US2840747A - Lightning arrester - Google Patents

Description

June 24, 1958 F v CUNNINGHAM 2,840,747

LIGHTNING ARRESTER Filed Ocl.. 15, 1954 'new INVENTOR. Frana's vV.' Gunn/:Zyban:

Uaifsdsares Patent i-,40j41 LIGHTNING ARRESTER Francis V. Cunningham, Milwaukee, AWis., assigner to McGraw-Edison Company, a corporation of Delaware Applieafin october "1s, 1954, serial No. 462,447 3 Claims. y (c1. 31a-231) This invention relates to electrical discharge devices, and more particularly to lightning arresters of the eX- pulsion type for use on electric power distribution systems.

Eifective operation of an expulsion type lightning arresterdepends upon the arresters ability to produce copious quantities of arc-extinguishing gases at a pressure sufficient for blocking follow current, while concurrently providing a relatively cool, de-ionizing atmosphere for quenching conductive arc-sustaining gases.

It is well known that the material, such as horn ber, used for producing the copious quantities of de-ionized gases is mechanically weak, and under operating conditions under which a relatively large pressure is built up within the interior of the arrester, the material may be further weakened, or in fact, destroyed with explosive force, endangering nearby electrical equipment and even the life of any person in the vicinity of the arrester. It has been conventional practice among lightning arrester manufacturers to provide an external sleeve for tubular 'housings which is of steel or other material capable of withstanding explosive forces exerted on the tubular arcextinguishing material. In addition, it is well known that such sleeves, being of conductive material, act as electro-static shields, redistributing the dielectric flux between the spaced-apart arcing electrodes and resulting in low and consistent sparkover of expulsion type arresters.

It has been the practice, during manufactu-re, to position one end of the sleeve in abutting relationship with the lower or ground electrode member, or iitting. This member is frequently threadingly fastenedto the outer tubular insulating member and tightened thereon to stop against the sleeve. It `will be understood that under the usual manufacturing tolerances, the various parts might conceivably leave la'nnute gap between the sleeve and the lower electrode.-y In addition, very often the sleeve member and theelectrode member are of dissimilar conductive materials which provide natural electrodes for galvanic action to take place. Since it is conventional practice to expose the arrester to the atmosphere, moisture from the atmosphere can permeate the minute gap` between sleeve and electrode member, this moisture serving as electrolyte for galvanic action. Obviously, as the material corrodes due to galvanic action, the oxides formed will bridge any minute gap which may exist between the abutting sleeve and electrode members. Flow of arrester charging current through this oxide produces radio and television interference. i

I have found that by positioning this electrostatic shield with a predetermined gap existing between the shield and the lower electrode,` rather than taking steps to insure securerengag'ement therebetween, the shield will remain effective to reduce and control sparkover. In addition, this method of insulating` the metal shield fromthev arrester terminal or electrode obviates4 the problem of corterial, preferably horn fiber.

y 21,840,747 Patented June 24, 1958 ICC 2 rosion, completely eliminating the problem of radio and television interference.

It is therefore an object of the present invention to provide an expulsion type lightning arrester having a strengthening and electro-static shield externally of the arc-extinguishing materials, which shield is positioned on the arrester in a manner that will provide a preselected and predetermined gap between the shield and an adjacent terminal or electrode of the arrester, whereby the problems of corrosion between dissimilar metals and radio and television interference is eliminated.

Referring now to the drawing which accompanies this specication:

Fig. 1 is a longitudinal View, partly in section, of the improved arrester.

Fig. 2 is a cross-section taken along lines 2-2 of Fig. l.

Fig. 3 is an enlarged fragmentary view of a portion of the arrester shown in Fig.V l.

Referring particularly to Fig. l, the arrester is preferably of the type described and claimed in U. S. Patent No. 2,429,533 granted to Herman O. Stoelting on October 2l, 1947, and assigned to the same assignee as the present invention.1 Although the invention is described in connection with the present embodiment, it is within the province of this invention toprovide the electro-static shield for any expulsion type lightning arrester having spaced apart arcing electrodes internally of an insulating housing.

The embodiment illustrated in Fig. l constitutes a housing 1 of dielectric materialfusually porcelain-having a chamber 2, which is nearly coextensive lengthwise "with the housing being open at the lower end and closed at the upper end except for an aperture 3, which aperture accommodates a stud member 4. The stud is electrically connected to a washer 5 to make contact with an elongated electrode member 6 clamped to the lower end of a petticoat insulator 7.l The insulator 7 is anchored to the upper end of the stud 4, which is firmly imbedded therein.

