US2964604A - Current-limiting fuses having compound arc-voltage generating means - Google Patents

Description

Dec. 13, 1960 P. c. JACOBS, JR.. ET AL 2,964,604 CURRENT-LIMITING FUSES HAVING COMPOUND ARC-VOLTAGE GENERATING MEANS Filed May 25, 1959 aa/ 12363.6. Eg.6.

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United States Patent CURRENT -LIMITING FUSES HAVING COM- ARC-VOLTAGE GENERATING Philip C. Jacobs, Jr., Newtonville, Mass., and Frederick J. Kozacka, South Hampton, N.H., assignors to The Chase-Shawmut Company, Newburyport, Mass.

Filed May 25, 1959, Ser. No. 815,728

6 Claims. (Cl. 200-120) This invention is concerned with current-limiting fuses for minimizing the eifects of fault currents.

A current-limiting fuse is a fusible protective device capable of generating an arc voltage sutliciently rapidly and of suflicient magnitude to preclude any fault current from ever reaching the available short-circuit current which the circu1t into which the fuse is inserted is capable of supplying.

'Vvnne all current-limiting fuses exhibit the above behavior, the performance characteristics of different types of current-limiting fuses difler widely from each other. This invention is concerned with fuses not only capable of exhibiting a current-limiting action but which are capable, in addition thereto, to minimize the effects of fault currents. Minimizing the effects of fault currents calls for relatively low peaks of the let-through current, relatively low fusing fi -d1 values, relatively low arcing fi -dz values, relatively short fusing times and relatively short arcing times.

These requirements can only be met if the arc voltage initially formed rises very rapidly, yet not so rapidly as to endanger the insulation of the circuit into which the fuse is inserted, and if the arc voltage upon having reached a predetermined permissible peak does not decay too rapidly. The latter might unduly increase the arcing time and the arcing fi -dt, provide insuflicient protection to apparatus intended to be protected by the currentlimiting fuse and ultimately result in a destruction of the fuse itself.

The basic problem underlying the design of currentlimiting fuses is that of control of the arc voltage which the fuse generates. All known or prior art current-limiting fuses are acceptable compromises in regard to their are voltage characteristics and generate arc voltages far from postulated ideal arc voltages.

It is, therefore, a general object of this invention to provide current-limiting fuses capable of minimizing the effects of fault currents and making it possible to control or tailor the arc voltage to a much greater extent than was possible heretofore.

The rapid build-up of arc voltage at the point of initial fusion of a fusible element depends upon the speed at which heat can be abstracted from that point. An optimum of heat abstraction can be achieved where the point of initial fusion of the fusible element is in immediate physical contact with an arc-extinguishing filler of quartz sand so as to expose an are formed upon fusion of the fusible element to the immediate quenching action of said filler. Such an arrangement is conductive to the formation of relatively rapidly rising and relatively rapidly decaying arc voltages rather than to relatively steady or sustained arc voltages.

It is, therefore, another object of this invention to provide current-limiting fuses wherein the initial rate of rise of the arc voltage tends to be less than in prior art current-limiting fuses and wherein the decay of the arc voltage upon having reached its first peak tends to be less than in prior art current-limiting fuses.

Another object of the invention is to provide currentlimiting fuses the fusible elements of which are adapted to form two different kinds of breaks, namely one kind of breaks tending to establish extremely rapidly upon occurrence of a fault current an arc voltage which rises rapidly and decays rapidly, and another kind of breaks tending to establish an arc voltage upon occurrence of a fault current which may rise less rapidly than that at the first mentioned kind of breaks but which is of a relatively stable nature, i.e. does not rapidly decay.

The aggregate arc voltage resulting from these two different kinds of breaks has a tendency to come much closer to ideal arc voltage requirements than the arc voltages achieved heretofore.

