Electric Fuse

Electric fuses having one or more fusible elements, in particular helically wound fusible elements, have generally supports of insulating material for supporting the fusible element or elements. In the past such supports have been the source of great difficulties. It is generally desirable to make such supports of a high grade ceramic material and, as a result, the cost of such supports is high. A further serious limitation consists in that their bulk either limits seriously the amount of arc energy absorbing filler that can be placed into a fuse casing of given size, or precludes size limitation or reduction which is often an essential requirement.

This situation gave rise to the problem of evolving fuses having one or more helically wound fusible elements which would not require a mandrel-like insulating support and yet allow provision of extremely thin, extremely fragile fusible elements wound according to any desired diameter. The process disclosed and claimed in my U.S. Pat. No. 3,848,214; Nov. 12, 1974 for METHOD OF ASSEMBLING ELECTRIC HIGH VOLTAGE FUSES AND SUBASSEMBLY THEREFOR makes it possible to manufacture fuses wherein the helically wound fusible element or elements are supported only by the granular arc-quenching filler by which they are surrounded in the absence of any mandrel-like fusible element support.

The performance of fuses manufactured in accordance with the above patent has proven to be satisfactory. However, the manufacture thereof may require under certain circumstances particular care. In a fuse structure wherein the healically wound fusible element or elements are permanently supported by mandrel-like structure the process of filling the casing of the fuse with granular arc-quenching filler is less critical than in a fuse lacking a permanent mandrel-like supporting structure wherein the function of supporting the fusible element or elements is ultimately solely assigned to the granular arc-quenching filler or quartz sand by which they are surrounded.

In fuses manufactured in accordance with U.S. Pat. No. 3,848,214 the helical windings of the fusible element or elements are initially formed on a squirrel-cage-like structure whose element supporting rods are subsequently withdrawn from the windings at a point of time of the manufacturing process when the windings have gained sufficient support from the granular arc-quenching filler by which they are surrounded. Rod withdrawal is an additional process step not normally encountered in the manufacture of highvoltage fuses. If badly performed rod withdrawal may result in damage to extremely fragile fusible elements. Rod withdrawal results in the formation of voids which voids must not exceed a certain critical volume and timely be filled with granular arc-quenching filler.

It is, therefore, a prime object of this invention to provide electric fuses having fusible elements, in particular one or more helically wound fusible elements, which fuses are not subject to the limitations of prior art fuses having mandrel-like fusible element supports, and which fuses can be manufactured at smaller cost than fuses manufactured in accordance with U.S. Pat. No. 3,848,214.

Another object of the invention is to provide electric fuses having one or more helically wound fusible elements that are supported by a low cost mandrel-like support combining a minimum of bulk with sufficient dimensional stability.

Other objects and advantages of the invention will become more apparent as this specification proceeds.

FIG. 1 is a diagrammatic representation of a fuse embodying the present invention;

FIG. 2 is a side elevation of a subassembly intended to form, and forming, a support for a helically wound fusible element;

FIG. 3 is a section along III--III of FIG. 2;

FIG. 3a shows the formation of a knot for affixing a cord or the like to a screw-eye, hook, or like fastener;

FIG. 4 shows a portion of a plug terminal of a fuse embodying the present invention and is a section along IV--IV of FIG. 5;

FIG. 5 is a side elevation of the structure of FIG. 4;

FIG. 6 is a top-plan view of a portion of another plug terminal of a fuse embodying the present invention;

FIG. 7 is a section along VII--VII of FIG. 6 and shows an adjustable screw eye in side elevation;

FIG. 8 shows a fuse embodying the present invention, partly in longitudinal section and partly in side elevation;

FIGS. 9a,9b;10a,10b and 11a,11b show the effect of winding features of the fusible element upon the performance characteristics of fuses embodying this invention; and

FIG. 12 shows in cross-section on a larger scale a detail of a modification of the structures previously described.

