IEC 60383-2: Insulators for Overhead Lines Above 1000V — Insulator Strings for AC Systems

How Many Insulator Discs Does a 500kV Line Need? Precision Calculation of Creepage Distance and Pollution Class

IEC 60383-2:1993 specifies overhead line insulators above 1,000V AC. Number of discs = (Um × specific creepage) / single-disc creepage. China Class I (clean): 16 mm/kV; Class IV (coastal/heavy industry): 31 mm/kV — for the same 220 kV line, Class IV requires nearly double the discs of Class I.

TNLab — Pollution class determines whether insulator strings are “short” or “long” — coastal substation strings can be twice the length of inland ones.