US3524157A

US3524157A - Electric current-limiting fuse

Info

Publication number: US3524157A
Application number: US748679A
Authority: US
Inventors: Erwin Salzer
Original assignee: Chase Shawmut Co
Current assignee: GOLUD INC A DE CORP; Gould Inc
Priority date: 1967-08-07
Filing date: 1968-07-30
Publication date: 1970-08-11
Anticipated expiration: 1987-08-11

Description

Aug. 11, 1910 .E. SALZER 3,524,157

ELECTRIC CURB IIIIIIII TING FUSE Original Filed Aug. '7. 1967 2 Sheets-Sheet 1 FIG. 2

5 v; *u I' JIM MIMI! I INVENTOR:

W- 5 E. SALZER 2 ,151

- ELECTRIC CURRENT-LIMITING FUSE I Original Filed Aug. 7. 196'? 2 Sheets-Sheet 3 40 BY ,4a 9

INVENTOR:

United States Patent 3,524,157 ELECTRIC CURRENT-LIMITING FUSE Erwin Salzer, Waban, Mass., assignor to The Chase- Shawmut Company, Newburyport, Mass.

Original application Aug. 7, 1967, Ser. No. 658,856, now Patent No. 3,413,586, dated Nov. 26, 1968. Divided and this application July 30, 1968, Ser. No. 748,679

Int. Cl. H01h 85 08, 85/10 US. Cl. 337-159 2 Claims ABSTRACT OF THE DISCLOSURE Electric current-limiting fuses include a ribbon fuse link, or ribbon fuse links, having a plurality of points of reduced cross-sectional area established by groove-like recesses and a local reduction of the thickness of the ribbon fuse link, or links, at the points where the groovelike recesses are located. The fuse link, or links, have such recesses on both surfaces thereof. The recesses form points of large bending compliance. The fuse link, or links, are bent at said points of large bending compliance to form an undulated fuse link structure.

This application is a division of my co-pending patent application Ser. No. 658,856, filed Aug. 8, 1967 for Electric Current-Limiting Fuse, and now Pat. No. 3,4l3,586.

SUMMARY OF THE INVENTION Fuses embodying this invention include a tubular casing of insulating material, a pair of electroconductive terminal elements closing the ends of said casing, and a pulverulent arc-quenching filler inside said casing. A ribbon fuse link of a current-limiting metal, i.e. silver or copper, is immersed in said arc-quenching filler and conductively interconnects said pair of terminal elements. The aforementioned fuse link has a plurality of points of reduced cross-sectional area and intermediate sections of relatively large cross-sectional area. Said plurality of points of reduced cross-sectional area are established by points of said link having a reduced thickness forming grooves extending transversely across said link and having open sides, side walls and botom walls. The latter have a smaller thickness than the thickness of said link at said sections of relatively large cross-sectional area.

The aforementioned plurality of grooves includes grooves at opposite surfaces of said fuse link forming on opposite surfaces of said fuse link strips of large bending compliance, and said fuse link being bent at said points or strips of large bending compliance in accordance with substantially equal radii of curvature to form an undulating structure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of a fuse link for a current-limiting fuse embodying this invention;

FIG. 2 is a top-plan view of a fuse link according to FIG. 1;

FIG. 3 is a side elevation of the structure of FIG. 2;

FIG. 4 is in part a side elevation and in part a sec tion along IVIV of FIG. 5 and shows a complete fuse embodying this invention; and

FIG. 5 is a section along VV of FIG. 4.

3,524,157 Patented Aug. 11, 1970 DESCRIPTION OF PREFERRED EMBODIMENT The fuse link structure 3 of FIGS. 1 to 3 includes a plurality of grooves 4a alternatingly arranged on opposite sides of ribbon fuse link 3. Fuse link 3 is bent in opposite directions at the point of each groove 4a, thus assuming generally an undulating or zig-zag shape. The bends are established at points where the thickness of the fuse link is reduced and its outer fibers are close to the neutral plane of the bottom portions, or closed ends of grooves 4a. Therefore the bending stresses resulting from bending fuse link 3 to undulating or ziz-zag shape are minimized.

The points where grooves 4a are situated are points of highest current density, minimal tensile strength and are subjected to high tensile stresses by magnetic action concomitant to occurrence of major fault currents. These stresses tend to break the link mechanically at the points of grooves 4a before the melting times; have elapsed which may be calculated from the melting F2 values of the fuse link structure.