For a better understanding of the invention reference may be had to the accompanying drawings wherein:

Fig. 1 shows a fuse embodying the invention designed for relatively small circuit voltages and is a section along the broken section line 1--1 of Fig. 2;

Fig. 2 is a section along 2--2 of Fig. 1;

Fig. 3 shows a fuse embodying the invention designed for higher circuit voltages than the fuse of Figs. 1 and 2 and is a section along the broken section line 33 of Fig. 4;

Fig. 4 is a section along 44 of Fig. 3;

Fig. 5 is a diagrammatic trace of the arc voltage buildup at one kind of break in fuses embodying the invention;

Fig. 6 is a diagrammatic trace of the arc voltage buildup at the other kind of break infuses embodying the invention; and

Fig. 7 is a diagrammatic trace of the aggregate arc voltage build-up in fuses embodying the invention obtained by algebraic addition of the voltage traces of Figs. 5 and 6.

Referring now to the drawings and more particularly to Figs. 1 and 2 thereof, the fuse shown therein comprises a tubular insulating casing l'closed on both ends thereof by terminal elements formed by copper plugs 2 tightly fitting into casing 1. Plugs 2 have extensions 2:: forming knife blade contacts. Transverse steel pins 3 establish a strong mechanical connection between casing 1 and plugs 2. Casing 1 is filled with an arc-extinguishing filler 4 of quartz sand. Such a filler has a high heat of fusion and hence a tendency to cause a rapid rise of the voltage of an are burning in a space filled with the filler. On the other hand such a filler is conducive to a very rapid decay of the arc voltage as arcing continues after fusion of the quartz filler has progressed to a certain point. A ribbon 5 of silver is immersed in filler 4. The ends of silver ribbon 5 are inserted'into a pair of grooves 2b formed in the inner faces of plugs 2, and soldered to plugs 2. Thus silver ribbon 5 establishes a conductive connection between plugs 2. Silver combines high conductivity with lower heat of fusion and thus tends to limit the peaks of the let-through currents and the fusing Il -dz. Ribbon 5 forms a pair of serially related points 5a, 5b of reduced cross-section. Both points 5a, 5b of reduced cross-section may be equal in size, i.e. may have the same crosssection and length. On occurrence of fault currents initial fusion of silver ribbon 5 will occur at the two points of reduced cross-section 5a, 5b. Assuming points 5a, 5b of reduced cross-section being equal in size, the fusing fi -dz thereof will be equal.

It will be appareht to anyone skiled in the art that the term fusing fi -dt as used in this context refers to a substantially constant value which obtains if the fusing times of points 5a, 5b are sufficiently short to make it permissible, when considering the temperature distribution in the fuse structure, to neglect any heat exchan e phenomena between its constituent parts. Fusing times in the order of 1 millisecond comply fairly well with this requirement. I

The size of ribbon '5 andof points 5a, 5b of reduced cross-section determine the current rating of the fuse, i.e. the current which the fuse is able to carry continuously without excessive heating of any of its constituent parts. The cross-section of the points 5a, 5b has relatively little effect on the current rating. The size of points 5a, 5b determines primarily the current at which the fuse blows at relatively rapidly rising fault currents. The cross-section of points 5a, 5b should be sufficiently small to cause fusion at these two points in substantially less than A cycle of a current of 60 c.p.s. having an R.M.S. value of less than 10 times the current rating of the fuse. With this standard of speed in mind the required cross-section of points, 5a, 5b may readily be determined empirically, i.e. by testing. It may be about A to. of the cross-section of the fuse link where its cross-section is widest.

The cross-section to be given to points 5a, 512 will depend to some extent upon the contemplated application of the fuse. The cross-section of points 5a, 5b will be particularly small where the fuse is intended to be serially connected with a semiconductor rectifier cell which is extremely critical, e.g. a germanium rectifier cell. This particular application of current-limiting fuses and the requirements to be met in that case have been described in the United States Patent 2,921,250 to Kenneth W. Swain, Jan. 12, 1960, for Coordinated Static Power Rectifiers and Current-Limiting Fuses. Another important application of fuses embodying the present invention is disclosed in the patent application of Philip C. Jacobs, Jr., et al., Ser. No. 578,251, filed April 16, 1956, for Circuit Interrupters With Back-Up Current-Limiting Fuses, now United States Patent 2,920,241, issued Jan. 5, 1960.

