US2501322A - Moisture-resistant lightning arrester valve block - Google Patents

Description

March 2,1, 1950 J. s. FERGUSON ETAL 2,501,322

MOISTURE-RESISTANT LIGHTNING ARRESTER VALVE BLOCK Filed Nov. '7, 1946 fly! WITNESSESZ INVENTORS J I 5.1" u an na' Havn-Egrczel 5r., BY C 72W 27ML f, ff. L

ATTOR Y Patented Mar. 2l, 1950 MOISTURE-RESISTANT LIGHTNING ARRESTER VALVE BLOCK John S. Ferguson,

Minneapolis, Minn and Howard E. Dyche, Jr., 'Wilkinsburgg Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsyivania Application November 7, 1946, Serial No.y 708,330

3 Claims. (Cl. 20l-76) The present invention relates to valve-type resistors, or valve blocks, for lightning arresters,

and, more particularly, to a moisture-resistant lightning arrester valve block.

Valve-type resistors, which are utilized as overvoltage protective devices, or lightning arrester blocks, consist of resistance material which has valve characteristics. 'Ihat is, it has very high resistance under normal voltage conditions, but when anv excess voltage is applied to it, its resistance decreases sharply to permit heavy currents to be discharged. When the voltage decreases to a normal value, the resistance rapidly increases again, reducing the current, so that only a very small leakage current can flow through the block.

Lightning .arrester valve blocks are frequently made of granular silicon carbide molded with a suitable binder, such as water-glass, or sodium silicate.- Such valve blocks have very desirable electrical characteristics, but the sodium silicate usually used as a binder is a hygroscopic material and readily absorbs moisture from the air. This is highly objectionable since absorption of water by the valve block changes its resistance characteristics, and if enough moisture is absorbed, the block loses its valve characteristic and is unable to sufiiciently increase its resistance after a discharge, so that a heavy current continues to now, resulting in failure of the block. There are two general ways in which this difllculty may be overcome. The housing in which the valve blocks are'enclosed can be sealed against moisture so that the blocks are not subjected to moisture-carrying air, or the blocks themselves may be made moisture-resistant so that they will not absorb any substantial amount of moisture from the air.

The principal object of the present invention is to provide a lightning arrester valve block, or overvoltage protective device, which is moistureresistant so that it can safely be exposed to moisture-containing airA without the risk of any adverse effect on the electrical characteristics. A further object of the invention is to provide a lightning arrester valve block consisting of granular silicon carbide with a binder of sodium silicate, or other Isuitable hygroscopic material, which has a protective moisture-resistantcoating covering all exposed surfaces of the block so that it cannot absorb any substantial amount of moisture. l

Other objects and advantages of the invention 2 scription, taken in connection with the accompanying drawing, in which:

'Figure 1 is atop plan view of a lightning arrester valve block embodying the invention;

and f Fig, 2.is a sectional view on the line II-fIl'. of Fig. l, the thicknesses of the various coatings being considerably. exaggerated for clarity of illustration.

'The lightning arrester block shown in the drawing consists of a mass of resistance material I having valve characteristics. The block shown is generally cylindrical, but it will be understood that the invention is not limitedv to any particular size or shape of block. The resistance material l preferably consists oi granular silicon carbide, which is mixed with a binder of sodium silicate and, if necessary, a small amount of water. Sodium silicate is the preferred material for the binder, but other -water-soluble silicates, or similar materials, might be used. The mixture is molded to the desired size and shape, and baked to drive oi'f the water, providing a rigid mass of resistance material which, as explained above, has valve characteristics and is very suitable for use as an overvoltage protective device. In order to facilitate electrical contact to the block, copper coatings 2 are applied to the opposite end surfaces of the block, preferably by spraying the copper directly onto the block. As shown in Fig. 2, the copper coatings do not cover the entire end surfaces, but terminate a short distance from the periphery, leaving an uncoated annular surface around the edge of the block.

A hard, strong, high-resistance coating 3 is preferably applied to the lateral surface of the block, in order to prevent ashover across the outside of the block, and to prevent certain types of block failures. This coating 3 consists of a refractory material mixed with a suitable binder, and is preferably composed of talc, or other finely-divided refractory material, mixed with l* sodium silicate, as described and claimed in a patent to W. E. Berkey, No. 2,253,360, issued August 19, 1941. As described in the patent, the talc and sodium silicate are mixed to forma vis- .cous material which is applied to the surface of the block as a paste. The coated block is then baked to expel the moisture from the coating 3,

and to convert it into a hard, mechanically strong coating which has very high electrical resistance as compared to the resistance of the block material I. The coating 3 provides mechanical support for the block material i to protect it against will be apparent from the following detailed destresses caused by discharges within the block,

and increases the dielectric strength over the edges of the block so as to prevent discharges or flashovers on the outside of the block. The coating I is usually desirable, although it may not always be necessary, and when used, it is essentially an integral part of the block itself insofar as the present invention is concerned.

