US2134470A - Method of and means for interrupting current flow - Google Patents

Description

N. J. CONRAD METHOD OF AND MEANS FOR INTERRUPTING CURRENT FLOW Oct 25, 1938.

Original Filed Oct. .15, 1932 3 Sheets-Sheet l N. J. CONRAD Oct. 25, 1938.

METHOD OF AND MEANS FOR INTERRUPTING CURRENT FLOW Original Filed 001;. 13, 1932 5 Sheets-Sheet 2 K N N Wm Oct. 25, 1938. N. J. CONRAD 2,134,470

METHOD OF AND MEANS FOR INTERRUPTING CURRENT FLOW Original Filed 001:. 13, 1952 s Sheets Sheet 3 Patented Oct. 25, 1938 UNITED STATES METHOD OF AND MEANS FOR INTERRUPT- ING CURRENT FLOW Nicholas J. Conrad, Winnetka, 111., assignor to Schweitzer & Conrad, Inc., Chicago, 111., a corporation of Delaware Application October 13, 1932, Serial No. 637,593 Renewed April 7, 1937 7 Claims.

My invention relates to a method of and means for interrupting current flow.

The invention involves the use of a fusible conductor or link which performs the two-fold func- 6 tion of first detecting the presence of excessive current, reacting accordingly, and second, establishing a gap in which conductivity to current flow is destroyed.

The primary object of the invention is to pro- 10 vide a method of and means for increasing the range of interrupting capacity of devices of the foregoing type.

Another object is to increase the reliability or certainty of operation of devices of the aforesaid 15 type.

A further object is'to secure increased capacity of interruption and/or certainty of operation, by the use of fusible elements which, of themselves, are of insufficient capability to accomplish the 20 desired purposes. For example, the capacity or characteristics of a power circuit, or the nature of the overload occurring therein, may be such.

that a fuse of a design or rating suitable for the circuits of a comparable voltage or rating finds 25 difficulty in interrupting the current flow. By the use of my invention certainty of interruption is secured, without change of design or rating of the fuse employed. Hence, with a limited number of standard forms of fuses, automatic circuit interrupters of widely varying ability may be constructed to suit various requirements.

The invention is particularly advantageous in the interruption 'of very heavy current flow.

According to my invention a main interrupter, 88 which automatically begins the action of mechanically opening the circuit upon the occurrence of overload, is bridged 'or shunted by a conducting path which is not adapted to carry the rated or normal current flow continuously, as 40 soon as the main interrupter has reached a certain stage where the shunting conductor becomes effective.

According to this invention the main circuit interrupter normally carries all or the major portion of the normal or rated current flow. The auxiliary circuit interrupter normally carries none or only a small part of the normal or rated current flow. These circuit interrupters are herein illustrated as liquid quenched fuses of a kind and type now on the market, but the teachings of the present invention are not to be limited to the use of fuses of that kind or type, for the actions and functions which are secured by my 55 invention may be attained through the use of different forms of fuses, as will be apparent from the detailed specification appearing hereafter.

The current flow which is normally carried by the circuit in which the interrupter of my invention appears, passes mainly or wholly through the 5 main fuse. Upon the occurrence of overload in the circuit the fusible link of the main fuse melts, and an arc tends to form. The spring then separates the terminals between which the are exists, and there is injected into the are an are extinguishing material the effect of which is a tendency to deactivate or deionize the gases which constitute the conducting path, or substance of the are. The combined effect of lengthening of the arc and deionization or deactivation of the substance of the arc tends to render the conductive path less conductive to the flow of current. In other words, the potential drop across the arc tends to become greater. The auxiliary fuse, which is bridged or shunted around the main fuse, is thereupon subjected to a greater impressed potential and, in the case where a gap is included in the bridge or shunt circuit, the gap is broken down and a conductive path about the main fuse is completed. This conductive path has suflicient capacity, for at least a brief period, to diminish the current flow through the main fuse, whereby the combined action of lengthening the main arc and the deionization of the same results in putting out the main arc and transfer of such current flow as persists to the shunt, or auxiliary circuit. Meanwhile, the separation of the terminals of the main fuse has proceeded to such a point, and the arc extinguishing material has so acted, as to prevent restriking of the are in the main fuse.

