US3479630A

US3479630A - High voltage fuse particularly for circuits including solid state devices

Info

Publication number: US3479630A
Application number: US736909*A
Authority: US
Inventors: Frederick J Kozacka
Original assignee: Chase Shawmut Co
Current assignee: GOLUD INC A DE CORP; Gould Inc
Priority date: 1968-05-14
Filing date: 1968-05-14
Publication date: 1969-11-18
Anticipated expiration: 1986-11-18

Description

Nov. 18, 1969 F. J. KozAcKA 3,479,630

HIGH VOLTAGE FUSE PARTIGULARLY FOR CIRCUITS INCLUDING SULID STATE DEVICES Filed May 14, 1968 2 Sheets-Sheet 1 INVENTOR ATTY . I uN m\ N Nov. i8, 1969 F. J. KozAcKA 3,479,530

HIGH VOLTAGE FUSE PAETIOULARLY FOR CIRCUITS INCLUDING SOLID STATE DEVICES y Filed May 14, 1968 2 Sheets-Sheet 2 'al hul 4I NVENTOR: FREDE ICK d. KOZACKA BY X ATTY.

United States Patent Or 3,479,630 HIGH VOLTAGE FUSE PARTICULARLY FOR CIRCUITS INCLUDING SOLID STATE DEVICES Frederick J. Kozacka, South Hampton, N.H., assignor to The Chase-Shawmut Company, Newburyport, Mass. Filed May 14, 1968, Ser. No. 736,909 Int. Cl. H01h 85/04 U.S. Cl. 337-159 10 Claims ABSTRACT OF THE DISCLOSURE Sub-assembly for making high-voltage fuses includes a ribbon fuse-link pre-bent 90 degrees at points of relatively large cross-sectional area alternating with intermediate points of reduced cross-sectional area to form an undulatory pattern. The fuse link means has extension spring characteristics, and its length exceeds substantially the shortest distance between its ends. It is stretched and mounted under longitudinal stress between a pair of terminal elements, and is self-supporting due to its spring characteristics, even if of considerable length, as required for circuits having circuit voltages in the order of kilovolts. Disclosed is also a high-voltage fuse resulting from the aforementioned sub-assembly.

BACKGROUND OF INVENTION In high-voltage fuses it is desired that the length of the fusible element by far exceeds the shortest distance between its ends or, in other words, exceeds the length of the casing of the fuse. This has been achieved heretofore in a number of different ways. One way consists in imparting a zig-zag-shape to the fusible element. Such fusible elements are relatively fragile and, therefore, difficult to handle. For some applications zig-zag-shaped fuse links result in a minimum ratio of shortest distance between ends to total length which is too large. A typical fuse having a fuse link of the aforementioned character is disclosed and claimed in U.S. Patent 3,319,029 to Philip C. Jacobs, J r., May 9, 1967, for High-Voltage Fuse Having Zig-Zag-Shaped Fuse Link. In order to make it easier to handle the long, relatively thin and fragile fuse links of high-voltage fuses the same are often mounted on a mandrel as shown, for instance, in U.S. Patent 2,877,321 to Philip C. Jacobs, Jr., Mar. 10, 1959, for High Voltage Fuses. The presence of a mandrel in a high-voltage fuse reduces the space available for pulverulent arc-quenching iller and, for this reason, should be avoided, Whenever possible. The ceramic material of which a fuse-link-supporting mandrel is made may also adversely affect the operation of a fuse. It is also possible to mount the fuse link on a mandrel and to gradually withdraw the mandrel from the casing of the fuse as the latter is being filled with pulverulent arc-quenching filler. This method of assembly is relatively complicated and its practice calls for highly skilled and consequently expensive labor.

SUMMARY OF INVENTION A sub-assembly for making high-voltage fuses embodying this invention includes a tubular casing of insulating material and a pair of terminal elements closing the ends of said casing. It further includes a ribbon fuse link means conductively interconnecting said pair of terminal elements. The fuse link means includes aligned substantially ice planar sections forming an undulating pattern. The aforementioned sections are bounded by straight edges tending to enclose angles of substantially degrees. The aforementioned sections have the largest cross-sectional area adjacent said edges, and a reduced cross-sectional area at points situated between said edges. Such fuse link means are capable of operating as extension spring means by virtue of the resiliency resulting from the bends forming said edges. The fuse link means is attached at each of the ends thereof to one of the aforementioned pair of terminal elements and is under tension and stretched to have in side elevation the shape of a family of aligned trapezoids. When in this state the fuse link means is selfsupporting in the absence of a fuse-link-means-supporting mandrel.

