Quantcast Chapter 3. Lighting Protection Subsystem

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MIL-HDBK-419A
CHAPTER 3
LIGHTNING PROTECTION SUBSYSTEM
3.1 THE PHENOMENON OF LIGHTNING.
Cumulonimbus clouds associated with thunderstorms are huge, turbulent air masses extending as high as
15 to 20 kilometers (9 to 12 miles) into the upper atmosphere. Through some means, not clearly understood,
these air masses generate regions of intense static charge.  These charged regions develop electric field
gradients of hundreds, or perhaps thousands, of millions of volts between them. When the electric field strength
exceeds the breakdown dielectric of air
volts/meter), a lightning flash occurs and the charged areas
are neutralized.
Electric field measurements indicate that the typical thundercloud is charged in the manner illustrated by
Figure 3-1 (3-1).  A strong, negatively charged region exists in the lower part of the cloud with a
counterbalancing positive charge region in the upper part of the cloud.  In addition to these major charge
centers, a smaller, positively charged region exists near the bottom of the cloud. Due to the strong negative
charge concentration in the lower portion of the cloud, the cloud appears to be negatively charged with respect
to earth -- except in the immediate vicinity underneath the smaller positive charge concentration.
Breakdown can occur between the charged regions within the cloud to produce intracloud lightning. It can also
occur between the charged regions of separate clouds to produce cloud-to-cloud lightning. Intracloud and
cloud-to-cloud discharges do not present a direct threat to personnel or structures on the ground and thus tend
to be ignored in the design and implementation of lightning protection systems. However, calculations of the
voltages which could be induced in cross-country cables by such discharges (3-2) indicate that they present a
definite threat to signal and control equipments, particularly those employing solid state devices.
The cloud-to-ground flash is the one of primary interest to ground-based installations. By definition, such
flashes take place between a charge center in the cloud and a point on the earth. This point on earth can be a
flat plain, body of water, mountain peak, tree, flag pole, power line, residential dwelling, radar or
communications tower, air traffic control tower, or multi-story skyscraper. In a given area, certain structures
or objects are more likely to be struck by lightning than others; however, no object whether man-made or
natural feature, should be assumed to be immune from lightning.
The high currents which flow during the charge equalization process of a lightning flash can melt conductors,
ignite fires through the generation of sparks or the heating of metals, damage or destroy components or
equipments through burning or voltage stressing, and produce voltages well in excess of the lethal limit for
people and animals. The objective of all lightning protection subsystems is to direct these high currents away
from susceptible elements or limit the voltage gradients developed by the high currents to safe levels.
3-1





 


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