Fuse element having parallel strips

Information

  • Patent Grant
  • 6194989
  • Patent Number
    6,194,989
  • Date Filed
    Tuesday, May 18, 1999
    25 years ago
  • Date Issued
    Tuesday, February 27, 2001
    23 years ago
Abstract
A fuse includes a tube, a pair of blade terminals projecting from opposite ends of the tube, at least one fuse element disposed in the tube and electrically coupled between the terminals, and a pair of metallic end caps disposed on opposite ends of the tube. Electrically insulative elements are disposed between the end caps and the terminals. The tube is filled with an arc-quenching material inserted through a fill hole that is plugged by a plastic drive rivet. Each terminal is attached to a metallic end plate by means of a staking tang inserted into a slot of the end plate, and by means of a separate solder joint. Each insulative element includes an axial sleeve through which a respective terminal extends for a part of its length. The fuse element comprises a one-piece metal element bent to form a pair of parallel, superimposed strips divided into sections by means of fusible weak points. The metal element also includes bridge elements which join sections of one strip to respective sections of the other strip, the bridges themselves being non-interconnected. End-most sections of one strip are fixedly joined to respective end-most sections of the other strip to define tabs for electrically connecting the fuse element to a circuit.
Description




BACKGROUND OF THE INVENTION




The present invention relates to fuses in general, and particularly to current-limiting, time-delay fuses.




A current-limiting time delay fuse


10


employs a built-in delay that allows temporary and harmless inrush currents to pass without the fuse being opened, but which is designed to open in response to a sustained overload and short circuit currents. Such a dual-element fuse is used in circuits subjected to temporary inrush current transients, such as motor starting currents, to provide both high performance short-circuit current protection and time-delay overload current protection.




One conventional type of such a fuse


10


, depicted in

FIG. 1

, comprises a body which includes an electrically insulative tube


12


formed for example of glass reinforced polyester, a pair of copper knife blade terminals


14


connected to respective brass end plates


16


, and a pair of steel end caps or ferrules


18


. The end caps


18


are attached to the tube


12


by screws


20


(or rivets) to close the ends of the tube and retain the end plates


16


. Each terminal


14


projects through a slit


24


formed in a radial portion


15


of a respective end cap


18


, and is supported or attached to the tube


12


by a flat pin or roll pin (not shown) extending through the terminal.




Alternatively, as shown in

FIGS. 2 and 3

, the terminals


14


A could be brazed to thick end bells


16


A which are inserted into respective ends of the tube


12


A such that radial holes


26


A formed in each end bell


16


A become aligned with respective radial holes


28


A formed in the tube


12


A. Cylindrical drive pins


30


A would be force-fit through respective pairs of holes


26


A,


28


A to secure the end bells to the tube.




Disposed within a cavity


32


formed by the tube


12


are fuse elements. Preferably, two types of fuse elements


34


,


36


are provided, namely, an overcurrent trigger mechanism


34


and a short circuit interrupting fusible element


36


. There is at least one of each type of fuse element. The cavity


32


is filled with an arc-quenching filler material


33


such as quartz sand.




Each overcurrent trigger mechanism


34


includes an alloy solder


38


for series-connecting the mechanism


34


to one of the fuse elements


36


, a trigger


40


, a coil compression spring


42


surrounding the trigger


40


, an absorber


44


surrounding the spring


42


, a heater element


46


, and an insulator


48


. The trigger mechanism


34


utilizes stored energy of the spring


42


to break the current in the event of low level overcurrents or overloads, and will hold an overload that is five times greater than the ampere rating of the fuse for a minimum time, e.g., about ten seconds.




Each short circuit fuse element


36


comprises a strip


50


of fusible metal, such as silver, copper, copper alloy, etc., having parallel rows


52


of perforations. Adjacently disposed perforations define therebetween current-carrying weak spots of substantially reduced cross-section designed to break in response to a short circuit overload current.




Although such fuses have performed acceptably, certain shortcomings exist. For instance, in the short circuit fuse elements


36


, the strips


50


are supported only by their weak spots which provide very little strength for the fuse element while being handled during the fuse-manufacturing process. Consequently, the fuse elements


36


are susceptible to mechanical fatigue and breakage due to normal handling during manufacture, as well as due to mechanical and thermal fatigue caused by steady state and transient current load current cycling.




