US2667549A - Electric fuse construction - Google Patents

Description

Jan. 26, 1954 FAHNQE ETAL 2,667,549

ELECTRIC FUSE CONSTRUCTION Filed May 29, 1952 27 Fig.2. [5

VENTORS H. Fohnoe 8| Patented Jan. 26, 1954 UNITED STATES 2, Kidd ATENT OFFICE ELECTRIC FUSE CONSTRUCTION Harold Fahnoe,-Wilkinsburg, and William A.

Gaskill, Turtle Creek, Pa, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa.., a corporation of Pennsylvania Application May 29, 1952, Serial No. 290,662

14 Claims. 1

This invention relates to electric circuit inter rupting devices in general, and, more particu larly, to electric fuse constructions.

In U. S. Patent 2,502,992, issued April 4, 1.950, to Herbert L. Rawlins and Harold H. Fahnoe, and U. S. Patent 2,495,70 i, issued February 7, 1950, to l-larold H. Fahnoe, both patents being assigned to the assignee of the instant application, there is set forth, and claimed, a particular type of fuse, known as a current-limiting fuse. This type of fuse consists of a plurality of fusible fuse wires connected in electrical parallel and disposed in a plurality of slots or grooves provided on the outside surface of an insulating rod. The rod extends axially of the fuse with the fuse wires being disposed within the longitudinally extending slots on the surface of the rod. The ends of the fuse wires are electrically connected to terminal disks, the latter being in turn connected to the terminal caps of the fuse, usually by means of some indicating structure, which will indicate when the fuse has blown.

To absorb the energy created during blowing of the fuse, an inert mass of finely divided material, such as sand, was disposed in proximity to the fuse wires. When such fuses were applied to circuits which were repeatedly switched on and off, as in motor-starting service, diificulty was encountered by reason of breakage of the fuse wires. We have found that the repeated heating and cooling of the wires, causing them, to expand and contract in the grooves, agitated the particles of sand so that it worked in behind. the fuse wires and into the lower ends of the slots provided on the surface of the rod. When the sand reached the lower end of the slot behind the fuse wire, it would prevent the fuse wire from. expanding and contracting uniformly and caused its breakage.

An object of the present invention is to improve and to prolong the life of a fuse when used on repeated motor-starting service. More specifically, this is accomplished by preventing access of the small particles of sand into the bottom of the fuse-wire grooves.

Another object is to provide a suitable thin insulating covering, or easily rupturable sheet separator, to protect the fuse wires within the grooves from the sand, or other finely divided inert insulating material, and yet to allow sufficient lateral movement of the fuse wires within the grooves to enable them to expand and contract freely during the repeated switching on ofheavy currents, as in motor-starting operations.

Still a further object of the invention is to improve fuse constructions by providing a thin insulating covering protecting the fuse wires within the grooves from the sand, which covering is of such a nature that upon blowing of the fuse it will disintegrate and thereby permit the sand to come into intimate contact with the fuse 2 wires to absorb the energy created during fuse rupture.

Further objects and advantages will readily become apparent upon reading the following specification, taken in connection with the drawing, in which:

Figure l is a side elevational View, partially in vertical longitudinal section, of a fuse incorporating the principles of our invention, shown in the unblown condition, and with the thickness of the covering 29 exaggerated for clarity;

Fig. 2 is a transverse sectional view taken along the line II-II of Fig. 1, again with the thickness of the covering 29 exaggerated;

Fig. 3 is a vertical longitudinal sectional view taken through a modified type of fuse construction, which is subjected to a clipping or spraying operation with the thickness of the coating 3| exaggerated for clarity;

Fig. 4 is a transverse sectional view taken through the modified type of fuse construction shown in Fig. 3;

Fig. 5 is a side elevational view, greatly enlarged, of a portion of one of the fusible wires, and;

Fig. 6 is a greatly magnified sectional portion of the insulating rod utilized in the construction of Figs. 1 and 2, showing the free movement possible of the fuse wires within the grooves, and yet utilizing a thin covering preventing the sand getting into the grooves.

