US3611007A

US3611007A - Current limiting spark gap assembly having electromagnetic means for retarding arc movement therein

Info

Publication number: US3611007A
Application number: US16161A
Authority: US
Inventors: Stanley A Miske Jr
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1970-03-03
Filing date: 1970-03-03
Publication date: 1971-10-05
Anticipated expiration: 1988-10-05
Also published as: SE383805B; DE2108311A1

Abstract

A current limiting spark gap that is normally operative to quickly lengthen and extinguish arcs formed therein is provided with a unique arrangement of its adjacent respective pairs of horn gap electrodes to afford an electromagnetic means for regulating the movement of arcs in the assembly. At least one electrode of a horn gapped spark gap is formed to have a sharply curved current conducting section that concentrates electromagnetic flux at a predetermined area in the arc-moving path of the current limiting spark gap adjacent thereto. The concentrated magnetic flux in this preselected area retards movement of an arc outward from said adjacent current limiting spark gap during the interval of time that a current in excess of a predetermined size is being discharged through the discharge circuit of the assembly.

Description

United States Patent [72] Inventor Stanley A. Mkke, Jr.

Pittsfield, Mass.

[21] Appl. No. 16,161

[22] Filed Mar. 3, 1970 [45] Patented Oct. 5, 1971 l 7 3 1 Assignee General Electric Company [54] CURRENT LIMITING SPARK GAP ASSEMBLY HAVING ELECTROMAGNETIC MEANS FOR RETARDING ARC MOVEMENT THEREIN [56] References Cited UNITED STATES PATENTS 3,361,923 l/1968 Osterhout 313/231 3,378,722 4/1968 Osterhout et al. 3,393,338 7/1968 Lee et al. 315/36X 3,504,226 3/1970 Stetson 315/36 ABSTRACT: A current limiting spark gap that is normally operative to quickly lengthen and extinguish arcs formed therein is provided with a unique arrangement of its adjacent respective pairs of horn gap electrodes to afford an electromagnetic means for regulating the movement of arcs in the assembly. At least one electrode of a horn gapped spark gap is formed to have a sharply curved current conducting section that concentrates electromagnetic flux at a predetermined area in the arc-moving path of the current limiting spark gap adjacent thereto. The concentrated magnetic flux in this preselected area retards movement of an arc outward from said adjacent current limiting spark gap during the interval of time that a current in excess of a predetermined size is being discharged through the discharge circuit of the assembly.

PATENTEUnm 51971 3511,00.

J fi ii zga CURRENT LIMITING SPARK GAP ASSEMBLY HAVING ELECTROMAGNETIC MEANS FOR RETARDING ARC MOVEMENT THEREIN The use of current limiting spark gap assemblies in lightning arresters to afiord both a low resistance discharge path and a means for rapidly clearing and resealing the arrester is already well known in the prior art. Normally, in the construction of present day current limiting lightning arresters, a plurality of such current limiting spark gaps are assembled in an insulating porcelain housing in series with blocks of nonlinear resistance valve material to provide a discharge path to ground for surge voltages that may be impressed across the arrester from a transmission line or other electrical system to which the arrester is connected to prevent surge voltages from damaging the insulation of the system. in these lightning arresters, the spark gaps of the assembly are designed to are over at a predetermined voltage which is below the safe insulating level of the components of the protected system. After such a surge voltage sparks over an arrester of this prior art type, electromagnetic arc-moving means within its spark gap assemblies operate to rapidly lengthen the arcs in the discharge path to stretch and cool them, thus causing them to be extinguished as quickly as possible so that the arrester discharge path can be rescaled to prevent power flow current from the system from being discharged to ground through the arrester. It is also common practice in prior art current limiting lightning arresters to provide a coil in series with the spark gap discharge circuit so that the coil is energized by a portion of the discharge current and creates a magnetic flux which aids in moving the arcs formed in the spark gaps of the assembly into arc extinguishing chambers. As is well understood by those skilled in the art, such coils are generally shunted, either with a resistor or with a spark gap, that acts as a voltage limiting element to protect the coil from flashover and insulation damage by the relatively large surge voltages discharged through the asembly. I