In fuses embodying this invention the tubular casing of electric insulating material is preferably closed by a pair of plug terminals. Fuses having plug terminals are well known in the art and are disclosed, for instance, in my above referred-to patent and in U.S. Pat. 3,851,289; Nov. 26, 1974 to Frederick J. Kozacka for HIGH-VOLTAGE FUSE HAVING HELICALLY WOUND FUSIBLE ELEMENT AND SUPPORT FOR HELICALLY WOUND FUSIBLE ELEMENT. In FIG. 1 a pair of such plug terminals has been indicated by rectangles 1. Rectangles 1 are cylindrical sections of a pair of coaxially arranged pair of plug terminals taken at regions situated about midway between the axes of the plug terminals and the periphery thereof. These sections have been developed into the plane of the paper on which FIG. 1 is drawn. The upper plug terminal 1 is provided with four screw-eyes or the like fasteners 2a,2b,2c,2d which are angularly displaced 90 degrees. In like fashion the lower plug terminal 1 is provided with four screw-eyes 3a,3b,3c,3d which are angularly displaced 90 degrees. The aforementioned screw-eyes are arranged in registering pairs 2a,3a; 2b,3b; 2c,3c; 2d,3d. The lines interconnecting the above pairs of screw-eyes are the longitudinal edges of a prism that is square in cross-section. Screw-eyes 2a,3a are tied together by a length of cord 4a of glass fibers. This length of cord 4a is placed under considerable stress in a direction longitudinally thereof. This may be achieved by various means which will be described below in detail. In like fashion the pair of screw-eyes or the like fasteners 2b,3b are tied together by a length of cord 4b of glass fibers, the pair of screw-eyes or like fasteners 2c,3c are tied together by a length of cord 4c of glass fibers, and the pair of screw-eyes 2d,3d are tied together by a length of cord 4d of glass fibers. The length of cords 4b,4c,4d of glass fibers are also under considerable stress in a direction longitudinally thereof as indicated by the arrows P. A fusible element 5, preferably in form of a perforated ribbon of silver, is wound substantially helically around the four stressed lengths 4a,4b,4c,4d of glass fiber cord. Fusible element 5 conductively interconnects the pair of plug terminals 1. Reference character 5a has been applied to indicate the point where fusible element 5 is conductively connected to the lower plug terminal 1 and reference character 5b has been applied to indicate the point where fusible element 5 is conductively connected to upper plug terminal 1.

It will be apparent that the glass fiber cord supporting structure 4a,4b,4c,4d for fusible element 5 could be formed by one single length of glass fiber cord having but two ends rather than four separate lengths of glass fiber cord of which each length has two ends. The latter alternative makes it relatively easier to impart high and equal stresses to the four lengths 4a,4b,4c,4d, which is of particular importance where the spacing between plug terminals 1 is considerable.

FIGS. 2 and 3 illustrate a sub-assembly for manufacturing fuses embodying this invention. In these two figures the same reference characters as in FIG. 1 have been applied to indicate like parts. Reference character 1 has been applied to indicate a pair of coaxially arranged cylindrical plug terminals intended to plug the ends of a tubular casing or fuse tube. Terminals 1 are spaced by a spacing column or center post 6. Lower plug terminal 1 has a recess 1a in the shape of a frustum of a cone into which the lower flexible end of spacing column or center post 6 projects. The upper terminal has in its center a bore 1b which is internally screw-threaded. Center post 6 has a screw-threaded portion 6a engaging the screw-threaded bore 1b in upper plug terminal 1. This combination of parts has been described in my aforementioned U.S. Pat. No. 3,848,214 in greater detail. As also shown in FIG. 1, the upper terminal element 1 is provided with four screw-eyes 2a,2b,2c,2d and the lower terminal element 1 is provided with four screw-eyes 3a,3b,3c,3d. Stressed spaced cords 4a,4b,4c,4d of glass fibers interconnect the pairs of screw-eyes 2a,3a;2b,3b;2c,3c;2d,3d.

FIGS. 1-3 do not show in detail how the cords or strings 4a-4d are attached to the screw-eyes 2a-2d and 3a-3d, respectively, which might also be replaced by rounded hooks. Any appropriate kind of knot may be used to this end. FIG. 3a shows a screw-eye and the formation of a knot for affixing the end of a cord to it.

Referring now to FIGS. 4 and 5, numeral 1 has been applied to indicate a portion of the lower of the two plug terminals 1 included in a fuse embodying this invention. Plug terminal 1 is provided with four radial bores 7 which are angularly displaced 90 degrees and of which but one is shown in FIGS. 4 and 5. A pin 8 which may be referred-to as winch pin is pivotally arranged in each bore 7. Cord 4b whose upper end is affixed to a screw-eye or hook-- as shown in FIGS. 1-3-- is wound around winch pin 8 and may be tightened to any desirable extent by pivoting winch pin 8 in counterclockwise direction, as seen in FIG. 5. When the desired stress is imparted to glass fiber cord 4b, winch pin 8 is held in position by means of set-screw 8', thus permanently maintaining cord 4b in the desired state of tension. Cords 4a,4c and 4d may be stressed and maintained in the desired state of stress in the same way as cord 4b.

