Electric Fuse for Elevated Circuit Voltages

It is often necessary to provide electric fuses with means which are responsive to blowing thereof. Such means may be used either to indicate that the particular fuse has blown, or to trip an automatic switch or circuit breaker arranged in series with the fuse.

As a general rule the means responsive to blowing of a fuse include a spring-biased pin normally maintained in a first, or non-indicating position, and caused when the fuse blows by its spring bias to move to a second, or indicating position.

Fuses of the kind under consideration include two parallel-connected fusible element means namely a first low resistance fusible element means and a second high resistance fusible element means. Normally the first fusible element means carries virtually the entire load current and the second fusible element means is virtually non-current carrying. When the current path through the first fusible element means is interrupted by overload currents, or short-circuit currents, the second fusible element means becomes current-carrying. This causes fusion thereof in rapid sequence to the overcurrent-caused fusion of the first fusible element means. Fusion of the second fusible element means liberates blown fuse responsive means allowing the same to move from their above referred-to first position to their above referred-to second position.

In low-voltage fuses the second or high-resistance fusible element establishes a current path which is normally continuous, extending from one terminal element of the fuse to the other terminal element thereof. It is common practice in high voltage fuses to include a break-down spark gap in the current path of the second or high-resistance fusible element. As a result of the presence of that break-down spark gap the current path of the second or high-resistance fusible element is normally interrupted at the point where the break-down spark gap is located. When the current path of the first or low resistance fusible element is interrupted on occurrence of an overload current or short-circuit current, a voltage appears across the break-down gap which voltage is sufficiently high to cause its break-down. As a result of this break-down and consequent arcing across the gap, a continuous current path is now established by the intermediary of the second or high resistance fusible element means from one terminal element of the fuse to its other terminal element. This causes fusion of the second fusible element at some predetermined point thereof which, in turn, causes operation of the blown fuse responsive device of the fuse.

It will be understood that in the aforementioned type of fuses the second or high resistance fusible element is subdivided into sections by the presence of the aforementioned spark gap. While the entire length of the high resistance shunt across the first low resistance fusible element may rightly be referred-to as a second fusible element, fusion thereof occurs, as mentioned above, only at a predetermined point of it at which point the positioning of the blown fuse indicating means is controlled by the mechanical integrity, or the absence of the mechanical integrity, respectively, of the second fusible element.

The present invention refers more specifically to fuses for elevated circuit voltages wherein the first low resistance fusible element is substantially helically wound around a mandrel of electric insulating materials, and one of the principal objects of this invention is to use that mandrel for purposes other than as fusible element support, thus greatly reducing the cost of manufacture of the fuse.

Another object of the invention is to adapt the aforementioned mandrel to define with a satisfactory degree of precision the break-down spark gap whose electric break-down causes energization of the high resistance fusible element, and operation of the blown fuse responsive means.

Still another object of this invention is to provide a high-voltage fuse wherein the reaction to the spring forces biasing the blown fuse responsive means are positively transmitted to the insulating mandrel. These spring forces may be very considerable, particularly if the blown fuse responsive means are not merely intended as blown fuse indicators, but are supposed to overcome the relatively large latch friction of a high voltage disconnect, or like piece of equipment. In the absence of firm and rugged means for anchoring the spring that biases the blown fuse responsive means, the latter may move simply by conventional handling of the fuse to a position intermediate their non-blown fuse position and their blown fuse position.

A further object of the invention is to provide fuses for elevated circuit voltages which have plug terminal inserted into the ends of the tubular fuse casing of insulating material wherein energization of the high resistance fusible element is controlled by the break-down of one or more spark gaps serially arranged therein, and wherein the blown fuse responsive means lends itself to be, and actually is, arranged in coaxial relation to the plug terminals.

Further objects and advantages of the invention will become apparent as the following description proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to, and forming part of, this specification.

