US2907910A - Protective electrical discharge devices - Google Patents

Description

Oct. 6, 1959 L. o. MARSTELLER PROTECTIVE ELECTRICAL DISCHARGE DEVICES Filed Aug. 20, 1956 w m wfiwv 1H dca maua a 5 y e da 6 ohflmm d 1 0 m a Z 2 INVENTOR. Leszer 0. NarszeZlez- Fig. 4. BY (0,4. M

H15 ATTORNEY Unite States Paten 2,907,910 PROTECTIVE ELECTRICAL DISQI-IARGE DEVICES Lester O. Marsteller, Forest Hills, Pa.-, assignor to Westinghouse Air Brake Company, Wilmerding, Pa., a corporation of Pennsylvania Appiication August 20, I956, Serial No. 6il4,863

8 Claims. or. 313-231) My invention relates to electrical discharge devices, and particularly to lightning arresters for protecting low voltage apparatus against voltage surges. My invention is particularly useful in protecting railway signaling and other low voltage circuits but is not necessarily limited to this particular type of application.

This application is a continuation-in-part of my copending application for Letters Patent of the United States, Serial No. 372,860, filed on August 7, 1953, now abandoned, for Protective Electrical Discharge Devices.

Railway signaling circuits are for the most part exposed to the hazards of electrical storms and require adequate and positive protection against voltage surges due to lightning to maintain the proper and safe operation of such circuits. Various types of lightning arresters have been available for protecting such circuits against volt age surges but it has been found that such arresters do not have a low enough breakdown voltage to protect sensitive equipment. In certain instances such arresters either disintegrated upon discharge, deteriorated in that the physical dimensions of the arresters were altered to a sufficient extent to change their electrical characteristics, fused, or short circuited. Any one of these types of failure of'a lightningarrester used in a railway signaling circuit could result in the improper functioning of the circuit and might result in undesirable operating conditrons.

It has been found that a lightning arrester, to aiford proper protection for the electrical apparatus of a low voltage circuit, must have a sufficiently low breakdown voltage, must not disintegrate, deteriorate, fuse, or short circuit, and must be capable of discharging successive voltage surges. Should'a lightning arrester disintegrate, :the circuit is unprotected in the event of a succeeding :surge. Should the electrodes of the lightning arrester .deteriorate in that the physical dimensions of the air ;gap increase with successive voltage surges, then voltages greater than the permissive amplitude may be applied :across the electrical appartus. In the event the electrodes of the lightning arrester fuse or are short circuited, then a condition exists which prevents proper functioning of the protected circuit.

In lightning arresters of the types heretofore available employing spaced electrodes separated by an air gap it has been noted that the location or configuration of the .air gap was frequently a factor in the failure of the arrester. Alightning arrester of this type in order to be .able to withstand successive surges must either be pro vided with electrodes which will not deteriorate, or if the electrodes do deteriorate, there must be provided some :means for maintaining a substantially constant air gap .upon successive discharges of the voltage surges. It has .been found that when discharges take place between the electrodes of an arrester of this type, gases are liberated, .and unless means are provided to permit the escape of ':these gases, damage to the arrester will result. It has .also been foundthat in many of the existing types of .arresters, the discharge of the voltage surge would take place between the metal brackets supporting the electrodes instead of within the air gap between the electrodes. The arc caused by the voltage surge tends to 'follow a straight line and the electrodes of previous protectors were mounted in such a way that the air gap between the electrodes was not in this straight line path.

It is therefore an object of my present invention to provide a protective electrical discharge device for electrical equipment capable of withstanding successive voltage surges without failure of the electrodes due either to disintegration, deterioration, or short circuiting.

Another object of my invention is to provide a protective electrical discharge device wherein a straight line conductive path is provided for the voltage surge from the terminals to the electrodes so that the discharge will take place only in the spacing between the electrodes.

Still another object of my present invention is to provide a series multi-surface gap discharge device wherein the breakdown voltage is less than half the breakdown voltage of prior art discharge devices having spacings between the electrodes of comparable widths.

