US3197592A - Time lag fuses with fuse links having link-severing overlays - Google Patents

Description

y 7, 1965 F. J. KOZACKA 3,197,592

TIME LAG FUSES WITH FUSE LINKS HAVING LINK-SEVERING OVERLAYS Filed Jan. 30, 1963 3 Sheets-Sheet 1 6' I 1 1'03! JKozacfltb,

uitiozweg y 7, 1965 F. J. KOZACKA 3,197,592

TIME LAG FUSES WITH FUSE LINKS HAVING LINK-SEVERING OVERLAYS Filed Jan. 50, 1963 2 Sheets-Sheet 2 Inverztofl: WJKozwia, .9 M/NW XM/ Wiley United States Patent M greases TME LAG FUSEES WETH FUSE LINKS HAVING LINEQSIEVERENG @VERLAYS Frederick .l. llozaclra, South Hampton, NHL, assignor to The Chase Shawmut Company, Newhuryport, Mass. Filed so, 1963, Ser. No. 255,732 12 Claims. (iCll. Z00123) This application is a oontinuation-in-part of my copending patent application Ser. No. 116,365, filed June 12, 1961, for Time Lag Fuses of the Blade Contact Type, now US. Patent 3,123,693, issued March 3, 1964.

This invention has reference to electric fuses, and more particularly to time-lag fuses.

it is a general object of this invention to provide improved time-lag fuses, and more particularly time-lag fuses having sufficient time-lag to preclude blowing thereof on motor starting inrush currents when included in a motor circuit.

it is another object of this invention to provide improved time-lag fuses wherein the required blowing delay on occurrence of overload currents is achieved by an overlay on a fuse link of .a metal having a lower fusing point than the base metal of which the fuse link is made, causing severance of the fuse link by a metallurgical re action when the fusing point of the overlay is reached.

Relatively long time-lags may be achieved by designing the fuse links of time-lag fuses of the above description in such a fashion that the fuse links comprise relatively large amounts of metal and have such a geometry as to effectively minimize the rate of rise of temperature of the overlay metal on occurrence of relatively large overload currents, and by further providing means tending to minimize heat losses of the fuse structure as a whole to the space surrounding the fuse structure on occurrence of relatively small overload currents. The latter means are in the nature of thermal insulators, minimizing heat exchange between the fuse structure and the space surrounding the same, and thus tending to minimize the minimum current required for causing fusion of the overlay and consequent severance of the base metal at the point where the overlay is located. Heat dissipation at relatively small overload current-s may be minimized by providing fuses with a composite ribbon fuse link structure comprising a first portion having about the same width as the blade contacts of the fuse and being arranged relatively close to the longitudinal axis of the casing, and further comprising a second portion arranged generally more remotely from said longitudinal axis and overlapping and shunting said first portion. The linksevering overlays of a metal having a lower fusing point than the base metal extend transversely across the above mentioned first portion and transversely across the above mentioned second portion of the ribbon fuse link structure. This geometry is, however, in itself not always sufficient to limit the heat exchange between the fuse structure as a whole and the ambient space to the extent required for obtaining a time-current characteristic of the type needed for time-lag fuses intended for application in motor circuits. To achieve this end it is further generally necessary to apply as a pulverulent arc-quenching filler a substance having a relatively small thermal conductivity. The thermal conductivity of the filler must generally be smaller than that of quartz sand in order to comply with the highest time la requirements, which requirements are in the order of ten seconds for overloads in the order of five times the rated current. An arcquenching filler having such a small thermal conductivity is, for instance, gypsum. Gypsum and other pulverulent arc-quenching fuse fillers having a thermal conductivity of the same order as gypsum tend to generate but rela- 3,197,592 Patented July 27, 1965 tively small arc voltages. This, in turn, tends to limit the interrupting capacity of a given fuse structure unless the fuse link structure is designed to form a relatively large number of series breaks on occurrence of large fault cur- 5 rents such as short-circuit currents. The provision of fuse link structures designed to form a relatively large number of series breaks on occurrence of relatively large fault currents is conducive to excessive watt losses during the extended periods of time during which a fuse link performs its normal current-carrying duty. Therefore it is desired to limit the number of series necks or serially related points of reduced cross-section forming series breaks on occurrence of major fault currents.

