US3504226A

US3504226A - Spark gap assembly for current limiting lightning arresters or like articles

Info

Publication number: US3504226A
Application number: US706911A
Authority: US
Inventors: Earl W Stetson
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1968-02-20
Filing date: 1968-02-20
Publication date: 1970-03-31
Anticipated expiration: 1987-03-31
Also published as: GB1254934A; FR2002244A1; SE390855B; DE1908152C3; DE1908152B2; CH496339A; DE1908152A1

Description

' March 31, 1970 v v E. w. 'STETSON I 3, 4,

Filed Feb. 20, 1968 'SPARK' GAP ASSEMBLY FOR CURRENT LIMITING LIGHTNING ARRESI'ERS 0R LIKE ARTICLES 3 Sheets-Sheet 1 SPARK GAP ASSEMBLY FOR CURRENT LIMITING March 31,- 1970 w STETSON I 3,504,226

' LIGHTNING ARRESTERS OR LIKE ARTICLES Filed Feb. 20, 1968 3 Sheets-Shget 2 251/ W Sfa/Zm March 31, 1970 E. w. STETSON 3,504,226

- SPARK GAP ASSEMBLY FOR CURRENT LIMITING LIGHTNING ARRESTERS OR LIKE ARTICLES Filed. Feb. 20, 1968 s Sheets-Sheet s [inf/2251:

ir/ W 6/6190);

United States Patent Office 3,504,226 Patented Mar. 31, 1970 SPARK GAP ASSEMBLY FOR CURRENT LIMITING ABSTRACT OF THE DISCLOSURE A spark gap assembly for a current limiting lightning arrester or like article. The assembly operates to rapidly and substantially limit power follow current through the assembly. The current limiting operation may be effected with or without the use of supplementary solenoid means to aid in increasing arc resistance. Characteristic features of the assembly include; matched pairs of electrodes arranged in overlapping relationship, mechanically simple means for readily and inexpensively securing main spark gap electrodes in precise predetermined position within the spark gap assembly, and preionizing means that atford reliable and inexpensive main spark gap preionization in a porous arc chamber.

This invention relates to a current limiting spark gap assembly, and, more particularly, to such an assembly formed of porous material and utilized in a lightning arrester.

It is common practice in the lightning arrester field to electrically connect a suitable spark gap assembly in series with a nonlinear valve resistor between suitable terminals disposed on opposite ends of an insulating lightning arrester housing containing these components. The spark gaps in such a lightning arrester are designed to spark over at a predetermined voltage level such that transient or surge voltages on the electric power system being protected by the arrester -will be shunted to ground rather than building up an excessive voltage on the system to injure insulation, or component parts, of the system. While the main object of lightning arresters is to protect a system from over-voltage surges; the arresters must also function to control the discharge of normal line current to ground after a transient voltage surge has passed. This current limiting action is provided in two ways; first, the nonlinear valve resistance in the arrester inherently presents a relatively high resistance at the usual line voltages of the system being protected; secondly, the resistance of the arcs in the spark gap assembly is increased by rapidly lengthening each arc. Heretofore, one technique for achieving such lengthening of the spark gap arcs has been to electrically connect a magnetic coil in series with the spark gaps and dispose it around the gaps in a manner such that a magnetic field is developed to drive the arcs in the spark gaps outwardly therefrom toward a tortuous arc chamber wall that serves to extend and cool the arcs and, thus, sharply increase their resistance.

It has also been recognized in the prior art that an improved rate of arc quenching could be afforded by lining the arc chamber with a porous arc quenching material. One construction of such a porous arc chamber is shown in US. Patent No. 3,151,273, issued Sept. 29, 1964, to E. W. Stetson et al. for Current Limiting Lightning Arrester With Porous Gap Structure, which is assigned to same assignee as the present invention. Such a porous construction for an arc chamber serves to absorb and, thus, control the plasma developed by a high current are so that a horn gap adjacent the arc can act effectively to move the arc toward its lengthened, high resistance position.

