Potential of a Transmission-Line Tower Top When Struck by Lightning [includes discussion]

When the ground wire on top of a transmission-line tower is struck by lightning the potential of the ground wire and the tower top is raised above the potential of ground. Because of electro-magnetic and electrostatic coupling between the ground wire and the phase wire, a part of this voltage appears on the phase wire. Thus, if the tower-top potential is E and the potential of the phase wire is KE, where K is the coupling factor (a number less than one), the voltage across the insulator string will be E(1-K). If this voltage exceeds the voltage-impulse-strength of the insulator string there will be a flashover from tower to phase conductor which may result in a serious outage on the system. Thus, in designing and protecting a transmission line against flashovers due to lightning, it is important to estimate the probable tower-top potential that might be expected for a given set of conditions. The derivation and calculation of this tower-top potential due to a direct stroke, as a function of time, is very lengthy and complex. In the interest of simplifying the problem, the tower-top potential is sometimes estimated by merely multiplying the probable crest value of tower current by the tower footing resistance. For very high values of footing resistance (100 ohms) this approach gives reasonable but optimistic answers, while for the low footing resistances, usually found in practice (10 ohms or less), this method gives very poor and very optimistic answers.