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. The smallest insulator unit standardized has a lighting impulse withstand voltage of 125 kV. 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. This standard may be regarded as a provisional standard for insulators for d.c. overhead lines. Note -For general definitions and methods of tests see IEC 383. -For dimensions of clevis and tongue couplings of string insulator units, see IEC 471. -For dimensions of ball and socket couplings of string insulator units, see IEC 120.