US2871390A - Lightning arrester - Google Patents

Description

United States Patent F 2,871,390 LIGHTNING aunus'rnn Application September 2 1950, Serial No. 182,994 7 Claims. c1. 313-231) The present invention relates to lightning arresters, and more particularly to a lightning arrester of the expulsion type having improved current-interrupting ability and long life.

Expulsion type lightning arr-esters consist essentially of spaced electrodes disposed ina vented arcing chamber, which is lined with, or contains, insulating material capable of evolving substantially tin-ionized gas when exposed to an electric arc. When a discharge occurs be tween the electrodes, a large quantity of gas is evolved which is expelled in a blast through the vent, blowing out the arc gases and deionizing the arc path to extinguish the arc, thus interrupting the power current which tends to flow through the arrester to ground following the discharge of a lightning surge. in the usual construction of arresters of this type, the arcing chamber consists of a tubular structure. of. hard fiber, or other suitable gas-evolving material, with electrodes disposed at the ends ofthetube, at least one of the electrodes extending into the tube, and at least one of the electrodes being vented. A cylindrical plug or filler member of gas-evolving material is often placed in the tube, between the electrodes, to restrict the area of the arc path and to increase the amount of gas-evolving material exposed to the ,arc, in order to improve the current-interrupting ability of the arrester. Such conventional fillers have been smooth-surfaced cylindrical members, and they may be either fixed in position, or free to move in the tube.

Filler members with smooth cylindrical surfaces, however, have the serious disadvantage that they are subject touneven wear or erosion. When a discharge occurs in .an expulsion arrester, gas is evolved by volatilization of the surface of the fiber, or other gas-evolving material, of which the filler and the tube wall are composed. With the conventional smooth cylindrical filler, this loss of material or erosion, on any particular discharge, occurs on one side of the filler only, causing the filler to wear unevenly and to lose its original cylindrical shape. Correspondingly uneven wear of the tube wall also oc cuts for the same reason, that is, any particular discharge takes place between the filler and the tube wall on one side of the filler only, so that the material on the other side of the filler is not substantially eroded. This uneven wear which causes the filler, and to some extent the tube, to lose their cylindrical shape results in a decrease in the area of gas-evolving material exposed to the arc, and an undesirable increase in the cross-sectional area of the arcpath, both of which effects tend (Olinpair the current-interrupting ability of the arrester. Thus, in an arrester using the conventional cylindrical filler, the current-interrupting ability decreases relatively rapidly on successive discharges and the useful life of the arrester is substantially reduced from what it would be if erosion of the filler and tube were'rnore uniform.

The principal object of the present invention is to'provide a lightning arrester'of the expulsion type, having a 2,871,390 Fatented Jan. 27, 1959 "ice generally cylindrical filler member, in which wear or erosion of the filler and tube during a discharge is ma terially decreased and is made substantially uniform over the surface of the tiller and tube, so that the current-interrupting ability of the arrester is reduced very slowly by repeated discharges, and the useful life of the arrester is greatly increased.

Another object of the invention is to provide an expulsion type lightning arrester having a filler member which is generally cylindrical, but which has its surface shaped to cause a circumferential flow of the gases pro duced during a discharge, so as to elongate the arc and make the erosion of the filler uniform over its entire surface, thus improving the current-interrupting ability of the arrester and increasing its life.

A more specific object of the invention is to provide an expulsion type lightning arrester having a generally cylindrical filler member with a helical groove cut in its surface to provide a path for the gases produced during a discharge which causes the gases to flow circumferentially to make erosion uniform. The circumferential flow of gas also has the effect of forcing the are into the helical groove to elongate the arc and thus facilitate its interruption with a minimum of arcing time, thereby reducing the amount of erosion, and increasing the life of the arrester.

The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawing, the single figure of which is a vertical sectional view of a lightning arrester embodying the invention.

