US2276732A - Lightning arrester - Google Patents

Description

March 17, 1942. L.. R. LUDWIG ET AL LGHTNING ARRESTER Filed Feb. 29, 1940 wlmEssEs; 6J QM".

Patented Mar. 17, 194? LIGHTNING ARRESTER kLeon n. Ludwig, wilkinsburg, Frederick B. John- SOD,

Murrysville, and William E. Bex-key,

Wilkinsburg, Pa., assignors to AWestinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 29, 1940,.',Serial No. 321,458

19 Claims.

Our invention relates to lightning ,arrestera and it has particular relation to lightning-arrester valve-elements utilizing silicon-carbide crystals as a basis, said crystals either being in loose form, packed between two terminal electrodes, or bound together in a solid mass, with a binder, preferably a ceramic binder.

The object of our invention is to improve the performance of these silicon-carbide lightningarrester valve-elements.

A more specific object of our invention is to provide such an arrester in which difliculties due to arrester-failure, or short life, particularly when subjected to long-tail waves, may be avoided by the admixture of suitable fine inert or relatively electrically non-conducting particles or binding material in a particle-size and quantity approximately sufllcienjt, or nearly sufilcient, to just ll the voids between the larger siliconcarbide particles which constitute the principal part of the lightning-arrester valve-element.

A still more specic object of our invention is to provide a ceramic molded block or lightningarrester valve-element, in which silicon-carbide crystals are held together by a relatively insulating arc-resisting binder, in the form' of clay or other ceramic binderfin quantities much smallerl than have heretofore been commonly utilized in clay-bound lightning arresters, and, in fact, in quantities so small that no substantial volumeshrinkage is encountered upon the firing of the block, in the process oi' manufacture, which means that the clay is not materially more than enough to fill the voids between the carbide crystals.

With the foregoing and other objects in view, our invention consists in the structures, compositions, combinations and methods hereinafter described and claimed, and illustrated in the accompanying drawing wherein:

Figure 1 is a vertical sectional view of a lightning-arrester embodying our invention in a loosegrain form, while Fig. 2 is a similar View illustrating an embodiment of the inventionin a ceramic-block form.

Recent studies concerning the discharge of surges through lightning-arrester valve-elements utilizing silicon-carbide crystals as the main discharging medium have resulted in the formulation of a more complete theory as to the mechanism of the discharge, and as .to the reasons l for the failure of the arrester,

When Voltage is applied to a column consistmg of two or more serially connected siliconcarbide crystals disposed in contact with each other, the current ilowing through the column will be limited by the internal resistance of the crystals and the external resistance of the junctions between the crystals. The part of the total voltage which `is consumed at the junction will be relatively independent of the amount of the current after the total current has increased beyond a critical cut off value which may be 25 amperes or less, whereas the part of the voltage which is consumed across the bodies of the crystals will vary with the current.

- The rough, irregular shape ci the silicon-carbide crystals, coupled with their hard, unyielding nature, results in small-area, so-called point contacts between adjacent crystals, so that, when, the total applied voltage across the arrester is low, practically all of that voltage is consumed at these point contacts. When the total applied voltage is increased, the amount of current which is required to be carried by these pointcontact joints increases until, at a certain critin cal voltage, there results a sutciently high current-concentration, at the points, to cause spart;- ing at the crystal junctions, thereby enabling the column to carry relatively heavy discharge-current, without appreciably increased voltages these junctions.