It is another object of this invention to provide time-lag fuses wherein interruption is effected in the overload range by a metallurgical reaction between an overlay metal having a relatively low fusing point and a base metal having a relatively high fusing point, which fuses lend themselves to be designed for the highest time-delay requirements which may be imposed in connection with motor circuits, and in which fuses the high thermal insulating action of the pulverulent arc-quenching filler is not achieved at the price of relatively small arc voltage generating performance thereof.

The design of time-lag fuses of the type having linksevering low fusing point metal overlays and adapted for application in motor circuits is particularly difiicult in the case of fuses having a relatively small voltage rating, e.g. a rating of 250 volts. This is due to the fact that fuses having a relatively small voltage rating comprise relatively compact casings, resulting in a closer proximity between fuse link and casing surface than in case of fuses having a relatively high voltage rating and relatively large casings, and that heat exchange with the ambient atmosphere tends to increase the closer the fuse link structure is arranged to the casing surface.

The following data taken from the Standards for Safety Fuses of the Underwriters Laboratories, Inc., are indicative of the relative size of the casings, and consequently of the relative proximity between casing surface and fuse link structure, in fuses having a voltage rating of 250 volts, and 600 volts, respectively.

It is, therefore, another object of the invention to provide time-lag fuses of the type having link-severing low fusing point metal overlays and adapted for application in motor circuits which fuses are designed for voltage ratings below 600 volts, say 250 volts, and which fuses have compact casings, and in particular casings of sizes standardized in the above referred-to fuse standard of the Underwriters Laboratories, Inc.

Still another object of the invention is to provide timelag fuses having the above referred-to link severing means capable of achieving relatively long blowing delays and including quartz sand as arc-quenching filler.

I have found that the two conditions of using quartz sand as :a highly effective arc-quenching medium and to provide a pulverulent thermally insulating filler which has a considerably smaller thermal conductivity than normally encountered with quartz sand are quite compatible rather than incompatible, as thought heretofore. These two conditions can be met by filling the casings of fuses with a mixture of quartz sand whose constituent particles are coated with a powder having a much smaller thermal conductivity than quartz sand, imparting to the composite filler a much smaller thermal conductivity than pure quartz sand. The mixture between quartz sand, or granular quartz, and an additional pulverulent arc-quenching filler having a substantially smaller thermal conductivity than quartz sand as such should be intimate. The additional arc-quenching filler which may, for instance, be gypsum should have a substantially smaller average particle size than the quartz sand. The intimate mixture of granular quartz and the additional pulverulenrt arc-quenching filler should embed all serially arranged points of reduced cross-section of the ribbon fuse link and the link severing low fusing point metal overlay thereon. On oc currence of major fault currents and formation of series breaks the granular quartz particles, though coated with a gypsum, or other medium having a thermal conductivity of the same order as gypsum, fuse rapidly, thereby absorbing relatively large amounts of heat and generating relatively high are voltages. As long as the fuse structure is intact and performs its normal current-carrying duty the thermal conductivity of the filler is relatively small because many particlcs of quartz sand are coated with, and thermally insulated from, the adjacent particles of quartz sand by layers of gypsum or a like relatively poor conductor of heat.

For a better understanding of the invention reference may be had to the accompanying drawings wherein FIG. 1 is substantially a vertical section of an electric fuse embodying the present invention showing some parts thereof, including the ribbon type fuse -link structure, in elevation rather than in vertical section;

FIG. 2 is a section along 22 of FIG. 1;

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

FIG. 4 is a photomicrograph of pure screened quartz sand; and

FIG. 5 is a photomicrograph of pure screened quartz sand coated with gypsum powder, the magnification of FIG. 5 being the same as that of FIG. 4.

