The measuring system of charges accumulated on an insulating plate is developed with an electrostatic probe whose sensing electrode is of 0.5 mm diameter. In the inverse calculation from the probe outputs to the charge distribution, Tikhonov's regularization technique is effectively used to suppress the excessive amplification of the noise. In the case of measuring a 2 mm thick PMMA plate, the spatial resolution of the measuring system reaches 1.0 mm. With this system, the residual surface charge distribution on an insulating plate just after the occurrence of a positive surface discharge at 25 kPa (190 Torr) air is measured. On the head of a positive surface streamer, there remains 10−11 C surface charge, which satisfies the criterion of an electron avalanche-to-streamer transformation. The charge density across a streamer takes a minimum on its centre and a maximum on its sheath. This fact suggests that a quasineutral channel exists in the centre of a streamer, and positive ions remain on the sheath of it.

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The phenomenon of accumulated charges on solid insulator surfaces is one of the critical parameters to consider at the insulation design stage, for ac electric power equipment as well as for dc equipment, so it is important to investigate the characteristics and predominant factors underlying various charging mechanisms. Several researches related to this theme have been reported, but independently, and cross-sectional comparison and evaluation from a unified viewpoint are meaningful. In this paper, the resistance of solid insulator is first discussed, showing that the resistances found by diverse measurements are in a fairly good agreement under similar conditions of the temperature and electric field. Next, three kinds of electric charging mechanisms, i.e. volume conduction, surface conduction and electric field emission are characterized in terms of the time constant, applied voltage and charge distribution. Then, eight cases of recent measurements on the charging time are investigated and their charging mechanisms are classified. Electric field emissions are likely to occur with model spacers made in routine GIS manufacturing process at the electric field level used. Further, three examples in cases with metallic particles are introduced, of simulating charge from edge on the tank, spacer surface charging phenomena, and influence of charge on spacer surface flashover. It is expected that this paper will be helpful for understanding charging phenomena e.g. on insulation spacers in gas insulated switchgears.