The auxiliary or bridged circuit includes a certain amount of resistance, preferably in the form of a resistance wire such as a nickel, chromiumalloy, commonly sold on the market as Nichrome or Chromel". Such resistance tends to absorb a part of the potential drop and reduces current flow in this circuit to a value which may be easily interrupted by the auxiliary fuse.

Instead of including a gap in the bridged or shunt circuit this circuit may be connected conductively in shunt or bridge with the main circuit at all times, and therefore a certain small part of the normal load current of the circuit may flow 0 through the bridge or shunt circuit, with its included auxiliary fuse, during normal conditions. Upon the occurrence of overload the operation is substantially identical with the foregoing, except that the transfer of current flow does not wait for created by sudden the breaking down of the scribed.

In either event, however, the transfer of the main flow of current to the shunt circuit depends upon producing sufficient potential drop across the first fuse to force the current through the shunt circuit. This cannot be produced by the are alone, inasmuch as an are, particularly that following the fusion of a metal conductor, generally has the capability of increasing its cross section to the amount required to produce the optimum conditions of current density. The arc, therefore, may be of lower resistance than the conductor which it replaces. It is therefore not possible to divert more current to the shunt circuit than the natural division due to initial resistance would compel unless some means of increasing the potential drop across the arc be provided.

When the two fuses are connected conductively without the intervening gap, the current initially tends to divide in proportion to the resistance of the circuits. When the resistance of the shunt circuit is low, this division of current may be sufficient to give great assistance to the first or main fuse, particularly on a heavy overload.

I conceive that in the operation of fuses of this character the projection or injection of the arc extinguishing material upon or into the arc acts to deactivate the ionized or activated gases which conduct the current. The rate of deactivation or deionization is high, and no doubt, in the drawing of an arc in a fuse of this type, there is always the tendency to extinguish the are quickly, but there appears to be a reactive, or after-voltage interruption which builds up to a relatively higher value than the potential drop across the arc, tending to reestablish the same. This may be accounted for by the fact that upon the cessation of current flow through the conductors of the circuit the potential drop which is due to the I. R. loss immediately ceases, with the result that live potential on the fuse terminals rises very sharply, tending again to reestablish the arc. The presence of the shunt or bridge circuit and the continuing current flow therethrough holds down this recrudescent voltage and prevents reestablishment of the are at the terminals of the main fuse. The result is that the current flow, instead of being sharply interrupted and again spilling over, is reduced in value to a quantum which is easily interrupted by the auxiliary fuse or interrupter.

The action is automatic. The separation of the main fuse terminals tends to increase, perhaps not greatly, but very definitely, the potential drop across the main fuse terminals. The potential across the main fuse terminals is, of course, limited by the I. R. drop through the shunt circuit. This increase in potential drop forces the current through the auxiliary fuse in the shunt circuit. The shunt circuit has a certain thermal capacity which permits the establishment of a large enough current flow to permit the main fuse to open its circuit definitely and completely. The blowing of the auxiliary fuse takes sufficient time to allow the main arc to be extinguished. Then the progressive action of separating the auxiliary fuse terminals, injecting deactivating or deionizing arc extinguishing materials into the are all cooperate to reduce and finally stop the flow of current in the auxiliary or shunt circuit. While the operations are described as occurring successively, the action as a whole is extremely rapid.

gap as previously de- Now, in order to acquaint those skilled in the art with the manner of constructing and operating a device embodying my invention, I shall describe, in connection with the accompanying drawings, a specific embodiment of the same and the actions involved therein.

In the drawings: a

Figure 1 is a side elevational view of a circuit interrupter of my invention Figure 2 is a diagram of the same;

Figure 3 is a side elevational view of another embodiment of my invention;

Figure 4 is a front elevational view of the same;

Figure 5 is a diagram of the circuit of the device shown in Figures 3 and 4;

Figures 6, 7, 8, 9 and 10 are diagrams illustrating successive actions in the operation of the device shown in Figures 3 and 4;

Figure 11 is a diagram illustrating another embodiment in which the gap is subject to the action of the gases evolved from the arc in the main fuse;

Figure 12 is a'diagram explaining the efl'ects of the gap in the circuit of the embodiments shown in Figures 3 to 11;

Figure 13 is a diagrammatic illustration of another embodiment in which the pressure in the main fuse switches in the auxiliary fuse; and

Figure 14 is a side elevational view, partly in section, of one form of fuse which may be employed in the practice of my invention.