It is also known to provide preionizing means for ionizing the spark gaps of lightning arrester spark gap assemblies thereby to improve the operating characteristics of the lightning arrester. Such preionizing means make it possible for the sparkover voltage of each spark gap in the assembly to be precisely determined so that the sparkover voltage can be closely set with respect to the voltage of the line being protected. Several effective preionizing means have been developed in the past but, of course, it would be desirable to provide an inexpensive and easily assembled preionizing means for use with porous arc chambers.

The sparkover voltages of spark gaps is determined primarily by the spacing of the electrodes defining such gaps. Prior to the present invention, it has been a common practice to space the individual electrodes within a spark gap assembly by utilizing measuring calipers or other suitable positioning means to obtain a precise spacing between given electrodes before they are glued or riveted in this carefully measured, spaced apart position. A desirable feature of my invention is that the several gaps in a given spark gap assembly are easily and precisely formed without requiring a separate measuring and positioning step to be performed on each pair of electrodes during the manufacture of the assembly.

In accordance with my invention, there is provided a new and improved current limiting spark gap assembly which is particularly adapted for use as an are interrupting and current limiting spark gap assembly in lightning arrester housings. In one form of the invention, a plurality of porous plate members are stacked to define a predetermined number of arc chambers therebetween. A plurality of dual-electrode elements are disposed within the assembly to define a separate spark gap within each arcing chamber. The respective electrodes are positioned in a manner such that they also form an arc lengthening horn gap adjacent each of the spark gaps. These horn gaps serve to drive the arcs formed in the spark gaps outwardly therefrom along the horn gap portions of the electrode elements thereby to extend the arcs and increase their resistance to limit the current flowing through the assembly. Accordingly, in many applications, no additional electromagnetic coil is required to lengthen the arcs and limit the power follow current through the spark gap assembly. Another feature of the invention is the provision of inexpensive and reliable electrode positioning means that serve to accurately space the respective electrode elements to form a spark gap of predetermined length in each arc chamber of the assembly without requiring a separate measuring and positioning operation for each spark gap during the manufacture of the assembly. The electrode elements utilized in the invention have a novel configuration that enhances the current limiting characteristics of the spark gap assembly while at the same time reducing the complexity of manufacturing operations required to fabricate the assembly. Still another feature of the invention is a unique preionizing means for the main spark gaps in the porous arc chambers of the assembly.

A primary object of my invention is to provide a new and improved spark gap assembly.

Another object of the invention is to provide a current limiting spark gap assembly which will rapidly increase the arc voltages after a surge voltage has been discharged, without requiring the use of an electromagnetic coil to lengthen the respective arcs formed across the spark gaps of the assembly.

A further object of the invention is to provide inexpensive and reliable means for securing separate electrodes in an arc chamber thereby to define a precisely predetermined length of spark gap between such electrodes.

Still another object of the invention is to provide inexpensive and highly reliable preionizing means for ionizing the main spark gaps of a spark gap assembly formed from porous material.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings and its scope will be pointed out with particularity in the appended claims.

In the drawings:

FIG. 1 is a side elevation, partly in cross section, of a preferred embodiment of the invention shown with respect to a schematically illustrated electric power conductor and a grounding terminal.

FIG. 2 is a perspective view of the preferred form of the spark gap assembly shown in FIG. 1 illustrating portions of its characteristic features in phantom, with the phantom view taken along the plane 2-2 of FIG. 1.

FIG. 3 is an exploded view of the embodiment of the invention shown in FIG. 2.

FIG. 4 is a perspective view of one of the plate members and electrodes used to form part of the assembly shown in FIGS. 2 and 3.

FIG. 5 is an enlarged, top elevation, of the main spark gap electrode elements utilized in the embodiment of the invention shown in FIGS. 2 and 3.

FIG. 6 is a side elevation of the electrode element shown in FIG. 5.

