US2901579A - Electric circuit breaker - Google Patents

Description

Aug. 25, 1959 W' SMPSON JR 2,901,579

ELECTRIC CIRCUIT BREAKER Filed Sept. 12, 1957 Figi 'lllllllllllllll'lln 'lllllllllllllll/ IT? e T1 t. O T` I Amos W. Simpson, Jr.,

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United States Patent O ELECTRIC CIRCUIT BREAKER Amos W. Simpson, Jr., Springfield, Pa., assignor to General Electric Company, a corporation of New York Application September 12, 1957, Serial No. 683,613

9 Claims. (Cl. 20D-147) This invention relates to an electric ci-rcuit breaker of the type which relies upon magnetic blow-out means for driving a circuit-interrupting arc into an arc extinguishing device, such as an arc-chute.

More specically, the invention is concerned with the type of arc chute in which a plurality of magnetic blowout coils are located along the conductive arc-runners of the chute for accelerating the movement of the arc along the runners into the interior of the ch-ute. An example of this general type of arc chute is shown in U.S. Patent No. 2,347,984, Baskerville, assigned to the assignee of the present invention.

When an arc is established in the type of arc-chute shown in the Baskerville patent, one terminal of the arc quickly transfers to an adjacent arc-runner after only a slight amount of contact-separation, whereas the other terminal transfers to the other arc-runner only after a considerable greater contact-separation. I have found that, as a result of this delay in transferring the other terminal of the arc to its runner, the blow-out coils along the rst runner have an opportunity to drive the iirst arc terminal into the chute considerably ahead of the other arc terminal. This tends to concentrate the arc-interrupting duty in the portion of the chute adjacent the first runner and, thus, tends to prevent the full interrupting ability ofthe chute from being utilized.

Accordingly, an object of my invention is to distribute this arc-interrupting duty more equally throughout substantially the entire chute and to prevent it from being concentrated in a localized portion of the chute.

Another object is to provide a magnetic blow-out arrangement which drives the arc along the runner which last receives the arc at a higher rate than the rate at which the arc is driven along the runner which rst receives the arc.

In carrying out my invention in one form, I provide an arc-chute which has conductive runners along which the terminals of an arc are adapted to be driven toward the interior of the chute so that the arc can be extinguished within the chute. The arc is established at the entrance of the chute by separable contacts which are disposed in such a manner that one terminal of the arc transfers to a iirst one of the arc runners after a relatively small contact separation and the other terminal transfers to the second arc runner after a larger contact separation. F or accelerating the motion of the arc into the interior of the chute, a tirst group of blow-out coils is located along said lirst runner and a second group along said second runner. The coils of the rst group are connected in series-circuit relationship, whereas at least some ofthe coils ofthe second group are connected in parallel. With respect to the second group of coils, preferably all of its coils except the initial coil, i.e., the coil nearest the arc-initiation region, are connected in parallel, and this parallel combination is connected in series with the initial coil. This arrangement of the second group of coils is capable of ice driving the arc terminal on the second runner at a higher speed than the first group of coils drives the other arc terminal along the first runner, thus enabling the second terminal to overtake the first terminal so that the arc moves into the interior ofthe chute along substantially its entire length.

F or a better understanding of my invention, reference may be had to the following description taken in conjunction with the accompanying drawing wherein:

Fig. l is a side elevational view partly in section showing an electric circuit breaker embodying my invention.

Fig. 2 is a cross sectional view taken along the line 2 2 of Fig. l.

Referring now to Fig. l, the circuit break-er shown therein comprises a pair of terminal bushings 1 and 2, both of which are xed in position relative to the supporting frame of the circuit breaker. The bushing 2 comprises a downwardly extending conductive stud 3 at the lower end of which a movable conductive switch blade 4 is mounted by means of a `fixed pivot 5. At its outer end, the blade 4 carries suitable circuit-controlling contacts such as a current-carrying contact 6 and an arcing contact 7 Bushing 1 comprises a conductive stud 1a to which a downwardly extendingconductive member 8 is electrically connected. Attached to this conductive member 8 is a curved contact-retaining member 9 which coacts with the member 8 to form a holding pocket for receiving the anchored ends of main current-carrying contact fingers 10. These lingers 10 are pivotally mounted on a curved portion 12. of the conducting member 8 and are biased for limited rotative wiping movement in a closing direction by means of suitable compression springs 13. These compression springs 13 provide for high pressure circuitclosing engagement between the stationary current-carrying contact 10 and the movable current-carrying contact 6.

