This part of IEC 61466 is applicable to composite string insulators units with a specified mechanical load (SML) of 40 kN and 70 kN to 600 kN for AC overhead distribution 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 on electric traction lines for railway applications. This standard applies to string insulator units of composite type with couplings fittings in accordance with IEC 61466-1. This standard prescribes specified specifies values for electrical and dimensional characteristics of for composite string insulators units for overhead distribution lines with a highest minimum lightning impulse withstand voltage (BIL) level of 325 kV up to 3100 kV and a specified mechanical load (SML) of 40 kN and 70 kN to 600 kN. NOTE General definitions and methods of testing are given in IEC 61109.