Heretofore, the fatigue problem due to handling has been solved by the use of special equipment, tool fixturing and procedures designed to reduce the amount of worker handling. Those measures, however, increase capital expenditures and slow the production rate.




Another shortcoming relating to a time delay current-limiting fuse, or to fuses in general, which are filled with an arc-quenching filler involves the need to plug a hole in which the filler has been introduced. In that regard, the filler is typically introduced through a hole which must be plugged or sealed, in order to retain the filler. A variety of methods of sealing or plugging have been used, such as metal drive plugs, set screws, steel balls, and metal cups, as well as adhesives and glues such as epoxy, but all suffer from various limitations. For example, drive plugs require costly fabrication machinery, set screws are also costly in that they require that the filler hole be machined to form a screw thread; balls and cups are held in place by an interference-fit and are less costly, but the interference-fit is not always reliable, whereby the balls or cups may become dislodged; adhesives are messy to apply and hard to control.




Additional shortcomings may result from the ability to provide the tubes of fuses with shorter lengths. If a fuse manufacturer is to incorporate shorter fuse tube lengths, then certain spacing requirements must be satisfied to ensure that a user can safely grip a fuse without simultaneously touching parts of the fuse which will produce an electrical shock. These spacing requirements are spelled out in the Underwriters Laboratory standards for electrical equipment that use these fuses in a covered device (i.e., disconnect switch). The spacing requirements specifically pertain to what is known as phase-to-phase and phase-to-ground distances between live and/or dead metal parts. A live metal part means a metal conductor at some voltage potential with respect to ground. A dead metal part means a metal conductor at no voltage potential with respect to ground.




In that regard, a common problem involving the application of shorter fuse tube lengths to a typical fuse design is that the longitudinal space between the live metal end caps is so short as to create spacing violations for phase-to-phase and phase-to-ground distances in existing equipment designed to specific Underwriters Laboratory standards. To overcome this spacing violation, several design approaches have been considered. One approach involved the use of heat shrink plastic wrap over the metal end caps, and another approach employed plastic end caps (e.g., see Swain U.S. Pat. No. 2,863,967). Both of those approaches proved either too expensive or impractical due to strength issues.




Yet another shortcoming involving the manufacture of shorter fuses is that in order to make the fuse body shorter the fuse blades must become longer to continue satisfying the dimensional requirements of the fuse. By making the fuse blades longer, a greater mechanical moment may be imposed during installation of the fuse. To accommodate this greater mechanical moment, a stronger mechanical system must be provided. The typical knife blade fuse depicted in

FIG. 1

does not provide the necessary mechanical system to support the force exerted on the longer blade of a short-body fuse. The fuse depicted in

FIGS. 2 and 3

, however, will support this force because of the added strength from the pinned mechanical system to the high strength tube. However, the cost of the pinned mechanical system is too high in cost to implement for all types of fuses, because it uses a very expensive tube material (e.g., glass melamine) and the fuse must be assembled on a C-shaped metal frame which is very labor intensive.




Therefore, it would be desirable to provide a fuse of the type containing an arc-quenching filler with a more effective fill-hole plugging arrangement.




It would also be desirable to provide a short-circuit fuse element which is less susceptible to mechanical and thermal fatigue due to handling as well as due to steady state and transient load current cycling.




It would also be desirable to provide a fuse which provides for strong reinforcement and closure of the ends of the fuse tube while ensuring that ample phase-to-phase and phase-to-ground distances are created.




SUMMARY




In accordance with the present invention, a fuse element comprises a body of metallic material including at least first and second parallel, superimposed strips, each strip including parallel rows of perforations dividing the strip into respective sections, adjacent perforations of each row being spaced apart to define weak points therebetween which secure adjacent ones of the sections together; a plurality of support bridges interconnecting adjacent edges of the first and second strips; each support bridge connecting one of the sections of the first strip to one of the sections of the second strip.











