This part of IEC 61466 is applicable to composite string insulator units with a specified mechanical load (SML) of 40 kN to 210 kN for a.c. overhead lines with a nominal voltage greater than 1 000 V and a frequency not greater than 100 Hz. It also applies to insulators of similar design used in substations or for railway applications. This standard applies to string insulator units of composite type with fittings in accordance with IEC 61466-1. This standard prescribes specified values for electrical and dimensional characteristics for composite insulators for overhead lines with a highest standard lightning impulse level of 1 050 kV and a specified mechanical load (SML) of 40 kN to 210 kN. NOTE General definitions and methods of testing are given in IEC 61109