ABSTRACT
External insulation is exposed to the environment. Typical external insulation is the porcelain insulators, supporting transmission line conductors. The insulators must withstand mechanical stresses for long periods of time. It is anticipated that a line or substation will operate for more than 20–30 years without changing the insulators. The insulators are attached to the tower and support the conductors. In a suspension tower, the insulators are in a vertical position or in a V-arrangement. In a dead-end tower, the insulators are in a horizontal position. The operating voltage fluctuates from changing load. The normal range of fluctuation is around ±10%. The line-to-ground voltage causes the voltage stress on the insulators. The insulator basic insulation level is determined by using standard lightning impulses with a time-to-crest value of 1.2 µs and time-to-half value of 50 µs. When an insulator is tested, peak voltage of the pulse is increased until the first flashover occurs.