BRIEF DESCRIPTION OF THE DRAWINGS




The objects and advantages of the invention will become apparent from the following detailed description of preferred embodiments thereof in connection with the accompanying drawing in which like numerals designate like elements, and in which:





FIG. 1

is a perspective view of a prior art knife blade fuse;





FIG. 2

is a side elevational view of another prior knife blade fuse, with a portion thereof broken away;





FIG. 3

is an exploded perspective view of the prior art knife blade fuse depicted in

FIG. 2

;





FIG. 4

is a perspective view of a knife blade fuse according to the present invention;





FIG. 5

is a sectional view taken through the fuse of

FIG. 4

along a plane extending parallel to blade terminals of the fuse;





FIG. 6

is a sectional view of

FIG. 4

taken along a plane extending perpendicular to the blade terminals;





FIG. 7

is a plan view of a blank used to make a fuse element according to the present invention;





FIG. 8

is a perspective view of the fuse element formed by the blank of

FIG. 7

;





FIG. 9

is a perspective view of a modified fuse element according to the present invention;





FIG. 10

is a plan view of a blank used to make yet another type of fuse element according to the present invention;





FIG. 11

is a perspective view of the fuse element formed by the blank of

FIG. 10

;





FIG. 12

is a perspective view of one end of an electrically insulative element according to the present invention;





FIG. 13

is a perspective view of the other end of the element depicted in

FIG. 12

;





FIG. 14

is a perspective view of a conventional plastic drive rivet;





FIG. 15

is another perspective view of the plastic drive rivet depicted in

FIG. 14

;





FIG. 16

is a sectional view taken through the end of the fuse depicted in

FIG. 4

as a drive rivet is initially inserted into a fill hole;





FIG. 17

is a view similar to

FIG. 16

after a plunger of the drive rivet has been driven to fixed the drive rivet within the fill hole;





FIG. 18

is an exploded perspective view of an end of the fuse according to the present invention;





FIG. 19

is a view similar to

FIG. 18

after a terminal has been joined to an end plate;





FIG. 20

is an exploded perspective view similar to

FIG. 19

after the end plate has been applied against an end of a tube; and





FIG. 21

is a sectional view taken through the end plate and terminal depicted in FIG.


19


.




FIGS.


22


(


a


)-(


d


), FIGS.


23


(


a


)-(


c


), and FIGS.


24


(


a


)-(


c


) illustrate alternative embodiments of fuse elements of the present invention.





FIGS. 25 and 26

illustrate alternative fuses according to the present invention.











DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION




A fuse


100


comprises an electrically insulative cylindrical tube


112


formed for example of glass reinforced polyester, a pair of metallic (e.g., copper) knife blade terminals


114


connected to respective metallic (e.g., brass) end plates


116


, and a pair of metallic (e.g., steel) end caps or ferrules


118


. Each of the end caps


118


includes a cylindrical portion


120


telescopingly arranged around an outer surface of the tube, and a radial portion


122


extending radially inwardly from an axially outer end of its respective cylindrical portion


120


. The end caps


118


are secured to the tube by forming conical indents or dimples


124


in the cylindrical portions


120


which create an interference fit with the outer surface of the tube


112


. The blade terminals


114


pass through slits


125


formed in the radial portions


122


of respective end caps.




Short Circuit Fuse Elements




Disposed within a cavity


132


formed by the tube


112


are fuse elements. Preferably two types of fuse elements


34


,


136


are provided, namely, an overcurrent trigger mechanism


34


such as the conventional mechanism


34


described earlier herein, and a short circuit interrupting fusible element


136


according to the present invention. There is at least one of each type of fuse element


34


,


136


. If a plurality of each type of fuse element is employed, such plurality shall preferably be an even number, e.g., two, four, six, etc. The cavity


132


is filled with an arc-quenching filler material


133


, such as quartz sand.




As described earlier herein, each overcurrent trigger mechanism


34


utilizes the stored energy of a spring to break the circuit in the event of low level overcurrents or overloads, and will hold an overload that is five times greater than the ampere rating of the fuse for a minimum time, e.g., about ten seconds.




Each short circuit fuse element


136


, which is also depicted in

FIG. 8

, is formed from a metallic (e.g., silver, copper, copper alloy, etc.) blank


138


depicted in FIG.


