US3414759A - Spark gap unit for lightning arresters - Google Patents

Description

. D66. 3, 1968 11R CONNELL ET AL 3,414,759

SPARK GAP UNIT FOR LIGHTNING ARRESTERS 5 Sheets-Sheet 1 I Filed Dec. 1, 1966 I INVENTOR L w L N Mr m 3 n P A RE sr MN AR Dec. 3, 1968 T R CQNNELL ET AL 3,414,759

SPARK GAP UNIT FOR LIGHTNING ARRESTERS 3 Sheets-Sheet 2 Filed Dec. 1, 1966 INVENT OR THOMAS R. CONNELL ROBERT E. PUTT 1'- lLLlHlllllhl FIG. 3

ATTORNEY Dec. 3, 1968 T, R, CQNNELL ET AL. 3,414,759

SPARK GAP UNIT FOR LIGHTNING ARRESTERS Filed Dec. 1, .l96 5 Sheets-Sheet 5 FIG. /0

INVENT OR THOMAS R. CONNELL ROBERT E. PUTT H648 m MM ATTORNEY United States Patent 3,414,759 SPARK GAP UNIT FOR LIGHTNING ARRESTERS Thomas R. Connell, Wadsworth, and Robert E. Putt,

Clinton, Ohio, assignors to The Ohio Brass Company,

Mansfield, Ohio, a corporation of New Jersey Filed Dec. 1, 1966, Ser. No. 598,467 11 Claims. (Cl. 315-36) ABSTRACT OF THE DISCLOSURE A spark gap unit for lightning arresters with a cen tral gap plate and an end plate on each side thereof defining spark chambers for two series connected spark gaps. It provides the functions and results of the prior art many-plate gap units with improved manufacturing procedures.

This invention relates to spark gaps for lightning arresters, voltage discharge devices, are switches, and the like.

A principal object of the invention is to simplify manufacture of spark gaps embodying stacked assemblies of ceramic gap plates.

Another object of the invention is to increase the mechanical strength of spark gaps and ceramic gap plate arrangements and to facilitate the use of porous ceramic materials in gap plates.

Another object of the invention is to increase the arc voltage and electrical strength of arc gaps for lightning arresters and the like.

In spark gaps for lighting arresters and the like, it is desirable to utilize large numbers of relatively small gaps to achieve the necessary voltage and current capability for a given arrester. A large number of parts, all of the same size and construction, may be manufactured and then associated in assemblies having electrical characteristics determined by the number of gap elements in the assembly. While providing many advantages, this type of spark gap presents certain design problems inasmuch as it is difiicult to associate the gap elements in a determinate way to produce a mechanically strong assemblage of arrester elements which can be provided with an insulated housing in a compact space. This is an important requirement since the arrester elements are arranged in stacks and columns and the various functions must be provided at an economically acceptable cost.

According to the present invention, three ceramic gap plates are associated to form two gap chambers and to provide an enclosure for two pairs of gap electrodes. One of the gap plates, referred to as the center plate, is formed on the opposite sides thereof to cooperate with the remaining two plates, referred to as the end plates, and the adjacent surfaces of the center plate and end plates are spaced apart to form two separate arc chambers. The gap electrodes are positioned so that the arc gap for initial sparking is near the center of the gap plate, and the two pairs of electrodes are rotated through 180 degrees in a modified helical arrangement in accordance with the teachings of US. Patent 3,019,367, issued Jan. 30, 1962. Assemblies of gap plates are arranged in stacks with the associated magnetic coils and valve blocks, as described in the patent.