A further object of the present invention is to provide a protective electrical discharge device wherein the gases resulting from electrical discharges are vented in a manner to prevent sufficientvpressure from building up to cause damage to the arrester and to assist in extinguishing the arc resulting from the discharge.

In carrying out the objects of my invention I provide two highly conductive electrodes rigidly secured in spaced relation. The two. electrodes are of substantial construction and are so formed that during the discharge of the voltage surge, no parts of the electrodes are either ruptured, broken off, or seriously eroded. In the preferred form of my invention, the electrodes are circular or disk-shaped. The electrodes of my lightning arrester are preferably so assembled that the conductive path to the electrodes is the shortest possible path between the terminal connectors. The assembly of the electrodes is further characterized by the provision of means for limiting the discharge of the voltage surge to a shallow peripheral groove between the electrodes. By providing the shortest possible conductive path for the voltage surges through the electrodes, the inherent tendency of the voltagesurges to form arcs across the shortest paths provided by the terminals or brackets of previous arresters has been eliminated. By confining the discharge to a shallow peripheral groove between the electrodes, the expanding gases and air due to the discharge are not trapped between or within cavities formed in the electrodes as in some prior devices of this type. The expanding gases thusblow outwardly and away from the electrodes any particles resulting from the discharge of the arc, which particles might tend to short circuit the electrodes.

The electrodes incorporated in the lightning arrester embodying my invention may be of various materials. I have found that electrodes made of metal such as brass or of a carbonaceous material impregnated with a conductive material such as silver, when spaced apart by a ember of insulating material capable of withstanding arcing temperatures, will withstand repeated discharges without appreciably altering the operational characteristics of the lightning arrester. In a lightning arrester employing such electrodes, I have found that after repeated discharges, the spacing remained substantially the same width, with the result that the breakdown voltage remains substantially constant.

The electrodes of my novel lightning arrested are separated by a washer of insulating material clamped therebetween, the thickness of the washer determining the spacing between the electrodes. The separating washer is of smaller diameter than the diameters of the opposing faces of the electrodes, thereby delimiting the spacing to a shallow peripheral groove. While washers of various insulating materials may be used in assembling the lightning arrester, I prefer to use a soft material having the characteristics of being resistant to carbon tracking and of accepting the deposition of generally individual minute particles of vaporized metal. One such material which I have found satisfactory is a tetrafluorethylene material sold under the trade name of Teflon.

Other objects and characteristic features of my invention will become apparent as the description proceeds.

I shall describe several forms of a protective electrical discharge device embodying my invention, and shall then point out the novel features thereof in claims.

In the accompanying drawings wherein similar reference characters designate similar parts throughout the several views:

Fig. 1 is an elevational view of a lightning arrester embodying my invention, the essential elements of the arrester being shown in vertical section.

Fig. 2 is a top plan view of the lightning arrester illustrated in Fig. 1.

Fig. 3 is a sectional view taken along the line III-III of Fig. 1.

Fig. 4 is an isometric view of an electrode forming a discharge element of the lightning arrester.

Fig. 5 is a fragmentary vertical section of the lightning arrester showing the spacing between two metallic electrodes after repeated discharges, while.

Fig. 6 is a fragmentary vertical section of a lightning arrester showing a V-shaped spacing between the two electrodes.

