This invention relates to electric insulators and particularly to means for reducing or preventing radio disturbance from insulators of the suspension type. Heretofore insulators have been provided on their surface with semi-conducting coatings extending over the surfaces of the insulators away from the metallic fittings to reduce or prevent disturbance caused by discharges from the fittings over the insulator surfaces, as disclosed in Patent 1,735,829, granted to A. O.
Austin, November 12, 1929, and assigned to The Ohio Brass Company of Mansfield, Ohio. Various forms of semi-conducting coatings have been employed in addition to those disclosed in the Austin patent, one form being disclosed in the patent of Hawley, No. 1,668,123, and another in the patent to Sleeman No. 2,154,387.
While the coatings disclosed in the prior patents reduce to some extent the surface discharges and radio disturbances of electric insulators, applicant has found that the effectiveness of such coatings can be greatly improved, especially on suspension insulators, by shaping the dielectric member so as properly to dispose the terminal edge portions of the coatings relative to each other and to the electrostatic field produced in the dielectric medium and surrounding atmosphere.
After extensive investigation, applicant has found that the effectiveness of the treatment depends, not so much upon the disposition of the separate coatings per se as it does upon the relation of the coatings on opposite sides of the dielectric member to each other and to the dielectric.
One object of the present invention is to provide an insulator in which semi-conducting coatings on opposite surfaces of the dielectric are so related as to coact upon each other to prevent disturbance from the edges of the coatings and consequently to reduce or eliminate radio disturbances.
Another object of the invention is to provide a device of the class named which shall be of improved construction and operation.
Other objects and advantages will appear from the following description.
The invention is exemplified by the combination and arrangement of parts shown in the accompanying drawing and described in the following specification and it is more particularly pointed out in the appended claim.
In the drawing: Fig. 1 is an elevation partly in section of a suspension insulator treated in accordance with the present invention.
Fig. 2 is a view similar to Fig. 1 showing a slightly different form of treatment for producing the same result as that produced in Fig. 1.
In both figures the numeral 10 designates a dielectric member forming the insulating body of a suspension insulator having a cap II and a pin 12 cemented thereto in the usual manner. Applied to the outer surface of the dielectric 10, adjacent the lower edge of the cap II, is a high resistance or semi-conducting coating 12'. This coating is indicated in the drawing by a heavy line and extends a sufficient distance beneath the cap Ii to make good electrical contact with the cement 13 and hence with the cap II. At its lower edge the coating extends out along the upper surface of the dielectric member 10 for a distance of approximately an inch the width of the coated zone depending upon the size of the insulator and the conditions under which it is to operate.
In the form of the invention shown in Fig. 1, the inner surface of the pin hole is not shown as covered by a conducting coating but is provided with a circumferential pocket 14 which is partially closed at its lower side by an inwardly inclined lip 15 on the lower surface of the dielectric member 10. The cement 16 which holds the pin 12 within the pin hole extends downwardly in the pocket 14 a sufficient distance that the cement terminates on the downwardly and inwardly inclined upper surface of the lip 15.
Since the cement 16 is partially conducting the outer surface of the cement will have the potential of the pin 12 and the points of greatest electrostatic stress will be located at the outer edge of the coating 12 and at the lower extremity of the contact of the cement 16 with the upper surface of the lip 15. The field of greatest electrostatic stress will be set up between the outer edge of the coating 12 and the terminal edge of the cement 16 and the lines of force extending between these two edges will be almost entirely confined to the dielectric member 10. Not only is this true, but these lines of force in each case are directed backwardly away from the terminal edge of the coated surface of the dielectric member, thus tending to restrain any discharge from -I these edges. This is not only true of the local electrostatic field set up by the conducting members on opposite sides of the dielectric flange but also true of the general electrostatic field set up by the conductor and supporting structure ;:, for the insulator string. The high resistance of the conductor coating 12 and of the cement 13 also tends to prevent discharge from the terminal edges of these elements, particularly if the resistance of the coating 12 is graded as suggested in the Austin patent cited above.
Of course, all of the electrostatic lines of force do not extend directly between the two terminal edges of the conducting members, but extend in curves of gradually increasing length at either side of the field of greatest stress. However, with the form and arrangement shown in the drawing only the very weak portion of the field at its outer fringe approaches a direction tangent to the surfaces of the dielectric member at the terminal lines of the conductors, and only the lines of force defining the very weak field extend into the surrounding atmosphere. This is due to the relation of the radially extending coating on the upper surface of the dielectric member to the terminal edge of the internal conductor on the inwardly and downwardly sloping surface of the internal pocket. In actual operation this particular relation has been found to give results far superior to all other arrangements in which this relation is not maintained.
In some cases it may not be desirable for mechanical reasons to extend the cement 16 into the pocket 14 but where this is true, the desired relation of the electrodes and dielectric member may be secured by providing the inner surface of the pocket 14 as shown in Fig. 2 with a semiconducting coating 17 similar to the coating 12.
The distribution of the electrostatic field and the operation of the insulator in this case will be similar to that produced by the arrangement shown in Fig. 1.
I claim: In combination, a dielectric member having a radial flange, and having a boss on the upper side of said member and a recess extending into said boss from the lower side of said member, a cap secured to said boss and terminating adjacent the surface of said flange, a pin secured within said recess, a semi-conducting coating electrically connected with said cap and extending over the upper surface of said flange away from said cap and terminating on the radially extending portion of said surface, the wall of said recess being shaped to form a circumferential pocket within said recess and adjacent the mouth thereof, said pocket being partially closed at its lower side by a downwardly and inwardly extending lip on the lower side of said dielectric member, and conducting means on the inner surface of said recess and pocket terminating within said pocket, the terminal edge of the portion of the inner surface of said recess in contact with said conducting means being directed inwardly toward the axis of said insulator while the terminal edge of the semi-conducting coating on the upper surface of the flange is directed outwardly away from the axis of the insulator so that electrostatic lines of force connecting the terminal edges of the conducting means on the opposite surfaces of said dielectric member will be con29 tained chiefly within the body of said dielectric member and will be directed away from the terminal edges of the conducting means on said surfaces.
JOHN J. TAYLOR.