The Choice of High Tension Insulators

THE CHOICE OF HIGH TENSION

INSULATORS.

BY H. C. KNOWLTON.

However important good material and workmanship may be in regard to the poles, cross arms, tie wires and pins of a high-tension line, it is imperative that the insulators be designed, built, tested and installed with the utmost skill and care, for the integrity of the line depends upon their adaptation to the service furnished. In the choice of insulators for circuits of 25,000 volts and upward, four essential features in design should be given special attention, viz., resistance to puncture; resistance to arcing, to pin or cross-arm; proper distribution of electrostatic strains and low capacity; and proper creepage surface.

The voltage required to puncture an insulator should be enough greater than the voltage required to arc over from wires to pin under any conditions to insure the insulator against injury in case of abnormal voltage. Wet tests, made by spraying a stream of water upon an insulator from a sprinkling hose inclined 30° from the horizontal and 4 or 5 feet away, usually indicate much greater liability to puncture than when the insulator is dry. Experience has shown that the relative mechanical forms of the petticoats exercise a marked influence upon the tendency to puncture. When the petticoats flare widely or are short there is usually less concentration of electrostatic strain on the inner petticoat, and consequently less liability to puncture.

Resistance to arcing is chiefly a function of the longest combined air and insulator surface path which the discharge can be forced to take in passing from the wire to the pin or to the cross-arm. If an insulator be designed to withstand a dry puncture test at double its working voltage, the arcing voltage under the wet test becomes less important as a factor influencing the construction and design. In making tests on high tension insulators their probable behavior under commercial operation must always be kept in mind, and the fascinating results which such experiments afford must not distract the testing engineer from his real purpose, which is to determine which particular insulator is best suited to his operating requirements. Insulators should always be compared under the worst conditions to be expected in practice. Dry tests chiefly indicate the relative static activity of insulators. The possibilities of resonance on the line will largely influence the importance of the arcing voltage. In general, a factor of safety of 3 on arcing voltage in normal operation is not too much to expect. This gives 1 1/2 in case of a breakdown of insulation on one side of the line. The puncture test should be twice the normal line voltage, or 120,000 volts on a 60,000-volt insulator.

In regard to electrostatic strains, the behavior of an insulator in the way of noise and display under test may be taken as an indication of its static capacity and effectiveness of material distribution. The point of visible static discharge should be kept at least as high as the full line voltage, in order that no insulator shall be subjected to undue strain. The area of creepage surface should be made as high as is consistent with reasonable design.

In testing high potential insulators careful notes should be taken of their behavior at all critical points in voltage application. Points to be specially recorded are: the first appearance of visible static discharge from the edges of the insulator under test; the point of first sparking, or discharge from one part to another; the point of general activity — lively and more general sparking snapping and crackling; and point of arcing over from wire to pin — or puncture in case of a faulty insulator. As a general rule these points are well defined.

It is well to realize that questions of price and delivery are not of the first importance in dealing with insulator problems. Probably no single device involved by modern engineering so well illustrates the saying that cheap goods are dear at any price. The selection of insulators for any high-tension line worthy to be placed in commercial operation as an example of the latest and best practice implies careful study of the dsign [sic] design, conversance with the general methods of manufacture, severe tests under unfavorable conditions, and the best installation which the judicious expenditure of money can accomplish.