Referring first to Figures 1 and 2, the circuit interrupter is indicated between conductors I and 2. This circuit interrupter includes a suitable insulated support comprising, as here shown, four stacks of insulators, namely, a top stack I which is connected to a suitable support above (not shown), the bottom stack 4 which is likewise connected to a suitable support below (not shown) and two intermediate stacks I and I which are mounted between stacks 3 and 4. The stacks 3 and l are of the same insulating value and are suitable for 220 k. v. The stacks I and 8 together are the equivalent, in insulating value,-

of one of the stacks 3-4, inasmuch as the voltage is divided between them.

Three liquid quenched fuses l2 and II are employed, the fuse H being connected through the resistors 1 and l in parallel with the fuse l2, conducting arms I and i0 being provided for supporting the fuse l2 at the ends of the insulator stack I.

Suitable fuse clips, indicated generally at H throughout, support the metallic terminals or ferrules of the fuses in well known manner, these fuse clips being shown in Conrad Patent No. 1,665,446, of April 10, 1928. The inner end of the arm 8 constitutes a metallic fitting II and provides clamping flanges at II and I! for the adjacent insulator stacks 3 and I, respectively, and provides also a clamping flange II for the resistance units, three of which are shown as constituting the resistance 1. These units are of the type shown in my copending application, Serial No. 244,015, filed December 31, 1927, and constitute metallic end members such as I! and 20, between which a porcelain or like core 22 is supported with a coil of nickel-chromiurn-ailoy wire 23, or its equivalent, wound upon the core 22 and connected to the end pieces II and 20. A glass enclosing sleeve 24 is secured between the metallic end members I! and 20, whereby an enclosed resistor, protected from atmospheric and outside influences, is provided. Obviously, other forms of resistance units or of resistancu may be employed. The resistance 8 is composed of similar units.

The inner end of the arm I0 is formed in a manner similar to the fitting I5. There is provided, however, in addition, an extension termihating in a clamping flange 25 to support the terminal of the spring clip It at the upper end of fuse 13. A metallic fitting 26 is supported between insulator stacks 4 and 6 and it provides a support for the lower fuse clip I4 of the fuse l3 through the flange 21. The line wire 2 is connectedto the terminal 28 of the fuse clip I4 at the lower end of fuss I3. In a similar manner line wire I is connected to the terminal of the fuse clip I4 at the upper end of fuse I2. The flanges 25 and 21 which support the clips for the fuse I3 are mounted upon extended arms similar to the arms 9 and I0 so as to support the fuse I3 away from the insulator stack 6, substantially as the fuse I2 is supported away from the stack 5. The connections for the above are shown in Figure 2.

The proportions of the fuses may, for example, be as follows:

Fuse Ii, ampere rating; fuse I2 25 ampererating, resistance of the resistors 55.5 ohms.

Another example of proportioning, for a difierent circuit, is as follows:

Fuse Ii, 5 amperes rating; fuse I2, 100 amperes rating; fuse I3, 100 amperes rating, with a resistance, in series, of 55.5 ohms.

Another example which I have employed successfully is:

Fuse I I, 5 amperes rating; fuse I2, 100 amperes rating; fuse I3, 200 amperes rating, with a resistance, in series with the fuse I I, of 4 ohms.

The desideratum in this case is to keep the rating of the fuse II low and include sufficient resistance that for normal current flow or rated capacity of the circuit the fuse II, which is conand fuse I3, 75 ampere rating, with the 1 and 8 consisting of stantly in circuit, will not be melted. The fuse I2 is intended to operate at a predetermined value of fault current and the fuse I3 is intended to operate at the same time as the fuse I3. During short circuit the fiow of current is interrupted in fuse I2 before the arc in fuse I3 is extinguished.

The are in fuse I3 is extinguished at the same time as the arc is extinguished in the fuse I I.

In the present embodiment the fuses II, I2 and I3 are of only 110 k. v. rating, hence the addition of the fuse I3 in series with the fuses II and I2, for 220 k. v. service.

The fuses II, I2 and I3 are preferably well known fuses now on the market. A suitable construction of fuse is shown in Figure 14. With various modifications the fuse of Figure 14 is disclosed in my copending application, Serial No. 470,416, filed July 24, 1930 now Patent No. 2,091,430. A suitable fuse for this purpose is also disclosed in my prior patents, No. 1,743,322 .of January 14, 1930, and No. 1,834,578 of December 1, 1931. i A sufficient description of such fuses and their operation is contained in the said prior patcuts, and a brief description of the fuse shown in Figure 14 will sufflce for present purposes.