Referring now again to Figs. 1 and 2 of the drawings, point 5a of fuse link 5 is in immediate physical contact with the quartz filler 4 to expose an arc formed upon fusion of point 5a, to the immediate action of filler 4. The immediate action of filler 4 upon said arc causing said arc to exhibit a relatively rapidly rising and a relatively rapidly decaying arc voltage has been illustrated in Fig. 5. The are voltage rises suddenly when link 5 is severed by fusion of point 5a and reaches its peak value very rapidly. Thereafter the arc voltage decays almost as rapidly as it has risen.

The point 5b of reduced cross-section of fuse link 5 is in intimate contact with an arc chute structure 6 impeding access of quartz filler 4 to an arc formed upon fusion of point 5b. Arc chute structure 6 is made. up of two plates 6a, 6b of a synthetic-resin-glass-cloth laminate sandwiching link 5 therebetween and being supported on link 5 by means of rivets 7. The arc voltage formed upon fusion of point 5b of restricted cross-section exhibits a relatively slow rise but is relatively stable, i.e. it does not decay rapidly. A typical trace of such an arc voltage has been illustrated in Fig. 6. If both points 5a, '5b of reduced cross-sectionare identical the arc voltage at point 5b. starts to rise a small fraction of a millisecond after the arc voltage. at point 5a begins to rise. This is due to the fact that arc chute plates 6a, 6b have a slight cooling action on point 5b. The rise in temperature of point Sb is so rapidthat the heat flow away from point 5b to plates 6a, 6b may almost be neglected. The sequence in which arcs are kindled at points 5a, 5b is, therefore, very rapid. The decay of the arc voltage which takes place adjacent the point 5a of restricted cross-section is due to the fact that the quartz particles by which the path of that are is bounded have become good conductors of electricity, or semiconductors. The relative stability of the voltage of the arc which takes the place of point 5b of restricted cross-section is due to the fact that plates 6a, 6b tend to keep particles of quartz sand away from the arc path. This mechanical tendency of plates 6a,

6b is enhanced by the outflow of are products which has been indicated in Fig. 2 by pairs of arrows.

Both points 5a, 5b are defined by V-shaped lateral incisions 5c in link 5. The products of arcing escaping through the fiat funnels formed by the V-shaped incisions 5c and bounded by plates 6a, 6b produce blast of gases away from the arcing zone sweeping with them any particle of quartz sand that may have penetrated into the gap formed between plates 6a, 6b. The gases evolved from plates 6a, 6b under the heat of the arc tend to increase the blast action inherent in the structure. Since plates 6a, 6b are preferably made of a synthetic-resin-glass-cloth laminate, the glass component of this material has an intense cooling action upon any are burning between plates 6a, 6b. The individual glass fibers of the material of which plates 6a, 6b are made may become conductive under the heat of the arc, yet they are more or less electrically insulated from each other by the synthetic resin component of the material and thus do not form an arc shunt of low resistance as the fused quartz particles do which are associated with the upper point 5a of restricted crosssection. Fig. 7 shows the arc voltage between terminal elements 2 obtained by superposition, or algebraic addition, of the voltage traces of Figs. 5 and 6.

In Figs. 3 and 4 the same reference numerals as in Figs. 1 and 2 have been applied to indicate like parts, however, with a prime added. Thus 1' indicates a tubular insulating casing closed by copper plugs 2 having blade contact extensions 2a and held in position by steel pins 3. Casing 1 accommodates the quartz sand filler 4 in which silver link 5' in form of a ribbon is immersed. Link 5' has four points 5a, 5b of reduced cross-section and is sandwiched between a pair of plates 6a, 6b of a synthetic-resin-glass-cloth laminate. Plates 6a, 6b are secured to link 5' by means of small rivets or eyelets 7'. Plates 6a, 6b have pairs of registering circular holes 8' exposing points 5a of link 5' to immediate physical contact with quartz filler 4. When points 5a fuse the arcs kindled at these points exhibit an arc voltage of the general character shown in Fig. 5. This is due to the fact that the arcs formed at the aforementioned points of break burn in paths which are immediately bounded by hot quartz sand or fused silica. Plates 6a, 6b impede access of quartz sand to the breaks at the points 5b, both by their mechanical shielding action and their blast producing action, as above described. As a result, the voltage of arcs kindled adjacent points 5b exhibits the general character, i.e. relative stability, of the voltage trace shown in Fig. 6. The compound are voltage generating means 4, 5a, 8, 5b, 6a, 6b yield an aggregate arc voltage of the general character shown in Fig. 7.