The normally intended operation of such prior art lightning arresters requires the impedance of the coil spark gap or resistor to be such that substantially all of the overvoltage surge is discharged through the assembly before the coil developes a strong enough magnetic field to move the arcs formed in the assembly outward on their respective horn gap electrodes any appreciable distance. Then, after the surge voltage has been discharged, the gap voltage should be rapidly increased by movement of the coil gap are thereby to produce a stronger magnetic field which operates to rapidly extinguish the arcs in the assembly and reseal it. However," it is frequently very difficult to accurately design and construct the component parts of current limiting spark gap assemblies so that they will perform these functions within narrow design limits. For example, a common problem that is encountered in this regard is to have the arc-moving coil attempt to drive the discharge circuit arcs to extinction while a relatively large discharge current is still flowing through the assembly. This type of operation has numerous disadvantages including a high risk to the protected system, the insulation of which may be punctured by the resultant arrester voltage when these high-current arcs are driven too forcefully into the chamber walls by a magnetic field that is too strong. in addition, such a strong coil field occuring with high arc currents increases the damage done to the arc chambers by the arc. This major problem has already been recognized and a solution for it has been proposed in a copending US. Pat. application, Ser. No. 13,317, filed Feb. 24, 1970, of Mr. Eugene C. Sakshaug, which is assigned to the assignee of the present invention. in this prior invention disclosed by Mr. Sakshaug, an auxiliary electromagnetic coil or sharply curved conductor is connected in series circuit relationship with the discharge circuit of a spark gap assembly and is mounted in a special insulated chamber adjacent the coil spark gap of the assembly so that the magnetic field developed by the auxiliary coil will resist arc movement in the coil gap in a desired fashion. Specifically, the auxiliary coil develops a strong magnetic flux that prevents the coil gap arc from moving outward on the horns of the gap until the entire surge voltage is discharged through the assembly, then the magnetic flux developed by the auxiliary coil weakens as only the smaller power follow current flows through it, thus freeing the coil gap arc to move rapidly to extinction so that a strong arc-driving flux is produced by the primary, gap-shunted coil of the assembly, thereby to rapidly extinguish all of the arcs in the assembly and reseal it against continued power follow current.

My invention is an improvement on the aforementioned invention by Mr. Sakshaug in that it affords a somewhat similar auxiliary electromagnetic means .for controlling movement of a coil gap arc, but accomplishes this result without requiring an extra electromagnetic coil and separate coil chamber in the spark gap assembly, as does his invention. Accordingly, my invention has inherent structural simplicity and resultant manufacturing economies that make it particularly suitable for commercial application.

Therefore, one object ofmy invention is to provide an improved spark gap assembly for lightning arresters in which the electromagnetic current limiting feature of the assembly is regulated by a unique auxiliary magnetic means.

Another object of the invention is to improve the operating efficiency of a lightning arrester spark gap assembly without increasing the size or cost of the assembly.

Yet another object of the invention is to provide a new and improved spark gap structure which will enable close control of the clearing and resealing voltage thereof in order to prevent damaging voltage surges from being applied to a pro tected system by an undesirable premature clearing of the assembly.

A further object of the invention is to provide a spark gap assembly that prevents a rapid buildup of arc voltages within the assembly until a surge voltage iscompletely discharged throughthe assembly, then rapidly builds up arc voltage to quickly clear the, assembly and reseal it against power follow current. r

A still further object of the invention is to provide a spark gap assembly having auxiliary magnetic means for controlling arc movement therein, said auxiliary magnetic means being of simplified design and structure thereby to facilitate its efficient manufa'cture.

In carrying out my invention in one preferred embodiment, a prior art type of electrodynamically controlled current limiting spark gap assembly is provided with at least one pair of main spark gap electrodes that is uniquely formed and mounted in relation to the coil spark gap of the assembly in a manner such that discharge current through the main electrodes of this main spark gap produces a strong magnetic field that resists outward movement by an are formed across the electrodes of the coil gap when a surge voltage is being discharged through the assembly. Specifically, one electrode of this main spark gap is formed with a sharply curved section that concentrates magnetic flux at a point spaced at given predetermined distance from the point of arc initiation of the coil spark gap therefore, when a discharge arc moves far enough outward on the horns of this main spark gap to cause discharge current to flow through said sharply curved section, the auxiliary magnetic field produced by this section in the coil spark gap arrests further arc movement and subsequent arc voltage buildup in the coil gap until after the surge voltage is completely discharged. Afier the surge voltage is completely discharged, the relatively smaller power follow current flowing through the curved section of the electrode produces a weaker magnetic fieldwhich is easily overcome by the electrodynamic forces tending to move the coil gap arc outward so that coil gap voltage buildup is rapidly increased and results in accelerated movement of all of the discharge arcs to extinguish these arcs and reseal the assembly.

