US2341865A - Fuse link - Google Patents

Description

Feb. 15, 1944. B. R. HERMANN FUSE LINK X LEAD w 6/L VER Filed May 12, 1942 W V! o M 4 w w m u E l I I I i l l I I l I i I I llllllllllilllllllr Inventor: Ben amm P. Hermann,

illllllillllllll Attorney Patented Feb. 15, 1944 FUSE LINK Benjamin R. Hermann,

signor to General E ration of New York Lanesborough, Mass, as-

lectric Company, a corpo- Application May 12, 1942, Serial No. 442,606

6 Claims.

My invention relates to electric fuse links and more particularly to fuse links which may be used with any type of fuse cutout or fused'circuit-interrupting device.

Electrical distribution circuits are usually provided wtih various protective means with the ultimate aim of eliminating to as large an extent as possible service interruptions. This is accomplished by sectionalizing the circuit so that, if a fault occurs anywhere thereon, it is possible to isolate a minimum portion while maintaining service on the remainder. Fuse cutouts are used to a large extent for the protection of distribution circuits and it is desirable for proper system coordination that their operation be correlated with relay controlled circuit breakers which are also used for protecting more important sections of the circuit. .The fuse link, which is the current sensitive part of a fuse cutout must be designed to have particular characteristics which afford the best balance between the somewhat conflicting present-day operating requirements of motors, relays, transformers and fuse cutouts. The time current characteristics of fuse links best suited to the average operating conditions have been calculated and it would be desirable to provide fuse links having these characteristics, which fuse links are simple, compact, and relatively inexpensive as well as dependable in their protective operation. In addition to having time current characteristics which may be properly coordinated with relay controlled circuit breakers, the fuse links should be designed so as to rupture within a very short time on fault currents and yet which will not rupture on transients, such asmotor starting currents and the like, to cause undesirable service interruption.

Fuse cutouts quite generally employ fuses of the expulsion type in which the fuse link is mounted within an expulsion fuse tube formed of a material which emits gas when subjected to the heat of an electric arc. Since any new and improved fuse links should be designed so that they can be installed in existing fuse cutouts, certain limitations are placed upon the size of the fuse link and especially the fusible element or conductor thereof. In the first place, the fusible conductor must not have too high a resistance to require a conductor diameter larger than will fit into existing expulsion fuse tubes. Also, fuse links must be able to carry the rated load current without undue temperature rise to cause charring and consequent destruction of the expulsion fuse tube within which they are used.

Accordingly, it is an object of my invention to provide a new and improved fuse link having a time current characteristic best suited to the average operating conditions which is relatively inexpensive and dependable in its operation.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this application.

For a better understanding of my invention, reference may be had to the accompanying drawing in which Fig. 1 is a sectional view of the fuse expulsion fuse including a fuse link embodying my invention which is adapted to be used therewith, and Fig. 2 is a sectional view of the fuse link shown in Fig. 1.

Although my invention is particularly concerned with the fusible element of fuse links, I have chosen, for purposes of illustration, to portray my invention as applied to the so-called drop-out type, or the reclosing type. Examples of these cutouts are well known and are represented by the following patents: 2,081,813,

or other suitable insulating material which emits gas for arc-extinguishing purposes when subjected to the heat of an electric arc. An upper metal ferrule or tubular contact terminal I! is provided which is threadedly mounted to the upper end of tube A closure cap l3 threaded to ferrule l2 closes the upper end of tube I. The lower contact terminal H of fuse holder I0 is fastened to tube H near the lower end thereof. This contact terminal includes a threaded extension l5 and a knurled clamping nut IE to clamp the flexible end or terminal of the fuse link embodying my invention to be described hereinafter. Fuse holder Ill is adapted to be utilized in fuse cutouts of the type illustrated in the above-mentioned paten Since it is common practice to use universal ed to be clamped between the ferrule 52 and cap l3 as is indicated in Fig. 1. The buttonhead terminal l! is constructed so that, when it is clamped between ferrule 12 and cap Hi, the upper end of the fuse tube H is completely sealed against the escape of gases.

As best illustrated in Fig. 2, the fuse link has a second or lower terminal which comprises a flexible cable portion l8 adapted to be connected to the lower contact terminal it of fuse holder II) as is clearly shown in Fig. 1. Electrically interconnecting these first and second fuse link terminals is a fusible element 19 to be described in greater detail hereinafter. This fusible element is is preferably soldered as at 20 to the lower fuse link terminal which includes a flexible cable portion It. The upper end of fusible element I9 is soldered as at 2! to upper buttonhead terminal H. For certain ratings, it may be desirable to provide a strain wire such as 22 connected in parallel with fusible element 19 to prevent mechanical stresses from damaging fusible element is which may have a low mechanical strength for low current ratings. Preferably, an insulating housing of tubular form, such as 23, is provided to protect the fusible element 09.

