US3029328A - Time-lag fuses - Google Patents

Description

Filed April 8, 1960 3,029,328 Patented Apr. 10, 1962 nice 3,029,328 TlME-LAG FUSErfi Frederick J. Kozacka, South Hampton, N.H., assiguor to The Chase-Shawmut Company, Ne'wburyport, Mass. Filed Apr. 8, 1960, Ser. No. 21,044 9 Claims. ((31. Zlltl-lZll) This invention relates to electric time-lag fuses, and more particularly to time lag fuses capable of withstanding, without blowing, the high inrush currents incident to starting of electric motors, but blowing on any overload tending to cause thermal damage to electric motors.

from anotherpoint ofview this invention refers to electric fuses capable of interrupting relatively small protracted overload currents as well as of interrupting relatively high fault currents in the nature of short-circuit currents.

Such dual function fuses, if adapted to be used in circuits of electric motors, are generally known as dual element fuses because they include two interrupting elements, one for elfecting delayed interruption on occurrence of relatively small protracted overloads, and another for erfecting rapid interruption on occurrence of relatively high fault currents in the nature of short-circuit currents.

Prior art dual element fuses are relatively complex structures which are relatively difficult and relatively expensive to manufacture.

it is, therefore, one object of this invention to provide novel fuse structures having, in substance, the same operating characteristics as prior art dual element fuses but being not ascornplex as this type of fuses, and being relatively easy and relatively inexpensive to manufacture.

It is possible to distinguish between two types of dual element fuses. In one type interruption is effected at the occurrence of relatively small protracted overloads by fusion and liquefication of a metal having a relatively low fusing point as, for instance, tin, and removal, by gravity, of the liquefied metal from the point in the current path where it was initially located, thus causing formation of a circuit-interrupting break. In the other type of prior art dual element fuses interruption is effected at the occurrence of relatively small protracted overloads by parting of a pair of contacts spring biased to the interrupting or spaced position thereof and normally held togather in their current-carrying engaged position by an intermediate current-carrying solder joint.

The first mentioned type of dual element fuses is subject to many limitations and drawbacks. One of its most serious drawbacks is lack of sufficiently positive breakforming means which is conducive to relatively long arcing times on occurrence of relatively small protracted overloads. In the second mentioned type of dual element fuses the spring bias of the contacts is a sufficiently positive break-forming means and thus, in this type of fuses, the danger of prolonged arcing when interrupting relatively small protracted overloads is minimized. The second mentioned type of dual element fuses is, however, subject to various other limitations. The presence of moving parts calls for relatively high manufacturing standards and skilled labor, if reliable operation is to be achieved. This type of dual element fuses involves relatively large masses, time lag being predicated on the heat absorbing capacity of a relatively large mass of a metal having a relatively low fusing point. Since these large masses should be arranged, and are generally arranged, substantially in the center region of the casing, or fuse tube, and supported by the axially outer ends of the fuse link, their presence is conducive to seriously impair the shock-resistance of this type of dual element fuses. In this type of dual element fuses the axially outer ends of the fuse link are provided with points of'restricted crosssection. These points of restricted cross-section have to perform two functions. They .must generate such amounts of heat as to cause fusion of the solder joint by which the contacts are conductivcly and mechanically inter-connected against the bias of the contact separating spring. These points of restricted cross-section of the axially outer ends of the fuse link must further be adapted to form breaks on occurrence of relatively high fault currents in the nature of short-circuit currents. The aforementioned two functions of the points of restricted cross-section of fuse links are not fully compatible. A point of restricted cross-section, generally referred to as a neck, designed to achieve the required heating function for the contact-joining solder joint cannot be designed to have a given or optimal performance on occurrence of relatively high fault currents in the nature of short-circuit currents. In other Words, in the type of dual element fuses under consideration there is such interdependence between the heating function of necks and their function of forming breaks on major faults as impose critical limitations on the design of the necks of their links. Hence neck design cannot amount to more than some compromise between two more or less incompatible neck functions. As a result, the interrupting capacity and the selectivity of dual element fuses of the type under consideration does not fully live up to requirements. Another limitation of dual element fuses of the type under consideration consists in that their solder joints call for alloys having relatively low fusing points, as a result of which the operatiton of the fuses is significantly affected by ambient temperatures and is, therefore, rather inconsistent or erratic with changes in ambient temperatures.