Referring now in detail to the drawings, the reference numeral 1 designates in general a lightning arrester embodyingmy invention. The lightning arrester 1 comprises two discharge elements or electrodes 2 rigidly secured in spaced relation to provide a spacing between the elements of suitable proportions for a given operating condition, the width of the spacing determining the breakdown voltage at which discharge will occur. The discharge elements or. electrodes are made of a suitable conductive material which will not short circuit or otherwise substantially deteriorate upon discharge of the device. I have found that a carbonaceous material such as graphite or graphitized carbon, or graphite or graphitized carbon having imbedded therein, either by impregnation or mixture, a latticework of a highly conductive material such as brass or silver, provide excellent electrode material. Graphitized carbon impregnated with silver in particular has been noted to'make excellent electrode material. It is applicants belief that, due to the high affinity of oxygen for silver at the fusion temperature and the subsequent release of the oxygen when the silver resolidifies, the consequent expansion of the gases resultquirement, and at the same time otherwise facilitates manufacture of the arrester, I prefer to make the'electrodes 2 in the form of a disk, as shown in Fig. 4. The electrodes 2 are formed with a central recess 2a and are mounted on a sleeve 3 of insulating material with the electrode recesses facing each other. A washer 4, of insulating material resistant to carbon tracking and capable of withstanding arcing temperatures without electrical breakdown, is interposed between the two electrodes. The material for the washer 4 should also be soft so that under pressure when the two electrodes 2 are clamped together in the manner hereinafter described, the washer will conform to any minute surface irregularities of the electrode faces and be in intimate engagement ing from the release of the oxygen aids in quenching the arc. It has been noted that such electrodes were cool to touch immediately after an arc discharge. I have also found that graphite and graphitized carbon provide a very stable material to act as the matrix for the imbedded latticework. The line particles of silver disposed in this matrix do not supply enough metal at one spot to build a metallic bridge across the spacing or melt out a crater on the faces of the electrode, so that appreciable pitting does not occur. Repeated discharges do not appear to remove enough of these materials to appreciably change the discharge voltage.

I have also found that electrodes made of brass are suitable for my lightning arrester. In Fig. 5 of the drawings I have illustrated two electrodes 22 made of brass, the condition of the two electrodes illustrated being after the arrester was subjected to thirty high voltage surges.

The discharge elements or electrodes 2 are of rugged construction and preferably have no protrusions, extensions or teeth formed on any of the faces or edges. This is to prevent any fracture or breaking off of the electrode material during discharge. Since a. disk meets this retherewith. The washer 4 should preferably be of a material which accepts the deposition of minute particles of vaporized metal for reasons hereinafter appearing. Of the insulating materials of this type available I have found that a washer made of Teflon, a tetrafluorethylene material, is preferable for the intended use.

The thickness of the washer 4 determines the spacing between the electrodes, washers of different thicknesses being interposed between the electrodes for specific breakdown voltages. For reasons hereinafter apparent, the diameter of the washer 4 is preferably made smaller than the diameter of the electrodes but greater than the diameter of the recesses 2a in the electrodes.

As will hereinafter appear, the electrodes 2 and the washer 4 are clamped together to form a rigid unit. The washer 4 is tightly clamped between the electrodes in the manner of a gasket so that upon discharge of the electrodes, no particles can or will be blown inwardly toward the center. The soft material of the washer 4 when made of Teflon aids in effectively sealing the recesses 2a in the electrodes 2. The recesses 2a formed in the electrodes provide a greater spacing between the central portions of the electrodes so that a greater breakdown voltage is necessary for discharge between the centers of the electrodes." In this way the discharge is limited to a shallow peripheral groove between the electrodes so that any particles resulting from the discharge will be blown outwardly and away from the electrodes to prevent such particles from forming short-circuiting paths between the electrodes.

While I have illustrated the spacing between the electrodes 2 as having parallel sides, I have found that excellent results are also obtainable when the opposing faces of the electrodes are slightly beveled as at 2b (Fig. 6) to provide a substantially V-shaped spacing. It will be noted in Fig. 6 that the inner parallel faces of the electrodes extend slightly beyond the area of contact with the insulating washer 4, the beveled surfaces which are approximately 10 with the vertical, terminating short of the washer 4. I thus provide a spacing having an inner portion of predetermined length for a particular breakdown voltage and an outer portion of increasing length which permits the discharge of the electrodes after its initial burst between the parallel faces of the air gap to travel outwardly toward the perimeter of the electrodes. The arcing of the electrodes and, as will hereinafter appear, the "expulsion of expanding gases and particles are thus in a radially outward direction, away from the center of the electrodes.