A high-voltage fuse embodying this invention is obtained filling the casing of the above sub-assembly with a pulverulent arc-quenching filler. The weight of the latter may slightly alter the precise geometry of the fuse link without, however, having any adverse effects on the mechanical and the electrical performance characteristics of the fuse.

BRIEF DESCRIPTION OF DRAWINGS FIG. l is a side elevation of a fuse embodying this invention, the center portion of the structure being broken away in order to shorten FIG. l;

FIG. 2 is a section along II--II of FIG. 1 shortened in the same way as FIG. l;

FIG. 3 is an end view of the structure of FIG. 1 seen in the direction of the arrow R of FIG. 1;

FIG. 4 refers to another embodiment of the invention showing the end portions and the center portion of a Subassembly for making a high-voltage fuse deleting the intermediate portions and thus shortening FIG. 4, FIG. 4 being a section along IV-IV of FIG. 5;

FIG. 5 is a section along V-V of FIG. 4 shortened in the same way as FIG. 4;

FIG. 6 is an end view of the structure of FIGS. 4 and 5 seen in the direction of the arrow S of FIG. 5;

FIG. 7 is a cross-section of a fuse link means for the structure of FIGS. 1 3 before being 4built into the fuse structure and stretched; and

FIG. 8 is a cross-section of a modification of the link structure of FIG. 7 before being built into the fuse structure, and stretched.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Referring to FIGS. 1-3 and 6, sub-assemblies and the fuses embodying this invention will now be described in connection with the manufacturing process thereof.

The point of departure is a ribbon fuse link 1, preferably of silver, having equidistantly spaced points 1a of reduced cross-sectional area and intermediate points 1b Whose cross-sectional area is relatively large. The points of reduced cross-sectional area may be formed by circular perforations, as shown in FIG. l. Fuse link 1 is bent 90 degrees at each point 1b of relatively large cross-sectional area (FIG. 6). The fact that fuse link 1 is bent to form substantially planar sections bounded by straight edges results in arranging a greater length thereof in a given space than the shortest distance between the ends of the fuse link. Bending at points of large cross-sectional area rather than at points of reduced cross-sectional area tends to keep stresses away from the points of reduced cross-sectional area which are particularly fragile, and thus to achieve a relatively high degree of dimensional stability of the fuse link structure. This has been explained more in detail in the above referredto U.S. Patent 3,319,029 to Philip C. Jacobs, Ir. Bending of fuse link 1 of FIG. 7 at angles of subbstantially 90 degrees imparts a high degree of resiliency to fuse link 1, making it capable of operating as a strong extenstion spring means.

The next step in the assembly process consists in spacing a pair of terminal elements for the purpose of securing to them the ends of fuse link means.

FIGS. 1-3 show a pair of terminal elements in form of cylindrical metal plugs 2, each being press-fitted into the end of a tubular casing 3 of insulating material, e.g. glass-cloth-melamine. Each terminal plug 2 is provided with a blade contact 2a and firmly maintained in position by four steel pins 2b projecting through casing 3 into metal plugs 2.

Preparatory to assembling the structure of FIGS. 1 3 metal plugs 2 must be spaced temporarily by means other than casing 3. This may be achieved by using spacing posts, or spacing rods, temporarily spacing plugs 2 before they are finally spaced and held in position by casing 3 and steel pins 2b. These temporary spacers can 'be removed after plugs 2 are being spaced and maintained in position by casing 3 and pins 2b. The method of assembling fuses by means of temporary spacers which are removed before the casing of the fuse is filled with a pulverulent arc-quencing filler is disclosed in detail in U.S. Patent 2,740,187 to Philip C. Jacobs, Ir. et al., Apr. 3, 1956, for Methods for Assembling Fuses, and reference may be had to this patent in regard to the use of spacers required for assembling the structure of FIGS. 1-3.

