US3848445A

US3848445A - Method of fabricating improved fuse elements

Info

Publication number: US3848445A
Application number: US00351921A
Authority: US
Inventors: F Cameron
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1972-01-21
Filing date: 1973-04-17
Publication date: 1974-11-19
Anticipated expiration: 1991-11-19
Also published as: JPS4891434U; US3747041A; JPS5422929Y2; CA937268A

Abstract

A cartridge type current limiting electrical fuse including at least one fuse element. The element is made from a length of fusible material which is tapered substantially along its length to a minimum central taper point or portion and then widened again, or which is varied in discrete steps along its length. The fusible material is then wound into a helical or spring-like shape. However, the effective diameter of the resulting springlike solenoid or helically wound coil is made to vary in accordance with the variation in the diameter of the fuse material. The smaller the diameter of the fuse material at any point along its length, the smaller is the diameter of the corresponding solenoid or coil at that point or portion. If the length of fusible material varies continuously between alternating maximum and minimum cross-sectional areas, the finished or overall helical coil or fuse element solenoid will vary continuously to form corresponding maximum and minimum alternating solenoid diameters. If the cross-sectional area of the fuse material varies discretely such as in steps, as would be the case if sections of fusible material of varying diameters or size were joined end-to-end, the corresponding solenoid or helical coil formed will also vary discretely. The size of the diameter for any section of wire depends upon the spring constant or spring rate developed in the solenoid or coil of fusible material. The spring rate is kept preferably substantially constant for any section of fusible wire regardless of the wire''s diameter by adjusting the diameter of the fuse to a predetermined value for the diameter of the wire. Consequently, a spring-like fuse element is ultimately formed having a generally constant spring-rate along substantiallly its entire length. If the spring shaped fuse element is then either expanded or contracted longitudinally within a reasonable, limit generally corresponding to Hooke''s Law, the pitch of the helical spring element will remain constant along the length of the spring although naturally it will change for each increment the spring is expanded or contracted.

Description

United States Patent [191 Cameron Nov. 19, 1974 METHOD OF FABRICATING IMPROVED FUSE ELEMENTS [75] Inventor: Frank L. Cameron, Irwin, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

22 Filed: Apr. 17, 1973 211 App]. No.: 351,921

Related U.S. Application Data [62] Division of Ser. No. 219,712,- Jan. 21, 1972, Pat. No.

Primary Examiner-Lowell A. Larson Attorney, Agent, or Firm-M. J. Moran [57] ABSTRACT A cartridge type current limiting electrical fuse including at least one fuse element. The element is made from a length of fusible material which is tapered substantially along its length to a minimum central taper point or portion and then widened again, or which is varied in discrete steps along its length. The fusible material is then wound into a helical or spring-like shape, However, the effective diameter of the resulting spring-like solenoid or helically wound coil is made to vary in accordance with the variation in the diameter of the fuse material. The smaller the diameter of the fuse material at any point along its length, the smaller is the diameter of the corresponding solenoid or coil at that point or portion. If the length of fusible material varies continuously between alternating maximum and minimum cross-sectional areas, the finished or overall helical coil or fuse element solenoid will vary continuously to form corresponding maximum and minimum alternating solenoid diameters. If the cross-sectional area of the fuse material varies discretely such as in steps, as would be the case if sections of fusible material of varying diameters or size were joined end-to-end, the corresponding solenoid or helical coil formed will also vary discretely. The size of the diameter for any section of wire depends upon the spring constant or spring rate developed in the solenoid or coil of fusible material. The spring rate is kept preferably substantially constant for any section of fusible wire regardless of the wires diameter by adjusting the diameter of the fuse to a predetermined value for the diameter of the wire. Consequently, a spring-like fuse element is ultimately formed having a generally constant spring-rate along substantiallly its entire length. If the spring shaped fuse element is then either expanded or contracted longitudinally within a reasonable, limit generally corresponding to l-lookes Law, the pitch of the helical spring element will remain constant along the length of the spring although naturally it will change for each increment the spring is expanded or contracted.

4 Claims, 13 Drawing Figures PATEmg rmvwsmu h 3348,44 5 SHEET 30F 3 ISOL / ISOR I52 159 M52 FIGJ I METHOD OF FABRICATING IMPROVED FUSE ELEMENTS This is a division. of application Ser. No. 219,712 filed Jan. 21, 1972, now Pat. No. 3,747,041.

