Electric Fuse

U.S. Pat. No. 3,979,709, Sept. 7, 1976 to Daniel P. Healey, Jr. relates to an electric fuse whose casing is manufactured by the so-called pultrusion process. by using that process as described in the above patent tubular casings may be obtained which have a great dynamic strength, i.e. a great strength to resist impulse loads as occurring in fuses, particularly current-limiting fuses, and which comply - so it seemed - with all other requirements of tubular fuse casings, or fuse tubes. It was, however, discovered that the mass consisting of glass fibers and cured polyester, of which pultruded tubing of the aforementioned kind is made, has a larger heat-transmitting conductivity than conventional fuse tube materials, such as, for instance, vulcanized fiber or convolutely rolled glass-cloth melamine. When using the tube material described by Healy U.S. Pat. No. 3,979,709, in order to obtain the same current rating as in an existing fuse, all fusible elements had to be redesigned or re-rated, or the wall tubing had to be given a thickness exceeding mechanical strength requirements. None of these solutions is economically acceptable.

It is well known to line fuse tubes with a layer of asbestos where a reduction of the radially outward heat flow from the fusible element, or elements, is desired. But asbestos is a carcinogen, i.e. a cancer-causing substance, and was ruled out for this reason from the very outset.

This invention is the outgrowth of a systematic investigation of materials which are non-carcinogenous, and are capable - though in form of thin lining sheets - to impart to pultruded glass cloth polyester tubings as described in Healey U.S. Pat. No. 3,979,709 about the same thermal conductivity as conventional fuse tube materials, e.g. glass-cloth-melamine laminates.

The product which meets the above requirements is a tough, paperlike product including highly purified asbestos in non-carcigenous form, and other insulation materials manufactured and sold by Johns-Manville under the trademark "QUINTEX II".

A fuse embodying this invention includes a fusible element, an arc-extinguishing filler embedding said fusible element, and a pair of electro-conductive terminal elements arranged at the ends of said fusible element and conductively interconnected by said fusible element.

A tubular casing or fuse tube supports said pair of terminal elements on the ends thereof. Said casing consists of a plurality of stripes of glass fiber material overlapping along the longitudinal edges thereof and impregnated with a polyester resin. A lining of Johns-Manville "QUINTEX II" is arranged inside said casing.

FIGS. 1 and 2 are substantially longitudinal sections along the casing of the fuse taken at 90.degree.; and

FIGS. 3 and 4 are substantially transverse sections across the casing of the fuse and deleting the arc-quenching filler of the fuse and the fusible element thereof.

Referring now to the drawings, reference character 1 has been applied to indicate a pair of fusible elements of sheet copper or sheet silver. Elements 1 are provided with circular perforations 2 which indicate the points of arc-inception at major fault currents. Elements 1 are provided with a low fusing point overlay 3, e.g. of tin, for severing fusible elements 1 on occurrence of overloads of inadmissible duration. The axially outer ends of fusible elements 1 are conductively connected to a pair of terminal elements in form of blade contacts 4. The latter project through the end surfaces of a pair of caps or ferrules 5. Blade contacts 4 are supported by ferrules 5 and by a pair of pins 6 which project transversely through blade contacts 4. The ends of pins 6 are fitted into bores 7 provided in tubular casing or fuse tube 8. In FIGS. 1 and 2 fuse tube 8 has been shown as consisting of but three layers, namely the radially outermost layer 8a of woven fiber glass, an intermediate layer 8b of fiber mat and the innermost layer 8c of woven fiber glass cloth. FIGS. 1 and 2 represent casing 1 in but diagrammatic fashion. For a more detailed description of fuse tube or casing 8 reference may be had to FIGS. 3 and 4 of the drawings and the context thereof. Layers 8a and 8c are formed of heavy strips of woven glass cloth. These strips are arranged so that the direction of their longitudinal edges is the same as the direction of the axis of casing 8. Each of layers 8a and 8c overlaps in the region of their longitudinal edges. The overlaps of layers 8a and 8b are angularly displaced, i.e. the overlaps of layer 8a may be situated at 0.degree. and the overlaps of layer 8c may be situated at 180.degree. Woven glass cloth is a relatively strong material having a high content of glass per unit while non-woven fiber mat is a mechanically relatively weak material being rather fluffy and having a relatively small fiber content per unit. Therefore, in spite of the overlaps formed by layers 8a and 8c which cause a local increase of numbers of layers and of the fiber density at the regions of overlap, the the give of the layer 8b allows to absorb the locally increased number and density of layers 8a,8c so that the wall thickness of casing 8 is substantially uniform along the entire periphery thereof. It is only the proportion of glass fibers to impregnating polyester 9 that changes from point to point of tubing 8, without having a significant effect on its dynamic strength.

Inside of casing 8 is a lining 10 of the Johns-Manville product known by its trademark "QUINTEX II". The liner of "QUINTEX II" may be held very thin, hardly thicker than an ordinary piece of paper, and yet its thermal insulating capacity is sufficient to impart to the fuse casing substantially the same thermal properties as, e.g. a casing of glass-cloth-melamine has. The minimal bulk of "QUINTEX II" allows filling of the casing 8 with a substantially equal amount of pulverulent arc-quenching filler or quartz sand 11', which was not possible heretofore in instances where asbestos was used to line a fuse tube because of the relatively larger bulk of asbestos.

Referring now to FIGS. 3 and 4 of the drawing, reference character 8a has been applied to indicate the outermost layer of woven fiber cloth overlapping at the region a, reference character 8b has been applied to indicate a layer of fiber mat overlapping at the region b and reference numeral 8c has been applied to indicate the innermost layer of woven fiber glass gloth overlapping at the region c. Layers 8a and 8c are provided with a coextensive lining layer 8a' and 8c' of glass fiber matte. These lining layers 8a' and 8c' allow to decrease the thickness of intermediate layer 8b, and in some cases to entirely eliminate the intermediate layer 8b. Reference numeral 10 has been applied to indicate the lining of "QUINTEX II".

In FIG. 4 the same reference characters as in FIG. 3 have been applied to designate like parts. Thus 8a is the outermost layer of woven fiber glass overlapping at a, 8c is the innermost layer of woven fiber glass overlapping at c and 10 is the lining of Johns-Manville "QUINTEX II" overlapping at 10a. FIG. 4 further shows layers 8a' and 8c' coextensive and integral with layers 8a and 8c formed by non-woven fiber glass mat. Between layers 8a and 8c a number of layers of non-woven fiber mat are arranged which have been designated by the common reference numeral 11. Layers 11 consist of non-woven mat material, the longitudinal edges of which overlap. A further layer 12 consisting of fiber glass rovings is arranged between layers 11. Rovings 12 are arranged to provide the necessary strength to the tubular composite when drawn through a pultrusion machine. The number of layers 11 may be decreased, and in some instances even made zero, in the presence of lining layers 8a' and 8c'. Also the layer of fiber glass rovings 12 may be dispensed with. The innermost layer 10 of Johns-Manville "QUINTEX II" must be present unless it is permissible to increase the heat transfer and thus to change the rating of the fuse.

For the sake of greater clarity the arc-quenching filler inside of the casing 8 of the fuse and the fusible element have not been shown in FIGS. 3 and 4.

The layer of Johns-Manville "QUINTEX II" has a very smooth surface and is likely to be displaced in a direction longitudinally of the casing of the fuse during assembly thereof. This is prevented by the knife-blade-supporting pins 6 forming abutments for lining layer 10 precluding any significant movement thereof in a direction longitudinally of the casing of the fuse.