It is, therefore, another object of the invention to pro vide time-lag fuses capable of interrupting overloads and major fault currents, having delay times of the same order as the above dual element fuses, but not being subject to the limitations and drawbacks of the above dual element fuses.

Another prior art type of time lag fuses is predicated upon a metallurgical reaction between a high fusing point base metal of which a fuse link is made, e.g. silver or copper, and a low fusing point overlay metal, e.g. tin. The above metallurgical reaction starts when the fusing point of the low fusing point overlay metal, such as tin, is reached. The metallurgical reaction causes the base metal to be ultimately severed, thus resulting in formation of a circuit interrupting break. Fuses of this type are free from the disadvantages and drawbacks of dual element time-lag fuses, but it was heretofore impossible with fuses of this type to achieve the long delay times required in circuits which include motors that cause high inrush current when started.

it is, therefore, another object of this invention to provide fuses having fuse links of a relatively high fusing point base metal provided with overlays of a relatively low fusing point metal adapted to sever the base metal by a metallurgical reaction when the fusing point of the overlay metal'is reached, which fuses are capable of achieving the long delay times required in circuits which include motors that cause high inrush currents when being started.

Another object of the invention is to provide time-lag fuses capable of achieving substantially the same timecurrent characteristics as prior art dual element fuses but being substantially faster than such dual element fuses in the range exceeding five times the rated current of the particular fuse. P

Considering time-lag fuses of the type wherein a fuse link of a high fusing point base metal is severed by a metallurgical reaction with a low fusing point overlay metal, the difiiculties encountered in achieving with such fuses long delay times increase as their design is being adapted for relatively small current ratings. A given design adapted for a current rating of many hundred amps. may be capable of achieving a time-current characteristic such as required for motor circuits, yet it is considerably more difficult to provide a design for a current rating of say one hundred amps., or less, capable of achieving the same time-current characteristics in terms of percent of overload above current rating.

It is,'therefore, another object of this invention to provide time-lag fuses of the type wherein a fuse link of a high fusing point base metal is severed by a metallugical reaction with a low fusing point overlay metal which fuses have current ratings as low as one hundred amps, and even less, capable of achieving time-current characteristics of the kind required for application in motor circuits.

Another object of the invention is to provide fuses having current ratings of one hundred amps, orless, capable of achieving by virtue of a delayed metallurgical reaction between a high fusing point base metal and a low fusing point overlay metal time-current characteristics of the kind required for application in motor circuits, which fuses are extremely fast in the range above five times current rating so as to effectively interrupt relatively small fault currents in the nature of relatively small shortcircuit currents, or arcing faults.

Other objects and advantages of the invention will, in part, be obvious and in part appear hereinafter.

For a more complete understanding of the invention reference may be had to the following detailed descrip tion thereof taken in connection with the accompanying drawing wherein:

FIG. 1 is a longitudinal section of a fuse embodying this invention;

FIG. 2 is an end view of the structure of FIG. I seen in the direction of the arrow R in FIG. 1 and showing the structure upon removal of one of its terminal caps;

FIG. 3 is a section along 3-3 of FIG. 1; and

FIG. 4 is an isometric exploded view of the structure of FIG. 1 upon removal of the pulverulent arc-quenching filler therefrom, showing the casing sectioned in a direction longitudinally thereof.