BACKGROUND OF THE INVENTION This invention relates to current limiting fuses in general and in particular to helical current limiting fuse links. A current limiting fuse element may be formed with its cross section along its length varied to cause the fuse element to form multiple arcs at predetermined points along its length as the melting temperature of the fuse material is reached such as disclosed in U.S. Pat. No. 2,157,907, issued to K. H. Lohausen May 9, 1939. This construction is employed to produce a plurality of voltage droping arcs which tend to limit the amount of current flowing through the fuse element during overload conditions. Occasionally in high voltage applications fuse elements of this type are required to be very long in addition to being tapered or stepped in size; however, it is impractical for reasons of overall size in such applications to employ a single length of straight fusible material in a cartridge type fuse. One solution has been to wind the fusible material into a helical coil so that the effective axial length of the fuse element is made suitable for mounting in a particular size of cartridge type fuse while the effective overall length of the fuse element is sufficient for a particular voltage application.

In the past, helically wound tapered or stepped fuse elements have been mounted within cartridge type fuses on mandrels or supports which are made of electrically insulating material. In such a construction, the fuse element is pre-assembled upon the support or mandrel, and the mandrel and the accompanying fuse element are assembled in a cartridge housing and surrounded by or embedded in a pulverulent arcquenching material, such as quartz sand, during final assembly. However, the use of fuse element supports or mandrels has proved to be expensive, time consuming and electrically poorer in certain applications. Consequently it has been proposed that lengths of fusible material containing stepped or tapered portions be pre-wound and removed from the mandrel upon which they were wound and stretched at both ends to be conveniently assembled in a cartridge type fuse. Unfortunately, regardless of how uniformly the fuse element may have been initially wound as it is stretched for mounting in the cartridge type fuse, the pitch of adjacent sections of fuse spirals would change as a function of the varying diameter or size of the fusible material from which the fuse element is formed. The portions of the fusible material with very small cross section tend to expand to a much greater degree than the ones with larger cross sections after removal from the fabricationmandrel on which the fuse element is initially wound. Consequently. it was found that although certain advantages may be achieved by eliminating a mandrel support for helical fuse elements, other disadvantages are encountered due to the fact that the fulgurite formed around different portions of the blown fuse element, may expand onto or overlap an adjoining closely spaced turn or helical portion of the fuse element in such a manner that the electrically insulating properties of the fulgurite or fused quartz sand material is not sufficient to electrically insulate the overlapped helical portion from remaining parts of the spent or melted fuse element. This may result in a short circuit or electrical conductivity between the remaining parts of the spent fuse element and what is left of a helical portion of the fuse element. This short circuit may be short lived or relatively permanent and may have the effect of changing the interrupting characteristics of the instantaneously melting fuse element or providing a permanent short circuit between what should be electrically insulated portions of an interrupted electrical circuit. Therefore, it is desirable to eliminate the need for mandrels for fuse elements of relatively long lengths which may be formed into the shape of a helical coil having a substantially constant minimum pitch between helical sections or portions.

