US2861150A - Fuse structures - Google Patents

Description

Nov. 18, 1958 K. w. SWAIN F USE STRUCTURES Filed Dec. 22, 1954 United States Patent FUSE STRUCTURES Kenneth W. Swain, Hampton Falls, N. H., assignor to The Chase-Shawmut Company, N ewburyport, Mass.

Application December 22, 1954, Serial No. 476,936

15 Claims. (Cl. 200-131) This invention relates, generally, to electric fuses and to the protection of electric circuits by means of electric fuses and it has particular relation to current-limiting fuses and to the protection of electric circuits having a low circuit voltage by means of such fuses.

It is one object of this invention to provide a currentlimiting fuse which is easy to manufacture and which has considerable dimensional stability.

Another object of this invention is to provide a currentlimiting fuse having a ribbon-type fuse link, or a plurality of such links, which link or links each have a portion of drastically reduced cross-sectional area to achieve a drastic limitation of fault currents, in which fuse the danger of fracture of the link, or links, at the very fragile portion of reduced cross-sectional area thereof is greatly reduced, or entirely eliminated.

Another object of the invention is to provide a currentlimiting fuse having a very steep time-current'characteristic or, in other words, a current-limiting fuse wherein the blowing time is very short in the low overcurrent range.

Another object of the invention is to provide a very compact current-limiting fuse particularly suitable for applications where current-carrying requirements and requirements as to current-limiting action are high and the circuit voltage is relatively low, say less than 100 volts.

Another object of the invention is to provide a currentlimiting fuse comprising terminal elements adapted to perform the dual function of boosting the current-carryingcapacity of the fuse and of assisting in quenching the arc formed therein upon blowing thereof.

Another object of the invention is to provide a currentlimiting fuse suitable for application in the A.-C. and D.-C. circuits of dry rectifiers, such as germanium rectifiers.

The foregoing and other general and special objects of the invention and advantages thereof will more clearly appear from the ensuing particular description of the invention, as illustrated in the accompanying drawings, wherein- Fig. 1 is a side elevation of a cartridge fuse embodying my invention;

Fig. 2 is in part a longitudinal horizontal section and in part a top plan view of the structure shown in Fig. 1;

Fig. 3 is a longitudinal vertical section of the structure shown in Figs. 1 and 2;

Fig. 4 is a transverse section along 44 of Fig. 2; and

Fig. 5 is a transverse section similar to that shown in Fig. 4 of another embodiment of the invention.

Referring now to the drawing, and more particularly to Figs. 1 to 4 thereof, reference numeral 1 has been applied to indicate a pair of spaced metallic terminal blocks, preferably made of copper. Terminal blocks 1, which are preferably cylindrical in shape, are arranged in coaxial relation to each other. Each of terminal blocks 1 is provided with a blade contact 2 at the respective side thereof remote from the gap formed between, or the space bounded by, the two terminal blocks 1. Blade contacts 2 2,861,159 Patented Nov. 18, 1958 are provided with holes or perforations 2a to enable the fuse to be secured to the spaced contacts of a fuse holder, or to a pair of spaced bus bars, etc., by means of screwthreaded studs (not shown).

In Fig. 2 the circuit with which the fuse is associated has been diagrammatically indicated by two lines to which the reference letter C has been applied. This circuit has a predetermined available short-circuit current. This term refers to the current which would flow in the circuit upon occurrence of a short-circuit formed by a conductor having practically zero impedance. The circuit of the fuse structure shown in Fig. 2 is a low-voltage circuit, in particular a circuit having a circuit voltage of less than 100 volts, e. g. of 65 volts D.-C. The figure of 65 volts D.-C. is given by way of example and not intended to limit the scope of the invention. The circuit of the fuse may be either a D.-C. or an A.-C. circuit.

