US3689995A

US3689995A - Electric fuses

Info

Publication number: US3689995A
Application number: US64347A
Authority: US
Inventors: Karl Lerstrup
Original assignee: Laur Knudsen Nordisk Elektricitets Selskab AS
Current assignee: Laur Knudsen Nordisk Elektricitets Selskab AS; Cooper Industries LLC
Priority date: 1970-08-17
Filing date: 1970-08-17
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12

Abstract

An electric fuse comprises a metal ribbon provided with indentations or openings to form zones of narrowed cross-section serving as fuse elements, the adjacent portions of the ribbon serving as heat sinks and cooling surfaces. According to the invention, one or more insulating stiffening members is/are attached to the ribbon to improve mechanical strength in the weakened zones without affecting arc formation. In a preferred method of manufacturing, the final formation of the fuse elements takes place after the stiffening member has been attached.

Description

United States Patent Lerstrup 1 Sept. 12, 1972 [541 ELECTRIC FUSES FOREIGN PATENTS OR APPLICATIONS Inventor: Karl Lerslrup, p g, 954,513 4/1964 Great Britain ..337/290 De 696,232 10/ 1930 France ..337/293 [73] Assignee: Aktleselskabet Pour. Knudsen Nor- Primary Examiner Bemard A Gilheany disk Elektncitets Selskab, Copenhagen D enmark Assistant ExammerDew1tt M. Morgan Attorney-Watson, Cole, Gnndle & Watson [22] Filed: Aug. 17, 1970 21 Appl.- No.: 64,347 [57] ABSTRACT An electric fuse comprises a metal ribbon provided with indentations or openings to form zones of nar- [52] US. Cl. ..29/623, 337/166, 337/297 h d 51 int. Cl....H01h 69/02, H01h 85/10, HOlh 85/14 cmsis'sectm sefvmg as F elements. e a [58] Field of Search 337/158 290 291 292 295 acent portions of the ribbon servmg as heat sinks and 337/231 293 cooling surfaces. According to the invention, one or 7 more insulating stiffening members is/are attached to the ribbon to improve mechanical strength in the [56] References Cited weakened zones without affecting arc formation. In a UNITED STATES PATENTS preferred method of manufacturing, the final formation of the fuse elements takes place after the stifien- 3,152,233 10/ 1964 Kozacka ..337/ 158 X ing member has been attached 3,465,275 9/1969 Swain ..337/295 X 1,601,726 10/1926 Eustice ..337/295 1 Claim, 6 Drawing Figures BACKGROUND OF THE INVENTION.

This invention relates to an electric fuse, and more particularly to the fuse element'or elements of such a fuse. The invention will be described as applied to a sand filledfuse with current limiting action, but the invention is not limited to such fuses.

For such fuses it has been customary to use one or more bands or ribbons of metal from which the fuse element or elements are formed by punching variously shaped holes or indentations'in the ribbon. In some cases it is intended that the major part of the ribbon shall melt during the operation of the fuse, that is, the ribbon itself may be considered the fuse element, but in other cases it is intended that only specific parts of the ribbon shall melt, while other parts remain intact during operation. In this case only the parts intended to melt are said to form the actual fuse elements, while the remaining parts of the ribbon form the terminals of the fuse elements and serve as heat sinks and cooling surfaces. The present inventionrelates in particular to this latter type.

To achieve this goal of having part or parts of the ribbon melt, while other parts remain solid during the entire fuse operation it is necessary that the cross-section through which the electric current flows is significantly smaller in the actual fuse element or elements than in the parts of the ribbon which serve as heat sinks and cooling surfaces. This difference in cross-section causes all mechanical stresses to be concentrated on the smaller cross-sections, and when the difference is large, the risk of overstressing the fuse elements during handling is likewise increased, and this forms a practical limit to the ratio of the largest to the smallest crosssection.