The invention, togethef with further objects, features, and advantages thereof, will be understood from the following detailed specification and claims taken in connection with the appended drawings in which:

FIG. 1 is a side elevation view, in section, of a lightning arrester embodying the spark gap of the instant invention;

3,414,759 Patented Dec. 3, 1968 FIG. 2 is a schematic diagram of the electrical circuit of the lightning arrester of FIG. 1 and the associated conductor and ground with which it is used;

FIG. 3 is a side elevation view of the spark gap of the invention;

FIG. 4 is a top plan view of the spark gap, taken in the direction 4-4 in FIG. 3;

FIG. 5 is a top plan view of the bottom gap plate of the spark gap of FIG. 3, taken along the line 5--5 in FIG. 3;

FIG. 6 is a section view of the bottom gap plate, taken along the line 6-6 in FIG. 5;

FIG. 7 is a section view of a gap plate assembly, taken in the direction 7-7 in FIG. 4;

FIG. 8 is a section view of a gap plate assembly, taken along the line 88 in FIG. 4;

FIG. 9 is a view of a center gap plate, taken along the line 99 in FIG. 3; and

FIG. 10 is a view of the opposite side of the center gap plate of FIG. 9, taken along the line 10-10 in FIG. 3.

Referring now to FIG. 1, the lightning arrester 10 embodying the invention comprises an elongated housing 11 of porcelain provided on, the outside with a plurality of skirts 12 for increasing the leakage distance along the outside of the housing, and formed on the interior with an opening 13 longitudinally through the housing for receiving the operative elements of the arrester. The ends of the housing 11 are closed by formed metal members 14 and 15 which engage the housing member along peripheral grooves 16 and 17 adjacent the end of the housing. The joints between the members 14 and 15 and the end faces of the housing 11 are sealed by means such as O- rings 18 and 19 for maintaining the interior of the housing separate from the exterior thereof. The end parts 14 and 15 are designed to enclose a stack of operative arrester elements 20 in the opening 13 of the housing, and the entire assembly is carried on end pieces (not shown herein) which are cemented over the ends of the housing and over the end pieces 14 and 15.

The operative elements of the arrester are arranged in a stack comprising two piles of non-linear resistors or valve blocks 21 and 22 and a plurality of spark gaps or gap assemblies 23, 24, 25, and 26, each assembly including a magnetic coil unit, a plurality of gap plates, and grading devices, as hereinafter described. The valve blocks and gap assemblies are electrically connected by mechanical contact between the adjacent elements by metal plates which are attached to the elements at the opposed faces thereof, the arrester elements being maintained in compressive relation with each other and with the end members 14 and 15 by means such as a spring 27.

FIG. 2 shows the circuit arrangement of the lightning arrester 10 connected between an electrical conductor 30 and a ground 31 according to its intended use for discharging overvoltages from the conductor 30 to the ground 31 in a protective circuit 32. The actual protective circuit includes the entire arrester 10 while the circuit 32 of FIG. 2 illustrates only that portion of the operative elements of the arrester 10 which includes the gap assembly 26 and valve blocks 22. The remaining ones of the gap assemblies 23, 24, and 25 are the same as the gap assembly 26, and the valve blocks 21 are the same generally speaking, as the valve blocks 22.

The spark gap apparatus 33, representing the gap assembly 26, comprises four main gaps 34, 35, 36, and 37 connected in series and having certain associated gaps, resistors, and capacitors connected in parallel to constitute four gap control stages, referred to generally as S1, S2, S3, and S4, all connected in series with a coil 38,

3 associated with the gaps 34 to 37. The valve blocks 22 are representative as a valve resistor 39.

The coil 38 comprises :a means for generating a magnetic field to elongate the arcs and the several main gaps 34 to 37. Two fixed gaps 40 and 41 are connected in shunt with the coil 38 and function during initial dis charge of energy from the line 30 to the ground 31, through the gaps 34 to 37. After a suflicient time interval, current flow in the coil 38 extinguishes the arcs in the gaps 40 and 41 so that an increasing magetic field is generated by the coil for moving the arcs in the gaps 34 to 37. The .gaps 40 and 41 may be replaced by arrangements of non-linear resistors, as is described in U.S. Patents 2,825,008 and 3,019,367 to J. W. Kalb.