FIG. 7 is a diagrammatic view of the electrical connections employed in the preferred embodiment of the invention shown in FIGS. 2 and 3.

Referring now to FIG. 1 of the drawing, there is shown a lightning arrester 1 having a hollow tubular housing 2 formed of suitable insulating material such as heat treated ceramic, and sealed at its opposite ends by terminal plates 3 and 4, respectively. The terminals 3 and 4 may be sealed to the housing 2 in any conventional manner, such as by providing a suitable cement 5 and '6 therebetween. Terminal 3 is electrically connected to a conductor 7 which completes a circuit to a transmission line conductor 8. The terminal 4 on the opposite end of the lightning arrester 1 is in contact with end cap 4a, which is connected to ground by a conductor 9 that is fastened to the end cap 4a in any suitable manner, such as by clamping it with a bolt and nut arrangement, 9a.

The functional components within the arrester housing 2 comprise a block of nonlinear resistance material 10 resting on the lower terminal 4 and supporting a spark gap assembly 11 having electrically conductive

metallic terminal plates

12 and 13 on opposite ends thereof. The

spark gap assembly 11 is maintained in electrically conductive relation with the nonlinear resistor 10 by a coil spring 14 that is compressed between the uppermost terminal 13 on spark gap assembly 11, and the terminal plate 3 on housing 2. It will be understood by those skilled in the art that the lightning arrester as described thus far embodies relatively conventional surge voltage discharging means connected in series with current limiting means. Accordingly, it will be appreciated that a wide variety of suitable materials and manufacturing techniques are known and may be utilized for forming the conventional components of such a lightning arrester structure.

In FIGS. 2 and 3 of the drawings, the spark gap assembly 11 is shown in greater detail. In the preferred embodiment of my invention, the assembly 11 comprises a plurality of stacked porous plate members A, B, C, D, and B. These plate members A-E are formed of granulated aluminum oxide that is molded with a suitable binding material to define arc chamber surfaces on the respective top and bottom surfaces thereof. It will be noted, by referring specifically to FIGS. 3 and 4, that each of the arc chambers defined between the respective upper and lower surfaces of the porous plate members A-E, are defined by pairs of complementary molded surfaces. Specifically, considering the arc chamber formed between plate members A and B, by way of example, the plate member A has a recessed semiannular portion around the major extent of its bottom peripheral edge 15 which is adapted to receive a complementary raised semi-annular portion 16 around the peripheral edge on the top surface of plate member B. The unrecessed semicircular portion 15' on the bottom peripheral edge of plate member A is shaped to complement surface portion 16 of the top surface of plate member B. In addition, the bottom surface of the uppermost plate member A has a substantially planar central portion 17 that extends axially below the recessed semi-annular peripheral portion 15 to form a lip 17 around its circumference. The lip 17 has a configuration complementary to the side wall of the semi-annular raised peripheral edge 18 on plate memberB. The extending lip portion 17 is only half of the depth 18 of the raised ring 16 on member B; accordingly, when plate member A is in operating position on top of plate member B, an arcing chamber is defined by the bottom planar surface 17 of plate member A and the top surface 19- of plate member B. The arc chamber thus formed is substantially closed by the peripheral sealing engagement afforded by the mating surfaces of the peripheral recessed portion 15 on member A, in engagement with the raised ring 16 on the top surface of plate member B. It should be understood that the vertical spacing between the surface of semi-circular portion 15' on member A and surface 16 on member B is at least equal to the vertical dimension of the electrode 20, i

so that the preionizing means, which will be described below, may function in an optimum manner. The remaining top and bottom surfaces, respectively, of plate members C, D, and E are identical in configuration to the top and bottom surfaces respectively of plate members A and B, and they coact When in juxaposition to define similarly shaped arc chambers between these surfaces. Of course, the bottom surface of plate member E will be substantially flat, just as the top surface of plate member A is, so that

terminals

13 and 12 respectively will seal smoothly thereon.