It is the object of the invention to provide a mechanical design of such fuse elements that permits a large ratio of cross-section without subjecting the actual fuse element to undue mechanical stress. It is a further object to provide for such sequence of manufacture that the first object can be achieved without necessitating extreme care during the process of manufacture.

SUMMARY OF THE INVENTION.

According to the invention, there is provided an electric fuse, having fuse elements integral with heat sinks and cooling surfaces, formed by the removal of parts of a metal ribbon, and provided with one or more insulating stiffening members fixed to the ribbon.

BRIEF DESCRIPTION OF THE DRAWING.

FIGS. 1 and 2 are diagrammatic figures showing certain aspects of the present state of the art.

FIG. 3 illustrates an aspect of the known art which forms an important part of the invention.

FIG. 4 serves to explain the basic aspects of the invention;

FIG. 5 illustrates a particular practical embodiment of the invention; and

FIG. 5a is a section taken along lines 5-5 as indicated in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS.

The known way of producing a fuse element of the kind to which the invention is related is shown in FIG. 1, where 1 is a band or ribbon of metal from which a fuse element is formed, 2 and 2' are indentations cut from the sides of the ribbon, leaving only a narrow part of the metal to form the actual fuse element 4, while the remaining partsof the ribbon l in the vicinity of the fuse element 4

form heat sinks

5 and 5 that at the same time provide or serve as cooling surfaces.

FIG. 2 shows how several such fuse elements can in known manner be formed in the same ribbon in such a manner that the melting of the fuse elements results in the formation of several arcs connected in series, the reference characters being the same as in FIG. 1.

If the ratio of the width of the basic ribbon 1 to the width of the individual fuse elements 4 becomes extremely high, as is desirable for certain kinds of fuses, for instance in the order of 10 to 1 or more, the fuses according to FIGS. 1 and 2 become mechanically weak and there is a grave risk that the fuse elements have been mechanically pre-damaged before the fuse is placed in service, and this seriously affects its electrical characteristics. The risk is the greater, the greater the ratio of the cross-sections, and the more fuse elements there are in series according to FIG. 2.

FIG. 3 illustrates a first step towards the alleviation of this weakness. This figure is directly comparable with FIG. 1 and again 1 is the ribbon from which the fuse elements are formed and 5 and 5' are the heat sinks and cooling surfaces, but in addition to the two indentations 2 and 2', which are not quite so deep, an opening 2" has been added between the two indentations. In this manner the original fuse element 4 of FIG. 1 has been replaced by two fuse elements 4' and 4" in parallel in FIG. 3. If now each of the two fuse elements 4' and 4" have half the width of the fuse element 4 of FIG. 1, then the two designs of FIG. 1 and FIG. 3 are equivalent with respect to reduction of cross-section, but it is clearly seen that the design of FIG. 3 has considerable mechanical advantages over that of FIG. 1.

In FIG. 1 any mechanical moment acting in the plane of the ribbon and tending to move the part 5 with respect to the part 5' will easily bend the narrow neck 4, while the two necks 4' and 4" of FIG. 3 will resist a considerably larger amount without permanent defor mation. However, the design illustrated in FIG. 3 also has to be regarded as belonging to the prior art, even if the motivation for its use has not been founded on mechanical considerations.

While the design exemplified in FIG. 3 represents an important improvement over that of FIG. 1 for any moment having a component in the plane of the ribbon, it is no better than that of FIG. 1 with respect to a moment tending to bend the ribbon out of the plane of the paper.

FIG. 4 shows a ribbon with fuse elements according to the invention. The selected example for illustration is a ribbon with several fuse elements in series and with the minimum of two sets of fuse elements in parallel, but the invention is by no means limited to this embodiment. The fuse elements have been formed on the principles illustrated in FIG. 3, but in addition a stiff insulating strip 6 is fixed to the ribbon in such a manner that it adds stiffness against bending of the fuse element in the direction out of the plane of the paper. It is noted that this stiffening member must not necessarily provide so much stiffness that it prevents bending of the ribbon, but only so much and of such a character that it prevents excessive bending at the location of the narrow necks forming the fuse elements.