The fuse comprises a glass sleeve 32 having metallic terminals at the ends thereof, the lower terminal 33 being in the form of a closed cap cemented on the glass sleeve, and the upper terminal 34 being in the form of a ferrule or sleeve likewise cemented on the upper end of the glass sleeve 32. The upper ferrule 34 is closed releasa-bly by a cap 35 which, in case of excessive pressure and upon heavy blowing of the fuse, is adapted to be removed by the internal pressure. The sleeve or ferrule 34 is counterbored to provide a shoulder for mounting the upper terminal plate 36, which has slotted spring fingers pressing radially against the walls of the counterbore and axially against the shoulder formed .by the counterbore. A terminal stud 31 has a threaded shank extending through the plate 36 and it is held in place by the nut 38 threaded on said shank. A cooperating movable terminal 39 is mounted upon the upper end of the tension spring All through the medium of a spring head 42, to which the spring M is anchored. This spring head 42 has a threaded coupling with the terminal 39 and also has a socket 43 to which the flexible cable 44 is firmly anchored, electrically and mechanically. 'The cable 44 shunts the spring 49. A silver fuse wire 45 formed in the shape of a helix with the ends straightened out axially is connected between the movable terminal 39 and the stationary terminal 31. A high tensile strength wire, preferably of nickel-chromium-alloy composition, indicated at 46, has loops formed at each end and these loops are engaged by pins 41 and 38 in the terminals 31 and 39, respectively. The ends of the fusible link 45 are mechanically anchored in slots in the terminals 31 and 39 to give a good mechanical and electrical anchorage, the edges of the slots being riveted or battered over to grip the fuse wire along a considerable length. A liquid director 49 is supported on radially extending arms 50 upon the terminal 39. An explosion chamber is provided about the fusible element by means of the sleeve 52, the upper end of which is pressed into or otherwise anchored in the barrier plate 53, which barrier plate is threaded into the bore of the ferrule 34 to form a transverse wall. The upper end of the liquid director 49 embraces loosely but fairly closely the upper end of the explosion chamber sleeve 52. The glass sleeve is filled with are extinguishing liquid to a point preferably just above the liquid director 49.

In operation, upon the occurrence of overload the fuse element 45-flrst melts, throwing the load over upon the high tensile strength wire 46, termed the strain wire. Thereupon 'the strain wire melts and lenses the coil spring 40 to retract the terminal 33. At the same time are extinguishing liquid is thrown into the explosion chamber'52, this stream of liquid beingrapidly vaporized and to some extent broken down, and discharged upwardly into the chamber formed above the barrier plate 53. Here the vapors and gases are condensed and chilled, and if the arc is interrupted before the pressure rises to too high a value the operation of interrupting current fiow occurs without blowing of the cap 35. If the operation is severe, as in a short circuit on a system of large capacity, the pressure in the fuse may rise to a value high enough to blow off the cap 35. The terminal plate 36 and terminal 31 may then also be expelled, and as the terminal of the fuse i2. Upon the occurrence of overload there is a tendency for the current to increase in proportion through both branches, including the fuses ii and I2. However, the fuse i2 is selected in rating so as to be melted before the fuse H is melted. Upon melting of the fuse I! it proceeds to lengthen the gap and inject arc extinguished material, this material being the liquid in the fuse heretofore described. It may be carbon tetrachloride, or any of the liquids disclosed in Reissue Patent No. 14,897 of June 22, 1920, or any equivalent material, solid, liquid or gaseous, for performing the same function, namely, of deionizing and extinguishing the are. It is desirable that there be interposed in the gap formed by melting of the fuse l2 3. medium of high dielectric value. As the length of the arc is increased and the tendency to extinguish the same makes it more diflicult for the flow of electric current to continue through the fuse l2, the proportions of the parallel circuits change, that is, the potential drop across the fuse l2 increases, tending to throw more current through the fuse ii. If, at the same time, the current flow is great enough to melt the fuse l3, it begins its operation almost immediately after the start of operationsof the fuse i 2.