Each of the juxtaposed surfaces 10 of terminal blocks 1 is provided with a system of straight parallel grooves 7, 8. Grooves 7 extend across the centers of the circular surfaces la. The additional grooves 8 on each surface 1a are arranged in spaced relation from, and on opposite sides of, groove 7. A ribbon-type fuse link 5, preferably of silver, is inserted into groove 7 in blocks, and conductively connected to blocks 1. This may be achieved by joints of soft solder (not shown in the drawing). Fuse link 5 has a neck or reduced cross-section portion 5a situated midway between blocks 1 and formed by a pair of lateral, substantially V-shaped notches, or incisions. The two converging edges of the V-shaped notches defining neck 5a enclose an acute angle substantially less than deg. The cross-section of the portion of reduced crosssectional area or neck 5a is less than A of the normal cross-sectional area of link 5, i. e. the cross-sectional area at any point of link 5 where it is laterally bounded by a pair of parallel edges. For electrical reasons neck So should be as narrow as possible, e. g. 34 of the width of link 5. As a result, link 5 is very fragile and must be handled with great care before being assembled in a fuse structure. The length of the aforementioned portion of reduced cross-sectional area or neck 5a is preferably very short, i. e. so short that the portion of reduced crosssectional area or neck 5a approximates a point heat source when link 5 is carrying an electric current. The small width of the portion of reduced cross-sectional area or neck 5a is conducive to an intense current-limiting action or, in other words, to a great reduction of let-through currents compared to the available short-circuit currents. The shortness of the portion of reduced cross-sectional area or neck 5a is conducive to small l .r losses in the fuse link, and therefore to a cool running fuse.

The two parallel spacing plates 6 inserted into grooves 8 maintain a predetermined spacing between terminal blocks 1 and thus establish the dimensional stability required for protection of the fragile necked link 5 after the same has become an integral part of the fuse structure. Spacing plates 6 are made of a heat resistant electric insulating material comprising organic and inorganic constituents as, for instance, a synthetic-resin-glass-clothlaminate. A tin rivet 5b may be inserted into link 5 if the fuse is intended to be used for protection against protracted overloads, in addition to protection against major fault currents. If the circuit voltage is in the order of 65 volts, link 5 does not need to be immersed in a pulverulent arc-quenching filler. At relatively higher circuit voltages it is generally advisable to immerse the fuse link in a pulverulent arc-quenching filler, such as quartz sand. The casing 4 which consists of a relatively heat resistant insulating material is mounted on the two cylindrical copper blocks 1 and encloses the spacing plates 6 and the fuse link 5. A pair of ferrules 3 of sheet metal may be mounted on the axially outer ends of easing 4. Casing 4 and terminal blocks 1 are attached to each other by a plurality of transverse steel pins 9.

The minimum length of a link of a current-limiting fuse is generally deemed to be equal to the normal backburning distance of the link under the most onerous interrupting conditions which may occur in the fuse when interrupting a specified circuit. As a rule, the length of a fuse link is made considerably longer than the minimum length to provide a safety factor against action of the are upon the terminal elements of the fuse.

I have found that under certain conditions the length of the link or links of a current-limiting fuse may be considerably shorter than the normal back-burning distance of the link or links under the most onerous interrupting conditions which may occur in the fuse when the same is interrupting a specified circuit, and that great advantages may be achieved by shortening a fuse link beyond the limits deemed heretofore to be necessary and imperative. A fuse link may be made shorter than the aforementioned back-burning distance if the circuit voltage is relatively moderate, particularly if the circuit voltage does not exceed the order of 100 volts, and if the terminal elements of the fuse are formed by substantial metal blocks having such an intense cooling action upon the are when contacted by the are as to cause instant extinction of the latter. In other words, the length of a link may be less than the normally required minimum back-burning distance if the terminal elements are capable of acting as are barriers and arc-quenchers, limiting further growth or elongation of the art and effecting instant quenching thereof. Such conditions are present in the fuse structure and the circuit shown in Figs. 1 to 4, inelusive.

The spacing S between the juxtaposed surfaces la of the cylindrical copper blocks 1 is in the order of two or three times the width of the V-shaped, neck-forming notches at the open end thereof, and less than the normal back-burning distance of link 5 under the most severe interrupting conditions which may occur in the fuse when interrupting the circuit indicated by the reference letter C. As a result of providing the large copper masses 1 immediately adjacent the point-heat-source-forming neck 50 of fuse link 5, a novel kind of fuse performance is achieved, i. e. the fuse is much faster in the low overload range than any other current-limiting fuse I am familiar with. This will become more apparent from what follows:

The performance of fuses may be described by their time-current characteristic. showing the blowing time plotted against the current carried by the fuse. For fuses of the type shown in Figs. 1 to 3 designed for shortcircuit protection only. the time-current-characteristics are a family of straight lines if both the abscissae and the ordinates are plotted on a logarithmic scale. Each line in this family of lines corresponds to one particular current rating. All lines referring to a given type of fuses or design thereof have the same slope.