In order to prevent formation of fulgurites at the points of reduced cross-section intended to exhibit an arc Voltage of the general character shown in Fig. 6, the points of reduced cross-section ought to be located at the region of and confined to the region of the longitudinal axis of the fuse link. This condition is met if the points of reduced cross-section are formed by a pair of lateral, substantially V-shaped incisions. A point of reduced cross-section might also be formed by stamping a circular hole into a fuse link, the center of the hole being situated in the longitudinal axis of the fuse link. Narrow current paths are formed in such a link adjacent the longitudinal edges thereof. This makes it relatively difficult to keep a quartz sand filler away from points of reduced cross-section thus formed.

The number of points of break generating a rapidly rising and rapidly decaying arc voltage and the number of points of break generating a relatively stable arc voltage does not need to be equal.

semble the voltage trace of Fig. 5, or the voltage trace The resulting, arc, voltagev characteristic may be controlled in such a way as. to re.--

aarsgaoa .5 of Fig. 6, depending upon which type of break predominates.

The cross-sectional areas of points a, 5b of reduced cross-section and their fusing fi 'dt values must not necessarily be equal. The cross-sectional area of points '5a, 5a may be slightly smaller than the cross-sectional area of points 512, 5b. This makes it poss'ible'to achieve a rising relatively steady are voltage at the breaks formed at points 5]), 5b at the time when the arc voltage at the breaks formed at points 5a, 5a is already decaying. The sequence of fusion at points 5a, 5a and 5b, 5b must be sufficiently rapid not to thermally overburden the areas around points 5a, 5a where arcs are initially kindled.

There must be a certain relation between the relative size of points 5a (Fig. 1) and 5a (Fig. 3) and of points 512 (Fig. 1) and 5b (Fig. 3). The cross-section of points 5b and 512 must be sufiiciently small to make the fusing fi -dt of points 5b and 5b smaller then the aggregate fusing and arcing fi -dt of points 5a, 5a. If this condition is not met no breaks are formed at points 5b and 5b, respectively.

The fuse structures which have been shown and described may be applied both in A.-C. circuits and in D.-C. circuits. Their performance in .D.-C. circuits is very satisfactory on account of the relative stability of their are voltage.

-It will be understood that the term are as applied in this context has not exactly the same meaning as that connoted with the term when applied in physics of gas discharges. An are burning in a relatively homogeneous gaseous medium not exposed to the action of other media situated close to the arc path exhibits a negative currentvoltage characteristic, whereas an are burning in quartz sand does not exhibit this typical characteristic. An are burning in quartz sand exhibits a negative or descending current-voltage characteristic limited to relatively small currents, where gas discharge mechanics predominate over the solid-state semiconductor mechanics imposed by the presence of the quartz sand. The descending branch of the current-voltage characteristic of an are burning in quartz sand is followed by an ascending branch or positve portion of the characteristic which corresponds to relatively high current intensities. This positive portion or ascending branch of the characteristic indicates the current range wherein solid state semiconductor mechanics predominate over gas discharge mechanics.

The longer an arc burns in a quartz sand filler, the smaller the heat absorbing capacity of the filler at any given point thereof, the smaller the arc voltage which any given point of the filler is able to generate. It is, therefore, not possible to draw a single current-voltage characteristic for an arc burning in quartz sand. Only a family of such characteristics can be drawn wherein arc duration appears as a parameter. Each charcteristic is more or less in the shape of an inverted V.

