US3123694A - High current-carrying-capicity cartridge - Google Patents

Description

March 3, 1964 F. .1. KozAcKA 3,123,694

HIGH CURRENT-CARRYING-CAPACITY CARTRIDGE FUSES WITH MINIMIZED LOSSES Filed Nov. 2, 1961 6 Sheets-Sheet 1 Mach 3, 1964 F. J. KozAcKA HIGH CURRENT-CARRYING-CAPACITY CARTRIDGE FUSES WITH MINIMIZED LOSSES 6 Sheets-Sheet 2 Filed Nov. 2, 1961 t im u Izavezzoaf.- Wedebi ellfozaca,

March 3, 1964 F. J. KozAcKA 3,123,594

HIGH CURRENT-CARRYING-CAPACITY CARTRIDGE FUSES WITH MINIMIZED LOSSES Filed Nov. 2. 1961 6 Sheets-Sheet 4 7 lwqrrs S F- IOO 200 400 600 Curren'r roring WCJHSBC. level of energy margin of safety absorblnq cop0c|ty amps.

critical current range by )www J'oadzzeg March 3, 1964 F. J. KozAcKA 3,123,694

HIGH cuRRENT-CARRYING-CAPACITY CARTRIDGE FUsEs WITH MINIMIZED LossEs Filed Nov. 2. 1961 6 Sheets-Sheet 5 aooo Y |000 5000 omps.X IO

.1a. 250volr, 400 and 600 amps.

2 3 x SYM RMS mp5./

0.ol 4 6 8 lo 2 3 45e eooA max. peck circuit can produce O LDQ'ION Od' N March 3, 1964 F. J. KozAcKA 3,123,694

HIGH CURRENT-CARRYING-CAPACITY CARTRIDGE FUSES WITH MINIMIZED LOSSES Filed Nov. 2. 1961 6 Sheets-Sheet 6 q/2 @Cl/2 *fn 16.11. 24, 800 pk. amps.

shorcircuit current voltage across fuse '2 Izavezoad.-

480 It 27,3;1) sym. omperes circuit.

arrasar Patented Mair. 3, 'iS-fl 3 123,694 HIGH CURRENT CRRYING CAPACITY CAR- TRIDGE FUSES WITH MINIMIZED LOSSES Frederick J. Koaacka, South Hampton, NH., assigner to The Chase-Shawmut Company, Newburyport, Mass. Filed Nov. 2, 196i, Ser. No. 149,554 7 Claims. (Cl. 20th-120) This invention relates to electric high current-carryingcapacity low-Voltage cartridge fuses, i.e. cartridge fuses designed to have a cur-rent rating of at least 400' amps. at voltages not exceeding 600 volts.

This invention relates more particularly to the type of fuses which are often designated in the trade as one-time fuses. The term one-time fuse has been coined to distinguish these fuses from fuses the fusible element of which can be renewed, and which, therefore, lend themselves upon renewal of their fusible element to performing a repeated interrupting duty. Such fuses are generlly referred to as renewable fuses. The term one-time fuse, as it is widely used in the trade implies, however, more than the fact 'that the particular fuse is capable of interrupting an electric circuit but one single time. Socalled one time yfuses are fuse structures which are less expensive to manufacture than current-limiting, highinterrupting-capacity fuses and are made of materials whose cost is less than the cost of materials of which current-limiting, high-interrupting-capacity fuses are generally made.

Current-limiting high-interrupting-capacity `fuses are generally lilled with quartz sand which is converted into a fulgurite when subjected to the heat of the arc incident to interruption of a circuit. ln other words, the sand is converted into a conglomerate of fused and sintered quartz particles forming a good conductor of electricity as long as the temperature of the conglomerate is high. Fuses which are provided with a quartz sand filler, or another pulverulent fulgurite-.torming arc-quenching filler, must be provided with casings of insulating materials which are heat shock resistant and capable of withstanding for some time the action of hot fulgurites. Such materials are either of a ceramic nature (steatite), or suitable synthetic resin laminates including inserts of woven glass iibers, or glass cloth. Casing materials of this description are relatively expensive and not used in manufacturing one-time fuses within the relatively narrow sense this term has acquired in the trade. One-time fuses, within the meaning this term is being used in the trade, and is here being used, are fuses adapted to effect but one interruption of a circuit and to be disposed of thereafter, i.e. not renewed, interruption by such `fuses being effected by means of a pulverulent non-fulgurite-forming arcquenching filler arranged in a tubular casing of a homogcnous organic insulating material. A characteristic 4feature of all non-fulgurite-forniing arc-quenching fillers such as gypsum, or chalk, resides in the fact that they evolve gases `under the heat of electric arcs. For this reason the terms non-iulgurite-forming filler and gas-evolving filler are often used as synonyms. The term homogeneous is used in this context in the sense of an antonym of composite synthetic resin laminates including inserts of woven glass, or glass cloth.

Electric fuses, when carrying current, generate heat, and the Watt losses occurring in electric fuses, and the heat generated in such fuses, increase with the currentcarrying capacity, or current rating, thereof. Heat losses are proportional to the second power of the current carried by the particular fuse, and watt losses and the concomitant rise in temperature become more and more of a problem the higher the current-carrying-capacity, or current rating, of a particular piece of equipment. The

problem of heat losses becomes rather critical at current ratings of 400 amps., and higher ratings. Fuses of relatively high current ratings are often housed in common enclosures `with other heat generating electrical equipment which adds heat to the relatively large amounts of heat generated by one-time fuses of high current-rating.

lt is one object of this invention to provide one time fuses, i.e. non-renewable fuses having pulverulent nonfulgurite-forming gas-evolving arc-quenching fillers and casings of homogeneous organic insulating materials, which fuses are adapted to carry high currents, yet have minimal watt losses and operate at relatively low temeratures.

