US2914742A

US2914742A - Valve block for lightning arresters

Info

Publication number: US2914742A
Application number: US664122A
Authority: US
Inventors: Donald L Heath
Original assignee: Ohio Brass Co
Current assignee: Ohio Brass Co
Priority date: 1957-06-06
Filing date: 1957-06-06
Publication date: 1959-11-24
Anticipated expiration: 1976-11-24

Description

Nov. 24, 1959 D. L. HEATH VALVE BLOCK FOR LIGHTNING ARRESTERS Filed June 6, 1957 Fig.l

INVENTOR. DONALD L. H EATH.

mum

ATTORNEY.

United States Patent 2,914,742 VALVE BLOCK FOR LIGHTNING ARRESTERS Donald L. Heath, Barberton, Ohio, assignor to The Ohio grass Company, Mansfield, Ohio, a corporation of New ersey Application June 6, 1957, Serial No. 664,122

8 Claims. (Cl. 33821) This invention relates to non-linear resistors for lightning arresters and the like, more particularly designated as valve blocks.

It is a principal object of the invention to increase the durability of valve blocks for lightning arresters.

Another object of the invention is to reduce the size and massiveness of valve blocks required for lightning arresters of a given current capability.

A specific object of the invention is to prevent flashover and failure along the outside surfaces of valve blocks for lightning arresters.

In the art pertaining to lightning arresters, it is known to use non-linear resistors in series with are gaps for limiting the flow of system energizing current, from the conductor to which the arrester is connected to ground, following the breakdown of the lightning arrester due to an over-voltage such as might be produced by a lightning discharge. Such non-linear resistors are designated as valve blocks and are ordinarily formed of silicon-carbide granules with a binder of ceramic or other material and have the property that the resistance decreases rapidly as the voltage across the block is increased. It is known, also, that upon the application of large impulsive voltages to such valve blocks, there is a tendency for the blocks to break down or are over along the external surface of the block.

Various measures have been utilized to minimize such are over or breakdown such as by applying coatings of lacquers or paints to the exterior surface of the block or by providing a ceramic coating on the exterior of the block. Such coating arrangements or collars, as designated by those in the art, while generally satisfactory, have been somewhat objectionable because of the high cost of manufacture. Particularly, when forming the painted collar, it has been necessary to spray a number of coats upon the block, allowing each coat to dry before the subsequent coat is applied. The ceramic collar is particularly satisfactory, but necessitates a complex forming operation prior to firing. Further, in manufacture, the production of valve blocks having either of these types of collars results in relatively large numbers of imperfect or reject blocks not satisfactory for utilization in an artester.

I have found that, in accordance with the present invention, a collar may be formed about the exterior of the valve block to produce a valve block of greater durability than has heretofore been possible and in which the valve block may be produced with consistently good quality and with a minimum of manufacturing variation which would tend to result in imperfect or reject blocks. In particular, in accordance with the invention, I use paraffin or carnauba wax as a coating material and apply the wax in such a manner that the wax penetrates a predetermined distance into the block from the exterior thereof. I have ICC found, moreover, that a depth of penetration from the exterior of the block inward in the order of A to of the diameter of the block results in a collar having the desired superior qualities.

The invention, together with further objects, features and advantages thereof will be more clearly understood by reference. to the following detailed specification and claims taken in connection with the appended drawing, in which:

Fig. l is a top view of a valve block;

Fig. 2 is a section view of the block of Fig. 1 taken along the line 22; and

Fig. 3 is a photograph showing the cross section of a valve block according to the invention and illustrating particularly the penetration of the collar wax into the body of the block.

Referring now to Figs. 1 and 2, the valve block 10 comprises a cylindrical body 11 having two flat metallic electrodes 12 and 13 disposed substantially over the ends of the block.

The body 11 is composed of silicon-carbide granules held in intimate engagement by a binder such that the body is a dense hard mass having semi-conducting properties due to the surface contacts of the granules. The electrodes 12 and 13 are in intimate electroconductive engagement with the silicon-carbide granules at the ends of the body and serve as terminals for connecting the valve block in an electric circuit for utilizing the semi-conductive properties of the body. Electrical devices employing valve blocks as non-linear conductive devices have spark gaps, fuses, or other mechanical or control devices connected thereto and it is intended that the valve block of the present invention be utilized in any of such applications.