It has more recently been discovered that, if the surge is of a long duration, there is a tendency for the sparking at the point-contacts to spreadl until the spark surrounds, bridges `or bypasses a crystal, thus shorting the crystal out of the column of which it is a part. The conventional arrester-element utilizing silicon-carbide crystals consists of a large number of crystals, so that the circuit from one terminal-electrode to the other consists of a large number of parallel paths, each of the parallel paths consisting of a column of crystals. 'I'hese parallel paths are solidly interconnected at each end, by the arrester-terminals, and are also interconnected at intermediate points, all along their length, by incidental sideways contacts to adjacent crystals. If, now, a single crystal, in a single one of these parallel columns, should become shorted out by a flashover resulting from either a long duration or an unusually high magnitude of the currentdischarge, it is obvious that the resistance of that particular column of crystals will be reduced to a value which is lower than the resistance of the other parallel columns, resulting in a concentration of current in the affected column. This, in turn, will aggravate the contact-conditions at other crystals in the same column,` and hence it will accelerate the ultimate complete failure of that particular column. When this occurs, the lightning-arrester element will be punctured by a low-resistance discharge-path which will destroy its usefulness as a lightning-arrester element.

It has been discovered that the probability as to whether this phenomenon will occur is a function of the time of duration of the surge, as well as the magnitude of the surge-current. Thus, it ha's been found that more severe conditions can be imposed by surges of long dura? tion, even though they are of only medium current, or even low-current magnitude, as distinguished from high surges of short duration. We believe that the reason for the time-element, as a contributing factor in the failure of crystaltype lightning-arrester elements, is that it takes some appreciable time for the sparking-conditions to spread from a point-contact joint between crystals, until the highly ionized sparking condition can completely surround or bridge a crystal, and from that time on it takes still further time for the number of spark-shunted crystals to multiply to the point where a breakdown or failure is imminent.

Our studies have disclosed that there are two means particularly available for retarding the rate of failure of carbide-crystal arrester-elements when subjected to surges of long duration. First, we have found that ashovers of the crystals become more difficult when we increase the size of the silicon-carbide crystals, so that an arrester valve-element made with 60-mesh silicon-carbide crystals will have a far greater ability to withstand the discharge of surges of long duration than a lightning-arrester valve-element made with 10G-mesh silicon-carbide crystals.

Another method of preventing or retarding the lashover of the individual crystals, in accordance with our invention or discoveries, is to ll, or partially fill, the voids or spaces between the larger or main contact-carrying crystals with an inert or relatively high-resistance arc-resisting material which will suppress the growth of the sparks initiatingl at the crystal-junctions. This void-filling material may be either finely-divided loose-material, such as silicon-carbide crystals of less than 250-mesh size, and preferably at least a neness of 325-mesh size, or even finer powders such as 60G-mesh silicon-carbide, or the void-filling material may be made from refractory, relatively insulating materials such as flint, other than silicon-carbide, although, in the loose form of silicon-carbide arrester-elements, the i fine silicon-carbide filler has so far proven to be the preferable filler-material. The iineness of the 250- or B25-mesh particles makes them interpose a larger number of serially connected contacts in any given length of arrester, so that they offer a higher electrical resistance to the discharge of current through the arrester than the coarse particles, so that the arrester-current is largely carried by the larger particles which are in shunt thereto.

In the case of molded silicon-carbide arresterblocks, the fine-particled void-filling material may be the binding material which holds the` silicon-carbide particles together in solid form. Here, again, our studies have shown a surprising circumstance in connection with the time-delayed spreading of the crystal-enveloping sparks which we have found to be the basic cause for failure of these lightning-arrester blocks. We have discovered that water-glass, which has, of

late years, been extensively utilized as the material for binding the silicon-carbide crystals together, in lightning-arrester blocks, is not a suitable heat-resistant binder for use as a filler material within the voids between the carbide crystals, for suppressing'the spread of the sparking from the joints to the spaces completely surrounding individual crystals. On the other hand, claybound crystal blocks, such as were known and utilized before the water-glass-bound blocks became customary, are not suitable for our purpose because such clay-bound blocks contained a very large quantity of clay, as compared with the quantity of,y the silicon-carbide crystals, so that the silicon-carbide conducting particles were relatively far apart. These old clay blocks were molded and lired in such a way that they were porous, andconducting material was introduced in the form of carbon or the like, so that the pores in the clay were lined with a conducting coating which would join the spaced siliconcarbide particles. The highly porous nature of these old clay-bound silicon-carbide blocks rendered them poorly able to withstand longsustained surge-discharges.

