US3819987A

US3819987A - Sparkover stabilizing means for an ungraded surge voltage arrester

Info

Publication number: US3819987A
Application number: US00326790A
Authority: US
Inventors: E Sakshaug
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1973-01-26
Filing date: 1973-01-26
Publication date: 1974-06-25
Anticipated expiration: 1991-06-25

Abstract

An ungraded surge voltage arrester having a plurality of series connected sparkgaps is provided with sparkover stabilizing means that are effective to raise the minimum power frequency sparkover level of the arrester without significantly changing its maximum impulse sparkover voltage. The stabilizing means includes a tapered arrangement of different length sparkgaps in combination with a sparkgap preionizer mounted to ionize the sparkgap having the shortest gap length. Maximum length sparkgaps are disposed adjacent the high voltage terminal of the arrester.

Description

United States Patent [191 Sakshaug SPARKOVER STABILIZING MEANS FOR AN UNGRADED SURGE VOLTAGE ARRESTER [75] Inventor: Eugene C. Sakshaug, Lanesborough,

Mass.

[73] Assignee: General Electric Company, Pittsfield, Mass. 22 Filed: Jan. 26, 1973" [211 Appl. No.: 326,790

[52] US. Cl. 317/70, 315/36 {51] Int. Cl. H02h 9/06 [58] Field of Search 317/70, 69; 315/36 [56] References Cited UNITED STATES PATENTS 2,032,566 3/1936 Earle 317/70 X 2,179,297 11/1939 Johnson...

3.259.780 7/1966 Stetson 315/36 X 1 1 June 25, 1974 3/1970 Osterhout 315/36 X 10/1971 Osterhout et al. 317/70 Primary Examiner-James D. Trammell Attorney, Agent, or Firm-F. X. Doyle; Vale P. Myles; V. R. Ulbrich l 5 ABSTRACT An ungraded surge voltage arrester having a plurality of series connected sparkgaps is provided with sparkover stabilizing means that are effective to raise the minimum power frequency sparkover level of the arrester without significantly changing its maximum impulse sparkover voltage The stabilizing means includes a tapered arrangement of different length sparkgaps in combination with a sparkgap preionizer mounted to ionize the sparkgap having the shortest gap length. Maximum length sparkgaps are disposed adjacent the high voltage terminal of the arrester.

11 Claims, 10 Drawing Figures PATENTEU JUN 25 I974 sum 1 0M SHEU 2 BF 4 LIIIEV GROUND V HOUSING [El/67b PATENTED JUN 25 1974 SHEET 3 OF 4 f lllllll |l w=ll1l T L T lllll |\l SPARKOVER STABILIZING MEANS FOR AN UNGRADED SURGE VOLTAGE ARRESTER BACKGROUND 1. Field of the Invention This invention relates to an improved surge voltage arrester, and more particularly, to a means for stabilizing the sparkover level of surge voltage arresters that are operated in environments that contaminate their insulating housings.

2. Description of Prior Art Modern surge voltage arresters of the type used to protect the insulation and apparatus on electric power transmission and distribution systems usually comprise a plurality of sparkgaps electrically connected in series with one or more blocks of nonlinear resistance electrical valve material that are housed within a weather resistant housing such as, glazed ceramic. Often the operating environment of such arresters are such that the exterior surface of the arrester housings become s'everely contaminated during their normal operating life. Due to the high voltage gradient that is applied across the arrester housings this type of surface contamination can operate to cause erratic sparkover of the arresters if some means is not provided to stabilize their sparkover voltage. The problems associated with contamination of arrester housings have long been recognized and a range of various solutions have been proposed. For example, it is common practice in the field of high voltage arresters to provide a voltage grading network electrically connected in parallel with the plurality of sparkgaps used in station-type surge voltage arresters to uniformly distribute the voltage gradient across each of the sparkgaps. Such voltage grading circuits have proven to be very successful in counteracting the problem of erratic sparkover due to contamination of the arrester housing; however, the cost of voltage grading circuits is sufficiently high to suggest the use of alternative means, if possible, to solve this problem; particularly in distribution type surge voltage arresters that would not otherwise require the use of a voltage grading circuit to assure proper operation of the arrester.