As soon as the current flow through the fuse it increases to a point where the fusible link thereof melts, the fuse H begins to separate the terminals and project are extinguishing material into the arc. However, the transfer of current flow from the circuit of the fuse l2 to the circuit of the fuse it allows the arc to be extingulshed in the fuse i2, and not become reestablished. The separation of the fuse terminals in the fuse unit I2 is accompanied by the interposition of the insulating liquid, which prevents any restriking of the arc in fuse unit I2 when the current flow is finally and completely interrupted in fuse unit H, or any time prior thereto. Thereupon, with the resistances '|8 in circuit, the current flow is diminished to a value which may easily be interrupted by the combined action of the fuses Ii and I3. Clearing the arc at either fuse II or I! is effective to interrupt current flow without reestablishment of the arc, since the two fuses are acting in series, and since they are both in series with resistances 1-8.

The flow of current initially divides between the two fuses H and I2, dividing in proportion to the resistance of the parallel circuits in which they are included. When a sudden increase of potential comes upon the terminals of the parallel circuit, as by a shorthircuit, the tendency of the current flow is to increase in the same proportion, since the resistance of the circuits remains substantially the same. The initial division of current is valuable for relieving the main fuse particularly on heavy overload.

The main fuse l2 invariably melts first. When the next cyclical current zero arrives, the arc in this fuse l2 goes out. Now the fuse ll protects the fuse from restriking an are upon the next potential wave. even though this is a recovery voltage wave of abnormal height, because the potential across the gap of main fuse I! cannot rise above the IR drop in the auxiliary fuse l I, even with resistance in said auxiliary circuit. It will thereby have an interval of time to become completely deionized and to present a much higher dielectric strength by the injection of arc extinguishing material in the gap, and by separation of the terminals. Meanwhile, assume that as soon as the arc in fuse 12 has ceased at cyclical current zero the next wave of current flow through fuse H produces an are by melting of the fuse wire. Then the fuse ll proceeds to iniect arc extinguishing material to deionize the arc and at some succeeding current zero the arc is finally extinguished in said fuse ll. Then the real stress of full line voltage comes upon both fuses. However, no arc can be reestablished at the main fuse l2, because it has had time to perform its static disconnecting function, i. e., interpose an adequate dielectric medium and separate its terminals while the second fuse is stil in its active or kinetic condition of discharging a gaseous arc extinguishing medium.- Also due to the resistance in the circuit of fuse l l, with consequent reduction in current flow, the recovery voltage will not be as violent as it would be without much resistance. Also the lag or phase angle is improved, and the second fuse has much less severe duty than would be the case if a single fuse were called upon to extinguish the full overload.

Thus the second fuse provides a certain timing function for the first fuse within which the first fuse may put itself in condition to withstand the reapplication of full line voltage. In brief, the first fuse initially protects the second fuse, and then the second fuse protects the first fuse in a unique manner not accomplished in prior devices. This cooperation also prevails in the construction when a gap is interposed in the auxiliary or shunt circuit.

Where the voltage rating of the fuses H and i2 is sufficient to withstand the full voltage between conductors l and 2, the fuse l3 may be dis-. pensed with, as its function in the present organization is largely to add sufficient dielectric gap to make up for 220 k. v.

In the embodiment shown in Figures 3 and 4, the main fuse I2 is connected between the conductors I and 2 and the auxiliary fuse ii is connected in shunt of the fuse l2 with the air gap in series. Resistance units 1 and 8, of a form similar to those previously described in connection with Figure 1, are mounted upon the insulators 50 and 01, the fuse clips l0--l4 for the fuse ll being connected to the metallic members ll-IO of the units 1 and 0. The corresponding fittings 20-20 at the inner ends of the reslstance units 1 and I are clamped to the metallic insulator caps 50 and 59. These members 20-20 are provided with extending terminals 0040. The terminal 00 of the resistance unit 0 is connected by a copper bar conductor 62 to the terminal 00, which terminalforms an extension of the fuse clip I 4 at the lower end of fuse I2. The terminal 20 at the base of the resistance unit 1 has connected thereto a bracket which supports a threaded rod 65, forming one terminal of the air gap 50. A bar of insulating material 80 is clamped on the opposite side of the terminal 20 and it forms a support for a corresponding bracket 60 carying a threaded rod 69 for the other electrode of the air gap 50. These terminals 05 and 60 are threaded in the corresponding brackets 08 and 04, respectively, and after being adjusted to the proper sparking distance between them are fixed in place by lock nuts or the like. The bracket 00 has a fiat bar conductor 10 clamped thereto, the opposite end of the bar I0 being fastened mechanically and electrically to the terminal 12, which forms an extension of the fuse clip H at the upper end of the fuse l2. Thus, as will be seen from the above description,

and as shown diagrammatically in Fi ure 5. the

fuse II is connected in series with the resistances I and 8 and air gap 55 in a circuit which forms a shunt around fuse I2. The air gap is adjusted to such a value that the voltage across the shunt circuit will not break down the gap until after the fuse l2 has begun to draw an are.