The movable arcing contact 7 cooperates with a stationary arcing contact 1S, which is mechanically and electrically connected to the conducting member 8 by suitable clamping means 16. The material of the arcing contacts 7 and 15 is capable of withstanding arcing and is also of a relatively high resistivity in comparison to the material of the current-carrying contacts 10` and 6. Accordingly, when the switch blade 4 is in the closed position shown, most of the circuit current ows through the current-carrying contacts. It is only when the switch blade 4 is driven counterclockwise to open the breaker that the arc* ing contacts carry appreciable current. During such opening action, the current-carrying contacts iirst part, thereby diverting current through the arcing contacts which are still in engagement due to their extensive wipe. Thereafter, the arcing contacts part and draw a circuit interrupting arc which is driven into an arc chute 20 and there lengthened, cooled, and extinguished in a manner soon to be described.

For driving the switch blade 4 counterclockwise to effeet circuit interruption, a reciprocable operating rod 24 pivotally joined to the switch blade at 26 is provided. When this operating rod is driven upwardly, it acts to move the switch blade counterclockwise to effect a circuit interrupting operation. The circuit can be re-established simply by driving the operating rod downwardly to return the switch blade 4 in a clockwise direction t0 the closed position shown. The operating rod 24, which is of insulating material, is actuated by means of a suitable conventional operating mechanism (not shown).

Referring now to Figs. l and 2, the arc chute 20 comprises a pair of sidewalls 21 and 22 constructed of appropriate arc-resistant insulating material. These sidewalls are clamped together in spaced-apart relationship by suitable means not shown. Each sidewall preferably comprises ribs 23 projecting toward the other sidewall and arranged -to mutually interleave with the corresponding projecting ribs on the other sidewall, thereby forming a sinuous or zigzag passage as viewed from the entrance end of the chute. As shown in Figure 2, these ribs taper toward the entrance of the chute and thereby provide a throat portion through which the arc must irst pass before entering the zigzag passage between the ribs 23. Generally speaking, this construction is of the type disclosed in U.S. Patent No. 2,293,513, Linde, assigned to the assignee of the present invention.

For facilitating movement ofthe arc into the arc chute, a pair of conductive arc runners 30 and 31 are provided along opposite edges of the chute. As shown in Fig. l, these runners 30 and 31 extend transversely to the path of the larc and in generally-divergent relationship with respect to each other from the region in which the arc is initiated.

The upper arc runner 30 is made up of a plurality of segments 32, 33, 34 and 35 disposed in end-to-end relationship, with the adjacent ends thereof separated by insulating spacers 36. Electrically bridging the spacer 36 nearest the arc-initiation region is a magnetic blow-out coil 37 having one terminal connected to runner segment 32 adjacent the spacer 36 and its other terminal connected to runner segment 33 immediately adjacent this same spacer 36. The other spacers 36 are respectively bridged by blow-out coils 38 and 39 connected between adjacent runner segments in a corresponding manner. The runner segment 32 located nearest the arc-initiation region is preferably of a generally U-shaped configuration and is electrically connected to the terminal stud 1a. This electrical connection is through a conductive adapter 41 and an additional blow-out coil 40, which has one temiinal connected to the adapter 41 and its other terminal connected to the innermost end of runner segment 32. Considering an electrical circuit which extends between the outermost runner segment 35 and the terminal conductor 1a, it will be noted that the blow-out coils 37-40 are connected in series-circuit relationship with each other as well as with the runner segments 32-35.

Referring now to the lower arc-runner 31, it will be noted that this runner also has four blowout coils mounted therealong. But, for reasons which will soon be pointed out, these coils are connected in a dilferent electrical relationship from the coils of the upper runner 30. In this regard and as will soon be described in detail, the outer three coils 46, 47 and 4S of the lower runner are connected in parallel-circuit relationship, and this parallel combination is connected in series with the remaining blowout coil 45 nearest the arc-initiation region. The parallel combination of coils 46, 47 and 48 is also connected in series with the two conductive segments 49 and 50 which constitute the lower arc-runner 31.