Each groove 4a has an open side, side walls and a bottom wall which has a smaller thickness than the thickness of the fuse link. Fuse link 3 is bent at each of its plurality of grooves 4a in such a direction that the open sides of grooves 4a are wider than the bottom walls thereof. This has been clearly shown in FIGS. 1 and 3.

Referring now to FIGS. 4 and 5, numeral 8 has been applied therein to indicate a tubular casing of electric innsulating material. Casing 8 is closed on both ends thereof by electroconductive terminals elements 9 in the form of caps or ferrules mounted on the ends of easing 8. Casing 8 is filled with a body of arc-quenching filler, preferably quartz sand, to which body reference numeral 10 has been applied. Terminal elements 9 are conductively interconnected by an integral ribbon fuse link 3 as shown in FIGS. 1-3. Fuse link 3 is of a currentlimiting metal, i.e. silver or copper, and fully immersed in the body of quartz sand 10. Ferrules or caps 9 form recesses filled with pools 9a of solidified solder, and the ends of fuse link 3 project through openings in caps or ferrules 9 into the aforementioned pools 9a of solder and are conductively connected by the latter with ferrules or caps 9.

In an current-limiting fuse the dimensions of the points of reduced cross-sectional area are determined by the required fusing i t value for any particular application, i.e. by electrical requirements and are, therefore not open to choice. It is, however, possible to give various shapes, or configurations, to the cross-section of a point of required cross-sectional area, and any particular configuration which is given to such a point has an immediate bearing upon the behavior of the fuse link. The bending stresses in a beam increase as the distance of the outermost fibers of the beam from the neutral axis thereof is increased. The thickness of the bottom wall of grooves 4a is considerably less than the total thickness of link 3 and, therefore, the distance of the outermost fibers from the neutral axis of the bottom wall of grooves 4a is very small. Because of the smallness of the above distance, the stresses set up by bending of link 3 a predetermined angle at one of its grooves 4a are small. In other words, the bending compliance of link 3 at its points of reduced thickness is large, and it can readily be bent in response to bending forces without ever exceeding the allowable unit stress at the extreme fibers. It will be apparent from FIGS. 1 and 3 that the radii of curvature at each bend or point of bend are virtually identical and that all bends are virtually identical. This results from the displacement of the grooves On one side of the link relative to the grooves on the other side of the link in a direction generally longitudinally of the link.

Considering now the behavior of the bottom wall of grooves 4a when link 3 is subjected to forces or twisting couples causing torsional stressing of the bottom walls of grooves 4a. There is a tendency for a fuse link for a fuse embodying this invention to better withstand torsional stresses than a comparable fuse link having point-heatsource-type necks and pre-stresses resulting from punching such necks.

The melting i t value of a fuse link depends upon the material of which it is made, i.e. its latent heat of fusion and its mean resistivity, and the cross-sectional area F of the point where melting initiates. Hence, if a given set of conditions requires a predeterminable melting i t value, having selected the appropriate metal of which the fuse link is to be made, this determines the cross-sectional area of the fuse link at the point where melting should initiate. While the cross-sectional area F of the point of initiation of melting and arcing is thus determined, the designer of the fuse is still free to select the geometry of the crosssection of the link where fusion and arcing is intended to initiate. Assuming that this cross-section is rectangular, then the sides of any rectangle having the area F to achieve a required melting i is given by the equation for an equilateral hyperbola, which is:

wherein x and y are the two sides of the rectangle. The circumference or perimeter U of any such rectangle is given by the equation U=2x+2y The circumference becomes minimum when x=y or x/y=1 (3) and i.e. when the rectangle having the cross-section F turns into a square. The larger the ratio of the longer side x of the rectangle to the shorter side y thereof, the larger the perimeter of the rectangle. The larger the perimeter of the aforementioned rectangle, the larger the area of interaction between a pulverulent arc quenching filler as, for instance, quartz sand, surrounding the fuse link, and the region of arc initiation resulting from the flow of a fault current.