As shown in FIGS. 2 and 3 terminal plugs are provided with Ibores 2c which are arranged in registry, i.e. each of the bores 2c in left plug 2 is arranged in coaxial relation to a 'bore 2c in right plug 2, the common axis of each coaxial pair of bores 2c being one of a pair of lines parallel to, and spaced from, the common axis of terminal plugs 2. During the process of assembly, prior to the provision of casing 1, terminal plugs 2 are spaced by a pair of parallel spacer rods each having ends inserted into one of bores 2c. In order to facilitate the subsequent removal of the aforementioned spacer rods (not shown), the same may have at the ends thereof a smaller outer dameter than that of bores 2c, and be radially expandable at the ends thereof by means of radial slits, an internal coaxial screw thread, and a coaxial spacerediameter-expanding screw co-operatively engaging the aforementioned screw thread. Upon having established a structural unit including plugs 2 and the two aforementioned spacer posts, or spacer rods, fuse link 1 of FIG. 6 is affixed to this structural unit. To this end one of the ends of fuse link 1 is inserted into one of the grooves 2d in one of the plugs 2 and secured therein by soft solder. Thereupon the entire structure is turned 180 degrees so that the plug 2 into which one end of fuse link 1 has been soldered is at a relatively high level, and the other plug is at a relatively low level. Now fuse link 1 is stretched in a direction longitudinally thereof, as a result of which fuse link 1 changes from the rectangular configuration shown in FIG. 7 or FIG. 8 to that shown in FIGS. 1 and 2. In the position and stretched condition shown in FIGS. 1 and 2 fuse link 1 has substantially the shape of a family of aligned trapezoids when seen in side elevation. Fuse link 1 is then, in effect, a stressed and stretched extension spring by virture of the resiliency resulting from the prebends of 90 degrees at points of largest cross-sectional area (see FIG. 7). After fuse link 1 has been soldered with its upper end to the upper terminal plug 2, the lower end of link 1 is inserted into the groove 2d provided in the plug terminal 2 which is at a relatively low level, and secured therein by a soft solder joint. Fuse link 1 is held in stretched position by external means until that solder joint has hardened. Now fuse link 1 is under tension and self-supported in the absence of a fuse-link-means supporting mandrel. Thereafter the structural unit including both plugs 2, their spacers (not shown) and fuse link means 1 are inserted under pressure into tubular casing 3 and secured therein by transverse steel pins 2b. Now casing 3 operates as a spacer means for terminal plugs 2, allowing removal of the spacer rods -by which terminal plugs 2 had been spaced heretofore. Thereupon bores 2c in left terminal plug 2 are closed by insertion into them of sheet metal caps 4, and thereafter casing 1 is filled through bores 2c in right terminal plug 2 with a pulverulent arc-quenching filler S, preferably quartz sand. The filling operation is performed while the fuse is in a substantially vertical position, the left terminal plug 2 closed by caps 4 being at a relatively low level, and the right open terminal plug 2 being at a relatively high level. The structure including casing 1, terminal plugs 2, fuse link 1 andiiller 5 is subjected to vibrations to cause settling of liller 5. The filling process and the vibrations to which the fuse is subjected may cause some bending of fuse link 1 so as to slightly change the geometry thereof shown in FIGS. 1 and 2. These slight changes have no significant effect on the mechanical and electrical performance characteristics of the structure of FIGS. 1-3.

It will be apparent from FIGS. l and 2 that the body 5 of pulverulent arc-quenching filler inside of casing 3 has substantially the volume of all of the space jointly bounded by casing 3 and the pair of terminal elements or terminal plugs 2. In other words, none of that space is occupied by a fuse-link-means-supporting mandrel, generally deemed necessary in high-voltage fuses including fuse link means of considerable length.

After the structure of FIGS. l3 has been thoroughly filled with pulverulent arc-quenching filler 1 and has lbeen vibrated, the bores 2c in right plug 2 are closed by appropriate closing means such as sheet metal caps 4.

The structure of FIGS. 1-3 includes a -blown fuse indicator, and if it is intended to include a blown fuse indicator the above described processes of assembly must be slightly modified.

The blown fuse indicator of FIG. 2 includes a cap 4a housing a compression spring 6 having one end resting against the perforated bottom of cap 4a and another end resting against an indicator cap 7. A rod 7a integral with cap 7 projects through the bottom of cap 4a into a space bounded by caps 4 and 4a. The end of rod 7a remote from cap 7 forms an eyelet through which restraining wire 8 is threaded and to which restraining wire 8 is att-ached. Restraining wire 8 is threaded through a perforation in sheet metal cap 4 and extends parallel to the axis of casing 3. The other or left end of taut restraining wire 8 is clamped in position by the upper left cap 4 of FIG. 2.

