US 3400235 A
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Description (OCR text may contain errors)
Sept. 3, 1968 HOWARD ET AL 3,400,235
CURRENT LIMITING FUSE Filed Aug. 21, 1967 United States Patent 79,048 5 Claims. (Cl. 337229) ABSTRACT OF THE DISCLOSURE A current limiting fuse whose fusible element is a strip of silver having along its length alternate straight portions and semi-circular bowed portions standing on the same side of the strip, the bowed portions each containing two relatively large apertures located on the longitudinal axis of the strip near the adjoining straight portions, the elements being under light tension and embedded throughout its length in granular arc extinguishing material.
This invention relates to high voltage fuses of the type adapted to limit the current when interrupting heavy overloads or short circuits.
High voltage fuses of the kind to which this invention applies are well known. Basically, this fuse consists of the following: a tubular casing made of an insulating material; a number of relatively long fusible strips which extend lengthwise in the casing and are supported therein spaced from one another and from the casing; a pair of metallic terminals which close the ends of the casing and are connected to the ends of the fusible strips; and a granular arc extinguishing material which completely fills the casing and embeds the fusible strips. In this particular construction, the fusible elements were straight strips of a fusible metal such as silver placed in light tension between the terminals. Changes in the current loading subjected these fuses to thermal cycling, resulting in repeated expansion and contraction of the fusible strips, and this, in turn, frequently led to premature failure of the strips from mechanical fatigue. Thermal cycling is particularly severe in fuses used to protect large motors because of the wide range of currents possible between starting, running and stopping. Consequently, the failure of fuses in motor circuits has been particularly high due to the extreme conditions leading to thermal cycling.
Fuses of this type are very often used to protect large motors which are subjected to abrupt changes in loading, sudden overloads and high starting currents. High currents, and in particular sudden changes thereof, subject the fusible elements to severe shock forces leading to cyclic stresses in the elements. Hence, fusible elements may fail from fatigue due to cyclic mechanical stresses as well as thermal cycling, or usually from a combination of the two.
The object of this invention is to provide a current limiting fuse of the foregoing type in which the fusible elements are resistant to fatigue failure.
According to the invention the fuse has its fusible elements constructed such that they readily yield longitudinally in response to cyclic forces or expansion and contraction from thermal cycling. Each element consists of a relatively thin, narrow and long strip of a fusible metal such as silver having along its length alternate straight portions and bowed portions standing on the same side of the strip, the structure being formed by bending short lengths of the strip to one side of the strip at spaced in tervals along its length. In a preferred form of the strip, each bowed portion is semi-circular in profile and con- 3,400,235 Patented Sept. 3,, 1968 tains two relatively large apertures located on the longitudinal axis of the strip near the adjoining straight portions. These apertures define regions of reduced cross section for initiating fusion of the strip. The elements are similarly arranged in the casing under light tension between supporting structures at the ends of the casing, which structure also spaces the element one from another and from the casing, and they are embedded in a granular arc extinguishing material which completely fills the casing.
A better understanding of the invention may be had from the description to follow taken with the accompanying drawing of a fuse constructed according to the invention.
In the drawing, which is a cutaway perspective view of an embodiment of the invention, there is shown a high voltage current limiting fuse 10 suitable for use in the protection of large electric motors and other loads of this kind. Fuse 10 has a tubular casing 11, a pair of end terminals 12 and 13, a number of fusible elements of which two are shown at 14 and 15, an indicating element 16, means 17 and 18 for supporting elements 14, 15 in spaced relation to one another and to the casing, a granular arc extinguishing material completely filling the enclosure defined by casing 11 and terminals 12, 13 and a device 20 for indicating that the fuse has blown. Casing 11 is a relatively long tube made of a strong, hard, heat resistant, electrical insulating material such as mat of glass fibers bonded together with a polyester resin. Terminal 12 has a relatively long skirt 21 which is an interference fit over the upper end of the casing, an inwardly formed shoulder 22, a short neck 23, and an outwardly flared end 24 closed by a cap 25 which has its outer edge rolled over the flared end and is adapted for supporting indicating device 20. The terminal, including its end cap, is made of a conductive metal such as copper and the regions where it makes contact with the fuse holder may be silver plated to ensure that a good electrical connection is made between the terminal and bolder. Terminal 13 is exactly the same as terminal 12 except that its end cap 26 is plain, and it is pressed onto the lower end of the casing in the same way that terminal 12 is pressed onto the upper end of the casing.
