US3489977A - Current limiting fuse - Google Patents

Description

Jan. 13, 1970 s. R. SMITH, JR 3,489,977

Y CURRENT LIMITING FUSE Filed Feb. 15, 1968 United States Patent 3,489,977 CURRENT LIMITING FUSE Sidney R. Smith. Jr., Myrtle Beach, S.'C., assignor to General Electric Company, a corporation of New York Filed Feb. 15, 1968, Ser. No. 705,798 Int. Cl. H01h 71/20, 73/22 U.S. Cl. 337-146 10 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to electrical fuses and more particularly to an improved fuse link which is exceptionally well adapted for use in providing overload current protection for electric power distribution systems thatvare often subjected to both sustained relatively lowcurrent overload conditions as well as to occasional highcurrent overload surges.

Current limiting fuses now commonly used for the protection of power distribution system components are usually designed to provide protection from either high current surges that must be interrupted immediately, or relatively small overloads that need only be interrupted if they are sustained for a substantial length of time at a current level that might generate enough heat to damage certain components of the system. It is also fairly common practice to provide such systems with fuses that afford both of these current limiting functions in a single fuse cartridge having a suitable series connected arrangement therein. However, the prior art series connected dual-element fuses presently available generally incorporate a fairly complex combination of fuse elements, each having different time-current characteristics, and often requiring the further combination of some type of heating arrangement for one of the elements. In addition to `their complexity and resulting expense, such arrangements have several operational shortcomings. For example, a common problem in such fuses is that in order to obtain rapid burn back of the low overcurrent element to assure clearing of the circuit when this element melts, it is necessary to make the separate fuse wires of very small diameter, which makes the wire dicult to work with during the manufacture of the fuse. A second inherent problem with such fuses has been that heretofore they have not been capable of accurately and consistently providing overload protection for very low values of sustained overcurrent.

A further problem encountered in prior art fuses for overload protection stems from the fact that frequent temperature cycling of the fuse element during its operational life often results in the relatively weak, reduced area portion of the fuse being broken as it is repeatedly forced to expand and contract. My invention overcomes the foregoing disadvantages of prior art fuse devices while atthe same time affording a simple and low cost fuse construction.

Accordingly an object of my invention is to provide an improved fuse that is adapted for use in power distribution systems and which will protect the components of such systems from damage due to either short duration high current transients or sustained relatively low overcurrent conditions in the system.

3,489,977 Patented Jan. 13, 1970 ICC Another object of the invention is to provide an improved fuse having a time-current characteristic which provides a relatively slow, low overcurrent limiting function.

A further object of the invention is to provide an improved fuse construction in which the fusible elements employed will not be subjected to the hazard of breakage during their normally long operational life in which the fuse elements are repeatedly heated and cooled.

Further objects and advantages will become apparent as the description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of the specification.

In the drawing,

FIG. l is a side elevation, partly in cross section, of a current limiting fuse embodying a preferred form of my invention.

FIG. 2 is a side elevation, partly in cross section, of a fuse embodying a second from of my invention.

FIG. 3 is a side elevation, partly in cross section of another embodiment of a fuse link constructed pursuant to the teachings of my invention.

Referring now to FIG. l of the drawing, there is shown a current limiting fuse illustrated as comprising a tubular casing 1 of insulating material, such as pressed organic fiber, glass or ceramic, upon the opposite ends of which are mounted metal terminals 2 and 2. The terminals 2 and 2 may be sealed to the tubular casing 1 in any conventional manner, such as by sealing them hermetically with a suitable epoxy glue. A granular arc quenching material 3, such as sand, is enclosed within the envelope formed by the tubular casing 1 and terminals 2 and 2. A first plurality of fusible conductive elements 4, which comprise silver wires of equal and substantially uniform diameter in the preferred embodiment of my invention, are wound in helical form around a cylindrical member 5, and are each electrically connected at one end to the terminal 2. The member 5 is formed of a nonlinear resistance material such as silicon carbide of the type commonly used in valve-type lightning arresters. The other ends of the fusible wires 4 are twisted to form a junction 6 with the respective ends of a second plurality of elongated fusible elements 7, which also comprise silver wires of substantially equal and uniform diameter in the preferred embodiment of my invention. The other ends of the silver wires 7 are electrically connected, respectively, to the terminal member 2 by being soldered thereto. A circular disc 8 formed of any suitable insulating material, such as glass, and having a plurality of equally spaced peripheral notches 8a therein, is disposed within the tubular casing 1 to help maintain the fusible conductors 7 in spaced relationship over a major portion of their length. As shown in FIG. l, each of the wires 7 is positioned in one of the notches 8a. It will be understood that fusible conductors 4 and 7 may be formed of other suitable eutectic material, such as a silver-copper alloy, and my invention is not limited to the use of silver wire for these elements.

