IEC 60433:2021 prescribes specified values for the electrical and mechanical characteristics and for the principal dimensions of string insulator units of the long rod type with insulating parts of ceramic material intended for a.c. overhead lines with a nominal voltage greater than 1 000 V and a frequency not greater than 100 Hz. It is also applicable to insulators of similar design, used in substations. Applies also to string insulator units: -of the long rod type either with a clevis at both ends for coupling with a tongue, or with a socket at both ends for coupling with a pin ball -with external metal fittings -for use on overhead lines situated in slightly polluted areas, and the creepage distances given in Tables I and II have been established accordingly -and insulators of similar design, used in substations.