In the diagrams of Figures 6 to 10, inclusive, I have indicated the operation of the device shown in Figures 3, 4 and 5. The fuses H and I2 may be of a type shown in the aforesaid patents and patent application. The fusible links 45 of the fuses H and I2 may be of the relative proportions heretofore indicated in connection with fuses H and I2 in the embodiment of Figure 1, and the resistance values of the resistors I and 8 may likewise be of the character previously referred to. The spark gap 55 is set to such a breakdown value as will prevent any flow of current through the shunt circuit during normal operation but will definitely be broken down upon the blowing of the main fuse l2.

There is another distinct virtue. in the use of the spark gap, or air gap 55, for it introduces a feature of very considerable value, as illustrated in connection with Figure 12, namely, of preventing any current flow through the fuse it until the succeeding voltage wave has risen to a value great enough to break down the gap 55. This assists in securing the desired timing of operations.

The spark gap 55 may be totally enclosed and may, if desired, be provided with a dielectric of greater strength than air between the electrodes. For example, it may be placed in an insulating liquid such as oil, or any of the are extinguishing liquids above referred to, or their equivalents.

The operation of the device 4 and 5 is as follows:

Assuming that the proportioning of the fuses II and I2, resistances 1 and 8, and the spark gap 55 have been properly selected, and this, as above explained, may desired cooperation of the elements and the timing of the same, the occurrence of overload upon the device causes the current flow in the fuse H to rise and melt the element 45 of main fuse H. An arc thereupon forms and the spring 40 of the fuse l2 draws the terminal down into the liquid and projects liquid into the arc space, as heretofore described, in connection with Figure 14. The action of lengthening the arc (see Figure 7) increases the potential drop between the fuse terminals 33 and 34. This imposes a greater E. M. F. upon the terminals of the gap 55 and, as shown in Figure 8, while the arc 13 persists, the gap 55 is broken down and current begins to flow through the fuse H. As soon as the conducting path in shunt of the are 13 is established the current subdivides and, together with the lengthening of the arc I3 and the projection of are extinguishing material into the same, the are 13 is extinguished as indicated in Figure 9, current flow persisting through the shunt circuit. The fuse terminals of the main fuse having been so far separated that the arc will not reform between them, all of the current now now passes through the fuse H. This, however, is limited, first by the resistors I and 8, next, by the spark gap 55, and finally, by the are 14 which has now formed by blowing of the fuse link 45 of the auxiliary fuse ll.

With the limiting resistances in series with the fuse H, current flow has been gradually reduced and the fuse ll therefore finds no great difficulty shown in Figures 3,

be varied to secure the.

in extinguishing the arc and in stopping current flow, the terminals of the fuse ll, like the terminals of fuse l2, being separated by so great a distance as to prevent restriking of the arc.

During the operation of clearing a circuit through the fuse ll the air gap 55 has a useful function in giving the fuse II a longer time period, when the current reverses, to clear the circuit and prevent resumption of current flow. This is illustrated in exaggerated form in Figure 12, where the curve I5 represents a sine wave of impressed voltage and the curve 15 represents the current flowing through the fuse H as modified by the air gap 55. The curve it illustrates in exaggerated form the voltage across the gap 55. As the current flow passes through zero, assuming in this case that the current flow is strictly in phase with the impressed voltage, that is, in phase with the curve I5, the current flow through the shunt circuit. including the fuse ll, tends to drop to zero because of the effect of the gap 55. The are tends to go out at the gap 55, as the current how in the shunt circuit approaches zero at each reversal. This is due to the deionizing effect of the surrounding medium (in this case air) upon the arc of the gap 55. As the arc goes out the voltage across the gap 55 rises, as indicated by curve Hi. In other words, the arc tends to go out as the current flow goes through zero. This is represented by the flat part ll of the heavy curve It in Figure 12. Then, after an appreciable period of time while the voltage is rising, tending to reverse current flow, the gap is not suificiently conductive to permit current to flow, with the result that for a short period of time, represented by the flat part I8 of the curve 16, current does not flow through the shunt circuit. Meanwhile, the voltage on the gap has risen, as shown by curve II. Due to this lengthened time of zero current flow it is much easier to deionize the arc in the fuse i i and extinguish the arc than would be the case if the gap 55 were not present. This action may be promoted by any preferred deionizing material at the gap 55, as above explained.