Arc extinction in a current-limiting fuse may be seen as a shift of the point of operation from members of the family relating to relatively short arc duration to members of the family relating to relatively long are duration.

The progressive decay of arc voltage at any given point of the sand filler with increasing arcing time is in part made up by the tendency of the arc voltage to increase With increasing burn-back of the fuse link. Burn-back takes time and increases the arcing fi -dt; hence the importance to achieve a sufiiciently high sufiiciently stable arc voltage by means other than burn-back, i.e. by a combination of points of break where quartz is in intimate contact with the arc and points of break Where such contact is precluded by the provision of appropriate arcchute means.

While, in accordance with the patent statutes, we have disclosed the details of a preferred embodiment of our invention, it is to be understood that many of these details are merely illustrative and variations in their preciserform will be possible or necessary depending upon the particularnature of application.

We desire, therefore, that our invention'be limited only to the extent set .forth in the appended claims and by the prior art.

"We claim as our invention:

1. A current-limiting fuse for'miuimizing the effects of short-circuit currents comprising a tubular insulating casing, an arc-extinguishing filler of quartz sand inside said casing, a pair of terminal elements one on each end of said casing, a silver'ribbon immersed in saidfiller conductively interconnecting said pair of terminal elements, said ribbon having a .pair of serially related points of reduced cross-section, the size of said ribbon and the size of said pair of points determining the current'rating of said fuse and the size of said pair of points being sufficiently small to cause fusion thereof in times substantially less than /1 of a .cycle of a current of 60 c.p.s. having an R.S.M. value less than 10 times said current rating, one of said pair of points being in immediate physical contact with said filler to expose an are formed upon fusion of said one of said pair of points to the immediate action of said filler whereby said arc formed upon fusion of said one of said .pair of points is caused to exhibit a relatively rapidly rising and relatively rapidly decaying arc voltage, the other of said pair of points being in immediate physical contact with an arc chute structure impeding access of said filler to an are formed upon fusion of said other of said pair of points whereby said arc formed upon fusion of said other of said pair of points is caused to exhibit a relatively stable arcvoltage, and the cross-section of said other of said pair of points beingsufiiciently small to make the fusing fi -dt of said other of .said pair of points smaller than the aggregate I fusing fi -alt and arching fi -dt of said one of said pair of points.

2. A current-limiting fuse for minimizing the effects of short-circuit currents comprising a tubular insulating casing, an arc-extinguishing filler of quartz sand inside said casing, a pair of terminal elements each on one end of said casing, a silver ribbon immersed in said filler conductively interconnecting said pair of terminal elements, said ribbon having a pair of serially related points of reduced cross-section located at and confined to the longitudinal axis thereof, the size of said ribbon and the size of said pair of points determining the current rating of said fuse and the size of said pair of points being sufficiently small to cause fusion thereof in times substantially less than A of a cycle at a current of 60 c.p.s. having an R.S.M. value of less than 10 times said current rating, one of said pair of points being in immediate physical contact with said filler to expose an are formed upon fusion of said one of said pair of points to the immediate action of said filler whereby said are formed upon fusion of said one of said pair of points is caused to exhibit a relatively rapidly rising and relatively rapidly decaying are voltage, the other of said pair of points being in immediate physical contact with an arc chute structure of synthetic-resin-glass-cloth laminate impeding access of said filler to an are formed upon fusion of said other of said pair of points whereby said are formed upon fusion of said other of said pair of points is caused to exhibit a relatively stable arc voltage, and the cross-section of said other of said pair of points being sufficiently small to make the fusing fi -dt of said other of said pair of points smaller than the aggregate fusing fusing ff -dt and arcing fi -dt of said one of said pair of points.

3. A current-limiting fuse as specified in claim 2 wherein each of said pair of points of reduced crosssection is defined by a pair of lateral substantially V-shaped incisions in said silver ribbon.