An important feature of my invention resides in the novel form of electrode elements utilized to form the main spark gaps in the respective arcing chambers of assembly 11. In order to describe the characteristic features of these electrode elements in detail, reference is made to FIGS.

connected together by an integral link 23. Each of the

electrodes

21 and 22 have a first generally linear are running surface 21' and 22 respectively that merges smoothly into second arc running portions 21" and 22" respectively. The particular angular relationship and unique operating characteristics of these arc running surfaces will be described in more detail below. However, in order to facilitate an understanding of the invention, some of its other maior features will be described now.

A second important feature of the invention resides in the unique positioning means utilized to secure the electrode elements 20 in their respective predetermined positions on the porous plate members A-E. Each of the

electrodes

21 and 22 of electrode elements 20 are provided with non-circular apertures 24 and 25 (see FIG. 5), respectively, therethrough. These

apertures

24 and 25, are adapted to engage bosses on one surface of the respective arc chambers as will be described more fully below. At this point, it is only necessary to understand that in my in vention the

apertures

24 and 25 in the electrode elements 20 are of a substantially noncircular configuration, so that when they are in engagement with their respective bosses, they securely lock the electrodes 20 in a predetermined position, and the electrodes 20 are prevented from rotating with respect to the porous plate member upon which they are mounted. It will be noted, by referring to FIG. 5 that the respective spaced apart

electrodes

21 and 22 on electrode element 20 are substantially identical in configuration and a major portion of their respective surfaces are in overlapping relationship. As will be seen more fully below, this configuration appreciably enhances the current limiting characteristics of the spark gap assembly 11 of my invention. At the same time, however, it is apparent that there is a tendency for the electrode elements 20 to rotate around a vertical axis through the center of

apertures

24 and 25; therefore, it is important as required above, that these apertures and their respective cooperating bosses have a non-circular profile to prevent such' rotation. In the preferred embodiment .of the invention, the

apertures

24 and 25 are formed in the elliptical configuration shown, having a major longitudinal axis approximately one and one-half times as long as the minor axis. It has been found that this configuration affords a strong and durable positioning means and the mating molded bosses on the surface of the porous plate members are not easily broken or distorted as might be the case with some other boss configurations. However, other basic polygon profiles such as squares, rectangles or star-shapes will provide suitable substitutes for .the elliptical configuration described herein, and other configurations are certainly Within the scope of my invention.

Referring again to FIG. 3, three of the electrode elements 20 are shown in operating position, mounted respectively on the porous plate members B, C, and D. More specifically, the uppermost electrodes 22 and the respective integral links 23 of each of these elements 20 are shown in FIG. 3, while the respective bottom electrodes 21 are obscured from view in this figure of the drawings. The top and bottom plate members A and E, respectively, have

separate electrodes

26 and 27 mounted thereon by having their respective integral link portions 26a and 27a electrically and mechanically connected to

terminal plates

12 and 13, respectively (see FIGS. 2 and 3). The

electrodes

26 and 27 are identical in configuration to

electrodes

21 and 22 and, in fact, may be formed from similar single pieces of bar stock, which are then severed at the end of one of the link portions, e.g. the end of link 23 in FIG. 6,

thereby to remove one of the electrodes from an electrode element 20. The remaining portion of the links, 26a and 27a, are then electrically and mechanically connected, respectively, to the

terminal plates

12 and 13. It will be appreciated that when the

electrodes

26 and 27, and their associated

terminals

12 and 13 are slid into position on the respective porous plate members A and E, they are not secured in position with respect to these plate members until they are assembled in operating position in spark gap assembly 11. More specifically, by referring to FIGS. 3 and 4 and plate members A and B depicted therein, it will be seen that electrode 26 has a nonrcircular aperture 24 therein which has a profile adapted to closely engage the boss 30 on the top surface of plate member B. A second boss 31 on the top surface of plate member B is adapted to engage the profile of aperture 25 in electrode 22 of the electrode element 20 and, thus, secure it in a predetermined position on the plate member B. Referring to the phantom view shown in FIG. 2, it will be seen that the