Referring to the drawing, and more particularly to Figs. 1, 2, 5 and 6 thereof, the reference numeral l designates a tubular fuse holder of insulating material, such as fibre or a synthetic resin. The tubular fuse holder l is provided with terminal caps 3 and 5 adapted to be secured over the opposite ends of the tubular holder l. Terminal caps 3 and 5 may be of any desired conducting material, such, for example, as copper or the like, and they are preferably secured in position by a pressed fit on the ends of the fuse holder l.

The fuse construction shown in Figs. 1, 2, 5 and 6 is similar to the fuse constructions set forth in the foregoing patents and has desirable current-limiting functions. The fuse comprises a fuse unit '5 including a rod 9' of an insulating material, which is capable of evolving an arcextinguishing gas when in proximity to an electric arc, such, for example, as fibre or a synthetic resin. The rod 9 has a plurality of helicallyextending, or spiral slots or grooves l3 disposed on its outside surface, and more clearly shown, in magnified fashion, in Fig. 6 of the drawing.

The longitudinally-extending slots or grooves it are cut in the periphery of the rod ii for the reception of one or more parallel fuse wires 55. Fig. 6 more clearly shows the disposition of the fuse wires 55 within the slots l3 provided on the external surface of the insulating rod 9.

The parallel-disposed fuse wires I are adapted to be electrically connected at their ends with terminal disks ll and IS in any desired manner, such as by soldering or the like. Rod 9 has a plurality of spaced washer-shaped insulating barrier disks 2| provided thereon, for a purpose to be described, and the entire unit I is supported within fuse holder I in spaced relation to tube I and to the end terminals 3 and 5.

As set forth in Patent 2,496,704, the opposite ends of insulating rod 9 are provided with bores for receiving reduced extensions on the terminal disks I! and I9. Electrically interconnecting the terminal disks I! with the terminal cap 3 is a serially-connected indicating means, which forms no part of our invention and, consequently, is not described. Reference may be had to Patent 2,496,704 forfeatures of this indicating means. It may be stated that upon rupture, or blowing, of the fuse assembly, the indicating means is actuated to permit the extension of an indicator disk 21, shown in Fig. l.

The lower terminal disk I9 is directly electrically connected to the lower terminal cap 5. Disposed within the fuse holder l and about the fuse unit l is a mass of finely divided inert insulating material 25, such, for example, as sand. As mentioned previously, the sand serves to absorb the energy created during rupture of the fuse unit I.

As previously stated, the fuse unit l, thus far described, is similar to the current-limiting structure described in the foregoing patents. Current limitation is accomplished in this structure by reason of the following factors. The fuse wire I5 is of special construction, and may be of silver, or other material having a high temperature coefficient of resistance, and is initially chosen to be of substantially the same size of that required to carry the rated current without undue heating, and yet melt on currents above the rated current. Such a wire is then provided with a plurality of reduced sections 23, more clearly shown in Fig. 5 of the drawing.

It has been found that in such a wire the fusion time is speeded up to correspond to that of a wire of substantially the same diameter as the reduced sections, at least for high values of current. This speeding up effect has been found to be roughly proportional to the ratio between the area of the large portions of the fuse wire l5 and reduced portions 23. As a practical matter, this ratio may be made as high as possible, being limited only by mechanical strength considerations. Such a fuse wire retains all the normal time-current characteristic desired in a fuse for time values in excess of approximately two cycles without appreciable reduction in current-carrying ability. 'Reduction in current-carrying ability varies roughly directly in proportion to the length of sections 23, so that it is desirable that these sections be made as short as possible. For short times of less than one-fourth cycle, fuse wire I5 has an exceptionally fast melting time.

Thus, with a fuse wire having a diameter of .036 inch and reduced portions of .0145 inch it has been found that the time current characteristic on low currents up to about 400 amperes is substantially the same as those of a fuse wire of uniform diameter equal to the large diameter portion of wire l5. However, on currents above 400 amperes, fuse wire I5 is much faster with its characteristics on these higher currents being substantially coincident with the characteristics of a fuse wire of uniform diameter which is substantially the same as the diameter of reduced sections 23. This means that fuse wire 15 has its continuous current carrying ability substantially unimpaired, by provision of reduced section 23, and even its time current characteristic on light overloads will be substantially unchanged. However, on heavy overloads, where it is desired to limit the current, fuse wire l5 operates much faster than it would without the reduced sections 23, which means that it is possible to initiate the current limiting action at a very early point in the first half cycle of fault current.

