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 MIL-HDBK-419A
Short-Circuit Current Induced at the End of a Semi-Infinite
Figure 10-4.
Above-Ground Wire by an Exponential Pulse
10.3.1.2 Long Buried Lines.
As noted in 10.3.1.1, imperfectly conducting soil does not completely reflect the incident field; some of the
incident wave is transmitted into the soil. This field in the soil can induce current in underground cables, pipes,
and other conductors. However, because the velocity of propagation of a wave is much less in soil than in air,
the bow-wave effect is almost negligible on buried conductors. Furthermore, the attenuation on buried
conductors is greater than on overhead lines because of the proximity of the lossy soil to the buried conductor.
For conductors in contact with the soil (i.e., buried bare conductor), the current at any observation point is
determined primarily by coupling within one skin-depth of the observation point. Current induced at points
farther away is so strongly attenuated by the soil that it adds little to the total current at the observation
point.
The current induced in a long buried cable is shown in Figure 10-5 for various depths of burial, as given by a
where d is the depth of burial. The current is normalized to the inductance per unit
depth parameter
length (L) of the cable, the magnitude of the incident exponential pulse (Eo), the decay time-constant
the
soil tine-constant
and a directivity function (D). The induced current is proportional to the incident
field strength
the 3/2 power of the decay time-constant
and the inverse square root of the soil
For a horizontally polarized, vertically incident pulse having
= 50 kV/m and
= 250 ns, a
S/m will have about 2.8 kA induced in it.
long cable buried near the surface (d = 0) in soil of conductivity
10-7
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