Two brackets 5 of suitable electrical conducting material are provided for rigidly securing the electrodes 2 in spaced relationship. Each of the brackets comprises a horizontal arm 5a formed with a slot 5b therein, and a vertical arm So having an opening 5d formed therein; the upper end 5e of the vertical arm being arcuate in form. The ends of the insulating sleeve 3 supporting the electrodes 2 and the washer 4 are received within the openings 5d formed in the vertical arms of the bracket; and a washer 6 of a soft, cushioning, conductive material such, for example, as lead or copper, is mounted on the sleeve and interposed between the electrodes 2 and the vertical bracket arms. The washer 6, however, is not essential to myinvention and may be omitted if desired.

It will be noted that the horizontal arms 5a of the brackets are in a straight line and that the lower portion of the spacing between the electrodes. is in line with the two bracket arms. Thus a straight line path is provided for the voltage surges through the spacing between the electrodes so that no shorter path exists over which discharge may occur as in previous arresters of this type.

Supported on the outer end of the insulating sleeve 3 which extends through a bracket opening 5d is a cover member 8, the cover member being provided with an integral end wall 8a formed with an opening 8b for receiving the extending end of the sleeve 3. A bolt 9 passing through the sleeve 3, a nut 10 and the usual washers 11 and lock washer 12 clamp the described elements together to form a rigid unit. It will be appreciated that the brackets, washers, cover member and electrodes may be secured together in any suitable manner; the sleeve 3 and bolt 9 being replaceable by a bolt made of an insulating material, or by an insulating bushing integral with the cover.

The cover member 8 is preferably made of a noncarbon-tracking, heat-resistant-material having a generally dome shaped upper portion terminating in downwardly extending skirts. The cover member cooperates with the brackets 5 to form a vented enclosure for the electrodes 2. It will be noted in Figs. 1 and 3 that the bracket arms 5d are smaller than the internal dimensions of the cover member so that an air space is provided around these elements as well as the electrodes 2. The enclosure formed by the brackets 5 and the cover member 8 is vented around the vertical bracket arm 50 as indicated generally at 7, and also vented at the bottom. The cover member extends slightly beyond the bracket arm 50 to provide a hood for the vent 7 to prevent the entry of deleterious material into the vented enclosure formed by the cover member and the brackets.

The cover member as herein illustrated is of an insulating material, but it will be understood that the cover member may be made of any suitable material if proper precautions are taken to prevent short circuiting the brackets or the electrodes.

A suitable base 13 of insulating material, such as a phenol condensation product, is provided for mounting the assembled discharge elements. The base 13, as shown, is a standard terminal block of the kind commonly employed in railway signaling, and is formed with a centrally disposed, elongated recess 14 in the top surface thereof, the recess being immediately below the opening in the cover member 8 and providing an exhaust space for the expanding gases and particles due to discharge of the electrodes. Spaced bolt openings 15 extend through the central portion of the base for receiving mounting bolts (not shown) for securing the base to any desired support. Molded or otherwise secured in the base 13 are two upright terminal posts 16. Received on the terminal posts are conductive washers 17 which are clamped against the base by nuts 18 threaded on the posts. The slots 5b in the horizontal bracket arms receive the terminal posts 16, the brackets being secured to the terminal posts between the nuts 18 and soft conductive washers 17 by the clamping action of the nuts 19. External connections for the lightning arrester are made to the terminal posts 16 by clamping such connections between conductive washers 20 by means of nuts 21.

The width of the spacing between the electrodes of some prior lightning arresters determines the minimum voltage at which discharge will occur. In protecting electrical equipment, the surge voltages which would damage the sensitive equipment are sometimes not very much 'larger in amplitude than the operating voltages. In low voltage equipment the damaging surge voltages may be :slightly above 250 volts. In previous lightning arresters, .a spacing of five mils required a breakdown voltage of from 500 to 600 volts,,while a spacing of 30 mils required a breakdown voltage of between 2000 to 2500 volts. As will hereinafter appear, lightning arresters made in accordance with my invention have breakdown voltages approximately less than half the breakdown voltages of prior art devices.