Referring now to the drawing, numeral 1 has been applied to indicate a tubular casing of insulating material closed on both ends thereof by terminal caps or ferrules 2 mounted thereon. Ribbon-type fuse link 3 is arranged within casing 1. Fuse link 3 comprises an axially inner portion 3a bent tubularly to envelop the longitudinal axis of easing 1, and axially outer portions 3b. The axially inner portion 3a of link 3 is bent into the shape of a prism which is square in cross-section, as can best be seen in FIG. 3. Link 3 is provided with three transverse lines of circular perforations 4 defining three points of reduced cross-sectional area. The space of the link 3 situated between two lines of perforations 4 is covered with an overlay 5 of a metal having a lower fusing point and the base metal of which link 3 is made and adapted to sever the axially inner portion 3:1 by a metallurgical reaction when said lower fusing-point is reached. Fuse link 3 may be formed of a copper ribbon, or copper sheet, and overlay 5 may consist of tin. As an alterantive, fuse link 3 may consist of silver in which case the overlay may likewise consist of tin or, if desired, of indium. The overlay 5 extends from one point of reduced cross-sectional area of the axially inner portion 3a of link 3 to another point of reduced cross-sectional area thereof, and the axially outer boundary lines of the overlay 5 are coextensive with points of axially inner link portion 3a where the current density in link 3 is highest when it is carrying current. ration the base metal of link 3 begins to dissolve into the overlay metal at the boundary line of the latter where the current density is highest, and this initiates severing On occurrence of overloads of inadmissible duof the axially inner portion 3a and kindling of a circuitinterrupting are. It will be apparent that the delay time of the structure tends to increase the more effectively heat is being abstracted from the points of reduced crosssection of link 3 by the adjacent overlay 5, thus delaying the time when the points of reduced cross-section reach the fusing or melting temperature of the overlay metal.

Distributing perforations transversely across a fuse link and abstracting the heat generated where the link is narrowest by an overlay of tin, or an equivalent link-severing metal, extending transversely across fuse link. 3 tends to achieve relatively long delay times. The delay times which can thus be achieved are, however, .not sufiiciently long to preclude premature or unnecessary blowing on heavy motor starting currents. Tubularly folding the fuse link in combination with the above geometry of transverse distribution of the perforations and the transverse arrangement of an overlay tends to result in increased delay times compared to those that are normally achieved in time-delay'fuses wherein delayed interruption of small protracted overloads is achieved by'a metallurgical reaction between an overlay metal having a relatively low fusing point and a base metal having a relatively high fusing point.

To further increase the delay times as required in mo tor circuits the flow of heat from the axially inner portion 3a of the fuse link 3 toward the terminal caps 2 is minimized, resulting in a derating of the fuse structure in regard to current-carrying capacity and a concomitant increase in delay times for given overload currents expressed in terms of percent of the current rating of the fuse structure.

In order to minimize the aforementioned flow of current and to impart to the link 3 a high heat-absorbing capacity the aggregate length of the axially outer portions 3b of link 3 exceeds the length of the axially inner link portion 3a. The axially outer ends of the axially outer link portions 3b are bent around the edges of easing 1 and turned to the outer surface thereof. The terminal caps 2 are mounted upon the ends of easing 1 and upon the axially outer ends of the axially outer link portions 3b which axially outer ends aree situated on the outer surface of casing 1. The axially outer link portions 3a define a pair of truncated pyramids resiliently supporting the axially inner prismatic link portion 3a and thus pre-, cluding damage to the latter, particularly at the points of smallest cross-section or necks thereof. The length of each of the two truncated pyramids formed by the axially outer link portions 3b exceeds the length of the axially inner portion 3a of link 3 which is in the shape of a prism.

Each cap 2 contains on the inside thereof a solder joint 6 conductively connecting the inner surface thereof with the points of the axially outer ends 315 of link 3 which are bent around the edges of easing 1. These so-- called blind solder joints are preferably established by placing into each cap a soldering laminate which comprises an inner layer of a fluxing agent sandwiched between a pair of outer layers of tin foil, and causing fusion of said soldering laminate by applying heat to the outside of caps 22. Upon fusion of the laminate the points of outer link portions 312 engaging the edges of easing 1 are firmly soldered to the inner surfaces of caps 2.

It will be apparent from the foregoing that link 3 is substan ially tubular and is arranged inside casing 1 in coaxial relation thereto. The axially outer portions 312 of link 3 have a plurality of cuts extending in a direc tion longitudinally of link 3 subdividing the axially outer portions into a plurality of separate strips.

