Applies to string insulator units of the cap and pin type with insulating parts of ceramic material or glass, intended for a.c. overhead lines with a nominal voltage greater than 1 000 V and a frequency not greater than 100 Hz and to insulators of similar design used in substations. Specifies values of mechanical and dimensional characteristics for string insulator units of the cap and pin type: 1) with ball and socket couplings designations U 40 BS to U 530 BL 2) with clevis and tongue couplings designations U 60 C to U 210 CL. Note: - For use in heavily polluted areas and for insulators for use on direct current systems, see IEC 438.