This standard is applicable to string insulator units of the long rod type with insulating parts of ceramic material intended for a.c. overhead lines with a nominal voltage greater than 1000 V and a frequency not greater than 100 Hz. The smallest insulator unit standardized has a lightning impulse withstand voltage of 125 kV.
It is also applicable to insulators of similar design, used in sub-stations.
This standard is applicable to string insulator units of the long rod type either with a clevis at both ends for coupling with a tongue, or with a socket at both ends for coupling with a pin ball.
This standard is applicable to string insulator units with external metal fittings (see Figures 1 and 2, page 12) or internal metal fittings (see Figures 3 and 4, page 14).
This standard is applicable to string insulator units for use on overhead lines situated in slightly polluted areas, and the creepage distances given in Tables I and II have been established accord-ingly. However, shorter creepage distances may be used in some non-polluted areas. On the other hand, it may be necessary to use longer creepage distances in polluted areas. In any case, the mechanical characteristics as well as the lengths L2 (see Clause 4) of this standard should be used unless the need for exceptionally long creepage distances requires values of L2 greater than those given in Tables I and II.
This standard may be regarded as a provisional standard for insulators for d.c. overhead lines.
Nole\ I. — The application of this standard to insulators of other materials may be considered l?ter.
2 — This standard is not applicable to insulators for electric traction lines which use special metal fittings. However, for these insulators it is recommended to use strength classes and distances between metal parts given in this standard.