See Also: Lightning
Some of the information for this topic came from: Electrical World, June, 1996, pages 22-26.
High-Impedance grounds, above 25 Ohms, (see Grounded, Ground Impedance) or an insufficient number of grounds are key contributing factors in
lightning-caused flashovers. Static Wire (see Conductor) protected lines are more sensitive to high ground-impedance than lines
protected with three-phase arresters. That’s because, with static-wire protection, the Voltage induced (see Induction) in the phase wires can be enough to cause phase-to-phase flashover. Other
factors contributing to outages include inadequate arrester protection, reduced
clearance problems, and excessive static Angles (an angle greater than 30 degrees between the static and the phase).
In an extreme lightning event line equipment and pole hardware may be
destroyed, but flashovers may also cause Insulation levels to degrade over time, resulting in additional wear and tear on Substation and line equipment.
With either static or arrester protection there are three main factors that
determine the peak voltage level after a lightning stroke - stroke intensity,
surge impedance, and ground impedance. There is no way to control the intensity
of a given stroke, and very little control over the surge impedance - which is
determined by the line Inductance and Capacitance. But we do have some control over ground impedance.
If a line in question is relatively clear of trees, it can be assumed that a
large percentage of momentary outages are caused by lightning.