AbstractSample cables having a few types of defects (such as voids, contaminants, and protrusions) with more volume than usual, were prepared intentionally to clarify how these defects affect separately insulation characteristics under the presence/nonpresence of moisture. In the case of void inclusion, insulation characteristics are even more susceptible to ac voltage than impulse voltage, and the distribution of lifetimes is classified into a wornout failure type due to discharge deterioration.While protrusions and some other contaminants can greatly deteriorate original dielectric strength, these defects have a relatively slight effect on long‐term endurance in the atmosphere. Therefore, the distribution of lifetimes is classified into an early‐failure type. With reference to the influence of moisture, in a strong electric field (exceeding 15 kV/mm) and in a short‐term region, there is an increase in harmfulness around defects with water absorbed, thereby inducing electrical trees and resulting in a breakdown. In the light electric field (no more than 15 kV/mm) and in a long‐term region, harmfulness of defects will not deepen to a considerable extent, and the growth of water trees very possibly is influential.