In the field of ungraded distribution-type surge voltage arresters it is generally known that when a plurality of sparkgaps are serially connected within the arrester between a line terminal and a ground terminal the sparkgaps nearest to the line terminal will be subjected to the greatest voltage gradient. This phenomenon is due primarily to the fact that all of the leakage current in the sparkgap assembly must flow through the line terminal sparkgap; whereas lesser amounts of leakage current flow through each of the other gaps due to the capacitive coupling between the respective sparkgaps and the insulating arrester housing. Accordingly, if some form of sparkgap preionization is used to stabilize the sparkover level of such a surge voltage arrester, the preionization means is usually mounted adjacent the sparkgap that is connected to the line terminal because it will normally be the first gap to sparkover.

It is also known by those familiar with surge voltage arresters that the sparkover level of an ungraded, multisparkgap arrester can be raised by increasing the gap length of the sparkgap, or sparkgaps, next adjacent to the arrrester line terminal. An example of such a surge voltage arrester is shown in US. Pat. No. 2,179,297 Johnson, which issued on Nov. 7, 1939. Another,

somewhat similar, prior art approach to the problem of erratic surge voltage arresters sparkover due to contamination of the arrester housing is shown in U.S. Pat. No. 2,688,715 Vorts et al., which issued on Sept. 7, 1954. In the arrester arrangement disclosed by Vorts et al., a large number of sparkgaps is arranged immediately adjacent to an arrester line terminal while a smaller number of sparkgaps is disposed closer to the ground terminal of the arrester thereby to at least partially counteract the inherent voltage distribution of the arrester due to leakage currents flowing through the sparkgap assemblies and the capacitive couplings to the arrester housing and to ground. One major shortcoming of such prior art attempts at counteracting the ef fects of arrester housing contamination is that they fail to provide a stable, relatively low impulse sparkover level while maintaining powerfrequency and impulse sparkovers that do not fall below a desired minimum when the arrester housing is contaminated. As pointed out above, it is common practice to stabilize the impulse sparkover level of ungraded distribution arresters by providing a sparkgap preionizer mounted in operative relationship to the sparkgap that is connected to the line terminal of the arrester. However, when the gap length of the spark gap directly connected to the line terminal is increased to such a point that the arrester minimum sparkover does not fall below the desired point when the arrester housing is contaminated, the maximum impuse sparkover of the arrester is increased to an undesirable level. Although it is generally known to mount sparkgap preionizing means adjacent sparkgaps that are remote from a line terminal sparkgap in a series discharge circuit, in the manner shown for example, in US. Pat. No. 3,169,208 Harrington, which issued Feb. 9, 1965, it is practically necessary from an economic standpoint to rely on the line voltage as a source of preionizer energy in distribution-type lightning arresters. Thus, relatively complex preionizing circuits as that disclosed in the Harrington patent are not generally suited for application in distribution class surge voltage arresters of the type commercially utilized to protect electric power distribution systems.

SUMMARY OF THE INVENTION It is a general object of this invention to provide a new and improved ungraded surge voltage arrester that has a stable impulse voltage sparkover level and that is highly resistant to erratic sparkover at voltages lower than its power frequency rating, even when the arrester housing becomes contaminated.

It is a further object of this invention to provide a new and improved surge voltage arrester, having the abovedescribed characteristics, which is economical to construct and simple to manufacture.

In a preferred form of the invention, a surge voltage arrester is provided comprising a hollow elongated insulating housing having a line terminal at one end and a ground terminal at the opposite end, with a plurality of sparkgaps mounted in spaced-apart, serial relationship inside the housing. A non-linear electrical valve serving as a current limiting resistor is electrically connected in series with the sparkgaps, and the sparkgaps and valve are electrically connected between the line and ground terminals to form an overvoltage discharge path. The sparkgap immediately adjacent and electrically connected to the line terminal has a gap length greater than the gap length of any other sparkgap in the arrester. The sparkgap immediately adjacent and electrically connected to the ground terminal has a sparkgap preionizer mounted in operative relationship with it to stabilize the impulse sparkover level of the arrester.