SUMMARY OF THE INVENTION ln accordance with the invention, a fusible element for cartridge type current limiting fuse is formed by winding variable cross-section fusible material into a helical coil. The effective diameter or size of the overall fuse element varies along the axial length of the fuse element as a predetermined function of the diameter of the fusible material. For example, a long length of fusible material which is in the form of tapered circular cross-section wire having a plurality of narrow neck portions of narrow cross-sectional portions where the fuse element will blow or melt initially is wound into a solenoid-like or helical coil fuse element having residual springiness. Those parts of the solenoid or coil corresponding to the smaller cross-sections of the wire are more tightly wound or in other words wound with a smaller diameter to form the coil or solenoid. Those parts of the larger diameter fusible wire near the ends of the coil or solenoid are wound with the largest effective diameter The ultimate aim in winding a fuse element of fusible wire or fusible material in this manner is to provide a helically coiled fuse element which may be expanded or compressed like an accordion but in such a manner that the pitch between adjacent sections of each contributing member of the spiral is always substantially the same. When the fuse element is fully expanded or tensioned, the axial spacing between each adjacent turn or helical sections is. relatively large but nevertheless substantially equal for every helical portion or section of the overall fuse element. Correspondingly, as the winding is compressed, the spacing between'wires or between adjacent sections or turns of the overall coil or solenoid becomes relatively smaller but again is substantially equal for the different adjacent sections or portions of the overall fuse element. The importance of this construction can be seen from the fact that when the fuse element is caused to melt or blow due to an overload current which causes the fuse element to heat the adjacent pulverulent arc-quenching material fuses into a cohesive conglomerate or fulgurite. Depending upon the voltageimpressed between opposite separated endsof the fuse element portions, the fuse element may burn away or melt in a plurality of current limiting sections which may generally correspond to the number of previously mentioned. thin necks or reduced cross-sectional portions along the length of the fuse element. As adjacent ends of each of these neck portions begin to burn away from each other due to the fusing action of the arc struck between them, a point is reached at which the distance between the ends of the blown portion is so large that the interposed fulgurite acts as a sufficient dielectric to prevent further electrical conduction between burned away ends of the remainder of the fuse element. This may happen substantially instantaneously depending upon the overload current since an entire helical portion of a fuse element may burn away within a fraction of a second. However, the expanding, heated fulgurite which has limited electrical conducting properties may overlap to make electrical contact with the next adjacent helical section or turn of the fuse element. The distance between one end of the melted fusible material and the next adjacent helical section may be considerably smaller than the distance between the two remaining end sections or end pieces of the fusible wire or material. A short circuit or electrically conducting path may then exist between the higher voltage end of the burned away fuse element and the next adjacent turn or helical portion which is electrically connected to the lower voltage end of the burned away fuse element. In some instances, this phenomena or operation may have the effect of modifying or distorting the interrupting characteristics of the fuse element or in another sense causing a permanent short circuit between supposedly interrupted ends of an electrical fuse if not prevented by the teachings of this invention.

Fuse elements similar to those previously described may be formed in accordance with the invention by using a length of fusible material comprising adjacent or serially connected sections of fusible wire which may be circular and which may have different diameters. Those wires having the smaller diameters or smaller cross-sectional areas are of course more likely to burn away or blow when an overload current condition exists in the cartridge fuse. A fuse element embodying the teachings of the invention may also be formed by winding the fusible wire or material in such a manner that the radius or diameter of the resultant solenoid or coil or spring-like fuse element varies in discrete steps. Those fuse elements formed from placing end-to-end sections of wire each having a different but substantially constant diameter may be pre-wound on a manufacturing mandrel in such a way that those sections of wire having relatively large circular cross-sectional areas will be wound in relatively larger diameter portions of the overall solenoid or coil and those sections of wire having relatively smaller diameters will be formed into those parts of the solenoid having relatively smaller solenoid diameters. The reason for this is to provide a substantially constant spring pressure or spring constant along the entire length of the solenoid or helical fuse element. A substantially uniform spring rate allows the wire to be expanded, for example, to a length suitable for mounting in a cartridge type fuse and concurrently having a constant pitch which provides adjacent sections or coils of the overall coil which are sufficiently spaced from other turns or sections so that a burned out or fused portion of the wire will not be electrically connected by a section of fulgurite to form an electrically conducting path or short circuit circuit between adjacent helical sections or turns of the overall fuse element.

The teachings of this invention also include the use of unique manufacturing or forming mandrels and a process for manufacturing the previously mentioned solenoids or spring-like coils of fuse elements. In the instance of a tapered fuse wire, assuming for example that the wire is tapered so that it is wider or larger at the ends than at the middle, a mandrel which is correspondingly wider or broader at the ends than in the middle may be formed by joining two sections of man drel together at the smaller inner area in any convenient fashion such as by threading one end of one mandrel and tapping a hole in the corresponding end of the other mandrel and screwing one mandrel into the other. The tapered fuse wire can then be wound on the mandrel forming a coil having gradually smaller diameter as the wire advances toward the middle of the mandrel and then a larger diameter as the wire advances away from the middle towards the other end of the mandrel. The mandrel can then be unscrewed and pulled out from the formed spiral or solenoid without distorting or in any way damaging the formed solenoid or coil. Similarly the same method can be employed with a stepped mandrel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a prior art fuse element formed in the shape of a solenoid or helical coil and having a melted or blown portion;

FIG. 2 shows a longitudinal section of a prior art tapered fuse element or wire with a blown portion;

