This part of IEC 61466 is applicable to composite string insulators units with a specified mechanical load (SML) of 40 kN and 210 kN for a.c. overhead lines with a nominal voltage greater than 1000 V and a frequency not greater than 100 Hz. It also applies to insulators of similar design used in substations or for railway specifications. This standard applies to string insulator units of composite type with fittings in accordance with IEC 61466-1. The standard prescribes specified values for electrical and dimensional characteristics for composite insulators for overhead lines with a highest standard lightning impulse level of 1050 kV and a specified mechanical load (SML) of 40 kN to 210 kN.