Alternative embodiments of the invention disclosed herein deal withthe lengths of other sparkgaps than the sparkgap adjacent the line terminal. Generally, in these embodiments, sparkgaps are positioned in a tapered arrangement decreasing from a maximum gap length at the top, or line terminal of the arrester to the sparkgap having the shortest gap length at the ground terminal end of the arrester.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the invention may be had by referring to the accompanying detailed description and drawings in which:

FIG. 1 is a sectioned side view of a surge voltage arrester illustrating a plurality of sparkgaps arranged with various different gap lengths, in combination with a preionizer mounted adjacent the ground terminal end of the arrester pursuant to the invention;

FIG. 2 is a schematic cirucit representation of the surge voltage arrester depicted in FIG. 1;

FIG. 3 is a sectioned side view of a surge voltage arrester shown to approximately true scale;

FIG. 4 is a schematic representation of the capacitance network of the arrester housing and internal parts;

FIGS. 5 and 6 are graphs of voltage distribution versus housing length for the arrester shown in FIG. 3; and

FIGS. 7, 8, 9 and 10 are schematic representations of further embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a surge voltage arrester 1 having an elongated hollow insulating housing 10, made of a suitable material such as glazed porcelain, which provides a strong, weather resistant protection for the arrester elements mounted in it. At the outset, it should be understood that well-known, conventional materials and structural configurations may be used in constructing an arrester embodying the teaching of the present invention. For example, housing, electrode and valve materials and configurations similar to those disclosed in U.S. Pat. No. 3,614,523 Stetson et al., which issued on Oct. I9, 1971 and is assigned to the same assignee as the present invention, may be employed when arranged as disclosed herein.

An electrically conductive, metal line terminal 11 at the upper end of the housing is adapted to be connected to an electrical power line conductor 50 as shown, thereby to operatively couple the arrester to such a source of high voltage. A similar conventional ground terminal 12 is mounted in watertight relationship over the lower end of the housing 10 and, as shown, is suitably adapted to be connected to ground potential.

A plurality of brass electrodes, 13 through 18, are mounted, respectively, in spaced-apart relationship between disc shaped insulators a, 20b, 20c, 20d and 20e, which have

apertures

21a, 21b, 21c, 21d, and 212 formed therein. A plurality of sparkgaps are formed between the raised, or boss-like, portions, of each pair of electrodes that face one another through one of the apertures. These sparkgaps have predetermined gap lengths that are established by the thickness of the insulating discs. For example, the pair of

electrodes

13 and 14 is maintained in a spaced-apart relationship by the insulator 20a and forms a sparkgap 22 that is the sparkgap closest to line terminal 11.

A plurality of nonlinear

electrical valves

24a, 24b and 240 are positioned within the insulating housing as shown. The valves may be formed of any conventional non-linear negative resistance material, such as molded discs of granular silicon carbide. As is well-known in the art, the purpose of the non-linear valves is to limit power follow current while affording a low resistance discharge path for high current surges.

A compression spring 28 mechanically supports the plurality of pairs of electrodes and the valve elements in series between the line terminal 11 and the ground terminal 12. As is well known, a shunting strap 28a may be connected across the spring 28 to maintain the low impedance of the surge current discharge path between the

terminals

11 and 12 through each of the sparkgaps and the valves. Thus, when an overvoltage of predetermined magnitude causes the sparkgaps to sparkover, an electrical current path is established from the protected line conductor 50, through the line terminal 11, the valve elements, the sparkgaps, and the ground terminal 12 to ground potential.

Pursuant to the invention, a preionizer 30 is located in an operative relationship with the sparkgap 23, which is the sparkgap closest to the arrester ground terminal 12. In the preferred embodiment being described, the preionizer 30 comprises a resilient metal electrode 30a having a pointed contact 30b mounted on it for contacting the insulator disc 20a. As shown, the preionizer electrode 30a is electrically connected to the electrode 18 so it is energized by the electric field gradient existing between the electrode and the disc 20e, when voltage is applied to arrester terminal 1 1. A slot 31 in disc 20e extends from the aperture 21e for the sparkgap 23 to receive the preionizer 30. Various types of self-energized preionizers have been found to be suitable for stabilizing the impulse sparkover voltage of the sparkgap (23) closest to ground terminal 12. Thus, alternative preionizer configurations may be used with other embodiments of the invention. For example, capacitively coupled preionizer of the type disclosed in US. Pat. No. 3,504,226 Stetson, which issued Mar. 31, 1970 is suitable for this purpose. It is important in selecting a preionizer to select one that will respond to the relatively low voltage gradient across the sparkgap 23 closest to ground terminal 12, so that the sparkover stabilizing objective of the invention can be attained.