Referring now to the drawings, and more particularly to FIGS. 1-3 thereof, numeral 1 has been applied to indicate a casing of insulating material closed on both ends thereof by terminal plates 2. Each terminal plate '2 supports one of a pair of blade contacts 5. Each blade contact 5 is provided with a pair of prongs adapted to be inserted into a pair of registering aperture-s in terminal plates 4 and to thereafter be upset in nivet fashion, thus firmly securing blade contacts 5 to terminal plates 2. Reference numeral 4 has been applied to indicate the upset ends of the aforementioned prongs of blade contacts 5. Casing 1 is provided with four internal reinforcement ribs lla which are angularly displaced 90 degrees and extend in a direction longitudinally of casing 1. Terminal plates 2 are screwed against the ends of easing .1 by screws 6 projecting axially into reinforcement ribs lla. Preferably asbestos washers (not shown) are interposed between the ends of easing 1 and terminal plates 2. A. pair of multiperforated fuse links L L L interconnects conductively the axially inner surfaces of terminal plates 2. The aforementioned fuse links are substantially channelshaped, each including a web portion L having substantially the same width as that of .blade contacts 5. 'Each web portion L has an axial extension L and the axial extensions L of links L L L flare outwardly toward terminal plates :2 having the axially outer end affixed to terminal plates 2'. Reference numeral .3 has been applied to indicate spot welds between the axially outer ends of link extensions L and terminal plates 2. Each of the channel-shaped fuse links or fusible elements L L L conductively interconnecting terminal plates 2 includes a pair of integral flange portions L each enclosing an angle of less than 90 degrees with the aforementioned Web portion-s L Web portions L and flange portions L are preferably made of an integral sheet of metal having a relatively high conductivity and a relatively high fusing point such as copper or silver. Each fuse link L L L is provided with an overlay 3- of a low-fusing point linksevering metal, preferably tin, or an alloy including tin. Each overlay extends tnansversely across the web portion L and the two flange portions L of each of the fuse links L L L Each fuse link L L L is provided with several, i e. three to five, transverse lines 9 of circular perforations, and with a larger number of longitudinal lines llZ of circular perforations. In other Words, the number of perforations in each transverse line 9 by far exceeds the number of perforations in each longitudinal line 12. Gverlays 8 extend parallel to the transverse center lines 9 of perforations and jut into the spaces defined by contiguous perforations of said lines, Without projecting significantly beyond the center lines of narrowest cross-section of links L L L defined by their respective transverse center line 9 of perforations. Overlays extend over the entire width of fuse links L L L i.e. they are coextensive with the width of the web portion L and with the width of the two flange portions L; of each fuse link. Casing It is filled with a pulverulent arc-quenching filler 11 indicated only in portions of FIGS. l3 and broken awa or deleted, in other portions thereof. \Filler 11 is an intimate mixture of granular quartz and of an additional pulverulent arcquenching filler having a substantially smaller thermal conductivity than granular quartz as such. Filler foil. embeds all of fuse links L L L including all their zones of reduced cross-section formed by their transverse lines of perforations 9 and also their link-severing overlays 8. The aforementioned additional filler has a substantially smaller average particle size than said granular quartz. The additional filler phase of mixed filler :11 is sufficiently high to extend the fusing time of overlays 8 at currents in the order of five times the rated current of the fuse to the order of several seconds, e.g. ten seconds. To achieve this end without increasing the size of casing ll beyond what the standards now prevailing in the United States of America prescribe the percentage of the additional small thermal conductivity filler must be relatively large, e.g. 30 percent, and the composite filler must have a thermal conductivity closer to that of the additional filler than to that of quartz sand as such.

The flange portions L form thermal shields precluding heat generated in web portions L from freely reaching casing l and being readily dissipated. Flange portions L form also shunts, shunting away from web portions L a substantial percentage of the electric current carried by blade contacts 5. Operating as thermal shields flange portions L keep the heat generated by the fuse links close to the axis region of casing l. The axial extensions L, of the fuse link have a smaller cross-section than the axially inner portions L L thereof and their ohmic resistance is consequently relatively high. As a result, axial extensions L form heat dams tending to limit heat exchange in a direction longitudinally of the fuse structure. These heat exchange limiting features of the fuse link structure L L L and its relatively large mass and heat absorbing capacity coupled with the relatively small thermal conductivity of the arc-quenching filler 11 account for the fact that the minimum fusing currents of structures of the type shown in FlGS.-1-3 are relatively small, and the time delays thereof relatively long, e.g. as long as ten seconds at protracted overloads in the order of five times the rated current. The concept of rated current is defined differently in various fuse standards and, therefore, this term should preferably be defined specifically wherever lack of such a definition may result in ambiguity. In this context, wherever the term rated current is used, this term is understood to mean minimum fusing current divided by 1.3, the figure 1.3 being known as the fusing factor. The statement that fuses embodying this invention achieve'time lags in the order of seconds, and as high as ten seconds, on occurrence of overloads in the order of five times the rated current may, therefore, also be expressed in terms of minimum fusing current. This statement then takes the form that fuses embodying this invention achieve time-lags in the order of seconds, and as high as ten seconds, on occurrence of overloads in the order of :13:38 times the minimum fusing current.