The washer 4, in addition to determining the width of the spacing between the electrodes of a lightning arrester embodying my invention, functions to limit the discharge area between the electrodes to the outer portions of the opposing electrode faces, and also provides a series, multisurface gap arrester.

The washer 4 in conjunction with the recesses 2a provided in the opposing faces of the electrodes prevents the discharge of the electrodes in a confined space as heretofore prevalent in most other discharge devices. Thus the arcing between the electrodes takes place in an open space from which the expanding gases and air due to the arcing may readily escape, the expanding gases and particles being blown radially outwardly from the air gap. By limiting the air gap between the electrodes to a shallow peripheral groove, a high gas expulsion rate is produced without building up dangerous pressures by trapping the expanding gases. The clearance and shape provided by the cover member 8 around the electrodes permit the expanded gases to force their way out through the open side and bottom of the enclosure formed, the recess 14 in the base providing additional space for the escaping gases and particles. Thus any dust which may result from the discharge is blown out of the spacing and away from the electrodes so that no leakage paths are formed between the electrodes. .The domed enclosure provided for the electrodes and vented in the manner described deflects most of the particles due to discharge downwardly into the base recess.

The particular blow-out feature of a lightning arrester embodying my invention is strikingly apparent when the electrodes are made of a metallic conductor such as brass, as in Fig. 5. Although metallic electrodes have been used in the past for electrode material in lightning arresters it has been found that such electrodes either fused readily or deteriorated. The fusing or shorting of metallic electrodes in previous arresters is believed to be due to the accumulation of conducting material across the spacing due to the particular construction of prior lightning arresters and possibly to the particular material used, including the separating washer.

In addition to providing a shallow peripheral groove between the electrodes of the lightning arrester, I have found that the washer 4, when made of a material such as Teflon, provides a series, multi-surface air gap which materially reduces the discharge voltage of the arrester. In an arrester constructed in the manner described, a 5 mil air gap requires a breakdown voltage of only 75 to 250 volts, while a 30 mil air gap requires a breakdown voltage of only 600 to 1000 volts.

I have found that when a discharge takes place between the electrodes of my lightning arrester two things occur which may explain the reason for this low breakdown voltage. After discharge, the periphery of the separating washer appears quite glossy as though a portion of the outer periphery of the washer were vaporized. There also appears on the periphery of the washer minute particles of vaporized metal, some of the metal particles being joined together but not to the extent that would short circuit the electrodes. It would appear from the examination of the washer that a series, multi-surface discharge gap is provided by the minute particles of vaporized metal, the discharge of the electrodes taking place along the peripheral surface of the washer from particle to particle. Apparently, after each discharge of the electrodes, a minute portion of the washer periphery is vaporized together with the particles of the deposited metal, a new peripheral surface and its attendant metal depositions being formed immediately thereafter. The

vaporization of the peripheral surface of the separating washer progressively changes the diameter of the washer, a diiference in the washer diameter being measurable after about thirty successive discharges of the device. The

decrease in the diameter of the washer is approximately equal to the deterioration of the electrodes at the point of'discharge so that the adjoining surfaces of the electerioration of the faces adjacent the outer edges of the electrodes did take place, but a section through the electrodes reveals that the spacing is cup-shaped, the width at the bottomv of the spacing being maintained by the washer 4. The electrodes are not undercut on the faces adjacent to the washer 4, the deterioration of the faces from their outer edges generally decreasing so that an a-rcuate section results at the bottom of the spacing. Thus by providing a lightning arrester with electrodes wherein the discharge is confined to the peripheral groove therebetween, which groove is vented to atmosphere, and a separating washer made of Teflon or the like, I am able to maintain the protective characteristics of the lightning arrester in spite of the deterioration of the opposing faces of the electrodes and the minute reduction in the diameter of the washer.