respective electrodes

26 and 22 secured in position by

bosses

31 and 30 on the top surface of plate member B define a spark gap therebetween. The spark gap comprises the lowest insulation point, or point of nearest juxtaposition, between the

respective electrodes

26 and 22, and the generally

linear portions

21 and 22 of these electrodes define a horn gap diverging outwardly from the spark gap. In the preferred embodiment of my invention, the horn gaps formed by the generally linear arc run ning surfaces 21 and 22' diverge from each other at an angle less than 50 degrees, and preferably in the range between 5 and 15 degrees.

Proceeding now to the arc chamber defined between the next successive plate members, B and C, it will be seen that electrode element 20 is further secured in position when the respective plate members A, B and C of the assembly 11 are in operating position by having the aperture 24 in electrode 21 (not shown) of the element 20 on plate member B in engagement with boss 300 on the top surface of porous plate member C. Then, in like manner, the electrode element 20 mounted on plate member C is positioned by the bosses 31c'and 3011. Also, the electrode 21 on the electrode element 20 mounted on porous plate member D is secured in position by boss 31d on the top surface thereof and boss 302 on the top surface of plate member E. A second boss 31a on the top surface of plate member E secures electrode 27 in position to define a spark gap of predetermined length between electrode 27, and electrode 21 mounted on the plate member D. It can thus be seen that an inexpensive and reliable positioning means is afforded for precisely securing each of the electrode elements 20 and the

end electrodes

26 and 27 in relative positions such that spark gaps of predetermined length are formed respectively therebetween.

In order to understand more fully the desirable features of the unique electrode element of my invention, reference is again made to FIG. 5 in conjunction with FIG. 7, of the drawings. In FIG. 5 it can be seen that the respective generally linear are running portions 21' and 22' defined by the respective spaced-apart

electrodes

21 and 22 are positioned at an angle of approximately 90 degrees with respect to each other. In the preferred form of my invention, the angle between the generally longitudinal axes of are running surfaces 21' and 22' is set at 90 degrees :15 degrees. By using this arrangement in conjunction with the overlapping configuration of the elements 20, described above, the arc resistance of the respective spark gaps formed between the various porous plate members A-E is maintained substantially equal. This effect is due to the fact that the closely spaced horn-gaps adjacent each spark gap develop strong electromagnetic forces that overcome the solenoid effect resulting from the arcing current being forced to flow in a helical path as it passes from electrode 26 to electrode 27. Since each of the horn gaps is formed of identically shaped electrodes and spaced by identically molded bosses, each of the arcs moves at the same rate outwardly from their respective sparks gaps up their associated horn gaps; accordingly, all of the arcs tend to increase in length at the same rate and thus maintain an equal voltage drop across each gap. This desirable operating characteristic of my spark gaps in the arc chambers between the plate members A-E at a predetermined voltage, preionizing means 32 are positioned in each arc chamber adjacent to the respective spark gaps. The preionizing means 32 comprise a pointed electrode 32a (shown in FIG. 4) made of resilient metal and adapted to be inserted in a preformed recess in the bottom surface of porous plate member A. The point on the pointed electrode 32a engages the flat surface'of a substantially impervious block 32b of insulating materials embedded in a suitable recess in the top surface of porous plate member B. In the preferred form of my invention, pointed electrode 32a is made of a thin strip of Phosphor bronze that is folded into a generally U shape at one end so that when the base of the U-shaped portion is forced into a generally rectangular recess in the surface of plate member A, the inherent resilience of the electrode member 32a retains it in position. The impervious insulating block 32b is formed of very small grains of aluminum oxide, which are preferably the mean diameter, or less, of the diameter of the aluminum oxide granules forming the body of porous plate members A through E. With this unique preionizing arrangement, the point on pointed electrode 32a which coacts with the surface on 32b to electrically stress the ambient gases adjacent the point of contact, is disposed in a direct line of sight view of the main spark gap defined in the arc chamber between plate members A and B. It will be appreciated that other materials, and different granular dimensions for the materials may be utilized without departing from my invention. Basically, it should be understood that in order to function in accordance with the teaching of my invention, it is necessary to form the block member 32b of a relatively hard, smooth, or substantially impervious material which will support the pointed end of electrode 32a in a position such that the electrically stressed area around the point on electrode 32a and the block 3211 will not be obscured from the spark gap by intervening insulating material associated with the walls of the arc chamber.