Another obiect of the invention is to provide electric one-time fuses having current ratings of at 'least 480 amperes, say 400 or 69()l amperes, which fuses `are less expensive to manufacture than prior art one-time fuses having an equal current rating and are subject to smaller watt losses than prior art one-time fuses having an equal rating and which `fuses operate at considerably lower ternperatures than prior art one-time fuses having an equal current rating.

Another object of the invention is to provide one-time fuses having a high current-carrying-capacity, which fuses involve `smaller material cost than current-limiting highinterrupting-capacity fuses, yet compare performancewise with current-limiting high-interrupting capacity fuses both in regard to low temperatures when carrying current, and in regard to interrupting capacity when clearing major faults.

l have made a comparative study of watt losses occuring in fuses having non-ferrous terminal elements and in fuses having ferrous terminal elements. The watt losses occurring in the former are less than those occurring in the latter, which is due to the fact that the presence of ferrous terminal elements tends to result in eddy currents in the terminal elements and concomitant so-called iron losses which are very .signiiicant when considering fuse structures designed to carry high currents such as, for instance, 400' amps., or even higher currents. lron losses are completely avoided in fuses wherein the casing is closed by a pair of terminal plugs of copper, `and the same is true in regard to renewable fuses including terminal elements formed of heavy brass parts. Copper terminal plugs as 4used in current-limiting I interruptingcapacity fuses and the heavy brass terminals as used in renewable fuses are, however, far too expensive to be used in one-time fuses within the restricted meaning this term has been defined above.

This invention is predicated upon the concept that relatively light :and inexpensive terminal caps or ferrules on a non-ferrous metal such as brass can be used in one-time fuses and thus the occurrence or iron losses completely eliminated therein, if it were possible to drastically limit the pressure occurring in such fuses incident to interruption of a faulted circuit, even in spite or the pressure of a gas-evolving arc-quenching ller which is necessarily present in such fuses.

Each and every means which tends to limit the generation of pressure in a fuse is conducive to some reduction of the mechanical strength requirements of the terminal caps or ferrules thereof. One-time fuses of relatively high current-carrying capacity can only be provided with relatively inexpensive or light terminal caps or ferrules of non-ferrous metals, and in particular brass, if the fuses include pressure-limiting means which are highly effective. Pressure can be limited to some extent by minimizing the mass of metal of which the fusible element or fuse link is made. To this end the fusible elements or fuse link ought to be made of a metal whose conductivity is high. Copper is a suitable metal since it has a relatively high conductivity and its cost are relatively loW. Fuses combining copper links and gas-evolving arc quenching fillers tend to generate very high transient pressures incident to interruption of a faulted circuit. Pressure generation incident to interruption of a faulted circuit can be further limited by limiting the melting fig-dt and the arcing fiz'dt, by imparting a current-limiting action to the fuse.

Since high current-carrying-capacity current-limiting one-time fuses had not been considered to date it became necessary to investigate whether current-limitation is a suiiiciently effective pressure-limiting means in the presence of a gas-evolving arc-quenching iler to allow on such fuses the provision of relatively light caps or ferrules of brass, or other nonferrous metal. It was found that current-limiting action is not suticient in itself, but that the problem can be solved by resorting to a predetermed minimum of current-limiting action.

There are a numbery of ways for defining numerically the current-limiting action of a current-limiting fuse. One characteristic quantity numerically defining degree of current-limitation is the current-limiting ratio. Current-limiting ratio is the ratio of the current required to cause melting of the fusible element or fuse link of a fuse in 0.01 sec. to the rated current of the particular fuse (see Philip C. Jacobs, Current-l4imiting Fuses: Their Characteristics and Applications, Technical Paper 56-772, American Institute of Electrical Engineers). To achieve the ends contemplated by this invention the current-limiting ratio must be less than 30.

In order to achieve this degree of current-limiting, or current-limiting action, it is necessary, or desirable, to resort to certain structural features as will be outlined below more in detail.

If the melting fiZ-dl or fusing j`i2-dr is to be minimized without resorting to relatively expensive fuse links of silver, the fuse links ought to be of copper. Current ratings of 400 amperes or in excess of 600 amperes call for ribbon type fuse links. To maximize the area of interaction between the arcs formed incident to interruption and the arc-quenching filler and to thereby minimize the arcing fi2dt and the duration of pressure generation by the arc-quenching filler a plurality of ribbon fuse links should be arranged in parallel. A pressure-generationlimiting arrangement of two fuse links in parallel which is easy to manufacture is obtained by sandwiching the axially inner ends of a pair of blade contacts projecting from the outside of the casing or fuse tube into the inside thereof between the axially outer ends of the ribbon fuse links. In order to avoid thermal damage to the casing or fuse tube of organic insulating material the rise in temperature of the fuse links while performing their current-carrying duty must be limited. This can be achieved by an overlay of a low-fusing-point link-severing metal, e.g. tin, on each of the aforementioned fuse links. Such overlays are well known in the fuse art and, therefore, do not need to be described in detail in the present context.

The current-carrying capacity of a pair of fuse links arranged to carry currents in parallel depends primarily upon the dimensions of the fuse links, their geometryand the nature (thermal conductivity) of the pulverulent arcquenching filler in which the fuse links are submersed. By controlling these parameters a pair of fuse links can readily he adapted to jointly carry continuously currents of at least 400 amperes, say 400 amperes, or 600 amperes. Each of said pair of fuse links should have a plurality of transverse lines of perforations forming zones of reduced cross-section or cross-sectional area. These zones can readily be adapted to establish a predetermined currentlimiting ratio. By adopting an appropriate geometry forthe transverse lines of perforations, and more particularly for the constituent perforations thereof, a very high degree of current-limiting action, or relatively low current-limiting ratio, can be obtained. It is thus readily possible to obtain current-limiting ratios of less than 30. Such low current-limiting ratios, i.e. current-limiting ratios which are less than 3), result in relatively low pressures during tion;

the interrupting process, even in the presence of pulverulent non-fulgurite-forming gas-evolving arc-quenching fillers.