The lightning arrester shown in the drawing is an expulsion type arrester having a tube or tubular structure 1 which forms the arcing chamber of the arrester and which is made of hard fiber, or other suitable insulating material capable of evolving substantially tin-ionized gas when exposed to an electric arc. An upper electrode member 2 is disposed in the upper end of the tube 1. The upper electrode 2 is shown as a steel sleeve which fits tightly in the tube 1 and is threaded into the tube at its upper end to hold it securely in position against the axial thrust which occurs when the arrester operates. A stud 3 is threaded in the upper end of the electrode 2 and an insulating sleeve or bushing 4 may be placed over the stud 3. A lower electrode member 5 is provided at the lower end of the tube 1, spaced from the electrode 2 to form a gap within the tube. The lower electrode 5 is shown as a generally cylindrical steel memher, internally threaded to engage corresponding external threads on the tube 1, and having a large central opening or bore 6 for the escape of gas from the tube.

The arrester is provided with a generally cylindrical filler member 7, which may be made of hard fiber, or other suitable gas-evolving, insulating material. The filler 7 is generally cylindrical, and it has a helical groove 8 cut in itssuriace extending from end to end of the filler, the groove being made of substantial cross-sectional area to provide a generally circumferential path for gas generated in the tube. The outside diameter of the filler 7 is made slightly less than the inside diameter of the tube 1, so that the filler fits in the tube 1 with a small radial clearance, providing a restricted initial arc path between the electrodes 2 and 5 in the annular space between the filler 7 and tube 1. The tiller '7 may be secured in a fixed position in the tube, if desired, but in the preferred embodiment shown, the filler 7 is permitted to float loosely in the bore of the tube, so as to be free to move in a random manner when a discharge occurs, to prevent establishing a preferential path for succeed ing discharges. The filler 7 is made somewhat shorter in axial length than the distance between the lower end at the upper electrode 2 and the lower end of the tube 1, to permit such movement, and it is supported on a steel disc or support 9 which is clamped between the lower electrode 5 and the bottom end of the tube 1. The disc 9 is provided with a plurality of peripheral vent openings 10 to permit the escape of gas from the interior of the tube.

The arrester is preferably enclosed in a generally cylindrical housing 11 of porcelain or other suitable weatherresistant insulating material. The housing 11 is supported on a washer 12 which rests on the upper end of the lower electrode 5 outside the tube 1, a gasket 13 preferably being interposed between the washer and the housing. The top of the housing is closed by a porcelain cap 14 with a gasket 51 between the cap and the housing. The stud 3, which provides the upper terminal of the arrester, extends through a central opening in the cap 14 and has a flange portion 15 extending over the central part of the cap 14, with a gasket 16 to seal the opening in the cap, and it will be seen that when the stud 3 is tightened, the parts of the assembly are firmly clamped together. A nut 17, or other suitable terminal means, may be provided on the upper end of the stud 3 for connecting a line lead to the arrester. The arrester may be mounted in position by any suitable means, such as by threading the lower electrode 5 into a grounded mounting plate or bracket 18 of any suitable construction. If desired, a deflector member 1% may be secured to the mounting bracket 18 to direct the hot, ionized gas expelled from the arrester in a desired direction.

In use, the upper terminal of the arrester is connected, through an external series spark gap 20 of any desired type, to a terminal of a protected device, such as a transformer, or to a line conductor 21 or other apparatus to be protected, the lower electrode 5 being grounded through the mounting bracket 18, or in any other desired manner. A metallic electrode 22 extending toward the stud 3 and connected to the line conductor 21 may be utilized to provide the series gap 20, or the series gap may be provided in any other suitable manner. When a lightning surge, or other excess-voltage surge occurs on the line to which the arrester or the protected apparatus is connected, the external spark gap 20 and the internal gap between the electrodes 2 and 5 spark over to discharge the surge to ground. The are in the arrester is initially formed in a substantially straight axial path between the electrodes 2 and 5 in the restricted clearance between the filler 7 and tube 1, and the heat of the arc causes the evolution of a large quantity of substantially un-ionized gas from the adjacent surfaces of the filler 7 and the tube 1. This large quantity of gas, which is generated in a very brief time, causes an extremely high pressure in the tube, which causes the gas to flow into the helical groove 8 of the filler 7 and circumferentially around the filler in the groove 8, the gas being discharged in a blast through the vent openings 10 and 6. This circumferential flow of the gas around the filler, in contact with the surfaces of the filler and tube, causes the wear or erosion of the filler and of the tube wall to be quite uniform circumferentially, so that the cylindrical shape of the tube wall is maintained, and there is no unevenness or non-uniformity of either the tube or the filler to materially reduce the area of gas-evolving material available to subsequent discharges. Thus, the currentinterrupting ability of the arrester is not greatly affected by a single discharge, and the life is greatly increased.

