Electric Current-limiting Low Voltage Fuse

Abstract

An electric current-limiting low voltage fuse for back-up protection of switching devices having relatively movable contacts and being controlled by excess current tripping devices includes low resistance fusible elements in ribbon form and a high resistance fusible element in form of a helically wound wire. On occurrence of major fault currents the low resistance and the high resistance fusible elements fuse and vaporize sequentially. Fusion and vaporization of the low resistance fusible elements limits the peak let-through current to a value way below the peak of the available short-circuit current. The relatively small current that flows through the high resistance fusible element affords sufficient time for the switching device to open the faulted circuit completely.

Description

BACKGROUND OF THE INVENTION

In view of the continuous increase of the magnitude of available major fault currents in industrial and other systems it is necessary, to an ever increasing extent, to protect electrical apparatus, e.g. circuit breakers, by means of current-limiting backup fuses.

It is possible to coordinate circuit breakers or other motor starting equipment and current-limiting back-up fuses for the latter either mechanically or electrically. In the first mentioned instance blowing of the fuse results in the release of a striker pin which operates a tripping bar which, in turn, unlatches the latch mechanism of the circuit breaker. The above referred-to mechanical coordination method has two limitations: It requires additional parts and requires a special circuit breaker operating mechanism. Conventional electrical coordination of a circuit breaker and current limiting back-up fuses calls for certain values of the let-through current, and more particularly certain i.sup.2. t values thereof which cannot always be met. Either the tripping or unlatching time of the circuit breaker may be too long to cause tripping of the circuit breaker by the let-through current of its back-up fuse, or the desired current-limiting action of the back-up fuses could not be achieved if they were selected with the object in mind of causing tripping of a given circuit breaker by the let-through current thereof.

The present invention resolves these conflicts.

SUMMARY OF THE INVENTION

Fuses embodying this invention include in addition to the current-limiting fusible elements a helically wound fusible element of resistance wire which fuses at a later point of time than the current-limiting fusible element, thus extending the duration of current flow to cause tripping of a relatively slow automatic switching device or circuit breaker. Both the current-limiting fusible elements and the helically wound resistance wire are submersed in the same or a unitary body of pulverulent arc-quenching filler. The helically wound fusible element is arranged close to a median plane of the fuse tube inside of a space bounded by the current-limiting fusible elements and sandwiched between the current-limiting fusible elements line 17, which form parallel current paths arranged symmetrically in regard to said median plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded isometric view of a fuse and fuse holder combination embodying this invention and also showing a portion of a molded case circuit breaker intended to be backed up on occurrence of major fault currents;

FIG. 2 is a diagrammatic top-plan view of a fuse holder as shown in FIG. 1 without any fuses therein;

FIG. 3 is a top-plan view of a fuse embodying this invention;