For a better understanding of the present invention together with other and further objects thereof, reference may be had to the following description, taken in connection with the accompanying drawings, and its scope will be pointed out with particularity in the appended claims:

Referring now to the drawings:

FIG. l is partly a longitudinal section and partly a side elevation of an Underwriters Laboratories standard size fuse for a 600 volt circuit embodying the present inven- FIG. 2 is a section taken along 2-2 of FIG. l;

FIG. 3 is a section taken along 3-3 of FIG. 2;

FiG. 4 is partly a longitudinal section and partly a side elevation of an Underwriters Laboratories standard Sile fuse for a 250 volt circuit embodying the present invention;

FIG. 5 is a sectionV taken along 5 5 of FIG. 4;

FIG. 6 is a section taken along 6--6 of FIG. 5;

FIG. 7 refers to a family of fuses and shows watt losses and equilibrium temperature plotted against current rating;

FIG. o refers to fuses embodying the present invention and is a block diagram showing temperatures in degrees C plotted against load in percent of the rated current;

FIG. 9 refers to fuses embodying the present invention and is a block diagram showing watt losses plotted against current rating;

FIG. l0 is a typical diagram showing energy converted into heat in a fuse of given design plotted against current in terms of multiples of the current rating of the particular fuse;

. FIG. ll shows peak let-through currents of fuses embodying the present invention plotted against available currents; i

FIG. 12 shows the time-current curves of two fuses embodying the present invention;

FIGS. l3-l5 are diagrammatic representations of ribbon type fuse links; and

FIGS. 16a and 16b are oscillographic records resulting from short-circuitrtests of fuses embodying the present invention.

Referring now to the drawings, and more particularly to FIGS. l-3 thereof, reference numeral 1,l has been applied to indicate a tubular casing of an Underwriters Laboratories standard size fuse having a voltage rating of 600 volts anda current rating of 600 amps. Casing 1 is made of a homogeneous organic insulating material, preferably vulcanized fiber. Casing 1 contains a pulverulent nonfulgurite forming arc quenching ller Z evolving gas under the heat of electric arcs. Filler 2 may be gypsum powder. A pair of blade contacts 3 extends in a direction longitudinally of casing 1 each projecting from the outside ofV casing 1 into the inside thereof. Reference numeral 4 has been applied to indicate a pair of ,ribbon fuse ,links of.copper submersed in filler 2. Fuse links 4 sandwich between the axially outer ends thereof the axially inner ends of blade contacts 3. Fuse links 4 are provided with live transverse lines 4a of circular perforations. The number of perforations in each line la of perforations exceeds the number of lines of perforations, i.e. the number ve. The diameter of the constituent perforations of the five lines of perforations 4a is of considerable importance in order to achieve a sufficiently small melting fi2-dt and a suiiiciently small arcing fz'2dt in spite of the fact that the de-ionizing action of the arc-quenching filler 2 is substantially less thanl that of quartz sand. Fuse links 4 are adapted to jointly carry continuously currents of at least 480 amps. In other words, the fuse structure of FIGS. l-3 has a current rating of at least 400 amps. In the particular embodiment of the invention shown in FIGS. l3 fuse links i are dimensioned in such a way that the fuse of which they form part is able to carry continuously currents as high as 600 amperes without causing any undue heating, or excessive heating, of its casing l tending to cause too rapid deterioration of the homogeneous organic insulating material of which the casing is made. The hottest points in the fuse under load conditions and overload conditions are the center lines of links 4. Each fuse link 4 is provided with an overlay 4b of a low-fusingpoint link-severing metal, eg. tin. When overlays 4 reach their fusing point they sever links 4 by virtue of a metallurgical reaction occurring between the melted overlay metal and the base metal, i.e. the copper of which links 4 are made. This limits the temperature ceiling, or highest temperature, to which links 4 are subjected when carrying overload currents, and thus limits the highest temperature to which casing l may be subiected when the fuse care ries overload currents. The constituent perforations of the five lines of perforations in define five Zones, or transverse lines, or reduced cross-section where the current density in fuse links 4 is highest. The cross-sectional area of these five zones of reduced cross-section determines the melting {i2-dr in terms of ampere square times seconds of fuse links 4, and hence also the current-limiting ratio of the fuse. This cross-sectional area is suiiiciently small to result in a current-limiting ratio of less than 30, i.e. the ratio of the current required to melt the zones of reduced cross-section in 0.01 sec. to the current rating of the fuse is less than 30. FIG. l2 shows the time current curves of fuses embodying the present invention making it -readily possible to read oil the current-limiting ratio of these two fuses. lt appears from FlG. l2 that the current-limiting ratio is about 25, which is a better figure than 30, the latter being about the highest permissible limit. The number of the zones of reduced cross-section of the structure of FIGS. 1 3, inclusive, and the geometry of the constituent perforations of the lines 4e of perforations determines the clearing ratio ot' the particular link structure when submersed in a given arc-quenching iller. The clearing ratio is the ratio between the melting fiz'd plus the arcing ft2-dt known as the total clearing ft2-dt to the melting ft2-dt (see above paper of Philip C. Jacobs, In). The clearing ratio tends to increase with decreasing arc voltage. The higher the arc voltage and the stabler the arc voltage the smaller the clearing ratio. The arc voltage tends to increase with the number of serially related Zones ot reduced cross-section- An increase of the diameter or" the constituent perforations of the lines of perforations 4a tends to increase and to steady the arc voltage and to reduce the current rating of the particular fuse as will be more apparent from what follows.