The invention which i desire to protect is particularly pointed out and distinctly claimed in the claims appended hereto. However, it is believed that this invention and the manner in which its objects and advantages are obtained, as well as other objects and advantages thereof, will be better understood from the following detailed description of a preferred embodiment of the invention considered in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a spark gap assembly that incorporates one embodiment of my invention.

FIG. 2 is a schematic circuit diagram showing the discharge circuit of the assembly illustrated in FIG. 1, in combination with a valve resistor.

FIG. 3 is an exploded perspective view of some of the component parts of the assembly depicted in FIG. 1 showing characteristic structural features of the invention relative to other operative components in the discharge circuit of the spark gap assembly.

FIG. 4 is a top plan view along the plane 4-4 of FIG. 3 and also showing in phantom the coil spark gap electrodes of the assembly depicted in FIGS. 1 and 2.

Referring now to FIG. 1 of the drawing, there is shown a spark gap assembly 1 having a plurality of

insulating plates

2, 3, 4, 5, 6 and 7. These plates 2-7 may be formed of any suitable insulating material such as molded, porous ceramic. As is well known by those skilled in the art, the individual plates 2-7 are each formed so that when they are assembled in the stacked arrangement shown in FIG. 1 arc-confining chambers are formed between each adjacent pair of plates.

A series discharge circuit is formed between terminals 8 and 9, that are mounted respectively adjacent opposite ends of the stacked assembly 1 (terminal 8 being shown in FIG. 1). This discharge circuit is shown in schematic form in FIG. 2 and comprises a plurality of pairs of main

spark gap electrodes

10, 10 and 11, 11 and 12, 12 and 13, 13 electrically connected by suitable circuit means in series with a coil-shunted current limiting spark gap comprising

horn gap electrodes

14 and 14. The current limiting spark gap 14-14 is shunted by electromagnetic coil 15. It will be understood by those skilled in the art that the electrodes forming the discharge circuit of the assembly 1 may be formed of copper or other suitable conductive material as is well known in the art. Also, except for the unique form and arrangement of one of the electrodes of either or both primary spark gap(s) 11-11' or 12-12 in relation to the arc path defined by current limiting spark gap 14-14', the spark gap assembly 1 may be constructed in the manner more fully described in U.S. Pat. 3,354,345 Stetson, which issued on Nov. 2 l 1967 and is assigned to the assignee of the present invention.

As is explained in the Stetson patent, the general principal of operation of such a discharge circuit is to allow a surge voltage impressed across the terminals 8 and 9 of assembly 1 to sparkover the respective spark gaps thereof, thus, forming a low impedance current path through the assembly to at least one series connected block of nonlinear resistance valve material, such as that shown schematically in the circuit of FIG. 2 as valve resistor 16, and thence to ground. During such a discharge, a relatively low voltage is impressed across coil 15 until such time as the electrodynamic arc-moving force developed by the

horn gap electrodes

14 and 14 of coil gap 14-14 moves an arc outward thereon to increase its length appreciably thereby building up the arc voltage to force a substantial current through coil 15. As the amount of current through coil 15 increases, the strength of the magnetic field developed by it increases and this stronger field drives arcs rapidly outward on the respective primary spark gaps 10-10, 1 1-11', 12-12 and 13-13 to extinguish the arcs and clear the discharge path. As noted above, it is very difficult to control the rate of voltage buildup on current limiting coil gap 14-14 for surge voltages of differing duration and magnitude, therefore, the auxiliary electromagnetic means described in the above-noted Sakshaug patent application, Ser. No. l3,3l7, filed Feb. 24, 1970, was developed to satisfy the need for better discharge and reseal characteristics for spark gap assemblies of a type similar to that disclosed herein as assembly 1. It should be understood that no claim is made herein to the basic circuit or operating principal disclosed in this prior art patent application of Mr. Sakshaug. The scope of the claims to my invention is specifically limited to an improvement over the basic structure and circuitry disclosed in that prior art patent application and to the extent that my claimed invention may overlap that prior invention, I hereby disclaim any right or interest in such overlapping area.