After application of the coating 3, the copper contact coatings 2 are preferably coated with a layer of solder 4. The solder may be applied by dipping the block into a solder pot and wiping off the excess, or it may be applied by spraying the solder onto the copper coating 2 and subsequently heating to cause the solder to flow and form a smooth, homogeneous surface for electrical contact with an adjacent blc-ck, or other element of a complete lightning arrester assembly. The sprayed copper contact coatings 2 may be somewhat porous, and the solder coatings 4 serve to seal the pores of the copper coatings and prevent penetration of moisture into the block through the end surfaces.

The block material l and the coating l both contain sodium silicate which, as pointed out above, is a hygroscopic material and will readily absorb moisture from the surrounding air. In order to make the block resistant to moisture, an outer protective coating 5 is provided on the lateral surfaces of the block and on the edges of the end surfaces, so that all exposed surfaces of the block are covered by the coating 5. The coating 6 may consist of any suitable non-conducting coating material which is substantially impervious to moisture, but it is preferred to use a moisture-resistant enamel or varnish of the type usually referred to as black baking varnish, which is a heavy varnish or enamel having an asphaltic base, often used as an insulating varnish or impregnant in electrical equipment. Any other suitable type of enamel or varnish might be used, however, which will form a nonconducting coating which is impervious to moisture and which will not crack when the block is heated by adischarge, or at low temperatures. Other suitable materials for this purpose, forexample, are varnlshes or enamels containing a synthetic resin base, such as phenolic resins, phenolic-tung oil resins, alkyd resins, or urea-formaldehyde resins. The protective coating 5 is preferably applied in two coats, the first coat being permitted to dry before the application of the second coat. A single thick coat might be used. however, with satisfactory results. The enamel coating forms a smooth, homogeneous protective coating overall exposed surfaces of the block which is substantially impervious to moisture, and which has no other effect on the electrical characteristics of the block.

The protective moisture-resistant coating l prevents the block material I from absorbing any substantial amount of moisture from the air, and thus it is not necessary to seal the block in a housing, since it can be exposed to the external air without danger of any adverse effect on its electrical characteristics. The use of the coating l is very effective in protecting the block material from moisture. Thus, for example, lightning arrester blocks made as described above have been subjected to an atmosphere of ninety percent relative humidity for seven days. and at the conclusion of this test, the blocks had absorbed only 0.16 percent moisture by weight which is a negligible amount for all practical purposes. Electrical tests showed that the characteristics of the scopic binder, a hard, mechanically strong,

block had not been affected in any measurable degree.

It should now be apparent that a moistureresistant lightning arrester block has been Drovided which is effectively protected against moisture and which has very desirable characteristics. A specific embodiment of the invention has been shown and described for the purpose of illustration, but it is to be understood that various changes and modifications may be made within the scope of the invention, and in its broadest aspects it includes all equivalent modifications and embodiments which come within the scope of the appended claims.

We claim as our invention:

1. An overvoltage protective device comprising a block of resistance material having valve characteristics such that it is capable of discharging high currents under excess-voltage conditions and of increasing its resistance to a relatively high value under normal voltage conditions, said block of resistance material including a hygrosoopic binder. metal contact coatings on the end surfaces of the block, and a coating of non-ccnductive material which is substantially impervious to moisture covering al1 other surfaces of the bloclr and overlapping said contact coating.

2. An overvoltage protective device comprising a block of resistance material having valve characteristicsy such that it is capable of discharging high currents under excess-voltage conditions and of increasing its resistance to a relatively high value under normal voltage conditions, said block including granular resistance material and a binder of a water-soluble, silicate, metal contact coatings on the end surfaces of the block. and a coating of non-conducting material which is substantially impervious to moisture covering all other surfaces of the block and overlapping said contact coating. 3. An overvoltage protective device comprising a block of resistance material having valve characteristics such that it is capable of discharging high currents under excess-voltage conditions and of increasing its resistance to a relatively high value under normal voltage conditions, said block of resistance material including a hygrohithresistance coating on the side surfaces of the block, said coating including a hygroscopic binder, metal contact coatings on the end surfaces of the block, and an outer coating of nonconducting material which is substantially impervious to moisture covering all other surfaces .of the block and overlapping said contact coating.

JOHN S. FERGUSON. HOWARD E. DYCHE, Js.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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