Quantcast Principles of Protection

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MIL-HDBK-419A
1.3 LIGHTNING PROTECTION FOR STRUCTURES.
1.3.1 Principles of Protection.
A structure, for lightning protection purposes, is defined as a building mast, tower, or similar self-supporting
object other than power lines, power stations, and substations. To provide minimum protection for structures
against direct lightning strikes, four requirements must be fulfilled:
an air terminal must be provided to intentionally attract the leader stroke,
a.
a path must be established that connects this terminal to earth with such a low impedance that the
b.
discharge follows it in preference to any other,
a low resistance connection must be made with the earth electrode subsystem, and
c.
a low impedance interface must be established between the earth electrode subsystem and earth.
d.
These conditions are met when a lightning discharge is permitted to enter or leave the earth while passing
through only conducting parts of a structure. The conditions can be satisfied by one of two methods, each
having specific applications. These methods are:
the installation of an integral protection system consisting of air terminals interconnected with roof
a.
and down conductors to form the shortest practicable distance to ground, or
b.
the installation of a separately mounted protection system of one of two types:
(1)  a mast type consisting of a metal pole which acts as both air terminal and down conductor (a
nonconductive pole may be used if provided with metal air terminals and down conductors connected to an earth
ground), or
(2)  two or more poles supporting overhead guard wires connected to an earth electrode subsystem
with down leads.
1.3.2 Integral Protection System. When designing and installing an integral system of protection, perform the
following steps:
Erect air terminals on the points of highest elevation and on other exposed areas to intercept the
a.
stroke before it has an opportunity to damage the structure or equipments or components mounted thereon.
The terminal points must be placed high enough above the structure to eliminate the danger of fire from the
arc.
Install roof and down conductors so that they offer the least possible impedance to the passage of
b.
stroke currents between the air terminals and the earth. The most direct path is the best. The radius of
conductor bends shall not be less than 8 inches nor shall the angle of such bends be less than 90 degrees.
Additional information may be found in para 3-12.5 of NFPA 78.
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