The parallel connection for the three blowout coils 46, 47 and 48' is provided by means of a conductive bar 52 electrically connected to one terminal of each of these coils and a second conductive bar 54 electrically connected to the other terminal of each of these coils. This parallel combination of coils electrically bridges an insulating spacer 55 which is located between the adjacent ends of the runner segments 49 and 50. In this regard, one of the conductive bars 52 is electrically connected to the segment 49 at one side of the spacer 55 and the conductive bar 54 is electrically connected to the runner segment 50 on the other side of the spacer 55. Aside from the electrical connection immediately adjacent the spacer S5, the conductive bar 54 is insulated from the runner segment 50 throughout its length. The physical location of this electrical connection is important for reasons which will soon be pointed out.

The blowout coil 45 nearest the arc-initiation region 1s' .connected in series with the runner segment 49 between this runner segment and the other terminal conductor 3 ofthe circuit breaker. In this regard, a conductive strap 56 is electrically connected between the terminal conductor 3 and one terminal of the blowout coil 45, whereas the other terminal of the blowout coil 45 is connected to the innermost end of the runner segment 49. An insulating spacer 57 is provided at the lower end of runner segment 49 to prevent the blowout coil 45 from being short circuited by the runner structure located across its terminals.

The purpose of the above-described blowout coils is to accelerate the movement of the arc along the runners into the interior of the chute. In this regard, each blowout coil is provided with a centrally-located core insulated from the coil and attached to pole pieces mounted on the outer surfaces of the sidewalls of the chute. For example, the coil 40 has a core 60 attached to pole pieces such as 6l shown by dotted lines in Fig. l. The coil 37 has a core 62 attached to similar pole pieces 63. In a like manner, all of the other blowout coils have similar cores and pole pieces, only some of which are shown. When a particular coil is energized, its pole pieces provide a magnetic iield transverse to the arc path, and this magnetic field reacts with the magnetic field surrounding the arc to produce a resultant force which drives the arc at high speed along the runners into the interior of the chute. The general manner in which these magnetic fields react to produce the arc-motivating force is wellknown and therefore will not be described in further detail.

When the switch blade 4 is driven counterclockwise to open the breaker, an interrupting arc is established at the arcing contacts 15 and 7 as soon as these contacts part. The upper terminal of this arc quickly transfers the runner segment 32, thereby connecting the blowout coil 40 in series with the arc. The energized coil immediately creates a magnetic effect which begins to drive the upper arc terminal along the upper runner 30 toward the interior of the chute. As the upper arc terminal moves along the runner 30 past the insulating spacers 36, it acts to successively insert the connected blowout coils in series with the arc, thereby progressively increasing the magnetic forces tending to drive the arc into the chute.

In the meantime, the movable switch blade 4 has swung rapidly away from the stationary contact 15. When the switch blade 4 moves downward into proximity with the runner segment 49 of the lower arc runner, the lower terminal of the arc transfer to the runner segment 49, thus inserting the lower blowout coil 45 in series with the arc. The energized coil 45 immediately creates a magnetic blowout effect which drives the lower terminal of the arc along the lower runner 31 toward the interior of the chute. When the lower terminal of the arc passes the insulating spacer 55, it acts to insert the parallel-combination of the three coils 46, 47, and 48 into series with the arc, thereby providing increased magnetic force for driving the arc into the chute. As the arc moves into the chute, it becomes elongated and cooled by the projecting ribs 23, and thus acts to quickly deionize and thereby extinguish the arc so as to interrupt the circuit.

In prior arc-chute constructions of this general type, for example, as shown in the above-noted Baskerville patent, it has been customary to connect all of the blowout coils for each runner in series, or, in other words, to connect the blowout coils for the bottom runner in the same electrical relationship as described above for the coils 37-40 of the top runner. I have found that with such prior arrangements, there is a tendency for the upper arc terminal to move into the chute considerably ahead of the lower terminal, thereby undesirably concentrating the interrupting duty in the top portion of the chute. I attribute this condition to the fact that the upper arc terminal is transferred to the upper arc runner before the arcing contacts can separate sufliciently to transfer the lower arc terminal to the lower arc runner. As a result, the magnetic blowout coils of the upper arc runner have an opportunity to drive the upper arc terminal well into the chute before the blowout coils of the lower runner are inserted into the circuit.

