US2960589A

US2960589A - Electric fuses

Info

Publication number: US2960589A
Application number: US790126A
Authority: US
Inventors: Salzer Erwin
Original assignee: Chase Shawmut Co
Current assignee: Chase Shawmut Co
Priority date: 1959-01-30
Filing date: 1959-01-30
Publication date: 1960-11-15
Anticipated expiration: 1977-11-15

Description

Nov. l5, 1960 E. sALzl-:R

ELECTRIC FUSES Filed Jan. 30, 1959 2 Sheets-Shea*I l /ombien temperature level INVENTOR.

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Nov. l5, 1960 E. sALzER 2,960,589

ELECTRIC FUSES Filed Jan. 30, 1959 2 Sheets-Sheet 2 limits of plOMl zNvENTOR.

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ELECTRIC FUSES Erwin Salzer, Wahan, Mass., assignor to The Chase- Shawmut Company, Newburyport, Mass.

Filed Jan. 30, 1959, Ser. No. 790,126

9 Claims. (Cl. 200-131) This invention has reference to electric fuses, and more particularly to electric fuses with improved small overload current interrupting performance.

Electric fuses interrupt best inside of a predetermined range of currents. Outside that range the interrupting performance of electric fuses tends to be less than required to meet specifications. The design of fuse structures whose interrupting performance is only satisfactory on occurence of major fault currents, or short-circuit currents, but unsatisfactory on occurrence of small protracted overload currents is readily feasible and does not involve particular difficulties. Nor are particular diiiiculties involved in the design of fuse structures whose interrupting performance is only satisfactory on occurrence of small protracted overload currents but unsatisfactory on the occurrence of major fault currents or shortcircuit currents. Where it is desired to achieve an interrupting performance which is satisfactory over the entire overcurrent range from small protracted overload currents to and including major fault currents, or shortcircuit currents, both designs of fuse structures may be serially connected and integrated into one single composite unit. Such composite units are, however, relatively complex, diicult to manufacture and expensive.

It is, therefore, one object of this invention to provide relatively simple electric fuses which are relatively easy to manufacture and are relatively inexpensive and are capable of interrupting satisfactorily the smallest overload currents as well as the largest fault currents, or shortcircuit currents, which may occur in the circuit for which the fuses are designed and intended.

The fusible elements or fuse links of fuses intended to interrupt relatively large fault currents are generally provided with a plurality of serially related zones of reduced cross-section. These zones of reduced cross-section form serially related multibreaks on occurrence of major fault currents, or short circuit currents. The aggregate arc voltage along such multibreaks is sufficiently high to rapidly reduce any major fault current, or short circuit current, to Zero. On occurrence of relatively small protracted overload currents the fusible element or fuse link forms but one single break generally situated at the point where the temperature is highest, which point is generally identical with the point of reduced cross-section situated at, or close to, the center of the fusible element, or fuse link. Where the circuit voltage is relatively high, the arc voltage generated at this single break is relatively small compared to the circuit voltage. Therefore the arc persists and causes back-burning of the fusible element, or fuse link. Back-burning and concomitant arc elongation tend to progressively increase the arc voltage at the point of break. A good deal of that potential gain or increase in arc voltage is, however, lost because as long as the arc persists, the arc path is heated by electric energy which continues to pour from the electric system into the fuse, thus heating the arc path, thereby lowering the voltage across the arc formed inside the fuse. This tendency lof the arc voltage to decay with prolonged arcing may Mice reduce to such an extent the tendency of the arc voltage to increase with progressive back-burning of the fusible element and concomitant arc elongation as to preclude, in extreme cases, the fuse from interrupting the circuit.

It is, therefore, another object of the invention to provide electric fuses which are not subject to the above limitations and drawbacks, which are capable of generating sufficiently high arc voltages on occurrence of relatively small protracted overload currents to rapidly interrupt such currents, and which are not subject to excessive arc voltage decay resulting from excessive arcing times.

When an electric fuse blown on occurrence of a small protracted overload current is unable to generate an arc voltage suiiiciently high for a suciently long time to bring the current down to zero, the fusible element burns back to the terminal elements or terminal caps of the fuse and then into these elements or caps. The arc may burn right through the terminal elements or terminal caps and form a considerable fire hazard when burning outside of the fuse enclosure in an atmosphere rich in metal vapors. If the terminal elements are of relatively heavy construction, the arc may not be able to burn through them, but the metal vapors formed inside the fuse enclosure as a result of prolonged arcing may reach such a pressure as to result in a volent explosion.