Pins 5 are formed by spirally wound sheet metal, thus imparting to these pins resiliency in radial direction thereof. Each pin 5 projects transversely through one of blade contacts Si and through the casing l, thus supporting blade contacts 3 on casing l. A pair of terminal caps 6 of a non-ferrous metal is mounted on the ends of casing l. Each terminal cap 6 defines a passage 6a for one of blade contacts 3. Gaskets 6b of a suitable fibrous material are interposed between the axially outer ends of casing l and the axially inner surfaces of caps d. Substantially nail-shaped fasteners 7 are driven through the lateral cylindrical surfaces of caps 6 into the inside of sheet rnetal pins 5 expanding the ends of pins 5 situated inside radial bores in casing 1. Fasteners 7 thus firmly secure caps 6 to casing l. Caps 6 are mounted sufliciently tightly on casing 1 to substantially preclude the emission ot hot products of arcing therefrom formed incident to interruption by the fuse of severe fault currents. The fuse is thus a non-vented fuse, i.e. there is no pressure relief by venting and, therefore, caps 6 must be able to withstand the transient pressures generated in casing l. during interruption of faulted circuits. If the caps ti were mounted relatively loosely on casing l, thus permitting the escape of arc products, the strength requirernents placed upon caps 6 would be greatly reduced.

However, the escape of hot products of arcing is a very undesirable operating characteristic for all indoor applications and shown, therefore, be avoided.

Caps 6 are preferably made of brass and have a wall thickness of less than 0.07 inch. Because the thickness of cap-s d is so small and the strength of the metal of which they are made is small, and since they must contain virtually all the products of arcing generated in casing l incident to blowing of the fuse, the generation of gases from the gas-evolving arc-quenching ller 2 must be drastically curtailed. The latter is achieved by a drastic limitation of the thermal energy released in the fuse during an interrupting process.

In FIGS. 4-6 the same reference characters as in FlGS. 1 3 with a prime sign added to them 'have been applied. Hence the description of liGS. 4-6 can be brief. Casing i is filled with a pulverulent arc-quenching filler 2.'. Casing l is made oi a homogeneous organic insulating material such `as vulcanized fiber and filler 2 is of the gassevolving variety, eg. gypsum. Blade contacts 3 extend in direction longitudinally of casing l from the outside to the inside thereof. Ribbon fuse links 4' of copper are submersed in filler 21. They sandwich between the `axially outer ends thereof the axially inner ends of blade contacts 3. Each of fuse `links 4' has three transverse lines of perforations, each line comprising about eight individual pertorations. The above number of lines of perforations and the above number of perforations per line has been round to `be necessary and desirable for circuit volta not exceeding 250 volts where the currents to lbe carried by the fuse and the current ratings intended to be assigned to the fuse are 4Gb amperes, or in excess of 4G() ampores, say 609` amperes. By giving appropriate dimensions to fuse links 4. a current rating of at least 400 amperes may be assigned to the fuse of FIGS. 4-6. Lines of perforations da define three related zones of reduced crosseection which are suiciently small to result in a current-limiting ratio of less than 30, preferably as low as Z5. rlhe width of fuse links 4 is only slightly less than the inner diameter of casing l', and the number of perforations per line 4a', which is about eight, coupled with the number of lines of perforations, which is 3, resuits in a gypsum filled fuse inserted into a circuit having a circuit voltage of 250 volts in a clearing ratio which, in the average, is less than 5. it will be understood that the clearing ratio is aliected by a number of variables, including the nature of the gas-evolving filler 2' provided in any particular instance. The severity of fthe circuit under interruption has also an important bearing on the `clearing ratio. All statements made herein refer 'to 60 cycle A.C. test .circuits having a low power factor and available fault currents up fto about 59,00@ arnperes. If a fuse according to the presen-t invention is tested in such a circuit its clearing ratio will vary with the fault angle. Considering a large number of tests wherein the fault angle varies at random, the clearing ratio will vary according to the fault angle :and thus an average clearing ratio will be obtained which is defined by the aforementioned test conditions. rthis average clearing ratio should be less than 5, and preferably less than 4.

Tests carried out with the structure of FIGS. l-4 at 480 volts rather than at 600l volts which is the voltage rating of that structure revealed average clearing ratios between 3 and 4. lf the circuit voltage is raised to 600 volts the clearing ratio increases without exceeding the upper limit value of 5.

Fl'G. 7 shows the temperature T in deg. C. and the watt loses W of a well designed modern `family of fuses when carrying their rated current plotted against current rating lr. This family of fuses had terminal elements or a non-ferrous metal and, therefore, the curves shown in FIG. 7 are not aiiected by eddy current losses which occur in fuses` having terminal elements of a ferrous metal, i.e. steel. Though FlG. 7 refers to a family of fuses in which `watt losses are minimized, it is apparent that watt losses W and temperature T increase significantly with increasing current ratings. Rise of temperature and Watt losses tend to become critical when fuses having a current rating of, or in excess of, 400 amps. and having terminal caps, or ferrules, of a ferrous metal, are housed in an enclosure. Hence it becomes necessary to give increased attention to watt losses wherever the current rating is in the order, or in excess of, 400 amperes and the fuses are intended .to be accommodated in an enclosure.

The block diagram of FIG. 8 refers to `a fuse structure substantially as shown in FIGS. 1-3, inclusive, having a current rating of 400i amps. and a voltage rating of 600 volts. This diagram shows temperatures in degrees C. plotted against load in percent of the rated current. Temperatures are indicated for two load currents, namely 100% and 110% of the rated current. The rectangles B indicate temperatures at the blade contacts and the rectangles C indicate temperatures at the center of the casing of a fuse according to FIGS. 1-3 having terminal caps or ferrules of steel. The rectangles B indicate temperatures at the blade contacts and the rectangles C indicate temperatures at the center of the casing of a fuse according to lFIGS. 1-3 having terminal caps or ferrules of brass. It is apparent from FIG. 8 that the substitution of brass for steel goes -a long way in obtaining a cool running fuse.

