Electric Fuse Having a Casing of a Synthetic-Resin-Glass-Cloth Laminate Including Rovings

The above patent application solves primarily the problem of providing electric fuses having casings that have a high degree of dynamic bursting strength, which casings lend themselves to be manufactured cost-effectively by the pultrusion process. The casing of tube structure disclosed in the above referred-to parent application includes a minimum of three re-inforced plies of glass fibers, namely (1) an outermost ply of woven glass fiber fabric, (2) an innermost ply of woven glass fiber fabric, and (3) an intermediate ply formed by a mat of substantially non-uniformly oriented glass fibers, or an intermediate ply formed by a non-woven glass fiber fabric. At least the outermost ply and the innermost ply have an extent in excess of 360.degree. and form overlaps. The three plies are integrated by a thermosetting resin into a tubular laminate having virtually uniform wall thickness along the entire periphery thereof. Woven glass fiber fabric is a relatively expensive material. Since the casing disclosed in my above patent application requires at least two re-inforcement plies of woven glass fiber fabric, its material cost are not minimized to the largest possible extent.

I have found that there are instances where the innermost reinforcement ply formed by woven glass fiber fabric may be dispensed with and a system of glass fiber rovings substituted for it, and yet a casing material or tubing material obtained that is entirely adequate for certain applications.

Fuses embodying this invention include a tubular casing of synthetic-resin-glass-cloth laminate housing the fusible element and the pulverulent arc-quenching filler, and supporting a pair of terminal elements for connecting the fusible element into an electric circuit. The aforementioned casing comprises but three re-inforcement plies. The outermost ply is of woven glass fiber cloth forming an overlap extending in a direction longitudinally of said casing, an innermost ply formed by spaced rovings of glass fibers extending in a direction longitudinally of said casing, and an intermediate ply formed by a mat of substantially non-uniformly oriented glass fibers, said overlap of said outermost ply and said rovings penetrating into said intermediate ply to such an extent that the outer surface and the inner surface of said casing are substantially smooth, substantially cylindrical coaxial surfaces.

FIG. 1 is a substantially longitudinal section of a low voltage fuse embodying the present invention;

FIG. 2 is substantially a side elevation of the fuse structure of FIG. 1 showing a small portion thereof in section;

FIG. 2a is a view of a non-woven glass fiber mat forming part of the structure of FIGS. 1 and 2;

FIG. 3 is an isometric diagrammatic representation of the constituent re-inforcement inserts of the casing structure of FIGS. 1 and 2;

FIGS. 4a-4c show separately the constituent re-inforcement inserts of the casing structure of FIGS. 1 and 2 developed into the plane of the drawing paper;

FIG. 5 shows as such in cross-section the casing of the structure of FIGS. 1 and 2; and

FIG. 6 shows a modification of the non-woven glass fiber mat of FIG. 4b which may take the place thereof.

Referring to FIGS. 1 and 2, numeral 1 has been applied to indicate a fusible element in ribbon-form provided with perforations 2 forming three serially related areas of reduced cross-section. The central of these areas is provided with a low fusing point overlay 3, e.g. of tin, for severing fusible element 1 on occurrence of small overloads of excessive duration. The axially outer areas of reduced cross-section are sandwiched between arc-voltage control plates 4 described in detail in U.S. Pat. No. 2,964,604 to P. C. Jacobs et al, Dec. 13, 1960 for CURRENT-LIMITING FUSES HAVING COMPOUND ARC-VOLTAGE GENERATING MEANS. In FIG. 1 fusible element 1 and plates 4 are shown in elevation rather than in section. Fusible element 1 is surrounded by pulverulent or granular arc-quenching means, preferably a body 5 of quartz sand. Fusible element 1 and the body of arc-quenching filler are enclosed in a tubular casing of a novel synthetic-resin-glass-cloth laminate generally designated by numeral 6. Casing 6 supports on the ends thereof a pair of terminal elements 7, e.g. in form of caps, or ferrules. Instead of being formed by caps or ferrules, the fuse might be provided with terminal elements in form of plugs inserted into the ends of casing 6, and pinned to the latter by steel pins. The fusible element 1 is bent over the rims of casing 6 to the outer surface thereof, and caps 7 are mounted on the portions of fusible element 1 situated on the outside of casing 6. Thus caps or ferrules 7 are conductively interconnected by fusible element 1. To minimize the resistance of the current path from one cap 7 to the other, the ends of fusible element 1 are preferably soldered to caps 7 in a fashion and by means well known in the fuse art.