Fuses embodying this invention include a tubular casing of electric insulating material housing therein a granular arc-quenching filler. The casing is closed at its ends by a pair of terminal elements. The terminal elements are conductively interconnected by a first substantially helically wound fusible element means submersed in said filler. Fuses embodying this invention are further provided with a spring-biased blown fuse responsive device arranged immediately adjacent one of the aforementioned pair of terminal elements. The operationof the blown fuse responsive device is controlled by a second high resistance fusible element means submersed in said filler and shunting at least a portion of said first fusible element means. The second fusible element means is sectionalized. It includes at least two spaced sections forming a voltage responsive break-down spark gap therebetween. The first fusible element means is supported by a mandrel extending in a direction longitudinally of the casing. The mandrel has a perforation through which spaced sections of the second fusible element means are threaded at spaced points of said perforation so that the aforementioned spark gap is formed by said perforation in said mandrel.

FIG. 1 is substantially a longitudinal section along I-I of FIG. 2 showing a fuse embodying this invention, FIG. 1 showing some parts in elevation rather than sectionalized, and FIG. 1 showing only the portions of the fuse including its terminal elements and the portions immediately adjacent the terminal elements, while deleting the intermediate portions of the fuse;

FIG. 2 is a section along II-II of FIG. 1;

FIG. 3 is an isometric diagrammatic view of a detail of the structure of FIGS. 1 and 2; and

FIG. 4 is a longitudinal section of the detail shwon in FIG. 3.

In the drawing numeral 1 has been applied to indicate a tubular casing of electric insulating material, e.g. a melamine-glass-cloth laminate, housing therein a pulverulent arc-quenching filler 2, preferably quartz sand. Filler 2 has been deleted in some portions of the drawing in order to expose to view other parts of the fuse structure. A pair of terminal plugs 3 is press-fitted into the ends of casing 1 and firmly held in position by steel pins 4 projecting radially through casing 1 into terminal plugs 3. Reference numeral 5 has been applied to indicate a first substantially helically wound low resistance fusible element means submersed in filler 2 conductively interconnecting terminal elements or terminal plugs 3. Fusible element means 5 is preferably a ribbon of silver having a plurality of points of reduced cross-sectional area (not shown). the current-carrying capacity of the structure shown may be increased by connecting several helically wound silver ribbons 5 in parallel. The ends of ribbon or ribbons 5 may be mechanically and electrically or conductively connected to the axially inner end surfaces of terminal plugs 3 by means of screws 5a and solder joints as shown in detail in U.S. Pat. No. 3,571,775 to F. J. Kozacka et al; 03/23/71 for HIGH VOLTAGE FUSE HAVING A PLURALITY OF HELICALLY WOUND RIBBON FUSE LINKS. Fusible element 5 is supported by a mandrel 6 of electric insulating material having radially outer edges 6a extending in a direction longitudinally of casing 1. Mandrel 6 is made-up of four plates 7 which are arranged at right angles. Two of plates 7 are provided with apertures of circular bores 8 and 9. Reference numeral 10 has been appllied to indicate generally a blown-fuse-responsive spring-biased device arranged immediately adjacent to, i.e. built into, one of terminal plugs 3. The circular bore 9 in one of plates 7 is arranged immediately adjacent to plug 3 into which device 10 is built, while the circular bore 8 is arranged in another of plates 7 immediately adjacent to the other of plugs 3. The fuse includes a high resistance fusible element means submersed in filler 2 and shunting at least a portion of fusible element 5. In the instant case the entire fusible element 5 is shunted by the aforementioned high reisitance fusible element means which is made up of three sections. The first section 11a extends from one of plugs 3 to circular aperture 8; the second section 11b extends from circular aperture 8 to circular aperture 9; and the third section 11c extends from circular aperture 9 to device 10. Section 11a is formed by a length of wire threaded through aperture 8. The ends of that length of wire remote from aperture 8 are conductively connected by means of a screw 5b to the axially inner end surface of plug 3 immediately adjacent to aperture 8. The second section 11b of the high resistance fusible element is formed by a resilient helically wound resistance wire which is in effect a tension spring. The ends of section 11b are substantially hook-shaped. One of these ends is hooked into aperture 8 and the other of these ends is hooked into aperture 9. The third section 11c is formed by a length of resistance wire threaded through a bore 10a in the flattened end of spring biased rod 10b of device 10 and also threaded through bore 9 in one of plates 7. The end of section 11c are twisted upon each other as indicated at 11c'.

