10.2.4 Comparison with Lightning.
Lightning and the EMP are often compared because they are both large electromagnetic phenomena and
because more people have experienced lightning in some form. Though they are generated by different
mechanisms, some aspects of their effects on systems are similar. Both can produce large electrical transients
in systems. Both interact with power lines and communication cables to excite systems served by these cables.
However, lightning and HEMP have important differences in their electromagnetic properties and in the way
they interact with systems. Lightning can deliver greater energy to a moderate impedance load, such as a
power transmission line, than can the HEMP. On the other hand, the HEMP has a larger rate of change of field
and induced currents and voltages than lightning, so that coupling phenomena that depend on dE/dt and dB/dt
(where E and B are the electric field intensity and magnetic flux density, respectively) are more important for
the HEMP excitation than they are for lightning. Because the HEMP appears to be a plane wave at the Earth's
surface, its interaction with long insulated conductors, such as overhead lines, can include a "bow wave" effect
in which the inducing wave propagates along the line synchronously with the induced current wave, building up
very large induced currents. The field produced by lightning decreases as l/r with distance, r, from the source,
so that the bow-wave effect is much less prominent for lightning than it is for HEMP.
Perhaps the most important difference between lightning and the HEMP is their area of coverage. Lightning
strikes one point in a large system such as a continental communication network, while the HEMP excites the
entire network almost simultaneously. Large networks have been designed to cope with single-point outages,
such as those that may occasionally occur because of lightning. We have no experience to assist us in
determining the effect of a large number of simultaneous outages that might accompany HEMP, and it is
virtually impossible to test hypotheses of system reactions with network-scale experiments. Furthermore, the
system is not exposed to the HEMP during peacetime; we get no feedback from a "protected" system on the
effectiveness of the protection. Thus, protecting large networks from the HEMP usually involves conservative
protection of individual parts of the network in the hope that network hardness will follow from component
10.3 HEMP INTERACTION WITH SYSTEMS. HEMP interaction with systems may be separated into long-line
effects and local effects. Long-line effects are the currents and voltages induced on long power lines,
communication cable links, or even other conductors, such as pipelines. Some of these HEMP effects may be
induced far away and guided to the facility along the conductor. Local effects are the currents and voltages
induced directly on the facility shield, building structure, wiring, equipment cabinets, etc. These local effects
are very difficult to evaluate analytically because of the complexity of the facility structure, the lack of
information on the broadband electrical properties of many of the structural materials, and the extremely large
number of interaction paths, facility states, and other complicating factors (10-2), (10-3). On the other hand,
the local interactions can be evaluated experimentally with simulated HEMP fields that envelop the facility.
The full length of the long lines connected to a facility can rarely be illuminated with simulated HEMP fields;
the HEMP interaction with the long lines must usually be estimated analytically and simulated as an external
excitation driving at the long-line port.