In order to apply a voltage across the preionizing means formed by pointed electrode 32a and insulating block 32b, the electrode 32a is positioned a predetermined distance from the main

spark gap electrodes

22 and 26. This arrangement provides capacitance coupling, shown by diagrammatic capacitors 33 and 34 (in FIG. 7), between the

electrodes

22 and 26 and the preionizing means. In the preferred form of my invention, this capacitance coupling results in adequate preionizing when electrode 32b is spaced approximately one-quarter of an inch from the respective main

spark gap electrodes

22 and 26. It will be understood that similar preionizing electrodes are utilized with the remainder of the main spark gaps in the spark gap assembly 11. However, since only the top surfaces of the remaining are chambers are shown, only the respective insulating blocks 32b for these remaining preionizing means are shown in FIG. 3.

In the operation of the spark gap assembly 11 and lightning arrester 1 of my invention, assuming the arrester 1 is electrically connected as shown in FIG. 1 between a power transmission line 8 by conductors 7 and 9 to ground; and further assuming a normal operating voltage is present on transmission line 8, the respective spark gaps in spark gap assembly 11 will not be conductive and no current will be discharged to ground through the arrester 1. However, when a high voltage surge is transmitted from conductor 8 through conductor 7 and terminals 3 and 13 to the spark gap assembly 11, the

capacitance couplings

33 and 34 between the respective main

spark gap electrodes

21, 22, 26 and 27 and the preionizing means 32a and 32b acts to raise the voltage stress on the ambient gas surrounding the pointed tips on electrodes 32a and the blocks of insulating material 32b which, thus, preionizes the main spark gaps. This preionizing effect causes the main spark gaps to break down essentially simultaneously so that the high voltage is immediately impressed across nonlinear valve resistor 10 which offers a low resistance discharge path to ground for the high voltage surge. After the high voltage surge has been discharged to ground, the relatively low frequency powerfollow current from conductor 8 is rapidly limited, and then completely terminated, by the combined action of the spark gap assembly 11 and the rapidly increasing resistance presented to the lower voltage follow current by nonlinear resistance 10. Referring to FIG. 7, it will be seen that when the arcing current is flowing across the respective

arc gap electrodes

21 and 22 from top electrode 26 to bottom electrode 27, a complete loop of arcing current is formed. The respective arcs between the electrodes of the four main spark gaps defined between plate member A, B, C, D and E are rapidly moved by the horn gaps associated with each spark gap along the generally linear are running horn gap portions 21' and 22 of the electrodes 20 thereby to increase the length of the arcs as they are driven toward the peripheries of the arc chambers where they contact the chamber walls and are cooled and extinguished.

Referring to FIG. 5, and FIG. 2, it will be seen that the respective electrodes 21 (26) and 22 defining the main spark gap between porous plate members A and B, for example, have first and second arc running surfaces 2122' and 21"-22" respectively, which each have generally linear portions that are disposed at an angle of more than 180 degrees with respect to one another. This particular electrode configuration causes an are moving up the horn gaps formed in assembly 11 to first run along the horn gap portion defined by are running surfaces 21'22' that are diverging at an angle of less than 50 degrees. After the arc has started to move it is driven more rapidly by the force of the increased concentration of magnetic flux behind it and as the arc bends from a nearly linear shape to a more arcuate shape, the horn gap angle increases rapidly from less than 50 degrees to more than 180 degrees, forcing the arc to be stretched linearly at the same time it is being arcuately extended. I have found that optimum arc lengthening action is afforded by my novel electrode elements 20 when the respective generally linear portions 21' and 22 are disposed at an angle of degrees 1-15 degrees with respect to each other. The unique electrode structure 20 in combination with the electrode positioning means, causes all of the four main spark gaps to move their respective arcs at substantially identical rates toward the periphery of their arcing chambers thereby to maintain a substantially equal voltage drop across each are so that the arcs are extinguished almost simultaneously and the chance of a restrike is greatly diminished.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that changes and modifications may be made therein without departing from the invention; therefore, it is intended by the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