A line terminal 8 is fastened to the upper extremity of the insulator -7, and is adapted to engage a conductor wire (not shown) connecting with the power line that is to be protected. An electrode 9 is secured to the upper end of the insulator 7 making an electrical contact with the terminal member 8, concurrently serving to hold this electrode member in operating position. An open spark gap 10 is formed by the spacing between the free ends of both electrode members 6 and 9. This spark gap is con- `ventional and prevents line lockout, isolating the expulsion tube from the line terminal 3 and preventing leakage currents from traveling to ground throughtheexpulsion tube and associated parts. Although an open; spark-gap arrangement is preferable, it is within the province of this invention to provide internal spark-gap electrodes (not shown). A gasket 1l. is provided which in combination with elongated electrode member 6 and porcelain housing llconstitutes a weather-proof seal of aperture 3.

The components shown assembled within the chamber 2 include a metal electrode plug l2 which is threaded ex ternally to engage with a complementary internally threaded tubular memberV 13 consisting of an insulating ma- The electrode plug 12 is drilled and tapped to receive the threaded stud member 4. The plug is also drilled at circumferentially spaced intervals from the top toV provide the well portions 14 for engaging a spanner wrench'during assembly. The sealing compound is an asphaltic bituminous compound and provides increased insulation between the plug 12 and the electro-static shield 31, thereby increasing the iiashover/sparkover, ratio. It is preferred to position an insulating sleeve, such as a paper tube 16, circumjacent to the stud 4 prior to pouring ofthe sealing compound`15.

The opposite end of theltubular retainer member 13 ispreferably threaded to receive the lower electrode or terminal fitting 17 externally thereof. This 'fitting 17 serves concurrently as a closure for the chamber 2, as a support for the expulsion tube hereinafter described, and as a lower terminal member to which-a ground wire (not shown)v may be connected. A cushioning gasket 18 is provided between the retainer member 13 and the terminal 17.

The conductive fitting 17 is provided with a discharge passage 19 for releasing gases to the atmosphere. A means is provided for connecting the arrester with ground, shown here as a solderless connector in .the form of abolt 2t) engaging the complementary threaded opening in the fitting 17. The internal surface`21 of the fitting 17 acts as a lower arcing electrode member for the internal components of the retainer member 13.

The expulsiontube 25 is best described with reference to both Figs. l and 2 and is preferably in contact with plug 12, comprising two semi-cylindricalhalf- portions 25 and 27. Gripped between the half-portions is a cylindrical metal insert 28 which serves as a spark and arc electrode. The half-portion is also appropriately slotted to receive and secure an intermediate electrode 29. This is `an elongate strip of metal, preferably of square crosssection, beveled at its two ends, the'lower end being shown in Fig. l, and disposed immediately adjacent the apex edge of passageway 30, as clearly depicted in Fig. 2being spaced from both the upper electrode 28 and the lower electrode comprising the surface Z1 of the fitting 17, to form two spaced spark-gapsV within the expulsion chamber.

One of the important features characterizing the arrester shown in Fig. 1 and more comprehensively de scribed and claimed in the aforementioned Stoeltingpatent, is the cross-sectional configuration of the expulsion chamber 3h. From an examination ofFig. 2 it will be seen that the configuration is slot-like, and is narrowly constricted along one edge portion and considerably distended along the opposite edge portion. The half- .portion 26 may be a true sector, whereas the'half-portion 27 may contain the milled slot along its longitudinal axis. Both half-portions are of an arc-extinguishing material, such as horn fiber. This material is capable of producing copious quantities of arc-extinguishing gases which cool and de-ionize any arcs produced between the electrode members 2S, 29 and the surface 21 of fitting 17.

The arrester operates in the conventional manner applicable to expulsion type arresters. When a surge occurs on the power line and is of a high enough potential, it will simultaneously spark over the external gap and the internal expulsion gaps between the electrodes 28 and 29, and 29 and 21, the latter being electrically connected with ground through the fitting 17.

Under certain conditions the surge may initiate the tiow of power follow-current making it necessary to extinguish this follow-currenty arc, in order that the arrester may return to its original operating condition. In the arrester described this normally 60 cyclecurrent produces an arc that tends to follow the path taken by the abnormal surge voltage which sparked over the arrester electrodes, and will tend to be confined to the relatively constricted portions of the chamber 30, where the pressures built up will tend to extinguish the arc, along with the cooling and de-ionization characteristics contributed 'will withstand relatively large forces exerted thereon, Yin wall thicknessesacceptable'forthe internal Ydiameter of the chamber 2 of the housing 1. It is the usual practice to prick punch the sleeve 31 to the tubular retainer member 13 in at least three circumferentially spaced positions 32 as shown in Figs. 1 and 2. The prick punch provides a means for fastening the sleeve to the member to prevent its movement in both axial and rotative directions.

