US2781434A

US2781434A - Current-limiting fuses comprising fuse links of silver and copper

Info

Publication number: US2781434A
Application number: US480080A
Authority: US
Inventors: Kenneth W Swain
Original assignee: Chase Shawmut Co
Current assignee: Chase Shawmut Co
Priority date: 1955-01-06
Filing date: 1955-01-06
Publication date: 1957-02-12
Anticipated expiration: 1974-02-12

Description

CURRENT-LIMIT'INGFUSES COMPRISING FUSE LINKS OF SILVER AND COPPER Feb. 12, 1957 K w SWAIN 2,781,434

Filed Jan. 6, 1955 2 Sheets-Sheet 1 1957 K. w. SWAIN 2,781,434 CURRENT-LIMITING FUSES COMPRISING FUSE LINKS OF SILVER AND COPPER Filed Jan. 6, 1955 2 Sheets-Sheet 2 5 All Wily/1111111 74 \v United States Patent CURRENT-LHVIITING FUSES COMPRISING FUSE LINKS OF SILVER AND COPPER Kenneth W. Swain, Hampton Falls, N. H., assignor to The Chase-Shawmut Company, Newburyport, Mass.

Application January 6, 1955, Serial No. 480,080

7 Claims. (Cl. 200-131) This invention relates to the protection of electric circuits, electric apparatus and electric machinery by electric fuses.

It is one of the objects of this invention to provide relatively inexpensive current-limiting fuses highly sensitive to major fault currents comprising composite fuse links of silver and copper.

Another object of the invention is to provide currentlimiting fuses comprising composite silver-copper fuse links wherein the amount of silver is minimized and which are nevertheless capable of limiting major fault currents at least to the same extent as comparable fuses having links consisting entirely of silver.

Still another object of the invention is to provide current-limiting high current-carrying capacity fuses comprising ribbon-type fuse links including a strip of silver and a pair of strips of copper bonded together to form a virtually unitary strip of sheet metal.

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

For a better understanding of the invention reference may be had to the accompanying drawing wherein Fig. 1 is a side elevation of the material used for manufacturing fuse links of the type used in fuses embodying my invention;

Fig. 2 is a section along 2-2 of Fig. 1;

Fig. 3 is a side elevation of a fuse embodying my invention;

Fig. 4 is substantially a longitudinal section through the fuse shown in Fig. 3 along 44 of Fig. 3;

Fig. 5 is a front elevation of a stamping to be used as fuse link for a high current-carrying capacity fuse em bodying my invention;

Fig. 6 is a section along 6-6 of Fig. 5;

Fig. 7 is a front view of a stamping which may be used to form a cylindrical fuse link;

Fig. 8 is substantially a longitudinal section through a fuse comprising a fuse link made of a stamping of the type shown in Fig. 7, and

Fig. 9 is a section along 9-9 of Fig. 8.

The material shown in Figs. 1 and 2 comprises a strip of silver 2 and two spaced strips of copper 1 each arranged on opposite sides of the strip of silver 2. Strips 2 and 1 are bonded together to form a substantially unitary strip of sheet metal. The processes for producing such composite metals are well known in the art, and therefore do not need to be described here in detail. The width of each strip of copper 1 is considerably larger than the width of 'the intermediate strip of silver 2. This tends to minimize the cost of the material required for manufacturing fuse links for fuses embodying my invention. A dash-and-dot line D in Fig. 1 indicates the way in which stampings may be made from the strip ICC therewith. Fuse link 3 extends in a direction longitudinally of the fuse and interconnects conductively the two ferrules or terminal caps 5. The two ferrules 5 are mounted on a tubular casing 4 of an appropriate insulating material such as, for instance, a glass-clothsynthetic-resin laminate. Each of the ferrules 5 is provided with a slot 5a for insertion of one end of fuse link 3. The axially outer ends of fuse link 3 which enter slots 5a may be slightly reduced in width compared to what may be called the normal width of link 3. Such a reduction in width may be desirable for manufacturing reasons but has no effect upon the performance of the fuse. Each slot 5a in each cap or ferrule 5 is substantially closed by a washer 6 to preclude the silicious pulverulent arc-quenching filler 7 within casing 4 from flowing through the slots 5a in ferrules 5 out of casing 4. Each ferrule 5 is soldered to one of the axially outer ends of fuse link 3 to minimize the resistance of the current-path formed by the fuse. An axially outer cap or ferrule 8 is mounted under pressure on each of the axially inner caps or ferrules 5, and both the axially outer and the axially inner caps or ferrules are crimped at their axially inner ends to produce a firm and gas-tight connection with casing 4. Each of the axially outer caps or ferrules 8 is provided with a blade contact 9 extending away from the casing 4 of the fuse.

