit fusible elements, besides the short circuit fusible elements  109  described above. 
       FIG.  1 C  shows the fuse  101  having another exemplary embodiment of trigger element  105 - 1   c . Compared to the trigger element  105  shown in  FIGS.  1 A and  1 B , where the metal material  110  is positioned between arc barrier material  120 , the arc barrier material  120  surrounds the metal material  110  in the trigger element  105 - 1   c , blocking or mitigating arcing from the metal material  110 . The arc barrier material  120  may be in other configurations that enable the arc barrier material  120  to function as described herein. 
       FIGS.  1 D and  1 E  show the fuse element assembly  108  of the fuse  101 - 1   d ,  101 - 1   e  having a plurality of fusible elements  100 , to increase the current rating of the fuse  101 . The plurality of fusible elements  100  are connected in parallel with one another. The fuse  101 - 1   d  includes two fusible elements  100  to double the current rating of the fuse  101 . The fuse  101 - 1   e  includes four fusible elements  100  to quadruple the current rating of the fuse  101 . For example, if the fuse  101 - 1   c  is rated at 100 A, by including two fusible elements  100  of the fuse  101 - 1   c , the current rating of the fuse  101 - 1   d  is increased to 200 A, and by including four fusible elements  100  of the fuse  101 - 1   c , the current rating of the fuse  101 - 1   e  is increased to 400 A 
       FIG.  2 A- 2 B  show the elongated metal element  115 .  FIG.  2 A  is a perspective view of the elongated metal element  115 .  FIG.  2 B  is a top view of the elongated metal element  115 . In the exemplary embodiment, the elongated metal element  115  is stamped from a planar sheet of metal or metal alloy. In some embodiments, the elongated metal element  115  is formed with an opening  210  in the planar sheet of metal or metal alloy  115 . 
       FIGS.  3 A and  3 B  illustrate the trigger element  105  formed with the elongated metal element  115  shown in  FIGS.  2 A and  2 B .  FIG.  3 A  is a perspective view of the trigger element  105 .  FIG.  3 B  is a cross-sectional view of the trigger element  105  taken along cross-sectional line  3 B- 3 B in  FIG.  3 A . In the exemplary embodiment, a portion of the elongated copper metal element  115  is overlaid with tin  110 . The planar sheet of metal or metal alloy  115  defines the opening  210  (also see  FIGS.  2 A and  2 B ), The elongated metal element  115  defines an upper major surface  305  and a lower major surface  310 . The tin  110  may be overlaid on both the upper major surface  305  and the lower major surface  310  of the planar sheet of metal or metal alloy  115 , covering the opening  210 . The opening  210  defines narrowed sections  202  ( FIGS.  2 A and  2 B ) in the elongated metal element  115 , which has a smaller cross-sectional area than the remaining of the elongated metal element  115  therefore current density at the narrowed section is higher than the remaining of the metal element. As a result, temperature rises are greater at the narrowed sections  202  where the metal material  110  locates, than the remaining area of the elongated metal element  115 , causing the metal material  110  to melt, diffuse to the elongated metal element  115 , and lower the melting temperature of the elongated metal element  115  at the areas  302  ( FIG.  3 B ) covered by the metal material  110  and/or adjacent the metal material  110 . After a predetermined time, the elongated metal element  115  at the areas  302  melts and disconnects the circuit, protecting the connected circuitry. Besides providing narrowed sections  202 , the opening  210  serves as a marker in positioning the metal material  110  during the manufacturing of the trigger element  105 . 
     In some embodiments, the trigger element  105  includes a plurality of openings  210 . A plurality of openings  210  increases current density at the narrowed sections  202  by reducing the cross-sectional areas at the narrowed sections, thereby reducing the response time of the trigger element  105 . 
       FIG.  4    shows another embodiment of the trigger element  105 . Compared to the trigger element  105  shown in  FIGS.  3 A and  3 B , which includes an opening  210  on the elongated metal element  115 , the trigger element  105  includes a gap  402  defined by two portions  404 - 1 ,  404 - 2  of the elongated metal element  115 . The gap  402  extends through the elongated metal element  115  along the radial or short direction of the elongated metal element  115 . The two portions  404  may both be planar sheets of metal or metal alloy. The two portions  404  may be fabricated by the same mechanism such as stamping. The metal material  110  is positioned over and covers the gap  402 . In operation, when overload current flows through the trigger element  105 , the metal material  110  melts first because tin has a lower melting temperature than copper of the elongated metal element  115 . The melted tin diffuses into copper of the elongated metal element  115 , lowering the melting temperature of the elongated metal element  115  at the area  302  overlaid by the metal material  110  and/or adjacent the metal material  110 . After a period of the time, the metal material  110  and the elongated metal element  115  at the area  302  are melted, enlarging the gap  402 . As a result, the circuit is disconnected. 
       FIG.  5    shows one more embodiment of the trigger element  105 . Compared to the trigger element  105  shown in  FIGS.  3 A- 4   , the elongated metal element  115  does not have an opening  210  or a gap  402 , and but the metal material  110  still overlays the elongated metal element  115 . In operation, when overload current flows through the elongated metal element  115 , the metal material  110  melts and diffuses into copper of the elongated metal element  115 , lowering the melting temperature of the metal at the area  302  overlaid by the metal material  110  and/or adjacent the metal material  110 . After a period of time, the metal material  110  and the elongated metal element  115  at the area  302  are melted, disconnecting the circuit. 
