Current limiting fuse with auxiliary element arcing clip spaced by nonporous dielectric member

Abstract

The fuse is of the general purpose type having a casing closed at the ends by terminal caps; a core inside the casing; a main fusible element wound about the core and connected between the caps; a pair of arcing clips mounted in spaced relation on the core adjacent the main fusible element; and, an auxiliary element also wound about the core and connecting together the arcing clips. The improvement comprises a nonporous dielectric sheet covering the exposed surface of the clips and being interposed between and in contact with the clips and the main fusible element for determining the arc gap between them.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to high voltage current limiting electrical fuses and particularly to such fuses of the general purpose type which can effectively interrupt both low and high currents.

High voltage fuses generally have an insulating tubular casing closed at both ends by metal terminal caps. An insulating fuse core typically extends between the caps inside the casing. Wound helically on the fuse core is a main fusible element, which may be one wire or ribbon, or several in parallel, connected at the ends to the terminal caps. The space in the casing around the main element is filled with a tightly packed arc-quenching filler, such as quartz sand.

The main fusible element is typically a silver ribbon which is perforated at regular intervals to provide cross-sectional "necks" which will melt prior to melting of the ribbon proper when the fuse carries a high fault current. Thus a high fault current results in the generation of a number of regularly spaced arcs which interact with the filler to limit the fuse current in a controlled fashion as the main element is consumed by arcing, until eventually all current ceases. The cross-sectional necks in the ribbon are an important feature for preventing the establishment of only a single arc, or merely several which might consume the ribbon entirely to the terminal cap and damage the cap to result in a failure mode for the fuse. The necks sufficiently increase the ribbon resistance locally that each neck is certain to generate an arc at high fault currents.

Low fault current operation of such a fuse is a somewhat different matter. At low currents, the distribution of melting I.sup.2 t, the product of the square of the current and time, in the main element is not sufficiently determined by the necks to assure that arcs will be generated at all their locations. The thermal conductivity of the filler and the thermal gradients in the fuse as a whole now play a major role in determining which neck will be first to melt. If the first to melt is near a terminal cap, the main element may be consumed adjacent to the cap, and this may result in a failure. Therefore, it is common practice to place near the center of the ribbon length an overlay of a lower melting point solder. Prior to any melting of the ribbon, the solder melts and reacts with the ribbon chemically to increase its resistance, thereby assuring the initiation of arcing at that central point.

It is desirable for the clearing characteristics of the fuse to establish additional arcs after the initial melting of the central ribbon segment. For this purpose, it is known to provide two metal arcing clips mounted on the core, each to one side of the center of the fuse, and half the distance to the cap and spaced a predetermined critical gap from the main ribbon element. The clips are connected together electrically by an auxiliary wire element also wound about the core. As the initial central segment arc becomes elongated and thus has an increasing potential difference between its ends, the ribbon portions opposite the clips also have this potential difference, and thus a voltage half that value appears across each of the gaps. The gap voltage increases until the air breaks down and arcing results to melt the ribbon opposite the clips. The arcing is then maintained between the severed ribbon ends by the fuse current until the fuse clears. Control of the gap spacing, the number of clips, and the spacing of the clips along the length of the ribbon thus affords control of the arc generation at lower fault currents. The gap in present fuses is generally an air gap, the spacing being determined by accurate positioning of the ribbon relative to the clip by stops of some sort, or, for example, by interposing a porous woven glass fiber tape between the ribbon and clip and pressing the covered clip outwardly against the ribbon. In this way, the tape thickness determines and maintains the gap spacing, with the air in the tape interstices providing the dielectric for the gap.

One problem with a gap structure as described above is that it is difficult with an air gap to accurately and reliably control the breakdown voltage at the desired lowest possible voltage to assure that arcing is initiated as early as possible after the central segment of the main ribbon is severed. The tolerance of the gap must be very close. The inherent resilience of a woven tape, for instance, could lead to variations in the breakdown voltage of the gap spaced by it as a result of varying pressure against the tape by the ribbon element as it undergoes thermal cycling. The effects of undesired variations of the spacing upon the breakdown voltage are aggravated by the very high dielectric constant of air, which makes spacing highly critical.

SUMMARY OF THE INVENTION

In the novel fuse the arcing clip is spaced from the main fusible ribbon element by a sheet of non-porous dielectric material. Thus, the breakdown of the gap is determined by the dielectric characteristics of the sheet, rather than those of the air around the gap. This results in improved and more reliable control of the breakdown voltage of the gap.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially exposed, partially sectioned side view of a current limiting fuse in accordance with a preferred embodiment of the present invention.

