US3491322A - Electric multifunction fuse - Google Patents

Description

. Jan. 20, 1970 F. J. KOZACKA 3,491,322

ELECTRIC MULTIFUNCTION FUSE Filed Sept. so. 1968 INVENTOR:

FREDERICK J. KOZACKA ATTY.

United States Patent O 3,491,322 ELECTRIC MULTIFUNCTION FUSE Frederick J. Kozacka, South Hampton, N.H., assignor to The Chase-Shawmut Company, Newburyport, Mass. Filed Sept. 30, 1968, Ser. No. 763,585

Int. Cl. H01h 71/20 US. Cl. 337165 4 Claims ABSTRACT OF THE DISCLOSURE An electric cartridge fuse having substantial time-lag in the over load range and a large interrupting capacity is provided with means capable of interrupting the circuit into which the fuse is inserted on occurrence of excessive heating at the point of engagement of its knife blade contacts with the cooperating contacts of a fuse holder. This is achieved by the provision of a normally current-carrying switch in the center region of the casing of the fuse spring biased to the open position thereof and normally held in the closed position by solder means having a softening point less than 300 F. The fuse further includes a pair of ribbon fuse link means for interrupting major fault currents. These fuse link means are arranged on opposite sides of the aforementioned switch and immersed in a pulverulent arc-quenching filler. Each of these fuse link means has an axially outer portion and an axially inner portion substantially coplanar with the blade contacts of the fuse. Each of these fuse link means further has an intermediate loop-shaped radially outwardly and radially inwardly extending portion defining at least one point of reduced cross-sectional area.

BACKGROUND OF INVENTION Fuseswhich are arranged in motor circuits are required not to blow on occurrence of harmless transients of relatively short duration, i.e., having the magnitude and duration of motor starting currents, to. blow with time lag if currents of such magnitude exceed a permissible time limit, to blow very fast when the current exceeds a given multiple of the rated current and/or is of major fault current proportions, and further to blow at very small overload currents not capable of doing any harm to equipment such as motors or transformers, if excessive heat is generated by such currents at the-interfaces between knife blade contactsof the fusesand fuse holder contacts engaged. by the knife blade contacts. Fuses having such performance characteristics will hereinafter be referred-to as multifunction fuses.

There are numerous prior art devices attempting to copewith the complex duties of multifunction fuses. None of these prior art devices isfully satisfactory. This invention offers a better solution to the above'referred-to complex duty requirements of multifunction fuses than the prior art has to offer.

Fuses wherein in case of. excessive overloads a circuit interrupting break is formed by a metallurgical reaction involving. metal interdiflfusion between a high fusing point base metal and a low fusing point overlay metal are generally not responsive'to excessive external heat generation at moderate overloads due to high contact resistance between the knife blade contacts of such fuses and the fuse holders receiving their knife blade contacts.

In order to achieve ready responsiveness to external heat sources as formed, for instance, by defective fuse holders, fuses have been provided with externalspring biased switching devices normally held in closed position bysolder joints, or such switching devices have been arranged close to the ends of the casings of the fuses. This design principle is shown, for instance, in German Patent 910,690, issued May 6, 1954. The arrangement of points of break outside of the fuse casing, and the provision of two solder-joint-controlled, spring-biased switching devices-each at one end of the casingare undesirable features of the aforementioned type of prior art fuses.

Other prior art fuses avoid the limitations and drawbacks of the type of fuses disclosed in the aforementioned German patent. This is, however, achieved by sacrificing current limitation in terms of peak let-through currents and. by sacrificing current limitation in terms of letthrough i -t values. These prior art fuses, therefore, do not qualify for the requirements imposed by the Underwriters Laboratory Standards on Class K-S fuses. It is, therefore, an object of this invention to provide multipurpose fuses which do not compromise one function at the expense of other important functions. In other words, it is an object of this invention to provide multipurpose fuses wherein responsiveness to external heat sources is not achieved by compromising in regard to current-limitation. To be more specific, it is an object of this invention to provide multi-purpose fuses fully complying with the conditions imposed by the Underwriters Laboratory Standards for Class K-S fuses, which means to comply with the conditions set forth in the table below.