Our studies have shown, on the other hand, that a clay, or other ceramic, binder is much better than waterglass, for holding the siliconcarbide particles together in a solid mass, and for, at the same time, resisting the spreading of the contact-point sparks, if the clay is utilized in an amount which is approximately correct for barely filling the spaces or voids between the A practical way of de o termining the proper amount of clay binder is silicon-carbide crystals.

to utilize the largest amount which will result in a block which does not materially or noticeably shrink in size during the firing of the clay, this criterion being available because of the notorious property of clay, in shrinking very materially during the firing process.

In Fig. 1, we show a practical form of embodiment of our invention, in a lightning arrester utilizing a porcelain housing I having a lineterminal 2 entering at the top, and a. groundterminal 3 at the bottom. The porcelain housing I is hollow, containing, at its lower end, a terminal-electrode 4 which is properly sealed, to the bottom of the housing, by a. sealing-gasket 5. Disposed over this bottom terminal-electrode 4 is a loose mass consisting of a large number of `silicon-carbide crystals 6 mixed, in accordance with our invention, and pressed down, at the top, by a terminal-electrode or contact-plate 1. In the particular embodiment illustrated, the contact-plate I is surmounted by a. thin metal disc 8, the edges of which are spun in place against the bore of the porcelain housing I, this thin metal disc 8 being strengthened or stilened by a backing-up plate 9. The two plates 'I and 9, with the spun-metal disc 8 between them, are preferably riveted together, as indicated -at II. Immediately above the spun-metal disc 8, the space near the bore of the porcelain housing I is further cemented by a gum seal I2 forfpreventing leakage of the fine silicon-carbide dust during shipment, when the arrester may be turned upsidedown.

Surmounting the valve-element part of the arrester, which is made up by the terminal platestructures 4 and 1, and the intervening mass of loose carbide crystals 6, we have shown our arrester as comprising a quench-gap portion I4, which is, in turn, surmounted by a spark-gap or switching-gap structure I5, which is, in turn,

ious parts together under pressure, said spring bearing up against a line-terminal electrode I1 at the top of the casing.

In accordance .with our invention, the mass of loose silicon-carbidecrystals 6 which constitute the valve-element of our arrester, is made up of a mixture which is composed, for the most part, of crystals of a certain mesh-rating, together with an admixture oi othercrystals of preferably (though not necessarily) at least three times as large a mesh-number. The relative proportions of the coarse and fine crystals are such as, by calculation or experiment, have been found to be requisite, sothat the ne material is present in about the right amount, or approximately nearly the right amount, to ll the spaces or voids between the larger crystals. As previously indicated, all of the particles are preferably of silicon carbide.

While we vare not limited to any particular` size of the relatively coarse silicon-carbide particles, our invention being advantageous even with the nest size of coarse particles which we have tried, there are advantages,` not unmixed withA certain disadvantages, in utilizing as large a size of the coarse particles as possible, and specifically a size which is known as SO-mesh, or even larger.

When we refer to different screen-sizes, such stood that a certain small admixture of both finer and coarser particles is tolerated in the definition of particles of a certain screen-mesh size or number, reference being made to standard screens having a certain designated number of threads per inch.

While the use of relatively large-sized "coarse particles is advantageous, from the standpoint of producing an arrester which will resist failure when subjected to surges of long time-duration, the increase in the particle-size should not be carried further than is absolutely necessary, because arresters must be built for certain voltageratings, referring to the voltages of the transmission lines to which the arresters are connected, and this voltage-rating must be lower than the so-called cut-off voltage at which the arrester ceases to discharge large currents at the termination of a surge-discharge; and this cutol characteristic, or socalled valve-characteristic of the arrester-element, is, in turn, a function of the number of crystal-contacts in series. An arrester of a given voltage-rating must, therefore, have approximately a certain prescribed number of serially connected crystal-contacts, regardless of the size of the crystals. lt thus follows that the overall height `of an arrester-column, having the prescribed number of crystal-contacts in series, is practically directly proportional to the crystal-size, which means that a large-grained crystal arrester of a given voltage-rating is taller, and hence more expensive, than a small-grained arrester.