4. A current-limiting fuse for minimizing the effects of short-circuit currents comprising a tubular insulating casing, 21 pair of terminal elements one on each end of said casing, an arc-extinguishing filler of quartz sand inside said casing, a silver ribbon immersed in said filler conductively interconnecting said pair of terminal elements, said ribbon having a plurality of serially related points of reduced cross-section, the size of said ribbon and the size of said plurality of points determining the current rating of said fuse and the size of said plurality of points being sufficiently small to cause fusion thereof in times substantially less than A of a cycle of a current of 60 c.p.s. having an R.M.S. value less than times said current rating, one portion of said plurality of points being in immediate physical contact with said filler to expose a plurality of arcs formed upon fusion of each of said one portion of said plurality of points to the immediate action of said filler whereby said plurality of arcs formed upon said fusion of each of said one portion of said plurality of points is caused to exhibit a relatively rapidly rising and relatively rapidly decaying arc voltage, a pair of plates of a synthetic-resin-glass-cloth laminate sandwiching said ribbon and impeding access of said filler to another portion of said plurality of points to cause arcs formed upon fusion of each of said other portion of said plurality of points to exhibit a relatively stable are voltage, and the cross-section of each of said other portion of said plurality of points being sufiiciently small to make the fusing fi -dt of each of said other portion of said plurality of points smaller than the aggregate fusing fi -dt and arcing fi -dt of each of said one portion of said plurality of points.

5. A fuse as specified in claim 4 wherein said one portion of said plurality of points is numerically different from said other portion of said plurality of points.

6. A current-limiting fuse for minimizing the effects of fault currents comprising a tubular insulating casing, a pair of terminal elements each on one end of said casing, an arc-extinguishing filler of quartz sand inside said casing, a silver ribbon immersed in said filler conductively interconnecting said pair of terminal elements, said ribbon having a plurality of points of reduced cross-section located at the region of and confined to the region of the longitudinal axis thereof, the size of said ribbon and the size of said plurality of points determining the current rating of said fuse and the size of said plurality of points being sufiiciently small to cause fusion thereof in times substantially less than A of a cycle of a current of c.p.s. having an R.S.M. value of less than 10 times said current rating; a pair of plates of a synthetic-resin-glasscloth laminate sandwiching said ribbon, said pair of plates having a plurality of pairs of registering perforations equal in number to one portion of said plurality of points exposing each of said one portion of said plurality of points to the immediate action of said filler whereby a plurality of arcs formed upon fusion of each of said one portion of said plurality of points exhibit a relatively rapidly rising and relatively rapidly decaying arc voltage, said pair of plates impeding access to another portion of each of said plurality of points to cause arcs formed upon fusion of each of said other portion of said plurality of points to exhibit a relatively stable arc voltage, and the cross-section of each of said other portion of said plurality of points being sufficiently small to make the fusing fi -at of each of said other portion of said plurality of points smaller than the aggregate fusing fi -dt and arcing fi -dt of each of said one portion of said plurality of points.

References Cited in the file of this patent UNITED STATES PATENTS 1,239,876 Burnham Sept. 11, 1917 2,833,890 Jacobs May 6, 1958 2,866,038 Kozacka Dec. 23, 1958 2,892,061 Kozacka June 23, 1959 I UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No. 2,964,604 December 13 1960 Philip C. Jacobs, Jr. et a1,

ppears in the above numbered pat- It is hereby certified that error a id Letters Patent should read as ent requiring correction and that the sa corrected below.

Column 6, line 35, for "arching" read arcing Signed and sealed this 9th day of May 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L LADD Commissioner of Patents Attcsting Officer UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No. 2,964,604 December 13 1960 Philip C. Jacobs, Jr. et a1.

appears in the above numbered pat- It is hereby certified that error aid Letters Patent should read as ent requiring correction and that the s corrected below.

Column 6, line 35, for "arching" read arcing Signed and sealed this 9th day of May 1961 (SEAL) Attest:

ERNEST W SWIDER DAVID L.: LADD Commissioner of Patents Attesting Officer

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