On occurrence of relatively small overloads of inadmissible duration the link-severing overlays 8 fuse initially at the radially inner portions L of the composite link structure and sever these portions by a metallurgical reaction. This greatly increases the current density and heating action of the radially outer portions L of the composite link structure, causing the same to be severed by a metallurgical reaction rapidly following that causing the radially inner portions L of the composite link structure to be severed.

On occurrence of major faults series breaks are formed, one at each transverse line 9 of perforations generating a sufficiently high aggregate are voltage to bring the fault current rapidly down to zero. Because of the particular geometry of the composite fuse link structure and its resulting magnetic action, the current density in the web portions L exceeds the current density in the flange portions L Consequently the initiation of the formation of each break begins in the web portion L; of each transverse line 9 of perforations and progresses to the flange portions L thereof. Therefore arc resistance is interposed into the circuit under interruption at a sufficiently small rate to preclude occurrence of dangerous surge voltages incident to interruption of major fault currents.

When quartz sand having a relatively narrow range of particle sizes is being mixed intimately with another nonsilicious arc-quenching filler having a much smaller thermal conductivity than quartz sand and evolving gases under the heat of electric arc, e.g. gypsum, the other filler having also a relatively narrow range of particle sizes, but the size of the particles of the other filler being much smaller than that of the particles of quartz sand, the particles of quartz sand are being coated by the other filler and form lumps the size of which is a multiple of their original size. This is readily apparent from FIGS. 4 and 5. If a relatively small amount of relatively fine gypsum powder is admixed to a relatively large amount of relatively coarse quartz sand, e.g. but 10% in volume of that of the quartz sand, then only a relatively small portion of the quartz grains will be gypsum coated, the balance of the particles of quartz sand remaining uncoated. There is a critical ratio of gypsum powder to quartz sand when about all the particles of the quartz sand are coated with gypsum powder, forming lumps, there being no significant excess of free gypsum powder, i.e. gypsum powder not forming part of a lump whose nucleus is a particle of quartz sand.

I have found that it is most desirable from the viewpoint of minimizing heat exchange between a current carrying fuse link and its surroundings and at the same time maximizing the cooling and deionizing action of the arc-quenching filler on blowing of the fuse to use a mixed filler having a considerably smaller content of gypsum than the above referred to critical ratio. The preferred grain size of quartz sand is 50 to 60 U8. Sieve Number, and the preferred grain size of gypsum powder is in the order of 18 US. Sieve Number. The preferred mixture of such quartz sand and such a gypsum powder is about 30 volume percent of the gypsum powder admixed to about 70 volume percent of such a quartz sand. If it is desirable to obtain a fuse structure having suificient timelag to be applicable in motor circuits, then the additional filler phase, or the gypsum content of the mixture, must be sufiiciently high to extend the fusing time of the linksevering overlay at currents in the order of five times the rated current of the fuse to several seconds, preferably as much as ten seconds. This can readily be achieved with a link geometry of the kind shown in FIGS. 1-3

and an arc-quenching filler being a mixture of about 70% quartz sand and about 30% gypsum powder. An increase of the content of gypsum powder beyond and above 30% of the volume of the content of quartz sand has virtually no effect upon minimum fusing current and rated current, respectively, but results in a significant reduction of arc voltage.

Referring now again to FIGS. l-3 of the drawing, it will be apparent that the function of the low thermal conductivity phase of the filler, e.g. the gypsum phase thereof, is to minimize heat exchange between the surface of casing 1 and the center zone of the fuse link L L L where the transverse center line of perforations 9 and the link-severing overlay S are located. This tends to derate the fuse link L L L calling for the addition of base metal, e.g. an increase in thickness, to restore the rating the fuse link had when rated in quartz sand. As a result, the blowing time at a given multiple of the rated current of the fuse structure is increased. The mere derating of reduction of the minimum fusing current by surrounding the center region of L L L including its link severing overlay 8, with a pulverulent arc-quenching filler having a relatively small thermal conductivity results in a reduction of the minimum fusing current, and hence of the current rating and, therefore, the fuse will blow within a given time at a higher multiple of the minimum fusing current, or the rated current, it had when filled with quartz sand rather than with a specific mixture of quartz sand and a finer arc-quenching filler having a much smaller thermal conductivity than quartz sand. This is but another expression of the fact that the timelag of a fuse having a filler of quartz sand is increased by admixing to the quartz sand another arc-quenching filler having a smaller particle size and a smaller thermal conductivity than quartz sand.