It is of importance for certain applications as, for instance. protection of germanium rectifiers, to increase the slope of the time-current-characteristic as much as possible. In other words. for certain applications it is desired that the first derivative of the time-current-characteristic be as high as po sible. The ideal for certain applications would be to have a time-current-characteristic virtually parallel to the axis of ordinates, i. e. parallel to the time axis. Such a fuse might be referred to as a ceiling device because of its ability to carry a given ceiling current and to interrupt instantly if that current is exceeded.

The fuse which has been shown in the patent drawing and has been described above is the closest approach to a ceiling device I am aware of.

I have conducted tests with identical fuses, keeping all variables unchanged, except the length of the link and the spacing between a pair of heavy terminal blocks of copper arranged at the axially outer ends of the link. It appears from these tests that the shorter the link and the smaller the spacing between the heavy terminal blocks of copper, the larger the current-carrying capacity of the link, and the larger the current rating thereof. As a result of the up-rating of the fuse caused by reduction of the length of the link and of the distance between the heavy terminal blocks to less than the normal back-burning distance of the link under the severest interrupting conditions which the circuit may impose upon the fuse, the fuse is much faster in the low overload range than any prior art fuse I am aware of. The low overload current range is deemed to be the range from about two times to about eight times the rated current of the fuse.

A fuse deemed to be very fast in the low overcurrent range is that disclosed in the copending patent application of Frederick J. Kozacka, Ser. No. 387,477, filed October 21, 1953, for Low-Voltage High-Capacity Current-Limiting Fuse, now United States Patent 2,734,111, issued February 7, 1956. The fuse disclosed in the copending Kozacka application has a blowing time in the order of hundredth of a second if carrying seven to eight times the rated current. A fuse built according to the teachings of the above copending patent application rated at 400 amps. blew within 22 seconds When caused to carry 1200 amps, i. c. three times the rated current thereof. The identical links as those present in this fuse were used in a number of tests in a circuit having a lower circuit voltage, i. e. 65 volts, permitting to shorten the length of the link and the spacing between the terminal blocks. Upon determination of the normal back-burning distance under the most severe interrupting conditions the circuit could impose upon the fuse, the length of the link and the spacing between the heavy terminal blocks was reduced to less than said critical back-burning distance. With this reduced length of link and distance between terminal blocks the rated current of the fuse was 300 amps. When caused to carry three times its rated current, i. e. 900 amps. the fuse blew within 2.5 seconds. By virtue of the low circuit voltage and the effectiveness of the terminal blocks as arc-quenchers at such low voltage levels, the fuse interrupted consistently in the absence of a pulverulent arc-quenching filler within its casing. It will be apparent that the minimum distance between the terminal blocks depends upon the circuit voltage, and even if full advantage is taken of the arcquenching ability of the terminal blocks, relatively more severe interrupting conditions may require provision of a pulverulent are quenching filler and some increase of the effective length of the link and of the spacing between the terminal blocks. Nevertheless, it is generally possible in circuits having a low circuit voltage to achieve blowing of a fuse at about two to three times the rated current thereof within five seconds, or at least less than ten seconds, provided that the spacing between the terminal blocks is less than the above referred-to critical back-burning distance, and provided that full advantage is taken of the arc-quenching ability of the terminal blocks and a pulverulent arc-quenching filler added, if necessary.

It follows from the foregoing that where a low voltage are is elongated by back-burning of a fuse link whose length is less than the back-burning distance required to extinguish the are under prevailing deionizing conditions, the are may be instantly extinguished if blocks of cold metal are arranged in the path of potential growth of the are, which decreases the arc temperature instantly below the temperature required for subsistence of the are and emission of electrons from the arc roots. To achieve the required cooling effect the masses of copper blocks 1 ought to be relatively large. It will be ob served from Figs. 2 and 3 that the copper blocks 1 which are arranged within casing 4 occupy a preponderant portion of the volume thereof.