In order to provide a fusible arrangement that will immediately interrupt a high-current overload without producing an unduly severe voltage kick, the combined diametrical cross-section of the fusible conductors 4 is substantially smaller than the combined diametrical crosssection of the fusible conductors 7. Accordingly, when a high overcurrent surge exists in the fuse, the fusible elements 4 immediately vaporize causing an arc to be formed between terminal 2 and the junction 6, along the length of the nonlinear resistance 5. The nonlinear resistance member S is thus placed in parallel with the resultant arc and, accordingly, conducts a portion of the current that flows as the fusible elements 7 are burned back; therefore, the voltage kick that would otherwise be transmitted to the system when the elements 4 fuse, is substantially reduced, by the simultaneous introduction of the nonlinear resistance member 5 into the circuit where wires 4 vaporize.

If the fuse device is subject to a low overload current condition of extended duration, the juxtaposed portion of fusible elements 4 and 7 formed at the twisted junction 6 will fuse and rupture, due partially to the fact that this novel arrangement serves to concentrate the heat generated by currents flowing through the respective conductors 4 and 7 at the junction 6. For the major portion of their respective lengths, the fusible elements 4 and 7 are surrounded by the arc quenching sand 3, therefore, over this portion of their lengths they are maintained in a relatively cool state by the heat-sink effect of the sand 3 so they will not heat as rapidly as thev junction 6 during sustained low current overload conditions. To further accelerate the rate of fusion of the junction 6, in the preferred embodiment of my invention, the twisted conductors at 6, are bonded together with a bead of eutectic material 9 comprising 25% cadimum, 50% tin and 25 lead. I have found this to be a particularly desirable eutectic alloy for the purposes of my invention because it melts at a much lower temperature than the silver wires 4 and 7, and causes these elements to form a low melting point alloy that fuses and ruptures at a temperature substantially below the melting point of pure silver. In addition, the alloy thus formed with silver wires 4 and 7 has a higher electrical resistance than pure silver. This formation of the alloy both lowers the melting point of the fusible wires 4 and 7, and simultaneously increases the amount of heat generated at the junction 6. This operating characteristic of my invention is also improved by providing an insulating sheath 10 around the twisted junction 6 to confine the heat generated at the junction during a sustained overload condition. Sheath 10 may be formed of a ber cylinder, as shown, or other suitable heat insulating means, such as insulating tape wrapped around the junction 6.

While it is known in the prior art to use cadmium-containing beads on fuse elements to lower their temperature of fusion, the above mentioned alloy has been found to be particularly desirable in practicing my invention because of the desirably low melting characteristics it provides. For example, with the structure disclosed it has been found that the melting point of the fusible elements at junction 6 can be lowered 50% below the melting temperature of the spaced apart, uninsulated portion of the fusible elements 4 and 7.

To further illustrate the features of my invention, reference is now made to FIG. 2 of the drawing wherein a modification of the invention is depicted. In FIG. 2, the component parts of the illustrated device similar to those in FIG. 1 will be designated with like reference numerals. Accordingly, there is shown a tubular casing 1 having electrically conductive terminals 2 and 2' sealed in a suitable manner at the opposite ends thereof. A first plurality of conductive fusible elements 4 are electrically connected at one of their respective ends to the terminal member 2 while a second plurality of fusible elements 7 are connected in a similar manner to the terminal element 2'. Circular, notched, insulating discs 8 are disposed in the tubular casing 1 between the respective elements 4 and 7 with the notches 8a each positioning one of the fusible elements to maintain it in spaced apart relationship with respect to the remainder of the elements. A twisted junction 6 is formed between the respective other ends of the fusible conductors 4 and 7 and this junction is encased in a heat insulating wrapping 10, such as insulating tape, that serves to maintain the heat generated at the junction in thermal proximity therewith. The interior of the envelope formed by the casing 1 and the terminals 2 and 2 is filled with a granular insulating material, such as the sand 3. This embodiment of the invention functions in much the same manner as the embodiment illustrated in FIG. 1; namely, the combined cross section of the plurality of fusible elements 4 is of smaller diameter than that of the fusible elements 7 and, thus, the elements 4 vaporize first when a high overcurrent surge is passed between the terminals 2 and 2. It will be noted that the nonlinear resistance element 5 which was present in the preferred embodiment of FIG. 1, is not employed in this embodiment of the invention. This omission of the nonlinear resistance causes a greater voltage kick to be induced `when the elements 4 fuse, but the heavier cross lsection of elements 7 causes them to burn back more slowly to thus reduce the magnitude lof the voltage kick somewhat. In a similar manner, it will be apparent that the insulating spacing discs 8 may be removed without departing from the scope of the invention; however, in order to provide for desirable radiant and conductive cooling of the fusible elements 4 and 7, I prefer to maintain them in substantially equidistant spacing and, therefore, utilize the discs 8 in the illustrated embodiment.