A great advantage of the liquid arc extinguishing material is that it conforms itself to every position of the fuse terminals, tending to close into the space in which the arc is formed. Obviously, a gas under pressure has the same ability. The liquid arc extinguishing material evolves gas by both vaporization and by decomposition under the heat of the arc, and this gas, together with the liquid, tends to break up and deactivate the substance of the are so that when the current flow passes through zero value and reverses the conductivity of the space previously occupied by the arc is so low that the voltage cannot restrike or again form the arc.

The present device, namely, the device of Figures 3 and 4, has the same advantage as previously explained of reducing the current flow before stopping the same, whereby the reestablished or recrudescent voltage rise does not reach so high a value as to break down the dielectric and again establish the arc.

The devices shown in Figures 3 and 4 automatically transfer the current flow from the main fuse to the shunt circuit, including the gap and auxiliary fuse H, as the voltage drop across the main fuse increases during the operation of clearing the main fuse. 1

In both the embodiment of Figure l and of Figures 3 and 4 the rated capacity of the fuse H is small as compared with the rated capacity of the fuse ii. In one example above given the rated capacity of the main fuse I2 is from 20 to 40 times the rated capacity of the auxiliary fuse H. The fuse [2 being of larger rating has a heavier fusible link, a larger diameter glass sleeve, more are extinguishing material, and is of heavier construction throughout. The fuse H, on the contrary, has a very small mass of metal in the fusible link, the sleeve is of smaller diameter and the construction throughout is less expensive and less massive than the fuse l2. There is a distinct advantage in the use of a small amount of metal in the fusible link I l, inasmuch as metallic vapor greatly assists in maintenance of an arc. 7

If it were possible to make the fuse without any metallic vapor that would obviously be highly desirable, but since suflicient conductivity must be present to carry the normal or rated current flow, the practical limit of reducing the mass of metal in the fusible link is quickly reached. I provide a silver fuse wire. Silver has a very high conductivity and has the further advantage of being relatively free from corrosion. By the use of silver I have decreased the mass of metal in the gap to the minimum for the required conductivity. By transferring the current flow from the circuit of the fuse l2, which circuit has very low resistance, to the circuit of the fuse II, which has appreciable resistance, with less metal in the are which results, the duty imposed on the time i i is much less severe than the duty which would be imposed upon the fuse I! if it were called upon alone to interrupt the current flow.

I may carry the method of interruption through more than the two steps, as, for example, by connecting a fuse of even less capacity and greater circuit resistance in shunt with the fuse H, with or without an additional air sap in series with such third fuse and its resistance. It can be seen, therefore, that I have provided a progressive system of increasing the resistance of the circuit and thereby decreasing the currentwhich must be interrupted and simultaneously decreas-- ing the metal of the are which results upon blowing of the auxiliary fuse or fuses.

The air gap may be subjected to the ionized gases released upon blowing of the fuse I2. Thus, in the embodiment of Figure I I, I have placed the gap 55 in such position with respect to the main fuse l2 that when the cap blows off the upper terminal .34 the arc gases will ionize the gap 55 and more quickly switch in the shunt circuit. The gap 55 may be made large enough that it will require blowing of the gases from the fuse I2 thereupon before the shunt circuit is switched in. In other words, if the overload can be handled by the fuse I2 without blowing of the cap 35, that is, if the violence is not so great as to require the use of the auxiliary fuse H, it will not be called into play. Therefore it will be seen that I have provided a selective method of including the shunt circuit only when the character of the overload issuch as to require an action greater than can safely be expected from the fuse I2.

In Figure 13 the fuse I2 is provided with an expansible member 19 which carriesa contact I. adapted to cooperate with the contact 52, forming the terminals of the gap II. In this case the pressure developed in the fuse l2 switches in the shunt circuit by either bringing the contacts II and 82 into engagement or bringing them near enough to each other that the voltage drop across the gap will break the gap down and switch in the shunt circuit.

vshunt of said arc,

Obviously I may build the two fuses into a common structure, and by employing an auxiliary contact I may cause the motion of the spring of the main fuse if to switch in the shunt circuit when the terminals of the fusible link of the main fuse have been separated a predetermined distance. Ihat is to say, by the mechanical separation of the main fuse terminals, the shunt circuit with the resistances and auxiliary fuse may be automatically connected.

