How the results of dielectric tests on external insulation in natural conditions may be predicted from indoor laboratory test results

Previous research¹ on 420 kV insulator strings, both in laboratory and in natural conditions, showed, on one hand, the large dispersion of the insulators´ switching impulse performance characteristics in natural conditions, especially with regard to negative polarity stresses and, on the other hand, the practical impossibility of evaluating the behavior of the insulation under switching surges in natural conditions by simple correction of laboratory data for real atmospheric conditions, and for real conditions of humidity and relative air density in particular.