As has been heretofore stated, it was the practice in the past to abut the edge 33 of the sleeve or shield 31 against the upper ksurface 34 of the fitting or electrode 17. It is the general practice in the trade to provide fittings as shown with a flanged portion 35 for supporting the porcelain housing 1 at the lower end. It will be obvious that this engagement had to be maintained in secure contact through a major portion of the abutting area, or radio and television interference is very likely to occur, andhas occurred in Vthe past.

The shield has two functions: One function is to provide strength to vthe relatively weak horn bre tubular member, and secondly, it is the function of the sleeve or shield 31 to provide a means of redistributing dielectric stressl by serving as a shunt capacitance bridging the arc chamber '30,! VThis produces increased dielectric stress of the external gap 10, thus controlling and reducing arrester sparkover.

Ifhave found that the metalshield need not'be solidly grounded to control arrester sparkover levels. The space or gap 36, shown in Figs. l and'3, may be of considerable dimension without affecting the shields ability to performits duty as a sparkover control. Having these two items spaced apartrather than positively connected electrically is of great convenience during manufacture. I have found that within an arrester length of 9 measured from the top Vof the tubular retainer member 13 tothe lower end of the fitting.17, the gap 36 may be Vas much as one-half inch. However, this is a maximum limit and itis most desirable to keep the gap spacing within 1/16" minimum to Ms" maximum.

It will be apparent that although the above-mentioned dimensions have been given, they are merely illustrative for the particular arrester embodiment described. The actual gap dimension applicable to other arresters is affected by factors such as the effective length of the of the'arrester in circuit with a cathode ray oscillograph to determine the degree of impulse sparkover control. It willbe obvious'that the maximum gap dimension is dependent von Asparkover 'control by the shield, while he minimum dimension is reflected by the radio noise evel.

It will be apparent from the above description, taken inrconnection with the drawing, that 'an improved expulsion type lightning arrester-has been provided, which arrester utilizes an electro-static shieldexteriorly of the expulsion tube and which shield is spaced a predetermined distance from its'cooperating electrode member to prevent corrosion and radio andtelevision interference.

I claim:

'1. In a lightningV arrester ,ofthe expulsion type including spaced electrodes, an insulating tubular retainer member having a bore,1arcfextinguishingmaterial contained within said bore, one ofjsaidelectrodes .being positioned at one end of said retainer member and having a portion ,extending radially outwardly therefrom; .a tubular electro-static shicldmem'ber formed, ofaconductive material'structurally stronger than saidretainer member and positioned externally circumjacent'tosaid insulating tubu- Ylar'retainer member intermediate said spaced electrodes to prevent bursting of said retainer member, the marginal edge of one end of said shield and said radially extending portion of said one electrode jointly defining a predetermined annular, spaced gap dimension therebetween.

2. In a lightning arrester of the expulsion type including spaced electrodes, an insulating tubular retainer member having a bore, arc-extinguishing material contained within said bore, one of said electrodes being positioned at one end of said retainer member and having a portion extending radially outwardly therefrom, and a tubular electro-static shield of a conductive material positioned externally circumjacent to said insulating tubular retainer member intermediate said spaced electrodes, and wherein said shield and said one electrode are of dissimilar metallic materials capable of acting as galvanic action electrodes in the presence of an electrolyte, the arrangement of said shield with respect to said one electrode wherein the marginal edge of one end of said shield and said radially extending portion of said one electrode jointly denes a predetermined annular, spaced gap dimension therebetween, whereby said galvanic action will be prevented from occurring.

3. In a lightning arrester of the expulsion type including spaced electrodes, an insulating tubular retainer member having a bore, arc-extinguishing material contained within said bore, one of said1 electrodes being positioned at `one end of said retainer member and having an annular ange extending radially outwardly therefrom; a tubular electro-static shield formed of a conductive metal that is structurally stronger than said retainer member and positioned externally circurnjacent to said insulating tubular retainer member intermediate said spaced electrodes to afford redistribution of the electrical flux while preventing bursting of said tubular retainer member, the marginal edge of one end of said shield and said radially extending ange of said one electrode jointly defining a predetermined annular, spaced gap dimension therebetween.

References Cited in the le of this patent UNITED STATES PATENTS 2,628,322 Yonkers Feb. 10, 1953 2,663,817 De Val Dec. 22, 1953 2,664,518 Eldridge Dec. 29, 1953

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