FIG. 3 shows a prior art fuse element assembly with parallel fuse elements or wires wound on a supporting member or mandrel;

FIG. 4 shows a prior art cartridge fuse structure with parallel fuse elements mounted in the associated housing without the benefit of a mandrel or supporting member;

FIG. 5 shows a prior art fuse element including stepped or discrete sections of fuse wires joined end-toend and a corresponding solenoid or fuse element coil that may be formed from such a fuse element;

FIG. 6 shows a stepped fuse wire similar to that shown in FIG. 5 but formed into a substantially constant pitch helically coiled fuse element in accordance with the invention;

FIG. 7 shows a section or length of tapered fuse wire and a corresponding prior art solenoid or coil type fuse element which may be formed from it;

FIG. 8 shows a second fuse element in accordance with the invention which may be formed from the ta pered fuse wire shown in FIG. 7 as a substantially constant pitch solenoid or coil;

FIG. 9 shows a cartridge type fuse similar to the one shown in FIG. 4 but with a substantially constant pitch solenoid or coil fuse element formed from fusible material of discretely varying size;

FIG. 10 shows a cartridge fuse similar to the one shown in FIG. 4 with a substantially constant pitch solenoid or coil fuse element formed from fusible material of continually varying or tapered diameter or size in accordance with the invention;

FIG. 11 shows a mandrel for forming a substantially constant pitch helically wound fuse element of discretely varying size in accordance with the invention;

FIG. 12 shows a mandrel for forming a constant pitch helically continuously varying spiral fuse section in accordance with the invention; and

FIG. 13 shows a helical describing vector system.

DETAILED DESCRIPTION OF THE DRAWING Referring now to the drawing and FIG. 1 in particular, a prior art generally helically wound fuse element is shown which is suitable for mounting in a current limiting cartridge type fuse. The fuse element 10 comprises a length of fusible material 12 wound into the shape of a resilient or spring charged helical coil having a diameter D. The size of the fusible material 12 may be of the tapered or stepped variety. The fuse element shown is typical of many known helically wound fuse elements, during interruption it may have a relatively high voltage V] applied at end or terminal 16 and a different voltage V2 applied at another end or terminal 14. A current I is shown flowing through a portion of the length of fusible material 12. Current I may be of sufficient magnitude to cause fusible material 12 to flow or melt at a predetermined weakened point or portion in the fusible 12 between points or fused

end sections

20 and 24. When the fusible material melts or blows, a high voltage arc is nearly instantaneously established between point or are anode 20 and are cathode 24 to limit the rate of magnitude of current l. The fusible material 12 is normally surrounded by a pulverulent arc quenching material 27 and as the heat of the are established between points or ends 20 and 24 melts or fires the arc-quenching material 27, a fulgurite or conglomerated mass of arc-quenching material 28 is formed. The are may cause the fusible material 12 to burn backwards such that points or ends 20 and 24, although shown in one instant of time, effectively process or move away from each other as the arc between them consumes the fusible material forming fuse wire 12. The are will cease or be interrupted when electrical point 20, which may be at high electrical potential V1, is sufficiently burned back or removed a distance L2 from electrical point 24, which may be maintained at a significantly lower electrical potential V2, to create an insulating gap filled with fused pulverulent material 28. However, the fulgurite material 28 which may have expanded outwardly and which may have sufficient electrical conductivity such that the distance represented by L] between

electrical points

20 and 32, which may also be maintained as voltages V1 and V2 respectively, is insufficient to prevent the continued arcing or flow of current I along path L1. In a short term sense or in the instant after the fuse element 10 has begun to melt the short circuiting of current I through path Ll may cause a drastic dhange in the current limiting characteristics of fuse element 10. In a long term or long time sense, the availability of the short circuit or conducting path L1 between the

points

20 and 32 will allow current I to flow between terminals 16 and 14 even though fuse element 10 ostensibly has blown or opened between the burned away points 20 and 24.

Referring now to FIG. 2, a view of a prior art fuse element 10' including electrically conducting fusible material 12' similar to fusible material 12 shown in FIG. 1 is depicted in the vicinity of a current limiting arc which has been established due to overcurrent in fusible material 12'. It will be noted that points or burn interfaces 20' and 24, which generally correspond to

points

20 and 24 in FIG. 1, have been burned back to the extent that they represent the limits of the material that previously existed between them and has burned away. The burned fusible material has been effectively replaced by a fulgurite or arc-quenching material 28' the melted ends of the fusible material 12 are separated by distance L2. A voltage or potential V1 exists at point or region 20 and the voltage or potential V2 exists at point or region 24. The electrically conducting properties of the fulgurite 28 may be sufficient in this geometric configuration to prevent further current flow between points 20' and 24. This is the way a fuse 5 normally provides an insulating gap in the electrical circuit in which it is connected.