When assembling the version of FIGS. l-3 including the blown fuse indicator, the first step in closing the bores 2c consist in inserting the upper right perforated cap 4 into upper right bore 2c. Wire 8 is then threaded through the aforementioned cap 4 and pulled with a hooked tool through the upper left bore 2c. Then cap 4a, spring 6, cap 7 and rod 7a are positioned, as shown in FIG. 2, wire 8 is stretched to be taut and its left end is clamped in position by insertion of upper left cap 4 into upper left bore 2c of left -terminal plug 2. Thereupon lower left cap 4 is inserted into lower left bore 2c of left terminal plug 2. Then casing 3 is filled with a pulverulent arcquenching filler 5 through the lower right bore 2c and finally this bore is closed by inserting into it a sheet metal cap 4.

Indicator wire 8 restraining spring biased indicator cap 7 has a relatively large resistance compared to that of fuse link means 1. Wire 8 becomes current-carrying only following thermal destruction of fuse link means 1. Then wire 8 fuses and releases spring biased indicator cap 7.

It is possible, and often desirable, to substitute for the fuse link means 1 of FIG. 7 the fuse link means -1 of FIG. 8. Both fuse link means lare of the ribbon type and have equidistantly spaced points of reduced crosssectional area. The fuse link 1 of FIG. 8 has points 1a' of reduced cross-sectional area and intermediate points 1b whose cross-sectional area is relatively large. Link 1 is preferably of silver and bent to form an undulatory pattern including planar sections enclosing angles of 90 deg. and bounded by substantially straight edges. These straight edges are coextensive with the points 1b' whose cross-sectional -area is relatively large. The link 1' includes planar sections which extend in a direction longitudinally of link 1 (and casing 3, respectively), and link 1 further includes planar sections which extend transversely to the aforementioned sections (and casing 3, respectively). Each of the first mentioned sections includes a single point 1a of reduced cross-sectional area, and each of the last mentioned sections includes three points 1a of reduced cross-sectional area. Since all points of reduced cross-sectional area are spaced equidistantly, the aggregate length of the transverse sections of link 1' by far exceeds the aggregate length of the sections that extend in a direction substantially longitudinally of link 1 and of casing 3 when the link 1' is mounted in the latter.

The fuse link 1 of FIG. 8 is intended to be mounted under stress between a pair of terminal elements 2 and stretched in a direction longitudinally thereof, as described more in detail in connection with FIGS. 1 to 3. Then fuse link 1 is laterally solely supported by the body S' of quartz sand in the absence of any fuse link means supporting mandrel.

The points of reduced cross-sectional area of the links of FIGS. 7 and 8 are formed by circular perforations, as

more clearly shown in FIG. l. The fuse link of FIG. 8 forms a family of aligned trapezoids when mounted in the structure of FIGS. 1 3, as explained above in considerable detail.

The link structure 1 of FIG. 8 makes it possible to arrange a longer link in a casing of given length than the link structure of FIG. 1. There is, however, a limitation in regard to the ability of this design to accommodate lengths of fuse link in a casing of given length. The transverse sec-tions of fuse link 1 must never be so close to each other as to allow the fulgurites which result upon blowing of the fuse from two contiguous transverse fuse link sections to merge.

The following table indicates data relating to fuses resulting from sub-assemblies embodying the present invention, -which fuses have been thoroughly tested in the power laboratory and found to be acceptable.

In ythe 5 kv. design the length of the fuse links exceeds by 25% the length of the casing, in the 71/2 kv. design the length of the fuse links exceeds by 50% the length of the casing, and in l5 kv. design the length of the fuse links exceeds by 75% the leng-th of the casing. Such favorable ratios of length of link to length of casing cannot be `achieved with zig-zag type fuse links as shown, for instance, in the above referred-to U.S. Patent 3,319,029.

The structure of FIGS. 4 to 6 is primarily intended for applications involving relatively high currents, and moderate circuit voltages. If the circuit voltage is relatively moderate it is sufficient to impart to but one portion of the fuse link means an undulatory shape, and the remaining portion of the fuse link means may be straight. This makes it possible to arrange ribbon fuse links 1" in the way shown in FIGS. 4 and 5 which arrangement of fuse links minimizes the required diameter of the casing 3". The latter is closed by a pair of terminal plugs 2 each having a blade contact 2a". The axially inner end surfaces of terminal plugs 2 are provided with grooves 2d each receiving one of the ends of ribbon fuse link means 1". The straight portion S of the upper fuse link means 1 of FIG. 4 is arranged adjacent the left terminal plug 2", and the undulatory portion U' of the upper fuse link means 1" of FIG. 4 is arranged adjacent the right terminal plug 2". The straight portion S of the lower fuse link means 1" is arranged adjacent the right terminal plug 2", and the undulatory portion U of the lower fuse link means 1" is arranged adjacent the left terminal plug 2". This arrangement effectively precludes the merger of fulgurities resulting from the fusion and vaporization of the fuse link means in the presence of a quartz sand filler.