In the apparatus 33, the gap control stage S1 is constituted by a main gap 34, a switching gap 42 having series connected switching resistors 43 and 44, and a grading resistor 45. Gap control stage S2 comprises a main gap 35, a switching gap 46 with series switching resistors 47 and 48, and a grading resistor 49. The gap control stage S3 comprises a main gap 36, a grading capacitor 50, and a grading resistor 51. The gap control stage S4 comprises a main gap 37, a switching gap 52 with series resistors 53 and 54, and a grading resistor 55. The described parts are connected in parallel circuits to constitute the designated stages.

The stages S1, S2, and S4 have the same grading resistances, that is, the grading resistors 45, 49, and 55 are equal, whereas the resistance of the grading resistor 51 of stage S3 is small relative to resistors 45, 49, and 55. The switching gaps 42, 46, and 52 are designed and adjusted to spark over at substantially the same voltage, less than the breakdown voltage of the main gaps, but the associated series resistors, that is, the resistors 43 and 44 of the gap 42, the resistors 47 and 48 of the gap 46, and the resistors 53 and 54 of the gap 52, are progressively smaller for the successive stages so that an unequal voltage division exists as between the main gaps 34, 35, and 37 after the'switching gaps have fired.

The switching gaps 42, 46, and 52 are small gaps having precisely dimensioned electrodes positioned adjacent the main gaps 34, 35, and 37. The switching gaps function as preionizers for the main gaps as well as a switching means for the several switching stages, and are adapted for discharge in a narrow, predetermined range of voltages over the range of design tolerances. In production, one of the gaps 42, 46, :and 52 is selected to spark over at a voltage close to the design voltage, and minor production variations in the remaining switching gaps do not materially alter the overall functioning, inasmuch as the gap 42 determines the firing sequence of stages because of the larger values of resistors 43 and 44.

In operation, the line voltages are divided between the several main gaps 34 to 37 in the same proportions as the resistance of the individual grading resistors 45, 49, 51, and 55 to the total grading resistance constituted by the four resistors. Accordingly, with the grading resistances heretofore described and, prior to discharge of the switching gap 42, the line voltage is divided with equal voltages across stages S1, S2, and S4 and a much lesser voltage across S3.

Upon the occurrence of an overvoltage on line conductor 30, the switching gaps 42, 46, and 52 spark over and connect the switching resistors 43 and 44 in parallel with the grading resistor 45, the switching resistors 47 and 48 in parallel with the resistor 49, and the switching resistors 53 and 54 in parallel with the grading resistor 55. The resistors 43 and 44 are substantially larger than the resistors 47 and 48, which :are in turn substantially larger than the resistors 53 and 54. Accordingly, after the switching gaps 42, 46, and 52 have fired, the distribution of line voltage as between the stages S1, S2, and S4 is altered so that a greater proportion of the line voltage appears across the main gaps 34, 35, and 37, in that order, in accordance with the inequality between the parallel resistances of the stages S1, S2, and S4. Accordingly, the gap 34 fires first, followed by the gap and the gap 37, whereupon all the line voltage appears across the gap 36 which is fired last. This completes the firing sequence and initiates the discharge sequence in which transient energy is discharged from the line conductor 30 to the ground 31, through the valve resistor 39. In the discharge sequence, the coil 38 functions to elongate the, arcs of the main gaps 34 to 37 to terminaate the discharge, all as described in U.S. Patents 2,825,008 and 3,019,367 heretofore referred to.

'Ilhe functioning of the spark gap apparatus just described occurs when the lightning arrester is subjected to line overvoltages which have slowly rising wave fronts, or switching surges, so called. For line overvoltages which have rapidly rising wave fronts, or impulse overvoltages, so called, the grading capacitor functions to distribute the line voltage between the gaps 34, 35, and 37 so that those gaps are fired, whereupon the line voltage is impressed across the gap 36 which is fired last. The construction and functioning of the gap control circuit, to gether with other circuit arrangements useful in the practice of the present invention, is described in detail in co-pending U.S. application, Ser. No. 585,846, filed Oct. 11, 1966.

The gap assemblies 23, 24, and 25 function in the same way as the gap assembly 26 just described. The several assemblies function simultaneously because of the series connections of the circuits and because of grading resistors, such as the resistors 56 and 57 connected across 1 the gap assemblies 23 and 26, for distributing the impressed voltage substantially equally between the several gap assemblies.

