Figure 3.8. Illustration of Process and Currents Which Occur
During a Lightning Flash to Ground
The lightning discharge involves the transfer of large amounts of electric charge between the cloud and the
earth. The typical flash transfers 15 to 20 coulombs (C) (1 coulomb equals 6.2 x
electrons) with some
flashes involving as much as 400 coulombs of charge. The energy per flash of lightning has been estimated to be
as high as watt-seconds. Table 3-1 summarizes the range of values for selected lightning parameters.
3.6.2 Mechanical and Thermal Effects.
The fast rise time, high peak amplitude current of the stroke can produce severe mechanical, thermal, and
electrical effects. The damage caused by these currents to objects in the discharge path is closely related to
the relative conducting power of the object. For example, metals generally receive a discharge with little
damage. In most cases, even slender conductors such as telephone and electric power cables handle the current
without fusing (melting) except at the point where the current enters or leaves the metal (where severe damage
may occur). Very strong discharges of high peak current (> 40 kA) and high coulomb values (>200 C), however,
can melt or burn holes in solid metal plates. This burning effect is not usually of primary concern for a typical
building or structure because, if an adequate protection system is installed, the principle effect will be a small
deformation at the tip of a lightning rod or a small melted area on the intercepting cable. Such effects are of
more concern where flashes to airplanes occur because such burning can perforate the fuselage to cause loss of
pressurization or penetrate the skin of fuel tanks and possibly ignite fuels. The burning or melting also presents
a threat to exposed tanks of volatile gases or fuels on the ground.