Since the arc is normally initiated in the center region of the fusible element, arc duration is longest in the center region of the fusible element. As a result the diameter of the fulgurite formed by fusion of an arcquenching quartz ller under the heat of the arc is largest in the center region of the fusible element. The continued radial growth of the fulgurite in the center region of the fusible element may result in immediate physical engagement between the hot fulgurite and the fuse tube or casing. The heat-shock to which the fuse tube or casing may thus be subjected may shatter or break the latter. Fuse tubes of synthetic-resin-glass-cloth laminates are not shattered or broken by the action of hot fulgurites, but fuse tubes of such materials are charred by hot fulgurites and then evolve gases trapped inside the enclosed fuse space whose pressure may increase to the point where it causes an explosion of the fuse.

It is, therefore, another object of this invention to provide electric fuses which are free from all the aforementioned limitations and drawbacks resulting from unsatisfactory low current interrupting performance.

As long as the circuit voltage is relatively 10W, say less than 200 volts, the single break formed in the center of a fusible element or fuse link incident to occurrence of relatively small protracted overload currents generates an arc voltage capable of extinguishing low-current arcs sufficiently fast before any harm is being done. Low overload current interruption becomes a problem when the circuit voltage is relatively high. At circuit voltages above 600 volts up to one or several thousands of volts the problem becomes one of significant proportions. The higher the voltage rating and the current rating of a particular fuse, the more difficult it is to interrupt small overload currents at the single break formed in the center region of the fusible element or fuse link.

It is, therefore, an important object of this invention to provide electric fuses for voltages exceeding 600 volts, and particularly high voltage fuses having relatively low voltage ratings, capable of interrupting protracted low overload currents sufficiently rapidly to preclude any harm to be done by excessive arcing.

The energy dissipated in a fuse while interrupting an electric current may be expressed by the term wherein z' are the instantaneous values of the current which ows through the fuse, e are the instantaneous Values of the voltage across the fuse, t1 is the time at which the overload current starts and t2 is the time at which the circuit interruption is completed by the operation of the fuse. The larger the smaller the ability of the fuse to operate selectively.

It is, therefore, another object of this invention to provide electric fuses tending to minimize the term on interruption of relatively small overload currents to thereby achieve selectivity in electric systems which comprise serially connected fuses.

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 description thereof taken in connection with the accompanying drawings, in which:

Fig. 1 is a longitudinal section of an electric fuse embodying this invention taken along 1-1 o-f Fig. 2;

Fig. 2 is a longitudinal section of the fuse shown in Fig. l taken along 2 2 of Fig. l;

Fig. 3 is a top plan view of a prior art ribbon-type fusible element and shows also in a general way the temperature distribution along such an element when immersed in a pulverulent arc quenching ller;

Fig. 4 is a hypothetical temperature distribution curve showing the temperature distribution along a fusible element this invention aims to achieve;

Fig. 5 shows per se the fusible element or fuse link comprised in the structure of Figs. l and 2 and the temperature distribution along that element when the latter lforms an integral part of the fuse;

Fig. 5a shows on a larger scale a portion of the fuse link of Fig. 5; and v Fig. 6 shows two modifications of the fusible element of Figs. l, 2 and 5 having a different geometry but operating in a similar way, and Fig. 6 shows also the temlperature distribution along these two elements when the latter form an integral part of a fuse.

It is possible to distinguish between two types of high voltage fuses, namely fuses having fusible elements whose length greatly exceeds the length of the fuse tube or casing and which are generally wound around a mandrel, and fuses having fusible elements whose length is substantially equal to that of the fuse tube or casing forming a direct connection between the terminal elements provided at the ends of the fuse tube or casing. This invention is more particularly intended to be applied to fuses of the latter type.

This invention is predicated on an investigation of heat ow and temperature distribution in electric fuses. This subject is of tremendous complexity and seems to defy a realistic mathematical treatment. The data given below are mainly of an empirical or experimental nature and were obtained by various methods, including infrared photography, use of so-called thermocolors, i.e. colors which change when a critical temperature is being reached, and so-called low temperature pyrometry.