FIGS. 4 to 6 illustrate a sub-assembly of a fuse structure which has not been filled with quartz sand as yet. The presence of such a ller has been diagrammatically indicated in FIG. 4 at 5". Both terminal plugs 2" are provided with axially extending eccentric passageways 2c. These passageways or bores 2c" are intended to receive spacer rods (not shown) for terminal plugs 2 during the process of assembly including the steps of soldering one end of fuse link means 1" into grooves 2d of one of the terminal plugs 2", of stretching and extending the former in a direction longitudinally thereof, and of soldering the other end of fuse link means 1." to to the other of terminalplugs 2". The bores 2c are plugged in one of terminal plugs 2. preparatory to filling casing 3 with a pulverulent arc-quenching filler 5" and the bores 2c in the other of the terminal plugs 2" are plugged after the casing 3" has been filled with a pulverulent arc-quenching filler 5, and thereafter the latter bores tare also plugged. Plugging of bores 2c" may be effected by metal caps, as described in connection with FIGS. l to 3. Reference character 2b has been applied to indicate steel pins projecting transversely through casing 3 into terminal plugs 2". FIG. 4 shows clearly that the right portion of the upper fuse link means 1" and the left portion of the lower fuse link means '1 have the shape of substantially planar plateau portions and substantially planar valley portions and intermediate substantially planar slanting portions when mounted under stress between terminal elements 2". This precise geometry of the fuse link means 1" may be slightly changed, or distorted, during the process of filling the fuse structure with quartz sand and subjecting the fuse structure to vibrations in order to compact the quartz sand. As mentioned above, these changes of the precise geometry of fuse links 1, or distortions thereof, have no effect on the mechanical and electrical performance of the fuse.

In the foregoing I have described my invention solely in connection with illustrative specific embodiments thereof. Since many vari-ations and modifications will be obvious to those skilled in the art, I prefer not to be bound by the specific disclosure herein contained, but only by the appended claims.

I claim as my invention:

1. A sub-assembly for making a high voltage fuse, said sub-assembly including in combination:

(a) ta tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing; and

(c) .ribbon fuse link means iconductively interconnecting said pair of terminal elements, said fuse link means including aligned substantially planar sections bent to define straight edges therebetween, said sections forming jointly an undulating pattern including flat top portions and flat valley portions, said sections having the largest cross-sectional area at said edges and having a reduced cross-sectional area at points situated between said edges, said fuse link means being capable of operating 'as an extension spring means by irtue of the resiliency resulting from the 2. A sub-assembly as specified in claim 1 wherein the..

length of said casing is at least 10 inches and the length of said fuse link means exceeds by at least percent the length of said casing.

3. A sub-assembly as specified in claim 1 including a pair of ribbon-fuse-link-means conductively interconnecting said pair of terminal elements, one of said pair of fuse-link-means including a irst portion arranged adjacent one of said pair of terminal elements and being substantially straight and a second undulating portion arranged adjacent the other of said pair of terminal elements, and the other of said pair of fuse-link-means including a first undulating portion arranged adjacent said one of said pair of terminal elements and a second substantially straight portion arranged adjacent said other of said pair of terminal elements.

4. A sub-assembly for making a high-voltage fuse, said sub-assembly including in combination:

(a) a tubular casing of insulating material having a predetermined length;

(b) a pair of terminal plugs closing the ends of said casing;

(c) la ribbon fuse link means of silver having a length exceeding at least by 25 percent said predetermined length conductively interconnecting said pair of terminal plugs, said fuse link means including aligned substantially planar sections forming an undulatory pattern, said sections being bounded by straight edges and said sections tending to enclose angles of substantially 90 degrees therebetween, said sections having the largest cross-sectional area adjacent said edges and having a reduced cross-sectional area at points situated between said edges, said fuse link means being capable of operating as extension spring means by virtue of the resiliency resulting from the bends forming said edges, said fuse link means being attached at each of the ends thereof to one of said pair of terminal plugs and being under tension and stretched to have in side elevation the shape of -a family of aligned trapezoids, and said fuse link means being self-supported in the absence of a fuse-linkmeans-supporting mandrel.