The arc-chute of the present application overcomes this problem by providing the lower arc-runner with a magnetic blowout arrangement which is capable of driving its associated arc terminal at a considerably higher speed than the magnetic blowout arrangement associated with the upper runner. This enables the lower arc terminal, in moving into the chute, to effectively overtake the upper arc terminal, with the net result being that the arc is effectively driven into both the upper and lower portions of the chute instead of being generally concentrated in the upper portion of the chute.

The speed of the arc terminal along the upper runner of the chute is limited, lirst of all, by the presence of spacers 36 interposed between ends of the runner segments. These spacers represent an obstacle in the path of the arc terminal and thereby tend to retard the motion of the terminal. In addition, when the arc terminal passes across one of the spacers, it is forced to overcome the voltage drop which exists across the newly-introduced blowout coil. This, too, tends to cause the arc terminal to hesitate before jumping across the spacer 36.

A considerably higher speed is obtained with the lower arc runner construction, first of all, because there is only one spacer (55) in the path of the arc terminal, as compared to the three spacers of the upper arc terminal. Moreover, when the arc terminal jumps across this one spacer (55) of the lower arc runner, a considerably smaller voltage drop is introduced into the circuit than is the case with each spacer of the upper arc runner. This follows from the fact that instead of a single coil connected across the spacer, three coils in parallel, and, hence, considerable less impedance is connected across the spacer of the lower runner. This lower impedance would, of course, result in the introduction of a smaller voltage drop when the arc-terminal jumped the spacer, and because of this, the arc terminal has considerably less hesitancy about jumping across this spacer.

Another factor which enables higher speeds to be obtained with the lower arc-runner construction is that as soon as the arc terminal moves across the rst spacer, three additional blowout coils yare inserted into the circuit as compared with the single additional coil which would be inserted with the upper runner construction. Thus, instead of the magnetic action of a single additional coil, that of three additional coils becomes immediately available to impel the arc toward the interior of the chute. The more intense magnetic action resulting from the three additional coils is effective to impel the arc at substantially higher speeds than is the case with the single additional coil.

While it is true that the three parallel-connected coils inserted into the circuit would each receive less current than a single coil inserted into the circuit, this factor does not materially detract from the eectiveness of the parallel connection in the arc-chute of the present invention. This is the case because the iron forming the magnetic circuit of each of the blowout coils is constructed to saturate at a value of current far below that which the breaker is normally called upon to interrupt. For example, in one typical breaker capable of interrupting 30,000 ampcres, saturation occurs at a value of approximately 1000 amperes. Thus, even though the amount of current owing through each of the parallel-connected coils is less than would be the case with a single seriesconnected coil, the amount of flux transmitted through the iron of each blowout coil remains substantially unchanged in spite of the reduced current. As a result, this reduction in current produces no major reduction in the magnetic force available from each coil to impel the arc.

To a minor extent, however, the increased magnetic forces available with the parallel connection are offset by the fact that reduced current through each coil results in reduced leakage ux from each coil, thereby decreasing the amount of leakage flux available for impelling the arc. The major increase in immediately-available iiux through the iron greatly outweighs this minor loss, at least when the arc is in a position to be acted upon by the added ux. The arc is in such a position once its terminal moves on to the second runner segment.

I have found, however, that when the arc terminal is on the first runner segment (49) the leakage uX plays a more important part in impelling the arc than is the case when the arc terminal has moved therebeyond. In this regard, when the arc terminal is on the first segment, the arc is relatively short and relatively remote from the magnetic circuits of the other coils. As a result apparently of this remoteness, the amount of flux being transmitted through the magnetic circuits of the other coils becomes of less importance than the leakage tlux in the immediate vicinity of the arc. In order to fully utilize the leakage flux when the arc is short, the first coil 45 has been left unshunted and therefore receives all of the circuit current, thus, making available the leakage flux needed for high speed motion of the arc when the arc is in this particular position.

The physical location of the connection between runner segment 49 and the parallel combination of coils 46-48 (i.e., at the end of segment 49 nearest the arcinitiation region) is important, first of all, because with the connection so disposed, as soon as the arc terminal moves on to the segment all of the coils become available to impel the arc toward the interior of the chute: and, second, because the current path formed by the arc and the runner segment remains of a U-shape bowing toward the interior of the chute. The magnetic repulsive forces between the arms of this U or loop act to further aid in driving the arc into the chute.

