US3317691A - Current-limiting fuse - Google Patents

Description

y 1967 E. BASSANI 3,317,691

CURRENT-LIMITING FUSE Filed Oct. 23, 1965 9. A I H9. 5

E l lump g M4 M04;

BY Ma M I NVENTOR ATTORNEY United States Patent 3,317,691 CURRENT-LIMITING FUSE Ermauno Bassani, Corso Porta Vittoria 9, Milan, Italy Filed Oct. 23, 1965, Ser. No. 503,623 Claims priority, application Italy, Dec. 19, 1964, 54,599/ 64 1 Claim. (Cl. 200-423) This invention relates to a cartridge fuse and more particularly to a cartridge fuse having an external envelope of refractory material, in which a fuse element is enclosed. The fuse element may consist of a metallic strip having in one or more positions a restriction of selected size appropriate to the fusing current.

It is known that one of the important characteristics of a fuse is its so-called nominal interrupting capacity. By nominal interrupting capacity there is intended the presupposed maximum electric current at which the fuse interrupts the passage of the electric current without damage to its supporting elements.

The nominal interruption capacity is limited by the capacity of the fuse to limit to a minimum the energy of an are which is generated at the instant of the interruption, and to absorb the thermal energy generated during said electric arc. In the case of short-circuit currents over the nominal interruption capacity, a considerable quantity of thermal energy is generated which by greatly raising the temperature in the region around the electric arc, generates within the ceramic material internal stresses which may cause explosions with consequential damage.

This drawback, besides of being dangerous, is wholly contrasting with the requisites of the nowaday electric plants of utilisation, wherein still higher short-circuit currents are required.

According to the present invention there is provided a cartridge fuse comprising a metallic strip provided with several critical sections parallel and equidistant from each other, which in operation can divide the current so that each critical section is only carrying part of the load.

There is obtained as an immediate consequence the advantage of an are which will be spent with a much greater speed, and a further advantage as well, consisting in a larger distribution of the so generated heat in all the critical sections, thus practically eliminating any danger of generation of stresses due to heat influence within the ceramic cartridge, causing cracks, explosions and the like.

A further optional feature of the invention is that the metallic strip is wound in the shape of a cylinder, and disposed axially inside the cartridge, so that powdered quartz can be disposed inside and outside the cylinder.

The geometrical disposition of the critical sections may be in parallel at the centers of circles whose circumferences are tangentially in contact with each other and with the internal wall of the cartridge.

Tests made have demonstrated that this disposition of the critical sections is such as to contribute in more quickly extinguishing the electric arc and also in rapidly dispersing the generated heat.

A further feature of the invention is that the end portions of the strip are subdivided into parallel end lamellae which, when the strip is enclosed within a cartridge, are bent onto the borders of the said cartridge, in order to ensure the electric connection, with end closing means, thus forming large areas of contact, and as a consequence therefrom giving an improved electrical conductivity.

Still another advantageous feature of this invention is that the critical sections in parallel position reciprocally are coated with an alloy of low melting point, capable of forming with the metal of the critical section (at a temperature) a eutectic at a temperature between the melting points of both the metals used for, so that the intervening of the fuse may be quicker.

In the matter of this last mentioned feature, the conventional known method was to place the low melting point alloy within an appropriate seating at the immediate neighbourhood of the critical section, and not thereonto, for which the molten metal had to flow against said critical section, but its influence was too little in respect of the time required for this flowing, and for the further drawback that said molten metal was absorbed by the powdered quartz before it could exert its proper action.

An illustrative embodiment of the invention will now be particularly described with reference to the accompanying drawings wherein,

FIG. 1 shows in plane view a strip of alloy used as a fuse element according to the invention;

FIG. 2 is a perspective view in a larger scale of the detail of the fuse enclosed within the circle A represented in FIG. 1;

FIG. 3 shows the disposition taken by the strip of FIG. 1 When enclosed within the interior of the ceramic cartridge;

FIG. 4 represents in cross-section a complete fuse as per the invention;

FIG. 5 is a cross-section along line L-L of FIGURE 4; and

FIG. 6 schematically shows the way followed by the short-circuiting electric current in a fuse of the kind of FIG. 4 in respect of an already known type of fuse.