"The gap assembly 26 is shown in an enlarged exterior View in FIG. 3. As there shown, a plurality of end gap plates 60, 61, 62, and 63 cooperate with associated center gap plates 64 and 65 to enclose and define spark gaps, associated arc chambers, and control and discharge circuits of the respective stages S1 to S4 of FIG. 2. The end plates 60 to 63 engage the associated center plates 64 and. 65, along peripheral ridge and recess means and interengage the end faces of a plate member 66 which carries the coil 38 at the center position of the six gap plates. The assembly is arranged in the form of a stack and the elements are connected in series with two end plates 67 and 68 which connect the gap assembly to the adjacent operative elements of the stack 20 or to the terminals of the arrester housing.

The invention will be understood by reference particularly to FIG. 5 and FIGS. 7 to 10 inclusive of the drawing illustrating the construction of the gap plates 62, 63, and 65. FIG. 5 illustrates the gap plate 63 which is substantially identical with gap plates 60 and 61 and identical with the gap plate 62 except in the substitution, in that plate, of the capacitor 50 for a switch gap and switching resistors in the plate 63.

In FIG. 5 the gap plate 63 comprises a circular ceramic plate having a circumferential ridge 69 with the interior surface 70 of the plate defining an arc chamber 71. Two main gap electrodes 72 and 73, comprising the main 1 gap 37, are carried by the plate 63 and the plate 65,

respectively, as is hereinafter described. Two elevated portions or ridges 74 and 75, integral with the body of the plate 62, extend above the surface 70 intermediate between the level of that surface and the level of the ridge 69. The ridges 74 and 75 extend from adjacent the interior of the electrodes 72 and 73, transversely of the gap plate to the ridge 69, and define the circumferential extremities of the arc chamber71 and the limit of elongating arc movement from the adjoining arcing faces of the electrodes 72 and 73, The ridge 69 extends circumferentially only to the ridges 74 and 75, and a recess 76 is disposed along the periphery of the plate 63 between the ridges 74 and 75 for receiving a ridge on the gap plate 65, as is hereinafter described.

A formed ceramic member 77 is received between the ridges 74 and 75 and is secured to a surface 78 between the ridges 74 and 75, disposed somewhat below the level of the surface 70. The member 77 carries two electrodes 79 and 80 which constitute the switching gap 52 and the switching resistors 53 and 54. The electrodes 79 and 80 are formed as cylindric bodies of resistive material which are received in recesses in radially extending portions of the member 77 and have the interior ends closely spaced at a predetermined distance apart to constitute the gap 52, the bodies of the electrodes constituting the series switching resistors 53 and 54. The grading resistor 55 is carried at the extremity of the member 77, and the lead wires of the resistor 55 are connected to the electrodes 79 and 80 and to the electrodes 72 and 73 to constitute the parallel circuit of stage S4.

The gap plate 63 is formed on the interior side thereof, illustrated in FIG. 5, to cooperate with the adjacent side of the gap plate 65, illustrated in FIG. 10. As shown in FIG. 10. the gap plate 65 is formed as a body having 'a fiat interior surface 81 which cooperates with the surface 70 of the plate 63 to form the gap chamber 71, a recess 82 from the surface 81 extending peripherally about the plate 65 for receiving the ridge 69 of the plate 63, two recesses 83 and 84 for receiving the ridges 74 and 75 of the plate 63, a surface 85 spaced from the surface 78 of the plate 63 for receiving the member 77 and the associated parts, and a ridge 86 extending along the peripheral extremity of the plate 65, between the recesses 83 and 84, for engaging the recess 76 in the plate 63. Very shallow recesses 87 and 88 are formed in the surface 81 of the plate 65 for receiving and positioning the electrodes 72 and 73 and the plates 63 and 65.