By referring once again to FIG. 1, it can be shown that if the distance Ll can be made larger than the distance L2, which is the maximum distance over which an arc may be sustained between

points

20 and 24 in the presence of a predetermined potential difference, an additional are along line Ll between

points

20 and 32 will not be possible. The minimum distance between point 20 and an adjacent point such as 32 in the next turn or helical portion is represented by the distance S; S being the measure of the pitch of the winding of the coil of fuse element 10.

Referring now to FIG. 3, a prior art cartridge fuse structure 40 is shown having electrically conducting end sections or

ferrules

44 and 46 with contacting

members

50 and 48 respectively. Tie-down or braid tie points or

end terminals

52 and 54 are shown mounted adjacent to ferrules or

end caps

46 and 44 respectively. A mandrel or electrically insulating supporting member 56 is disposed between end portions or end

caps

44 and 46. An electrically insulating cylindrical or tubular casing 43 surrounds the mandrel 56 and an interposed arc quenching material 57. As illustrated there are five parallel fuse elements or fuse wires spaced from one and connected between the comoon points 52 and 54 of fuse section 40 as indicated by fusible conducting wires or

elements

12a, 12b, 12c, and 12d and l2e. The minimum distance between any two wires S can be quite easily maintained at a value sufficient to prevent flashover between adjacent turns or

helical sections

53 and 55 for example. The fixed distance S is maintained by relatively rigidly winding the

wires

12a, 12b and 120 and 12d and l2e on the supporting mandrel 56.

Referring now to FIG. 4, a prior art fuse structure similar to the one shown iin FIG. 3 is depicted. End sections or ferrules 44' and 46' are shown having a cylindrical electrically insulating main housing 43' disposed between and supporting them. A pulverulent arc quenching material 57' such as quartz or silica sand is enclosed by cylinder 43 and end sections 46 and 44'. A spring loaded electrically conducting plunger 48' is also provided. Helically wound or charged fuse elements of

fusible material

60 and 62 are mounted within casing 43. Helical coil or solenoid is supported at ends 64 and 66 and solenoid or helically wound fuse element 62 is supported at ends 68 and 70. The helically wound fuse elements and 62 have been prefabricated on a mandrel, removed from the mandrel and stretched spring'loaded or charged to be placed or assembled in fuse structure 56. Pitch S varies along the entire axial length of both

solenoids

60 and 62. This is because the overall diameter of the springlike winding generally constant along its length and, because the mandrel upon which the winding was pre-fabricated has a uniform diameter. However in most circumstances the diameter of the fusible material from which the fuse elements are formed is varied either continuously or discretely in steps in order to establish points or areas where arcs may begin upon the heating of the

fuse elements

60 and 62 due to overcurrent. But this causes the smaller sections of

fuses

60 and 62 of cross sectional area to be pulled apart or elongated to a larger extent by the tensioning or stretching of the spring-

like fuse elements

60 and 62, than the portions of said fuse elements where there may be more corsssectional area in the fusible material. Consequently, a minimum predetermined spacing such as spacing S as shown in FIG. 3 cannot be maintained and an arc-over between adjacent spiral sections of either

fuse piece

60 or 62 may occur similarly to that which was described with respect to FIG. 1.

Referring now to FIG. 5, a prior art fuse element of stepped fusible material 72 is shown, Fusible material 72 comprises, in this example, three fuse wires of generally circular cross-section abutted or joined end-toend in some convenient fashion such as brazing. Wire 74, having a diameter d1 is abutted at point 80 to fuse wire 76 having diameter d2 and fuse wire 76 is abutted at point 82 to fusible material 78 having diameter d3. When the composite length of fusible material 72 is wound into the shape of a helical coil which has a single overall diameter D1, the pitch of adjacent sections of spiral may vary depending upon the strength or corsssectional size of the fuse material making up each particular part of the winding. For example, the pitch S1 existing between

points

90 and 92 of fusible material 74 may be greater than the pitch S2 existing between

points

94 and 96 of the fusible material 76 which, in turn, may be different than the pitch S3 existing between points 101 and 102 of fusible material 78. It will be noted that the solenoid or spring-like fuse element 73 has a common centerline or longitudinal axis 84.