7


. That blank


138


comprises a pair of strips


140


A,


140


B each having parallel rows


142


of perforations


144


. Formed between adjacently disposed perforations


144


are current-carrying weak spots


146


of substantially reduced cross section designed to break in response to a short circuit overload current.




The two strips


140


A,


140


B are interconnected by support bridges


148


, each support bridge being joined to an edge of a strip


140


A or


140


B along a region


150


thereof disposed between adjacent rows


142


of perforations. The support bridges


148


are non-interconnected. To form the blank


136


B into a fuse element


136


, the strips


140


A,


140


B are folded along parallel fold lines


152


defined by the juncture of the support bridges and strips, whereupon the strips become arranged in spaced apart, superimposed relationship, with the support bridges


148


oriented perpendicular to the strips. Also, the end-most sections


154


,


156


of the strips are bent and joined to one another by spot welding, soldering, etc., to form connecting tabs


158


,


159


. The tab


158


is joined by solder


38


to a trigger


40


of a respective overcurrent trigger mechanism


34


. The other tab


159


is joined in a suitable fashion to a respective end plate


116


.




Because of the presence of the support bridges


148


, and the interconnected end sections


154


,


154


and


156


,


156


, which provide mechanical strength to the adjacent strips


140


A,


140


B, the strips are no longer supported solely by their weak spots and thus are less susceptible to breakage while being handled. Furthermore, the joining of the end sections to form connecting tabs


158


,


159


serves as a convenient means to secure the blank in its folded, fuse-forming state. Moreover, when the fuse element


136


is connected in an electrical circuit and conducts current, the support bridges


148


(since they are non-interconnected) produce an equal distribution of current densities to each of the parallel current paths defined by the weak spots and thereby increase the current capacity for increased time-delay characteristics. Such increased time-delay characteristics, combined with an enhanced heat transfer area contributed by the support bridges, allow for a minimal cross-sectional area of the weak spot region to exist for the purpose of reducing the short-circuit I


2


t and peak let-through current I


p


to satisfy the UL requirements for maximum allowable I


2


t and I


p


for a particular class of fuse.




The short circuit fuse element can assume different configurations other than that shown in FIG.


8


. For example, the end sections


156


could be equal in length to the other end sections


154


and folded to form identical connecting tabs


158


,


159


′ as shown in the fuse element


136


′ depicted in FIG.


9


.





FIG. 10

illustrates a blank


160


B for forming a short-circuit fuse element


160


depicted in FIG.


11


. That fuse element


160


is similar to that of

FIG. 9

, with the principal differences being that four strips


162


A-D are provided, instead of two strips, and each connecting tab


164


,


164


′ is formed by interconnecting four end sections


166


A-D instead of two end sections. As in the case of

FIGS. 8 and 9

, the strips of each adjacent pair of strips


162


A-D are interconnected by support bridges


168


A-C situated along only one edge of a respective strip, and the support bridges are noninterconnected. To form the fuse element


160


, the blank


160


B is bent into an S-shape, whereby the support bridges


168


A and


168


C are situated on one side of the fuse element


160


, and the support bridges


168


B are situated on the opposite side.




The fuse element


160


exhibits the same advantages relating to improved mechanical strength, current density distribution, and heat dissipation exhibited by the fuse elements


136


and


136


′.





FIGS. 22 through 24

illustrate three alternative embodiments of a short circuit fuse element


334


,


434


, and


534


that can be used in accordance with the present invention. Elements


334


,


434


, and


534


are made and function substantially as described above with respect to the first embodiment short circuit element


134


.




End Cap Insulation




As observed earlier, the end caps


118


are formed of metal to provide suitable reinforcement and strength in securing the end plates


116


to the tube


112


. It will be appreciated, however, that the mutually adjacent inner ends


170


of the end caps constitute the most closely arranged external metallic pieces of the fuse


100


. Hence, in the case when the end caps are electrically connected to the terminals


114


or end plates


116


, there exists a risk to a user if his fingers bridge both end caps. That risk becomes greater if a relatively short tube


112


is used. In the present invention, however, that risk is completely eliminated, regardless of the length of the tube


112


, by the provision of insulating elements


172


for respective end caps. Since both of the insulating elements


172


are the same, only one will be explained in detail. With reference to

FIGS. 12 and 13

, each one-piece insulating element


172


includes a radial washer


174


, a cylindrical axial flange


176


projecting from an outer peripheral edge of the radial washer


174


, and a hollow sleeve


178


projecting axially from a slit


180


formed in the radial washer


174


.