The back-burning distance of a fuse link depends upon the design of the fuse of which the link forms an integral part, the circuit into which the fuse is inserted (circuit voltage, available current, frequency, power factor, number of phases) and upon the characteristics of the fault. The two most important characteristics of any fault are the fault starting angle and the voltage angle at melting of the neck of the link. In a circuit which is highly inductive the available current will be fully asymmetrical if the fault occurs at voltage zero. All other conditions remaining unchanged, the maximum severity in a fuse Occurs if arc initiation at the neck or restricted crosssection portion occurs slightly before or at peak voltage. If the length of link 5 and the spacing between copper blocks 1 is less than the back-burning distance of link 5 under the most severe interrupting conditions which may occur in the fuse when interrupting a specified circuit, and if the fuse is called to interrupt the circuit under less severe interrupting conditions than the circuit is capable of producing, only a portion of the link rather than the entire link may be consumed by the are formed incident upon blowing of the fuse.

Referring now to Fig. 5, this figure shows a cross-section of a fuse structure comprising a pair of spaced terminal elements of the same type as those shown in Figs. 1 to 4, inclusive. The juxtaposed surfaces of the terminal elements are each provided with two parallel spaced link-receiving grooves 7' and three parallel spaced grooves 8 for receiving three insulating barriers or partitions 6'. The barriers or partitions 6' operate as spacers for the terminal elements in the same fashion as the spacing plates 6 shown in Figs. 2 to 4, inclusive. The barriers or partitions 6' of the structure shown in Fig. 5 have the additional function of precluding formation of one single semi-conducting fulgurite by the pulverulent siliceous arc-quenching filler within the easing 4'. It will be noted from Fig. 5 that three fuse links 5 are arranged to the left and three fuse links 5 to the right of the central barrier 6' of insulating material which thus precludes the formation of a unitary fulgurite by the fuse links and the arc-quenching filler 10 arranged at different sides of central barrier 6'. The two radially outer insulating barriers 6' are primarily used as spacers between the terminal elements of the fuse. Their fit in casing 4' is a relatively loose one and metal vapors resulting from the vaporization of fuse link 5 are able to pass through the gaps formed between casing 4' and the radially outer barriers or spacing plates 6 and to reach the body of pulverulent filler or quartz sand 10 situated beyond the radially outer plates 6'. Thus any portion of the body of quartz sand 10 is effectively used to deenergize the products of arcing.

In both embodiments of the invention shown the linkrecetving groove or grooves and the spacing-plate-receiving grooves may be cut simultaneously by means of a suitable gang-type milling cutter. This reduces the manufacturing cost substantially compared to the manufacturing cost involved in manufacturing fuses comprislng spacing posts for the terminal elements rather than a plurality of spaced spacing plates. Structures of the kind shown in the patent drawing comprising a plurality of spaced spacing plates extending between the terminal elements tend to have a higher degree of dimensional stability than fuse structures having merely a spacing post between the terminal elements thereof.

The first step in assembling fuses of the type shown in the patent drawing consists in making a dimensionally stable sub-assembly comprising only the heavy terminal elements and their spacing plates of insulating material. This sub-assembly forms a self-contained structural unit. The fuse link or links may thereafter be inserted into the link-receiving grooves of this unit or sub-assembly without danger of injury to their very fragile neck or necks by a slight change in the relative position of the two terminal elements of the fuse structure.

The fact that the length of the link or links and the spacing between the terminal elements is less than the normal back-burning distance of the link or links under the most severe interrupting conditions which the circuit is capable of imposing upon the fuse tends to keep the size of the fuse very small, requiring a minimum of bulk per ampere current-carrying capacity.

Whenever the term length of link or similar language is used in this context, it is intended to refer to the effective length which is the length situated within the gap formed between the juxtaposed surfaces 1a of terminal blocks 1 rather than to the actual length which includes the portion of link 5 beyond the surfaces In, i. e. the portion of the link within groove 7. The actual length of a fuse link may be considerably longer than its effective length.

Whenever the term rated current or similar language is used in this context, it is intended to refer to a current which the fuse can carry continuously without undue heating. It appears from this definition that a current rating indicates a range of currents rather than a precise current value. The above referred-to fuse which was rated at 300 amps. was actually able to carry currents of 350 amps. for an indefinite period of time without undue heating. In other words, its rating of 300 amps. is rather conservative. The minimum fusing current of this fuse was about 400 amps. Blowing thereof occurred at an overload of 1000 amps, i. e. 2.5 times the minimum fusing current, in .2 second which is again much less than any prior art current-limiting fuse I am familiar with. Speaking in terms of rated current rather than minimum fusing current, the fuse blew in .2 second at 3.33 to 2.87 times the rated current, and the rated current was .3 to .35 times the current required to cause blowing of the fuse within .2 second.