The circumferential flow of the gas also has another very important effect. The circumferentially flowing gas drags or forces the are into the flow path of the gas, which follows the helical groove 8, so that the arc is moved from its initial, more or less straight axial path, between the electrodes 2 and 5 into a helical path, which greatly elongates the are and facilitates its interruption, the are being extinguished by the combined effects of 1 deionization of the arc path and elongation of the are. This movement, or dragging, of the are into a helical path is very readily obtained because of the high degree of mobility of an arc in the presence of a blast, or rapidly moving stream, of gas. The movement of the are, therefore, occurs very rapidly and the are changes rather abruptly from the initial short straight path between the electrodes to the much longer helical path of in the groove 8. Oscillograms taken during actual tests show that the transfer of the arc to the helical path occurs in a time of the order of 400 microseconds or less from the initial establishment of the are, or less than one-fiftieth of a cycle, on a 60-cycle basis. This rapid transfer of the arc to the longer path enables the arrester to interrupt the power current in a small fraction of the time required by an arrester utilizing a conventional filler, in which the current is usually not interrupted until it passes through zero, which may be as long as a full half-cycle after the initial spark-over. The are voltage is substantially increased when the arc transfers to the helical path, because of the increased length of the path, and the high are voltage has a strongly current-limiting effect, reducing the magnitude of the power current and thus further facilitating interruption of the arc. The result of these effects is that the arrester interrupts the power current in an extremely short time and has greatly improved current-interrupting ability. The greatly reduced arcing time and the limited power current reduce the energy input to the arrester and thus reduce the amount of erosion of the gas-evolving material which occurs during a discharge. The life of the arrester is thus greatly increased, since the current-interrupting ability is only very slightly affected by a single discharge.

As indicated above, the erosion or wear of the filler 7 and tube 1 is substantially uniform and even over their entire surfaces, and no unevenness occurs which would reduce the area of gas-evolving material exposed to the are, or which would undesirably increase the area of the arc path. Repeated operations of the arrester, however, result in erosion which, even though uniform, tends to reduce the current-interrupting ability, as in any expulsion arrester. Even when a substantial amount of erosion has occurred, however, after a large number of discharges, the helical groove 8 still causes the gas to flow circumferentially, and the centrifugal force thus produced results in relatively high gas pressures inthe region immediately adjacent the wall of the tube 1, which forces the are into the helical groove 8 so that it is elongated as described above. This effect tends to offset the decrease in current-interrupting ability resulting from erosion of the gas-evolving material, and thus increases the useful life of the arrester, since the number of operations which can occur before the current-interrupting ability is too greatly reduced for further use is greatly increased.

It should now be apparent that an expulsion type lightning arrester has beenprovided in which greatly improved characteristics, both as to current-interrupting ability and life, are obtained by the use of the helically grooved filler. A particular embodiment of the invention has been shown and described for the purpose of illustration, but it will be apparent that various other embodiments are possible, and that various modifications may be made within the scope of the invention. It is to be understood, therefore, that the invention is not limited to the specific details of construction shown and described, but in its broadest aspects it includes all equivalent embodiments and modifications which come within the scope of the appended claims.