A surface 89 recessed from the surface 81 of the plate 65 cooperates with a like surface 90 recessed from the surface 70 of the plate 63 to constitute an arcing chamber about the adjacent portions of the electrodes 72 and 73, and the recesses 89 and 90 converge to the surfaces 81 and 71 along surfaces 91 and 92, respectively, to constitute an arc entrance chamber connecting the arcing chamber with the arc elongation chamber defined by the surfaces 70 and 81 extending radially outward from the electrodes 72 and 73. The surfaces 70 and 81 are parallel and spaced apart throughout the extent of the arc elongation chamber, as set forth in co-pending application Ser. No. 624,375, filed Mar, 20, 1967 and the patents previously referred to.

As will be evident from FIG. 7, FIG. 8, and FIG. 10, that portion of the gap chamber 71 just referred to as the arc elongation chamber extends radially from the electrodes 72 and 73 throughout the circumferential extent of the gap plates between the barrier ridges 74 and 75. Adjacent the radial extremity of the plates, the gap chamber 71 is defined by a surface 93 constituting the interior surface of the recess 82 of the plate 65, and a surface 94 comprising the interior surface of the ridge 69 of the plate 63. As shown in FIG. 7, the surfaces 93 and 94 extend generally in the longitudinal direction of the gap assembly 26 from the radial extremity of the surfaces 70 and 81, and converge together from the parallel spacing of the surfaces 70 and 81. The convergent surfaces 93 and 94 define an arc extinguishing chamber of the gap chamber 71.

The arcing chamber defined by the surfaces 89 and 90, the arc entrance chamber defined by the surfaces 91 and 92, and the arc elongation chamber defined by the surfaces 70 and 81 correspond, respectively, to the arc recess, arc entrance chamber, and arc extinguishing chamber of the spark gaps described in US. Patents 2,825,008 and 3,019,367 above referred to. Accordingly, it will be understood that the arc elongation chamber defined by the surfaces 70 and 81 performs the function performed by that portion of the spark gaps described as the arc extinguishing chamber in the subject patents and that the function of the arc extinguishing chamber defined by the surfaces 93 and 94 is in addition thereto.

The gap stage S3 is constituted by the gap plate 62 and the gap plate 65, which define the gap chamber 96 for the gap electrodes 97 and 98. The gap chamber 96 and electrodes 97 and 98 (FIG. 7 and FIG. 8) are rotated about the longitudinal axis of the gap assembly 26, through an angle of 180 degrees with respect to the gap electrodes 72 and 73 and are chamber 71. The relative disposition of the two surfaces of the plate 65 will be understood from FIG. 9 and FIG. 10, which are representations taken by rotating the opposed faces of the plate 65 about the lateral axis and into the plane of the paper. The electrodes 97 and 98 constitute the gap 36, and the grading resistor 51 and capacitor 50 are mounted on a member 99, similar to the member 77 carried by the plate 63, in the positions of the electrodes 79 and and resistor 55, respectively, of the plate 63 and member 77. The gap chamber 96 is formed in the same way as the gap chamber 71, including arcing chamber, arc entrance, arc elongation chamber defined by surfaces 100 and 101, and an arc extinguishing chamber defined by surfaces 102 and 103, of the plates 62 and '65 respectively.

According to the invention, and referring to FIG. 7, the gap electrodes 72 and 73 are secured to the end plate 63, the gap electrodes 97 and 98 are secured to the end plate 62, and the gap electrodes 73 and 98 are secured together to hold the gap plates 62, 63, and 65. To this end, the gap electrodes 72 and 97 are secured to the metal end plates 68 and 104 by metal rivets 105 and 106 extending from the end plates through the bodies of the gap plates 63 and 62 to the gap electrodes. The gap electrodes 73 and 98 are secured together and to the center plate 65 by means of a rivet 107 extending through the electrodes and through the body of the center gap plate 65.