Referring now to FIG. 6, the disclosed invention is illustrated in a fuse element 73'. In this construction, the length of fusible wire 72 as was shown in FIG. is once again wound into a helically shaped fuse element 73'. However, the pitch between adjacent sections of each fuse element is maintained at a substantially constant or predetermined distance which may be S4 as shown in this case. In other words, the distance between adjacent corresponding points of fusible material or wire 74, as shown by S4, between

points

90 and 92 is generally the same as the distance S4 which may exist between corresponding points or turns 94' and 96 of fusible material 76. This also applies for the distance S4 between points 100' and 102' of the fusible material 76. A common centerline or longitudinal axis 84 exists for the helical fuse element. An important difference between

fuse elements

73 and 73 is that the overall diameter of the formed helix or fuse element 73' varies as a function of the thickness or diameter of the fusible material or wire that is being wound into the helix shape. Consequently, wire 74 having a diameter d1 is formed into a helical fuse element portion having a helical diameter D4. Likewise, fusible wire 76 having a diameter d2 is formed into a helical portion having a helical diameter D2 and finally fusible wire 78 having a diameter d3 is formed into a helical fuse element portion having a diameter D3. Consequently, it can be seen that the diameter of the helical section of stepped or discretely varying fuse material is made substantially proportional to the actual diameter of the wire forming the particular portion of the overall helical fuse element. This fuse element construction provides a generally constant spring rate along the length of the solenoid or helical section 73'. As a result, a fuse element can be formed with great accuracy so that the distance between adjacent corresponding turns or points such as 94' and 96 in the same longitudinal plane (provided that longitudinal plane contains centerline 84') will be the same for any degree of extension or flexing of the helical fuse element 73'. If helical fuse element 73 is compressed in an accordion-like manner, the distance between adjacent corresponding points of the helical fuse element will become smaller but will nevertheless remain substantially uniform or constant along the entire axial length of the helical fuse element. The same result applies for an extension of the spring-like fuse element 73'.

Referring now to FIG. 7, a prior art fuse element including a length of continously tapered fusible material 104 is shown. Fusible material 104 is circular and has a relatively larger diameter at its ends 108 and and a relatively smaller diameter in the middle 106. The variations between

points

108, 106 and 110 are continuous rather than discrete, as was shown in FIG. 5 for the fusible materials employed. Consequently, the helical section or spring-like section 105 of silver composition fusible material when stretched or extended will provide varying spaced, distances or gaps between adjacent corresponding sections or turns of the helical section 105. For example distance S5 between

points

114 and 116 on the helix 105 is shorter than the distance S6 between

points

118 and 120. This is true because the helically wound fusible wire in the vicinity of the narrower cross-sectional area at 106 is more easily stretched in a longitudinal or axial direction than the fusible wire in the vicinity of the relatively larger

size end sections

108 and 110. If the distance S5, as an example, were smaller than the minimum breakdown voltage path through the formed fulgurite between

points

116 and 114, a short circuit or conducting path may result between these points even though the fuse element had blown elsewhere either in the vicinity of point 116 or in the vicinity of point 114. Referring now to FIG. 8, to compensate for the problem shown in FIG. 7, the fusible wire 104 in accordance with the invention may be wound into a helically shaped coil 105 having a varying overall diameter. For example, the diameter of the helix 105 in the vicinity of the relatively larger cross-sectional areas of wire 106 near ends 108 and 11.0 is shown as D6 which is larger than the overall diameter D7 of the helix or fuse element portion shown in the vicinity 104' of the relatively smaller crosssectional portion of the wire 106. The effect of this construction is to maintain substantially uniform spacings S7 between adjacent corresponding turns or sections of the helical fuse element 105' which are equal for any given extension of the helix or fuse element 105. For example, the distance or spacing S7 between points 114' and 116' is generally equal to the distance S7 between the points 118' and 120 even though the cross-sectional area of the fusible material in wire 106 is larger in the vicinity of points 114' and 116' than the cross-sectional area of the material in the wire 106 in the vicinity of the ends 118 and 120'.

