US3735317A - Electric multibreak forming cartridge fuse - Google Patents

Abstract

A fuse for elevated circuit voltages has a fuse link provided with a link-severing low fusing point overlay which forms a break on the occurrence of very small overloads the duration of which is in the order of, or exceeds, one hour. The fuse further includes means forming series breaks upon formation of an initial break in the fuse link. These means include a shunt across the overlay portion of the fuse link adapted to generate relatively large amounts of heat at the ends thereof conductively connected to the fuse link and adapted to generate relatively small amounts of heat per unit of shunt length in the center region of the shunt.

Description

United States Patent 1191 1111 3,735,317 Jacobs, Jr. May 22, 1973 [541 ELECTRIC MULTIBREAK FORMING 2,416,428 2 1947 Boothe ..337 291 CARTRIDGE FUSE 3,621,433 11/1971 Belcher ..337/244 [75] Inventor: llafilip C. Jacobs, Jr., Newtonville, Primary Examiner Bemard A Gilheany Assistant Examiner-F. E. Bell [73] Assignee: The Chase-Shawmut Company, Att0rneyErwin Salzer Newburyport, Mass. 22 Filed: May 1, 1972 [57] ABSTRACT A fuse for elevated circuit voltages has a fuse link pro- [211 Appl' 249707 vided with a link-severing low fusing point overlay which forms a break on the occurrence of very small [52] US. Cl. ..337/291, 337/161, 337/296 overloads the duration of which is in the order of, or [51] Int. Cl. ..H0lh 85/04 exceeds, one hour. The fuse further includes means [58] Field of Search ..337/ 161,291,293, forming series breaks upon formation of an initial 337/296 break in the fuse link. These means include a shunt across the overlay portion of the fuse link adapted to 1 References Cited generate relatively large amounts of heat at the ends thereof conductively connected to the fuse link and UNITED STATES PATENTS adapted to generate relatively small amounts of heat 876,273 1/1908 Hall ..337/29l per unit of shunt length in the center region of the 1,818,382 8/1931 Conrad ..337/29l shunt,

2,243,107 5/1941 Lindell ..337/29l 2,341,865 2/ 1944 Hermann ..337/291 4 Claims, 5 Drawing Figures 111 111 b b b l l a l O O O O 0 O O O o C III a Pmimiw 3', 7 35.317

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ELECTRIC MULTIBREAK FORMING CARTRIDGE FUSE BACKGROUND OF THE INVENTION The interruption of very small overload currents in high-voltage fuses, e.g., fuses having a voltage rating of 5-15 kV is, in many instances, effected by means of an arc gap formed by separation of a pair of relatively movable or separable contacts. The provision of such switching means considerably complicates the design and manufacture of fuses.

It is, therefore, one object of the invention, to provide fuses for elevated circuit voltages, capable of interrupting overload currents as small as the currents which must be carried for one or several hours to cause interruption or clearing of the overloaded circuit by the particular fuse, without relying on relatively movable or separable contact to achieve this end.

SUMMARY OF THE INVENTION Electric fuses embodying this invention include a tubular casing of electric insulating material and a pair of metallic terminal elements closing the ends of the casing. A first fusible element having a relatively small resistance and forming a plurality of serially related points of reduced cross-sectional area interconnects conductively said pair of terminal elements. Said first fusible element has means at a predetermined point thereof for causing formation of a break therein at the occurrence of small protracted overload currents, the fuse further includes a second fusible element having a relatively high resistance shunted across said predetermined point of said first fusible element. Said second fusible element has ends immediately adjacent said first fusible element conductively connected to said first fusible element. The aforementioned ends of said second fusible element have a smaller resistance per unit of length than the center portion of said second fusible element to cause melting of, and consequent arc initiation by, said second fusible element at points thereof located immediately adjacent said first fusible element.

BRIEF DESCRIPTION OF THE DRAWINGS BRIEF DESCRIPTION OF PREFERRED EMBODIMENT Before describing the preferred embodiment of the invention, a brief summary of the pertinent prior art is given below.

In FIGS. la-lc reference characters have been applied to indicate diagrammatically a pair of metallic terminal elements, e.g., a pair of ferrules or a pair of terminal plugs.

According to FIG. 1a terminal elements a are conductively interconnected by a ribbon fuse link b and by a fusible wire c. The former has a relatively small resistance and may be provided with serially related points of reduced cross-sectional area. The fusible wire c has a relatively high resistance and its cross-section is uniform along its entire length. Normally wire c does carry hardly any current. Upon fusion of link b and interruption of the current path formed by it wire c becomes current carrying and melts, thus completing the interruption of the faulted circuit initiated by fusion of ribbon link b. The arrangement of FIG. 1 has various applications, but does not lend itself to effectively interrupt extremely small overload currents whose duration is, or exceeds, 1 hour.

