Abstract
The decay time of residual dc charge in a 500kV transmission line had once been measured during five fine and dry days of winter season. The results showed a large scattering without depending on the simultaneously observed weather conditions, such as temperature or relative humidity. Then the authors have performed an additional experiment in a laboratory to discuss the factors that affect the residual dc charge leakage in a dry condition focusing on the moisture in the air and the dusts floating in the atmosphere. It is shown that absolute humidity alone decides the decay time without scattering under clean and calm condition. The floating dusts blown up by the wind, however, reduce the decay time and bring a large scattering. The dusts should be a charge carrier moving freely in the atmosphere.