Referring now to FIG. 9, a fuse structure 56 generally similar to the fuse structure 56 in FIG. 4 is shown. There are end caps 44" and 46" and a spring loaded conducting plunger 48". A pulverulent arc quenching material, such as silica sand 57" is enclosed within an electrically insulating cylinder or housing 43" and around the

helical fuse elements

73 and 73B". Fuse element 73T" is stretched and connected between electrical contact points or terminals 64' and 66' which are connected to electrical end caps or ferrules 46" and 44 respectively. In a similar manner, helical fuse element 738" is connected at the ends 68' and 70' to the end ferrules or caps 46" and 44" respectively. It will be noted that the helical fuse elements 73T" and 73B" are of the discrete or stepped variety with respect to size as depicted in FIG. 6. The helical electrical fuse element of 73T" for example is divided in five easily identifiable discrete regions or portions each one having a different solenoid diameter associated therewith.

Regions

124 and 132 have the relatively largest diameters; regions 126 and 130 have smaller diameters and region 128 has the smallest diameter. It is important to note that the pitch or spacing between adjacent corresponding points or turns of each of the fuse elements 73T" and 73B" is substantially uniform or equal along the axial length of each of said fuse elements.

Referring now to FIG. 10, a similar current limiting fuse structure 56" is shown having a plurality of helical fuse elements 105T and 1058" with continuously varying solenoid diameters. Helical fuse element 105T" is suspended or connected between end points or terminals 64" and 66" and helical fuse element 1058" connected between end points 68" and 70'. It will be noted that the pitch of each of the helical fuse elements 105T" and 1058" is the same across the entire length of said helical fuse elements. However the amplitude or the diameter of the overall portions of helical section vary in proportion to the diameter of the fusible wire forming the helical portion at any point. Consequently, the diameter of the helical portion near, 105" for example, points 134 and 136 is large because the diameter of the wire is relatively larger in those areas and the diameter of the helical portion in the region of point 133 is relatively smaller because the diameter or size of the wire is tapered or smaller in that area.

Referring now to FIG. 11, a manufacturing mandrel is shown for prefabricating or forming a variable diameter fuse element helix. In this case, a stepped mandrel 150 has left section 150L and a right section 150R which may be joined by the screwing or turning of a threaded member 156 into a taped hole 154 of section 150R. The discrete steps are shown at points 152.

Referring now to FIG. 12 a similar manufacturing mandrel 160 for pre-fabricating or forming fuse elements from continously varying or tapered fusible wire is shown. Mandrel 160 may have left section 160L with a threaded protrusion 166 and a right section 160R with a tapped hole 164. Section 160L is screwed into the hole 164 in section 160R. A continuous taper results which decreases towards the middle is shown along the surface 162.

Referring now to FIG. 13, a means for theoretically showing the generation of the shape ofa helical fuse element is shown. A centerline or longitudinal axis 170 is established between en d points 171 and 172. A rotatable or revolving vector V is mounted at one of its ends 173 to the imaginary centerline or axis 170 and is capable of rotating about centerline 170 with ar i angular velocity (.Q). The other end 174 o f-vector V traces the path of th helical coil. Vector V is capable of being moved with the velocity (v) from point 171 to 1 7 2 or vice versa. As it moves longitudinally the vector V rotates angularly about centerline 170 with an angular velocity Q. The length of vector V is L(d), where d is the diameter of the fuse wire. L(d) is variable such that the distance between hinge point or pivot point 173 and end point 174 of vector V may vary as vector V is moved longitudinally and rotatably along line 170. As

vector V is moved point 174 traces the path of the helix. The distance between

points

171 and 172 is l. 5 It is to be understood that a generally circular type helix is contemplated in practicing the disclosed invention; however, helical coils of oblong cross-sectons or elliptical cross sections may be provided where desired. It is also to be understood that the discrete type variable diameter wire may have many discrete sections and the corresponding helical section will reflect the corresponding size of the wire along the length of the fuse material. It is also to be understood that the tapered type fuse helical section may have alternating large tapers and small tapers providing multiple areover points for the fuse material and correspondingly the helix form may have alternating large and small diameters along its length. It is to be understood that any type of fusible material may be used which is suitable for fusing or melting under overload conditions such as silver or silver alloys. It is also to be understood that the cross section of the wire itself may vary along its entire length and need not necessarily be circular and it is also to be understood that the mandrels as shown in F 16S. 11 and 12 are only aids in forming the helical sections disclosed and need not necessarily be the means for fabricating the helical sections. The mandrels sections may be joined in any convenient manner. The helix shape may vary continuously and is not limited to a tapered or stepped helix shape. The voltages V1 and V2 may be of any value and V2 may be higher than V1.