Referring now to Figs. l and 2, the electric fuse embodying this invention comprises a fusible element or fuse link 1 made of a metal having a relatively high conductivity and a relatively low heat of fusion. Silver is the metal best complying with these requirements. For some applications copper complies suciently well with these requirements. Both silver and copper are metals having a relatively high fusing point, the fusing point of the former being 960.5 deg. C. and the fusing point of the latter 1083 deg. C. The heat of fusion of silver is 25 CaL/grarn and the heat of fusion or copper is 50 cal./ gram. Fuse link 1 is of the ribbon type and provided with a plurality of circular perforations 1a which are spaced in a direction longitudinally of fuse link i. Fuse link 1 is further provided with a pair of lateral incisions 1b adjacent the axially outer ends thereof defining two zones of restricted cross-section. The ends of fuse link 1 are threaded through asbestos washers 3 and metal washers 3' and clamped between the outer surface of a fuse tube or casing 4 and the inner surface of terminal elements or terminal caps 2. The inside of fuse tube or casing 4 contains a pulverulent arc-quenching filler 7, such as quartz sand, in which the fuse link 1 is submersed.

Referring now to Fig. 3, the fuse link l' shown therein has a plurality of perforations la' arranged in a similar fashion as the perforations of the fuse link of Figs. l and 2. A fuse link having a plurality of serially related zones of reduced cross-section to establish multibreaks on occurrence of major fault currents, or short-circuit currents, has generally a temperature distribution such as that indicated in Fig. 3 by the reference character T. A ternperature peak is formed at each point of restricted crosssection. The temperature peaks are highest near the center of the fuse link and decrease progressively toward the outer ends thereof. The curve E is the envelope of the peak temperatures.

Curve T represents the temperature distribution under stationary conditions, i.e. for a given current, e.g. the rated current, when there is a balance between the heat generated in the fuse link and the heat dissipated by conduction, convection and radiation. On occurrence of major fault currents, or short-circuit currents, the temperature at each zone of restricted cross-section starts to rise at a certain rate of rise, depending primarily upon the magnitude of the fault current or short-circuit current. Assuming that all zones of restricted cross-section have the same geometry, and that heat exchange phenomena may be neglected on account of the shortness of the interval of time between fault inception and thermal destruction of the fuse link, the rate of rise of ternperature will be the same at all zones of restricted crosssection. The center zone of restricted cross-section having the highest initial temperature will first reach the fusing point of the metal of which the link is made. Thereafter the two zones of restricted cross-section immediately adjacent to the center of the fuse link will reach the fusing point of the metal of which the fuse link is made. The two axially outer zones of restricted cross-section having the lowest starting temperature will be the last to reach the fusing point of the metal of which the fuse link is made. The sequence of fusion at the various zones of restricted cross-section is very rapid. Fusion occurs at the zones which fuse after the center fuses shortly after the center fuses. The time intervals elapsing between fusion at the center, fusion at the two zones of reduced cross-section immediately adjacent to the center, and ultimate fusion at the two zones of reduced cross-section adjacent to the ends of the link are very short. The rapidity of the sequence in which arcs are being kindled at the various zones of reduced cross-section accounts for the formation of series breaks along the fuse link.

Considering now the performance of the same structure at the occurrence of small protracted overload currents, say protracted overload currents in the order of 3 times the rated current, heat exchange phenomena cannot be neglected any longer because the times involved in the rise from pre-existing temperatures to the temperature at which the metallic current path through the link is severed by fusion and arcing are relatively long. Since heat dissipation or cooling is less adjacent the center of the fuse link than at the axially outer ends thereof the rate of rise of temperature at the center perforation will be highest. Initial fusion will occur at this point while the ti other points are still at temperatures way below the fusing point of the metal of which the fuse link is made. It is the formation of one single break slowly expanding in axial direction rather than formation of multibreaks which accounts for the difficulties encountered in interruption of small protracted overload currents.