The table below indicates in column A delay times of conventional prior art time-lag fuses wherein delayed interruption of small protracted overloads is achieved by a metallurgical reaction between an overlay metal having a relatively low fusing point and a base metal having a.

Q. Li.

relatively high fusing point. The same table indicates in column B delay times of fuses embodying this invention and designed as illustrated in the drawing. The data in columns A and B refer to fuses having a current rating of 60 amps.

Fuses performing as stated in column A are not adapted to be applied in circuits involving high inrush currents, whereas fuses performing as stated in column B are ex cellently suited to be applied in such circuits. No time delay is required, or desired, at overload currents exceeding 500% of the rated current of the particular fuse. At currents exceeding 500% of the current rating or higher currents the delay action of overlay becomes insignificant, i.e. series breaks are then formed substantially simultaneously at all three points of .restricted crosssection of link 3. The fuse then operates as a quick acting fuse rather than as a time-lag fuse and is able to clear short-circuit currents of considerable magnitude. To this end the casing l is filled with a pulverulent arcquenching filler '7 which has a substantialde-ionizing action, resulting in the build-up of considerable arc voltages inside of the fuse. The necks or points of reduced crosssection where breaks are formed and are voltage is generated a-re confined to. the axially inner portion 3a of link 3, which is considerably shorter than the total length of the casing. Hence it is necessary to provide highly effective arc voltagegenerating means at the axially inner portion 3a in order to generate at this portion in spite of the short length thereof the arc voltages required for interrupting both high fault currents and protracted overload currents. The pulverulent tiller 7 must perform the dual function of effectively quenching arcs and of minimizing heat flow away from the axially inner portion of link 3. Gypsum (CaSO serves this purpose well.

A more complete understanding of the above described behavior and of the entire behavior of the fuse may be obtained from what follows:

Considering a fuse link comprising a plurality of points of reduced cross-section or necks, and further comprising an overlay of a metal having a lower fusing point than I the base metal of which the link is made adapted to sever the link by a metallurgical reaction when said lower fusing point is reached. In such a fuse link an initial break is formed on occurrence of protracted overloads at the point of the link where the overlay is located. As the overload current is increased there is a critical current at which the point of initial break-formation shifts from the point where the overlay is located to another point of the link. This shift of the point of initial break occurs at a critical rate of heat generation, i.e. when heat generation is such that it takes less time for said other point of the link to reach the fusing point of the base metal than required by the overlay'to reach the fusing point of the overlay metal. The time-current curve of such a fuse link, or of a fuse having such a fuse link, has a sharp bend at the critical current at which the point of initial break shifts. In other words, the time-currentcurve comprises two portions of which one applies to currents below the critical transfer current and one applies to currents above the critical transfer current. This makes it possible to design such fuses in such a way that 'they are relatively slow at currents below the critical transfer current and relatively fast at currents above the critical transfer current.

The fuse structure which has been described above and illustrated in the drawing shares thes features with prior art fuses. Aside from their unique time-lag features and structural features the performance of fuses according to the present invention differs from prior art as will be shown below:

It has been observed that the metallurgical reaction by which the base metal of the link is ultimately severed initiates at or adjacent the bouundary line between the overlay metal and the base metal. In view of this fact the overlay metal is arranged on the base metal in such a Way that the boundary line of the overlay metal is coextensive with the necks or points of reduced cross-section of the fuse link, where the current density is highest. Speaking more specifically, the overlay 5 is arranged between a pair of axially spaced neck zones and its boundary lines are coextensive with the pair of neck zones. Under overload conditions the temperature at or adjacent the neck zones will be substantially equal and hence there will be a tendency that double breaks will form when the fuse blows on occurrence of overload currents. The over-.

lay has a cooling action on the neck zones with 'which it is associated, delaying the time at which the overlay reaches its fusing point and delaying the time at which the link-destroying metallurgical reaction is initiated. At a given critical current density in the necks the rate of heat generation in the necks exceeds the rate of heat absorption by the overlay. Under such conditions the overlay does not operate any longer as a time delay means, i.e. the necks rapidly reach the fusing point of the overlay metal and of base metal and the fuse then operates as a fast fuse. Under major fault current conditions, or under overload conditions above 5 times current rating, the link 3 is severed along its periphery at three points spaced in a direction longitudinally of link 3. In other words, the same necks which form breaks at small protracted overloads are also used to form breaks at major fault currents, and high overload currents. This dual function aspect of the necks makes it possible to confine the necked or perforated portion of the link to its axially inner portion which, in turn, tends to limit heat losses by axial heat flow to caps 2, tends to derate the link and to impart to the fuse the delay time current characteristic required in circuits with motors which give rise to high inrush currents.