FIG. 2 is a schematic diagram of the surge voltage arrester described in FIG. 1 showing the line terminal 1 1; the ground terminal 12; a single non-linear valve 24, which is intended to reference all the valve elements of the arrester; the electrodes 13, l4, 15, 16, 17 and 18; and the preionizer30. The sparkgap 22 closest to the line terminal 11 is formed by the pair of

electrodes

13 and 14, and the pair of

electrodes

17 and 18 form the sparkgap 23 closest to the ground terminal. Thus FIG. 2 clearly illustrates the overvoltage discharge path from the line terminal 11 through the series sparkgaps to the ground terminal 12.

FIGS. 3, 4, 5 and 6 graphically depict the adverse effect created by the presence of moisture or contamination on the surface of housing 10, as well as showing how the present invention overcomes this adverse effect to secure consistent, stable sparkover. FIG. 4 is a schematic diagram showing the capacitance network of the arrester. Capacitances a through a represent the inherent capacitance of the insulating housing. Capacitances c1, c2, and 03 represent the capacitance from the housing to ground. Capacitances b through bl7 represent the capacitances from the internal parts of the arrester to the housing, and capacitances d1 through d7 represent the capacitances across the arrester gaps. None of the capacitance values are large, but capacitances d are larger than the others. While the capacitances a, b, c and d are shown as discrete capacitors for the sake of clarity in the following discussion, it is well understood that the capacitance is actually distributed rather than lumped at discrete points.

Under clean and dry conditions, curve 35a of FIG. 5 gives the approximate housing voltage without the internal parts, at the reference points shown in FIG. 3. The voltage gradient is highest near the top of the arrester because the upper capacitances must carry the current for the capacitances to ground as well as the current through the lower capacitances of the housing. For instance, capacitance a3 must carry the current through a4 as well as the current through cl. Therefore, the voltage drop across the section of housing having capacitance a3 is greater than the voltage drop across the section of housing having capacitance a4. The higher voltage gradient near the top of the housing is shown by the steeper slope of the curve 35a between terminal 11, the 0 point in the housing, and point 1 than between

points

2 and 3.

The voltage on the internal parts is illustrated by curve 38 of FIG. 5. The voltage at point 1 is very nearly the same as the voltage at terminal 11 because the valve disc 24a is an extremely low impedance compared to the impedance of the capacitance network. Similarly, the voltage'at point 2 is almost exactly the same as the voltage of the ground terminal because of the low impedance of valve discs 24b and 240 and spring 28. The potential of the housing near point 3 is also ground because the hanger bracket is grounded, as shown in FIG. 3. Almost all of the arrester voltage appears across the arrester gaps between

points

1 and 2.

When the complete arrester including housing and internal parts is connected to a source of high voltage as shown in FIGS. 1 or 3 the housing voltage becomes approximately as shown by curve 35 of FIG. 5. The shape of the voltage changes from that given by 35a because of current flow through capacitances b between the housing and the internal parts. Current flowing in capacitances b, b1, b2 etc. is larger than the current in capacitances c to ground. For this reason, the current through a3 is less than the current through a4, and the gradient at the upper end of the porcelain housing is decreased. Similarly the gradient at the lower end of the housing is decreased.

Curve 38 between

points

1 and 2 is not a straightline. The slope near point 1 is greater than the slope midway between

points

1 and 2 because gap capacitance d1 must carry the current through b8 in addition to the current through d2. Similarly, capacitance d7 carries current from both bl2 and d6 so that the voltage across the bottom gap is greater than the voltage across the gap above it. The slope of curve 38 does not deviate very much from a straightline because capacitances d are larger than capacitances b, but as shown previously, the housing voltage is changed by the internal parts because the capacitances b between internal parts and housing are greater than the capacitances between housing and ground. The direction of current flow in capacitances b is not the same at the top of the gap column as it is at the bottom of the column. As illustrated in FIG. 5, the voltage on the internal parts at point 1 (see curve 38) is higher than the voltage of the housing at point 1 (see curve 35). Assuming a positive line voltage, current flows from the gaps toward the housing through capacitances b7, b8, b9. The voltage of the gaps near point 2 is less than the voltage of the housing (compare curve 38 with curve 35 in FIG. 5) and current flows from the housing toward the gaps through capacitances bl l, b12, bl3.