The structure of the body 11 is not continuous, but is composed of silicon-carbide crystals with the binder interdispersed between the grains and holding the grains, each with the other, in intimate contact to form the semi-conducting current paths. In accordance with the present invention, the body 11 is impregnated with wax or parafiin to fill the voids between the grains of the body from about the outermost extremity of the grains of the block and inwardly into the interior of the block for a predetermined distance indicated, for example, at 14 in Fig. 2. The body of wax or paraffin impregnant is referred to herein as a collar for the block. The wax extends inwardly from the outer extremity of the block, defined principally by the extremity of the outermost silicon-carbide grains of the hardened mass, for a distance to V of the diameter of the block.

The wax functions to fill the spaces between the grains so as to provide a body having a relatively high dielectric strength in the voids between the grains, e.g. the spaces which have not been filled by the binder during the forming and firing of the block. The wax, of course, displaces the air which would otherwise occupy the voids and thereby produces a great increase in the resistance toflashover as between the individual grains of the mass. Because of the high dielectric strength of the wax, the current flow adjacent the lateral extremities of the body of the block must necessarily be from granule to granule of siliconcarbide and the breakdown which would otherwise occur in the spaces between the granules is prevented.

I have found that the depth of penetration of the wax into the body between the silicon-carbide granules is broadly critical, that is, that it is important that there be an actual penetration between the grains and not a mere formation of the coating on the exterior surface of the body or simply between the grains of the immediate outer extremity of the block. The collar does, of course, include a layer of wax over the outside of the outermost silicon-carbide grains because of the forming process.

Excessive penetration of the wax into the body is, however, not effective to produce the desired advantageous results. A block which is completely impregnated, that is, in which the wax is penetrated throughout the granular mass, will flashover immediately upon the application of the high impulse voltages as though no collar whatsoever had been provided under like circumstances. This is a particularly important aspect of the invention and the depth of penetration of the wax into the body is controlled in the manufacturing process as hereinafter set forth.

The binder of the valve block body referred to herein may constitute either the so-called phosphate binder, such as described in U.S. Patents 2,206,792 and 2,589,157 to B. Stalhane, or a porcelain binder, such as is described in my co-pending application Serial No. 711,926, filed January 29, 1958.

l have designated the impregnating materials employed herein as waxes, that term designating a class of materials having the necessary viscosity and dielectric characteristics to accomplish the intended purposes. i prefer to use paraffin or carnauba wax or beeswax, although any of the synthetic waxes such as the polyethylene waxes or carbo waxes are satisfactory. Moreover, I exclude, by that term, the shellacs and similar materials employed in the prior art in coating the exterior of the valve block body.

in manufacturing the valve block of the invention, I first prepare the mixture of silicon-carbide granules with the binder, e.g. the phosphate materials or clay mixes previously referred to, and mold the mix into the desired form by the application of pressure. The molded mix is then fired to form the semi-conducting body for the valve block. Thereupon, after cooling sufficiently, the body is immersed in a bath comprising a molten mass of wax for a period of ten seconds or thereabouts, depending upon the binder which is utilized in the block. The body is then removed from the wax bath, the excess wax allowed to drain off the body, and the wax is then allowed toharden and set. The electrodes 12 and 13 are then formed on the ends of the block by spraying the metal directly onto the block. No preparation of the body or removal of wax is necessary, inasmuch as the molten metal will fuse the wax to permit engagement of the metal with the silicon-carbide grains.

If any substantial interval occurs between the time in which the body is fired and the impregnating operation takes place, the valve block body should preferably be heated in order to thoroughly dry the body and drive off any moisture which has penetrated into the body. l have found that a block temperature of about 250 F, prior to immersion, permits uniform penetration of the wax without buildup on the external surfaces.

The following table indicates typical immersion times for paraffin or carnauba wax for a block of the type shown in Fig. 3:

Time, Temperature, Penetration,

sec. F. inches The figures above are given for a circular block having a diameter of 3" and a height of 2 /2. The block of Fig. 3 had a collar of paraffin penetrated into the block a distance of A2. from the outer surfaces thereof. The

4 electrodes extended to within from the edges of the block.