If a fuse structure of the kind illustrated in FIGS. l-3 and described in connection therewith blows on major fault currents, or short-circuit currents, the action of the quartz phase of the mixed filler becomes significant and results in the generation of a relatively high arc-voltage at each point of break, i.e. at the region of each of the transverse lines of perforations 9.

It will be apparent from the foregoing that an intimate mixture of relatively coarse quartz sand and relatively fine gypsum powder forms a more effective thermal barrier between the peak temperature region of fuse link L L L when the latter is carrying current and which is situated in the median plane thereof and the radially juxtaposed region of easing 1 than a filler of quartz sand only. 0n the other hand, a mixture of coarse quartz sand and fine gypsum powder is capable of generating higher are voltages than an arc-quenching filler of gypsum.

It will be apparent to those skilled in the art that various changes and modifications may be made in the fuse structure illustrated and described without departing from the spirit of the invention, as set forth in the appended claims.

I claim as my invention:

1. An electric time-lag fuse comprising in combination:

(a) a pair of spaced terminal elements;

(b) a tubular casing of insulating material closed on both ends thereof by said pair of terminal elements;

(c) a ribbon fuse link of a metal having a relatively high fusing point conductively interconnecting said pair of terminal elements, said fuse link defining a plurality of serially related zones of reduced crosssection;

(d) an overlay on said fuse link of .a metal having a relatively low fusing point arranged in close proximity to one of said plurality of zones of reduced cross-section to cause severance of said fuse link by a metallurgical reaction on reaching by said overlay the fusing point of said low fusing point metal; and

(e) an intimate mixture of granular quartz and of an additional pulverulent arc-quenching filler embedding said plurality of zones of reduced cross-section of said link and embedding said overlay, said addi tional filler having a substantially smaller thermal conductivity than said granular quartz as such, said additional filler further having a substantially smallor average particle size than said granular quartz, and said mixture having a suiiiciently high content of said additional filler to coat the preponderant portion of the particles of said granular quartz and to extend the fusing time of said overlay at currents in the order of five times the rated current the fuse to the order of several seconds.

2. An electric time-lag fuse comprising in combination:

(a) a pair of spaced terminal elements;

(b) a tubular casing of insulating material closed on both ends thereof by said pair of terminal elements; (c) a ribbon fuse link of a metal having a relatively high fusing point conductively interconnecting said pair of terminal elements, said fuse link defining a plurality of serially related zones of reduced crosssection;

((1) an overlay on said fuse link of a metal having a relatively low fusing point arranged in close proximity to one of said plurality of zones of reduced cross-section to cause severance of said fuse link by a metallurgical reaction on reaching by said overlay the fusing point of said low fusing point metal; and

(e) an intimate mixture of granular quartz and of an additional pulverulent arc-quenching filler embedding said plurality of zones of reduced cross-section of said fuse link and said overlay, said additional filler having a substantially smaller thermal conductivity than said granular quartz, said additional filler further having a substantially smaller average particle size than said granular quartz, and the additional filler phase of said mixture occupying a volume Within said casing substantially less than the volume occupied by the granular quartz phase thereof in excess of of the volume occupied by said granular quartz phase thereof.

3. An electric time-lag fuse comprising in combination:

(a) a pair of spaced terminal elements;

(b) a tubular casing of insulating material closed on both ends thereof by said pair of terminal elements;

(c) a ribbon fuse link of a metal having a relatively high fusing point conductively interconnecting said pair of terminal elements, said fuse link defining a plurality of serially related zones of reduced crosssection;

(d) an overlay on said fuse link of a metal having a relatively low fusing point arranged in close proximity to one of said plurality of zones of reduced crosssection to cause severance of said fuse link by a metallurgical reaction on reaching by said overlay the fusing point of said low fusing point metal; and

(e) an intimate mixture of granular quartz and a pulverulent gypsum filler embedding said plurality of zones of reduced cross-section of said fuse link and said overlay, said gypsum filler having a substantially smaller thermal conductivity than said granular quartz, said gypsum filler further having a substantially smaller average particle size than said granular quartz, and gypsum filler of said mixture being in the order of percent of the Volume of said granular quartz.