An important feature of my invention resides in the fact that the low current fusible portion of the junction 6 is mechanically strong due to the fact that its mechanical cross section is greater than the mechanical cross section of the respective fusible conductors 4 and 7 that feed into it. Accordingly, unlike other prior art fuse devices that provide a low overcurrent fusing function by reducing the mechanical cross section of the fuse element, and thus weaken the element at that point, the fuse of my invention can withstand repeated temperature cycling with the resultant expansion and contraction of the fusible element, without any danger of rupture due to mechanical failure at the low overcurrent fusible junction 6.

In both of the embodiments of my invention illustrated in FIGS. l and 2, the fusible elements 4 and 7, on opposite sides of the respective junctions 6, comprise separate elements. It should be understood from the foregoing discussion that the fusible elements 4 and 7 could be; formed of different materials, shaped to different respective diameters, or varied in number, so that one of the groups of elements consistently initiates fusion when a high overload current is passed through the fuse, while the other group of elements serves to reduce voltage kick by affording a slower rate of burn back. Although the above described embodiments of my invention are its more preferred forms, yet another modification of my invention `affords a simplified, inexpensive-to-manufacture structure that also possesses the desirable operating characteristics of my invention.

Referring to FIG. 3, this modication of my invention is shown in the form of a fuse link `comprising a plurality of silver Wires 4' that are maintained in a spaced-apart relationship for a major portion of their lengths, as in zones A and B, and held in juxtaposition for a short portion of their lengths, as in zone C. It will be understood that other conventional fusible elements than the silver wires embodied in this form of the invention can be used without departing from the scope of the invention. For example, a plurality of fusible ribbons or bands formed of an eutectic alloy might be employed in other embodiments of the invention. Of course, any suitable spacing means, such as those employed in the modifications depicted in FIGS. l and 2 in the form of discs 8, may be used to maintain the substantially equidistant spacing of wires 4' in zones A and B; therefore, no specific spacing means is illustrated in FIG. 3. In the zone C, a heat generating junction 6 is formed by the closely spaced wires 4. This junction `6 retards heat dissipation to the surrounding environment, due to the relatively small surface area of the wires 4 in contact with that environment, as explained above with regard to the modifications of the invention shown in FIGS. 1 and 2. Also, an insulating housing 10" is disposed around the junction 6 and a band of cadmium alloy 9, comprising 25% cadmium, 50% tin and 25% lead is deposited around the junction 6. In addition to providing a low melting point eutectic alloying material at junction 6', the band of alloy 9 serves to hold the wires 4 together prior to the time housing 10 is put in place. It will be noted that a significant feature of this form of the invention is that the fusible elements 4 are continuous over their respective lengths, including the portion thereof forming the junction 6'; therefore, it will be appreciated that such a fuse link is easily fabricated by simply twisting a plurality of such elements 4' to form a juncton 6', then depositing a band of solder 9 around the junction and encasing it is an insulating housing 10'.

In operation, the modification of the invention illustrated in FIG. 3 functions -as follows: when a low overcurrent is passed through the fuse for a predetermined sustained length of time, enough -heat is generated in housing 10 by the juxtaposed wires 4' at junction 6 t0 melt the cadmium alloy band 9. The melted alloy band then forms an alloy with the wires at junction 6. This alloy has a low fusing point and relatively high resistance which combines to cause the wires to fuse at a temperature level approximately 50% below the temperature at which the portions of wires 4', in zones A and B, will fuse. Accordingly, an arc is formed at the midpoint of junction 6' and it burns back the fusible elements 4 to effect an interruption of any circuit in which the fuse link may be connected.