When the housing is coated with a conducting contaminant, the voltage gradient along the arrester length between terminal 1 1 and the grounded bracket tends to be uniform as shown by curve 35b of FIG. 6. Because the current through the conducting contaminant is usually in the order of milliamperes, while the steady current through the distributed capacitances is in the order of microamperes, the voltage gradient of the housing is affected very little by the internal parts when the arrester is contaminated. The housing capacitances a through a20 are not effective in determining the housing voltage because of the low impedance of the conducting layer. The current through the capacitances b near the ends of the gap column is increased because the voltage difference between the housing and the ends of the gap column is increased as shown by the difference between the

curves

38 and 35 of FIG. 5 and curves 38 and 35b of FIG. 6. Because of this increase in current, the voltage gradient across the end gaps is increased and the voltage of the internal parts becomes as shown by curve 35d of FIG. 6. Assuming a positive half cycle of line voltage, the current through d1, is the sum of the current through d2 and b8. Similarly, the current through d7 is the sum of the current flowing in d6 and M2.

As the layer of contaminant begins to dry, the housing voltage may begin to assume a shape such as given by curve 350 of FIG. 6. Under such conditions, the voltage across the gap column will be as illustrated by curve 35c of FIG. 6. Comparing curves 35c and 352 it can be seen that the voltage of any point of the gap column is then equal to or greater than the housing voltage opposite that point. Therefore, the current in capacitances b7, b8, etc. from the gap column to the housing flows in the same direction in all capacitances. The current in capacitance d1 is then equal to the current flowing in d2 plus b8, the current in d2 is equal to the current in d3 plus b9, and a disproportionate share of voltage appears across the upper gaps while the voltage on the lower gap remains normal.

From the above description, it will be clear that FIGS. 3, 4, 5 and 6 essentially disclose the voltage and current distribution within and on the housing of a surge arrestor. The housing lengths show in FIGS. 5 and 6 are obviously the various points on the housing in FIGS. 3 and 4. As will be understood, the curves of FIG. 5 are generally for the clean and dry condition of the housing while the curves of FIG. 6 are generally for the contaminated condition of the housing. However, curve 38 which illustrates the voltage on the internal parts is included in both FIGURES as is curve 35b, which is the voltage gradient across the housing when coated with conducting contaminants. A comparison of the curves, as above discussed, shows that the voltage gradient of the top gaps is greater than the gradient over the lower gaps.

Because the voltage gradient on the top gaps is increased by contamination of the arrester housing, the uppermost sparkgap 22 in the present invention is formed to have a greater gap length than any other sparkgap in the arrester. The greater gap length of sparkgap 22 enables it to withstand more voltage without sparkover.

The voltage gradient across the sparkgap 23 does not change greatly under contaminated conditions. Accordingly, pursuant to the objectives of the present invention, the gap length of sparkgap 23 may be decreased slightly to make it possible to cause this gap to sparkover in a stable manner by suitably preionizing it with the preionizer 30 as explained above. In fact, it has been demonstrated that by use of the unique tapered gap length arrangement combined with a preionized bottom sparkgap, as taught by the present invention, it is possible to raise the random sparkover voltage of a given arrester with a given level of surface contamination, under applied line voltage conditions, by 35 to 40 percent, while maintaining the impulse sparkover rating of the arrester essentially constant within a very stable and narrow range of voltages.

Some-alternative embodiments of the invention are schematically illustrated in FIGS. 7, 8, 9 and 10. It is to be understood that these schematic diagrams represent actual surge voltage arrester structures that may be somewhat similar to the structure shown in FIG. 1. In all the schematic representations, the sparkgap 22 closest to the line terminal 11 has a greater gap length than any of the other sparkgaps of the arrester for the purposes of the invention, as explained above and a preionizer 30 is located in operative relationship with the sparkgap 23 closest to the ground terminal 12.

In modern surge voltage arresters, it is common practice to utilize a plurality of substantially identical sparkgaps, each of which have a similar sparkgap length, such as a length of 60 mils. Unlike such conventional arresters, according to the present invention the gap lengths of different sparkgaps are changed; for example, the sparkgap 22 closest to the line terminal has a length in the range of 85 to 95 mils, while sparkgap 23 closest to the ground terminal has a length in the range of 45 to 55 mils in the embodiments of the invention shown in FIGS. 1, 2 and 7.