With reference to

FIG. 20

, it can be seen that an outer peripheral edge


182


of the end plate


116


is recessed radially inwardly with respect to an outer periphery


184


of the tube


112


to form an annular recess


186


. The dimensions of that recess


186


in the radial and axial directions are the same as the radial thickness T and axial length L of the flange


176


of the insulating element


172


(see FIG.


12


). Therefore, when the insulating element


172


is placed against an end of the tube


112


, the flange


176


thereof precisely occupies the recess


186


, and the outer surface of the flange


176


is flush with the outer surface


184


of the tube


112


, as can be seen from

FIGS. 5 and 6

.




Furthermore, the radial washer


174


of the insulating element


172


overlies the end plate


116


, and the terminal


114


extends through the sleeve


178


at the point where the terminal passes through the slit


125


of the end cap


118


. It will thus be appreciated that the flange


176


of the insulating element


172


electrically insulates the axial portion


120


of the end cap


118


from the end plate


116


; the radial washer


174


electrically insulates the radial portion


122


of the end cap from the end plate


116


; and the sleeve


178


electrically insulates the radial portion


122


of the end cap from the terminal


114


, and also provides insulation and support along a portion of the length of the terminal.




The insulating element


172


can be formed of any suitable electrically insulative material, such as a glass reinforced thermoplastic molding compound.




Filler Hole Plug




As explained above, the cavity


132


of the tube


112


is filled with an arc-quenching filler material, such as quartz sand


133


. The quartz sand is introduced through one or more filler holes each defined by aligned openings in the radial portion


122


of an end cap


118


, the radial washer


174


of the insulating element


172


, and the end plate


116


, respectively, as shown in FIG.


16


.




It becomes necessary to close the filler hole


192


after the quartz sand has been introduced. In accordance with the present invention, the filler hole


192


is closed by a plug formed by a plastic drive rivet


194


. Such plastic drive rivets are conventional and are typically used to interconnect parts. The drive rivet


194


, depicted in

FIGS. 14 and 15

, is of one-piece construction and includes a generally frusto-conical flange


196


, a plurality of expansion fingers


198


projecting from one side of the flange


196


, and a plunger


200


projecting from an opposite side of the flange.




To install the rivet


194


after the cavity


132


has been filled with quartz sand


133


, the fingers


198


are inserted axially through the filler hole


192


until the flange


196


abuts the radial portion


122


of the end cap


118


(the flange


196


being of larger diameter than the filler hole). Then, the plunger


200


is driven axially through the flange


196


and into a cavity


199


formed by the fingers


198


. The plunger


200


expands the fingers radially outwardly into tight contact with a surface of the filler hole, whereby a maximum diameter formed by the free ends of the fingers is greater than the diameter of the opening of the end plate


116


and is situated inwardly of that opening (i.e., to the left of the opening in FIG.


17


).




Accordingly, there results a highly reliable interference fit between the fingers and the inner surface


202


of the end plate


116


, preventing dislodgement of the rivet. There thus results a tight and reliable plugging of the filler hole


192


by a relatively inexpensive element.




Furthermore, since the rivet


194


is formed of plastic (i.e., an electrically insulative material) the end cap


118


will not become electrically connected to the end plate


116


as would occur if the filler hole were instead plugged by drive plugs, set screws, balls or cups, which are all typically formed of conductive metal.




Terminal Reinforcement




As explained earlier herein, when a short tube


112


is used in a fuse, the blade terminals


114


must be lengthened in order to continue satisfying the dimensional requirements for the fuse. Lengthening of the terminals means that the terminals will be subject to greater mechanical moments.




The present invention provides additional reinforcement for a portion of the length of the blade terminals by means of the sleeves


178


of the insulating elements


172


, as previously mentioned. In addition, an end


208


of each terminal is constructed with an integral staking tang


210


as shown in FIG.