Fusible elements 14 and 15 are relatively thin, narrow and long strips of a fusible metal such as silver. They are two of a large number which can be used, depending on the current rating of the fuse. In the fuse illustrated up to twelve strips such as 14, 15 can be placed in parallel between terminals 12 and 13. These strips are supported inside the casing in an annular array parallel with the casing, spaced therefrom and from one another by means of support means 17 and 18 gripping the respective ends of the strips. An end portion of each strip extends above support means 17 in between the flared end 24 of terminal 12 and its end cap 25 where the ends are firmly secured to the terminal structure, these ends having been brought out over the flared terminal end before the cap is applied. Since the ends of the strips are squeezed between the flared surface and the rolled edge of the cap, a good electrical connection is, therefore, made between the ends of the strips and the terminal. The other ends of the strips extend below support means 18 and are secured to terminal 13 in exactly the same way.
Means 17 and 18 for supporting the strips 14, 15 in spaced relation are two identical discs of an insulating material. Disc 17 is fitted inside the skirt of terminal 12 at the upper end of casing 11, and is secured to these components by having its outer edge wedged between the end of the casing and the shoulder 22 on the terminal. Disc 18 is secured to the lower end of the casing and terminal 13 in exactly the same way. Each disc, as particularly indicated for disc 17, has a large central opening 27 and a plurality of slits 28 radiating from the periphery of the opening at equally spaced intervals. The width of the slit is a little greater than the thickness of the strip and its depth about equal to the width of the strip. Hence the strip fits freely into the slit where it is held with its flat sides radial with respect to the tubular casing. Each strip is also held under light tension between discs 17, 18 by means of a pair of dents 29 and 30 in the strip resting against the outer surfaces of the discs. The slits in the two discs are aligned axially of the casing so the strips are parallel with the axis of the casing.
Current limiting fuses of this type invariably include a fusible element such as 16 which is of much higher resistance and fusing temperature than elements such as 14, 15. This element introduces resistance into the circuit as elements 14, 15 blow, and immediately after all the elements 14, 15 have blown, it too blows and triggers the indicating device 20 into displaying a flag which gives a visual indication that the fuse has blown. This feature is well known in the art and is applicable to fuses using many kinds, of fusible elements in place of 14 and 15.
The granular arc extinguishing material may be a sand well known in the art for its arc quenching properties. The fuse is completely assembled except for cover 26 before it is filled with the sand. Then apply the sand, the fuse is placed upright with its closed end down, the reverse of that illustrated, and vibrated while dry sand is poured into it through its open end. After all the spaces inside the enclosure defined by casing 11 and terminals 12 and 13 are completely full of sand and the fusible elements well embedded therein, cover 26 is applied to complete the assembly.
The novel fusible elements 14 and 15 will now be described. As already pointed out, there is usually more than just the two elements identified at 14 and 15. However, as they are all alike no matter how many are used, only one, that identified by numeral 14, will be described. As well as being alike, the elements are placed in a like manner inside the casing so that the parallel circuit formed by the elements is balanced both electrically and mechanically.
Fusible element 14 is a relatively thin, narrow and long strip of a fusible metal such as silver having along its length between support means 17 and 18 straight portions 31, 32, 33, 34, etc., alternating with bowed portions 41, 42, 43, 44, etc. The straight portions lie in the original plane of the flat strip and the bowed portions are bent out of the flat strip to one side of the plane. Preferably, the straight portions are relatively short and the bowed portions much longer and semi-circular in profile, appearing along the edge of the strip as a series of humps spaced apart along one side of the strip. Moreover, the straight portions are all alike, appearing rectangular in plan form, and the bowed portions are also all alike and uniform across the strip. By having a uniform arrangement of straight and bowed portions, the strip is rendered uniformly resilient throughout its supported length, thus enabling it to yield uniformly as the structure changes temperature. It has already been pointed out that the strips are stretched a little between their support means so as to place them under light tension. They are also arranged similarly in the casing, that is, the humps on one strip point into the hollows of the next adjacent strip. By lightly stretching the strip, its straight portions 31, 32, 33, 34, etc., are held in alignment so that the bowed portions of one strip are located in a definite relation to the bowed portions of the adjacent strips. The straight portions serve another very useful purpose. They hold the strips in their proper relation while the fuse is filled with sand. It is very important that the strips be not distorted by the sand because this can seriously impair the rupturing capabilities of the fuse.
