Page "Insulator (electricity)" Paragraph 33

Standard disc insulator units are in diameter and long, can support a load of 80-120 kN ( 18-27 klbf ), have a dry flashover voltage of about 72 kV, and are rated at an operating voltage of 10-12 kV.

However, the flashover voltage of a string is less than the sum of its component discs, because the electric field is not distributed evenly across the string but is strongest at the disc nearest to the conductor, which will flash over first.

Metal grading rings are sometimes added around the lowest disc, to reduce the electric field across that disc and improve flashover voltage.