The body may also be rolled in hot wax and a spraying process may be used. However, it is difficult to control the depth of penetration of the wax when these processes are utilized. Particularly in the case of spraying, the wax tends to form a surface layer without sufficient penetration of the wax into the body.

I have found, also, that the treatment of the block after immersion will effect the penetration of the wax into the body. If the fired body is placed in an oven after immersion, some of the wax will continue to penetrate into the block, although this effect will be minimized if the body is heated before immersion. If the body is at room temperature before immersion, the wax will build-up on the surface of the body and penetrate into the body upon subsequent heating. The density of the block will, of course, be a determining factor in the extent of penetration for a given temperature and the given wax. Further, the penetration will ordinarily be greater from the ends of the body than from the sides if the body is totally immersed. This is primarily due to the greater density and orientation of the granules on the lateral surface of the block.

It is to be understood that the foregoing description is not intended to restrict the scope of the invention and that various modifications thereof may be resorted to, giving effect to a liberal interpretation to the claims as herein set forth.

I claim:

1. A valve block for lightning arresters and the like, comprising a body of high resistance semi-conducting material including silicon-carbide granules in a ceramic binder, the said body having a non-linear voltage-current characteristic, metallic electrodes at the ends of the body adhered to the silicon carbide granules thereof, and a collar of wax filling the interstices of the body from the outer surface thereof inward for a distance of not more than to of the diameter of the body for preventing flashover between the silicon-carbide grains adjacent the exterior of the block and between the metallic electrodes.

2. The invention in accordance with claim 1, in which the wax is selected from the group which includes parafiin and carnauba wax.

3. The method of manufacturing a non-linear resistive device, which comprises forming a semi-conducting body consisting of silicon-carbide granules fired with a binder to produce a hardened mass with the granules in intimate engagement, and impregnating the fired body with wax to a depth of about to 6 of the transverse dimension of the body inward from the exterior thereof.

4. The method in accordance with claim 3, in which the impregnation is accomplished by immersing the body in a mass of molten wax for a period of time sufficient to produce the required penetration of the wax.

5. The method in accordance with claim 3, in which the impregnation is accomplished by immersing the body in a mass of molten wax at a temperature of in the order of 240 to 300 Fahrenheit for a period of about ten to thirty seconds.

6. The method in accordance with claim 3, in which the impregnation is accomplished by immersing the body in a mass of molten wax selected from the group consisting of paraffin and carnauba wax at a temperature of in the order of 250 Fahrenheit for a period of about ten to thirty seconds.

7. The method of manufacturing a non-linear resistive device, which comprises forming a semi-conducting body consisting of silicon-carbide granules fired with a binder to produce a hardened mass with the granules in intimate engagement, impregnating the fired body with wax to a depth of about to 1 of the transverse dimension of the body inward from the exterior thereof and form- :ing metallic electrodes over the ends of the body.

8. A block for a non-linear resistive device comprising a body of high resistive semi-conductive material inm eluding silicon carbide granules in a ceramic binder, the said body having a non-linear voltage current characteristic, metallic electrodes at the ends of the body, and a collar of wax filling the interstices of the body from the outer surface thereof inward for a distance of about to of the diameter of the body for preventing flash-over between the silicon carbide grains adjacent the exterior of the block and between the electrodes.

References Cited in the file of this patent UNITED STATES PATENTS Read July 9, 1907 Lind Feb. 12, 1924 Earle Apr. 24, 1945 FOREIGN PATENTS Great Britain Jan. 5, 1928 Australia Nov. 11, 1953

Claims

1. A VALVE BLOCK FOR LIGHTNING ARRESTERS AND THE LIKE, COMPRISING A BODY OF HIGH RESISTANCE SEMI-CONDUCTING MATERIAL INCLUDING SILICON-CARBIDE GRANULES IN A CERAMIC BINDER, THE SAID BODY HAVING A NON-LINEAR VOLTAGE-CURRENT CHARACTERISTIC, METALLIC ELECTRODES AT THE ENDS OF THE BODY ADHERED TO THE SILICON CARBIDE GRANULES THEREOF, AND A COLLAR OF WAX FILLING THE INTERSTICES OF THE BODY FROM THE OUTER SURFACE THEREOF INWARD FOR A DISTANCE OF NOT MORE