FIG. 4 is substantially a longitudinal section of the fuse of FIG. 3, and

FIG. 5 are traces illustrating the operation of the composite circuit breaker and fuse unit shown in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the drawings, numeral 1 has been applied to designate a tubular casing or fuse tube of a current-limiting fuse. Fuse tube 1 is of an electric insulating material, e.g. melamine glass cloth. The ends of casing 1 are closed by a pair of plug terminals 2, which are press-fitted into casing 1. Steel pins 3a, 3b, 3c,3d project transversely through casing 1 into plug terminals 2, thus affixing firmly the latter to the former. Plug terminals 2 are conductively interconnected by current-limiting fusible elements or fuse links 4 in ribbon form having a relatively low electric resistance. The particular configuration of the fuse links 4 shown in FIG. 4 is more fully disclosed in U.S. Pat. No. 3,394,333 to P. C. Jacobs, Jr., July 23, 1968 for ELECTRIC FUSE HAVING STRESS-REDUCING FUSE LINK MEANS. Each fusible element or fuse link 4 is sandwiched between a pair of plates 4a of insulating material for controlling the arc-voltage generated incident to blowing of the fuse, as more fully explained in U.S. Pat. No. 2,964,604 to P. C. Jacobs, Jr. et al., Dec. 13, 1960 for CURRENT LIMITING FUSES HAVING COMPOUND ARC-VOLTAGE GENERATING MEANS. The axially inner surfaces of plug terminals 2 are provided with grooves engaged by the axially outer ends of fuse links 4. The aforementioned grooves are filled with soft solder to conductively connect fuse links 4 and plug terminals 2. Fuse links 4 are shunted by a high resistance restraining wire 6 for a blown fuse indicator of the kind more fully disclosed in U.S. Pat. No. 3,391,369 to F. J. Kozacka, July 2, 1968 for HIGH VOLTAGE FUSE. Fuse links 4 are further shunted by a helically wound resistance wire 5 of Nichrome. The ends of the latter are inserted into recesses or bores in plug terminals 2 and conductively connected to the latter by masses of a low fusing point metal. This may be a tin-free solder. It is not possible to form a solder joint between the metal of which plugs 2 are made and the ends of Nichrome wire 5. However, the masses of low fusing point metal surrounding the ends of Nichrome wire 5 exert such friction on wire 5 as to firmly position the ends thereof inside the bores or recesses into which they are inserted. This resistor 5 is our joint invention, but the fuse holder described below is not our joint invention. It is claimed in a copending patent application of Bernard DiMarco et al., filed Jan. 16, 1973, Ser. No. 324,193 for FUSE HOLDER WITH REJECTION FEATURE. The upper plug terminal 2 is provided with a wire connector generally indicated by reference numeral 7 and including the set-screw 8. The lower plug terminal 2 is provided with a substantially Z-shaped terminal strap 9 -- i.e., a connector strap similar in shape to an inverted Z -- affixed to it by screws 10. Connector straps 9 include web-portions extending parallel to the axis of casing 1 and pairs of flange portions extending at angles of 90.degree. in opposite directions away from the web portions. The upper flange portion of each terminal strap 9 forms a lug intended to engage a corresponding terminal in the molded case circuit breaker 11 shown in FIG. 1. Reference numeral 12 has been applied in FIG. 1 to indicate a screw intended to screw the upper flange portion of terminal strap 9 against a cooperating contact surface forming part of the molded case circuit breaker 11. The molded case circuit breaker 11 is provided with a fuse holder or pouch generally indicated by reference numeral 13. Fuse holder or pouch 13 is a molding of a plastic or synthetic resin forming three substantially tubular receptacles 13a for insertion of a fuse into each of them. Each receptacle 13a may be provided with one or more internal groove means extending in a direction longitudinally thereof. One or more of the fastener pins 3a-3d for fastening the lower plug terminal 2 (FIGS. 3 and 4) to casing 1 project radially outwardly beyond the cylindrical surface of casing 1. Therefore a receptacle 13a can only receive a fuse conforming to its shape and size if there is a groove 14 juxtaposed to pins 3a-3d projecting beyond the general outline of the fuse. It is possible to vary the number of projecting pins and the position thereof, and to provide fuse holders 13 which have grooves 14a whose number and position correspond to the number and position of the pins of the particular fuse that is intended to be accepted by the fuse holder.

The top-plan view of the fuse holder 13 shown in FIG. 2 is diagrammatic. The fuse holder forms 3 receptacles 13a closed at the bottom and open at the top. Each receptacle 13a is closed at the front thereof and is provided with a slot 15 on the rear thereof allowing the upper flange portion of terminal straps 9 to project out of receptacles 13a into engagement with corresponding terminals of molded case circuit breaker 11. Slots 13 and straps 9 determine the position of each of the three fuses relative to the fuse holder 13, and thus a fuse can only be inserted into the fuse holder if both have corresponding projecting pins 3a-3d and pin-engaging grooves 1' . . . 4'.

In FIG. 2 the possible positions of four grooves 14a have been indicated and numbered 1',2',3',4'. Considering a fuse holder 13 with a single groove having the position 1' and a fuse having one single projecting pin 3a, pin 3a cannot engage the groove positioned at 1' because the fuse is positioned relative to fuse holder 13 by straps 9 of the former and the slots 15 of the latter. A fuse having a single projecting pin 3a positioned as shown in FIG. 3 can only be inserted into a fuse holder having groove means positioned at 4'.

The resistor 5 of FIG. 4 makes it possible to achieve electrical coordination of a circuit breaker and of its back-up current limiting fuses even in instances where the circuit breaker is relatively slow and where the let-through i.sup.2. t value of the fuses is relatively small.