In manufacturing the assembly comprising the gap plates 62, 65, and 63, the gap plates are first made by pressing particulate ceramic material into the shape and form shown. The formed gap plates are then fired until the desired degree of rigidity and porosity is achieved. The end gap plates 62 and 63 are prepared by coating the adjacent surfaces of the electrodes and gap plates with epoxy resin, and then assembled with the metal end plates. The assembly is heated to a predetermined temperature to cure the resin and bond the metal parts to the ceramic gap plate. In the gap plate 63, for example, the pin 106 of the end plate 63 is then riveted into place to connect the gap electrode 72 to the end plate '68 and to secure the electrode and end plate to the gap plate 63. The end gap plates are then tested to determine the breakdown volt-age, and those plates having breakdown voltages close to the design voltage are segregated for use in stage S1. The remaining gap plates, within a wider range of breakdown voltages, are used for stages S2, S3, and S4.

The assembly comprising the member 77 with associated electrodes 79 and 80 and grading resistor 55, or grading capacitor for stage S3, is attached to the end gap plate by bonding the member 77 to the body of the end plate 63 by means of epoxy resin. The resin is cured and the leads from the electrodes 79 and 80 soldered to the electrodes 72 and 73.

The end gap plates are then assembled with the center gap plate, the adjoining faces of the peripheral ridges and recesses 69 and 82 and 76 and 86 being coated with epoxy resin to bond the gap plates together. The pin 107 is then inserted through the openings 108 and 109 in the end plates and the opening 110 in the center plate, and riveted into place to secure the gap electrodes 73 and 98 to the center plate and hold the end plates against the center plate while the resin along the adjoining surfaces of the ridges and recesses hardens to form a bond between the parts and to seal the interior of the gap. Alternatively, the plates are held in a suitable jig while the resin is cured, and the heads are then formed on the pin 107 to make an electrical or an electrical and mechanical connection between the gap electrodes 73 and 98. The openings 108 and 109 are then sealed with porcelain or other ceramic plugs and epoxy resin.

In another embodiment, the pin 107 may be formed of a cOntact or other conductive material having suitable elastic properties and the head omitted.

The plate assembly or spark gap comprising the plates 62, 65, and 63 is then combined with a suitable plate assembly, comprising the gap plates 60, 64, and 61, and a coil assembly comprising plate 66 and coil 38 to constitute the gap assembly 26.

The several gap assemblies 23 to 26 are arranged in stacked superimposed relation in the housing 11, with the gradin resistor 57 connected to the metal end plates 67 and 68 of the assembly 26, and similar grading resistors connected to the remaining gap assemblies. The gap assemblies are connected in series with valve blocks 21 and 22 to constitute the operative elements of the lightning arrester 10, and all connected to the end parts 14 and 15. The housing is then closed and sealed, and suitable end parts comprising terminal and support members are cemented in place over the ends of the housing.

It will be understood that the gap assembly 26 has important advantages in the construction of the lightning arrester of FIG. 1. First, the arrangement of the plate assembly with two end plates cooperatin with a single center plate formed on opposite sides thereof to constitute an arc chamber with each of the associated end plates, provides a spark gap unit having a relatively short extent along the longitudinal axis of the stack in relation to the arc voltage generated by the gap and in relation to the energy dissipating capability of the gap. Again, the three-plate construction embodies relatively thick gap plates and permits the use of materials of relatively low modular strength so that the gap plates may be constructed of porous ceramic materials. The resultant structure utilizes the desirable properties of porous materials in arc voltage generation and are extinction and provides height saving of importance in lightning arrester-s for very high voltages.

Another advantage of the three-plate construction described herein is that manufacture of the plates themselves is simplified and expedited. Thus, the end plates 62 and 63 are of identical construction so that only two kinds of dies are required, one for the end plates and one for the center plates. Again, the modular arrangement of the parts, in which the end plates are formed with one fiat surface on one side and elevated surfaces or projections from a central surface on the other side, and the center plate is formed with recesses on both sides, simplifies die design and facilitates molding and pressing of particulate ceramic materials.

It is to be understood that the foregoing description is not intended to restrict the scope of the invention and that various rearrangements of the parts and modifications of the design may be resorted to. The following claims are directed to combinations of elements which embody the invention or inventions of this application.

