The ground wire is placed above the power conductor at such a position that practically all lightning-stroke paths will be to it instead of to the power conductor. Stroke current then flows to the ground, most of it passing through the tower footing ground resistance awhile a smaller part goes down the line and to ground through the adjacent tower footings. The tower rises in voltage due to the current Ix through the resistance Rx to a value which is
U1= I1R1
Approximately this voltage appears between the tower and the power conductor (which was not struck). If this voltage is less than that required to cause insulator flashover, no trouble results. Protection by this method is improved by using two carefully placed ground wires and by making tower footing ground resistance of low value.
The lightning record of lines supported on towers 80 to 90 feet tall substantiates the simple theory of line protection just presented. The poorer record of lines on towers over 100 ft in height indicates that other factors, perhaps the inductance of the path down the tower, should be considered.
Indirect strokes produce relatively low voltages on lines. They are of real concern on low-voltage lines supported on small insulators. They are of little importance on high-voltage lines whose insulators can withstand hundreds of kilovolts without flashover.

地线应该放在电力管道上面，所以所有的电击路线都会通过地线而不是电力管。 电击这个时候会流到地面上，多数电击会通过塔底的地面，少数会因为电阻而通过线流到临近的塔， 塔提升电压直到Ix 通过电阻Rx到一定的数值U1， 这个电压大概显示在电塔河电力管道之间。如果由于绝缘体跳火了，而导致这个电压小于他所需要的电压，则没有什么问题。保护这个的措施现在是用2条地线，让塔底地面的电阻保持在一个很低的值。
上述的保护措施已经被在80到90尺高的塔的电力记录所证实，然而如果那条线在100英尺高的塔上的话则要考虑另外的一个因素，塔向下的感应系数。
间接雷击会产生低压在电线上面，他们所关心的是哪个小的绝缘体能否存活在这条低电压线上，他们现在考虑的重点是，高压线上的绝缘体能够经得起上百次千伏电压的冲击而不坏掉。


我不是专业。可能有些地方翻译的不够准确，希望能帮到你。