The relatively large circumference of the points of reduced cross-section in the structures of FIGS. 1 to 3 results in a relatively large heat flow from these points when the fuse is carrying current. Therefore, the geometry of fuse links according to FIGS. 13 tends to increase the current rating of the fuse, or requires a smaller mass of metal for a given current rating. The geometry of fuse links according to FIGS. 1-3 further tends to result in a relatively large area of cooling following are initiation and are extinction. This, in turn, tends to reduce the arcing i t of the fuse and to increase the cooling of the fulgurite resulting from the fusion of the surrounding quartz sand.

There are fundamental differences in regard to heat transfer during the melting time and during the arcing time of a current-limiting fuse. It is justifiable to assume that the melting i t is a constant for any given fuse structure as long as the melting time is less than milliseconds. This constancy law results from the fact that heat dissipation from a metallic conductor in solid or liquid state is negligible in extremely short intervals of time such as 10 milliseconds and less. Once the continuity of the metallic current path of a fuse link is broken and an are or electric gas discharge substituted for the metallic current path, the situation in regard to heat transfer is entirely different from that prevailing during the melting period of the fuse link. During the arcing period the heat transfer occurring in intervals of time in the order of microseconds may mean the difference between failure or success of a current interrupting device. The melting temperature of silver is slightly less, and that of copper slightly more, than 1000 deg. centigrade. When an arc is kindled the temperature of the arc path is in the order of many thousand degrees and thus the temperature gradient is greatly increased and the area of heat flow becomes significant. The same remark in regard to the significance of the area of heat flow applies also to the postarcing period of dielectric recovery involving much smaller temperature gradients than the arcing period, but relatively longer times during which dielectric recovery of the hot fulgurite is effected by heat conductionaway from the latter.

During the arcing period cooling and deionization is effected by recombination of ions and diffusion of ions. Wherever there is a concentration gradient of ions, there is a flow of ions from regions of high concentration to regions of low concentration. The rate of change of ion concentration is analogous to heat flow. Therefore the relative increase of the boundary area between quartz sand and the ionized metal vapors which form the arc path, and the relative increase of the boundary area between fused quartz sand aud relatively cool quartz sand, which both result from the geometric configuration of the fuse link shown in FIGS. 1 to 4, result in an increase of heat dissipationduring and following the arcing period.

It is thus apparent that the geometry of fuse links according to FIGS. 1 to 4 results in desirable mechanical performance characteristics as well as in desirable electrical and thermal performance characteristics.

There are a number of ways for making fuse links in accordance with this invention of which the combination of photosensitive resists and etching or chemical machining are the most desirable. This process is well known in the art and, therefore, does not need to be described in detail in this context. As a general rule, fuse links of current-limiting fuses are made of silver in which instance etching should preferably be performed with chromium trioxide sulfuric acid solutions (see Kury, P. F., Etching Silver With Chromium Trioxide Sulfuric Acid Solutions, Journal of the Electrochemical Society, April 1956).

It will be understood that while the fuse structure illustrated herein is a preferred embodiment of my invention, the same may take forms other than those specifically shown and described herein. It will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. An electric current-limiting fuse including in combination:

(a) a tubular casing of electric insulating material;

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

(c) a pulverulent arc-quenching filler inside said casing;

and

(d) a ribbon fuse link of a current-limiting metal immersed in said arc-quenching filler conductively interconnecting said pair of terminal elements, said fuse link having on each of the opposite sides thereof a plurality of grooves extending transversely across said fuse link and having open sides, side walls and bottom walls, said bottom walls having a smaller thickness than the thickness of said fuse link at points thereof remote from said plurality of grooves, said plurality of grooves on one side of said fuse link and said plurality of grooves on the other side of said 5 6 fuse link being displaced in a direction longitudinally References Cited of said fuse link and forming on opposite surfaces UNITED STATES PATENTS of said fuse link points of large bending compliance, v and said fuse link being bent at said points of large 3,213,242 10/ 1965 Cameron 337-158 XR bending compliance in accordance'with substantially 3, 9/ 1964 F an et al- 337295 equal radii of curvature to form an undulating struct BERNARD A. GILHEANY, Primary Examiner 2. An electric fuse as specified in claim 1 wherein said 1 MORGAN, Assistant Examiner link is bent at each of said plurality of grooves in such a direction that said open sides of said plurality of grooves 10 US. Cl. X.R. are wider than said bottom walls thereof. 337295