I claim as my invention:

l. A lightning arrester comprising a tubular structure having an internal surface of insulating material capable of evolving gas when exposed to an electric are, spaced electrode means disposed at opposite ends of the tubular structure, at least one of said electrode means extending into the tubular structure and at least one end of the tubular structure being vented, and an elongated filler member disposed in the tubular structure between the electrodes, said filler member being made of an insulating material capable of evolving gas when exposed to an electric arc and having a groove in its surface extending continuously around the filler from end to end thereof, said groove providing an elongated arc path of greater length than the straight initial arc path between the electrode means and being adapted to cause circumferential movement of gas produced upon the occurrence of an arc in said initial arc path to cause the gas to flow through the groove and to cause the arc to transfer to said elongated arc path in the groove.

2. A lightning arrester comprising a tubular structure having an internal surface of insulating material capable of evolving gas when exposed to an electric are, spaced electrode means disposed at opposite ends of the tubular structure, at least one of said electrode means extending into the tubular structure and at least one end of the tubular structure being vented, and an elongated filler member disposed in the tubular structure between the electrodes, said filler member being made of an insulating material capable of evolving gas when exposed to an electric arc and having a helical groove in its surface extending continuously from end to end of the filler.

3. A lightning arrester comprising a tubular structure having an internal surface of insulating material capable of evolving gas when exposed to an electric are, spaced electrode means disposed at opposite ends of the tubular structure, at least one of said electrode means extending into the tubular structure and at least one end of the tubular structure being vented, and a generally cylindrical filler member of insulating material capable of evolving gas when exposed to an electric arc, said filler member being disposed in the tubular structure between the electrode means and fitting in the tubular structure with a small clearance, and the filler member having a groove in its cylindrical surface extending continuously around the filler from end to end thereof, said groove providing an elongated arc path of greater length than the straight initial arc path between the electrode means and being adapted to cause circumferential movement of gas produced upon the occurrence of an arc in said initial arc path to cause the gas to flow through the groove and to cause the arc to transfer to said elongated arc path in the groove.

4. A lightning arrester comprising a tubular structure having an internal surface of insulating material capable of evolving gas when exposed to an electric arc, spaced electrode means disposed at opposite ends of the tubular structure, at least one of said electrode means extending into the tubular structure and at least one end of the tubular structure being vented, and a generally cylindrical filler member of insulating material capable of evolving gas when exposed to an electric arc, said filler member being disposed in the tubular structure between the electrode means and fitting in the tubular structure with a small clearance, and the filler member having a helical groove extending continuously from end to end of its cylindrical surface.

5. A lightning arrester comprising a hollow insulator body forming a housing for a plurality of metallic electrodes positioned in spaced relationship to provide an air gap, and a dielectric structure formed of arc extinguishing material and confining said air gap, said dielectric structure comprising a plurality of members having adjacent cooperating surfaces, one of said members having a relatively smooth surface and another having a convoluted surface cooperating with said smooth surface to delineate a tortuous air passage extending continuously from end to end of said confined air gap and forming a path of travel for a discharge are bridging said confined air gap, said smooth surface on one of said members forming a shorter path of travel for the discharge are to insure its initial establishment along said shorter path, the resistance to continued travel of the discharge are along said initially established path being substantially increased immediately upon the establishment of the discharge are through the action of the are upon the dielectric structure, the resistance thus produced attaining a magnitude greater than the resistance offered by said tortouous air passage to efiect the transfer of the discharge are to said tortuous passage wherein it is extinguished by the process of deionization, and venting means for relieving internal pressure evolved during the presence of the discharge arc.

6. A lightning arrester constructed as set forth in claim 5 in which the dielectric structure comprises a pair of telescopically arranged members.

7. A lightning arrester constructed as set forth in claim 5 in which the dielectric structure comprises a pair of telescopically arranged members having, respectively, smooth and convoluted surfaces cooperating to provide separate paths of travel for a discharge are.

References Cited in the file of this patent UNITED STATES PATENTS 2,164,720 Pittman July 4, 1939 2,239,940 Stroup Apr. 29, 1941 2,304,848 Roloson Dec. 15, 1942 2,338,479 Ackermann Jan. 4, 1944 2,418,791 Pittman Apr. 8, 1947 2,434,010 Pittman Jan. 6, 1948 2,591,370 Nelson et al. Apr. 1, 1952

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