The apparatus embodying the teachings of this invention have many important advantages. For example one advantage lies in the fact that a helical fuse element may be provided without the use of a supporting mandrel which would therewise be mounted within a cartridge type fuse. A substantially uniform or constant minimum pitch or distance or spacing between adjacent corresponding portions of a fuse element is provided so that as the fuse element blows or is melted a short circuit or electrically conducting path is not provided between adjacent fuse elements due to proximity and the formation of fulgurite material which may temporarily act as a conducting medium. Since a mandrel is not required to be used to maintain the exact spacing between adjacent spiral sections of fuse elements, the heat dissipated during the fusing or melting process may be more easily absorbed by the arc quenching and heat absorbing pulverulent material such as silica or quartz sand. In addition since the diameters of the respective sections of a length of fusible material may vary, when forming the helical fuse element, the fuse element may be compressed or stretched to any reasonable length, not exceeding l-lookes Law, while maintaining relatively constant distance between each adjacent section of the fuse element for the compressing or stretching of the helical section. This aids in the convenient assembly and manufacture of the overall fuse structure. It is also to be understood that in some instances the fuse elements may be pre-fabricated on mandrels which are separable after prefabrication in such a manner that the mandrels can be removed without destroying the shape of the formed helical fuse section. Another advantage lies in the fact that since the tension between adjacent sections of fuse material is retained at a constant spring rate or tension rate the likelihood of a tearing, breaking or failure of the fuse element for reasons other than the flow of overcurrent may be minimized. Another advantage lies in the fact that if the fuse element is pre-stressed or charged at a certain pitch diameter when the fuse element blows there is less tendency for random longitudinal oscillation to occur among adjacent isolated parts of the blown helical element during the interrupting interval which might cause unusual and unpredictable interrupting characteristics. Another advantage lies in the fact that the use of a helical fuse element as disclosed provides an easy way to mount long lengths of fusible wire within a cartridge type fuse configuration.

I claim as my invention: 1. A method for fabricating a fuse element, comprising the following steps:

joining the narrowest end of each piece of a twopiece mandrel having an outer periphery which is variable with mandrel length to form a complete mandrel which is smaller in cross-sectional area at the point said narrow ends are joined than at opposite ends of the completed mandrel; winding fuse material which has a variable crosssectional area along the length thereof, at a constant pitch on said completed mandrel to form a wound fuse element of generally constant pitch,

said latter cross-sectional area of said fuse material being generally proportional to the radius of curvature of said periphery of any point along said mandrels length;

disconnecting said two-piece mandrel while said wound fuse element is disposed thereon; and removing said separated mandrel pieces from said wound fuse element.

2. The method of claim 1 wherein said outer periphery of said mandrel is generally circular in shape, the diameter of said latter mentioned circular periphery being variable with mandrel length from a larger circular periphery at one end of said mandrel to a smaller circular periphery in the region where each piece of said two piece mandrel is joined to the other piece, said wound fuse element being generally helically shaped when completed.

3. The combination as claimed in claim 1 wherein said outer periphery of said mandrel is generally continuously variable with said mandrel length.

4. The combination as claimed in claim 1 wherein said outer periphery of said mandrel is generally discretely variable with said mandrel length.

Claims

1. A method for fabricating a fuse element, comprising the following steps: joining the narrowest end of each piece of a two-piece mandrel having an outer periphery which is variable with mandrel length to form a complete mandrel which is smaller in cross-sectional area at the point said narrow ends are joined than at opposite ends of the completed mandrel; winding fuse material which has a variable cross-sectional area along the length thereof, at a constant pitch on said completed mandrel to form a wound fuse element of generally constant pitch, said latter cross-sectional area of said fuse material being generally proportional to the radius of curvature of said periphery of any point along said mandrel''s length; disconnecting said two-piece mandrel while said wound fuse element is disposed thereon; and removing said separated mandrel pieces from said wound fuse element.