A second disadvantage of an unnecessary increase in the sizes of the crystal-grains is that the protective ratio of the arrester is impaired by increasing 'the grain-size, this protective ratio beingthe ratio between the crest-voltage, which appears across the arrester during its rated current-discharge, and the cut-olf voltage of the arrester, or the ratio between the crest-voltage and the rated line-voltage of the arrester. It is obviously desirable that this so-called protective ratio should be as small as possible. However, since arresters of the larger grain-sizes are taller, or longer electrically, than arresters of the smaller grain-sizes, the large-grain arresters have a larger amount of the total overall voltage consumedin the internal resistance of the silicon-carbide grains or crystals, so that the crestvoltages of these arresters are somewhat increased.

Our expedient of filling, or partially filling, the voids between the so-called coarse particles, which constitute the major portion of the volume of the arrester, is very useful, therefore, as a means whereby we are enabled to get along with large grains which are not as large as they would have to be if advantage were not taken of the spark-suppressing qualities of the fine fillermaterial which we utilize, in accordance with our invention.

Various relative amounts of the fine-mesh silicon-carbide crystals, as compared with the volume of the large-mesh silicon-carbide crystals, may be utilized. While We are not definitely limited to any exact ratios, it may be noted that successful results have been obtained with 100 parts of 40-mesh silicon-carbide crystals and 30 'parts, by volume, of B25-mesh silicon-carbide crystals; or with 100 parts of 60-mesh siliconcarbide crystals and 25 parts, by volume, of 325- mesh silicon-carbide crystals. With 100 parts of 100-mesh silicon-carbide crystals we have tried an admixture of 20 parts by volume, of325-mesh silicon-carbide crystals, and have found some irnprovement, but our tests indicate that we should have used much finer fine-mesh crystals, such as, for example, 60G-mesh crystals. As indicating the variations in these proportions, which could be varied between wide limits, it may be noted that one of our better-performing 40 and `I300-mesh samples had only 15 parts of the ne material to 100 parts of the coarser material, although we anticipate that there might be a certain tendency for uneven packing of the fine material during shipment, in case the relative quantity of the fine material was as small as in the last-mentioned sample.

In operation, when an excess-voltage surge is applied to an arrester as shown in Fig. 1, the series gap-devices i4 and l5 first break down, applying practically theV full line-voltage to the valve-typecpart, consisting of the mass of silicon-carbide crystals 6, and the surge-current is discharged to ground through these crystals. At the expiration of the surge-discharge, it is believed that the sparks at the crystal-contacts are extinguished, and at any rate the apparent internal resistance of the dis-charge more or less suddenly increases, at what is known as the cutoil voltage at the arrester, so that the discharge-current is reduced to a value at which the iinal current-interruption is brought about in one or both of the series gap-devices i4 and l5, at a voltage which is only slightly higher than the rated line-votage of the arrester.

The explanation of the operation, as vjust given, makes no reference to the time of duration of the discharge, which is a particularly impor-- tion, without arrester-failure, than has been heretofore possible.

broader aspects thereof.

In Fig. 2, we show our invention in a form oi' embodiment in which, instead of utilizing a mass or owable quantity of loose silicon-carbide crystals 6, we utilize a block, or a plurality of blocks A26, of molded silicon-carbide crystals. In molding these blocks, we utilize silicon-carbide crystals corresponding to the relatively coarser crystals which were utilized in the loose mix 5 of Fig. l, such as (iO-mesh, {iO-mesh, or G-mesh silicon-carbide crystals. without being limited, of course, to these parti-cular sizes, as we may utilize either coarser or finer-mesh particles, or mixtures of different meshes. We prefer, however, to have the silicon-carbide crystals of a single mesh-rating, such as GO-mesh or 80-mesh, and we mix, with these silicon-carbide particles, a certain amount of clay, or other ceramic material, which will operate both as a binder and as a filler to close the pores or voids between'the grains of silicon carbide.

