US3789256A

US3789256A - Shielded spark gap device

Info

Publication number: US3789256A
Application number: US00250000A
Authority: US
Inventors: N Osmundsen
Original assignee: Westinghouse Electric Corp
Current assignee: ABB Inc USA
Priority date: 1972-05-03
Filing date: 1972-05-03
Publication date: 1974-01-29
Anticipated expiration: 1991-01-29
Also published as: CH558986A; DE2321726A1

Abstract

A shielded spark gap device which is essentially independent of external influences. The gap is enclosed in a metallic shielding enclosure which is discontinuous between opposite ends of the gap, so as to provide a capacitance in parallel with the gap, and which shields the gap in such a manner that its sparkover characteristics are not affected by any change in external conditions which may affect the electric field in the region of the gap.

Description

United States Patent 11 1 [111 3,789,256 Osmundsen Jan. 29, 1974 SHIELDED SPARK GAP DEVICE 2,392,679 1/1946 MacCarthy 313/243 x 3,504,221 3/1970 Osterhout.... Inventor. Normal! K. osmulldsen, Kawiecki Bloommgwn, 2,132,175 10/1938 Machlett 313/313 [73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa. Primary Examiner-John K. Corbin Filed y 3 1972 Att0rney,Agent, 0r Firm-G. H. Telfer 21 Appl. No.: 250,000 {57] ABSTRACT [52] U S Cl 313/313 zoo/144 B 313/242 A shielded spark gap device which is essentially inde- [51] 1/52 pendent of external influences. The gap is enclosed in [58] Fie'ld 326 a metallic shielding enclosure which is discontinuous 3l3/ 356 219 5 1 between opposite ends .of the gap, so as to provide a capacitance in parallel with the gap, and which shields 56] References Cited the gap in such a manner that its sparkover characteristics are not affected by any change in external condi- UNITED STATES PATENTS tions which may affect the electric field in the region 3,430,015 2/1969 Crouch et al. 200/144 B of the a 1,651,876 12/1927 Evans 313/313 X g p 2,240,229 4/1941 Scharli 313/313 X 10 Claims, 2 Drawing Figures PAIENIEI] JAN 2 9 18M SHIELDED SPARK GAP DEVICE BACKGROUND OF THE INVENTION The present invention relates to spark gap devices, and more particularly to a shielded gap which is essentially independent of external influences.

Spark gaps are commonly used for protection of electrical circuits and apparatus against overvoltage, and in particular they are used in lightning arresters. In a lightning arrester, one or more spark gaps are connected in series with a suitable number of valve blocks, or nonlinear resistors, and upon the occurrence of a predetermined overvoltage the gaps spark over to permit the surge or other excess voltage to be discharged to ground, the valve blocks limiting and reducing the power current following the surge so that the gaps can interrupt the current after the surge has been discharged. Obviously, for the 'best possible protection, the spark-over voltage of the gaps must be accurately determined and must be consistent throughout the life of the arrester so the the level of protection afforded by the arrester remains unchanged. The conventional spark gaps that have usually been used in arresters, and for other protective applications, however, are subject to being influenced by external conditions which may affect the electric field in the region of the gap. For this reason, relatively large and elaborate grading rings are frequently necessary in high voltage arresters to control the electric field and to provide the desired voltage distribution in order to stabilize the performance of the gaps. The presence of contamination on the outer surface of the porcelain housing of the arrester may also affect the distribution of the electric field, and other external conditions may have similar effects which change the sparkover of the arrester. No satisfactory means have been available heretofore to effectively stabilize the performance of spark gaps in lightning arresters, and their sparkover voltages have been subject to the effect of external influences to an undesirable degree.

SUMMARY OF THE INVENTION In accordance with the present invention there is provided a spark gap device which is completely shielded in such a manner that it is essentially independent of external influences and its sparkover voltage remains unchanged under any conditions. This result is achieved by a new type of shielding in which the gap is enclosed in a conducting, preferably metallic, shielding enclosure which surrounds the gap but which is discontinuous between opposite ends of the gap, so that a substantial capacitance is provided in parallel with the gap. The gap itself may be either a single gap or a multi-gap structure and is preferably of a sealed type. Theeffect of the enclosure is to completely shield the gap from any external influences such as contamination in the arrester housing, or other influences that may disturb or affect the electric field, and the performance of the gap is thus completely stabilized and remains the same unaffected by any external influences.

BRIEF DESCRIPTION OF THE DRAWING The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawing, in which:

FIG. 1 is a sectional view of a sealed gap device embodying the invention; and

FIG. 2 is a view, partly in elevation and partly in section, showing a multiple gap structure embodying the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS As previously indicated the invention provides a spark gap device which is essentially completely independent of external influences. A preferred embodiment of the invention in a sealed single gap device is shown in FIG. 1. As there shown the gap may consist of two dish-shaped electrodes 1 and 2 which maybe made of brass or other suitable metal and which have flat opposed central portions forming a spark gap 3 between them. The'electrodes l and 2 are separated and spaced apart by a generally cylindrical spacer 4 which may be an insulator such as porcelain or a resistance spacer for controlling the voltage distribution across a series of gaps. The peripheries of the electrodes 1 and 2 engage the ends of the spacer 4 as shown and are sealed thereto in any suitable manner to provide a sealed gap. If desired a tube 5 may be provided in one of the electrodes for the purpose of filling the gap with a suitable gas such as nitrogen, or a mixture of nitrogen and hydrogen containing a small amount of tritium to obtain low and consistent sparkover voltage, the tube 5 being pinched off and sealed after the gap is filled.