In FIG. 8, between the uppermost and lowermost sparkgaps are a predetermined number of intermediate sparkgaps, shown for example by

sparkgaps

40 and 41. The number of intermediate sparkgaps may vary, but each intermediate sparkgap must be less in length than the gap length of sparkgap 22 and greater in gap length than the length of sparkgap 23. In addition, the intermediate sparkgaps 40 and 41 may be respectively equal in length, or of different lengths. A typical example of this situation would be equal gap length intermediate sparkgaps in the range from greater than 55 to 65 mils.

In FIG. 9, the

sparkgaps

43, 44, 45 and 46 comprise a plurality of intermediate sparkgaps. In this embodiment the intermediate sparkgaps include a first group of intermediate sparkgaps adjacent the sparkgap 22 closest to line terminal 1 l and a second group of intermediate sparkgaps adjacent the sparkgap 23. The first group comprises

sparkgaps

43 and 44, and the second group comprises sparkgaps and 46. FIG. 9 illustrates two sparkgaps in each group but a greater or lesser number may be employed in each group. The sparkgaps comprising the first group are respectively equal in length; the sparkgaps of the second group are respectively equal in length and the gap lengths of the sparkgaps in the first group is greater than the gap lengths of the sparkgaps in the second group. The first and second groups might, for example, have sparkgap lengths as follows: The

sparkgaps

43 and 44 comprising the first group have equal gap lengths in the range from greater than 75 to 85 mils; and the sparkgaps 45 and 46 comprising the second group have equal lengths in the range from greater than to mils.

A final embodiment is illustrated in FIG. 10 which includes the

intermediate sparkgaps

43, 44, 45, 46, 47 and 48. Just as in FIG. 9, the

sparkgaps

43 and 44 comprise a first group and the sparkgaps 45 and 46 comprise a second group. A third group of intermediate sparkgaps shown, for example, by the sparkgaps 47 and 48 is located between the first and second groups. The sparkgaps of the third group are respectively equal in gap length, less in length than the gap lengths of the sparkgaps in the first group, and greater in gap length than the sparkgaps of the second group. The sparkgap lengths of the embodiment in FIG. 10 includes, for example, the gap lengths described for the corresponding sparkgaps in FIG. 9 while the sparkgaps of the third group have equal lengths in the range from greater than 65 to mils.

Although certain embodiments of the invention have been shown and described, those skilled in the art will perceive changes and modifications without departing from the invention, and it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of thisinvention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

l. A surge voltage arrester, comprising an elongated hollow insulating housing, a line terminal and a ground terminal respectively mounted adjoining opposite ends of said housing, a plurality of pairs of electrodes, each of said pairs of electrodes being mounted within said housing in spaced apart relationship to define a sparkgap therebetween which is ungraded by voltage grading means, at least one nonlinear electrical valve, and means for electrically connecting in series said plurality of pairs of electrodes, said valve and said terminals thereby to form an overvoltage discharge path between said terminals through each of the sparkgaps and said valve; including the improvement wherein:

the sparkgap closest to said line terminal has a gap length substantially greater than the gap length of the sparkgap closest to said ground terminal; in combination with a single sparkgap preionizer in said arrester mounted in operative relationship to ionize the sparkgap adjacent said ground terminal, thereby to stabilize its sparkover voltage, whereby the sparkover voltage of the arrester is maintained relatively stable even when the insulating housing becomes contaminated thereby enabling an appreciable and erratic leakage current to flow across the housing surface from the line terminal to the ground terminal.

2. The surge voltage arrester as defined in claim 1 wherein said sparkgap closest to the ground terminal has a gap length less than the length of any other sparkgap in the arrester.

3. The surge voltage arrester as defined in claim 2 wherein the sparkgaps include an intermediate sparkgap having a gap length less than the gap length of the sparkgap closest to the line terminal and greater than the length of the sparkgap closest to the ground terminal.

4. The surge voltage arrester as defined in claim 2 wherein the sparkgaps include a plurality of intermediate sparkgaps between the sparkgaps respectively closest to said terminals, each intermediate sparkgap having a length less than the length of the sparkgap closest to the line terminal and greater than the gap length of the sparkgap closest to the ground terminal.

5. The surge voltage arrester as defined in claim 4 wherein said plurality of intermediate sparkgaps are of equal gap length.

