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High-speed photography has clearly demonstrated that lightning is actually composed of both a cloud component and an oppositely charged ground component. During "cloud-to-ground" lightning, these oppositely charged components usually "meet" somewhere in the atmosphere well above the earth to equalize previously unbalanced charges.

Lightning strikes can cause the following types of damage:

  • Injuries to living beings due to step and touch potential;
  • Physical damage (fire, explosion and mechanical destruction) due to lightning current effects,
  • Failure of internal systems due to lightning electromagnetic impulse (LEMP).

No lightning protection system can guarantee absolute safety from lightning, because of the high energy and current levels associated with lightning (currents can be in excess of 150,000 amps), and the very rapid rise time of a strike. If lightning travels through porous and water-saturated materials, these parts of a building may literally explode if their water content is flashed to steam by the heat produced from the lightning current. Lightning current will divide to follow every conductive path to ground, and even the divided current can cause damage. Secondary "side-flashes" can be enough to ignite a fire, blow brick, stone, or concrete apart, or injure occupants within a structure or building.

The fundamental principle used in lightning protection systems is to provide a sufficiently low impedance path for the lightning to travel through to reach ground without damaging the building. This is accomplished by surrounding the building in what is effectively a Faraday cage. Lightning protection systems are designed for high frequency transients with lightning current peaking over a very short time period.

In a wooden building, the structure and its contents may be ignited by the heat generated by lightning current conducted through parts of the structure. A basic lightning protection system would provide a conductive path between an air terminal and earth, so that most of the lightning's current will follow the path of the lightning protection system, with substantially less current traveling through flammable materials. In a steel-framed structure, members are bonded to earth to provide lightning protection. A metal flagpole with its foundation in the earth is its own extremely simple lightning protection system (However, the flag(s) flying from the pole during a lightning strike may be completely incinerated!).


Step Potential: Voltage between the feet of a person. When current from the lightning strike is flowing from the structure to the earth ground, the ground potential rises at the structure. A voltage gradient will occur based on the resistivity of the soil, resulting in a potential difference between two points on the ground. This is called a Step Potential as it can cause voltage between a person’s feet.

Touch Potential: Voltage between energized object and feet of a person. If the ground connection between the structure and the soil is high resistance (common with some soil conditions), the structure itself (and any conductive item touching the tower) can be energized. This causes a voltage difference between the energized object and the feet of a person in contact with the object.

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