The structure shown in FIGS. 6 and 7 is intended to achieve the same end as that shown in FIGS. 4 and 5, i.e. to put any of cords 4a-4c under stress and to maintain it in the desired degree of stress. The screw-eyes or hooks 2a-2d in the upper terminal plug 1 may be fixed and the screw-eyes 3a-3d in the lower terminal plug 1 adjustable (See also FIG. 1). FIGS. 6 and 7 illustrate such an adjustable screw-eye 3a in lower terminal plug 1 for tightening glass fiber cord 4a. Terminal 1 is provided with four bores 1c which are angularly displaced 90 degrees and of which but one bore 1c has been shown in FIGS. 6 and 7. The shank of screw-eyes 3a is screw-threaded, projects through bore 1c and supports an adjustment nut 9 arranged in a cavity 10 of terminal plug 1. Nut 9 abuts against plug 1 and turning of nut 9 allows to impart the desired stress to glass fiber cord 4a. Cavity 10 shown in FIG. 7 and all cavities in terminal plug 1 that correspond to cavity 10 are closed by the common connector strap 11 shown in more detail in FIG. 8.

FIG. 8 shows a complete fuse embodying this invention. The structure of FIG. 8 is obtained from the sub-assembly shown in FIGS. 2 and 3 by mounting the tubular casing 12 on terminal plugs 1, affixing the former to the latter by transverse steel pins 13, removing center post 6 and thereafter filling casing 12 with quartz sand 14, or another granular arc-extinguishing filler. The filling operation is achieved by means of screw-threaded bore 1b in upper terminal plug 1(See FIG. 2). The fuse structure of FIG. 8 is completed by affixing connector strap 11 to the axially outer surfaces of terminal plugs 1. The lower connector strap 11 is affixed to the lower terminal plug 1 by a screw 15, and the upper connector strap 11 is affixed to upper terminal plug 1 by a screw 16 having a shank (not shown) that engages the internally screw-threaded bore 1b (See FIG. 1) in upper terminal plug 1. The lower terminal plug 1 must be provided with an internally screw-threaded bore for screw 15. A screw-threaded bore may be substituted for the cavity 1a, and the lower end of center post 6 provided with an external screw-thread that fits the thread of said bore. In that instance, the same internally screw-threaded bore may serve the dual purpose of receiving the lower end of center post 6 and of receiving the shank of screw 15.

The glass fiber strings 4a-4d may have a certain tendency to be bent radially inwardly under the pressure of fusible element 5. This tendency may be limited by arranging annular bracing elements 17 along strings 4a-4d which take up the inward pressure exerted by fusible element 5 upon glass fiber strings 4a-4d. The annular bracing elements 17 are preferably made of a melamine glass cloth laminate. When the casing 12 is filled with quartz sand 14 the latter drops readily through the central circular apertures defined by bracing elements 17.

Referring now to FIGS. 9a,9b,10a,10b and 11a,11b, FIG. 9s is an elevational view of a fusible element 5 wound helically around four stressed cords 4a,4b,4c,4d of glass fibers which define the edges of a prism that is square in cross-section. FIG. 9b is a top plan view of the structure of FIG. 9a. Considering a specific turn of fusible element 5 beginning at the point 1' on cord 4a, then a full turn includes the four quarter turns or quarter sections:

from point 1' on cord 4a to point 2' on cord 4b

from point 2' on cord 4b to point 3' on cord 4c

from point 3' on cord 4c to point 4' on cord 4d

from point 4' on cord 4d to point 1' on cord 4a.

The narrowest spacing along each of cords 4a-4d of points subject to arcing is equal to the height of one full turn, or a distance equal to that between points 1' and 4'. The areas of cord 4a adjacent points 1',4' are more or less subject to the intense heat of the arc taking the place of fusible element 5 and may liquefy more or less as a result of that heat thus truning into a semi-conductor. The length of the cord 4a between points 1' and 4' is considerable and not all of this length is heated significantly, and hence retains its initial insulating properties.

