Lightning

There is little doubt that lightning is dangerous. The average peak electrical current is 20kA and can go as high as 150kA. A lightning strike can last from 20-200 µs and is usually a combination of multiple strikes. Lightning is also very common. At any one time there are 2000 lightning storms in progress, with a daily occurrence of 5000 storms. The incidence of lightning varies widely over the earth's surface. The relative frequency of lightning worldwide is shown in Figure 1 . The highest incidence is over central Africa but other areas that show high amounts of lightning are South America and the south eastern United States , in particular the state of Florida.

Figure 1 Relative lightning frequency worldwide (US National Space Science and Technology Center)

Electrical stress from lightning can enter a building's electrical system in a variety of ways other than a direct strike to the structure. Lightning striking overhead power transmission lines can directly couple the current into the building's AC power conductors. Lightning striking a nearby hill will force large currents to flow in the ground. These currents will affect the ground potential around the building. This can be aided by underground utility services such as water or gas pipes which are electrically bonded to the buildings earth ground. A lightning strike to a flag pole next to a building can induce current on conductors within the building such as the phone or cable connected to the PC.

The variety of paths that lightning stress can enter a building illustrates why it is necessary to add surge protection at multiple locations. Surge protection is required by building codes at the point of entrance of AC power and telephone service. Additional surge protection at the location of sensitive electronics such as personal computers and entertainment centers can be well worth the added expense.

Figure 2 Paths of Lightning Stress

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