A significant difference exists between the high overcurrent operating characteristics of this form of my invention and those of the forms illustrated in FIGS. 1 and 2. Since the cross sectional area of t-he current conducting fusible elements 4 remains unchanged over the entire length of the fuse link, in the FIG. 3 embodiment, a high current surge vaporizes the portions of wires 4' in both zones A and B, so only the thick junction 6 serves to retard the rate of burn back.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that changes and modifications may be made therein without departing from the invention, and therefore, it is intended by the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A current limiting fusible device comprising first and second spaced apart terminals, a first plurality of fusible conductors each connected at one of their ends to said first terminal, a second plurality of fusible conductors each connected at one of their ends to said second terminal, said first and second plurality of conductors being arranged to maintain a substantial lateral spacing between their respective conductors for a major portion of their lengths, means forming an electrical junction between the respective other ends of said first and second pluralities of fusible conductors, said first and second pluralities of conductors being arranged in juxtaposition for a short portion of their respective lengths adjacent the point of their electrical junction, the timecurrent characteristic of said first plurality of fusible conductors being substantially faster than the time-current characteristic of said second plurality of fusible conductors whereby said first plurality of conductors ruptures first when a high overcurrent is passed through the device.

2. A current limiting fusible device as defined in claim 1 wherein said means forming an electrical junction cornprises `a layer of eutectic material bonded around the respective other ends of said first and said second pluralities of fusible conductors.

3. A current limiting fusible device defined in claim 2 wherein said other ends of said first and second pluralities of fusible conductors are twisted together for a relatively short portion of their overall respective lengths.

4. A current limiting fusible device as defined in claim 1 including a body of granular arc quenching material disposed between said first and second terminals and around all of said fusible conductors.

5. A current limiting fusible device as defined in claim 3 including an insulating material disposed around the twisted junction of said conductors, said material being effective to thermally insulate said connection.

6. A fuse including first and second spaced apart terminals, a tubular insulating casing disposed between said terminals and sealed thereto to form an envelope, a first plurality of fusible conductors each connected at one of their ends to said first terminal, a second plurality of fusible conductors each connected at one of their ends to said second terminal, an electrically conductive nonlinear resistance member disposed within said envelope, each conductor of said rst plurality of conductors being tightly wrapped on said member and arranged to maintain a substantial lateral spacing respectively therebetween said conductors form a plurality of parallel hellical paths around said member, means forming an electrical connection between the respective other ends of said first and second pluralities of fusible conductors, said first and second pluralities of conductors being arranged in heat exchanging juxtaposition for a short portion of their respective lengths adjacent the point of their electrical connection.

7. A fuse as defined in claim 6 wherein said means forming an electrical connection comprises a solder containing 50% tin, 25% lead and 25 cadmium mutually bonded to the respective other ends of said first and second pluralities of fusible conductors.

8. A fuse as defined in claim 6 including a body of granulated arc quenching material disposed in said envelope around said bonded solder connection.

9. A fuse as defined in claim 2 wherein said layer of eutectic material contains more than 10% by weight of cadmium.

10. A fuse as defined in claim 2 wherein said layer of eutectic material comprises an alloy of approximately 50% tin, 25 lead and 25 cadmium.

References Cited UNITED STATES PATENTS 1,692,445 11/1928 Hope 337-291 2,876,312 3/1959 Frederick 337-163 X FOREIGN PATENTS 1,021,069 2/ 1966 Great Britain.

BERNARD A. GILHEANY, Primary Examiner H. B. GILSON, Assistant Examiner U.S. Cl. X.R. 337-163, 291

fjg" UNHED STATES PATENT of CERTIFICATE OF CORRECTION Patent No. 3,489,977 Dated January I3, |970 Invencor(s) Sidney R. Smith, Jr

It is eettified that error appears in the above-identified patent and that said Lettere Patent are hereby corrected as shown below:

Cobumn 6, I inefl -4- Si gned and sealed this 9th day of Maly` 1970 (SEAL) Attest:

EDWARD IWFLETCHERJR. WILLIAM E. SCIIUYLER, JR.

Commissioner of Patents Attesting Officer

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