As mentioned above it is often desirable to make the neck or reduced cross-section portion very narrow. A fuse link having a neck width of of the normal link width imposes less difficult problems than a link wherein the ratio of neck width to normal link width is considerably smaller. With decrease of that ratio the links become increasingly fragile, and decrease of that ratio may also tend to cause transient overvoltages incident to blowing of the fuse since it may result in increased rates of change of current.

Where utmost dimensional stability on account of particularly small neck dimensions is indicated, it is desirable to apply a high local pressure to opposite sides of blocks 1 at right angles to the planes of spacing plates 6 in order to drive or displace some of the copper of which blocks 1 are made radially inwardly into grooves 8, thereby reducing the cross-sectional area thereof. This locks spacing plates 6 firmly into their grooves 8. Such a sub-assembly has sufficient dimensicnal stability to safely receive the most fragile ribbon-type fuse link.

I have found that in current-limiting fuses having fuse links with necks closely approximating a point heat source the current-limiting action may be so drastic as to require a means for decelerating current decay from the limited peak of the let-through current down to zero. Omission of an aroquenching filler within the casing of the fuse combined with a reduction of the size of the arcing chamber have been found to be the simplest means to control the rate if current-decay in such a way as to preclude dangerous voltage surges. By assigning the preponderant portion of the volume of the casing 4 to the terminal blocks 1, and omitting an arc extinguishing filler within the small arcing and deionizing chamber left within the casing 4 of the fuse, the rate of current decay may be kept within the relatively narrow limits imposed by very critical applications such as, for instance, protection of dry rectifiers, e. g. germanium rectifiers.

It will be understood that I have illustrated and described herein two 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:

1. A fuse comprising a pair of spaced metallic cylindrical blocks, a plurality of substantially spaced grooves in each of the juxtaposed surfaces of said pair of blocks, a plurality of spacing plates of an electric insulating material inserted into said plurality of grooves to maintain a predetermined spacing between said pair of blocks. fuse link means spaced from said plurality of spacing plates conductively interconnecting said pair of blocks, and a casing of insulating material mounted on said pair of blocks enclosing said plurality of spacing plates and said fuse link means.

2. A fuse comprising a pair of spaced cylindrical metal blocks, three spaced parallel grooves in each of the juxtaposed surfaces of said pair of blocks, a pair of parallel spacing plates of an electric insulating material inserted into the outer of said grooves to maintain a predetermined spacing between said pair of blocks, a ribbon-type fuse link having a portion of restricted cross-sectional area situated between the axially outer ends thereof inserted into the inner of said grooves and conductively connected to each of said pair of blocks, and a casing of insulating material mounted on said pair of blocks enclosing said pair of spacing plates and said fuse link.

3. A fuse comprising a pair of spaced coaxially arranged substantially cylindrical metal blocks each provided with a blade contact projecting from the axially outer surface thereof, three spaced grooves in each of the juxtaposed surfaces of said pair of blocks, a pair of parallel spacing plates of a heat resistant electric insulating material having organic and inorganic constituents inserted into the outer of said grooves to maintain a pre determined spacing between said pair of blocks, a ribbontype fuse link having a portion of restricted cross-sectional area less than of the normal cross-sectional of said link inserted into the inner of said grooves to conductively interconnect said pair of blocks, and a tubular casing of insulating material mounted on said pair of blocks enclosing said pair of spacing plates and said fuse link.

4. A fuse comprising a pair of spaced coaxially arranged substantially cylindrical metal blocks each provided with a blade contact projecting from the axially outer surface thereof, a first groove in each of the juxtaposed surfaces of said pair of blocks extending across the center thereof, a pair of additional grooves in each of said juxtaposed surfaces, each of said pair of additional grooves being arranged in relation on opposite sides of and parallel to said first groove, a pair of spacing plates of a heat resistant synthetic-resinglasscloth-laminate inserted into said pair of additional grooves in said pair of blocks to maintain a predetermined spacing thereof, a ribbon-type fuse link having a portion of restricted cross-sectional area less than A; of the normal cross-sectional area thereof inserted into said first groove in each said pair of blocks to conductively interconnect said pair of blocks, and a tubular casing mounted on said pair of blocks enclosing saicl pair of spacing plates and said fuse link.

