US2876312A - Fuse link for a time-lag fuse and method of constructing the link - Google Patents

Description

March 3, 195 9 FREDERICK FUSE LINK FOR A TIME-LAG FUSE AND METHOD OF CONSTRUCTING THE LINK Filed Sept .17, 1956 Fig.2.

Inventor: 1 Oscar C."F-r-ede-r-ick, A by W M His 'btorneg.

2,876,312 Patented Mar. 3, 1959 FUSE LINK FOR A TllVIE-LAG FUSE AND METHOD OF 'CONSTRUCTIN G THE LINK Oscar C. Frederick, Springfield, Pa., assignor to General Electric Company, a corporation of New York Application September 17, 1956, Serial No. 610,327

Claims. (Cl. 200-435) This invention relates to a time-lag type of electric fuse and, more particularly, to a link for such a fuse and to a method of constructing such a fuse link.

The invention is especially concerned with the type of time-lag fuse in which the fuse link is constructed of dissimilar metals which react metallurgically upon protracted circuit overloads to initiate link-destruction and consequent circuit interruption. Normally, such fuse links include a main portion formed of a highaconductivity, high-fusing point metal and a second portion located immediately adjacent the first portion and formed of a metal having lower conductivity and a lower fusing point. When the metal with the lower fusing point fuses, the other metal dissolves in it to form an alloy. This alloy has a relatively high resistivity and causes a relatively rapid destruction of the link once the process of alloy-formation has begun.

An important problem in connection with fuses of this type is the matter of how the, low fusing-point metal is to be applied to the main portion of the fuse link in order to provide for link-destruction after time intervals which can be predetermined with a high degree of accuracy- Generally, unless there is intimate contact between the low fusing point metal and the main portion of the link, then it is most difiicult to accurately predict the time which will be required for a predetermined overload to destroy the link. As a means for providing the required intimacy of contact, it has been customary during the assembly operation to moderately heat the low fusing point metal and to fuse at least the surface portion thereof which engages the main portion of the link. This, however, has not been an altogether satisfactory solution to the problem because the heating operation produces a limited amount of alloying action between the two metals. I have found that minor variations in the extent of this premature alloying action produce appreciable variations in the time-overcurrent response of the fuse, and, as a result, the heating operation must be controlled very carefully in order to control the extent of such premature alloying action. construction process.

In the ribbon-type of fuse link, it has been customary to apply the low fusing point metal in the form of a clip crimped about the edge of the ribbon or in the form of a rivet suitably attached to the ribbon. In either case, the assembly operation is awkward and time-consuming and generally involves the carefully-controlled application of heat for effecting the desired intimate contact between the clip or rivet and the ribbon.

Accordingly, one of the objects of my invention is to applylow-fusing-point metal to a metallic fuse element by an inexpensive process which produces no significant alloying action between the two metals but yet produces intimate contact therebetween.

Another object is to provide a fuse link with alloyforming means which responds to predetermined overloads to produce link-destruction after time intervals This, of course, complicates the fuse-' which can be predetermined with a high degree of accuracy.

In carrying out my invention in one form, I utilize a metal-spraying process for applying to the fuse element a coating of low-fusing-point metal which intimately contacts the fuse element along a portion of its length. I control the process in such a manner that it heats the fuse element only to a minor degree which is insufficient to produce any significant alloying action between the coating and the fuse element. As a result, there is no premature alloying action to detrimentally affect that desired alloying action which occurs during actual fuseoperation just prior to the interruption of overload currents.

For a better understanding of my invention, reference may be had to the following specification taken in connection with the accompanying drawing, wherein:

Fig. 1 is a cross-sectional view of an electric fuse having a link which embodies my invention in its preferred form.

Fig. 2 is a cross-sectional view taken along the line 2-2 of Fig. 1.

Figs. 3 and 4 illustrate certain steps in the preferred method which is used for constructing the fuse link of Fig. 1, Fig. 4 being a sectional view taken along the line 4-4 of Fig. 3.

Referring now to Figs. 1 and 2, the fuse shown therein comprises a tubular casing 10 formed of suitable insulating material and provided at its'opposite ends with electricallyconductive terminals 12 and 13. Aribbontype fuse link 15 is disposed within the casing 10 and electrically interconnects the spaced-apart terminals 12 and 13. Thislink 15 is centrally located within the casing 10 by means ofsuitable washers 17 located at opposite ends of the casing. As shown in Fig. 2, the ends of thelink 15 pass through centrally-located apertures in thewashers 17 and extend over the outer surface of the casing 10, where they are mechanically fastened by crimping theterminals 12 and 13 about the casing.