FIG. 5 explains more in detail the operation of shunt resistor 5. FIG. 5 is a plot of current versus time, i being the trace of an available short-circuit current i' being the trace of the actual short-circuit current or, in other words, that of the available short-circuit current as modified by the operation of the fuse. The actual short-circuit current i' rises initially at the same rate as the available short-circuit current i and the former decays rapidly following melting and vaporization of the fusible elements 4. After reaching an initial current zero the fault current limited by resistor 5 continues to flow for a period of time sufficient to cause tripping of the molded case circuit breaker 11.

For fuses embodying this invention the problem of interrupting the current flowing through resistor 5 is not of a serious nature as long as the current rating of the fuses is relatively low. This problem becomes more serious, the larger the current-carrying capacity or current rating of the fuse. Upon fusing of the resistor 5 series breaks exist in the molded case circuit breaker 11 and the fuse F. It is desired to properly apportion the interrupting duty to both aforementioned devices and to minimize the interrupting duty of each. The interrupting duty of fuse F is much more severe than that of a conventional back-up fuse because the presence of resistor 5 results in a drastic increase of metal vapor incident to blowing of the fuse and because that metal vapor has a relatively low ionizing potential. Considering relatively high current ratings of fuse F, the positioning of the helically wound resistors 5 relative to the other parts of the fuse becomes a factor of considerable significance. To be more specific, in such instances the resistor 5 ought to be arranged as shown in FIG. 4, i.e., resistor 5 ought to be arranged relatively close to a median plane of the fuse tube 1 and the fusible elements 4 ought to be arranged relatively remote from said median plane sandwiching resistor winding 5. As a result of this configuration a zone of relatively high and uniform temperature is formed adjacent the center region of resistor winding 5. This results in a tendency of formation of a very long initial break, or of series breaks, at the aforementioned zone of relatively high and uniform temperature.

It may happen incident to some fault condition that fusible elements 4 fuse and are vaporized by resulting arcing, that the letthrough current causes opening of the backed-up circuit breaker or other motor starting device, but that the resistor 5 remains intact, i.e., is not melted and vaporized. Reclosing of the circuit breaker or other motor starting device then causes re-energization of resistor 5. Under such conditions it is necessary to effect rapid interruption of the current flow through resistor 5 by melting and vaporization of the latter.

It may be pointed out that a molded case circuit breaker or other motor starting device and a back-up fuse may be designed from the very outset to form a matched pair. In that case the problem underlying this invention does not arise.

Claims

We claim as our invention:

1. An electric current-limiting low voltage fuse for backup protection of switching devices having relatively movable contacts and being controlled by excess current tripping devices, said fuse including in combination

a. a fuse tube of electric insulating material;

b. a pair of terminal elements closing the axially outer ends of said fuse tube;

c. a unitary body of pulverulent arc-quenching filler inside said fuse tube;

d. low resistance current-limiting fusible elements in ribbon form conductively interconnecting said pair of terminal elements submersed in said unitary body of filler and being arranged symmetrically to opposite sides of a median plane of said fuse tube to form separate current paths situated to opposite sides of said median plane and relatively remote from said median plane; and

e. a high resistance fusible element in form of a helically wound wire conductively interconnecting said pair of terminal elements, submersed in said unitary body of filler and being arranged relatively close to said median plane of said fuse tube inside of a space bounded by said low resistance current-limiting fusible elements so as to be sandwiched between said low resistance fusible elements.

2. A fuse as specified in claim 1 wherein said helically wound wire is of Nichrome.

3. An electric current-limiting low voltage fuse for back-up protection of switching devices having relatively movable contacts and being controlled by excess current tripping devices, said fuse including in combination

a. a fuse tube of electric insulating material;

b. a pair of terminal elements in the form of a pair of plugs plugging the axially outer ends of said fuse tube, the axially inner end surfaces of said pair of plugs being provided with a pair of juxtaposed bores;

c. a pulverulent arc-quenching filler inside said fuse tube;

d. low resistance current-limiting fusible elements in ribbon form conductively interconnecting said pair of terminal elements, submersed in said filler and being arranged relatively remote from a median plane of said fuse tube; and

e. a high resistance fusible element in form of a helically wound Nichrome wire conductively interconnecting said pair of plugs submersed in said filler and being arranged relatively close to said median plane of said fuse tube and being sandwiched between said low resistance fusible elements, said pair of juxtaposed bores in said axially inner end surfaces of said pair of plugs receiving the axially outer ends of said helically wound wire of Nichrome, and said bores further containing a mass of low fusing point metal frictionally positioning said ends.