4. An electric time-lag fuse comprising in combination:

(a) a pair of spaced terminal elements;

(b) a tubular casing of insulating material closed on both ends thereof by said pair of terminal elements;

(c) a ribbon fuse link of a metal having a relatively high fusing point conductively interconnecting said pair of terminal elements, said fuse link defining a plurality of serially related zones of reduced crosssection;

(d) an overlay on said fuse link of a metal having a relatively low fusing point arranged in close proxiinity to one of said plurality of zones of reduced cross-section to cause severance of said fuse link by a metallurgical reaction on reaching by said overlay the fusing point of said low fusing point metal; and

(e) an intimate mixture of quartz sand having a grain size in the order of 50 to 60 US. Sieve Number and of gypsum powder having a grain size in the order of 18 US. Sieve Number embedding said plurality of zones of reduced cross-section of said link and embedding said overlay, and the gypsum powder phase of said mixture occupying a volume within said ca ing about one third as large as the volume occupied by said quartz sand phase thereof.

5. electric time-lag fuse comprising in combination:

(a) a pair of spaced terminal elements;

(b) a tubular casing of insulating material closed on both ends thereof bysaid pair of terminal elements;

(0) a ribbon fuse link of a metal having a relatively high fusing point conductively interconnecting said pair of terminal elements, said fuse link defining a plurality of serially relatedzones of reduced crosssection;

(d) an overlay on said fuse link of a metal having a relatively low fusing point arranged in close proximity to one of said plurality of zones of reduced crosssection to cause severance of said fuse link by a metallurgical reaction on reaching by said overlay the fusing point of said low fusing point metal; and

(e) an intimate mixture of a plurality of pulverulent arc-quenching fillers embedding said plurality of zones of reduced cross-section of said fuse link and embedding said overlay, said mixture comprising a quartz sand and a non-silicious phase of a substance decomposing under the heat of electric arcs having a substantially smaller average particle size and a substantially smaller thermal conductivity than said quartz sand, and the non-silicious phase of said mixture being sufficiently high to coat the preponderant portion of the particles of said quartz sand and to ex tend the fusing time of said overlay at currents in the order of five times the rated current of the fuse to the order of several seconds.

6. An electric time-lag fuse comprising in combination:

(a) a pair of spaced terminal elements;

(b) a tubular casing of insulating material closed on both ends thereof by said pair of terminal elements;

(0) a pair of ribbon fuse link structures inside of said casing conductively interconnecting said pair of terminal elements, said pair of fuse link structures comprising a base metal having a relatively high fusing point and each of said pair of fuse link structures defining a plurality of serially related zones of re duced cross-section, each of said pair of fuse link structures including a first portion arranged relatively close to the longitudinal axis of said casing and each of said pair of fuse link structures further including a second portion arranged generally more remotely from said longitudinal axis and overlapping and shunting said first portion;

(d) an overlay on each of said pair of fuse link structures of a metal having a relatively low fusing point arranged in close proximity to one of said plurality of zones of reduced cross-section to cause severance of each of said pair of fuse link structures by a metallurgical reaction on reaching by said overlay the fusing point of said low fusing point metal; and

(e) an intimate mixture of granular quartz and of an additional pulverulent arc-quenching filler embedding all of said plurality of zones of reduced cross-section and said overlay of each of said pair of fuse link structures, said additional filler having a substantially smaller average particle size than said granular quartz and a substantially smaller thermal conductivity than said granular quartz, the volume of the granular quartz content of said mixture being at least twice about five times the rated current of said fuse to the the volume of said additional filler and the volume of order of ten seconds. said additional filler being sufficiently large to coat 10. An electric time-lag fuse having a predetermined substantially all particles of said granular quartz with rated current and comprising in combination: said additional filler. (a) a pair of spaced terminal elements; 7. An electric time-lag fuse as specified in claim 6 ('o) a tubular casing of insulating material closed on wherein said additional arc-quenching filler is gypsum both ends thereof by said pair of terminal elements;