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

1. A spark gap assembly comprising a plurality of stacked insulating plate members, electric terminal means disposed adjacent the top and bottom surfaces respectively of the top and bottom plate members in said stacked assembly, the remaining top and bottom surfaces of said plate members being formed to define arc chambers between each of the juxtaposed plate members in the stocked assembly, electrode positioning means on each of said plate members disposed on one of the surfaces thereof defining an arc chamber, each of said electrode positioning means being adapted to secure two separate electrodes in a predetermined spaced-apart relation thereby to form a spark gap between said electrodes, a

first electrode disposed in the arc chamber defined partly by the bottom surface of said top plate member, means electrically connecting said first electrode to the terminal means adjacent the top surface of said top plate member, a second electrode disposed in the arcing chamber defined partly by the top surface of said bottom plate member, means electrically connecting said second electrode to the terminal means adjacent the bottom surface of said'bottom plate member, a plurality of electrode elements each comprising a pair of spaced-apart electrodes connected by an integral electrically conductive link, the spacedapart electrodes of each of said electrode elements being disposed respectively in adjacent arcing chambers of said stacked assembly in a manner such that two separate electrodes are secured in each of said chambers by the respective electrode positioning means therein whereby a spark gap is defined between said separate electrodes in each of the chambers of the stacked assembly, each of said spark gaps being electrically connected in series by the integral links in said electrode elements.

2. A spark gap assembly as defined in claim 1 wherein each of said electrode positioning means comprises bosses on said one are chamber defining surface of each plate member said bosses being adapted to respectively engage the pair of separate electrodes disposed in the arc chamber therewith and secure said electrodes in a spaced-apart relation thereby to form one of said spark gaps between said electrodes.

3. A spark gap assembly as defined in claim 1 wherein each of said electrodes is provided with an are running surface, said arc runing surfaces being formed to define horn gaps respectively adjacent each of said spark gaps, the arc running surfaces of each of said horn gaps having first generally linear portions that diverge from each other at an angle of less than .50 degrees, the arc running surfaces of said horn gaps having second curvilinear portions that diverge from each other at progressively greater angles varying from less than 50 degrees to more than 180 degrees.

4. A spark gap assembly as defined in claim 3 wherein the respective first generally linear portions of the are running surfaces on the spaced-apart electrodes of each of said electrode elements are positioned to define an angle of 90 degrees, plus or minus 15 degrees, between the respective longitudinal axes of said generally linear portions, whereby the longitudinal axis of each horn gap defined by said generally linear first portions is substantially perpendicular to the longitudinal axis of each adjacent horn gap adjacent their respective spark gaps.

5. A spark gap assembly as defined in claim 1 wherein said electrode positioning means comprises a plurality of sets of bosses, each of said sets of bosses being disposed respectively in different arcing chambers on one surface of one of the plate members thereof in a manner such that each arc chamber contains one set of said bosses.

6. A spark gap assembly as defined in claim 5 wherein each of said sets of bosses comprises at least a pair of separate bosses, each of said separate bosses having a non-circular profile, complementary profile defining means on each of said electrodes adapted to closely engage said bosses and be secured in a predetermined position thereby.

7. A spark gap assembly as defined in claim 6 wherein each of said complementary profile defining means comprises means defining at least one aperture in each of said electrodes having a profile that closely complements the profile of a non-circular boss adapted to be closely engaged therewith.