The characteristic features of my invention will now be explained in detail with reference to FIG. 3 of the drawing. As shown in FIG-3, the

electrodes

10 and 10 of the upper primary spark gap 10-10 are horn gap electrodes of a current limiting type which are designed to move an arc formed therebetween outward from its point of initiation into contact with the castellated peripheral wall of the arc-confining chamber 17 formed between insulating plates 2 and 3 to stretch, cool and extinguish the arc. The other end main spark gap 13-13 is similarly formed and in this embodiment of the invention, main spark gap 12-13' also comprises such a conventional horn gap arrangement. However, pursuant to my invention, one electrode 11 of the primary spark gap "-1 I is formed to have a sharply curved section 11a that is carefully mounted in a unique position relative to the path of arc movement defined by

electrodes

14 and 14 of coil spark gap 14- 14'.

The exact relative position of electrode 11 with respect to coil gap electrodes 14 and 14' can best be seen in FIG. 4 of the drawing. As shown in FIG. 4, the point of closest spacing of coil gap electrodes 14-14' is identified by the numeral 18 and this point will be the point of sparkover or arc initiation when a surge voltage in excess of the sparkover rating of assembly 1 is impressed across terminals 8 and 9 of the assembly. When the current limiting spark gap 14-14 is sparked over in this manner, the electrodynamic are moving force developed by the horns 14a and 14b thereof drives the are from its point of initiation 18 outward toward the center of arc-confining chamber 19. At the same time, an are formed between the point of minimum spacing or are initiation 20 of main spark gap 11-11' is moved outward by the electrodynamic arc-driving force of its horn gap electrodes 11 and 1 1' acting in combination with the relatively weak arc-moving magnetic field produced by coil 15. This movement of the arc 21 outward on

electrodes

11 and 11 continues until it moves past the sharply curved portion 11a of electrode 11. At this point in the operation of the assembly 1, discharge current flows from terminal 8 across the spark gap 10-10' and through electrode 1 1 including the sharply curved portion Ila thereof, across are 21, thence through electrode 11' and coil gap 14-14' and the remaining primary spark gaps 12-12' and 13-13 to nonlinear valve resistor 16 and then to ground. It is important to note that in following this discharge path the entire discharge current is forced to pass through the sharply curved portion 11 a of electrode 11. Thus, it will be seen that a very strong magnetic field is-formed by the flux concentrated at the inner radial surface of the curved portion 11a of electrode 11. In the preferred embodiment of the invention depicted herein, this concentrated magnetic field is opposed to the magnetic field developed by coil 15, therefore, as an arc is moved outward on the horns 14a and 14b, its movement is strongly resisted by the magnetic field developed by the curved portion 11a of electrode 11. Due to the strength of this magnetic field, it arrests movement of the arc moved outward from are initiation point 18 along horns 14a and 14b at a given point is shown generally by the spaced a predetermined distance from arc initiation point 18. This given point is shown generally by the graphically illustrated are 22 at the point where it is stopped in its outward movement between current limiting

spark gap electrodes

14 and 14.

In the operation of this preferred embodiment of my invention, assuming the arc initiation and movement just described, further movement of are 22 between electrodes 14 and 14' of current limiting gap 14-14' will be arrested as long as discharge current through the assembly I exceeds a predetermined value that is sufiicient to maintain the magnetic field developed by sharply curved portion 11a of electrode 11 strong enough to overcome the combined arc-driving electrodynamic force of horn gap 14-14' and the electromagnetic filed produced by coil 15. After such a surge voltage is completely discharged, the power follow current falls below this predetermined current size, therefore, the magnetic field developed by sharply curved portion 11a of electrode 11 weakens sufficiently to allow are 22 to be moved rapidly outward in the arc-confining chamber 19 into contact with the castellated peripheral wall thereof, where it is rapidly cooled and extinguished. According y, the' voltage across coil 15 rapidly increases and produces a strong magnetic field that quickly drives the arcs (such as are 21) outward on the respective main spark gaps -10, 11-11, l2--12 and 13- into contact with the peripheral walls of their respective arcconfining chambers to extinguish these arcs and completely clear the assembly 1. Of course, an advantage of such operation is that the assembly 1 is prevented from prematurely extinguishing high-current discharge arcs that could cause an undesirable voltage peak to be impressed upon the system to which the assembly 1 is connected as a protective device, normally as a component in a lightning arrester. This and other advantages are more completely described in the aboveidentified patent application of Mr. Sakhaug', therefore, further enumeration of such advantages will not be given herein.