Although best results have been obtained by connecting the first blowout coil of the lower runner in series with the remaining parallel-connected coils of the lower runner, as shown in the drawing, it should be understood that improved results over prior constructions can also be obtained by connecting all of the lower coils in parallel. The parallel arrangement enables a continuous uninterrupted arc runner to be used, and along such a runner the arc terminal moves at a higher speed than along a segmented runner having many insulated spacers therein, as is the case with prior constructions where all of the coils are connected in series.

Although l have described my invention only in connection with an arc-chute arrangement in which the arc moves horizontally into the chute, it is to be understood that the invention is also applicable to arc-chute arrangements w-hich are otherwise oriented. For example, it will be apparent that the principles of my invention would still apply even if the disclosed arc-chute were inverted or oriented at right angles relative to its illustrated position. In any such case, the arc runner generally designated 31 would be the last to receive its arc-terminal, but its blowout coils would impel this arc terminal at a higher rate than the rate at which the blowout coils of the other runner impel the other arc terminal, thus enabling the arc to be driven into the interior of the chute along substantially the entire length of the arc.

In connection with the illustrated embodiment, I have been particularly concerned with pointing out how full utilization can be obtained of the type of arc-chute which relies upon a plurality of blowout coils along each arc runner. lt is to be understood, however, that certain aspects of my invention are applicable to other types of arc chutes. In this regard, a broader aspect of my invention, irrespective of the specific magnetic blow-out arrangement used, is that of impelling the arc along the runner which last receives its arc terminal at a higher speed than the speed at which the arc is impelled along the runner which first receives its terminal.

lt should therefore be understood that my invention is not limited to the specific details of construction and arrangement herein illustrated, and that changes and modifications may occur to one skilled in the art without departing from the spirit of my invention.

What I claim as new and desire to secure by Let-ters Patent of the United States is:

l-. Any electric circuit breaker comprising an arc chute into which a circuit-interrupting arc is adapted to be driven for the purpose of extinguishing the arc, said arc chute comprising a pair of side walls extending generally perpendicular to the path of the arc and a pair of conductive arc runners spaced apart within said chute and extending transversely with respect to said arc path, means including separable contacts for establishing said arc in such a manner that a first terminal of the arc transfers to a first one of said arc runners after a relatively small contact separation and a second terminal of said arc transfers to a second one of said arc runners after a larger contact separation, means for accelerating the motion of said arc into the interior of said chute comprising a first group of magnetic blowout coils located along said first runner and a second group of magnetic blowout coils located along said second runner, means effective when said first terminal is transferred to said first runner for connecting said first group of blowout coils in circuit with said arc in such a manner as to drive the first terminal of said arc into the interior of said chute at a predetermined first speed, and means effective when saidsecond terminal is transferred to said second runner for connecting said second group of blowout coils in circuit with said arc in such a manner as to drive said second arc terminal into the interior of said chut at a speed which substantially exceeds said first spee 2. The electric circuit breaker of claim l in which said last means comprises means for first inserting one of the coils o-f said second group in series Iwith said arc and means for thereafter inserting the parallel combination of a plurality of other coils of said second group in series with said arc and said first coil in response to movement of said second arc terminal along said second runner into the chute.

3. In the circuit breaker of claim 1, means for connecting at least some of the coils of said second group in parallel with each other, and means for electrically connecting said parallel combination to at least a portion of said second runner in such a manner that said runner portion is located electrically between said arc and said parallel combination when said second terminal is on said runner portion, the electrical connection between said runner pontion and said parallel combination being located solely at the end of said runner portion adjacent the region in which said arc is initiated.

4. In the circuit breaker of claim 2, said arc accelerating means comprising magnetizable structure induotively coupled to the coils of said parallel combination for forming magnetic circuits for arc-motivating flux derived from said coils, said magnetic circuits being saturable in response to the ow of a predetermined current through the coil to which said magnetic circuit is coupled, said predetermined current being substantially less than the usual value of current interrupted by fthe breaker.