FIG. 10a shows, in substance, the same structure as FIG. 9a in the same fashion as FIG. 9a except for the fact that in FIG. 10a two fusible elements 5 are shown to be connected in parallel. Furthermore in FIG. 10 the relation of the cords for supporting the fusible element 5 and the latter have been shown diagrammatically in the interest of greater clarity. FIG. 10b is a top plan view of the structure of FIG. 10a. It is apparent from FIG. 10a that the increase in the number of fusible elements 5 results in an increase of the number of points along cords 4a-4d which are subject to arcing and intense heating. As shown in FIG. 10a the lengths of glass fiber cords 4a-4b whose insulating properties are not likely to be impaired by arcing are still considerable.

FIG. 11a shows in substance the same structure as FIGS. 9a and 10a in the same fashion as FIGS. 9a and 10a, except for the fact that three rather than but two fusible elements are shown to be connected in parallel. In FIG. 11a the relation of the cords 4a-4d supporting elements 5 and of the latter has been shown but diagrammatically as in FIG. 10a. FIG. 11b is a topplan view of the structure shown in FIG. 11a. The increase in the number of fusible elements 5 increases further the portion of the total length of cords 4a-4c that is subject to intense heat and reduces the portion of the total length of these cords that tends to retain its insulating character. There is a limit condition which depends on the pitch of the windings 5, their number, the arc energy released during interruption and other parameters when the stressed glass fibers 4a-4d fail dielectrically.

One way to cope with this situation is to avoid its occurrence. Another better way is to impregnate the element supports 4a-4d of stressed glass fibers with a medium that evolves gases under the heat of electric arcs. Such gases form thermal insulating shields around the element supports 4a -4d that preclude the latter from reaching dangerous temperatures, i.e. temperatures tending to seriously impair the insulating characteristics thereof. All other conditions remaining unchanged, the conductivity of a stressed glass fiber support at elevated temperatures increases with the cross-sectional area of the support. Imparting an excessive cross-sectional area to the stressed glass fiber support may lead to failure of the fuse. The tensile strength of glass fiber is so high as to make it possible to stress the fibers sufficiently to impart the necessary degree of dimensional stability to the support, without resorting to excessive cross-sectional areas. In instances where the glass fibers are impregnated with a synthetic resin, e.g. melamine resin, evolving protective, or arc-quenching, gases under the heat of electric arcs, the ratio of synthetic resin to glass has an important effect on performance. It is further desirable to add inorganic, non-tracking substances to the synthetic impregnating resin, a measure which is generally known in the art.

The prime purpose of the impregnation of the glass fibers under stress is the interposition of a gaseous thermal insulation between the arcs and the glass fiber supports. The gases resulting from such impregnation may be selected to have desirable secondary effects. Impregnation with melamine resins is desirable on account of the fact that it can readily be achieved and that the gaseous media resulting from the breakdown of melamine resins are nitrogen rich. The gaseous media resulting from vaporization of substances used for impregnating stressed cords 4a-4d diffuse into the arc and thereby contribute to the de-ionization of the arc path. If desired, the stressed cords 4a-4d may be impregnated with well known substances evolving electronegative gases.

The selection of the nature of the stressed glass fiber support may have an effect upon the performance of the fuse. It is impermissible to use glass fibers that are impregnated with a substance that has a tendency to track.

FIG. 12 shows a cross-section of two parallel glass fiber cords 4a' which may take the place of a single stressed glass fiber cord 4a,4b,4c,4d shown in FIGS. 1,2,3, and 8. As shown in FIG. 12, a body of gas-evolving material 18 surrounds cords 4a' and fills the narrow gap which is left therebetween.

A glass fiber cord may be under considerable stress and exhibit a considerable degree of stiffness during the manufacturing process of a fuse but develop slack upon completion of the assembly process of the fuse and when the fuse is heated by current flow through its fusible element or elements. This has no adverse consequences because at this time the fusible element or elements of the fuse derive adequate support from the granular arc-quenching filler 14 if the filling process has been carried out adequately.

The physical properties of glass fibers in form of rovings, cords, etc. are well known. The curves formed by fusible element supports 4a-4d are catenaries. This enables to readily calculate fusible element support structures embodying the present invention.