In said drawings FIG. 1 shows an embodiment of this invention in which a strip 1 of adequate wideness is used; this strip is punched so as to receive an intermediate portion showing three critical sections a, a, a" in reciprocal parallel position equidistant from one another, and of a considerably restricted width. Further critical sections may of course be provided when necessary along said intermediate portion, and also at critical sections b, b, b on one side 0, c, c" on the other side, in a reciprocal parallel relationship. At the ends of the strip, tabs 2 and 3 are cut to an adequate length.

At least in the intermediate zone, as shown in FIG. 2, and onto the critical sections in parallel, a metallic alloy, or a single metal, as at 12, of a considerably lower melting point, and consituting the fusible strip, is applied by casting or the like thereon. This said alloy has the purpose to form a eutectic with the metal of the critical sections at an intermediate temperature between both the heat melting values of the two kinds of the contacting metals.

The metallic strip so-prepared is wound to a cylindrical body, as shown in FIG. 3, and is introduced in this shaped form into a cartridge 4 (FIG. 4) so as to take a position exactly coaxial with the cartridge itself.

The end terminal portions 2 and 3 of the strip are bent and pressed upon the borders of the cartridge 4, this with the view to hold the cylindrical portion of the strip in its central position, and to obtain at the same time a wide surface 'of contact between said end terminal portions and metallic caps 9, 10 and 11 which ensure the strong closure of the cartridge at both its ends.

As shown in FIG. 5, the wideness of the strip, and the intervals between the critical sections are such that the latter dispose themselves geometrically at the centers of circumferences 5, 6, 7 which are located tangentially to each other and to an internal wall 8 of the cartridge 4.

In FIG. 6 the full line represents a short circuiting current with respect to time in a fuse as illustrated and the broken line represents the current with respect to time in conventional prior art fuse. As it may be observed from the represented diagram, the electric current in the ordinary fuse and at a time T after the initiation of the interruption, continues to increase due to the delay in extinguishing the electric arc formed by the short-circuit.

In the fuse of this invention, in contrast, extinguishing of the electric arc is almost instantaneous, both due to the subdivision of the short-circuiting current in several branches, and to the factors explained above, for which there will be a development of the situation as indicated by the continuous line of the diagram. In this second case there will of course be the matter of a proportionally smaller generation of energy, and consequently a very limited generation and development of heat, thus permitting the fuse to be used under a higher nominal interruption capacity than was the case with the former kinds of fuses.

What I claim is:

A current-limiting fuse comprising, in combination, an outer tubular casing of insulating material; an annular fuse link disposed within said tubular casing concentric therewith, said annular fuse link consisting of an elongated metallic strip comprising a plurality of longitudinally arrayed portions successive ones of which are joined by respective groups of critical sections, the critical sections in each group being arranged parallel with one another transversely of the elongation of said strip and each critical section being also arrayed with the corresponding critical sections of all other groups as seen in longitudinal direction of said strip, said critical sections being arranged equidistantly from one another as seen in the respective direction of said strip, the Width of said strip and the spacing between the parallel sections of the respective groups being such that in annular condition of said strip the sections of each group are located at the centers of respective circles Whose circumferences tangentially touch each other and an inner face of said outer tubular casing; a coating of an alloy having a melting point lower than that of said strip, provided on said critical sections so as to cause formation of a eutetic at a temperature intermediate the melting points of said strip and said alloy; and an arc-quenching filler material surrounding said critical sections in said tubular casing within and withoutsaid annular fuse link.

References Cited by the Examiner UNITED STATES PATENTS 569,803 10/1896 Scott 200-135 1,215,992 2/1917 Schweitzer 200-135 2,665,348 l/1954 Kozacka 200-131 2,681,398 6/1954 Kozacka et a1. 200- 2,777,033 1/ 1957 Kozacka 200-120 2,866,037 12/1958 Stewart 200-131 2,934,628 4/1960 Massar et al 200-142 3,143,615 8/1964 Kozacka 200-120 BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner.

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