The clay should be present in quantities not substantially greater than enough to fill these pores or voids. Practically, clay may be present in quantities varying between 30 and 40% of the total weight of the nnished molded arresterblock.` A proper amount of clay is the largest amount useable without allowing or causing the f block to shrink during iiring. In the course of manufacture, the silicon carbide and clay are thoroughly mixed together, with the addition of a small amount of/water, after which the mixed mass is molded into blocks and fired in a furnace in a non-oxidizing or reducing atmosphere, in accordance with a practice which is well known except for the relatively small quantity of clay which we utilize.

In operation, a lightning arrester made with molded valve-element blocks 26, as shown in Fig. 2, operates the same as in the case of the loosecrystal arrester of Fig. 1, the electrical conductivity through the block or blocks 26 being through the grains of the silicon carbide, and across the crystal-to-crystal contact-points, as in the loose-grain arrester, the clay binder being utilized jointly as a means for drawing the crystals together in a tightly held mass, and as a means for resisting the spread of the arcs or sparks which originate yat the crystal-junctions, so that individual crystals are not, in general, entirely enveloped in, and short-circuited by, a

- spark or highly ionized space,

our invention, in its broader aspects, may alternatively utilize, as its principal constituent, that is, for its larger particles, av large number of "M-mesh granules of some other simi-conducting material, where M is no smaller than 40 and no largerthan 120.

While we have illustrated our invention in two different illustrative forms of embodiment, and whilewe have explained the preferred limits in the application of our materials and mixtures, we wish .it to be understood that we are not altogether limited to these details, as many changes or variations may be adopted by those skilled in the art, without departure from the essential principles of our invention, particularly in the We desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their lan- Iguage and the prior art.

We claim as our invention: 1. An excess-voltage protective device comprising two spaced terminal-electrodes and a valve-element means bridging the space between said terminal-electrodes, characterized by said valve-element means comprising a large number of silicon-carbide granules which are for the most part of one size-range of a larger size than 10U-mesh granules, and a smaller quantity, by volume, of smaller-sized particles disposed in the spaces between said rst-mentioned granules, said smaller-sized particles being smaller than the spaces between said first-mentioned granules and offering relatively a considerable resistance to the passage of current therethrough, characterized by said smaller-sized particles being of such small size and of roughly the approximate quantity nearly sufficient to ll the voids in the coarser material.

2. An excess-voltage protective device comprising, in combination, an insulating vessel having two spaced terminal-electrodes associated therewith,C and a iiowable quantity of loose particulate matter disposed within said vessel in the space between said terminal-electrodes, said loose particulate matter comprising graded sizes of granules consisting mainly of silicon-carbide, a major portion of the silicon-carbide granules, by weight, being for the most part of one size-range, the remainder of the silicon-carbide granules' being of a considerably smaller size-range and being of such small size and of roughly the approximate quantity nearly sufficient to ll the voids in the coarser material.

3. An excess-voltage protective device comprising, in combination, an insulating vessel having two spaced terminal-electrodes associated therewith, and a flowable quantity of loose particulate matter disposed within said vessel in the space between said terminal-electrodes, said loose particulate matter comprising graded sizes of granules consisting mainly of silicon-carbide, having such relative proportions of relatively large and small particles that the smaller particles are of such small size and of roughly the approximate quantity nearly suflicient to ll the voids in the coarser material.