The blown fuse responsive device 10 includes compression spring 10c biasing rod 10b and the latter is normally restrained by wire section 11c from moving under the action of spring 10c from left to right, as seen in FIG. 1 While one end of spring 10c rests against the end of rod 10b remote from aperture 10a, the other end of spring 10c rests against a cap 10d. Another cap 10e through a coaxial bore in which wire section 11c is threaded precludes entry of filler 2 into a chamber housing rod 10b and cap 10d. That chamber is formed by a hollow shaft 12a of a screw 12 by which one ferrule 13 is screwed against the axially outer end surface of one of plugs 3. The other ferrule 13 is screwed by a screw 14 against the axially outer end surface of the other of plugs 3.

A first spark gap is formed between sections 11a and 11b and a second spark gap is formed between sections 11b and 11c. Incident to blowing of the fuse an electric arc takes the place of low resistance fusible element 5. When the arc voltage is sufficiently high, both aforementioned spark gaps break down. This establishes a current path from screw 5b on one of plugs 3 through wire section 11a, across aperture 8 and through wire section 11b and across aperture 9 through wire section 11c and rod 10 to the other of plugs 3. As a result of the flow of current through that current path section 11c fuses, allowing compression spring 10c to move rod 10b from left to right, as seen in FIG. 1.

The breakdown voltage of spark gaps at 8 and 9 depends upon the diameter of these two apertures. Therefore the desired breakdown voltage of the spark gaps at 8 and 9 can readily be determined by the selection of the diameters of the spark gaps formed at apertures 8 and 9. Where the arc voltage generated incident to blowing of the fuse is relatively limited, the spark gap at 8 may be dispensed with, in which instance section 11a may be dispensed with and section 11b directly conductively connected by means of screw 5b to the plug 3 immediately adjacent thereto.

In order to fixedly position plates 7, plugs 3 are provided with groove means 3a at the axially inner end surfaces thereof. These groove means 3a receive the transverse relatively short edges of the elongated plate means 7. While the radially outer longitudinal edges 6a of plates 7 support fusible element 5, the radially inner edges of plates 7 are separted from each other (see FIG. 2) and bound a space wherein fusible element section 11b is arranged in coaxial relation to, but widely separated from, low resistance fusible element means 5. The aforementioned way of arranging wire section 11b is far better than winding the same around the radially outer edges 6a of plates 7 because it allows to maximize the insulating spacing between contiguous turns of fusible element means 5.

As long as the force of spring 10c is relatively moderate, affixing wire section 11c -- which is the restraining wire for blown fuse responsive device 10 -- to aperture 9 in one of plates 7 does not present any particular problem. The situation is quite different if spring 10c exerts a considerable force upon rod 10b. In that instance it is vitally important to maximize the friction of frictional forces between restraining wire section 11c and the plate 7 provided with perforation 9 in order to avoid any slight unintentional movement between pin 10b and plates 7 under the action of spring 10c. In order to positively prevent such movement wire section 11c is threaded to the perforation 10a in rod 10b so that two ends of wire section 11c extend beyond perforation or aperture 10a. Then one of these ends is threaded through aperture 9 from one side thereof to the other and the other of these ends is threaded through aperture 9 from said other to said one side thereof. As a result, both ends of wire section 11c cross each other inside aperture 9 and exit from aperture 9 at different sides of the plate 7 in which aperture 9 is provided. The ends of wire section 11c leaving aperture 9 on opposite sides of the plate 7 in which aperture 9 is provided are tied together, preferably by twisting, as indicated at 11c'.

When the spark gaps coextensive with apertures 8 and 9 break down, arcs are kindled between wire sections 11a and 11b and wire sections 11b and 11c. As a result, a current flows from one terminal plug 3 to the other terminal plug 3 by a current path that includes high resistance wire sections 11a, 11b, 11c, rod 10b and cap 10d. the portion of that current path designed to fuse before any other portion thereof reaches its fusing temperature is wire section 11c. When wire section 11c fuses, spring 10c propels rod 10b from left to right, as seen in FIG. 1.