Actual current limitation is effective by providing a high are voltage upon fusion of fuse wires 15, which are voltage is built up substantially instantaneously, to thereby exert a limiting or choking effect on the current and prevent its rise beyond a predetermined value. The particular structure disclosed provides a high are voltage per unit length of unit I, which means that the overall length of the fuse is small. One Way of providing such a high are voltage per unit of arc length is to provide a relatively large number of reduced sections 23 in fuse wire 15 so that when these reduced sections melt, a plurality of serially related arcs will be found and the summation of the arc voltages of these series arcs will then be greater than the arc voltage across a single arc. Generally speaking, the arc voltage per unit of length is directly proportional to the number of series arcs or restrictions 23 per unit of length. This suggests that maximum arc voltage per unit length may be obtained by providing as many restrictions 23 per unit of length as is physically possible. However, as the number of restrictions 23 per unit of length is increased, a point is eventually reached where a further increase results in a decrease in current carrying ability. It has been found that at least for voltages above 600 volts, at least two restrictions 23 per inch of fuse wire I5, or a total of ten should be provided to obtain an effective rise in arc voltage with a fuse wire which is not of excess length. This corresponds approximately to a spacing of restrictions 23 not to exceed about 15 times the large diameter of the fuse wire. It is desirable, however, to use the maximum number of restrictions possible without substantially impairing the current carrying ability. The most desirable number of restrictions is dependent on the size of the fuse wire, and appears to correspond roughly to a spacing of about 3% times the largest diameter of the wire. This is an optimum spacing, since spacings as low as about twice the largest diameter of the fuse wire may be employed with good results.

Because fuse wires l5 are in close proximity to the walls of slots l3, it will be apparent that the are formed upon interruption will cause the evolution of arc-extinguishing gas from the walls of the slot, and this gas will blast laterally through the arc to perform three functions, all of which act to increase the arc voltage and to extinguish the arc. First, the blast of arc-extinguishing gas acts to sweep metal vapor out of the arc stream and out of slots l3 into the mateincrease the resistance of the arc path and to 1 extinguish the are at current zero. A third function of the transverse gas blast is to cause the series arcs to be looped outwardly toward filling Secondly,

material. 25, thereby lengthening. the arc: path.

and, consequently, increasing the: resistance thereof and the voltage drop; across, it. One function of washers 2% on supporting rod 9 will now be apparent, aspreventing escape of the arcs from slot i3 and, consequently, from proximity with the gas-evolving material and from the restricting action from the narrow slot, and. thus preventing the series arcs from restriking asa single are outside the slots.

From the foregoing, it is apparent that efficient. current limitation of the first half cycle. of the.

arcing current may be obtained with the. structures disclosed, because (1) the fusible wires themselves are capable of melting to. establish. an are at least on such high currents which itis desired to limit, in a very short time, that is m a very small fraction of a half cycle, (2) as soon as the arc is established it is subjected to all of the factors enumerated above to create an arc: voltage high enough to prevent any further rise of a fault current.

"When a fuse structure, such as that set out above, is applied on repeated motor-starting. operations it is necessary for the fuse wires [5 to be able to expand and contract freely within the slots E3, since they are heated. by the high motor in-rush current each time the motor is started. To prolong the life of the fuse on repeated motorstarting, the notched-silver wires i5 were placed in the spiral grooves it, which permitted the wire to expand uniformly without buckling. This improved the performance of the fuse but it was found that the small particles of sand. which surround the interrupter were getting into the grooves. Also these small particles of sand were getting behind the wire it when it expanded and prevented the wire from returning to its original position upon cooling, thus causing the wire to break. ticles improved the performance to the extent that 20s or 300 motor starts could be obtained before failure. This was not sufficient improvement, and the performance depended upon the amount of fine sand remaining after sifting, and also upon the amount of fine sand produced. by the sand breaking into small particles.

We have discovered a method of eliminating the breakage of the fusible elements l5 under, normal motor starting conditions regardless of the particle size of the sand. A thin barrier or frangible covering 29 of material possessing. considerable strength is placed over the grooves to keep the sand out of the grooves 13 in which the notched wires it are located. This permits the wire 5 to expand and contract freely. During the interruption of short circuits the arc disintegrates the barrier, and the sand is immediately in contact with the arc to absorb the energy.