In FIG. l9 watt losses are plotted against current rating. FIG. 9 refers to a structure of the kind shown in FlGS. l1-3. losses obtaining if the structure is provided with terminal caps, or ferrules, of brass, while tl e columns S represent watt losses obtaining if the structure is provided with terminal caps, or ferrules, of steel. Itis apparent from FlG. 9 that substitution of brass for steel results in an outstanding performance improvement.

Electric fuses convert during circuit interruption electric energy into heat. The design of the particular fuse and in particular the geometry of its fuse link means and the nature of its pulverulent arc-quenching filler as well as the nature of the circuit under interruption determine the amount of heat generated in a given fuse under specified excess current conditions. FiG. 10l shows energy converted into heat in a fuse of given design plotted against current in terms of multiples of the current rating of the particular fuse. The diagram of FIG. 10 is typical of the operation of any current-limiting fuse.

" There is la vcritical range where energy dissipation or thermal stresses are highest, and this critical range is less than the maximum current which the fuse is'capable of safely successfully interrupting. The heat generated in a fuse is not highest at the highe-st currents which the fuse may be called to successfully interrupt but has its peak region at currents Way below the peak currents which the fuse may be called to interrupt. `lt is rather difficult to experimentally determine the pressure prevailing in a fuse casing and acting upon the terminal .caps or ferrules and stressing the latter. However, the energy dissipated in the casing, or converted into heat therein, is a fair measure of the pressure generated in a fuse casing during an interrupting process. -As long as a fuse is current-limiting and the peak of heat generation lies withinV the currentlimiting range of the fuse, the strength requirements of terminal caps, or ferrules, can be reduced significantly by so designing the fuse that its current-limiting action is substantial.

The pressure which is developed within the casing of a fuse and which acts upon the terminal caps or ferrules of the latter and tends to deform and damage the same depends upon a number of parameters. Gne of these parameters is, for instance, the grain size of the pulverulent arc-quenching filler which is arranged Within the casing of the fuse. Another very important parameter governing internal pressure during severe interruptions is the internal volume of the casing and the geometrical configuration thereof. The aforementioned internal pres- The rectangular columns marked B represent watt sure decreases inversely to the internal volume. Considering the internal volume of a fuse designed in accordance with the Underwriters Laboratories standards as point of departure, the volume of such a standard fuse may either be decreased or increased (see Standards for Safety Fuses UL 198 Underwriters Laboratories, Inc., reprinted `uly 1959). Progressive decrease of the volume of a standard fuse may result in such an increase of pressure during severe interruptions that the pressure cannot be contained any longer by any feasible means. Similarly, progressive increase of the volume of a standard fuse beyond any feasible dimensions may result in such a decrease of pressure during severe interruptions that containing the pressure within the casing does not present a problem any longer. What has been stated above in regard to the objects of this invention and what is stated below in regard to the means being employed in connection with this invention refers specifically to Underwriters Laboratories standards size fuses, i.e. fuses having casings of the sizes defined in the above standards. T he most important dimensions derived from the above standards defining casing size are length over caps designated A in FiG. 1, maximum width measured parallel and/ or at right angles to the plane of the blade contacts designated B in FIG. 3, and minimumv thickness of casing wall designated C in FlG. 2. Length over caps A is substantially equal to the length of the tubular casing. The values for the quantities A, B and Cas defined above are set forth in the table below:

Dimensions of cartridge-enclosed fuses in inches Electrical rating Volts Amps. B C

Regarding the relation between the pressure on the walls and the caps of a cartridge fuse and grain size, there is a definite or critical grain size resulting in minimum pressure. As a general rule, the grain size will be selected with a view to achieving certain electrical performance characteristics irrespective of resulting pressure generation.

While the pressure Within a fuse depends upon arc energy, there is no simple law relating both quantities, pressure depending on arc energy as well as on the rate at which arc energy is being released.

Static and transient pressures in a fuse can be determined in a number of ways, yet it is quite difficult to reliably carry out transient pressure measurements. in developing the structures of FIGS. 1-6 no numerical pressure measurements were made. The fuses were tested as to whether or not the pressure on the caps or ferrules was sufficiently low to preclude any deformation thereof, and to preclude shearing of the casing or fuse tube by the pressure transmitted from the caps or ferrules through the blade-contact-supporting pins 5 and 5' to the casing.

Most tests including non-ferrous caps were conducted with caps having a wall thickness of less than 0.07 inch.

Y in order to achieve economic designs the wall thickness must be reduced to this order, and the transient pressures within the casing must be so controlled, or'limited, to allow the use of non-ferrous caps which are asthin as that. The preferred non-ferrous metal used is red brass containing copper and 15% zinc. Fuses having a voltage rating of 250 volts can be provided with brass caps having a wall thickness substantially less than 0.07, e.g. a wall thickness of less than 0.045 inch, both for the 400 amp. and the 600 amp. rating. ln that particular instance a wall thickness of 0.04 inch would be satisfactory. Fuses having a voltage rating of 600 volts do not call for caps of increased wall thickness as long as the current rating thereof is not in excess of 400 amps. If the current rating is as high as 600 amps. and the voltage rating as high as 600 volts, the wall thickness should be increased to the order of 0.07 inch. The copper content of the brass should be at least in the order of 80 percent, irrespective of the particular voltage rating and current rating.

Referring now to FlG. 11 which applies to the voltage ratings of 250 volts and 600 volts, it is apparent from that figure that fuses having a current rating of 400 amps.

egin to limit shortcircuit currents if the latter are in excess of l0,000-1l,000 amperes, and that fuses having a current rating of 600 amperes begin to limit short-circuit currents if the latter are in excess of 16,000 amperes.

lG. 12 shows that fusion occurs in about 0.01 sec. at the occurrence of short-circuit currents in the order of 10,000 amperes, and 16,000 amperes, respectively.