Link 15 is formed of a high-conductivity metal, such as silver, and is imbedded in abody 19 of pulverulent arc-extinguishing filler, such as quartz sand. The link preferably has a number of spaced-apart regions of reduced cross-section which are formed by opposed V-shaped notches 20 provided in the edges of the link. The specific configuration of the illustrated link is the subject matter of and is claimed in a copending patent application S. N. 683,864, Wilfred F. Skeats, filed on September 13, 1957, and assigned to the assignee of the present application. When such a fuse is subjected to currents .of short circuit proportions, the regions of reduced crosssection immediately vaporize to initiate arcing, and the resulting are or arcs are quickly extinguished by the action of the arc-quenching filler. In the current-limiting type of fuse such as shown, arc-extinction occurs in appreciably less than a half-cycle, and the current permitted to flow is limited to a value appreciably lower than the maximum available short circuit current.

In a fuse of the type shown, it is most desirable that upon protracted overloads of inadmissible duration, the fuse blow after certain predetermined time intervals which aredependent upon the magnitude of the overload current. These time intervals are customarily controlled by the use of an overlay of low-fusing point metal applied to the surface of the fuse link in a region near its point of minimum crosssection. When the temperature of this overlay reaches the metal fusing point, the metal of the overlay reacts metallurgically with the metal of ribbon to form an alloy. This alloy has a relatively high resistivity and causes a relatively rapid destruction of '1 the link once the process of alloy-formation has begun;

In the disclosed fuse, I have provided an overlay of this general type by relying upon a particular metalspraying process, soon to be described, to coat a selected portion of the fuse link with low-fusing point metal. This overlay or coating is shown at 25 in Fig. l and, preferably is formed of tin or of a eutectic alloy of tin and lead.

The metal-spraying process is carried out by means of a conventional metal-spraying gun in which a wire or rod of the metal being sprayed is melted by means of a flame and the molten metal is atomized by means of a stream of air which is projected through the usual nozzle of the gun. Such a gun is illustrated at 34 in Fig. 4, where a spray of atomized metal is shown issuing from itsnozzle 35.

Because the fuse link is only several thousandths of an inch in thickness and consequently has a very low mass, it has a pronounced tendency to become heated by the flame during the metal-spraying process. This heating tends to fuse the portion of the coating contacting the ribbon and, hence, tends to produce the premature alloying action which I have found to be detrimential to subsequent accurate operation of the fuse during circuit overloads. I overcome this problem by controlling the spray-gun in such a manner that the temperature of the fuse element during the spraying-operation rises to only a minor degree which is insufficient to produce any significant alloying action between the metal of the coating and that of the fuse element. 1 have found that overlays applied in this particular manner are capable of producing link destruction after overload-time periods which can be predetermined with an exceptionally high degree of accuracy. I attribute this to the fact that the above spraying process produces highly intimate contact between the metal of the overlay and that of the ribbon and yet produces no significant alloying action between the two metals, thus freeing the fuse from the previouslydescribed adverse effects of premature alloying action. It is also significant to note that no intervening fluxes or bonding agents are present to impair the intimacy of contact between the two metals.

Among the numerous factors which can be suitably controlled to prevent undesirable heating of the ribbon during the spraying process are: the distance of the nozzle of the spray gun from the fuse element being coated; the size and characteristics of the flame within the gun; and the rate at which the metal wire is fed into the gun.

Applying the overlay by the above-described metalspraying process not only results in the highly-consistent fuse performance described above but also results in considerably reduced manufacturing costs for fuse links of this general type. In this latter respect, the metal-spraying process readily lends itself to mass-production techniques which involve very little individual handling of the fuse links. This not only permits the fuse links to be manufactured at considerably higher speed with considerably less effort than heretofore but also minimizes the possibility of breaking or otherwise damaging the thin ribbons as a result of handling during the manufacturing operation.

Specifically, this manfacturing process, in its preferred form, is as follows: first, a large number of ribbons such as are stamped from a thin sheet of conductive material such as silver. Thereafter, the ribbons are immersed for a short time in an etching and cleaning fluid, such as dilute nitric acid. This superfiicial etching etching prepares the ribbon surface for the ensuing metalsprayingoperation by roughening it sufliciently to insure a high degree of adherence of the sprayed metal. By utilizing this etching procedure, instead of some mechanical procedure such as shot-blasting, I am able to sufficiently roughen the surface without subjecting the thin ribbon to damaging mechanical stresses and without sig- '4 nificantly altering its thickness and resultant electrical properties.