Instead of having the resistances 1-8 in sep arate parts they may be combined into one unit, and also they may be embodied in the fuse construction itself, as is shown in Figure 15 of Patent No. 1,215,722, issued February 13, 1917.

I do not intend to be limited to the specific details which I have above described. I do not wish to limit the arc extinguishing material, nor the specific form of the fuses, to the forms above described for the reason that those skilled in the art will at once appreciate that the method which I have set forth and the mode of operation which I have explained may be embodied in forms structurally quite different but mechanically the equivalent of the aforesaid.

I claim:

1. The method of interrupting current flow through a circuit including a fusible link, which comprises fusing the link and establishing an arc, then progressively increasing the length of the arc and simultaneously injecting an are extinguishing medium into the arc to deionize the arc and by said lengthening and deionizing of the arc increasing the potential drop across the arc, thereafter extinguishing said arc, substituting for the arc in the circuit a fluid dielectric of a strength greater than air to resist breakdown by the circuit voltage, by increase of potential drop across said arc breaking down a gap in creating an arc in series with said gap, then progressively increasing the length of said second arc and simultaneously injecting 2. In a circuit interrupter, the combination of a pair of circuit terminals, fusible links having terminals connected in parallel between said terminals, one of said links being of a current carrying capacity lower than the other, means including an arc gap connected in series with said link of lower capacity to delay the fusing of the same upon overload being appliedto said terminals, and spring means for separating the fuse terminals of said fusible link of higher capacity to prevent the restoration of potential in the circuit following interruption of current by the link of lower capacity from breaking down the gap at the terminals of the fusible link of higher capacity.

3. An automatic alternating current interrupter for connection in an electric circuit and through which current normally flows comprisin combination, a pair of terminal conductors, a pair of fusible elements connected in parallel between said terminal means independent of the fusible elements for determining the order in which they are fused by excess of current flow in said electric circuit, said means comprising a resistor connected in series circuit relation with only one of said fusible elements and an arc gap in series circuit relation with said resistor.

4. An automatic alternating current interrupter for connection in an electric circuit and conductors, and

through which current normally flows comprisance element, means including a spark gap coning, in combination, a pair of terminal conducnecting said resistance element and said second tors, a pair of fusible elements connected in fuse in series circuit relation with each other parallel between said terminal conductors, and and in parallel circuit relation with said first fuse means independent of the fusible elements for whereby no current is carried by said second fuse determining the order in which they are fused until said first fuse is blown by excess of current by eXcess of current flow in said electric circuit, flow in said electric circuit, and means for autosaid means including an arc gap connected in matically increasing the voltage breakdown value series circuit relation with only one of said fusible of said first fuse after it operates so that sub- I elements. sequent interruption of current flow by said 10 5. An automatic alternating current intersecond fuse does not result in reestablishment rupter for connection in an electric circuit and of current flow through said first fuse. through which current normally flows compris- '7. A circuit control device comprising, in coming, in combination, a pair of terminals, a first bination, a pair of circuit terminals, a first circuit circuit connecting said terminals, a second cirinterconnecting said terminals and including an cuit parallel to the first circuit and also conautomatic circuit interrupter adapted to carry meeting said terminals, fusible links of different the entire circuit current under normal operating current carrying capacity in said circuits, and a conditions and to automatically open said first gap connected in series circuit relation with the circuit under predetermined overload conditions, fusible link of lower current carrying capacity and a second circuit interconnecting said terwhereby no current is carried thereby until the minals including a permanent arc gap and a fusible link of higher current carrying capacity fusible element in series circuit relation, said arc is blown by excess of current flow in said electric gap being adapted to prevent the flow of current circuit. through said fusible element under normal 0p- 6. An automatic alternating current intererating conditions until the arc gap is broken rupter for connection in an electric circuit and down on operation of said automatic circuit inthrough which current normally flows compristerrupter in said first circuit. ing, in combination, a first fuse, a second fuse of small capacity relative to the first fuse, a resist- NICHOLAS J. CONRADZ

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