Replace clause 1 with the following: This part of IEC 61466 is applicable to composite string insulator units with a specified mechanical load (SML) of 40 kN to 550 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 minimum lightning impulse withstand voltage (BIL) up to 2700 kV and a specified mechanical load (SML) of 40 kN to 550 kN. NOTE General definitions and methods of testing are given in IEC 61109.