In the construction set out in Figs. 1, 2, 5 and 6, one layer 29 of .00176 thick resin tape, such as a regenerated cellulose, or cellophane tape, was used around the fibre rod 9 between each pair of fibre washers 2i. Fuses assembled with the cellophane tape 29 around the interrupter were tested with simulated motor starting conditions and were not damaged'after 2,000 motor starts. Interrupting tests were then taken, and the interrupting ability was in no way impaired.

Another method of producing the thin barrier or covering is shownin Figs. 3- and i of the drawing. Here the thin barrier or covering BI is sprayed on the unit 1, or the unit 1' is dipped in a plastic material which will completely cover the top of the grooves without interfering with sifting the sandto remove the small parthe free movement of the fusible elements. i5: Preferably the material 3i. must not flow down on the fibre walls. 33 (Fig. 6) of the slots It or the. interrupting characteristic of the fuse is changed, Such. flowing down would not only interfere with the free movement of the fuse wire it within the slot l3, but also the gasevolving. characteristic of the wall 33 would be changed by the presence of the foreign material, namely the plastic.

We have discovered a suitable material providing a web finish is a plasticized vinyl resindissolved in a. lzetone solvent. The material is such that it may be easily stripped off of the unit. I; and is. similar to that which the United States Government utilized in. encasing. military equipment for storage.

From the foregoing description it will be apparent that we have prevented the finely divided material, such as sand, from entering into the bottoms of the slots iii in back of the wirel5, either by taping between the washers 2|, or by dipping or spraying the entire unit I with a suitable thin covering 3|. The result has been to prolong the operational life of the fuse and yet not to interfere with the efficient operating characteristics of the fuse.

The films 29, 3! used in the practice of the invention may comprise resinous film forming. polymers. Examples thereof are vinyl polymers, such as polyvinyl acetate, polyvinyl butyrate and polyvinyl chloride, polyvinylidene chloride, cellu lose, cellulose esters and others, such as cellulose acetate, cellulose butyrate and ethyl cellulose, polystyrene, and synthetic elastomers. Thefilms 29, 3! may be applied from solutions or dispersions of the polymers or as preformed film or sheets. As an example we have sprayed and dipped a solution of a copolymer of vinyl acetatevinyl chloride plasticized with dibutyl phthalate in a ketone solvent. Preformed films of regen erated cellulose such: as cellophane and cellulose acetate may be wrapped on the fuse.

Even different forms of paper may be used with suitable adhesives, the only prerequisite being that the sand, or finely divided inert material, is kept out of the grooves, and the arc may rupture the covering to contact the inert material to facilitate arc extinction.

Although we have shown and described specific.

structures, it is to be clearly understood that the same were merely for the purpose of illustration and that changes and modifications may readily be made therein, by those skilled in the art, without departing from the spirit and scope of the invention.

We claim as our invention:

1. Afuseincluding a fuse holder of insulating material having spaced terminals thereon, current-limiting circuit-interrupting means connected between said terminals including one or more fusible conductors, a rod of insulating material having one or more grooves provided there'- in, the one or more fusible conductors being disposed withinthe. one or more grooves, a thin rupturable covering over the open side of the groove so as to enclose the fusible conductor within, the groove, and an inert insulating material outside of the covering. to assist in are extinction.

2. A fuse including a pair of spaced terminals, conducting means electrically connecting. the spaced terminals including one or more fusible conductors, insulating means forming a slot, within which. one of the fusible conductors is disposed, a thin rupturable insulating covering for the slot, and an inert insulating material outside of the covering to assist in arc extinction.

3. A fuse including a, pair of spaced terminals, conducting means electrically connecting the spaced terminals including one or more fusible conductors, an insulating rod forming a groove, one of the fusible conductors being disposed within the groove, a thin frangible insulating covering for the groove, and finely divided inert insulating material about the covering to assist in are extinction.

4. A fuse including a pair of spaced terminals, conducting means electrically connecting the spaced terminals including one or more fusible conductors, an insulating gas-evolving rod forming a groove, one of the fusible conductors being disposed within the groove, a thin frangible insulating covering for the groove, and finely divided inert insulating material about the covering to assist in arcextinction.