6. The surge voltage arrester as defined in claim 4 wherein said plurality of intermediate sparkgaps comprises a first and a second group of intermediate sparkgaps, each group having at least one sparkgap, said first group being adjacent to the sparkgap closest to said line terminal and the second group being adjacent to the sparkgap closest to said ground terminal, each of the sparkgaps of said first group being of equal gap length, each of the sparkgaps of said second group being of equal gap length, and the sparkgaps of said first group having a gap length greater than the gap length of the sparkgaps of said second group.

7. The surge voltage arrester as defined in claim 6 wherein the plurality of intermediate sparkgaps includes a third group of sparkgaps between said first and second groups, said third group including at least one sparkgap, each sparkgap of said third group being of equal gap length, the gap length of the sparkgaps of said third group being less than the gap length of said first group and greater than the gap length of the sparkgaps of said second group.

8. The surge voltage arrester as defined in claim 1 wherein the gap length of the sparkgap closest to said line terminal is in the range from 85 mils to 95 mils.

9. The surge voltage arrester as defined in claim 2 wherein the gap length of the sparkgap closest to said line terminal is in the range from 85 mils to 95 mils and the gap length of the sparkgap closest to the ground terminal is in the range from 45 mills to 55 mils.

10. The surge voltage arrester as defined in claim 3 wherein the gap length of the sparkgap closest to the line terminal is in the range from 85 mils to 95 mils and the gap length of the sparkgap closest to the ground terminal is in the range from 45 mils to 55 mils.

11. The surge voltage arrester as defined in claim 6 wherein the gap length of the sparkgap closest to said line terminal is in the range from 85 mils to 95 mils, the gap length of the sparkgap closest to the ground tenninal is in the range from 45 mils to 55 mils, the intermediate sparkgaps comprising said first group have a gap length in the range from greater than mils to mils, and the intermediate sparkgaps comprising said second group have a gap length in the range from 55 mils to 65 mils.

i "UNITEDSTATES PATENT omicE CERTIFICATE OF CO-RRECTIQN Patent NO. :9 I Dated JUDGES, 197 4 EUGENE SAKSHAUG Inventofls) It is certified that etror appears in the abpve-identified patent and that said Letters Patentare hereby corrected as shown below:

"1 Col. 4, .Line 36 "20ashould be 2oe--.

Signed and 'seal ed this 7th day; 'of January 1975.-

(SEAL) I Attest:

McCOY M. GIBSON JR. 'c. MARSHALL DANN Attesting Officer C onupissioner of Patents FORM po-soso (10.69)

. UNITED ATEs PATENT OFFICE CERTIFICATE OF I CO-RRECTIQN I Patent NO. 9:9

Dated June'25, 197 4 EUGENE SAKSHAUG Inventofls) It is certified tha t error appears in the above-identified patent and that said Letters P a tent are hereby corrected as shown below:

v" Col; Line 36,- "2083 should be 2oe-- Signed and qealegzl this 7th dag of January 1975,

(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Comissioner 'of Patenrs

Claims

1. A surge voltage arrester, comprising an elongated hollow insulating housing, a line terminal and a ground terminal respectively mounted adjoining opposite ends of said housing, a plurality of pairs of electrodes, each of said pairs of electrodes being mounted within said housing in spaced apart relationship to define a sparkgap therebetween which is ungraded by voltage grading means, at least one nonlinear electrical valve, and means for electrically connecting in series said plurality of pairs of electrodes, said valve and said terminals thereby to form an overvoltage discharge path between said terminals through each of the sparkgaps and said valve; including the improvement wherein: the sparkgap closest to said line terminal has a gap length substantially greater than the gap length of the sparkgap closest to said ground terminal; in combination with a single sparkgap preionizer in said arrester mounted in operative relationship to ionize the sparkgap adjacent said ground terminal, thereby to stabilize its sparkover voltage, whereby the sparkover voltage of the arrester is maintained relatively stable even when the insulating housing becomes contaminated thereby enabling an appreciable and erratic leakage current to flow across the housing surface from the line terminal to the ground terminal.

11. The surge voltage arrester as defined in claim 6 wherein the gap length of the sparkgap closest to said line terminal is in the range from 85 mils to 95 mils, the gap length of the sparkgap closest to the ground terminal is in the range from 45 mils to 55 mils, the intermediate sparkgaps comprising said first group have a gap length in the range from greater than 75 mils to 85 mils, and the intermediate sparkgaps comprising said second group have a gap length in the range from 55 mils to 65 mils.