The ribbons are then rinsed, and a large number are placed in a fixture such as schematically shown at 30 in Fig. 3. Thisfixture 30 comprises abase 31, on which the desired number of ribbons are suitably located, and a pair of spaced-apart suitably-mountedmasking plates 32 disposed atop the ribbons. The exposed portion of each ribbon which is located between the twomasking plates 32 corresponds to the region Which is to receive the overlay. As shown in Fig. 4, aconventional metalspraying gun 34 having its nozzle '35 aligned with the exposed portions of the ribbons is then used for spraying a stream ofatomized low-fusing point metal on to these exposed portions in the controlled manner previously described. As thenozzle 35 is moved parallel to the gap between the plates, each successive ribbon is given the desired coating of low-fusing-point metal. The thickness of this sprayed metal coating can be accurately controlled by suitably controlling the spraying time. When this metal-spraying operation has been completed, themasking plates 32 are lifted from the coatedribbons 15, and the coated ribbons are then lifted from the base 31-, after which the ribbons are ready for incorporation into the finished fuse.

To insure that, after the spraying operation, the thin ribbons are not bent or otherwise damaged in lifting the maskingplates 32 or in lifting theribbons 15 from thebase 31, I have found that it is most important to prevent the metal-spraying process from bonding the ribbons either to the masking plates or to the base. Any such bond tends to interfere with free removal of the partsand thus tends to result in undesirable bending of the thin ribbons. As between the maskingplates 32 and theribbons 15, I avoid such bonds by providing the face F of each masking plate with arecess 33 which extends along the lower edge of each plate in the region of the sprayed overlay. Theserecesses 33, which are best seen in Fig. 4, have a height substantially greater than that of the finished overlay so that the upper surface of the overlay does not become bonded to the horizontal upper surface of therecess 33 during the spraying process. The vertical surface of eachrecess 33 is also sufiiciently removed from the overlay to prevent a bond from being established at such surface. Although the illustratedrecess 33 is of rectangular cross-section, it is to be understood that other cross-sectional shapes are also suitable for the practice of this invention.

As between theribbons 15 and thebase 31,] avoid undesirable bonding by providing the base 31 with anelongated recess 33a that has a length extending across the width of the ribbons so that the lateral edges of the ribbons are out of contact with the base 31 in the region of therecess 33a. The recess has a width W appreciably greater than the width of the overlay (as measured along the length of the ribbon) so that the maskingplates 32 leave exposed only an intermediate zone of the section of the link extending across therecess 33a. During the coating operation, therecess 33a receives that sprayed metal which issues from thenozzle 35 when thegun 34 is passing over the space betweenadjacent ribbons 15. This prevents the sprayed metal from building up' about the edge of each ribbon, as would be the case if therecess 33a were not present and the upper surface of the base 31 were continuous.

The coating which is produced by this metal spraying process has the general structural characteristics typical of most sprayed-metal coatings. That is, it has a stratified appearance as microscopically viewed in cross-section. It contains internally-located oxide films and has a density somewhat less than that of the same metal in its cast state before spraying. As described hereinabove, the junction between the coating and the ribbon is devoid of any alloy of the two contacting metals.

Another feature which is believed to contribute to the exceptionally accurate time-overload response of my fuse is the configuration of theoverlay 25 relative to the opposed notches. As shown in Fig. 1, the opposed notches define between their apices an area of minimum cross-section which is spaced a small distance away from the location of the overlay. Since the overlay extends completely across the width of the ribbon, it has a relatively large amount of metal available near this minimum cross-section portion of the link. 'When the overlay metal fuses, the converging edges of the opposed notches act to guide or funnel the fused metal into the minimum cross-section region of the link, where the alloying action will be most effective in producing the desired rapid link-destruction. There are no corners or circuitous paths to impede the flow of fused metal into the minimum cross-section region. Thus, this combination of factors enables a near maximum quantity of overlay metal to flow into this critical region with a minimum of delay.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

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

1. A fuse link for a time-lag fuse comprising a conductive metallic ribbon, and a stratified, internallyoxidized sprayed-metal coating disposed in intimate metal-to-metal contact with a surface portion of said ribbon, said coating being formed of a metal which has a lower fusing point and lower conductivity than the metal of said ribbon and which is capable of reacting with the metal of said ribbon to form an alloy having a higher specific resistance than the metal of said ribbon, the junction between said coating and said ribbon normally being devoid of said alloy.