5. In combination an electric circuit having a circuit voltage less than 100 volts and a short-circuit current, a current-limiting fuse arranged in said circuit for limiting major fault currents therein to values less than said available short-circuit current, said fuse comprising a fuse link having a portion of restricted cross-sectional area less than 1 of the normal cross-sectional area of said link, and a pair of spaced metal blocks arranged at the axially outer ends of and conductively connected with said link. the spacing between juxtaposed surfaces of said metal blocks being less than the normal back-burning distance of said link under the most onerous interrupting conditions which may occur in said fuse when interrupting said circuit.

6. In combination an electric circuit having a circuit voltage of less than volts and a predetermined available short-circuit current, a current-limiting fuse arranged in said circuit for limiting the major fault currents therein to values less than said available short-circuit current, said fuse comprising a fuse link having a short portion of restricted cross-sectional area approximately a point heat source when said link is carrying an electric current, a pair of spaced copper cylinders coaxially arranged at the axially outer ends of and conductively connected to said link, grooves in juxtaposed surfaces of said copper cylinders, spacing plates of insulating material inserted into said grooves to maintain a predetermined spacing between said juxtaposed surfaces, said predetermined spacing being less than the normal back-burning distance of said link under the most onerous interrupting conditions which may occur in said fuse when interrupting said circuit.

7. In combination a circuit having a circuit voltage of less than 100 volts and a predetermined available shortcircuit current, a current-limiting fuse arranged in said circuit for limiting major fault currents therein to values less than said available short-circuit current, said fuse comprising a fuse link having a short portion of restricted cross-sectional area approximating a point heat source when said link is carrying an electric current, a pair of spaced metal cylinders coaxially arranged at the axially outer ends of and conductively connected to said link, a first pair of grooves each in one of the juxtaposed surfaces of said pair of metal cylinders and each receiving one of said axially outer ends of said link, a pair of additional grooves in each of said juxtaposed surfaces, each of said pair of additional grooves being arranged in spaced relation on opposite sides of and parallel to one of said first pair of grooves, a pair of parallel spacing plates inserted into said pair of additional grooves in said juxtaposed surfaces to maintain a predetermined spacing therebetween, said predetermined spacing being less than the normal back-burning distance of said link under the most severe interrupting conditions which may occur in said fuse when interrupting said circuit.

8. In combination an electric circuit having a circuit voltage of less than 100 volts and a predetermined available short-circuit current, a current-limiting fuse arranged in said circuit for limiting major fault currents therein to values less than said available short-circuit current, said fuse comprising a ribbon-type fuse link having an extremely short portion of restricted cross-sectional area approximating a point heat source when said link is carrying an electric current, a pair of spaced metal blocks arranged at the axially outer ends of and conductively interconnected by said link, the length ofsaid link and the spacing between said pair of metal blocks being less than the normal back-burning distance of said link under the most severe interrupting conditions which may occur in said fuse when interrupting said circuit.

9. in combination an electric circuit having a predetermined available short-circuit current, a current-limiting fuse arranged in said circuit for limiting major fault currents therein to values less than said available shortcircuit current, said fuse comprising a fuse link having a portion of restricted cross-sectional area less than of the normal cross-sectional area of said link, a pair of spaced metallic terminal blocks arranged at the axially outer ends of said link sufficiently close to said portion of restricted cross-sectional area to impart a current-rating to said fuse in the order of .3 times the current required to cause blowing of said fuse in the order of .2 second, and a pair of spacing plates of insulating material arranged on opposite sides of and substantially spaced from said link to maintain the spacing between said pair of terminal blocks.

10. In combination an electric circuit having a circuit voltage of less than 100 volts and a predetermined shortcircuit current, a current-limiting fuse arranged in said circuit for limiting major fauft currents therein to values less than said available short-circuit current, said fuse comprising a fuse link having a portion of restricted cross-sectional area less than 4 of the cross-sectional area of said link, a pair of spaced metallic terminal blocks arranged at the axially outer ends of said link sufiiciently close to said portion of restricted cross-sectional area to impart a current rating to said fuse in the order of .3 times the current required to cause blowing of said fuse within about .2 second, and a pair of spacing plates of a heat resistant synthetic-resin-glass-cloth-laminate arranged on opposite sides of said link in substantially spaced relation thereto to maintain the spacing between said pair of terminal blocks.

