Electric Fuse Capable of Interrupting Small Currents

The patent application of Frederick J. Kozacka, filed 03/07/77; Ser. No. 775,248 for ELECTRIC ALL PURPOSE FUSE suggests to limit the axial spacing between a plurality of parallel connected fusible elements to a critical distance at which an electric breakdown can occur, but a self-sustained high current discharge, or arc, cannot occur. The flashover damages the fusible elements, and results in break formation at the damaged point or points by additional i.sup.2.r losses at this point, or points, thus establishing the required series breaks.

This process has a number of limitations. While it is desirable to maximize the axial spacing between parallel connected turns of the helical windings of a fusible element, Kozacka calls for reduction of that spacing to a critical value. Another limitation of Kozacka resides in the fact that the point where the breakdown will occur is not pre-determined, but can occur randomly, at any point of the fusible elements. It may, for instance, occur near one of the terminal elements where an arc should not be initiated.

The primary object of this invention is to provide electric fuses which are not subject to the above limitations. Other objects will become apparent as this specification proceeds.

Fuses according to this invention include a tubular casing of electric insulating material, a pair of terminal elements closing the ends of said casing, and a granular quartz filler inside said casing. A group of fusible elements including a plurality of fusible elements conductively interconnects said pair of terminal elements. Said plurality of fusible elements is electrically connected in parallel, wound in substantially parallel helical paths around a common geometrical axis in rectangular turns and embedded in said quartz filler. The pitch of said turns remote from said pair of terminal elements is less than the pitch of said turns adjacent said pair of terminal elements whereby series multibreaks are produced at regions of said reduced pitch following initiation of an arc adjacent the centers of said group of fusible elements.

FIG. 1 is in part a front elevation and in part a longitudinal section of a fuse embodying the present invention;

FIG. 2 is a top-plan view of the structure of FIG. 1;

FIG. 3 is a section along 3--3 of FIG. 1;

FIG. 4 is an elevational view on a larger scale of the fusible element of FIG. 1; and

FIG. 5 is a planar winding diagram of the fusible element obtained by cutting the winding along one of its support rods and spreading the winding into the plane of the paper on which FIG. 5 is drawn.

Referring now to the drawing, numeral 1 has been applied to indicate a tubular casing of electric insulating material. Casing 1 is closed by a pair of terminal elements 2 which are in the shape of terminal plugs 2. Terminal plugs 2 are pinned to casing 1 by steel pins 3. Casing 1 is filled with a granular arc-extinguishing filler 4 of quartz sand. A group 5 of fusible elements conductively interconnects terminal plugs 2. The constituent fusible elements of the group 5 are electrically connected in parallel and wound helically in quarter turns.

The fuse may be manufactured according to teaching in my U.S. Pat. No. 3,848,212, Nov. 12, 1974 for METHOD OF ASSEMBLING ELECTRIC HIGH-VOLTAGE FUSES AND SUBASSEMBLY THEREFOR, in which case the support rods 6 are removed after the fuse has been completed as more fully set forth in the above patent.

In the present drawing I have shown four rods 6 of insulating material which form a permanent part of the fuse structure. Rods 6 support the group 5 of fusible elements comprising four fusable elements. Rods 6 are secured with the ends thereof to terminal elements 2 and the fusible elements 5 are wound helically in rectangular quarter turns around rods 6. Some of the windings have a relatively large pitch .alpha., and some of the windings have a relatively small pitch .beta..

Referring to FIG. 1 windings 5 begin at A, go from there to B,C,D and E.sub.o. The quarter turns such as from B to C, or from D to E.sub.o, all have the same pitch .alpha.. The pitch of the quarter turns of which the groups of fusible elements 5 is made up decreases in the quarter sections

E.sub.o -E.sub.1

E.sub.1 -e.sub.2

e.sub.2 -e.sub.3 and

E.sub.3 -e.sub.4.

a similar decrease in pitch occurs in the upper portions of the fuse between the quarter sections

E.sub.o '-E.sub.1 '

E.sub.1 '-e.sub.2 '

e.sub.2 '-e.sub.3 '

e.sub.3 '-e.sub.4 '.

the variations or reduction of the pitch provide points where the dielectric strength between turns of winding 5 is weakened and where a dielectric breakdown may occur. Such a breakdown will occur after an initial arc gap has been formed in the center region of fusible elements 5 (not shown in FIG. 5).

FIGS. 1 and 2 show prismatic recesses 2a in terminal plugs 2 intended for the insertion of blade contacts (not shown).

It will be understood that the centers of the group 5 of fusible elements may or may not be provided with a so-called M-effect spot, i.e. an overlay of a low fusing point link-severing metal, e.g. tin.

Referring now particularly to FIG. 5, thw winding begins at A' and proceeds to B' and C'. At C' and E' there is a drastic decrease of the pitch, each time for four consecutive quarter turns a,b,c,d. In other words, the spacing between quarter turns is smallest at C' along rod 6' and at E' along rod 6' where a sudden change of pitch occurs. The consecutive turns of the windings that begin at A' on terminal 2 and end at G' at terminal 2' have been marked M.sub.2,M.sub.1,K,M,M.sub.1, M.sub.2. The pitch of turns K is reduced, that of intermediate turn M is normal. FI is the point where tin, or another low fusing point link-severing overlay, is located. Since the point FI is located in the center of the fuse, melting will be initiated at point FI of each of the fusible elements 5'. Thus an initial break is formed in each of them by the overlay FI. Since neither the fusible elements nor their overlays are absolutely identical, fusion of the overlays at FI and the fusible elements 5' will not occur simultaneously, but in a rapid sequence. The melting of each fusible element will give rise to a voltage surge -L (di/dt). If the gaps at C' and E' are slightly less than the breakdown voltage -L (di/dt), the gaps will break down and form two additional breaks in the fusible elements, i.e. two breaks in each fusible element 5' in addition to the breaks formed at FI. The discharges at C' and E' ought to be a momentary low current discharge, rather than a high current arc discharge.

Since this invention is predicated on the breakdown of a spark gap by -L (di/dt), it is obvious that voltage must exceed the breakdown voltage of the spark gap. If this does not occur, a single break will be formed near the centers of fusible elements 5' and the fuse is likely to fail on the one hour fusing current, or other small currents in that range.

On the other hand, the voltage must be sufficiently small to avoid thermal emission and formation of an arc between turns, resulting in the formation of a fulgurite that short-circuits the turns. Among the various gas discharges that may cause a proper breakdown the surge breakdown in prominent. The points where the wire-like ribbon of which the fusible conductors 5 are formed (see FIG. 4) causes an electric field concentration. The sparking potential is a function of the product of pressure and gap length only. This fact plus an approximate knowledge of the range of -L (di/dt) that occurs allow a determination of the pitch of the low pitch turns required to achieve the desired effect.