A primary novel advantage of my invention is that a very effective arc movement controlling means is afforded in a spark gap assembly, such as assembly 1, without requiring the addi tion of more than the usual number of components parts, such as the use of an auxiliary electromagnetic coil. Moreover, since no additional circuit components are required by my invention, it is not necessary to enlarge the assembly 1 to house and insulate such components, so that the cost of assembly need not be increased in order to enable it to operate in the unique manner disclosed herein. It will be apparent to those skilled in the art that various modifications may be made in my invention without departing from the true spirit and scope thereof. For example, the relative spacing of the sharply curved section 110 of horn gap 11 can be varied relative to the arc movement path of current limiting gap 14-44 to change the given point at which an arc (22) is arrested in its outward movement in the manner described above. Also, the relative strength of the arc-moving electrodynamic forces created by current limiting spark gap l4-l4 in combination with the magnetic field produced by coil 15 can be varied as desired to effect a similar movement of the given point at which the are 22 will be arrested by the concentrated magnetic field produced by arcuate section 11a of electrode 11 when a discharge current flows through it. All such modifications and improvements are intended to be encompassed within the scope of the following claims.

What I claim and desire to secure by Letters Patent of the United States is:

l. A spark gap assembly comprising a plurality of insulating plates arranged in a stack, each of said plates being formed to define an arc-confining chamber with the plate next adjacent to it when the plates are in their stacked arrangement, at least two pairs of main horn gap electrodes, each of said pairs of main electrodes being mounted in spaced-apart relationship respectively in one of said arcing chambers to form main spark gaps between the electrodes of said pairs, a current limiting spark gap formed by a pair of horn gap electrodes mounted in spaced-apart relationship in an arcing chamber and positioned in the stacked arrangement between two of said main spark gaps, a pair of tenninals mounted respectively adjacent opposite ends of said stack, circuit means connecting said main spark gaps and said current limiting gap in a series circuit between the respective terminals of said pair of terminals, one horn gap electrode of one of said pairs of main spark gaps between which the current limiting gap is positioned being shaped to form a sharply curved current conducting path that develops a magnetic field of high flux density adjacent the midpoint of the radially inner surface thereof when current is discharged through the spark gaps of the assembly, said radially inner surface of said horn gap electrode being sitioned ad acent the current limiting spark gap a pre etermined distance outward from the point of closest spacing of the horn gap electrodes thereof, whereby movement of an arc fonned between the electrodes of said current limiting gap away from said point of closest spacing responsive to the arc-driving effect of the horn gap is resisted by said magnetic field with a force that is directly proportional to the size or arcing current being discharged through the series circuit of said assembly.

2. A spark gap assembly as defined in claim 1 including a coil of wire mounted on the assembly around said current limiting gap and electrically connected in parallel relationship with said current limiting gap, said coil being operable when energized to develop an arc-driving magnetic field that moves arcs formed between the respective electrodes of each of the spark gaps outward from the respective points of initiation of the arcs.

3. A spark gap assembly as defined in claim 2 wherein the magnetic field developed by the sharply curved horn gap electrode is opposed to the magnetic field developed by said coil.

4. A spark gap assembly as defined in claim 2 wherein said magnetic field of high flux density is stronger at said predetermined distance from the point of closest spacing of the horn gap electrodes of the current limiting gap than the magnetic field developed by said coil when a discharge current in excess of a given size is discharged through the series circuit of the assembly.

5. A spark gap assembly as defined in claim 4 wherein said magnetic field of high flux density is weaker at said predetermined distance from the point of closest spacing of the electrodes of the current limiting gap than the magnetic field developed by said coil when a discharge current smaller than said given size is discharged through the series circuit of the assembly.

