IEC 60660 Ed. 2.0 b:1999

Applicable to post insulators of organic material for indoor service in electrical installations or equipment operating in air at atmospheric pressure on alternating current with a nominal voltage greater than 1 000 V up, but not including, 300 kV, as defined by range I of IEC 60071-1, and a frequency not greater than 100 Hz. Composite insulators are not covered by this standard.