5. A high-voltage fuse including in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) a body of quartz sand inside said casing, said body of quartz sand having substantially the same volume as the volume of all of the space jointly bounded by said casing and said pair of terminal elements; and

(d) ribbon fuse link means of silver conductively interconnecting said pair of terminal elements, said fuse link means being bent to form an undulatory pattern including sections enclosing angles and bounded by substantially straight edges, said sections having the largest cross-sectional area adjacent said edges and having a reduced cross-sectional area at points situated between said edges, the aggregate length of said sections extending in a direction substantially longitudinally of said casing being substantially less than the aggregate length of said sections extending in substantially transverse direction, said fuse link lmeans being attached at each of the ends thereof to one of said pair of terminal elements and being laterally solely supported by said body of yquartz sand in the absence of a fuse-link-meanssupporting mandrel.

6. A high-voltage fuse as specified in claim 5 wherein said fuse link means is provided with equidistantly spaced points of reduced cross-sectional area, each of said sections extending in la direction substantially longitudinally of said casing having a smaller number of points of reduced cross-sectional area than each of said sections extending in substantially transverse direction.

7. A high-voltage fuse as speciiied in claim 6 wherein said sections of said fuse link means extending in a direction substantially longitudinally of said fuse link means have one single circular perforation, and wherein said sections of said fuse link means extending in substantially transverse direction have more than one single circular perforation.

8. A high-voltage fuse including in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) a body of pulverulent arc-welding filler inside said casing; and

(d) a pair of ribbon fuse link means embedded in said body of arc-quenching liller conductively interconnecting said pair of terminal elements, each of said pair of fuse link means including a first substantially straight portion and a second undulating portion serially connected with said straight portion, said straight portion of one of said pair of fuse link means being arranged adjacent one of said pair of terminal elements and said undulatory portion of said one of said pair of fuse link means being arranged adjacent the other of said pair of terminal elements, and said undulatory portion of the other of said pair of fuse link means being arranged adjacent said one of said pair of terminal elements and said straight portion of said other of said pair of fuse link means being arranged adjacent said other of said pair of terminal elements.

9. A high-voltage fuse including in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) a body of pulverulent arc-quenching iiller inside said casing, said body of arc-quenching ller having substantially the same volume as all of the space jointly bounded by said casing and said pair of terminal elements; and

(d) a ribbon fuse link means embedded in said body of arc-quenching filler condu-ctively interconnecting said pair of terminal elements, said ribbon fuse link means defining equidistantly spaced points of reduced cross-sectional area, and the shape of said ribbon fuse link means being undulatory including substantially planar plateau portions and substantially planar valley portions and intermediate slanting portions, the ratio of the aggregate number of said points of reduced cross-sectional area defined by said intermediate slanting portions to the aggregate number of said points of reduced cross-sectional area deiined by both said plateau portions and said valley portions being at least 2:1.

,10. A high-voltage fuse including in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) a body of pulverulent `arc-quenching liller inside said casing, said body of arc-quenching filler having substantially the same volume as the volume of all of the space jointly bounded by said casing and said pair of terminal elements; and

(d) a ribbon fuse link means embedded in said body of arc-quenching filler conductively interconnecting said pair of terminal elements, said fuse link means' deiining equidistantly spaced points of reduced crosssectional area and being undulating pre-bent 90 degrees at points situated between points of reduced 10 `cross-sectional area and said fuse link means being 3,113,195 12/ 1963 Kozacka 337-160 mounted under stress in a direction longitudinally of 3,213,242 10/ 1965 Cameron 337-159 said casing between said pair of terminal elements 'and assuming the shape of substantially planar FOREIGN PATENTS plateau portions and substantially planar valley por- 5 706,455 5 /1941 Germany. tions and intermediate substantially planar slanting 541,116 11 /1941 Great Britain. portions when mounted under stress between said pair 0f terminal elements- BERNARD A. GILHEANY, Primary Examiner References Cited 10 H. B. GILSON, Assistant Examiner UNITED STATES PATENTSv U S C1 X'R.

2,877,321 3/1959 Jacobs 337-159 337-295