8. A lightning arrester of the type comprising a tubular housing of insulating material, a pair of electric terminals disposed respectively adjacent opposite ends of said tubular housing, a spark gap assembly disposed in said housing, a valve resistor disposed in said housing, and means 10 electrically connecting said spark gap assembly in series with said valve resistor between the pair of terminals adjacent opposite ends' of said housing; wherein said spark gap assembly is as defined in claim 1.

9. A lightning arrester of the type defined in claim 8 wherein said spark gap assembly is as defined in claim 5..

10. A lightning arrester of the type defined in claim 8 wherein each of said insulating plate members is formed of porous, granular material, said porous plate members being adapted to receive in the interstices between its granules a major portion of the gas developed by arcs in the arc chambers defined by said plate members.

11. A spark gap assembly as defined in claim 1 wherein the electrodes on each of said electrode elements are in overlapping vertical alignment over a substantial portion of their respective surface areas.

12. A spark gap assembly as in claim 11 wherein each of said electrode elements is substantially identical in configuration.

1 3. A spark gap assembly as defined in claim 1 wherein each of said electrode elements is formed respectively of a single piece of conductive, generally flat, bar stock having a substantially uniform thickness, the pair of spaced-apart electrodes on each of said elements comprising enlarged portions of predetermined configuration at opposite ends of the element, said enlarged electrode portions being held in parallel planes by the integral link connected therebetween in a manner such that substantial portions of the respective electrodes overlap one another.

14. A spark gap assembly as defined in claim 13 wherein each of said enlarged electrode portions is of substantially identical configuration.

15. A spark gap assembly comprising at least two arc chambers stacked in substantial axial alignment, said chambers each having at least one inner wall formed of porous insulating material, a pair of spaced-apart electrodes defining a main spark gap in each of said chambers, means for electrically connecting the electrodes defining said spark gaps in series to an electric circuit, ionizing means for preionizing at least one of said main spark gaps, said ionizing means comprising; a pointed electrode disposed adjacent at least one of said main gaps, a relatively impervious block of insulating material positioned in contact with the point on said pointed electrode, and means for electrically energizing said ionizing means thereby to cause it to ionize the main spark gap.

16. A spark gap assembly as defined in claim 15 wherein said means for electrically energizing said ionizing means comprises a wireless capacitance coupling between at least one of the electrodes defining said main spark gap and the pointed electrode forming said ionizing means.

17. A spark gap assembly as defined in claim 15 wherein said arc chamber is defined between the top and bottom surfaces respectively of two porous plate members, said top and bottom surfaces each having recesses therein adapted respectively to receive and retain in operating position the relatively impervious block of insulating material and the pointed electrode defining said ionizing means.

18. A spark gap assembly as defined in claim 17 wherein said pointed electrode is formed of resilient conductive material and adapted to be force-fitted into the recess on one of said porous surfaces, said resilient pointed electrode being held in compression between the inner walls of said recess and said relatively impervious block of insulating material when said plate members are positioned in juxtaposition to define an arc chamber therebetween.

19. A spark gap assembly as defined in claim 17 wherein the porous plate members and the relatively impervious block are formed of particles of substantially the same substance, the particles forming said plate members having. a mean .diametrical dimension at least 25 times greater than the mean diametrical dimension of the particles forming said relatively impervious block.

20. A lightning arrester of the type comprising a tubular housing of insulating material, a pair of electric terminals disposed respectively adjacent opposite ends of said tubular housing, a spark gap assembly disposed in said housing, a valve resistor disposed in said housing and means electrically connecting said spark gap assembly in series with said valve resistor between the pair of terrninals adjacent opposite ends of said housing; wherein said spark gap assembly is as defined in claim 15.

References Cited UNITED STATES PATENTS 12/1959 Cunningham 31536 7/1966 Stetson 31536 X 9/1966 Sacer 315--36 1/l968: Qsterhout BIS-36 X U.S. C1. X.R.