Fuse links are sometimes provided with a mass of a low fusing point metal which absorbs heat and tends to delay interruption on occurrence of relatively small protracted overload currents. Thus the fuse link 1' shown in Fig. 3 may be provided with a tin rivet which is inserted into the center perforation thereof. The presence of such a rivet has no substantial effect on the distribution of temperature as long as the conditions under consideration are of a stationary nature. Considering now transient conditions, e.g. an increase from the rated current initially carried by the link to three times the rated current. Due to the heat absorbing capacity of the aforementioned rivet, or an equivalent thereof, the rate of rise of temperature at or adjacent the center of the link will be less than in the absence of the rivet, or its equivalent. When the tin rivet fuses the base metal of which the link is made is corroded by the formation of tin alloys having a high resistivity, resulting in interruption of the metallic current path through the fuse link. While the presence of a heat absorbing mass at the center of the link results in a relative decrease of rate of rise of temperature, the presence of a mass of tin or the like corrosive metal reduces also the temperature required for interrupting the current path through the link. The latter fact is conducive to an interruption of the current path formed by the link at the point thereof where the lower fusing point mass is located before any other point of the link reaches the fusing point of the base metal, provided that the current carried by the link is less than a critical current intensity It. Current It may be referred to as the transfer current for reasons which will be shown below more in detail. In other words, for overload currents less than the critical transfer current It a single break is formed near the center of the link where the link-corroding timelag mass is arranged, and a limited arc voltage generated yat this point may not be sufficient to achieve a satisfactory interruption of the overload current. At currents in excess of the critical transfer current It the rate of rise of temperature at two axially outer zones of restricted cross-section is so high that the fusing temperature of the high fusing point link metal is reached at these points before the low fusing point mass at the center of the link reaches its respective fusing point. Therefore the point of arc-initiation shifts from the center of the link to two axially outer zones of restricted or reduced cross-section. (This explains the term transfer current which has been used above.) Hence there is a tendency for currents exceeding the transfer current toward formation of two points of break in series. The arc voltage generated at these two points of break may, or may not, be sufficient to eect a satisfactory interruption of the circuit.

Bepending on the particular way in which the centernear mass of low fusing point metal is arranged on the fuse link, the aforementioned mass may effect a relative decrease of the rate of rise of temperature not only at one but at a plurality of serially related zones of reduced cross-section. Assuming, for instance, that the fuse link shown in Fig. 3 is provided with an overlay of tin extending into the space formed between the center perforation la and another perforation immediately adjacent thereto. This will, to some extent, delay the rate of rise of temperature at both perforations. The arrangement `of the overlay may be such as to be closer to the center perforation than to the perforation next to the center perforation `and thus to effect a greater relative decrease of the .rate of rise of temperature adjacent the first mentioned perforation than adjacent the last mentioned perforation. If the rate of rise of temperature adjacent two contiguous perforations is affected by the presence of a heat absorbing mass of a metal having a relatively low fusing point, the point of arc initiation may shift from one perforation to another as the fault current is being incr-eased and eventually shift to the region of the perforations on the axially outer end of the link not affected by the delaying action of the lag means situated near the centerof the fuse link. Such a shift of the point of arc initiation toward one end of the link may be particularly dangerous because this tends to minimize the backburning distance available between the point of arc initiation the terminal element or terminal cap immediately adjacent thereto.

It is thus apparent that neither fuses comprising the multi-perforated prior art links shown in Fig. 3 nor fuses comprising prior art modifications of such fuse links involving low fusing point metals operate entirely satisfactory. As mentioned above, the limitations to which prior art fuses are subjected are more severe with increasing voltage ratings and current ratings.

Fig. 4 illustrates a hypothetical static temperature distribution along a fuse link comprising a plurality of serially related zones of reduced cross-section. All the peaks of temperature distribution curve T are situated in a horizontal line E which forms the envelope of the ternperature distribution curve T. A temperature distribution curve of the character shown in Fig. 4 will be obtained where the heat flow in a direction longitudinally of the link is negligibly small in comparison to the radial heat flow. This condition would prevail if the length of the link were infinite and the zones of restricted crosssection where peak temperatures are established were situated in the center region of the link. in such a fuse the rate of rise in temperature at each point of reduced cross-section of the fuse link depends only upon the current carried by the link, assuming all points of reduced cross-section to be identical. On occurrence of transient conditions the envelope E of the temperature peaks would be displaced parallel to itself, irrespective of whether' the transient current is relatively high, or relatively low. Hence all points of the link would simultaneously reach the fusing point of the metal of which the link is made, and breaks simultaneously formed at all zones of restricted cross-section of the link irrespective of whether the transient current is relatively high, or relatively low.