The strip 2 of silver of which fuse link 3 is made is arranged in the center region of easing 4, i. e. midway between the ends thereof. Silver strip 2 has a point of reduced cross-sectional area 2a coextensive with the center of casing 4 and with the hot spot formed on said link 3 when carrying an electric current. Each of the strips of copper 1 of which link 3 is made begins at opposite axially outer boundary lines 2b of silver strip 2, and each strip of copper 1 provides a direct currentpath from the silver strip 2 to one of the terminal elements 5, 8. The point of minimum cross-sectional area 2a is situated exactly in the middle between boundary lines 2b. Each copper strip 1 is considerably longer than the intermediate silver strip 2, i. e. the length of each copper strip 1 is a multiple of the length of silver strip 2. The length of copper strips I ought to be at least twice the length of silver strip 2.

The above referred-to point of reduced cross-sectional area 2a of silver strip 2 is produced by two lateral substantially V-shaped notches 20 substantially coextensive with the center of, and the hot-spot formed on, link 3 when the latter is carrying current. The edges of notches 2c enclose an acute angle. The length of the silver strip 2 is in the order of the width of notches 20 at the wide open end thereof. The depth of each notch 20 is almost equal to one half of the width of strip of silver 2 where its width is greatest; consequently the cross-sectional area of the point of reduced cross-sectional area 2a formed by notches 2c is very small. The length of the portion of minimum cross-sectional area 2a of silver strip 2 is also very small. As a result of this configuration of silver strip 2, its point of reduced cross-sectional area 2a approximates closely a point heat source when link I 3 is carrying an electric current. It will be apparent from the foregoing that in the fuse illustrated and described the point of the fuse link where the maximum heat is generated is arranged at a point if heat generation is substantially localized at the center of. the casing. Itis, therefore, possible to producefuses having a relatively high operating temperature at the center of the casing and of the link, which nevertheless run. cool on account of small I -r losses required to achieve the high hot-point temperature on the link. Since the neck or reduced cross-section portion 2a of the fuse link. operates normally at a high temperature and since it is made of the low fusing. energy metal silver but. little additional energy is required subsequent to fault inception to heat the neck 20 of the link from its normally high operating temperature to fusing temperature. The fuse is therefore very fast, i. e. tends to limit the peak of major fault currents way below the peak of. theravailable short-circuit currents. This is achieved in spite of the fact that theprepondcrant portion of the length of the link consists of copper which has a relatively high fusing energy.

In this fuse structure silver is used to initiate the are as early after fault inception as possible to reduce the peak of the let-through current to as low a level as possible. Once arcing is initiated and the current-limiting action of insertion of arc resistance into the circuit started, the terminals of the arc may be safely transferred by back-burning from the axially inner small silver portion of the link to the relatively long portion thereof which consists of copper. As far as current-limiting action is concerned, the link functions in the same way as a well pre-heated all-silver link. As far as all other phases of the operation of the link are concerned, including its current-carrying ability, the link behaves, in essence, in the same way as an all-copper link. Because of the small quantity of silver involved, the cost of the link is almost as low as that of an all-copper link, though the current-limiting action thereof is equal to that of an all-silver link designed for drastic currentlimiting action.

The silver strip 2 and the narrow neck 2a formed therein operate as supersensitive arc-kindling means, i. e. they kindle an arc and initiate the process of insertion of arc resistance at an early time following fault inception. The silver strip 2 does not need to be as long as the required ave-rage back-burning distance of the link, i; e. the back-burning distance of the link in the given surrounding medium and in a given circuit under conditions of average severity as determined by the fault angle, etc. For reasons of economy the length of the silver strip 2 ought to be less than the average backburning distance of the link 3. The two strips of copper 1 forming direct axial extensions of the strip of silver 2 are adapted to support additional back-burning upon complete vaporization of silver strip 2 under the heat of the arc formed upon blowing of the fuse. Where the severity of interruption is small, the current-flow through the circuit protected by the fuse may cease before all of the silver strip 2 is heated to vaporization temperature and vaporized.

It will be apparent from the foregoing that the strip of silver 2 decreases progressively in width from a point of largest width to a point of minimum width and increases progressively in width from said point of minimum width to a point of largest width. Fuse links with gradual change of cross-sectional area compare favorably with fuse links wherein changes in cross-sectional area are abrupt.

In some instances it is necessary or desirable to provide for fuse protection not only in cases of major faults but also in cases of small protracted overloads. To this end an element of a link-destroying low fusing point metal may be arranged on the link 3 at a point thereof closer to the point of reduced or minimum cross-sectional area 2a than to the axially outer boundary lines 212 of silver strip 2. In Fig. 4 a rivet 10 of tin, or indium, or of a tin-indium alloy has been shown as the preferred a form of adding a link-destroying low fusing point ele' mentto fuse link 3.