It will be understood that I have illustrated and described herein a perferred embodiment of my invention, and that .various alterations may be made in the details thereof without departing from the spirit of the invention as defined in the appended claims.

I claim as my invention:

1. A time-lag fuse comprising a tubular casing of insulating material; a fuse link arranged inside said casing having an axially inner portion bent to envelop the longitudinal axis of said casing and axially outer portions, the aggregate length of said axially outer portions exceeding the length of said axially inner portion, said axially outer portions of said link being bent around two edges of said casing and turned upon the outer surface thereof; an overlay on said axially inner portion of a metal having a lower fusing point than the base metal of said link adapted to sever said axially inner portion thereof by a metallurgical reaction when said lower fusing point is reached, and a pair of terminal caps mounted upon the ends of said casing and mounted upon said axially outer portions of said link on the outer surface of said casing.

2. A time-lag fuse comprising a tubular casing of insulating material; a fuse link arranged in said casing hav ing an axially inner portion bent substantially into the shape of a prism and axially outer portions defining a pair of truncated pyramids, the length of each of said pair of truncated pyramids exceeding the length of said portion in the shape of a prism, the axially outer ends of said axially outer portions being bent around the edges of said casing and turned upon the outer surface thereof; an overlay on said axially inner portion extending transversely over all sides thereof, said overlay including a metal having a lower fusing point than the base metal of said link and adapted to sever said axially inner portion by a metallurgical reaction when said lower fusing pomt is reached; and a pair of terminal caps mounted upon the ends of said casing and mounted upon said axlally outer ends of said axially outer portions.

3. A time-lag fuse comprising a tubular casing of insulating material; a fuse link arranged in said casing havlng an axially inner portion bent substantially into the shape of a prism square in cross-section and axially outer portions defining a pair of truncated pyramids, the axially outer ends of said axially outer portions being bent around the edges of said casing and turned upon the outer surface thereof; an overlay on said portion in the shape of a prlsm extending transversely over the four sides thereof, said overlay including a metal having a lower fusing point than the base metal of said link adapted to sever said portion in the shape of a prism by a metallurgical reaction when said lower fusing point is reached; a pair of terminal caps mounted upon the ends of said casing and mounted upon said axially outer ends of said axially outer portions; and solder joints inside said pair of caps conductively connecting the inner surfaces thereof with the points of said axially outer ends of said axially outer link portions bent around said edges of said casing.

4. A time-lag fuse comprising a tubular casing of insulating material; a fuse link arranged in said casing having an axially inner portion bent substantially into the shape of a prism and axially outer portions defining a pair of truncated pyramids, said axially inner portion being perforated along a plurality of axially spaced circumferential lines and the axially outer ends of said axially outer portions being bent around the edges of said casing and turned upon the outer surface thereof; an overlay on said axially inner portion occupying the space bounded by a pair of said circumferential lines, said overlay consisting of a metal having a lower fusing point than the base metal of said link and adapted to sever said axially inner portion by a metallurgical reaction when said lower fusing point is reached; and a pair of terminal caps mounted upon the ends of said casing and mounted upon said axially outer ends of said axially outer portions.

5. A time-lag fuse comprising a tubular casing of insulating material; a fuse link arranged in said casing having an axially inner portion bent substantially into the shape of a prism substantially shorter than the length of said casing and axially outer portions, said axially outer portions flaring radially outwardly in said casing and having ends bent around the edges of said casing and turned upon the outer surface thereof; an overlay on said axially inner portion of a metal having a lower fusing point than the base metal of said link adapted to sever said axially inner portion thereof by a metallurgical reaction when said lower fusing point is reached; and a pair of terminal caps mounted upon the ends of said casing and mounted upon said ends of said axially outer portions of said link turned upon said outer surface of said casing.

