US3523265A - Fuse elements - Google Patents

Description

Aug. 4, 1970 I Y FUSE ELEMENTS Filed March 29, 1968 J. FEENAN ETAL 3,523,265

United States Patent 3,523,265 FUSE ELEMENTS John Feenan, Ronald Vincent Wafer, and Eric Jacks, Liverpool, England, assignors to The English Electric Company Limited, London, England, a British com- Filed Mar. 29, 1968, Ser. No. 717,211 Claims priority, application Great Britain, Mar. 30, 1967, 14,467/67 Int. Cl. H01h 85/08 U.S. Cl. 337-290 4 Claims ABSTRACT OF THE DISCLOSURE A fusibleelement for use in a high voltage fuse link, which element has a number of elongated apertures spaced apart along its length. The cross-sectional area and length of the intermediate portions of the element between the apertures and the cross-sectional area and length of the reduced portions alongside the apertures are carefully designed and so related that the relatively large mass of metal in these intermediate portions retards the rate at which the element is consumed by an are upon overload fault inception. This retardation of the element consumpton is sufiicient to encourage arcs to strike adjacent other apertures in the element whereby to realise an unstable multiple arcing condition conducive to their extinction.

Fuses designed in accordance with the invention are suited to interrupt fault current in a controlled manner in response to both overload and short-circuit conditions.

This invention relates to fuse elements for high voltage fuse links.

According to the invention, there is provided a fusible element of silver strip for a high voltage fuse link, the strip having formed therein a plurality of elongated apertures spaced apart along its length and extending laterally of the strip, the cross-sectional area and length of the intermediate portions of the strip between the apertures and the cross-secitonal area and length of the reduced portions of the strip alongside the apertures, being so related that the relatively large mass of metal in said intermediate portions reduces the rate at which the element is consumed by an arc struck adjacent only one of said apertures upon overload fault inception, this reduction encouraging arcs to strike adjacent the other apertures whereby to realise an unstable multiple arcing condition conducive to their extinction.

By a high voltage fuse link we mean fuse links which are suitable for protecting systems in which the effective voltages may be between, say, 660 volts and 33kv.

The apertures may be rectangular in form so ""as to promote an even burn back as a result of arcing, and to constrain the arc to cover a Wide area, the width of the strip, the length and width of each aperture and the lengthwise spacing between the apertures typically being in a ratio of the order of 5:3%:1:12.

A high voltage fuse element according to this invention is thus suited to interrupt fault current in a controlled manner in response to both overload and short-circuit conditions since, in the latter conditionsjthe same multiple arcing condition is realised, but by reason this time of the current density in the reduced portions being such that arcing commences more or less simultaneously in all these portions.

Hitherto, the performance of this dual duty in high voltage fuse links has not been satisfactory since whereas they may be designed to be readily capable of interrupting short-circuit currents, under overload conditions the cur- 3,523,265 Patented Aug. 4, 1970 p CC rent density may be sufiicient to fuse, and cause an arc to be struck across, only one of the reduced portions and not infrequently this results in the generation. of a long single are which persists for an appreciable period. Such an arc is, of course, more persistent in fuses designed to interrupt high voltages than the lower voltage types, and in particular this phenomena is realised by the element being burnt back towards its ends from the position in which the initial arc was struck so that the arc is continuous and is elongated from that position, the arc frequently being extinguished only after it has burnt its way through the end caps of the fuse link.

In such fuses the inherent fault is that the element is consumed too rapidly to permit any controlled interruption of the arc current, but with this invention the rate at which the initial arc extends is retarded by the relatively large masses of the silver element in the aforesaid intermediate portions sufficiently to encourage arcing at a multiplicity of the reduced portions which results in the arcs becoming unstable and finally extinguished.

In order that the invention can be fully understood, some embodiments thereof will now be described with reference to the accompanying drawings in which:

FIG. 1 illustrates a plain strip fuse element; and

FIGS. 2(a) to 2 (c) illlustrate a strip fuse element which has been bent into various forms to become selfsupporting.

Referring now to FIG. 1, a plain silver strip 3 has a number of rectangular apertures 4 formed in it at spaced intervals. These apertures may be formed, for example, by photo-etching or press techniques.

