Overhead Power Line

Protection of overhead electric power lines from the deleterious effects of lightning may be achieved by one or a combination of the following methods: (1) use of the highest economically reasonable insulation levels. (2) use of overhead ground wires with good connections to Earth at the most closely spaced intervals physically possible and economically reasonable and (3) use of arresters between the phase conductors and the neutral, spaced as closely along the line as economically reasonable and at locations of sensitive line hardware

Either a direct lightning strike on a power line or an induced voltage from a nearby strike may lead to line "flashover" (electrical breakdown between the wires of the line), and/or failure of arresters, transformers, insulators, or other line hardware. Flashovers or equipment failure can result in an out-of-service line, an "outage." Direct strikes are the most difficult to protect against because the associated overvoltages can be many millions of volts. typical return-stroke peak current of 30 kA (where 15 kA Hows in each direction from the strike point) multiplied by the parallel combination of the typical line surge impedance "seen" in each direction (about 500 ohms) yields a voltage at the strike point of 7.5 x 10⁶ volts. On distribution lines the line insulation can generally withstand only between 100 and 300 kV. Voltages induced on over­head lines by nearby strikes are thought to be less than about 300 kV, so induced voltages are easier to protect against, although there are no reliable data on the launch vehicle protection by the long overhead wires grounded at each end. We now discuss in more detail the lightning protection afforded by overhead ground wires, followed by a discussion of the lightning protection provided by lightning arresters and circuit breakers.

Grounded wires located above the phase conductors are referred to by a variety of names: overhead ground wires, shield wires, and sky wires. Overhead ground wires are most commonly used for the lightning protection of transmission lines where they are typically attached to the metal towers supporting the lines.

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