Casing 6 includes a first or intermediate ply 6a formed of a non-woven glass fiber mat material as shown in FIG. 2a, a second radially inner ply 6b of spaced glass fiber rovings and a third radially outer ply 6c of woven glass fiber fabric. The intermediate ply 6a is sandwiched between plies 6b, 6c. Casing 6 further includes a thermosetting resin integrating plies 6a, 6b, 6c into a tubular laminate.

The configuration of the glass fiber reinforcement inserts forming plies 6a, 6b and 6c are shown diagrammatically in FIG. 3.

FIG. 3 shows separately the insert-forming outer ply 6c of woven fiber glass fabric. The insert or ply 6c has a circumferential extent in excess of 360.degree.. Ply 6c forms an overlapping area c where two layers of the insert or ply 6c are superimposed.

As diagrammatically shown in FIG. 3 the ply 6a of a non-woven glass fiber fabric or a glass fiber mat made up of non-uniformly oriented fibers has a circumferential extent of 360.degree. and thus does not form an overlap. Ply 6a may, however, form an overlapping area where to layers of ply 6a are superimposed. Such an overlapping area of ply 6a should be angularly displaced about 180.degree. relative to the area c shown in FIG. 3.

As diagrammatically shown in FIG. 3, the radially inner ply 6b of glass fiber rovings has a circumferential extent of 360.degree.. The constituent rovings of ply 6b form a squirrel-cage-like structure which is compressed into the intermediate relatively flexible ply 6a. As a result, the inner surface of casing 6 is in the form of a substantially smooth cylindrical surface as clearly shown in FIG. 5. The same applies also to the outer surface of casing 6. It is apparent from FIG. 5 that due to the pressure exerted by the pultrusion machinery, or pultrusion die, the ply 6b of rovings is pressed into the ply 6a of mat material and that due to the pressure exerted by the pultrusion machinery, or pultrusion die, the thickness s of ply 6c at the region of overlap c is less than the thickness 2s' (s < 2s'), s' being the thickness of ply 6c remote from overlap c. FIG. 5 further shows clearly that overlap c is pressed by the action of the pultrusion machinery into ply 6a so that the thickness of ply 6a is less in the region of overlap c than at any point remote from that region. The ratio of glass-to-synthetic resin in the region of overlap c by far exceeds the ratio of glass-to-synthetic resin remote from overlap c, i.e. at any other region of casing 6. The constituent glass fiber rovings of ply 6b have two functions. One of their functions consists in drawing the radially outer plies 6a, 6c through the pultrusion die, and the other function is to increase the mechanical strength provided by the radially outer plies 6a, 6c.

FIG. 4b has been drawn on the assumption that the intermediate ply 6a does not form an overlap such as the overlap c of ply 6c. As mentioned above, the intermediate ply 6a may be provided with an overlap region and the presence of such a region contributes to the mechanical strength of casing 6. Ply 6a may also be formed of several superimposed layers or webs of mat or non-woven glass fiber fabric, each forming an overlap and the several overlaps of intermediate ply 6a being angularly displaced.

FIG. 6 shows a ply of non-woven glass fiber mat 6a' which may take the place of ply 6a shown in FIG. 4b and be combined with plies 6c and 6b shown in FIGS. 4a and 4c, respectively. The intermediate ply 6a shown in FIG. 6 has an extent in excess of 360.degree. and forms an overlap a' extending in a direction longitudinally of casing 6 shown in FIGS. 1 and 2. Overlap a' of intermediate ply 6a' is angularly displaced relative to overlap c of the outermost ply 6c (see FIG. 4a) of woven glass fiber fabric.

As mentioned above the three plies 6a, 6b, 6c are integrated into a tubular laminate by means of an appropriate thermosetting resin. The wall thickness of casing 6 is substantially uniform or equal along its perimeter of 360.degree..

The tubing or casing material which has been described above is not limited to use in connection with electric fuses, but may be applied wherever like requirements obtain.