In accordance with the present invention the gap is shielded to make it independent of external influences. The shielding means provided for this purpose consists of members 6 and 7 disposed on opposite ends of the gap. The shield members 6 and 7 are made of sheet metal, or any-suitable conductive material, and are dish-shaped as shown. Each of the shield members 6 and 7 extends over one end of the gap structure so as to be in electrical contact therewith and its peripheral portion extends toward the opposite end of the gap device. As shown in FIG. 1, however, the peripheries of members 6 and 7 are spaced apart so that while the gap is enclosed in a metallic enclosure, the enclosure is discontinuous between the ends of the gap because of the space 8 between the shield members. The edges of the members 6 and 7 may be rolled if desired as indicated at 9. The length of the space 8 is not critical but it must be such that this space will not spark over under an excess voltage before the gap 3 itself sparks over.

It will be seen that the gap device is enclosed in a metallic shielding enclosure which surrounds the gap but is discontinuous between the ends of the gap. The space between the two metallic shield members results in a substantial capacitance in parallel with the gap, and the effect of the metallic enclosure together with this capacitance is such that the gap itself is made essentially independent of any external influences. The voltage across the gap at which sparkover will occur is fixed by the spacing of the gap 3 and by the characteristics of the shielding enclosure, so that when the gap has once been assembled it is independent of any external changes or variations in the electric field around the gap and its sparkover remains constant.

7 The invention is shown in FIG. 2 applied to a multiple gap assembly which may be of any desired type but which is shown as being of the type disclosed in a U.S. Pat. to Osterhout No. 3,504,221. As more fully described in that patent, the gap assembly consists of a number of gap plates 10 having spark gaps between them and assembled in a stack between metal end plates 11. A coil 12 is preferably provided to produce a magnetic field upon the occurrence of a surge to move the arcs in the gaps in a manner to facilitate their extinction. The detailed structure of the gap device itself is not a part of the present invention and it may be of any desired type, one representative multiple gap construction being shown in the above-mentioned patent.

In accordance with the present invention the gap is provided with shielding means consisting of shielding members 13 and 14 on opposite ends. The members 13 and 14 are metallic cup-shaped shields which extend over the ends of the gap assembly in contact with the metal end plates 11. The shield members 13 and 14 extend toward each other and are separated by a substantial space 15 which is made large enough to avoid any possibility of the sapce l5 arcing over before the series of gaps themselves arc over. It will be seen that the shielding members 13 and 14, as in the previous embodiment, comprise a metallic enclosure surrounding the gap device but which is discontinuous between the ends of the gap so that a substantial capacitance is provided in parallel with the gap itself. If desired the edges of the shielding members 13, 14 may be rolled as indicated at 16 to somewhat enhance the effectiveness. It will be seen that the construction of FIG. 2 is essentially the same as that of FIG. 1 but is applied to a multiple gap assembly. The effect, however, is the same in making the gap device completely independent of external influences as previously described.

The gap assemblies described are particularly suitable for use in lightning arresters, although their usefulness is obviously not so restricted and they may be used for other overvoltage protective applications. The shielded gas filled gap of FIG. 1 is particularly suitable for use as a control gap in high voltage lightning arresters of the type disclosed in the U.S. Pat. to Kennon Nos. 3,534,221 and Osterhout et al. 3,611,044, for example. The multiple gap structure of FIG. 2 is also suitable for use as a main gap device in this type of high voltage arrester, although it will be understood that either of the gap devices shown may be used in any type of arrester. It will also be apparent that various modifications are possible within the scope of the invention. Thus, in some cases it may be possible to provide a shielding member at one end only of the gap structure which extends towards the opposite end to provide a discontinuous metallic enclosure in the same manner and with the same effect as described above. The configuration of the shield members may of course be modified as desired, and they may be made of any conductive material including perforated or mesh materials and metal screen, although sheet metal as shown is preferred.

As previously explained, the metallic shielding enclosure, with its discontinuity providing a parallel capacitance, has the effect of completely shielding the gap and making it essentially completely independent of any external influence that may affect the electric field in the region of the gap. This has important advantages arising from the effective isolation and unchanging characteristics of the gap. Thus, in many cases it is possible to eliminate grading rings, or in the case of high voltage arresters to greatly reduce the size of the required rings and simplify the construction. Similarly, the presence of contamination on the arrester housing will not affect the performance of the new gaps as they are completely independent of changes in the electric field caused by the presence of contamination and their performance is completely stabilized and remains constant throughout the life of the arrester. A new gap device has been provided, therefore, which is unaffected by external influences and even under the most severe conditions of changes or variations in the surrounding electric field, the spark-over level and other performance characteristics of the gap will not change appreciably.