Fuses according to this invention can be assigned Underwriters Laboratories current ratings. Whenever the terms current rating, or rated current are being used in this context, these terms have been used within a broader meaning, i.e. with reference to a current which a fuse can carry for an indenite period of time without excessive heating.

ln conventional low-voltage current-limiting high-interruptingcapacity fuses the arc-quenching filler is formed by quartz sand. Because of the extremely high de-ionizing or cooling action of that arc-quenching medium, it is in quartz-sand-lled fuses relatively easy to achieve a sufficiently elevated arc-voltage for the particular purpose in hand. Where the pulverulent arc-quenching iiller has a smaller de-ionizing and cooling action than quartz sand, the problem Vto achieve a suillciently elevated and sullciently stable arc-voltage requires particular consideration. The geometry of the ribbon type fuse links, and more particularly the number and size of their perforations, must be carefully determined if acceptable results, or best results, as regards arc-voltage and energy dissipation during the interrupting process are to be obtained.

The basic considerations concerning the design of fuse links with a view of generating the right kind of arcvoltage are outlined below. FIG. 13 shows a ribbon fuse link having but one single central perforation. This is a simplified version of the fuse links illustrated in FIGS. 1 6. Fuse links have a given length, a given width and a given thickness and these parameters determine primarily the current carrying capacity of a fuse link in a `given pulverulent arc-quenching iiller. ln other words, the current-carryingcapacity, or current rating, changes when the link is submersed in another arc-quenching filler. The specific heat of the metal of which the link is made and its latent heat of fusion together with the minimal cross-section q of the fuse link determine the melting ft2-dt or fusing ft2-dt and, therefore, also the currents required to cause fusion within given times not exceeding 0.01 sec. Thus the concept of current-limiting ratio can readily be translated into terms of dimensions of a fuse link, or terms of fuse link geometry.

The fuse link of FlG. 13 las a temperature gradient in a direction longitudinally of the link and a temperature gradient in transverse direction. The temperature within the plane of minimal cross-section is highest at the points T and smallest at the points t. Hence fusion is initiated at the points T and progresses toward the points t. Thus the link is initially severed by an incision progressively expanding from points T to points t. The result of this initial severing process is the formation of a short arc gap which may be in the order of 1/0 inch. Upon formation of that gap the period of back-burning begins, i.e. the period of gap growth in a direction longitudinally of the link. The arc voltage generated during the period of back-burning, or longitudinal gap growth, depends largely upon the diameter of the perforation or perforations of the link, as will become more apparent from the ensuing analysis of this phenomenon.

FIGS. 14 and l5 illustrate two links which are identical, except for the geometry and size of their center perorations. ln both fuse links the minimal cross-section q is equal and, therefore, the fusing fiZ-a't is equal in both instances. The same minimal cross-section has been obtained, in one instance, by providing one single center perforation having a relatively large diameter, and in the other instance, by providing a pair of center perforations having a relatively small diameter. The difference of two extreme typical designs in terms of arc voltage can be explained as follows:

The higher the current density in an electric arc, the higher the voltage drop across the arc or the are voltage. The higher the voltage drop across an arc, or the arc voltage, the higher the speed of burnoack from the relatively hot point of arc initiation to areas where the pulverulent arc-quenching filler is relatively cool. The higher the speed of burnback, the greater the stability of the arc voltage, as contrasted with an arc voltage which decays rapidly upon having reached an initial peak. Stable arc voltages tend to accelerate the decay of the current to zero and thus to reduce the arcing fig-dt as well as the transient pressure within the casing oi the fuse.

Leaving aside magnetic eld action upon current density, it is apparent from FTGS. 14 and l5 that the current densities at the time ci arc initiation will be about the same in both instances since the minimal crosssection q is the same in both instances. ln the case of FIG. l5 the current density will drop to a minimum upon a short burnhack equal to the relatively small diameter d of each of the two perlorations in the link. The aforementioned minimum current density largely depends upon the ratio of the cross-section of the iuse link to the instantaneous current being carried by it. lt is apparent from 14 referring to a link having one single rela tively large perforation whose diameter is D that in this case the current density does not decay to its minimum value upon as short a burnback as in the case or FIG. l5. ln other words, perforations of relatively large size tend to maintain the arc voltage at a relatively high evel. Thus it is in thinterest oi minimizing the arcing ,lf2-dt as well as the transient pressure to provide relatively large pertorations in the fuse link. lf the size of the pertorations is excessive this tends, however, to reduce the currentcarryingcapacity of the link. Thus the size or diameter of the perforations must be determined by striking a judicious compromise between arc voltage and pressure equirements, on the one hand, and current-carrying-capacity requirements, on the other hand.

The current trace illustrated in 16a and the voltage trace illustrated in FIG. 161i) show that fuses embodying the present invention have a current-limiting action, and that the latter is quite different from non mally encountered in current-limiting high-interruptingcapacity fuses having fulgurite-forming arc-quenching quartz fillers therein. The voltage trace illustrated in FIG. 16a is characterized by a relatively small rise of the arc voltage at the time t1 of arc inception. The initial arc voltage rises to less than 1.5 times circuit voltage. During a large portion of the arcing time the arc voltage rises while the current gradually decays to zero. This rise occurs immediately alter t1. After reaching a peak the arc voltage decreases. The current becomes zero at t2 and at t2 the voltage across the fuse is equal to the system voltage. The initial decrease of current intensity, i.e. the decrease in current intensity immediately following the time of arc inception t1 is relatively small and thereafter the current decays approximately at a constant rate. There is no signiiicant rise of tne voltage across the fuse in the interval of time between fault inception at to and arc inception at t1.