An essential feature of this stiffening strip is that it is so located on the ribbon that it does not interfere with the formation of arcs when the fuse elements melt. This can be achieved, as illustrated in FIG. 4, by a simple strip that is narrow compared to the width of the openings 2 in' the ribbon so that the necessary distance from the fuse elements to the strip is secured, but it is also possible to use a wider strip provided it has indentations of sufficient width and depth to provide the desired distance from the fuse elements, or, if it is wide enough, is provided with openings at the-location of the fuse elements.

It is within the scope of the invention to fix stiffening strips or plates to both sides of the ribbon, and it is also within the scope of the invention to let the insulating stiffening member cover the fuse elements partly or completely, but in that case only stiffening on one side can be used.

The stiffening member can be fixed in relation to the ribbon of metal by any suitable means. Examples of fixing means that have been found suitable are gluing, riveting or deformation of part of the ribbon to clamp the strip, but any practical means of fastening is considered within the scope of the invention.

FIG. and FIG. 5a illustrate a specific method of fixing the stiffening strip to the metal ribbon. FIG. 5 repeats part of FIG. 4 with the fixing means added and FIG. 5a further illustrates this by showing a cross-section of the ribbon and the stiffening strip, but for the sake of clarity the dimensions are out of proportion. Again 1 is'the metal ribbon, 4' and 4" the fuse elements located on either side of the stiffening strip 6, which is fastened to the ribbon by means of wire staples 7. In the illustration, the staples surround the stiffening member, but pierces the metal ribbon. However, such details are immaterial for the invention.

In the examples used to illustrate the invention a design according to FIG. 3 with two parallel fuse elements has formed the basis. However, to the man skilled in the art it will be obvious that the invention also covers designs having more parallel fuse elements and a single stiffening strip, as well as the use of several stiffening strips or stiffening members on the same ribbon, including the use of a single fuse element, provided the stiffening strip and its fixation to the ribbon are such that it can transmit the necessary forces without undue stress to the fuse element.

However, in case the ratio of the cross-sections is extremely high there still remains the risk of mechanical damage to the fuse elements during the interim between the shaping of the ribbon and the fixing of the stiffening member and in such cases a special manufacturing procedure may be necessary and forms part of the invention. On principle it would be desirable to fix the stiffening member before the ribbon is punched to give it the desired shape, but obviously this is impossible seeing that in the embodiments illustrated the stiffening mem ers over rgart of the ptpenings or indentatrons orme int e rib on. This d1 iculty is overcome by forming in a first manufacturing step only the openings to be covered by the stiffening member, fixing the stiffening member in a second step, and forming in a final step the actual fuse elements which in this embodiment cannot be covered by the stiffening member. Another solution is to make the stiffening member so wide in relation to the width of the metal ribbon that it is possible to have openings in the stiffening member wider than the width of the ribbon.

Iclaim: 1. A method of making a fuse element strip for a sand-filled fuse, comprising the steps of:

punching a row of spaced holes longitudinally along a metal ribbon wherein the spacing of said holes provides sufficient mechanical strength of said metal ribbon for subsequent processing; attaching at least one insulating strip having a width less than the transverse dimension of said holes to at least one surface of said metal ribbon with the edges of said insulating strip spaced from the outer transverse portions of the edges of said holes; and punching additional material portions from said metal ribbon to form narrow necked fuse element portions spaced from said insulating strip.

Claims

1. A method of making a fuse element strip for a sand-filled fuse, comprising the steps of: punching a row of spaced holes longitudinally along a metal ribbon wherein the spacing of said holes provides sufficient mechanical strength of said metal ribbon for subsequent processing; attaching at least one insulating strip having a width less than the transverse dimension of said holes to at least one surface of said metal ribBon with the edges of said insulating strip spaced from the outer transverse portions of the edges of said holes; and punching additional material portions from said metal ribbon to form narrow necked fuse element portions spaced from said insulating strip.