Because of the fact that element 10 is arranged close to the neck or point of reduced cross-section 2a of silver strip 2, the temperature of element 10 will always be relatively high, though always lower than that at the neck or point of reduced cross-section 2a. Consequently, the neck 2a tends to be at a higher temperature than element 10 when element 10 reaches its fusing temperature on account of a protracted small overload. Upon fusion element 10 tends to flow to the immediately adjacent point of reduced cross-section where the prevailing temperature is higher. There a very rapid metallurgical reaction between the low fusing point metal and the silver takes place. This reaction results in the formation of an alloy which has a considerably higher resisitivity than silver, which in turn is conducive to increased generation of heat at the neck portion 2a, and consequently in linkdestruction at an accelerated rate.

If element ,10 is made of induim, or a tin-indium alloy, relatively longer delay times are achieved than if the element 10 is made. of tin, or any other low fusing point metal having the required link-destroying properties. Indium alloys readily with silver at the fusing point of the former but does hardly alloy with copper; hence the desirability of providing a fuse link which consists prepond'erantly of copper with a small section of silver performing the function of imparting to the linkthe intense current-limiting action of an all-silver link, and increasing the range of time lags by allowing the use of indium, or indium alloys, as link-destroying, or derating, agents.

Referring now to Figs. 7 to 9, reference numeral 1' has been applied to indicate a pair of spaced strips of copper and reference numeral 2 to indicate an intermediate silver strip. Strips 2' and 1' are bonded. together to form a virtually unitary strip of sheet metal. Silver strip 2 is provided with a line of diamond-shaped perfo rations 20- which take the place of the V-shapcd notches in the embodiment of the invention shown in Figs. 3 and 4. A very narrow current-path is produced between each pair of immediately adjacent diamond-shaped perforations, resulting in a line of high spot-temperatures when a link made up of stamping 1', 2' is carrying current.

Fig. 8 shows a fuse comprising a fuse link 3 made up of a stamping of the kind shown in Fig. 7 rolled to form a substantially tubular conductor of current. The fuse structure shown in Fig. 8, except for that of the link, is more fully disclosed in the copending patent application of Frederick J. Kozacka, Ser. No. 436,398, filed June 14, 1954, for Power Fuses with Tubular Links and Pressure-Type Link Connections, now U. S. Patent 2,777,033 and reference is made to this patent for a -more complete disclosure of this structure.

The structure shown in Fig. 8 comprises, in essence, a pair of coaxial cylindrical copper blocks 11 arranged in spaced relation and conductively interconnected by tubular fuse link 3. Each copper block 11 is provided at the axially inner end thereof with a cylindrical projection 11' on which link 3 is mounted. Split rings 12 are mounted on both ends of link 3 and pressed firmly by the outer rings 13 against the projections 11' of blocks 11. Blocks 11 are held in spaced relation by casing 14 pinned to blocks 11 by transverse steel pins 15. Each block 11 is provided with a blade contact 16 for insertion of the fuse into a fuse holder, or the like. The cylindrical space within tubular link 3' and the toroidal space between link 3 and casing 14 may be filled with a pulverulent arcquenching filler 17. To indicate the two alternatives, i. e. presence or absence of filler 17, a portion of Figs. 8 and 9 has been shown with the filler added, and another portion with the filler omitted.

Although this invention has been described in considerable detail, it is to be understood that such description is intended illustrative rather than limiting, as the invention may be variously embodied, e. g. in multi link rather than single link fuses, and is to be interpreted as claimed.

I claim:

1. In a current-limiting fuse the combination of a substantially tubular casing, a pair of terminal elements each arranged on one end of said casing, a ribbon-type fuse link comprising a strip of silver and a pair of strips of copper bonded together to form a virtually unitary strip of sheet metal, said strip of silver being arranged in the center region of said casing and having a pair of lateral substantially V-shaped notches coextensive with said center region and the hot-spot region on said link when carrying current, the width of the wide open end of each of said pair of notches being substantially equal to the length of said strip of silver, the ed es of each of said pair of notches enclosing an acute angle, and said pair of notches defining a short length of reduced cross-sectional area on said strip of silver closely approximating a point-heatsource, each of said pair of strips of copper beginning at opposite axially outer boundary lines of said strip of silver and each providing a direct current-path from said strip of silver to one of said pair of terminal elements.

