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High reliability of electric power supply is requested by the rapid evolution of information-oriented and computerized society.On the other hand, it is important to consider the cost down on power distribution lines in power companies. From this point of view, the authors developed an analytical method that can calculate flashover rate caused by lightning considering both the reliability and cost performance. As the results of calculations, it is clear that the flashover rate is significantly influenced by various parameters such as the lightning current waveform and nearby buildings. Moreover it is clear that the flashover rates of power distribution lines for various lightning protection designs are much different even if the constructing costs of power distribution lines are the same.

The authors have examined over a period the basic characteristics 01 discharge when an open wire fitted as an overhead ground wire is located near a tree. to clarify the attachment manner in which direct lightning hits power distribution lines which are in close proximity to trees. Previously, a focus was made on how lightning attachment emerges to the open wire or the tree. In this latest experiment, made at a testing yard using an experimental distribution line and a tall tree, a lightning impulse waveiorm and a switching impulse waveiorm were applied individually to a rod electrode. The different manners of attachment of sparkover to the distribution line and the tree were observed. along with manners of prebreakdown phenomena. This result coincided with the lightning attachment status previously achieved. This experiment revealed that the discharge Characteristics shown in the previous study are valid.

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