III — Designs and economics of suspension insulator strings

IN THE PAST the effectiveness of transmission line insulation has been judged almost entirely by its 60-cycle flashover characteristics. On this basis the wide changes in spacing and diameter of suspension insulator units have affected the over-all flashover values but little. Recently, however, it has been realized that the criterion for insulation should be based upon impulse flashover characteristics rather than upon the 60-cycle behavior. Limited laboratory tests using impulse voltages showed that the impulse flashover voltage of insulator strings is changed appreciably by varying the diameter and spacing of the individual units. Economic studies show that, in general, low cost is associated with large diameter units spaced close together.