Direct lightning strokes to lines as shown in Fig. la are of concern on lines of all voltage class, as the voltage that may be set up is in most instances limited by the flashover of the path to ground, Increasing the length of insulator strings merely permits a higher voltage before flashover occurs. The most generally accepted method of protection against direct strokes is by use of the overhead ground wire (Fig. 2) .For simplification only one ground wire and one power conductor are shown.

图1a显示了所有的电压等级的电线的直接雷击情况. 因为电压的设定很大程度上局限于对地放电的情况.当放电存在时增加绝缘子的长度对提高电压没多大作用,比较常用的办法是用架空接地线(图2), 简单起见图中只显示了一个接地线和一个火线的情况.

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