The influence of atmospheric conditions on the dielectric strength of external insulation is a complicated matter. Investigations carried out resulted in a correction procedure for atmospheric conditions that is currently adopted by the IEC standard 60060-1/1989. Despite its widespread applicability, many of the factors employed in this standard are still under consideration and several authors have argued about its validity especially on short air gaps. This paper presents data concerning the breakdown characteristics of rod-plane gaps under positive impulse voltages. Emphasis is given on the influence of atmospheric conditions, the effect of gap spacing and the wavefront duration of the applied impulse voltages. Empirical expressions relating to these parameters are presented. An optimised atmospheric correction procedure is introduced which employs an iterative concurrent correction for both air density and humidity estimating more accurately than the IEC procedure the influence of atmospheric conditions.
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