2. A fuse link for a time-lag fuse comprising a conductive metallic ribbon formed with a pair of opposed generally V-shaped notches defining between their apices a region of minimum cross-section, and a stratified, internally-oxidized sprayed-metal coating disposed in intimate contact with a surface portion of said ribbon and extending across the width of said ribbon adjacent to but spaced from said region of minimum cross-section, said coating being formed of a metal which has a lower fusing point than the metal of said ribbon and which is capable of reacting with the ribbon metal to form an alloy having a higher specific resistance than the ribbon metal.

3. A fuse link for a time-lag fuse comprising a conductive metallic ribbon containing at least one notch which is shaped to gradually reduce the cross-section of the ribbon down to a region of minimum cross-section, and a stratified, internally-oxidized sprayed-metal coating intimately contacting a surface portion of said ribbon in the gradually reduced region of the ribbon, said coating extending across the width of said ribbon at a location spaced from said region of minimum cross-section, the metal of said coating having a lower fusing point and lower conductivity than the metal of said ribbon and being capable of reacting with the metal of said ribbon to form an alloy having a higher specific resistance than the metal of said ribbon.

4. The fuse link of claim 3 in which the junction between said coating and said ribbon is normally devoid of said alloy.

' 5. A method of constructing a ribbon-type fuse link for a time-lag fuse comprising: cleaning and chemically etching at least a portion of the surface of the ribbon, and then coating said surface portion with low-fusing-point metal applied by means of a metal-spraying process so controlled that the ribbon is insufficiently heated to produce alloying action between the metal of the ribbon and that of the coating.

6. A method of constructing a fuse link comprising: cleaning and chemically etching a surface area of said link. masking predetermined portions of said surface area to leave a portion of said area exposed, and then coating said exposed portion with low-fusing metal applied by means of a metal-spraying process.

7. A method of constructing a ribbon-type fuse link comprising: providing a base which contains a recessed portion, positioning said ribbon-type link upon said base with a section of the link having its lateral edges extending across said recessed portion, masking selected surface portions of said link in such a manner as to leave an intermediate zone of said link section exposed, and then coating said intermediate zone with a low-fusing point metal applied by means of a metal-spraying process.

8. A method of constructing a ribbon-type fuse link comprising: providing a base which contains a recessed portion, positioning said ribbon-type link upon said base with a section of the link having its lateral edges extending across said recessed portion, positioning upon said link a pair of spaced-apart masking members having opposed faces defining therebetween a gap which registers with an intermediate zone of said link section, each of said opposed faces being recessed across the width of said link in the region immediately adjacent said link, and then coating said intermediate zone with a low-fusing point metal applied by spraying the metal in atomized form through said gap.

9. A method of constructing a ribbon-type fuse link comprising: positioning a thin metallic element upon a base, positioning upon said element a pair of spacedapart masking members having opposed faces defining therebetween a gap which registers with a predetermined zone of said metallic element, each of said opposed faces being recessed across the width of said metallic element in the region immediately adjacent said element, and then coating said predetermined zone with a low-fusing point metal applied by spraying the metal in atomized form through said gap.

10. The process of constructing a ribbon-type fuse link for a time-lag fuse comprising the steps of: cleaning and chemically etching at least a portion of the surface of the ribbon, heating and melting a relatively lowfusing point metal, atomizing said molten metal by means of a stream of gas, and spraying said atomized metal on said surface portion of the ribbon, the metal-spraying process being so controlled that the ribbon is insufficiently heated to produce significant alloying action between the metal of the ribbon and that of the spray.

References Cited in the file of this patent UNITED STATES PATENTS 1,256,599 Schoop Feb. 19, 1918 1,303,404 Simon May 13, 1919 1,473,284 Feldkamp Nov. 6, 1923 2,270,404 Bitter Jan. 20, 1942 2,471,176 Von Hoorn May 24, 1949 2,703,352 Kozacka Mar. 1, 1955 2,734,111 Kozacka Feb. 7, 1956 2,739,911 Andriulis et a1. Mar. 27, 1956 2,832,868 Kozacka Apr. 29, 1958 FOREIGN PATENTS 473,335 Great Britain Oct. 7, 1937 484,782 Great Britain May 10, 1938 488,446 Great Britain July 7, 1938 Great Britain July 25, 1941

Images