powder. (c) ribbon fuse link means of a metal having a relatively 8. An electric fuse comprising in combination: high fusing point conductively interconnecting said (a) a pair of spaced terminal elements; 10 pair of terminal elements, said fuse link means having (b) a tubular casing of insulating material closed on both ends thereof by said pair of terminal elements; (0) a pair of ribbon fuse links inside said casing conductively interconnecting said pair of terminal elea plurality of transverse lines of perforations defining a plurality of serially related points of reduced crosssection, and said fuse link means further having a pair of lateral current-carrying fin portions bent out ments, said pair of fuse links comprising a base metal having a relatively high fusing point and each of said pair of fuse links defining a plurality of serially re lated zones of reduced cross-section, each of said pair of fuse links being substantially channel-shaped and additional filler having a substantially smaller average particle size than said granular quartz and a substantially smaller thermal conductivity than said of the general plane defined by said fuse link means and enclosing a predetermined acute angle with said general plane;

(d) overlay means on said fuse link means having a relatively low fusing point arranged in close proximity each of said pair of fuse links including a relatively to one of said transverse lines of perforations to cause Wide web portion and a pair of relatively narrow inteseverance of said fuse link means by a metallurgical gral flange portions each enclosing an angle of less reaction on reaching by said overlay the fusing point than 90 degrees with said web portion thereof; of said low fusing point metal; and

(d) an overlay on each of said pair of fuse links of a (e) an intimate mixture of granular quartz and of an metal having a relatively low fusing point extending additional pulverulent :arc-quenching filler inside said transversely across said Web portion and across said casing embedding all of said plurality of lines of pair of flange portions of each of said pair of fuse transverse perforations of said fuse link means and links, to cause severance of each of said pair of fuse said overlay means thereof, said additional filler havlinks by a metallurgical reaction on reaching by said ing a substantially smaller thermal conductivity than overlay the fusing point of said low fusing point said granular quartz as such, said additional filler metal; and having a substantially smaller average particle size (e) an intimate mixture of granular quartz and of an than said granular quartz, and said additional filler additional pulverulent arc-quenching filler embedding being present in sufiiciently high proportion to coat all of said plurality of zones of reduced cross-section substantially all the grains of said quartz thereby and said overlay of each of said pair of fuse links, said 3 5 substantially increasing the size thereof and to extend the fusing time of said overlay on currents of about five times said rated current of said fuse to the order of ten seconds.

granular quartz, the volume of the granular quartz H. An electric time-lag fuse having a predetermined content of said mixture being at least twice the volume rated current and comprising in combination: of said additional filler and the volume of said addi- (a) a pair of spaced terminal elements; tional filler being sufficiently large to coat substantial- (b) a tubular casing of insulating material closed on ly all particles of said granular quartz with said adboth ends thereof by said pair of terminal elements; ditional filler. (c) a pair of ribbon fuse link means of a metal having 9. An electric time-lag fuse having a predetermined a relatively high fusing point arranged inside said rated current and comprising in combination: casing and conductively interconnecting said pair (a) a pair of spaced terminal elements; of terminal elements, each of said pair of fuse link (b) a tubular casing of insulating material closed on means having a plurality of transverse lines of perboth ends thereof by said pair of terminal elements; forations defining a plurality of serially related points (c) a pair of ribbon fuse links of a metal having a 59 relatively high fusing point conductively interconnecting said pair of terminal elements, said pair of fuse links comprising a base metal having a relatively high fusing point and each of said pair of fuse links having a plurality of transverse lines of perforations,

of reduced cross-section, and each of said pair of fuse link means being bent transversely to define a cavity extending in a direction longitudinally of said casing limiting heat exchange between said fuse link means and a medium surrounding said casing;