5. A fuse including a pair of spaced terminals, conducting means electrically connecting the spaced terminals including one or more fusible conductors, insulating gas-evolving means forming a slot, within which one of the fusible conductors is disposed, a thin rupturable insulating covering for the slot, and an inert insulating material outside of the covering to assist in arc extinction.

6. A fuse including a fuse holder of insulating material having spaced terminals thereon, current-limiting circuit-interrupting means connected between said terminals including one 01' more fusible conductors, a rod of insulating gasevolving material having one or more grooves provided therein, the one or more fusible conductors being disposed within the one or more grooves, a thin frangible covering over the open side of the groove so as to enclose the fusible conductor Within the groove, and an inert insulating material outside of the covering to assist in arc extinction.

'7. A fuse including a fuse holder of insulating material having spaced terminals thereon, current-limiting circuit-interrupting means connected between said terminals including one or more fusible conductors, a rod of insulating material having one or more grooves provided therein, the one or more fusible conductors being disposed within the one or more grooves, a thin rupturable covering over the open side of the groove so as to enclose the fusible conductor Within the groove, an inert insulating material outside of the covering to assist in arc extinction, and a plurality of spaced insulating transverselyextending barriers disposed about the rod in said inert insulating material to prevent the formation by the heat of an arc of any continuous conductor or semi-conductor in said inert material.

8. A fuse including a pair of spaced terminals, conducting means electrically connecting the spaced terminals including one or more fusible conductors, insulating means forming a slot, within which one of the fusible conductors is disposed, a thin frangible insulating covering for the slot, an inert insulating material outside of the covering to assist in arc extinction, and a plurality of spaced insulating transversely-extending barriers disposed about the insulating means in said inert insulating material to prevent the formation by the heat of an arc of any continuous conductor or semi-conductor in said inert material.

9.'A fuse including a pair of spaced terminals,

conducting means electrically connecting the spaced terminals including one or more fusible conductors, an insulating rod forming a groove, one of the fusible conductors being disposed within the groove, a thin insulating covering for the groove, finely divided inert insulating material about the covering to assist in arc extinction, and a plurality of spaced insulating transversely-extending barriers disposed about the rod in said inert insulating material to prevent the formation by the heat of an arc of any continuous conductor or semi-conductor in said inert material.

10. The combination in a fuse structure of means for extinguishing an arc therein including a rod of insulating material, one or more slots disposed substantially longitudinally of the rod, at least one fusible conductor disposed within a slot, finely divided insulating material disposed about the rod of insulating material, and thin rupturable covering means of insulating material over the slot to prevent the entrance of said finely divided insulating material into the slot.

11. A fuse including a pair of spaced terminals, conducting means electrically connecting the spaced terminals including one orv more fusible conductors, insulating means forming a slot, within which one of the fusible conductors is disposed, a thin frangible film of insulating resinous material over the slot, and an inert insulating material outside of the thin film to assist in arc extinction.

12. A fuse including a pair of spaced terminals, conducting means electrically connecting the spaced terminals including one or more fusible conductors, insulating means forming a slot, within which one of the fusible conductors is disposed, a thin film of insulating material over the slot, and an inert insulating material outside of the thin film to assist in arc extinction, wherein the thin film of insulating material comprises a vinyl polymer.

13. A fuse including a pair of spaced terminals, conducting means electrically connecting the spaced terminals including one or more fusible conductors, insulating means forming a slot, within which one of the fusible conductors is disposed, a thin film of insulating material over the slot, and an inert insulating material outside of the thin film to assist in arc extinction, Wherein the thin film of insulating material is selected from the group consisting of cellulose, cellulose esters and cellulose ethers.

14. A fuse including a pair of spaced terminals, conducting means electrically connecting the spaced terminals including one or more fusible conductors, insulating means forming a slot, within which one of the fusible conductors is disposed, a thin film of insulating material over the slot, and an inert insulating material outside of the thin film to assist in arc extinction, wherein the thin film of insulating material is selected from the group consisting of cellulose, cellulose esters and cellulose ethers, wherein the thin film comprises regenerated cellulose.

HAROLD I-I. FAHNOE. WILLIAM A. GASKILL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,523,139 Owens Sept. 19, 1950 2,571,492 Schultz Oct. 16, 1951 2,586,171 McMahon Feb. 19, 1952

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