18


. Likewise, each end plate


116


is provided with a through-slot


212


sized to receive the staking tang


210


.




In addition, each end plate


116


is provided with a pair of through-holes


214


arranged on opposite sides of the slot


212


such that the through-holes


214


will be covered by the end


208


of the terminal when the staking tang


210


has been inserted into the slot


212


, as shown in FIG.


19


. By the application of heat or mechanical force, an inner end of the staking tang becomes deformed, as shown in

FIG. 21

, thereby staking the terminal to the end plate


116


. Also, solder


216


is applied to the through-holes


214


in order to mechanically and electrically couple the terminal to the end plate. The combined support produced by the tang


210


, the solder


216


, and the sleeve


172


, results in an effective strengthening and reinforcing of the blade terminal.




Alternative Embodiments





FIG. 25

illustrates a ferrule-type fuse


610


that is similar to the fuse described above, except that it uses ferrules


614


as the electrical contacts, instead of knife blades. The fuse


610


includes a conventional overcurrent trigger mechanism


634


and the short circuit fuse element


434


, as disclosed hereinabove.





FIG. 26

illustrates another ferrule-type fuse


710


that is similar to the fuses described above, particularly the fuse


610


of FIG.


25


. The fuse


710


includes a conventional overcurrent trigger mechanism


734


and the short circuit fuse element


534


, as disclosed hereinafter.




Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims.



Claims
  • 1. A fuse element comprising a body of metallic material including at least first and second parallel, superimposed strips, each strip including parallel rows of perforations dividing the strip into respective sections, adjacent perforations of each row being spaced apart to define weak points therebetween which secure adjacent ones of the sections together; the metallic body further including a plurality of support bridges interconnecting adjacent edges of the first and second strips; each support bridge electrically connecting one of the sections of the first strip to one of the sections of the second strip; adjacent bridges being noninterconnected; and wherein the support bridges are arranged at an angle with respect to the first and second strips.
  • 2. The fuse element according to claim 1, wherein an end-most section of the first strip is fixedly joined to an end-most section of the second strip to define a connecting tab for connecting the fuse element to an electrical circuit.
  • 3. The fuse element according to claim 2, wherein there are four strips.
  • 4. The fuse element according to claim 1, wherein there are four strips.
  • 5. A fuse element comprising a body of metallic material including at least first and second parallel, superimposed strips, each strip including parallel rows of perforations dividing the strip into respective sections, adjacent perforations of each row being spaced apart to define weak points therebetween which secure adjacent ones of the sections together; the metallic body further including a plurality of support bridges interconnecting adjacent edges of the first and second strips; each support bridge electrically connecting one of the sections of the first strip to one of the sections of the second strip; an end-most section of the first strip being fixedly joined to an end-most section of the second strip to define a connecting tab for connecting the fuse element to an electrical circuit; and wherein the support bridges are arranged at an angle with respect to the first and second strips.
  • 6. A fuse element comprising:a body of metallic material including at least first and second parallel, superimposed strips; each strip including parallel rows of perforations dividing the strip into respective sections, adjacent perforations of each row being spaced apart to define weak points therebetween which secure adjacent ones of the sections together; the metallic body further including a plurality of support bridges interconnecting adjacent edges of the first and second strips, each support bridge electrically connecting one of the sections of the first strip to one of the sections of the second strip, adjacent bridges being noninterconnected; and a connecting tab at each end of each of the strips for connecting the fuse element to an electrical circuit.
CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patent application Ser. No. 09/004,443, filed on Jan. 8, 1998, now U.S. Pat. No. 5,905,426, which is a divisional of U.S. patent application Ser. No. 08/670,559, filed on Jun. 27, 1996, now U.S. Pat. No. 5,736,918.

US Referenced Citations (7)
Number Name Date Kind
3743994 Kozacka Jul 1973
4153892 Kozacka May 1979
4179678 Perreault Dec 1979
4319213 Reid Mar 1982
5077534 Douglass Dec 1991
5736918 Douglass Apr 1998
5905426 Douglas May 1999
Continuation in Parts (1)
Number Date Country
Parent 09/004443 Jan 1998 US
Child 09/313769 US