(d) a pair of overlays each on one of said pair of fuse each of said plurality of fuse links being substantially li k ean aid air of overlays being made of a channel-shaped and each of said pair of fuse links inmetal having a relatively low fusing point and being eluding a relatively wide Web portion and a pair of arranged in close proximity to one of said plurality relatively narrow flange portions each enclosing an of transverse lines of perforations of one of said pair acute angle with Said Web Portion ihfileof; 0 of fuse link means to cause severance thereof by a (d) a pair of overlays each on one of said pair of fuse metallurgical reaction on reaching by said pair of links each of a metal having a lelfltivel 10W f g overlays the fusing point of said low fusing point point and each arranged in close proximity to one of m tal; and said plurality of transverse perforations to cause a intimate mixture of granular quartz sand and severance of said pair of fuse links by a metallurgical of an additional pulverulent arc-quenching filler reaction on reaching by said pair of overlays the within said casing inside and outside of said cavity f g Point Of Said 10W fusing Point metal; and defined by said fuse link means, said additional filler (e) an intimate mixture of granular quartz and of an having a substantially smaller thermal conductivity additional pulverulent arc-quenching filler embedding than aid granular quartz as such, said additional said plurality of transverse lines of perforation of said fill r having a ub tantially maller average particle pair of fuse links and said pair of overlays, said addisize than said granular quartz and said additional tional filler having a substantially smaller thermal filler being present in sufficiently high proportion to conductivity than said granular quartz as such and coat the grains of said quartz thereby substantially being present in a sufiiciently high proportion to exincreasing the size thereof and to extend the fusing tend the fusing time of said overlay at currents of time of said overlay on currents of about five times l l l 2 said rated current of said fuse to the order of ten (e) an intimate mixture of granular quartz and of an seconds. additional pulverulent araquenching filler embedding 12. An electric time-lag fuse comprising in combinasaid plurality of zones of reduced cross-section and tion: embedding said overlay and separating said overlay (a) a pair of spaced terminal elements; 5 from the radially juxtaposed region of said casing, (b) a tubular casing of insulating material closed on and said additional filler having a substantially both ends thereof by said pair of terminal elements; smaller average particle size and a substantially (c) a unitary fuse link of a metal having a relatively smaller thermal conductivity than said granular high fusing point inside said casing extending from quartz.

one of said pair of terminal elements across the me- 10 Rate enees C'ted la the Exarn'ner dian plane of said casing to the other of said pair of T E y )1 terminal elements, said link defining a plurality of UNITED STATES PATENTS serially related zones of reduced cross-sect on and 2,223,959 12/40 Lohausen 200-435 establishing a peak temperature region situated sub- 2 592,399 4/ 52 Ed ll t 1, 200-135 stantially at the median plane of said link; 15 2,599,646 6/52 Kozacka 200-120 (d) an overlay on said link of a metal having a rela- 3,123,693 3/64 Kozacka 260-120 tively low fusing point arranged in close proximity to said median plane of said link to cause severance POREKGN PATENTS of said link by a metallurgical reaction on reaching 392,258 5/33 Great Britain.

by said overlay the fusing point of said low fusing 29 i t t l; d BERNARD A. G-ILHEANY, Primary Examiner.

Claims (1)

1. AN ELECTRIC TIME-LAG FUSE COMPRISING IN COMBINATION: (A) A PAIR OF SPACED TERMINAL ELEMENTS; (B) A TUBULAR CASING OF INSULATING MATERIAL CLOSED ON BOTH ENDS THEREOF BY SID PAIR OF TERMINAL ELEMENTS; (C) A RIBBON FUSE LINK OF A METAL HAVING A RELATIVELY HIGH FUSING POINT CONDUCTIVELY INTERCONNECTING SAID PAIR OF TERMINAL ELEMENTS, SAID FUSE LINK DEFINING A PLURALITY OF SERIALLY RELATED ZONES OF REDUCED CROSSSECTION; (D) AN OVERLAY ON SAID FUSE LINK OF A METAL HAVING A RELATIVELY LOW FUSING POINT ARRANGED INCLOSE PROXIMITY TO ONE OF SAID PLURALITY OF ZONES OF REDUCED CROSS-SECTION TO CAUSE SEVERANCE OF SAID FUSE LINK BY A METALLURGICAL REACTION ON REACHING BY SAID OVERLAY THE FUSING POINT OF SAID LOW FUSING POINT METAL; AND (E) AN INTIMATE MIXTURE OF GRANULAR QUARTZ AND OF AN ADDITIONAL PULVERULENT ARC-QUENCHING FILTER EMBEDDING SAID PLURALITY OF ZONES OF REDUCED CROSS-SECTION

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