In order to initiate fusion of the strip at specific points along its length, it is customary to reduce the cross section of the strip at these points. When the fuse now blows, the
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4 strip ruptures at different points, enabling the sand to quench the series of arcs which follow. In the case of a strip such as 14, good rupture characteristics have been obtained by reducing the cross section of each bowed portion in tWo places, a reduction near its juncture with each of the two adjoining straight portions. The reductions in cross section illustrated consist of a pair of apertures in each bowed portion near the beginning and end thereof, for example, apertures 51 and 52 in bowed portion 41, apertures 53 and 54 in bowed portion 42, apertures 55 and 56 in bowed portion 43, etc. These apertures are relatively large, of the same size and shape, and are located in the middle of the strip so the two edge portions left are as closely as possible to being equal in cross section.
Fusible elements of the configuration illustrated at 14 and applied in a structure of the kind described are found to have good rupture characteristics and resistance to fatigue. An alternate arrangement of straight and bowed portions such as this leads to a fairly equal distribution of the flexing of a strip throughout its length, thereby preventing concentrations of flexure which eventually cause failure of the strip from fatigue. Uniform flexing among the bowed portions keeps the stress intensity well within the endurance range of the metal at all points along the length of the strip. When the fuse blows, the series of restricted cross sections fuse to provide a series of arcs, the sum of which produces a high are voltage. This fuse is well suited for use in motor circuits in that it can accommodate the high voltages associated with motor starting and the varying current conditions to which the motor is subjected.
While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention, and therefore it is intended by the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.
What we claim as new and desire to secure by Letters Patent is:
1. A fuse having a tubular casing, a terminal structure closing each end of the casing, a plurality of elongated fusible elements inside said casing extending lengthwise thereof from one terminal structure to the other, means located at the ends of said casing for retaining said elements in light tension and in said spaced relation to one another and to the casing, and a granular arc extinguishing material filling the casing and embedding the elements said elements being alike, similarly located in the casing, and each one comprising a fiat, relatively thin, narrow and long strip of a fusible conducting material having along its length between said retaining means straight portions lying in the plane of the original flat strip alternating with bowed portions lying on the same side of said plane and appearing in edge profile as a series of humps extending across the strip at spaced intervals along its length, said straight portions being relatively short in length and said bowed portions being considerably longer lengths bent out of the strip to one side thereof, said alternate straight and bowed portions imparting uniform resilience to the strip for the accommodation of thermal cycling and cyclic stressing of the element to overcome failure thereof from fatigue.
2. The fuse defined in claim 1 wherein the means for retaining the fusible strips in light tension and in spaced relation comprises the following: two annular discs located one at each end of the casing and secured thereto along the outer periphery of the disc, each one of said discs having a plurality of slits equally spaced around its inner periphery and extending radially outward therefrom a distance approximately equal to the width of the fusible strip; each one of said strips having its ends located in a corresponding one of the slits in the two discs; two dents in each strip near the ends thereof bearing against the 5 outer surfaces of the discs from the light tension in the strip.
3. The fuse defined in claim 1 wherein each strip is reduced in cross section at spaced intervals between said retaining means.
4. The fuse defined in claim 1 wherein each one of said bowed portions contains two relatively large apertures located in the middle of the strip near the adjoining straight portions.
5. The fuse defined in claim 1 wherein each one of said straight portions contains a relatively large aperture located in the middle of the portion.
References Cited UNITED STATES PATENTS 2,734,111 2/1956 Kozacka 200-120 3,080,463 3/1963 Kozacka 200 -120 2,988,620 6/1961 Kozacka 200l35 XR BERNARD A. GILHEANY, Primary Examiner.
H. B. GILSON, Assistant Examiner.