For low current ratings, it may be desirable to provide mechanical means for insuring high speed separation of the fuse link parts upon rupture of the'fusible element i9 and, to this end, I have illustrated a tension spring 24 suitably connected between insulating tube 23 and one end of fusible element 89.

The particular arrangement of the fuse link illustrated in Fig. 2 forms no part of my present invention which is concerned primarily with the construction of the fusible element I9. I have discovered that, if the fusible element I9 is constructed of a eutectic alloy of lead and silver comprising substantially 97 /2 per cent lead and 2 /2 per cent silver, a fuse link is provided having a time current characteristic substantially identical with that calculated as best suited to the average operating conditions. The fusing r blowing characteristic of such a fuse link may be expressed by the following equation:

where I is the current in amperes, t is the time in seconds, D is the diameter of the fusible element in mils, and K is a constant which should preferably be in the range between 3X10- and X 10-. I have discovered that, when the fusible element of a fuse link is constructed of the eutectic alloy of lead and silver mentioned above, the constant K in the above equation is approximately 5.5x 10- If the fuse link is constructed of an alloy having the above composition, the conductivity will be such that the diameter D in the abovementioned equation is small enough to be used in all existing installations in a very satisfactory manner without causing undue temperature rise to char the fuse tube H of the fuse holder it shown in Fig. 1. Furthermore, the metals necessary to form the alloy are both relatively easy to obtain and also relatively inexpensive, Consequently, a fuse link embodying my invention may be constructed at low cost. Furthermore, with a fusible element constructed of the eutectic alloy referred to above, not only is a very high degree of dependability obtainable but a time current characteristic substantially identical with asaaees the best characteristic for average operating conditions is obtained. Also, since the melting point of the eutectic alloy is low being in the neighborhood of 304 degrees centigrade, no damage will be done to the tube of a fuse holder as is the case with fusible elements constructed of a material having a higher melting point.

I have discovered that a solder comprising a lead-silver alloy with a substantial percentage of bismuth added thereto to lower the melting point thereof is very satisfactory for use as at 20 and iii of Fig. 2 to solder the fusible element is to the terminals of the fuse link.

From the above description, it will be obvious that I have provided a new and improved fuse link which is relatively inexpensive, which is very dependable in operation, and which has a time current characteristic satisfactory for proper system coordination.

Having described a preferred embodiment of the invention in accordance with the patent stat= utes, it is desired that the invention be interpreted as broadly as possible, and that it be not limited to this particular embodiment, inasmuch as it will be obvious, particularly to persons skilled in the art, that many changes, especially of form, of this embodiment may be made without departing from the broad spirit and scope of this invention. Therefore, it is desired that the invention be interpreted as broadly as possible and that it be limited only by the prior art and by what is specially set forth in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A fuse link comprising a pair of terminals and a fusible element interconnecting said terminals, said fusible element comprising the eutectic alloy of lead and silver.

2. A fuse link comprising a first terminal, a second terminal, and a conductor formed from the eutectic alloy of lead and silver interconnecting said terminals, said conductor being fusible when a current of a predetermined value flows therethrough.

3. A fuse link comprising a first terminal, a second terminal, a fusible conductor formed from the eutectic alloy of lead and silver interconnecting said terminals, and an insulating tube sur rounding at least the portion of said fuse link comprising said fusible element.

4. A fuse link comprising a first terminal, a second'terminal, a fusible conductor formed from the eutectic alloy of lead and silver lnterconnect= ing said terminals, a strain wire connected \in parallel with said fusible element, and an insu= lating tube surrounding at least the portion of said fuse link comprising said fusible element.

5. A fuse link comprising a first buttonhead terminal, a second terminal including a flexible cable portion, a fusible conductor formed from the eutectic alloy of lead and silver interconnecting said terminals, and an insulating tube sunrounding at least the portion of said fuse link comprising said fusible element.

6. A fuse link comprising a first terminal, a

second terminal, a fusible element consisting of substantially 97 /2 per cent lead and 2 per cent silver interconnecting said terminals, and an in sulating tube surrounding atleast the portion of said fuse link comprising said fusible element.

BENJAMIN R. HERMANN.

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