The next step is to impart such a geometrical configuration to a real fuse link that its temperature distribution comes sufficiently close to the hypothetical or idealized temperature distribution of Fig. 4.

Figs. 5 and 6 indicate how this can be achieved. The fusible eiernent or fuse link l shown in Fig. 5 comprises eleven equidistantly arranged circular perforations 1a defining eleven zones of reduced cross-section. Each of the axially outer ends of link l is provided with a pair of lateral incisions fb leaving between them a very narrow neck portion, say 1/5 of the normal width of the link, i.e. the width thereof at points where its crosssection is not restricted. The ohmic resistance of such a neck portion is proportional to its length. The incisions or slits fb are as narrow as possible to minimize the ohmic resistance of the necks or points of reduced cross-section which are being formed by them. A point or zone of reduced cross-section having such a geometrical configuration is not capable of generating a relatively large arc voltage upon fusion thereof, but it is not the object of the necks formed between incisions or slits 1b to generate arc voltage upon fusion thereof. The object of the points of reduced cross-section or necks adjacent the axially outer ends of the link is to contain the heat generated in the link at the portions thereof situated relatively near to its center, and thus to atten the apex portion of the peak temperature envelope E. In other words, incisions or slits 1b are heat dams minimizing the dow of heat from the axially inner portions of the link to the ends thereof. The length of the portions L of the link situated between its ends and the heat-damforming incisions 1b exceeds substantially the spacing 1 between adjacent circular perfo-rations la. The axially outer ends of link 1 are substantially at ambient temperature. The temperature gradient becomes abruptly steep in the region of heat-dam-forming incisions lib. The envelope of the peak temperatures shown in Fig. comes much closer to a line parallel to the axis of abscissa than the envelope E of peak temperatures shown in Fig. 3. In other words, due to the presence of heat dams 1b the radius of curvature of the envelope E shown in Fig. 5 is considerably larger than the radius of curvature of the envelope E shown in Fig 3. In Figs. 3 and 5 the difference in temperature between the highest peak and the peak next to the highest peak has been indicated by Atl and All', respectively, and the difference in temperature between the highest peak and the peak two steps lower than the highest peak have been indicated by Atz and Atz, respectively. It is apparent that Ar1 At1, and that A2I A2 Under certain conditions the heat dam means lb provided in Fig. 5 combined with the relatively long nonperforated end portions of the link suiice to obtain a practically sufficient approximation to the at top envelope characteristic of Fig. 4 to achieve a satisfactory interruption over the entire current range, including the smallest overcurrents, and the largest short-circuit currents, which the fuse is supposed to interrupt.

Referring now to Fig. 5a, letter p has been applied to indicate the width and letter m has been applied to indicate the length of the neck or zone of reduced crosssection defined by the two lateral incisions lb. Reference letter q has been applied to indicate the width and reference letter n has been applied to indicate the length of a zone of reduced cross-section defined by a circular perforation la. It is apparent that P 1 and that man Where it is desired to obtain an even more perfect attening of the apex or top of the envelope E this may be achieved by designing the fuse link in the ways indicated in Fig. 6. In Fig. 6 two different links have been shown which are based on the same principle and achieve the same end. The principle involved consists in progressively decreasing the cross-section of the zones of reduced cross-section from the center of the fuse link toward the axially outer ends thereof. One of the links shown in Fig. 6 comprises, in addition to the heat dam means' 1b, eleven circular perforations la increasing in size from the center of the link toward the axially outer ends thereof forming zones of reduced cross-section decreasing in size from the center of the link toward the axially outer ends thereof. The link is of uniform Width throughout its length, except where the points of reduced cross-section la and lb are located. The other of the links shown in Fig. 6 comprises, in addition to the heat dam means 1b, eleven circular perforations of identical size. The width of the link decreases pnogressively from the center thereof toward its axially outer ends and thus the cross-section of the points of reduced cross-section formed by perforations 1a increases progressively from the axially outer ends of link l toward its center, where the cross-section of the point of reduced cross-section of the link is largest. The curvature of the envelope E of Fig. 6 is less than the curvature of envelopes E in Fig-s. 5 and 3. in Fig. 6 the difference in temperature between the highest peak and the peak next to the highest peak has been indicated by ^.t1 and the difference in temperature between the highest peak and the peak two steps lower has been indicated by hr2. It is apparent that At1" nt1; that At2" At2; and that At1" At1 and that AI2 AI2.