2. In a current-limiting fuse the combination of a substantially tubular casing; a silicious pulverulent arcquenching filler Within said casing; a pair of terminal elements each arranged on one end of said casing, a ribbontype fuse link comprising a strip of silver and a pair of strips of copper bonded together to form a virtually unitary strip of sheet metal submersed in said filler, said strip of silver being arranged in the center region of said casing and having a pair of lateral sub-stantially V-shaped notches, the edges of each of said pair of notches enclosing an acuate angle and said pair of notches defining a short length of reduced cross-sectional area on said strip of silver approximating a point-heat source coextensive with said center region of said casing and the hot-spot region on said link when carrying an electric current, the length of said strip of silver being in the order of the width of said pair of notches at the wide open end thereof, and each of said pair of strips of copper beginning at opposite axially outer boundary lines of said strip of silver and each providing a direct current-path from said strip of silver-to one of said pair of terminal elements.

in a fuse the combination of a substantially tubular casing of insulating material, a pair of terminal elements each arranged on one end of said casing, a ribbon-type fuse link comprising a strip of silver and a pair of strips of copper bonded together to form a virtually unitary strip of sheet metal, said strip of silver being arranged in the center region of said casing and having a pair of lateral substantially V-shaped notches defining a short length of reduced cross-sectional area closely approximating a pointheat-source when said link is carrying an electric current, said short length of reduced cross-sectional area being coextensive with the center of said casing and the hotspot on said link when said link is carrying an electric current, each of said pair of strips of copper beginning at opposite axially outer boundary lines of said strip of silver and each providing a direct current-path from said strip of silver to one of said pair of terminal elements, and a rivet of a link-destroying low fusing point metal on said strip of silver at a point thereof closer to said short length of reduced cross-sectional area than to said axially outer boundary lines.

4. In a current limiting fuse the combination of a substantially tubular casing; a silicious pulverulent arcquenching filler within said casing; a pair of terminal elements each arranged on one end of said casing, a ribbontype fuse link comprising a strip of silver and a pair of strips of copper bonded together to form a virtually unitary strip of sheet metal submersed in said filler, said strip of silver comprising a portion of reduced crosssectional area arranged in the center of said casing, the length of said strip of silver being less than the average back-burning distance of said link, each of said pair of strips of copper beginning at opposite axially outer boundary lines of said strip of silver and providing a direct current-path from said strip of silver to one of said pair of terminal elements, and the length of each of said pair of strips of copper exceeding the length of said strip of silver.

5. In a current-limiting fuse the combination of a substantially tubular casing, a pair of terminal elements each arranged on one end of said casing, a ribbon-type fuse link comprising a strip of silver and a pair of strips of copper bonded together to form a virtually unitary strip of sheet metal, said strip of silver being arranged in the center region of said casing and having a point of reduced cross-sectional area midway between the opposite axially outer boundary lines thereof, each of said pair of strips of copper beginning at said opposite axially outer boundary lines and providing a direct current-path from said strip of silver to one of said pair of terminal elements, and the length of said strip of silver being less than the average back-burning distance of said link.

6. In a current-limiting fuse the combination of a substantially tubular casing; a silicious pulverulen-t arcquenching filler in said casing; a pair of terminal elements each arranged on one end of said casing, a ribbon-type fuse link submersed in said filler, said link comprising a strip of silver and a pair of strips of copper bonded together to form a virtually unitary strip of sheet metal, said strip of silver being arranged in the center region of said casing and having a pair of lateral substantially V-shaped notches each comprising a pair of edges enclosing an acute angle and defining a region of reduced cross-sectional area of said link, each of said pair of strips of copper beginning at an opposite axially outer boundary line of said strip of silver and each providing a direct current-path from said strip of silver to one of said pair of terminal elements, the length of said strip of silver being less than the average back-burning distance of said link and the length of each of said pair of strips of copper being a multiple of said length of said trip of silver.

7. A current-limiting fuse comprising a substantially tubular casing; a silicious pulverulent arc-quenching filler within said casing; a pair of terminal elements each arranged at one end of said casing, a fuse link made of an axially inner strip of silver and of a pair of axially outer strips of copper bonded together to form a virtually unitary strip of sheet metal submersed in said filler and conductively interconnecting said pair of terminal elements, said strip of silver having a portion of reduced cross-sectional area arranged in the center of said casing to cause arc-initiation thereat, said strip of silver being shorter in length than the required average back-burning distance of said link, and said pair of strips of copper forming direct axial extensions of said strip of silver adapted to support additional bacbburning upon complete vaporization of said strip of silver under the heat of the are formed upon blowing of the fuse.

References Cited in the file of this patent UNITED STATES PATENTS 348,048 Perkins Aug. 24, 1886 713,831 Badeau Nov. 18, 1902 1,215,999 Schweitzer Feb. 13, 1917 2,703,352 Kozacka Mar. 1, 1955 FOREIGN PATENTS 349,519 Great Britain May 27, 1931 468,916 Great Britain July 15, 1937