The lack of formation of a high voltage spike immediately upon arc inception at t1 may mainly be explained by the fact that the heat absorbing capacity of gas-evolving arc-quenching nllers such as gypsum is less than the heat absorbing capacity of quartz sand. The voltage gradient across an arc gap formed in quartz sand drops very rapidly as arcing continues and such a rapid decay of the voltage gradient does not seem to occur in current-limiting fuses having a gas-evolving non-fulguriteforming arc-quenching filler. Thus the relatively great stability of the arc voltage may be explained, at least in a large part, by the characteristics of the arc-quenching filler. he overlays db and 4b may have some slight delaying action in regard to fusion under short-circuit conditions, causing fusion to occur at the center lines of perforations shortly after fusion has occurred at the axially outer lines of perforations. Another factor tending to stabilize the arc voltage is the judicious selection oi the right diameter of the individual periorations of the fuse links, as explained above more in detail.

The relatively great uniformity and stability of the arc voltage makes it possible to keep the arcing figur within relatively narrow limits and thus to compensate for the tendency to generate relatively high pressure as a result or" the presence of arc-quenching lillers that evolve relatively large amounts oi gas under the heat of arcs. The limitation of the melting fizdt and of the arcing f2'dt make it possible to use inexpensive casings having a relatively limited rnechanical strength and to close the samek in a substantially gas-tight fashion by relatively light, or unsubstantial, caps or ferrules of a non-ferrous metal, preferably red brass, thus drastically reducing the watt losses occurring in such fuse structures.

Having disclosed preferred embodiments of my invention, it is desired that the same be not limited to the particular structure disclosed. it will be obvious to any person skilled in ti e art that many modifications and changes may be made without departing from the broad spirit and scope of the invention as defined by the following claims.

I claim as my invention:

l. An Underwriters Laboratories standard size lowvoltage fuse comprising:

a tubular casing of a homogeneous organic insulating material;

a pulverulent non-fulgurite-forming gas-evolving arcquenching filler inside said casing;

a pair of blade contacts extending in a direction longitudinally of said casing each projection from the outside of said casing into the inside thereof;

ribbon fuse link means of copper submersed in said iller conductively interconnecting the axially inner ends of said pair orV blade contacts, said fuse link means having a plurality of transverse lines of perforations forming serially related zones of reduced cross-section sufficiently small to result in a currentlimiting ratio of less than 30, and said fuse link means having dimensions adapted to impart to said fuse a current rating of at least 400 amperes;

a loW-iusing-point linkevering metal overlay on said fuse link means;

a pair of terminal caps each delining a passage for one of said pair of blade contacts and each closing one end of said casing sufii iently tight to substantially preclude the emission of hot products of arcing therefrom, and said pair of caps consisting of a non-ierrous metal.

2. An Underwriters Laboratories standard size lowvoltage fuse comprising:

a tubular casing of a homogeneous organic insulating Y material;

a pulverulent non-fulgurite-orrning gas-evolving arcquenching iiller inside said casing; Y

a pair oi blade contacts extending in a direction longitudinally of said casing each projecting from the outside of said casing into the inside thereof;

ribbon fuse link means oi copper submersed in said filler conductively interconnecting the axially inner ends of said pair of blade contacts, said fuse link means having a plurality of transverse lines of perforations forming serially related zones of reduced cross-section suiiciently small to result in a currentlimiting ratio of less than 3i), and said pair of fuse link means having dimensions adapted to impart to said fuse a current rating of at least 400 amperes;

a low-fusing-point link-severing metal overlay on said fuse link means;

a pair of terminal caps each delining a passage for one of said pair of blade contacts and each closing one end of said casing suliiciently tight to substantially preclude the emission of hot products of arcing therefrom, and said pair of caps consisting of brass.

3. An Underwriters Laboratories standard size lowvoltage fuse comprising:

a tubular casing of a homogeneous organic insulating material;

a pulverulent non-fulgurite-forming gas-evolving arcquenching filler inside said casing;

a pair of blade contacts extending in a direction longitudinally of said casing each projecting from` the outside of said casing into the inside thereof;

ribbon fuse link means of copper submersed in said filler conductively interconnecting the axially inner ends of said pair of blade contacts, said fuse link means having a plurality of transverse lines of perforations forming serially related zones of reduced cross-section sufficiently small to result in a currentlimiting ratio or less than 30, and said fuse link vmeans having dimensions adapted to impart to said fuse a current rating of at least 400 amperes, and the number of said zones of reduced cross-section and the geometry of the constituent perforations of said lines of perforations determining an average clearing ratio of less than 5;

a low-fusing-point link-severing metal overlay on fuse link means;

a pair of terminal caps each defining a passage for one of said pair of blade contacts and each closing one end of said casing suiciently tight to substantially preclude the emission of not products of arcing therefrom, and said pair of terminal caps consisting of a non-ferrous metal. Y

4. A one-time fuse having a voltage rating of 250 volts comprising:

a tubular casing of liber having a length of approximately 8% inches, a diameter of less than 2.406 inches and a wall thickness of at least 1A inch.

a pulverulent non-iulgurite-forming gas-,evolving arcquenching filler inside said casing; Y

a pair of blade contacts extending in a direction longitudinally of said casing each projecting from the outside of said casing into tie inside'thereof;

ribbon fuse link means of copper submersed in said filler conductively interconnecting the axially inner ends of said pair of blade contacts, said link means being adapted to impart to said fuse a current rating of 400 amperes, said fuse link means having three transverse lines of circular perforations forming serially related zones of reduced cross-sectional area, said zones being suihcently small to result in a current-limiting ratio of less than 30 and the diameter of the constituent perforations of said lines of perforations determining an average clearing ratio of less than 5;

an overlayor a low-fusing-point link-severing metal arranged on said fuse link means immediately adjacent the center line of said transverse lines of perforations;

a pair of terminal caps each deiining a passage for one of said pair of blade contacts and each closing one end of said casing suiiciently tight to substantially preclude the emission of hot products or arcing therefrom, and said pair of caps consi ing of brass.