I claim:

1. A spark gap device having a pair of electrodes, spacer means for maintaining the electrodes in fixed spaced relation and enclosing the space between them, and shielding means for protecting the electric field distribution in the gap from external influences comprising a metallic enclosure surrounding the gap, the elec trodes and the spacer means, said enclosure being in electrical contact with electrodes at opposite ends of the gap and being discontinuous between opposite ends of the gap.

2. A spark gap device as defined in claim 1 in which the shielding means comprises metallic members in contact with the electrodes at each end of the gap, said metallic members surrounding the gap and extending toward each other with a space between them.

3. A spark gap as defined in claim 2 in which the length of said space is such that it will not spark over before the gap sparks over.

4. A spark gap device as defined in claim 1 having a plurality of electrodes forming a plurality of seriesrelated gaps.

5. A spark gap device as defined in claim 1 in which the shielding means comprises cup-shaped metallic members in contact with the electrodes at opposite ends of the gap, said metallic members extending toward each other but being spaced apart to form a discontinuous enclosure for the gap.

6. A spark gap device as defined in claim 1 in which the shielding means comprises at least one cup-shaped metallic member extending-from one end of the gap toward the other end to form an enclosure incorporating a predetermined capacitance in parallel with the gap.

7. A spark gap device comprising: a pair of electrodes; spacer means for maintaining said electrodes in a fixed spaced relationship to form a spark gap therebetween, said spacer means enclosing the space between said electrodes; a first electrically conductive member electrically contacting a first one of said electrodes and enclosing said first electrode and a first portion of said spacer means; and a second electrically conductive member electrically contacting a second one of said electrodes and enclosing said second electrode and a second portion of said spacer means, said first and second electrically conductive members extending toward each other with a space therebetween that has a length sufficiently great to preclude sparking over at this space prior to spark over of the spark gap, while pro viding a predetermined capacitance in parallel with the spark gap.

8. A spark gap device as claimed in claim 7 wherein said first and second electrically conductive members are metallic and cup-shaped, with the bottom of the cup engaging the respective electrode and the sides thereof extending toward each other, the space between said metallic members being defined by the outermost ends of the extending sides.

9. A shielding arrangement for a spark gap device having a pair of electrodes and spacer means for maintaining the electrodes in a fixed space relationship to form a spark gap therebetween, the spacer means enclosing the space between the electrodes, comprising: a first electrically conductive member electrically contacting one of the electrodes and enclosing a first portion of the spark gap device comprising a first one of the electrodes and a first portion of the spacer means; and a second electrically conductive member electrically contacting the other electrode and enclosing a second portion of the spark gap device comprising the other electrode and a second portion of the spacer outermost ends of the extending sides.

Claims

1. A spark gap device having a pair of electrodes, spacer means for maintaining the electrodes in fixed spaced relation and enClosing the space between them, and shielding means for protecting the electric field distribution in the gap from external influences comprising a metallic enclosure surrounding the gap, the electrodes and the spacer means, said enclosure being in electrical contact with electrodes at opposite ends of the gap and being discontinuous between opposite ends of the gap.

4. A spark gap device as defined in claim 1 having a plurality of electrodes forming a plurality of series-related gaps.

6. A spark gap device as defined in claim 1 in which the shielding means comprises at least one cup-shaped metallic member extending from one end of the gap toward the other end to form an enclosure incorporating a predetermined capacitance in parallel with the gap.

7. A spark gap device comprising: a pair of electrodes; spacer means for maintaining said electrodes in a fixed spaced relationship to form a spark gap therebetween, said spacer means enclosing the space between said electrodes; a first electrically conductive member electrically contacting a first one of said electrodes and enclosing said first electrode and a first portion of said spacer means; and a second electrically conductive member electrically contacting a second one of said electrodes and enclosing said second electrode and a second portion of said spacer means, said first and second electrically conductive members extending toward each other with a space therebetween that has a length sufficiently great to preclude sparking over at this space prior to spark over of the spark gap, while providing a predetermined capacitance in parallel with the spark gap.

9. A shielding arrangement for a spark gap device having a pair of electrodes and spacer means for maintaining the electrodes in a fixed space relationship to form a spark gap therebetween, the spacer means enclosing the space between the electrodes, comprising: a first electrically conductive member electrically contacting one of the electrodes and enclosing a first portion of the spark gap device comprising a first one of the electrodes and a first portion of the spacer means; and a second electrically conductive member electrically contacting the other electrode and enclosing a second portion of the spark gap device comprising the other electrode and a second portion of the spacer means, said first and second electrically conductive members extending toward each other with a space therebetween that has a length sufficiently great to preclude sparking over at this space prior to spark over of the spark gap, while providing a predetermined capacitance in parallel with the spark gap.

10. A shielding arrangement as claimed in claim 9 wherein said first and second electrically conductive members are metallic and cup-shaped, with the bottom of the cup engaging the respective electrode and the sides thereof extending toward each other, the space between said metallic members being defined by the outermost ends of the extending sides.