11. In combination an electric circuit having a circuit voltage of less than 100 volts and a predetermined available short-circuit current, a current-limiting fuse arranged in said circuit for limiting major fault current therein to values less than said available short-circuit current, said fuse comprising a casing of insulating material, a pair of spaced metal terminal blocks arranged on opposite ends of said casing and occupying a preponderant portion of the volume thereof, a ribbon type fuse link having a portion of restricted cross-sectional area adapted to approximate a point heat source when said link is carrying an electric current conductively interconnecting said pair of metal blocks, said pair of metal blocks being arranged sufliciently close to said portion of restricted cross-sectional area of said link to impart a current rating to said fuse of more than .3 times the current required to cause blowing of said fuse in about .2 second, and a pair of spacing plates of insulating material each arranged on opposite sides of said link to maintain the spacing between said pair of metal blocks.

12. In combination an electric circuit having a predetermined available short-circuit current, a current-limiting fuse arranged in said circuit for limiting major fault currents therein to values less than said available short-circuit current, said fuse comprising a fuse link having a portion of restricted cross-sectional area less than the normal cross-sectional area of said link and sufliciently short to approximate a point heat source when said link is carrying an electric current, a pair of metallic terminal blocks conductively connected by said link equally spaced from said portion of restricted cross-sectional area and spaced at distance from each other less than the back-burning distance of aid link under the most onerous interrupting conditions which may occur in said fuse When interrupting said circuit, and a pair of spacers of insulating material each arranged to one side of said link for rigidly maintaining said distance between said pair of terminal blocks.

13. In combination an electric circuit having a circuit voltage of less than 100 volts and a predetermined available short-circuit current, a current-limiting fuse arranged in said circuit to limit major fault-currents therein to values less than said available short-circuit current, said fuse comprising a substantially tubular casing in insulating material, a pair of spaced terminal blocks of metal arranged within at opposite ends of said casing and occupying a preponderant portion of the volume thereof, a ribbon-type fuse link having a portion of restricted crosssectional area less than $5 of the normal cross-sectional area of said link conductively interconnecting said pair of blocks, the arcing chamber defined by said casing and the juxtaposed surfaces of said pair of blocks being solely filled with atmospheric air as an arc-extinguishing medium, and the spacing between said juxtaposed surfaces being less than the normal back-burning distance of said link under the most severe interrupting conditions which may occur in said fuse when interrupting said circuit.

14. A current-limiting fuse comprising a pair of spaced metallic terminal elements, a plurality of grooves in each of the juxtaposed surfaces of said pair of terminal elements, a plurality of spacing plates of an electric insulating material inserted into said plurality of grooves to maintain a predetermined spacing between said pair of terminal elements, means for locking said plurality of spacing plates in said plurality of grooves including portions of the metal of which said pair of terminal elements is made displaced into said plurality of grooves to reduce the cross-sectional area thereof, a ribbon-type fuse link conductively interconnecting said pair of terminal elements, and a casing enclosing said plurality of spacing plates and said fuse link.

15. In combination an electric circuit having a predetermined available shortcircuit current at a given point thereof, a current-limiting fuse arranged in said circuit at said given point for limiting fault currents therein to values less than said available short-circuit current, said fuse comprising a ribbon-type fuse link having a pair of substantially V-shaped lateral notches, said pair of notches jointly defining a short portion of restricted crosssectional area between the closed ends thereof approximating a point heat source when said link is carrying an electric current, a pair of spaced coaxial cylindrical metal blocks arranged at the axially outer end of and conductively interconnected by said link, the length of said link and the spacing between said pair of metal blocks being sufficiently small to impart a current rating to said fuse in the order of .3 times the current required to cause blowing of said fuse in about .2 second, and a substantially tubular casing housing said link mounted on said pair of metal blocks.

References Cited in the file of this patent UNITED STATES PATENTS 652,748 Cote July 3, 1900 1,388,269 Kramer Aug. 23, 1921 1,953,011 Cote Mar. 27, 1934 2,665,348 Kozacka Jan. 5, 1954 2,670,418 Kozacka Feb. 23, 1954 2,734,111 Kozacka Feb. 7, 1956 2,734,112 Kozacka Feb, 7, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,861,150

November 18, 1958 Kenneth W. Swain pecification n requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 50, for "l .r" read I .r column 8, line 9,

for "approximately" read approximating Signed and sealed this 3rd day of March 1959.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents

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