Figs. 5 and 6 have been drawn to better illustrate the invention and the latter will take various dierent forms depending upon the voltage rating and the current rating intended to be given to a particular piece of equipment. The significance of the basic link geometry shown in Figs. 5 and 6 will become more apparent when considering the application of this geometry to fuses having a relatively small voltage rating on the one hand, and to fuses having a relatively high voltage rating, on the other hand.

Fuses designed to have a relatively small voltage rating have fuse links of relatively short length and the number of perforations or points of reduced cross-section along a link of relatively short length is generally relatively small. The fuse links shown in Figs. l, 2, 5 and 6 are links for fuses having a voltage rating of a few kilovolts. Such a voltage rating calls for relatively short fuse links provided with a relatively small number of points of reduced cross-section. In the arrangement shown in Figs. 5 and 6 three or more zones of reduced cross-section in the center of the link form series breaks on occurrence of relatively small overload currents of inadmissible duration, whereas all the Zones of restricted cross-section la and 1b form series breaks on occurrence of major fault currents, or short-circuit currents.

Manufacturing tolerances may be responsible for the formation of fewer breaks on occurrence of small overload currents of inadmissible duration than the number of breaks expected and desired to be formed under such operating conditions. Even with one or more potential points of break failing on account of manufacturing tolerances, the low overcurrent interrupting performance of fuses embodying the invention is greatly improved in comparison to that of prior art fuses. This improvement i-s due to the relative flatness of the peak temperature envelope E. Because of this feature the temperature of the link adjacent each initial point of break is' relatively close to the fusing temperature, and thus but little additional energy has to be supplied by the circuit to achieve rapid back-burning from any point of initial break. In other words, back-burning from any point of initial break occurs at a relatively high speed, and thus the arcing time and the arc energy are significantly reduced. In fuses intended for relatively high current ratings a number of links of the general character illustrated in Figs. 5 and 6 is connected in parallel into an electric circuit. The larger the number of fuse links connected in parallel into an electric circuit, the less significant an accidental failure of a break to form. Fuses comprising a relatively large number of fuse links connected in parallel into an electric circuit lend themselves also particularly well to interrupting relatively small protracted overloads' because of the relatively high current owing in the last fuse link to melt and arc.

Considering now fuses for relatively high circuit voltages having relatively long links provided With a relatively large number of points of reduced cross-section, in such fuses it is easier tov produce a section of considerable length including a considerable number of points of reduced cross-section which will reach fusing temperature substantially at the same time and form a sufficient number of series breaks on occurrence of relatively small overcurrents of inadmissible duration.

On occurrence of major fault currents the axially outer points ib of reduced cross-section form breaks before any of the axially inner points 1a of reduced cross-section fuse. ri'he arc voltage at these two points 1b of break is generally insignificant in comparison to the circuit voltage. Axially inner points of break are formed before any substantial burn-back can occur at the two breaks initially formed at the axially outer ends of the fuse link.

At the occurrence of relatively small overloads of inadmissible duration a plurality of breaks is formed in rapid sequence adjacent the center region of the fuse link. Burnback from any point of break occurs toward regions of the fusible element which are at a relatively high temperature, and therefore burnback and concomitant generation of arc voltage occurs at a rapid rate, tending to reduce both arcing time and arc energy.