Although I have herein shown and described several forms of a lightning arrester embodying my invention, it is to be understood that various changes and modifications may be made therein Within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. A protective electrical discharge device comprising, two conductive electrodes, an insulating member made of a tetrafluorethylene material separating said electrodes to provide a spacing between the electrodes lim ited to a shallow peripheral groove, and bracket means for supporting said electrodes and member, said bracket means including aligned supporting arms which are substantially in line with a portion of the shallow peripheral spacing between the two electrodes.

2. A protective electrical discharge device comprising, two spaced brackets of conductive material, two conductive electrodes secured to said brackets in predetermined spaced relation, a Teflon spacer clamped between said electrodes, and a dome-shaped member secured to but insulated from one of said brackets and overlying said electrodes and a portion of the other of said brackets with some clearance providing a vented enclosure for said electrodes permitting the escape of the expanding gases and particles upon discharge of the electrodes.

3. A protective electrical discharge device comprising, two spaced brackets of conductive material, tWo conductive electrodes secured to said brackets in predetermined spaced relation, a Teflon spacer clamped between said electrodes, and a dome-shaped cover member open at one side and at the bottom and secured to but insulated from one of said brackets; said member overlying said electrodes and the other of said brackets with some clearance, said cover member forming a vented enclosure for said electrodes which is vented at one side and at the bottom to permit the escape of expanding gases and particles due to the discharge of the electrodes, said dome-shaped member extending beyond said other bracket and forming a hood for the vented side of the enclosure formed.

4. A protective electrical discharge device comprising, a pair of electrodes, a spacer of insulating material separating the opposed faces of said electrodes and limiting the discharge area between said electrodes to a shallow groove, and a plurality of conductive particles adhering to the surface of said spacer in the groove between said electrodes forming a series multi-surface discharge gap between said electrodes. 1

5. A discharge device comprising, a pair of electrodes, a spacer of insulating material separating said electrodes, and a plurality of particles of conductive material adhering to the surface of said spacer between said electrodes forming a series multi-surface discharge gap between said electrodes.

6. A discharge device comprising, a pair of electrodes, and a washer of insulating material separating theopposing faces of the electrodes and limiting the discharge area between the two electrodes to a shallow peripheral groove, the material of said washer having an aiiinity for the deposition of particles of vaporized metal, and a plurality of metal particles forming a seriesmultisurface discharge gap on the periphery of the washer between said electrodes.

7. A discharge device comprising, a pair of electrodes, and a Washer of insulating material separating the opposing faces of the electrodes and limiting the discharge area'between the two electrodes to a shallow peripheral groove, the material of said washer being soft to conform to minute surface imperfections of the opposing surfaces of the electrodes and having an affinity for the deposition of particles of vaporized metal, and a plurality of metal particles forming a series multi-surface discharge gap on the periphery of the washer between said electrodes,

8. A discharge device comprising, a pair of electrodes, and a spacer of Teflon material separating the opposing faces of said electrodes, the material of said spacer having an affinity for the deposition of particles of vaporized metal and a plurality of metal particles forming a series multi-surface discharge gap on the surfaces of said spacer between said electrodes.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. A PROCTECTIVE ELECTRICAL DISCHARGE DEVICE COMPRISING, TWO CONDUCTIVE ELECTRODE, AN INSULATING MEMBER MADE OF A TETRAFLUORETHYLENE MATERIAL SEPARTING SAID ELECTRODES TO PROVIDE A SPACING BETWEEN THE ELECTRODES LIMITED TO A SHALLOW PERIPHERAL GROOVE, AND BRACKET MEANS FOR SUPPORTING SAID ELECTRODES AND MEMBER, SAID BRACKET MEANS INCLUDING ALIGNED SUPPORTING ARMS WHICH ARE SUB-

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