Oct 8, 1946. J. E. M. MITCHELL‘ ' 2,408,990 BUSHING INSULATOR Filed July 16, 1941 2 Sheets-Shee't 1 INVENTOR. (1.15M M/fmg //;l Get. 8, 1946. J. E. 2,408,990 MITCHELL BUSHING INSULATOR Filed July 16, 1941 2 Sheets-Sheet 2 w8 \ 0.17%) \ \ \ \ \\\ \\ \\\‘.Tn?za) \ \.\ /nhu/. / /// Patented Oct. 8, 1946 2,408,990 UNITED STATES PATENT OFFICE 2,408,990 BUSHING INSULATOR John E. M. Mitchell, Parkersburg, W. Va., as signor to Porcelain Products, Inc., Findlay, Ohio, a, corporation of Delaware Application July 16, 1941, Serial No. 402,671 3 Claims. (Cl. 174-31) 1 2 The invention described in detail below relates primarily to insulators of the bushing type and more particularly to bushings made of porcelain tablishing intimate connection with the porcelain body and at the same time producing improved voltage distribution without local differences of potential existing between the glaze and the adja cent porcelain. This glaze is moreover high in silica reaching its optimum at approximately or other suitable material and employing oil or other insulating media. Such bushings are de signed for insulating the conductor which car ries electric current through a grounded zone such as the cover of a transformer tank. Under such circumstances there is a concentrated Ilux or potential difference which may give rise to sparkover or even corona formation in quantities sufficient to cause disturbances for example, in“ terference with the reception of radio frequency currents. Numerous means have been suggested 70 %’ so that the resultant compound has a coeffi cient of expansion conforming closely to that of the porcelain body and thus there is no tendency to “crazing” or “shivering.” Glazes of this type are said to “?t” the porcelain. It is to be under stood that when the glaze is thus attached to the porcelain body there results an actual increase in the mechanical strength of the porcelain mem for the purpose of improving the voltage gradient 15 ber. In fact the glaze becomes an integral part as Well as the distribution of the ?ux density. of the porcelain. . p Prominent among these is the idea of applying a In respect to the conducting character of the conductor of relatively low resistance for an ex~ silicate glaze it is to be noted that the resistance betweenpoints one-half inch apart range from one-tenth up to ?fty megohms which is an ex ceptionally low resistance compared with com tended distance along the high tension conductor. When this takes the form of a metallic sleeve around the conductor and within the bushing, it parable electricalporcelain glazes. adds materially to the complication of the device The application of this conducting glaze may and its cost. Furthermore, it in turn gives rise be made to any portion of the insulator where to stresses existing between this sleeve and the nearest insulating ba?ie or other part of the 25 there is likelihood of over-stressing or abnormally high voltage gradient.‘ In the example illustrated bushing. Where it has been the practice to‘bond on the attached drawing I have shown it applied such a metallic sleeve to the porcelain of the to several areas any one of which may in itself bushing or its baffle, this can be accomplished only be the locality of sparkoverv or discharge. at the expense of. the strength of the porcelain 30 The protection of any one of these areas is which in the treatment is materially weakened. bene?cial by itself. At the same time the pro My invention has been developed for the pri mary purpose of obtaining a high electrical effi tection of all of these areas is an added advantage ciency as well as attaining higher mechanical em ciency and greater strength. and improvement in operating characteristics. An important area to be protected by the con quately eliminate the generation of voltage of the type causing radio interference. A still further object of my invention is to ob ducting glaze from overstressing, as has already been suggested, is the surface of the bushing and speci?cally its internal baille, where it surrounds the high tension conductor. I have obtained tain a more improved voltage gradient than’ is improved voltage gradient as well as distribution It is a further object of my invention to ade present in similar devices. . ' The method. used in this invention, in addition to greatly improving the voltage gradient and of stresses by giving the inner surface of the baffle surrounding the conductor a uniform and complete coverage of the conducting silicate glaze. Another area likely to be affected by abnormal eliminating to a greater degree over stressed voltage characteristics, is the outer surface of areas, actually increases the mechanical strength 45 the insulating bushing extending downwardly of the dielectric porcelain members. from the mounting ?ange to below the surface Speci?cally my invention applies a special glaze, of the oil surrounding the bushing. By applying upon insulating parts of the bushing. Thisglaze a glaze of good conductivity over‘ the outer sur is selected for the special purpose of materially face of the bushing and extending this from below reducing the'electrical resistance and this tak ing the place of any grounding sleeve or other 50 the surface of the liquid upwardly to the support— metallic conducting member. This glaze is prinj cipally formed of silicates containing. conducting“ of a discharge between any part of the bushing materials and which ?res to an integral part of and the adjacent areas. in. g ?ange or other structure I avoid the possibility This eliminates the necessity of a special outer the porcelain body without the reduction of the conducting metallic oxides present, thereby cs 55 grounding sleeve and also grading down the con 2,408,990 4 3 ductivity of the latter at its lower extremity as in Rorden 2,209,003. It also eliminates the neces is given a glaze 6. This glaze is a silicate spe sity of providing special flux control structure known to have de?nitely low resistance. at the lower end of the grounding sleeve as in glaze is fired on the bushing