assassin If any point of the fuse link is provided with a corrosive low fusing point metal overlay means for initiating blowing at lower temperatures than the fusing point of the base metal of which the link is made, then such means must extend over a significant portion of the link to produce formation of several breaks in rapid sequence on occurrence of small protracted overload currents. The corrosive means for initiating blowing may be formed by an overlay, preferably one produced by plating, occupying the space between at least two perforations 1a. Tin plating about lAOOO" thick on fuse links of silver is generaily satisfactory. As indicated in Fig. 6 the plating, or other overlay as, for instance, one produced by hot metal spraying, should cover a sufliciently long portion of the axially inner region of fusible element 1 to cause formation of series breaks on occurrence of relatively small protracted overload currents, yet said plating or other overlay ought to be limited to the axially inner region of element 1, and extend only to points thereof spaced from lateral incisions 1b. In other words, the necks or heat dams formed by incisions 1b ought to be bare and remain intact under overcurrent conditions until they reach the fusing point of the base metal of the fuse link. This precludes on occurrence of small protracted overcurrents initial melting and arc formation at the necks defined by incisions lb which would be highly undesirable. At the occurrence of major fault currents initial melting and arc formation occur at the necks formed by incisions 1b, but there is no harm to that because additional series breaks are then formed in rapid sequence, immediately relieving the breaking duty imposed on the two points of initial melting and arcing.

While this invention is applicable to both fuses with fusible wire elements and fuses with fusible ribbon elements, the prefer-red embodiments thereof are fuses including fusible ribbon elements.

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

I claim:

l. An electric fuse including a pair of spaced terminal elements, a fusible current-carrying element of a metal having a relatively high conductivity interconnecting said pair of terminal elements, said element comprising an axially inner region wherein said element forms a plurality of zones of restricted cross-section spaced a predetermined distance from each other in a direction longitudinally of said element, said element further comprising axially outer regions, a pair of relatively short separating zones more restricted in cross-section than any of said plurality of zones of restricted cross-section separating said axially inner region from said axially outer regions, and the shortest distance between each of said pair of terminal elements and one of said pair of separating Zones adjacent thereto substantially exceeding said predetermined distance.

2. An electric fuse including a pair of terminal elements, a fusible current-carrying element of a metal having a relatively high fusing point conductively interconnecting said pair of terminal elements, said element comprising an axially inner region wherein said element forms a plurality of Zones of restricted cross-section having a predetermined spacing in a direction longitudinally of said element, said element further comprising axially outer regions, a pair of heat dam means separating said axially inner region from said axially outer regions to minimize heat exchange between said axially inner region and said axially outer regions, the shortest distance between each of said pair of terminal elements and one of said pair of heat dam means immediately adjacent thereto substantially exceeding said predetermined spacing, and metal means having a relatively low fusing point supported by said axially inner region and adapted to form lil series breaks on said axially inner region when said metal means reach said relatively low fusing point.

3. An electric fuse having a predetermined current rating and including a casing of a synthetic-resin-glass-cloth laminate, a pulverulent quartz filler inside said casing, a pair of terminal elements closing the ends of said casing, a fusible current-carrying element of a metal having a relatively high conductivity and a relatively low heat of fusion conductively interconnecting said pair of terminal elements, said element comprising an axially inner region wherein said element forms a plurality of Zones of restricted cross-section spaced in a direction longitudinally thereof, each of said plurality of zones having a predetermined minimum cross-section and a predetermined length, said element further comprising axially outer regions, and a pair of Zones of restricted cross-section separating said axially inner region from said axially outer regions, each of saidpair of Zones having a minimum cross-section less than said predetermined minimum cross-section, and the spacing between each of said pair of zones and the end of said element immediately adjacent thereto being a multiple of the spacing between each of said plurality of zones.

4. An electric fuse including a pair of terminal elements, a ribbon of silver conductively interconnecting said pair of terminal elements, said ribbon comprising an axially inner region wherein said ribbon is provided with a plurality of perforations forming a plurality of zones of restricted cross-section spaced equidistantly in a direction longitudinally of said ribbon a predetermined distance from each other, each of said plurality of zones of restricted cross-section having a predetermined minimum cross-section and a predetermined length, said ribbon further comprising axially outer regions, a pair of separating zones of restricted cross-section each having a minimum cross-section less than said predetermined minimum cross-section and each having a length less than said predetermined length separating said axially inner region from said axially outer regions, the shortest distance between each of said pair of terminal elements and one of said pair of separating zones adjacent thereto substantially exceeding said predetermined distance, and overlay means on said element of a metal having a lower fusing point than the fusing point of silver adapted upon fusion thereof to corrode the silver of which said element is made, said overlay means extending along a suciently long portion of said axially inner region to cause formation of series breaks on occurrence of relatively small overcurrents.