said

an overlay of a loW-fusing-point link severing metal arranged on said fuse link means immediately ad- ]acent the center line of said lines of perforations;

mately 10% inches, a diameter of less than 2.906 inches and a wall thickness of at least 3/16 inch,

a pulver-nient non-fulgurite-forrning gas-evolving arcquenching iiller inside said casing;

a pair of blade contacts extending in a direction lontudinally of said casing each projecting from the outside of said casing into the inside thereof;

ribbon fuse link means of copper submersed in said iiller conductively interconnecting the axially inner ends of said pair of blade contacts, said fuse link means being adapted to impart to said -f-use a current rating of 400 amperes, said fuse link means having five transverse lines of circular perforations forming serially related zones of reduced cross-sectional area, said zones being sufficiently small to result in a current-limiting ratio of less than 30, and the diameter of the constituent perforations of said lines of pertorations determining an average clearing ratio of less than 5;

a pair of terminal caps each defining a passage for 5 one said pair of blade contacts and each closing one end of said casing suiiciently tight to substantially preclude the emission of hot products of arcin-g therefrom, and said pair of caps consisting of brass.

7. A one-time fuse having a voltage rating of 600 volts,

a tubular casing of tiber having a length of approxin mately 13S/8 inches, a diameter of less than 3.419 inches and a Wall thickness of at least 1A inch;

a pulverulent nonaf-ultgurite-forming gas-evolving arcquenching ller inside said casing;

rating of 600 amperes, said fuse link means havl5 a pair of blade contacts extending in a direction loning three transverse lines of circular perforations gitudinally of said casing each projecting from the forming serially related zones of reduced cross-secoutside of said casing into `the inside thereof; tional area, said zones being sufciently small to ribbon fuse link means of copper submersed in said result -in a current-limiting ratio of less than 30I and rler conductively interconnecting the axially inner the diameter of the constituent perforations of said ends of Said pair of blade contacts, said fuse link lines of perforations determining an average clearing means being adapted to impart to said -fuse a current ratio of less than 5; rating of 600 amperes, said ruse link means having an overlay of a low-fusing-point link severing metal ve transverse lines of perorations forming searranged on said fuse link means immediately adrially related Zones of reduced cross-sectional area, jacent the center line of said lines of perforations; Said ZOnes being suiiciently small to result in a cura pair of terminal caps each deining a passage for rent-limiting ratio of less than 30, and the diameter one of said pair of blade contacts and each closing of the constituent perforations of said lines of perone end of said casing sufficiently tight to substan- `fOratiOns determining an average clearing ratio of tially preclude the emission of hot products of arcing -less than 5; therefrom, and said pair of caps consisting of brass. an overlay of a loW-fusing-point link severing metal 6. A one-time fuse having a voltage rating of 600 arranged 0n said fuse link means immediately advolts comprising: jacent the center line of said lines of perforations; a tubular casing of fiber having a length of approxi- 'f1 P311' 0f tfmmfll CaPS each deing a Passage `OI mately 11S/s inches, a diameter of less than 2.906 3f one 0f Sad Pa.1I 0f Piaf-le COQHCS and Cach Closing inches and a Wall thickness of at least 9%16 inch, o 0F15 end of Sad Camlg Suclly tight t0 Substana pulverulent non-.fulgurite-forming gas-evolving arcliyfpcludedthe.mlsilonfof hot profluts d anfmg quenching uer inside Said casing; ere rom, an sai pair o caps consisting of biass. a Plil'dff llladef couacts extendg in a tfecon ll' 40 References Cited in the le of this patent gitu ma y o sai caslng eac proyec ing rom e outside of said casing into the inside thereof; UNITED STATES PATENTS ribbon fuse link means of copper submersed in said 2582x587 Burt et al- June 29, 1954 ller conductively interconnecting axially inner 283268 Kozaclfa A1913 29, 1958 ends of said pair of blade contacts, said fuse link redeflck Maf 3, 1959 means being adapted to impart to said `ruse a current 2:988620 Kg OTHER REFERENCES Jacobs: Current Limiting Fuses: Their Characteristics and Operations, AIEE Transactions, Part III, vol. 75,

1956, pages 988-993.

Claims (1)

1. AN UNDERWRITERS'' LABORATORIES STANDARD SIZE LOWVOLTAGE FUSE COMPRISING: A TUBULAR CASING OF A HOMOGENEOUS ORGANIC INSULATING MATERIAL; A PULVERULENT NON-FULGURITE-FORMING GAS-EVOLVING ARCQUENCHING FILLER INSIDE SAID CASING; A PAIR OF BLADE CONTACTS EXTENDING IN A DIRECTION LONGITUDINALLY OF SAID CASING EACH PROJECTION FROM THE OUTSIDE OF SAID CASING INTO THE INSIDE THEREOF; RIBBON FUSE LINK MEANS OF COPPER SUBMERGED IN SAID FILLER CONDUCTIVITY INTERCONNECTING THE AXIALLY INNER ENDS OF SAID PAIR OF BLADE CONTACTS, SAID FUSE LINK MEANS HAVING A PLURALITY OF TRANSVERSE LINES OF PERFORATIONS FORMING SERIALLY RELATED ZONES OF REDUCED CROSS-SECTION SUFFICIENTLY SMALL TO RESULT IN A CURRENTLIMITING RATIO OF LESS THAN 30, AND SAID FUSE LINK MEANS HAVING DIMENSIONS ADAPTED TO IMPART TO SAID FUSE A CURRENT RATING OF AT LEAST 400 AMPERES; A LOW-FUSING POINT LINK-SEVERING METAL OVERLAY ON SAID FUSE LINK MEANS; A PAIR OF TERMINAL CAPS EACH DEFINING A PASSAGE FOR ONE OF SAID PAIR OF BLADE CONTACTS AND EACH CLOSING ONE END OF SAID CASING SUFFICIENTLY TIGHT TO SUBSTANTIALLY PERCLUDE THE EMISSION OF HOT PRODUCTS OF ARCING THEREFROM, AND SAID PAIR OF CAPS CONSISTING OF A NON-FERROUS METAL.

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