4. An excess-voltage protective device comprising, in combination, an insulating vessel having two spaced terminal-electrodes associated therewith, and a flowable quantity of loose particulate matter disposed within said vessel in the space between said terminal-electrodes, said loose particulate matter comprising graded sizes of granules consisting mainly of silicon-carbide, having a material proportion, by volume, of the particles at least as large as Sil-mesh, and another material, but smaller, proportion, by volume, of the particles at least as small as 250- mesh; the two groups of large and small particles, at least as large as BO-mesh, and at least as small as 250-mesh, respectively, together constituting a major portion of the loose particulate matter, the aforesaid small particles being present in roughly the approximate quantity necessary to approximately ll the voids between the aforesaid large particles.

5. An excess-voltage protective device comprising, in combination, an insulating vessel having two spaced terminal-electrodes associated therewith, and a flowable quantity of loose particulate matter disposed within said vessel in the space between said terminal-electrodes, said loose particulate matter comprising graded sizes of poorly conducting granules, a major portion of said granules, by weight, being for the most part of one size-range, the remainder of said granules being of a considerably smaller size-range and being of such small size and of roughly the approximate quantity nearly suiilcient to lill the voids in the coarser material.

6. An excess-voltage protective device comprising, in combination, an insulating vessel having two spaced terminal-electrodes associated therewith, and a flowable quantity of loose particulate matter disposed Within said vessel in the space between said terminal-electrodes, said loose particulate matter comprising graded sizes of poorly conducting granules having such relative propor. tions of relatively large and small particles that the smaller particles are of such small size and of roughly the approximate quantity nearly sufcient to lill the voids in the coarser material.

'7. An excess-voltage protective device comprising, in combination, an insulating vessell having two spaced terminal-electrodes associated therewith, and a flowable quantity of loose particulate matter disposed within said vessel in the space between said terminal-electrodes, said loose particulate matter comprising graded sizes of poorly valve-element means bridging the space between -said terminal-electrodes, characterized by said valve-element means comprising at least one ilred ceramic block comprising a large number of silicon-carbide granules which are `for the most part of one size-range,/and a ceramic binder, the silicon-carbide granules including a majority of its total mass in particles at least as large as 80-mesh, and the quantity of ceramic binder beingapproximately the largest amount Y which can be utilized without causing the block conducting granules having a material proportion, by volume, of the particles at least as large as BO-mesh, and another material, but smaller, proportion, by volume, of the particles at least as small as 250-mesh; the two groups of large and small particles, atleast as large as SO-mesh, and at least as small as 250-mesh, respectively, together constituting a major portion of the loose particulate matter, the aforesaid small particles being present in roughly the approximate quantity necessary to approximately ll the voids between the aforesaid large particles.

8. An excess-voltage protective device comprising two spaced terminal-electrodes and a valveelement means bridging the space between said terminal-electrodes, characterized by said valveelement means comprising at least one moldedcomposition block comprising a large number of silicon-carbide granules which are for the most of one size-range, and a ceramic binder, the quantity of ceramic binder being sufficient to hold the block together but not substantially greater than enough to ll the voids between the silicon-carbide granules.

9. An excess-voltage protective device comprising two spaced terminal-electrodes and a valve-element means bridging the space between said terminal-electrodes, characterized by said valve-element means comprising at least one red ceramic block comprising a large number of silicon-carbide granules which are for the most part of one size-range, and a ceramic binder, the quantity of ceramic binder being approximately the largest amount which can be utilized to appreciably shrink during iiring.

12. An excess-voltage protective device comprising, in combination, an insulating vessel having two spaced terminal-electrodes associated therewith, and a flowable quantity of loose particulate matter disposed Within said vessel in the space between said terminal-electrodes, said loose particulate matter comprising graded sizes of granules, a major portion of the granules, by weight,l being M-mesh granules of semi-conducting material, where M is no smaller than 40 and no larger than 120, the remainder 'of the granules being of such small size and of roughly, the approximate quantity nearly suiiicient to lill the voids in thefcoarser material, said smaller-sized granules oiering relatively a considerable resistance to the passage of current therethrough.

