Applies to insulator strings and insulator sets comprising string insulator units of ceramic material or glass for use on a.c. overhead power lines with a nominal voltage greater than 1 000 V and a frequency not greater than 100 Hz. This part of IEC 60383 also applies to insulator strings and insulator sets for use on d.c. overhead electric traction lines. This part of IEC 60383 also applies to insulator strings and insulator sets of similar design when used in substations.