     Whether to include an opening or gap, the number of openings, and the sizes of the opening or gap may be adjusted to adjust the response time of the trigger element  105 . Current density at the areas adjacent the opening  210  is increased compared to other areas of the elongated metal element  115  or compared to the elongated metal element  115  that does not have an opening, thereby reducing the melting time of the M-effect element  105  and shortening the response time of the fuse  101  to an overload condition. Having the gap  402  also reduces the response time of the trigger element  105  because tin has a lower melting temperature than the eutectic material of tin and copper and melts and disconnects the circuit faster than the trigger element  105  having the opening  210  ( FIGS.  3 A and  3 B ) or not having the opening  210  or the gap  402 . 
     Compared to the known mechanical trigger, trigger element  105  may be used for a range of current ratings without adjustment of the size of the elongated metal element  115  for every current rating. For example, a trigger element  105  for a fuse rated at 100 A may be the same as the trigger element  105  for a fuse rated at 90 A. In some embodiments, the thickness of the elongated metal element  115  may be adjusted by increasing the thickness of the elongated metal element  115  for a higher current rating. The mass of the metal material  110  may remain the same across different current ratings. Compared to the known mechanical trigger, which may include 0.2 g of tin, the trigger element  105  includes greatly increased mass of tin, e.g., 1.7 g, in the metal material  110 . The metal material  110  serves as a heat sink to slow down the melting of the elongated metal element  115  during the overload condition. Although heat increases as current increases, the effect of the changes in heat on the metal material  110  is not as noticeable as on the known trigger element such that redesign of the trigger element  105  is not necessary for the trigger element  105  to meet the performance requirements under a standard, such as a UL standard. Under the UL standard, the response time of a trigger in a dual-element fuse should be 10 s or more when 500% of the rated current flows through the fuse, should be 480 s or more when 200% of the rated current flows through the fuse, and should be 7200 s or more when 135% of the rated current flows through the fuse. The trigger element  105  disclosed herein may be used in fuses that have a current rating from 35 A up to 600 A and meet the UL standards. For example, the fuse  101  having a current rating from 35 A to 600 A and including the trigger element  105  provides a minimum of 10 s delay when 500% of rated current flows through the fuse. In some embodiments, the fuse  101  has a voltage rating of 600 VAC for alternate current (AC) and 300 VDC for direct current (DC). In one embodiment, the fuse  101  has an interrupting rating of 300 kA Vac RMS (root of mean squared). 
       FIG.  6    is a flow chart illustrating an exemplary method  600  of assembling a dual-element electrical fuse. Dual-element electrical fuse may be the electrical fuse  101  described above. The method  600  includes providing  605  a short circuit fusible element. The short circuit fusible element may be the short circuit fusible element  109  described above. The method  600  also includes providing  610  a trigger element  105 . The trigger element  105  may be assembled by a molding process. For example, a mold of metal material  110  is placed at the elongated metal element  115 . In some embodiments, an upper mold and a lower mold are provided and placed above the elongated metal element  115  and below the elongated metal element  115 , respectively. In one example, the opening  210  serves as a marker, and the upper mold and the lower mold are provided and placed above the elongated metal element  115  or below the elongated metal element  115 , covering the opening  210 . Melted tin or tin mixture is poured into the mold. Once the metal material  110  is cooled and hardened, the mold(s) are removed. The method  600  further includes connecting  615  the trigger element in series with the short circuit fusible element. 
     The benefits and advantages of the present disclosure are now believed to have been amply illustrated in relation to the exemplary embodiments disclosed. 
     At least one technical effect of the assemblies and methods described herein includes (a) chemically activated dual-element electrical fuse; (b) a dual-element electrical fuse without adjustment to the trigger element for each current rating; and (c) a simplified trigger element of a dual-element electrical fuse. 
     An embodiment of an electrical fuse is disclosed. The electrical fuse includes a short circuit fusible element and a trigger element connected in series with the short circuit fusible element. The trigger element is chemically activated rather than mechanically activated to interrupt a predefined overload condition with a predetermined time delay. 
     Optionally, the trigger element includes a freestanding planar sheet metal element. The sheet metal element is a stamped planar sheet of metal or metal alloy. The sheet metal element includes copper. A portion of the copper is overlaid with tin. The sheet metal element defines an opening, and wherein the opening is covered by the tin. Alternatively, the sheet metal element does not define an opening that is covered by the tin. Optionally, the sheet metal element defines a gap separating the sheet metal element into a first portion and a second portion, and the gap is covered by the tin. The electrical fuse further includes an arc barrier material, the arc barrier material extending between the tin and the short circuit fusible element. The arc barrier material includes a silicone material. Alternatively, the silicone material is a room temperature vulcanizing material. The arc barrier material extends on opposing sides of the tin. Alternatively, the arc barrier material surrounds the tin. The sheet metal element includes an upper major surface and a lower major surface opposing the upper major surface, and wherein portions of each of the upper and lower major surfaces are overlaid with the tin. The short circuit fusible element includes at least first and second perforated walls fabricated from a metal or metal alloy, the first and second perforated walls extending in spaced apart but generally parallel planes to one another. The short circuit fusible element further including a third perforated wall extending perpendicularly to and between the first and second perforated walls. The trigger element extends parallel to the first and second perforated walls. The trigger element includes a sheet metal element provided with a Metcalf-effect element. The electrical fuse has an amperage rating of 35 A to 600 A. The trigger element is operable with a minimum 10 second time delay at 500% of rated current. 
     While exemplary embodiments of components, assemblies and systems are described, variations of the components, assemblies and systems are possible to achieve similar advantages and effects. Specifically, the shape and the geometry of the components and assemblies, and the relative locations of the components in the assembly, may be varied from those described and depicted without departing from inventive concepts described. Also, in certain embodiments, certain components in the assemblies described may be omitted to accommodate particular types of electrical fuses or the needs of particular installations, while still providing the needed performance and functionality of the electrical fuses. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.