d and becomes an integral part of this. It is characteristic of such ?red glazes that the porcelain may thus be given ci?cally one of the silicate compositions which are This Figure 3 of Rorden 2,188,417. In these prior structures, the lower ends of the grounding sleeves an increased strength of as much as 40% over are maintained at ground potential, and the entire similar unglazed articles. This glaze is of a sili voltage drop between the high tension conductor cate type similar to those applied to the other passing through the insulator bushing and ground is concentrated within the oil space between the 10 portions of the bushing, with the exception that it possesses conductivity to the degree already lower end of the ground sleeve and the lower terminal of the high tension conductor. The con mentioned. Glazes of this type may be prepared by intro ducing titanium dioxide one or more other for a different distribution of the voltage drop between the lower terminal of the high tension 15 metallic oxides in a silicate glaze which would be a non-conducting glaze without the addition of conductor and the supporting ?ange of the insu these particular oxides. As examples, titanium lator, and the present invention is distinguished dioxide with iron oxide or chromium oxide, or from these prior structures by the fact that instead combinations of these oxides, will produce the of concentrating the entire voltage drop below the surface of the oil, the glazed outer surface of 20 required conductivity of the glaze. The resistivity of the glaze may be varied between wide limits hy the insulator bushing provides for a substantial the variation of the content of the titanium dioxide voltage drop along the outer surface of the bush~ and the other metallic oxides introduced for the ing within the air space between the top of the purpose of increasing conductivity. Dependent oil and the supporting ?ange. The remainder upon the proportions of the oxides, the resistance of the voltage drop is taken up below the surface of the glaze over a one-half inch surface will of the oil. This distribution of the potential drop vary from one-tenth megohm to ?fty megoh'ms. reduces the amount of concentration of voltage The bottom of the bushing shell 4 is closed by at any one point and thereby reduces the tendency means of a ring ‘I. This is separated from the for the development of corona discharge. These bushings contain an insulating oil up to 30 bottom of the tubular bushing shell 4 by means of the oil-proof gasket 8 made of synthetic resin within a short distance from the top. such as Corprene (from neoprene) or the like An electrostatic charge may develop within and the ring is cemented or otherwise attached the bushing and thus subject the space above to the outer wall of the shell as shown in Fig. 1. the oil to an overstress. To reduce this condi Within the bushing the ring ‘I is extended to tion I propose to provide an oil reservoir at the 35 form a ?ange support upon which a gasket 9 rests. top of the bushing. This reservoir will be of con A vertical tubular baffle ill rests upon the gasket ducting material, for example, a metal casting. 9 and extends to the top of the bushing. As the It will have the same potential as the lead-in ducting glaze of the present invention provides conductor and as high as any potential Within the bushing itself. Hence there will be no differ ence in potential to cause a discharge, Where the size of the bushing does not require a reser voir and the oil level does not come above the outer porcelain wall of the bushing then the latter may have a conducting glaze applied from a point below that oil level upwardly to the metal cap of the bushing and electrically bonded to this metal cap. In like manner the surface inside of the bush ing may be subject to overstress in the area oppo site the fiange. To reduce this I may provide a conducting glaze on this surface extending above and below the ?ange and also on the opposite surface of the inner bai’?e. To illustrate my invention I have shown it in shell and baiiie are concentric, there is thus pro 40 duced an annular oil chamber H. The inner surface of the tubular baffle I!) has a conducting glaze it, This is coextensive with the inner surface of the ba?le. It is also fired upon the baffle in the manner already indicated. 45 The ba?ie provides a passageway for the high U311“ sion conductor l3. The metallic cap M is provided to rest upon the shell 4, to center the inner baiiie it, to sup port the high tension conductor I3 and ?nally 50 to permit ready inspection of the oil with which the bushing is ?lled. This cap i4 is supported upon an oil-proof gasket i5 which in turn rests upon the upper edge of the shell 4. The exten sion l6 of cap [4 is cemented indicated at 55 I‘! around the upper end of the shell 6. The cap I4 is substantially cylindrical and pro its preferred form in the accompanying drawings vides a chamber l3 which is in communication in which with the space I i in the shell 4. Fig. 1 is a vertical half section of the improved One portion of the side wall of the cap 14 is bushing partly reduced in length; Fig. 2 is an enlarged plan view of the reservoir 60 recessed as shown at H! to receive a vertical gauge glass 20. This gauge glass rests upon the gasket with the cover removed and 2| in the seat 22. Its upper end projects through Fig. 3 is an enlarged vertical section of the the hole 23 at the top of the cap I4. A passage cap on the line 3-3 of Fig. 2. way 24 connects the lower end of the gauge glass The bushing is mounted upon a ?ange I. This ?ange is of suitable form and shape to be attached to the chamber H3. to the top of a transformer tank A or the like. The ?ange has an upstanding collar 2 upon which a gasket 3 rests. The outer tubular bush The cap [4 has a plurality of inwardly extend ing lugs 25 upon which an open spider 20 rests and is bolted by means of screws 21. This open spider 26 rests upon an oil-proof gasket 28 which to the collar 2 by means of a body of cement 5 70 is thus clamped ?rmly against the end of the baffle l0 and compresses the latter against the or the like. Commencing with the enlargement of the bush gasket 9 at the bottom of the bushing. In this ing shell 4 which rests upon the gasket 3, and way the chamber If is rendered substantially extending to a point below the oil level B of the leak proof at the bottom and top of the inner transformer tank, the outer surface of the bushing 75 baffle. ‘ ing shell 4 rests upon the gasket 3 and is attached 5 2,408,990 6 Another oil-proof gasket 29 overlies the spider ‘ 26 and the top of the cap l4. A metallic cover 30 ?ts over the gasket 29 and has a rim 3| ?t ting within a stepped portion 32 of the side wall of the cap M. Bolts 33 pass through the cover 39 and seal the top of the cap M. A passage way 34 is provided to connect the upper part of the gauge glass 20 with the chamber l8 through gasket 29. A suitable vent may be provided in complished by providing a zone of intermediate conductivity. ~ While I have illustrated by way of example my preferred form of an oil ?lled bushing insulator having integral stress-reducing facilities, the principle involved in the invention may be em bodied in numerous other speci?c arrangements of details and materials and the invention is to be considered as limited only by the scope of the the cover 3%}. 10 appended claims. A spring contact 35 is screwed to the cover 30 In the annexed claims the term “high resist by means of screw 36. Y The free end of the con ance path” is intended to apply to a path of sum tact rests against the inner surface of the ba?le ciently high resistance to produce an appreciable H) where it is in contact with the conducting voltage drop along its length, as distinguished glaze E2. The high tension conductor [3 has a a low resistance path provided by the screw-threaded extension 31 extending through 15 from grounding sleeve or metalized sleeve used in the a center opening in the cover 39. A nut 38 on prior art where there is substantially no voltage the extension 31 supports the conductor I3 on drop along the length of the sleeve. the cover. What I claim is: A metallic cap nut 39 is also threaded on the 1. In electrical apparatus having a conducting 20 extension 31 and by means of the oil-proof gasket housing containing insulating liquid and provided 49 seals the passageway through the cover 30. with an opening above the level of said liquid The usual terminal 4! is formed integrally with through which a high voltage conductor extends the cap nut 39. to a point below the level of said liquid, said con This arrangement of a bushing insulator is a great simpli?cation over existing types. In the 25 ductor being supportedby an insulator bushing mounted in said opening and extending below ?rst place by the use of a conducting glaze on the the level of said liquid, the combination of means inner surface of the baf?e, it is possible to elimi for establishing a substantial voltage drop along nate entirely the conducting metal sleeve which the outer surface of said bushing between said has ordinarily been used for ?ux distribution at this point. Additionally the use of the conduct 30 opening and the level of said liquid, said means comprising a high~resistance path formed solely ing glaze gives added mechanical strength to the porcelain. The top of the oil adjacent to the bushing is of a conducting glaze forming the exterior sur face of said bushing and having electrical contact said opening and extending - with said housing at maintained at the same ground potential as the mounting ?ange by means of the conducting glaze 6. The glaze coatings 6 and I2 extend parallel to a point below the level of said liquid. reducing materially local electrical stresses. housing containing insulating liquid and provided 2. Electrical apparatus according to claim 1 wherein said conducting glaze has a resistance for a considerable distance and thus distribute value not substantially less than one-tenth meg the ?ux in large degree. The arrangement described in detail above, per 40 ohm and not substantially greater than ?fty megohms as measured between points separated mits the support of a high tension conductor by a distance of one-half inch. through a partition such as the cover of a trans 3. In electrical apparatus having a, conducting former casing, a metallic deck or the like while Thus the outer shell of insulating porcelain which would otherwise establish a zone of high electrical 4 .3 to a point below the level of said liquid, said con stress, is sheathed with a conducting glaze 6 in contact with the oil or other media surrounding it. This prevents objectionable electrostatic dis ductor being supported by an insulator bushing turbances. The same property is possessed by the con ducting glaze 12 extending throughout the length of the inner ba?le l3. Not only does each con; ducting glaze surface perform a useful purpose by itself but also the extension of these surfaces in parallel serves to distribute the stresses be tween them and vavoid electrostatic disturbances. In some situations, for example where the con ducting glaze is carried beneath the level of the oil, a corona glow may still occur if the glaze ends abruptly. Hence the glazed areas may be grad ed off in conductivity at their edges. This is ac with an opening above the level of said liquid through which a high voltage conductor extends 50 mounted in said opening and extending below the level of said liquid, the combination of means for distributing a substantial portion of the volt age of said conductor along the outer surface of said bushing between said opening and the level of said liquid, said means comprising a conduct ing glaze forming the exterior surface of said bushing and having electrical contact with said housing at said opening and extending to a point below the level of said liquid, said glaze constitut ing a resistance path having a resistance value not less than one-tenth megohm as measured be tween points separated by a distance of one-half inch. JOHN E. M. MITCHELL.