Allowable peak let- Allowable let-through through current ifit Fuse rating (kilo amps) (amp.'-'sec.)

This invention relates more specifically to a modification of the structure of US. Patent 2,321,711 which is responsive to heat generated by external heat sources, i.e.', heat generated at the interfaces between the blade contacts of the fuse and cooperating fuse holder contacts, and which fuse structure is capable of generating relatively high are voltages, thus tending to maximize its interrupting capacity.

SUMMARY OF THE INVENTION Fuses embodying this invention include a tubular casing of electric insulating material, and a pair of terminal caps mounted on the axially outer ends of the casing, and closing the casing. A pair of knife blade contacts project from the outside of the casing through the pair of terminal caps into the inside of the casing. A spring biased switching device is arranged inside of the casing in the center region thereof. Solder means normally maintain the switching device in the closed position against the spring bias thereof, allowing opening of said switching device under the action of said spring bias upon reaching a predetermined softening point. The softening point of said solder means is less than 300 F. The fuse further includes a pair of ribbon fuse link means each immersed in a pulverulent arc-quenching filler inside the casing of the fuse. Each fuse link means connects conductively the axially inner end of one of said pair of blade contacts to said switching device. Each of said pair of fuse link' means includes an axiallyouter portion substantially coplanar with said pair of blade contacts, an axially inner portion substantially coplanar with said pair of blade contacts, and an intermediate loop-shaped radially outwardly and radially inwardly extending portion defining at least one point of reduced cross-sectional area.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a longitudinal section of a multipurpose fuse embodying this invention taken substantially along 11 of FIG. 2;

FIG. 2 is a side elevation of the structure of FIG. 1;

FIG. 3 is an isometric view of the region where. the fuse link means is attached to the blade contacts; and

FIG. 4 is an end view of the structure of FIGS. 1 and 2 seen in the direction of the arrow R of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, numeral 1 has been applied to indicate a tubular casing, or fuse tube, of insulating material, e.g., a synthetic resin. A pair of terminal elements 2 in the form of caps close the axially outer ends of easing 1. A pair of knife blade contacts have axially outer ends 3a outside of casing 1 and axially inner ends 3b inside of casing 1. Caps 2 have rectangular apertures 2a in the end surfaces thereof, and blade contacts 3 project through these apertures from the outside of casing 1 into its inside. A tubular spring enclosure, or spring housing, 4 of metal is arranged inside of easing 1 in the center region thereof. The left side of spring enclosure, or spring housing, 4 is closed by a disc-shaped metal cover 4a. A helical spring 5 is arranged inside of the space bounded by spring enclosure, or spring housing, 4. Spring 5 acts upon the flange of a plunger 6. One end of plunger 6 is arranged inside of enclosure 4, and the other end thereof projects outside of enclosure 4. Spring 5 biases plunger 6 to further move into spring housing, or spring enclosure, 4. Parts 4, 5 and 6 are arranged in coaxial relation. The structure further includes a pair of solder joints 7, 8. Solder joint 7 normally maintains plunger 6 in position against the bias of spring 5. Solder joint 8 is arranged on one end surface of plunger 6 for electrically connecting plunger 6 to a fuse link means. The softening point of solder joints 7 and 8 is less than 300 F. Best results were obtained with solder softening at 286 F. Casing 1 is filled with a pulvenllent arcquenching filler 9 in which parts 4, 4a, 6, 7 and 8 are immersed. The fuse structure further includes a pair of ribbon fuse link means 10, 11 which are also immersed in the pulverulent arc-quenching filler 9. Ribbon fuse link means 10 conductively connects the axially inner end 3b of one blade contact 3 to spring enclosure 4. To be more specific, the fuse. link 10 connects left blade contact 3 conductively with the cover 4a of spring housing 4 by means of a solder joint 12. Ribbon fuse link means 11 conductively connects the axially inner end 3b of right blade contact 3 to plunger 6, this conductive connection including solder joint 8. Ribbon fuse link means 10 and 11 both include an axially inner portion 10a and 11a, respectively, and an axially outer portion 10b and 11b, respectively. The intermediate portions 10c and 11c, respectively, of ribbon fuse link means 10, 11 are arranged between their axially inner portions 10a, 11a and their axially outer portions 10b, 11b. The axially inner portions 10a, 11a and the axially outer portions 10b, 11b of fuse link means 10, 11 are arranged substantially in the common plane of blade contacts 3, 3. In other words, parts 3, 3, 10a, 10b, 11a, 11b are substantially coplanar. The portions 100, 11c of fuse link means 10, 11 form radially outwardly and radially inwardly extending loops and portions 10c, 11c define at least one point of reduced cross-sectional area.