13. An excess-voltage protective device comprising two spaced terminal-electrodes and a valve-element means bridging the space between l said terminal-electrodes, characterized by said valve-element means comprising at least one molded-composition block comprising, as its principal constituent, a large number of M-mesh granules of semi-conducting material, whe/re M is no smaller than 40 and no larger than 120, and a ceramic binder, the quantity of ceramic binder f being sufficient to hold the block together but not without causing the block to appreciably shrink but not substantially greater than enough to iill the voids between the silicon-carbide granules.

ll. An excess-voltage protective device comprising two spaced `terminal-electrodes and a substantially greater than enough to iill the voids between the granules.

14. An excess-voltage protective device comprising two spaced terminal-electrodes and a valve-element means bridging the space between said terminal-electrodes, characterized by said valve-element means comprising graded sizes of poorly conducting granules having a material proportion, by volume, of the particles at least as large as SO-mesh, and another material, but smaller, proportion, by volume, of the particles at least as small as Z50-mesh; the two groups oi large and small particles, at least as large as mesh, and at least as small as 25o-mesh, respectively, together constituting a major portion oi the valve-element means, the aforesaid small particles being present in roughly the approximate quantity necessary to approximately iill the voids between the aforesaid large particles.

l5.-An excess-voltage protective device comprising two spaced terminal-electrodes and a valve-element means bridging the space between said terminal-electrodes, characterized by said valve-element means comprising graded sizes of granules consisting mainly of silicon-carbide, having a material proportion, by volume, of the particles at least as large as 80-mesh, and another material, but smaller, proportion', by Volume, of the particles at least as small as 250- rnesh; the two groups of large and small particles, at least as large as Sil-mesh, and at least as small as Z50-mesh, respectively, together constituting ainajor portion of 'the valve-element means, the aforesaid small particles being present in roughly the approximate quantity necessary to approximately ll the voids between the aforesaid large particles.

16. An excess-voltage protective device comprising two spaced terminal-electrodes and a valve-element means bridging the space between said terminal-electrodes, characterized by said valve-element means comprising graded sizes of poorly conducting granules having a material proportion, by volume, of the particles at least as large as 80-mesh, and another material, but smaller, proportion, by volume, of the particles at least as small as 250-mesh, characterized by said smaller-sized particles being of such small size and of roughly the approximate quantity nearly suflicient to ll the voids in the coarser` material.

17. A excess-voltage protective device comprising two spaced terminal-electrodes and a valve-element means bridging the space between said terminal-electrodes, characterized by said valve-element means comprising graded sizes of granules consisting mainly of silicon-carbide, having a material proportion, by volume, of the particles at least as large as SO-mesh, and another material, but smaller, proportion, by volume, of the particles at least as small as 250- mesh, characterized by said smaller-sized particles being of such small size and of roughly the approximate quantity nearly suflicient to fill the voids in the coarser material.

18. An excess-voltage protective device comprisng two spaced terminal-electrodes and a valve-element means bridging the space between said terminal-electrodes, characterized by said valve-element means comprising graded sizes of poorly ,conducting granules having a material proportion, by volume, of the particles at least as large as -mesh, and another material, but smaller, proportion, by volume, of the particles at least as small as 250mesh, characterized by said smaller-sized particles including a ceramic binder which holds the valve-element means together in one or more solid masses, said smallersized particles being of such small size and of roughly the approximate quantity nearly sufficient to ll the voids in the coarser material.

19. An excess-voltage protective device comprising two spaced terminal-electrodes and a valve-element means bridging the space between said terminal-electrodes, characterized by said valve-element means comprising graded sizes of granules consisting mainly of silicon-carbide, having a material proportion, by volume, of the particles at least as large as 80-mesh, and another material, but smaller, proportion, by volume, of the particles at least as small as 250- mesh, characterized by said smaller-sized particles including a ceramic binder which holds the valve-element means together in one or more solid masses, said smaller-sized particles being of such small size and of roughly the approximate quantity nearly suflicient to fill the voids in the coarser material.

LEON R. LUDWIG. FREDERICK B. JOHNSON. WILLIAM E. BERKEY,

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