The structure of FIG. 1b is similar to that of FIG. 1a, except that ribbon fuse link b is provided with an overlay b of a low fusing point metal, e.g., tin, capable of severing upon fusion thereof fuse link b by a metallurgical reaction, and except that high resistance wire c' shunts but the center portion of fuse link b with its overlay b rather than its entire length. Upon formation of a break at b' the entire current is carried by wire c which then fuses relatively rapidly and thereby effects final interruption or clearing of the faulted circuit. The structure of FIG. lb is inapplicable to interruption of very small overload currents at relatively high circuit voltages, e.g., 5-15 kV because the single point of break formed at b is likely to break down following fusion of wire 0'.

The structure of FIG. 10 includes ribbon fuse link b having a low fusing point link-severing overlay b near its center. Reference character c" has been applied to indicate a wire which is normally not conductively connected into the circuit of the fuse. The ends of wire 0 are spaced from ribbon fuse link b and form a pair of spark gaps d. Upon fusion of overlay b fuse link b is severed at its center resulting in kindling of an are. When its arc voltage is sufficiently high, spark gaps d break down and the resulting arcs formed at spark gaps d sever fuse link b at the two points b", thus producing multibreaks, e.i., three series breaks are ultimately formed in fuse link b.

To achieve the desired result the potential of resistance wire 0" cannot be floating. The fuse must be provided with means not shown maintaining wire c" at such a potential that arc gaps d break down following formation of a break at b and following a predetermined burnback of the arc kindled at b'. A more serious limitation of the structure of FIG. 10 resides in the fact that its operation depends critically upon the gaplength of arc gaps d and on the configuration of the surfaces of parts b and c" at which the voltage gradient between them is largest. It is difficult to establish in fuses arc gaps d whose breakdown voltage is sufficiently uniform. Another limitation of the arrangement of FIG. 1c resides in the fact that a large portion of the heat generated in were 0" by F-r losses is lost inasmuch as it is not used to sever fuse link b at points b". Only the heat of the arcs across gaps d is used for forming series breaks b" in fusible element b, but the heat dissipated from wire c" into the pulverulent arc-quenching filler (not shown) surrounding wire c" serves no purpose.

The structure of FIG. 2 is not subject to the aforementioned limitations of the structure of FIG. 10. As shown in FIG. 2, ribbon fuse link b is provided at its center region with a low fusing point link-severing overlay b. The portion of ribbon fuse link b supporting overlay b is shunted by a wire shunt c. The ends of shunt c are conductively connected e.g., -spotwelded or soldered to ribbon fuse link b at the points b thereof. The ends of shunt c' conductively connected at b to ribbon fuse link b have a relatively high resistance per unit of length and generate a relatively large amount of heat when shunt c' becomes currentcarrying following formation of a break at b. The center portion of shunt c has a relatively small resistance per unit of length and generates a relatively small amount of heat when shunt c becomes currentcarrying following formation of a break at b. As shown in FIG. 2 the ends of shunt connected to fusible element b are formed by single wires, and the center portion of shunt c' is formed by two or more wires connected in parallel and has a much smaller resistance than the single wire ends of shunt c.

The ends of wire shunt 0 formed by a single wire are much shorter the center of wire shunt c formed by a plurality of wires. As a result, the generation of heat is concentrated to relatively small regions immediately adjacent ribbon fuse link b.

Upon formation of a break at b shunt 0 becomes current-carrying. This results in fusion of its high resistance ends conductively connected to ribbon fuse link b at the two points b". The resulting arcs sever ribbon fuse link b at points b. Points 12" where shunt c' is conductively connected to ribbon fuse link b are preferably points where the cross-sectional area of the latter is reduced.

In certain instances it is possible and desirable to provide ribbon fuse link b at points b with low fusing point overlays capable of severing ribbon fuse link b by a metallurgical reaction when these overlays reach the fusing point of the overlay forming metal, e.g.,- tin. The provision of such overlays at points b' is possible and meaningfull in instances where the temperature distribution along fusible element b is generally more or less of a flat top nature or, in other words, where the temperature at points b is substantially equal to, or but slightly less than, the temperature at point b. Then the generation of heat at the ends of shunt 0" may raise the temperature at points b of ribbon fuse link b sufficiently fast to sever fuse link b at points b' by a metallurgical reaction between the overlay metal at points b and the base metal at points b only shortly after such a metallurgical reaction has taken place at b.

Referring now to FIG. 3, the structure shown therein is substantially the same as that shown and described in U.S. Pat. 3,621,433 to Richard A. Belcher, Nov. 16, 1971 except for the addition of the low overload current interrupting multibreak-forming means shown in FIG. 2 and described in connection with that figure.