As shown in FIG. 3, the axially outer end of fuse link means 10 and the intermediate loop-shaped portion 10c of fuse link means 10 are. bifurcated, thus forming a point of reduced cross-sectional area. Fuse link means 10 defines two additional points of reduced cross-sectional area 10 and 10". These additional points of reduced crosssectional area are formed by a pair of parallel lines of circular perforations. Fuse link means 11 has the same configuration as fuse link means 10 and, therefore, does not need to be described in detail.

Reference character 10 has been applied to indicate the aperture in fuse link means 10 by which its third point of reduced cross-sectional area in intermediate portion 10c is formed. A pin 13 projects through aperture 10" in portion 10c of fuse link 10. In a like fashion a pin 13 projects through an aperature in the intermediate portion 11c of fuse link means 11. Pins 13 are circumferentially flexible and project through registering circular apertures in blade contacts 3 and in casing 1, thus supporting blade contacts 3 by casing 1.

The axially outer ends of portions 10b, 11b of fuse link means 10, 11 may be spot-welded to blade contacts 3, as indicated at 10d in FIG. 3.

' Parts 4, 4a, 5 6, 7 and 8 form a heat sink absorbing the heat generated by fuse link means 10 and 11. When solder joints 7 and 8 soften at a relatively low temperature, plunger 6 is moved from right to left under the action of helical spring 5, thus forming a circuit-interrupting break between plunger 6 and the axially inner portion 11a of fuse link means 11.

On occurrence of major fault currents breaks are formed at the points of reduced cross-sectional area 10,

10", of fuse link means 10. Like breaks are formed in fuse link means 11. The cross-sectional area at the bi furcated ends of fuse link means 10, 11 is sufficiently small to contribute significantly to the generation of heat in fuse link means 10, 11, but is much larger than the cross-sectional area of the two axially inner points of reduced cross-sectional area of fuse link means 10, 11, and so large as never to result in fusion at said bifurcated ends. Thus, even under severe interrupting conditions, arcing is kept away from the resilient blade-contact-supporting metal pins 13. Y

The loop-shaped portions and of fuse link means 10, 11 tend to increase the arc voltage since they increase the length of the fuse link means 10 and 11 and the permissible length of burn-back. Because of the relative increase in length of fuse link means 10, 11 and the relative increase of the number of points of reduced cross-v sectional area, the configuration of fuse link means 10, 11 tends to decrease the current rating-of the fuse structure.

Hence, in order to achieve a given current rating, the

holder and there is excessive heat generation at therinterfaces between the contacts of the fuse holder (not shown) and one or both of blade contacts 3, there is an additional heat flow which is directed axially inwardly toward parts 4, 4a, 5 and 6 and toward solder joints 7' and 8.'If the fuse carries but a relatively small current, not capable of causing softening and yielding of solder joints 7, 8, and the heat flow caused by i r losses in fuse linkmeans 10, 11 is increased by heat externally applied from the interfaces between blade contacts 3 and the fuse holder contacts engaged by blade contacts 3, the solder joints 7 and 8 reach their relatively low softening and yielding point of less than 300 deg. F. This causes opening of'the circuit in situations where the heat generated inside of the fuse is insufficient to cause interruption of the circuit before damage is done by heat which is generated outside of the fuse structure.

As long as a fuse holder is in good condition, the heat generated at the interface between the blade contacts of the fuse and the fuse holder contacts is relatively small, and cannot result in damage of any sort. But where a distribution system is not properly maintained, where the pressure of the fuse holder contacts upon the blade contacts of a fuse is too small, and the fuse holder contacts and the blade contacts of the fuse badly oxidized, there is a need for a multifunction fuse as described above, and illustrated in the accompanying drawings.