We claim:

1. A spark gap for an overvoltage protective device comprising three gap plates of rigid ceramic material, including an integral center plate, a first end plate on one side of the center plate, and a second end plate on the opposite side of the center plate and each cooperating therewith, a first pair of two gap electrodes arranged between the first end plate and the center plate having laterally spaced outwardly divergent arcing faces, and the adjacent faces of the first end plate and center plate being spaced apart to constitute an arc chamber for receiving an are formed between the two gap electrodes, a second pair of two gap electrodes arranged between the second end plate and the center plate, the adjacent faces of the second end plate and center plate being spaced apart to constitute an arc chamber for receiving a second are formed between the two gap electrodes, means securing each pair of electrodes to the adjacent end plate, means extending through the center plate from one electrode of one pair of electrodes to one electrode of the remaining pair of electrodes electrically connecting the said one electrode, and means securing the two end plates to the center plate.

2. A spark gap in accordance with claim 1, in which the last two named means comprise a pin mechanically connecting the said one electrode.

3. A spark gap in accordance with claim 1, in which the last two named means comprises a metal pin extending through the said one electrode and center plate, and heads on the pins engaged with the exterior surfaces of the said electrodes for compressively holding the electrodes against the center plate.

4. A spar-k gap in accordance with claim 3, in which there are openings in the end gap plates aligned with the pin for receiving the pin during assembly of the gap plates.

5. A spark gap in accordance with claim 1, in which the said two electrodes are each bonded to the associated end plate in predetermined spaced relation for predetermining the sparkover voltage of the spark gap after assembly thereof.

6. A sparkgap in accordance with claim 5, in which there is a metal end plate over the exterior surface of the ceramic end plate, and the remaining one electrode of each pair of electrodes is electrically connected to the metal end plate by a metal pin extending through the ceramic plate.

7. A spark gap in accordance with claim 1, 'with interengaging ridge and recess means extending along the peripheral extremities of the gap plates for closing the lateral extremities of the gap chambers, and the lastnamed means comprises means bonding the plates along the adjacent faces of the ridge and recess means and closing the arc chambers thereof.

8. A spark gap for an o vervoltage protective device comprising three gap plates having the bodies thereof formed of rigid ceramic material, including an integral center plate, a first end plate on one side of the center plate, and a second end plate identically the same as the first end plate on the opposite side of the center plate and each cooperating therewith, a first pair of two gap electrodes arranged between the first end plate and the center plate having laterally spaced outwardly divergent arcing faces, and the adjacent faces of the first end plate and center plate being spaced apart to constitute an arc chamber for receiving an are formed between the two gap electrodes, 21 second pair of two gap electrodes arranged between the second end plate and the center plate, the adjacent faces of the second end plate and center plate being spaced apart to constitute an arc chamber for receiving a second are formed between the two gap electrodes, means securing each pair of electrodes to the adjacent end plate, means on the end gap plates comprising n'dges formed of ceramic material integral with the body of the plate projecting from the said adjacent face of the end gap plate, and means on the center plate formed as recesses in the body of the plate from the said adjacent faces of the center plate for receiving the ridges of the end plates, and means bonding the plates along the adjacent faces of the ridge and recess means.

9. A spark gap in accordance with claim '8, in which the gap electrodes, are chamber, and ridge and recess means of the second end plate and center plate are rotated through degrees about a longitudinal axis through the plates with respect to the gap electrodes, are chamber, and ridge and recess means of the first end plate and center plate.

10. A spark gap in accordance with claim 9, in which the gap electrodes are in contact with the center plate and the associated end plate adjacent the transverse cen- 9 10 tral portion of the plates for mutual reinforcement of the References Cited plates along the said longitudinal axis thereof. UNITED STATES P 11. A spark gap in accordance with claim 8 or claim 9, in which the electrodes are bonded to the end plates, the 3 07 114 1 19 3 Hicks 315 3 X two pair of electrodes are connected in series by means 5 3,242 37 3 19 Schultz 315 3 extending through the center plate from one electrode of one pair of electrodes to one electrode of the remain- JAMES LAWRENCE P'lmary Exammer' ing pair of electrodes. C. R. CAMPBELL, Assistant Examiner.

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