Numeral 1 has been applied in FIG. 3 to indicate a tubular casing of electric insulating material closed on both ends thereof byterminal plugs 2 press-fitted into casing 1. Terminal plugs 2 are firmly held in position bysteel pins 3 projecting through casing 1 into terminal plugs 2. The axially inner sides ofterminal plugs 2 are provided withgrooves 2a which receive the axially outer ends of fourceramic plates 4 of a heat-shock resistant material.Plates 4 form a spider, or mandrel, for supporting a ribbon fuse link b. The latter is helically wound around the radially outer edges of thespiderforming plates 4 and conductively interconnects terminal plugs 2. The plate-mandrel structure of FIG. 3 is disclosed more in detail in U.S. Pat. No. 3,599,138 to Frederick J. Kozacka, Aug. 10, 1971 for HIGH- VOLTAGE FUSE. The ribbon fuse link b of silver is provided in its center region with a low fusing point link-severing overlay b. Shunt c extends across a section of ribbon fuse link b to both sides of overlay b. The ends of shunt 0" having a relatively high resistance per unit of length and a relatively small crosssectional area are aflixed to points of ribbon fuse link b where the cross-sectional area of the latter is reduced by means of one of a plurality of circular perforations provided therein. Casing 1 is filled with a pulverulent arc-quenchingfiller 5, preferably quartz sand. Both terminal plugs 2 are provided in the center thereof with an internally screw-threaded bore of which each receives aplug screw 7. One ofplug screws 7 is provided with a recess receiving a spring-biased blown fuse indicator and the other ofplug screws 7 receives a clamping means for anchoring the end of a restraining wire for the blown fuse indicator remotefrom the blown fuse indicator. The details of these features are described in the above referred-to U.S. Pat. No. 3,621,433 to Richard A. Belcher. The hexagonal heads ofplug screws 7 clamp the axially inner end surfaces of a pair offerrules 8 against the axially outer end surfaces of terminal plugs 2.

On occurrence of overload currents the structure of FIG. 3 operates in the fashion described above in connection with FIG. 2 first forming one break at b and shortly thereafter forming two additional series breaks at b. Major fault currents result in the formation of series breaks, one at each point of reduced crosssectional area of fuse link b.

I claim as my invention:

1. An electric fuse adapted to form multibreaks at the occurrence of small overload currents including a. a tubular casing of electric insulating material;

b. a pair of metallic terminal elements closing the ends of said casing;

c. a first fusible element having a relatively small resistance and forming a plurality of serially related points of reduced cross-sectional area conductively interconnecting said pair of terminal elements, said first fusible element having means at a predetermined point thereof for causing formation of a break therein at the occurrence of small protracted overload currents;

e. a second fusible element having a relatively high resistance shunted across said predetermined point of said first fusible element and having ends immediately adjacent said first fusible element conductively connected to said first fusible element, said ends of said second fusible element having a smaller resistance per unit of length than the center portion of said second fusible element to cause melting of, and consequent arc initiation by, said second fusible element at points thereof located immediately adjacent said first fusible element.

2. An electric fuse as specified in claim 1 wherein said first fusible element is formed by a ribbon of silver supporting at said predetermined point a metallic overlay having a relatively low fusing point and capable of severing upon fusion thereof the current path through said ribbon of silver, and wherein said second fusible element is formed by wire means having a smaller electric conductivity than silver.

3. An electric fuse as specified inclaim 2 wherein the ends of said second fusible element immediately adjacent to said first fusible element are formed by single wire means and wherein the center portion of said second fusible element is formed by a plurality of wire means connected in parallel.

4. An electric fuse as specified in claim 1 wherein the length of said ends of said second fusible element is relatively short in comparison to the length of said center portion thereof in order to concentrate heat generation by said second fusible element to relatively small portions thereof immediately adjacent said first fusible element.

Claims (4)

1. An electric fuse adapted to form multibreaks at the occurrence of small overload currents including a. a tubular casing of electric insulating material; b. a pair of metallic terminal elements closing the ends of said casing; c. a first fusible element having a relatively small resistance and forming a plurality of serially related points of reduced cross-sectional area conductively interconnecting said pair of terminal elements, said first fusible element having means at a predetermined point thereof for causing formation of a break therein at the occurrence of small protracted overload currents; e. a second fusible element having a relatively high resistance shunted across said predetermined point of said first fusible element and having ends immediately adjacent said first fusible element conductively connected to said first fusible element, said ends of said second fusible element having a smaller resistance per unit of length than the center portion of said second fusible element to cause melting of, and consequent arc initiation by, said second fusible element at points thereof located immediately adjacent said first fusible element.

2. An electric fuse as specified in claim 1 wherein said first fusible element is formed by a ribbon of silver supporting at said predetermined point a metallic overlay having a relatively low fusing point and capable of severing upon fusion thereof the current path through said ribbon of silver, and wherein said second fusible element is formed by wire means having a smaller electric conductivity than silver.

3. An electric fuse as specified in claim 2 wherein the ends of said second fusible element immediately adjacent to said first fusible element are formed by single wire means and wherein the center portion of said second fusible element is formed by a plurality of wire means connected in parallel.

4. An electric fuse as specified in claim 1 wherein the length of said ends of said second fusible element is relatively short in comparison to the length of said center portion thEreof in order to concentrate heat generation by said second fusible element to relatively small portions thereof immediately adjacent said first fusible element.

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