5. An electric circuit breaker comprising an arc chute into Which a circuit-interrupting arc is adapted to be driven for the purpose of extinguishing fthe arc, said arc chute comprising a pair of sidewalls extending generally perpendicular to the path of the arc and a pair of conductive arc runners spaced apart within said chute and extending transversely with respect to said arc path, means including separable contacts for establishing said arc in such a manner that a first terminal of the arc transfers to a first one of said arc runners after a relatively small contact separation and a second terminal of said arc transfers to a second one of said arc runners after a larger contact separation, first magnetic blowout means effective when said first arc terminal is transferred to said first runner for'driving the first terminal of said arc into the interior of saidchute at a predetermined first speed, and second magnetic blowout means effective when said second arc terminal' is t-ransferredl to saidv second runner for driving said second terminal into the interior of said chute at a speed which substantially exceeds said first speed, said second magnetic blowout means being effective to drive said second arc terminal at a speed' exceeding said first speed irrespective of the points in the current Wave at which said arc terminals are transferred to said arc runners.

6. An electric circuit breaker comprising an arc chute into which a circuit interrupting arc is adapted to be driven for the purpose of elongating, cooling, and thereby extinguishing the arc, said arc chute comprising a pair of sidewalls extending generally parallel to the path of the arc and a pair of conductive arc runners spaced apart Within said chute and extending transversely with respect to the arc path, means including separable contacts for establishing said arc in such a manner that one terminal c-f the arc transfers to a first one of said arc runners after a relatively small contact separation and the other arc terminal transfers to a second one of said arc runners after a larger contact separation, a first group of magnetic blowout coils located along said first runner and a second group of magnetic blowout coils located along said second runner for accelerating the motion of said arc into the interior of said chute, means for successively connecting the coils of said first group in series circuit relationship with each other and with said arc as said one arc terminal is driven along said first runner into the interior of said chute, means for connecting one of the coils of said second group in series with said arc when the other terminal of said arc is first transferred to said second runner, and means for thereafter connecting the parallel circuit combination of the remaining coils of said second group in series with said arc and said first coil as said other arc terminal moves along said second runner into the interior of said chute.

7. An electric circuit breaker comprising an arc chute into which a circuit interrupting arc is adapted to be driven for the purpose of extinguishing the arc, said arc chute comprising a pair of sidewalls extending generally parallel to the path of the arc and a pair of conductive arc runners spaced apart within said chute and extending transversely with respect to the arc path, means including separable contacts for establishing said arc in such a manner that one terminal of the arc transfers to a first one of said arc runners upon a relatively small contact separation and the other terminal transfers to a second one of said arc runners after a larger contact separation, a first group of magnetic blowout coils located along said first runner and a second group of magnetic blowout coils located along said second runner for accelerating the motion of said arc into the interior of said chute, means for successively connecting the coils of said first group in series-circuit relationship with each other and with said arc as said one arc terminal is driven along said first runner into the interior of said chute, and means for connecting the parallel circuit combination of at least some of the coils of said second group in series with said arc as said other arc terminal moves along said second runner into the interior of said chute.

8. An electric circuit breaker comprising an arc chute into which a circuit-interrupting arc is adapted to be driven for the purpose of extinguishing the arc, said arc chute comprising a pair of sidewalls extending generally parallel to the path of the arc and a pair of conductive arc runners spaced apart within said chute and extending transversely with respect to the arc path, means including separable contacts for establishing said arc in such a manner that one terminal of the arc transfers to a first one of said arc runners after a relatively small contact separation and the other terminal transfers to a second one of said arc runners after a larger contact separation,

means for accelerating arc-motion along said runners comprising a rst group of magnetic blowout coils located along said rst runner and electrically connected to first one of said contacts and a second group of magnetic blowout coils located along said second runner and electrically connected to the other of said contacts, means for stuccessively connecting the coils of said rst group in series circuit relationship with each other and with said arc as said one arc terminal is driven along said rst runner into the interior of said chute, said second runner com prising a pair of segments having their adjacent ends insulated from each other in the region along which said other arc terminal travels, means for connecting a plurality of the blowout coils of said second group in parallelcircuit relationship with each other, means for electrically connecting said parallel combination across the adjacent ends of said pair of segments, and an additional coil of said second group connected in series with said parallel combination and interconnecting said other contact and the conductive segment of said second runner nearest the arc-initiation region.

9. The circuit breaker of claim 8 in which the electric connection between said parallel combination and the segment of said second runner remote from the arcinitiation region is physically located solely at an end of said remote segment adjacent said arc-initiation region.

References Cited in the le of this patent UNITED STATES PATENTS

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