6. In a current limiting spark gap assembly having a plurality of pairs of spark gap forming horn gap electrodes mounted in insulated arcing chambers and electrically connected into a surge voltage discharge circuit, and having an electromagnetic coil mounted thereon and electrically connected in shunt relationship with at least one of said spark gaps, the improvement comprising means mounting one horn gap electrode of a spark gap next adjacent the spark gap shunted by said coil so that said one horn gap electrode has a sharply curved radially inner conductive section thereof positioned adjacent a given point in the path of arc movement of said spark gap shunted by the coil, said sharply curved radially inner conductive section of said horn gap electrode being a part of its horn portion spaced away from the point of arc initiation of the spark gap so that arcing current will not flow therein until an arc has moved outward on said horn portion, whereby the sharply curved section of said horn gap electrode develops a magnetic field that resists movement of an arc outward on the horns of the spark gap shunted by said coil only after'an arc has moved outward on said horn gap electrode past the sharply curved section thereof.

7. The invention defined in claim 6 wherein the magnetic field developed by the sharply curved section of said horn gap electrode is sufficiently strong to arrest movement of an are between the electrodes of the coil-shunted spark gap when a current in excess of a predetermined size is passed through the discharge circuit of the assembly, and wherein said magnetic field is not strong enough to arrest outward movement of said arc when a current smaller than said predetermined size is passed through the discharge circuit.

Patent No.

Inventor(s) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,611,007 Dated October 5, 1971 Stanley A. Miske, Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer line ll, develops is misspelled.

line 39, "12-12" should be -l2-l2' line 11, "13-" should be 13-13 Signed and sealed this 2nd day of May 1972.

ROBERT GOTTSCHALK Commissioner of Patents

Claims

1. A spark gap assembly comprising a plurality of insulating plates arranged in a stack, each of said plates being formed to define an arc-confining chamber with the plate next adjacent to it when the plates are in their stacked arrangement, at least two pairs of main horn gap electrodes, each of said pairs of main electrodes being mounted in spaced-apart relationship respectively in one of said arcing chambers to form main spark gaps between the electrodes of said pairs, a current limiting spark gap formed by a pair of horn gap electrodes mounted in spaced-apart relationship in an arcing chamber and positioned in the stacked arrangement between two of said main spark gaps, a pair of terminals mounted respectively adjacent opposite ends of said stack, circuit means connecting said main spark gaps and said current limiting gap in a series circuit between the respective terminals of said pair of terminals, one horn gap electrode of one of said pairs of main spark gaps between which the current limiting gap is positioned being shaped to form a sharply curved current conducting path that develops a magnetic field of high flux density adjacent the midpoint of the radially inner surface thereof when current is discharged through the spark gaps of the assembly, said radially inner surface of said horn gap electrode being positioned adjacent the current limiting spark gap a predetermined distance outward from the point of closest spacing of the horn gap electrodes thereof, whereby movement of an arc formed between the electrodes of said current limiting gap away from said point of closest spacing responsive to the arc-driving effect of the horn gap is resisted by said magnetic field with a force that is directly proportional to the size or arcing current being discharged through the series circuit of said assembly.

6. In a current limiting spark gap assembly having a plurality of pairs of spark gap forming horn gap electrodes mounted in insulated arcing chambers and electrically connected into a surge voltage discharge circuit, and having an electromagnetic coil mounted thereon and electrically connected in shunt relationship with at least one of said spark gaps, the improvement comprising means mounting one horn gap electrode of a spark gap next adjacent the spark gap shunted by said coil so that said one horn gap electrode has a sharply curved radially inner conductive section thereof positioned adjacent a given point in the path of arc movement of said spark gap shunted by the coil, said sharply curved radially inner conductive section of said horn gap electrode being a part of its horn portion spaced away from the point of arc initiation of the spark gap so that arcing current will not flow therein until an arc has moved outward on said horn portion, whereby the sharply curved section of said horn gap electrode develops a magnetic field that resists movement of an arc outward on the horns of the spark gap shunted by said coil only after an arc has moved outward on said horn gap electrode past the sharply curved section thereof.

7. The invention defined in claim 6 wherein the magnetic field developed by the sharply curved section of said horn gap electrode is sufficiently strong to arrest movement of an arc between the electrodes of the coil-shunted spark gap when a current in excess of a predetermined size is passed through the discharge circuit of the assembly, and wherein said magnetic field is not strong enough to arrest outward movement of said arc when a current smaller than said predetermined size is passed through the discharge circuit.