5. An electric fuse including a pair of terminal elements, a fusible element of a metal having a relatively high conductivity and a relatively low heat of fusion conductively interconnecting said pair of terminal elements, said element comprising an axially inner region wherein said element forms a plurality of zones of restricted cross-section spaced in a direction longitudinally thereof and decreasing in cross-section from the center arca of said axially inner region in a direction longitudinally thereof, said element further comprising axially outer regions, a pair of relatively short separating zones more restricted in cross-section than any of said plurality of zones of restricted cross-section separating said axially inner region from said axially outer regions, and the shortest distance between each of said pair of terminal elements and one of said pair of separating zones immediately adjacent thereto substantially exceeding the spacing between said Zones of restricted cross-section.

6. An electric fuse including a casing of a syntheticresin-glass-cloth laminate, a pulverulent quartz ller inside said casing, a pair of terminal elements closing the ends of said casing, a fusible current-carrying element of a metal having a relatively high conductivity and a relatively low heat of fusion conductively interconnecting said pair of terminal elements, said element comprising an axially inner region wherein said element forms a plurality of zones of restricted cross-section spaced in a direction longitudinally thereof and decreasing in cross-section ll from the center area of said axially inner region in a direction longitudinally thereof, said plurality of zones of restricted cross-section including one pair of points of restricted cross-sectio-n having a predetermined minimum cross-section and a predetermined length, said element further comprising axially outer regions, a pair of sepa rating zones of restricted cross-section each having a cross-section less than said predetermined minimum crosssection and each having a length less than said predetermined length separating said axially inner region from said axially outer regions, and the shortest distance between each of said pair of terminal elements and one of said pair of separating zones adjacent thereto exceeding the spacing between said zones of restricted cross-section.

7. An electric fuse as specied in claim 6 wherein said separating zones are bare and said axially inner region is plated approximately 1/1000" thick with a metal having a relatively low fusing point.

8. An electric fuse including a tubular casing, terminal elements closing the ends of said casing, a fusible currentcarrying silver ribbon of uniform width conductively interconnecting said terminal elements, said ribbon comprising an axially inner region wherein said ribbon is provided with a plurality of spaced perforations progressively increasing in size from the center of said axially inner region in a direction longitudinally thereof to form zones of reduced cross-section progressively decreasing in size from said center in a direction longitudinally of said ribbon and including a pair of zones of minimum crosssection, said element further comprising axially outer regions, a pair of separating zones of restricted crosssection each smaller in cross-section than said minimum cross-section separating said axially inner region from said axially outer regions, the length of said axially outer regions substantially exceeding the spacing between said perforations, and overlay means on said element of a metal having a lower fusing point than the fusing point of silver adapted upon fusion thereof to corrode the silver '12 of which said element is made, said overlay means extending along a portion of said axially inner region equal yto a multiple of the spacing of said plurality of perforations.

9. An electric fuse including a tubular casing, terminal elements closing the ends of said casing, a fusible currentcarrying silver ribbon having a larger width at the center than at the ends thereof and progressively decreasing in width from said center to said ends thereof, said ribbon comprising an axially inner region wherein said ribbon is provided with a plurality of spaced perforations of equal size forming a plurality of zones of reduced cross-section progressively decreasing in size from said center in a direction longitudinally of said ribbon and including a pair of axially outer zones of minimum cross-section, said element further comprising axially outer regions, a pair of separating zones of restricted cross-section each smaller in cross-section than said minimum cross-section separating said axially inner region from axially outer regions, the length of said axially outer regions substantially exceeding the spacing between said perforations, and overlay means of a metal having a lower fusing point than the fusing point of silver adapted upon fusion thereof to corrode the silver of which said element is made, said overlay means covering an appreciable length of said axially inner region to cause formation of series breaks on occurrence of relatively small overcurrents by corroding said axially inner region at a plurality of spaced points thereof.

References Cited in the file of this patent UNITED STATES PATENTS 1,856,317 Clark May 3, 1932 2,502,747 Popp Apr. 4, 1950 2,662,140 Kozacka Dec. 8, 1953 2,832,868 Kozacka Apr. 29, 1958 FOREIGN PATENTS 514,916 Great Britain Nov. 21, 1939