6. A time-lag fuse comprising a tubular casing of insulating material; a substantially tubular fuse link arranged in coaxial relation inside said casing, said link including an axially inner portion perforated along a plurality of axially spaced circumferential lines and axially outer portions, said axially outer portions being provided with a plurality of cuts extending in a direction longitudinally of link subdividing said axially outer portions into a plurality of strips, the axially outer end of each of said plurality of strips being bent around one of the edges of said casing and turned upon the outer surface thereof; an overlay on said axially inner portion of said link adjacent one of said plurality of circumferential lines, said overlay being a metal having a lower fusing point then the base metal of said link adapted to sever said axially inner portion thereof by a metallurgical reaction when said lower fusing point is reached; and a pair of terminal caps mounted upon the ends of said casing and mounted upon said axially outer end of each of said plurality of strips.

7. A time-lag fuse comprising a tubular casing of insulating material; a fuse link arranged inside said casing having an axially inner portion bent to envelop the longitudinal axis of said casing and axially outer portions, said axially inner portion defining a plurality of serially related points of reduced cross-section where the current density is highest when said linkis carrying current, said axially inner portion having an overlay of a metal having a lower fusing point than the base metal of said link adapted to sever said axially inner portion thereof by a metallurgical reaction when said lower fusing point is reached, said overlay having a boundary line coextensive with one of said points of reduced cross-section, said axially outer portions of said link flaring out radially outwardly and having ends being bent around the edges of said casing and turned upon the outer surface thereof; and a pair of terminal caps mounted upon the ends of said casing and upon said ends of said axially outer portions of said link on said outer surface of said casing.

8. A time-lag fuse comprising a tubular casing of insulating material; a fuse link arranged inside said casing having an axially inner portion bent to envelop the longitudinal axis of said casing and axially outer portions, said axially inner portion defining a plurality of serially related points of reduced cross-secticn where current density is highest when said link is carrying current, a

said axially inner portion having an overlayof-a metal having a lower fusing point than the base metal of said link adapted to sever said axially inner portion by a metallurgical reaction when said lower fusing point is reached, said overlay being arranged between two of said plurality of points of reduced cross-section and having a pair of boundary lines co-extensive with said two points, said axially outer portions flaring out radially outwardly and having ends being bent around the edges of said casing and turned upon the outer surface thereof; and a pair of terminal'caps mounted upon the ends of said casing and upon said ends of said axially outer portions of said link on said outer surface of said casing.

9. A time-lag fuse comprising a tubular casing of insulating material; a fuse link arranged in said casing having an axially inner portion bent substantially in the shape of a prism substantially shorter than the length of said casing and axially outer portions, said axially inner portion defining a plurality of serially related points of re duced cross-section where current-density is highest when said link is carrying current, said axially outer portions flaring radially outwardly in said casing and having ends bent around the edges of said casing and turned upon the outer surface thereof; an overlay on said axially inner portion of a metal having a lower fusing point than the base metal of said link adapted to sever said axially inner portion thereof by a metallurgical reaction when said lower fusing point is reached, said overlay being arranged between two of said plurality of points of reduced crosssection and having a pair of boundary lines coextensive with said two points; and a pair of terminal caps mounted upon the ends of said casing and upon said ends of said axially outer portions of said link turned upon said outer surface of said casing.

References Cited in the file of this patent UNITED STATES PATENTS 1,290,860 Bender Jan. 7, 1919 2,832,868 Kozacka "Apr. 29, 1958 FOREIGN PATENTS 1 338,890 Switzerland July 31, 1959 510,343 Germany Oct. 18, 1930 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0. 3,029,328 April 10, 1962 Frederick J. Kozacka It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 58, for "and" read than column 4, l nes 34 and 35, for "current" read heat column 7, l l e 63 before "link" insert said line 70, before a first occurence, i'n sert of Signed and sealed this 7th day of August 1962.

(SEAL) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer Commissioner of Patents

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