It will be understood that the parts 4, 4a, 5 and 6 are a spring-biased switching device arranged in the center region of casing 1, and involving but a very small voltage drop on account of the small resistance of solder joints 7, 8 and 12.

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

I claim as my invention:

1. An electric fuse including:

(a) a tubular casing of electric insulating material;

(b) a pair of terminal caps mounted on the axially outer ends of said casing and closing said casing;

(c) a pair of knife blade contacts projecting from the outside of said casing through said pair of terminal caps into the inside of said casing;

(d) a spring biased switching device connecting said contacts and arranged inside of said casing in the center region thereof;

(e) solder means normally maintaining said switching device in the closed position against the spring bias thereof and allowing opening of said switching device under the action of said spring bias upon reaching a predetermined softening point, the softening point of said solder means being less than 300 F.;

(g) a pair of ribbon fuse link means each immersed in a pulverulent arc-quenching filler inside of said casing and each conductively connecting the axially inner end of one of said pair of blade contacts to said switching device, each of said pair of ribbon fuse link means including an axially outer portion substantially coplanar with said pair of blade contacts, an axially inner portion substantialy coplanar with said pair of blade contacts and an intermediate loop-shaped radially outwardly and radially inwardly extending portion defining at least one point of re duced cross-sectional area.

2. An electric fuse as specified in claim 1 wherein said point of reduced cross-sectional area of each of said pair of ribbon fuse link means is formed by an aperture in said intermediate portion thereof, and wherein a pin projects through said aperture in said intermediate portion of each of said pair of ribbon fuse link means and supports the axially inner end of one of said pair of blade contacts.

3. An electric fuse as specified in claim 1 wherein said spring biased switching device includes a tubular closed spring housing, a helical biasing spring inside of and arranged in coaxial relation to said housing and a plunger arranged in coaxial relation to said housing and said spring and having an end inside said housing and an end outside said housing and being biased by said spring to move further into said housing.

4. An electric fuse including (a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the axially outer ends of said casing;

(c) a pair of knife blade contacts having axially outer ends outside of said casing and having axially inner ends inside of said casing, said pair of knife-blade contacts being positioned in a common plane;

(d) a tubular spring enclosure arranged inside of said casing in the center region thereof, said spring enclosure housing a plunger and a helical spring biasing said plunger to further move into said spring enclosure;

(e) a pair of solder joints, one of said pair of solder joints normally maintaining said plunger in position against the bias of said spring and the other of said pair of solder joints being arranged on one end surface of said plunger for electrically connecting said plunger to a fuse link means, both of said pair of solder joints having a softening point of less than 300 F.;

(f) a pulverulent arc-quenching filler inside said casing immersing said spring enclosure; and

(g) a pair of ribbon fuse link means each immersed in said pulverulent arc-quenching filler, one of said pair of ribbon fuse link means conductively connecting the axially inner end of one of said pair of knife blade contacts to said spring enclosure and the other of said pair of ribbon fuse link means conductively connecting the axially inner end of the other of said pair of knife blade contacts to said one end surface of said plunger by the intermediary of said other of said pair of solder joints, each of said pair of ribbon fuse link means including an axially outer portion and an axially inner portion and an intermediate portion between said axially outer portion and said axially inner portion, said axially outer portion and said axially inner portion of both of said pair of ribbon fuse link means being arranged substantially in said common plane of said pair of knife blade contacts and said intermediate portion of both of said pair of fuse link means forming a radially outwardly and radially inwardly extending loop and defining at least one point of reduced cross-sectional area.

References Cited UNITED STATES PATENTS 2,300,142 10/1942 Wood 337 2,300,620 11/1942 Duerkob 337-165 2,321,711 6/1943 Taylor 337165 2,773,961 12/1956 Sundt 337166 3,253,103 5/1966 Fister 337158 X H. B. GILSON, Primary Examiner US. Cl. X.R. 337-158, 166

Images