In order to produce the desired characteristics to which reference has been made, for a fuse link of a particular rating e.g. 5 kv., 50 A., the apertures in an element of width A=0.32 inch and thickness 0.0025 inch were spaced apart by a distance B=0.75 inch, the apertures lying centrally and having a length C=0.2'1 inch transversely of the strip and a width D=0.06 inch. About 19 apertures were formed along the length of the strip.

This plain form of strip element may conveniently be wound along a helical path about a ceramic former in a fuse link for support and more than one element may be wound on the same former. Alternatively, however, the strips may be made self-supporting by bending them to produce the required degree of rigidity.

Thus, with reference to FIGS. 2(a) and 2(b), the strip may be pierced with two apertures alongside one another and bent lengthwise between the apertures into either an L-shaped section (FIG. 2(a)) or a V-shaped section (FIG. 2,(b) In particular, with the L-shaped section, a fuse link of rating 5 kv., 225 a. was made from elements of strip material 0.004 inch thick, the dimensions of each limb and its apertures being substantially the same as those shown in FIG. 1, one of the limiting factors in this design being that the ratio between the reduced-tofull cross-sectional areas cannot be increased beyond about 3:1 otherwise the element would not be sufliciently selfsupporting. The apertures were again formed at 19 spaced positions along the length of the element and six ele ments were connected together in parallel to produce the desired rating.

Other self-supporting shapes are also possible and the number of apertures pierced at the reduced sections may again be different. For example, three apertures may be pierced at each reduced section and the element bent into channel form, (FIG. 2(0) With" these self-supporting elements the width of each element is two or more times the width of the element shown in FIG. 1 in order for them to be sufficiently rigid, especially where long lengths are involved, and in some instances a transverse solder fillet is applied mid-way along its length in order to promote arcing in the centre of the element.

In each of the above cases, it will be appreciated that the width of the element is far greater than the width of previous elements designed for duty in high voltage fuses of comparable rating, and the result of this is that the wiring of the fuse link can be more concentrated e.g. a fuse link of given current rating may contain eight conventional elements but only four elements of the type constructed in accordance with this invention.

Although a number of detailed embodiments of specific fuse elements have been described, it is to be understood that the invention is not limited to elements of the precise form and disposition shown, and the proportion of the reduced to full sections, and the pitching of the apertures, may be varied to compromise between the theoretical ideal and practical cases which result in an element which is robust enough to handle during manufacture.

We claim:

1. A fusible element for a high voltage fuse link, said element comprising a strip of silver defining a plurality of rectangular apertures spaced apart along its length whereby to define intermediate strip portions between the apertures and reduced strip portions alongside the apertures, the width of the strip, the length and width of each aperture and the lengthwise spacing between the apertures being in a ratio of the order of 5:3 /2:l:l2 such that the relatively large mass of metal in the intermediate portions reduces the rate at which the element is consumed by an arc struck adjacent only one of the apertures upon overload fault inception, this reduction encouraging arcs to strike adjacent the other apertures whereby to realise an unstable multiple arcing condition conducive to their extinction.

2. A fusible element according to claim 1, wherein, with a strip thickness of 0.0025 inch, the width of the strip is 0.32 inch, the length and width of each apertures is 0.21 inch and 0.06 inch, respectively, and the spacing between the apertures is 0.75 inch.

reduced strip portions in all the groups so as to be self-supporting, the width of each bent strip section, the length and width of each aperture and the lengthwise spacing between the groups of apertures being in a ratio of the order of 5:3 /2:1:12 such that the relatively large mass of metal in the intermediate portions reduces the rate at which the element is consumed by an arc struck adjacent only one of the apertures upon overload fault inception, this reduction encouraging arcs to strike adjacent the other apertures whereby to realise an unstable multiple arcing condition conducive to their extinction. 4. A fusible element according to claim 3, wherein the strip s bent into right-angle, channel or V-sections along its length.

References Cited UNITED STATES PATENTS 3,317,691 4/1967 Bassani 337-295 XR 3,281,556 10/1966 Salzer et al 337-159 2,777,033 1/1957 Kozacka 337-295 XR FOREIGN PATENTS 807,347 I 1/1959 Great Britain.

BERNA-RD GILHEANY, Primary Examiner D. M. MORGAN, Assistant Examiner US. Cl. X.R.

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