FIG. 2 is an exaggerated elevational view of a fragment of the fuse of FIG. 1, showing an arcing clip and its associated structures in greater detail.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the invention is the general purpose fuse 10 shown in FIG. 1 of the drawing. The fuse 10 has an insulating tubular casing 12 of glass fibers and epoxy with an inside diameter of about 5cm (centimeters) and sealed at both ends by bronze terminal caps 14. Situated between the caps 14 and extending along the interior of the fuse 10 is a stepped, gas-producing core 16 which is centered in the caps 14 by metal connector clips 18 having tabs 20 welded to the inside wall of a sleeve 19. A pure silver fusible ribbon element 22 is wound helically on the outermost steps of the core 16 along the axis of the fuse 10 and connected at its ends to the connector clips 18. The ribbon element 22 is about 0.0125cm thick, about 0.47cm wide and provided at 1.27cm intervals with round, regular perforations 23 about 0.32cm in diameter for cross-section necks. A central segment 24 of the ribbon 22 has two additional perforations 26, one of which is hidden from view, located midway between a central perforation 30 and the regular perforations 23 to either side, for surface area necks. The surface on the outer side of the ribbon 22 between approximately the midpoints of the additional perforations 26 is covered with a solder overlay 31 of about 60% lead and 40% tin, by weight.

An auxiliary fusible silver wire element 32 about 0.025cm in diameter is wound helically about the innermost steps of the core 16 between two metal arcing clips 34, one of which is shown in more detail in FIG. 2, which press outwardly on a portion of the ribbon 22. The clips 34 are each located about midway between the central segment 24 of the ribbon 22 and one connector clip 18. Covering the surface of each arcing clip 34 and interposed between it and the ribbon is a piece of non-porous aromatic polyamide paper dielectric tape 35 about 0.05 millimeters thick, and having a dielectric strength of about 900 volts per mil (per 25.4 micrometers).

The remaining interior space of the fuse 10 around the ribbon 22, the wire 32 and the core 16 is filled with a tightly packed arc-quenching quartz sand filler 36 which is bound into a rigid matrix by colloidal silica particles.

When the fuse 10 passes a low fault current, as the temperature of the solder overlay 31 increases to near its fusion point, the solder overlay begins to melt and to diffuse into the ribbon 22 to form an alloy having a greater resistance than the ribbon element 22. This causes the segment 24 to rapidly melt and initiate arcing. As the ribbon 22 burns back and the voltage across the arc is increased by the increased arc length, the voltage of the arc appears across the tape 35 of the arcing clips 34 which are maintained at the same potential by the wire element 32. When the voltage across the tape 35 reaches a critical value, the tape 35 breaks down and arcing between the clips 34 and the ribbon 22 melts the tape 35 at the clips 34 to also establish arcs there to more rapidly limit the current.

The dielectric strength of the non-porous tape 35 permits accurate and reliable control of the breakdown voltage of the gap at the desired lowest possible voltage to assure that arcing is initiated as early as possible after the central segment of the main ribbon is severed. The control is afforded by utilizing the dielectric breakdown of the tape material itself to determine the breakdown, rather than using a porous tape only as a mechanical spacer for an air gap.

Although for the fuse 10 of the preferred embodiment the arc gap tape 35 is of aromatic polyamide resin paper it should be recognized that other non-porous dielectric materials may be used, provided they have a predictable and stable breakdown potential. Also, a fuse utilizing the present invention may have only one clip, with the other end of the auxiliary element connected to a terminal cap, or it may have more than two clips.

While in the fuse 10 of the preferred embodiment there were two clips 34, it is known in the art that an auxiliary element with only one clip may be used and the other end of the element connected to a terminal cap. Fuses may also be provided with more than two clips if it is desired.

Claims

1. A current limiting fuse of the type having a tubular insulating casing;

first and second conductive terminal members closing the first and second ends, respectively, of said casing;

an insulating core extending longitudinally inside said casing between said terminal members;

a main fusible element wound helically on said core and connected at its ends to said terminal members;

at least a first conductive arcing clip mounted on said core adjacent said main fusible element; and,

an auxiliary conductive element wound around said core and having one end connected to said arcing clip, the improvement therein comprising:

a non-porous dielectric member covering a major portion of the exposed surface of said first arcing clip interposed between and in physical contact with both said first arcing clip and said main fusible element.

2. The fuse defined in claim 1 and wherein said dielectric member is a sheet of aromatic polyamide resin paper.

3. The fuse defined in claim 2 and wherein said paper has a thickness of about 0.05 millimeters and a dielectric strength of about 900 volts per mil (per 25.4 micrometers).

4. The fuse defined in claim 1 wherein said auxiliary conductive element is connected at its other end to a second arcing clip mounted on said core adjacent said main fusible element remote from said first arcing clip, and a non-porous dielectric member covering a major portion of the exposed surface of said second arcing clip interposed between and in physical contact with both said second arcing clip and said main fusible element.

5. The fuse defined in claim 4 and wherein said first and second arcing clips are spaced substantially equidistant along said main element to either side of the midpoint of said main element.

6. The fuse defined in claim 4 wherein said dielectric member is a sheet of aromatic polyamide resin paper having a thickness of about 0.05 millimeters and dielectric strength of about 900 volts per mil (per 25.4 micrometers).