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 MIL-HDBK-419A
Another protective measure is to reduce the flux density within the pickup loop by providing magnetic shielding.
Because the coupling field is primarily magnetic in nature, a shielding material having a high permeability such
as iron or nickel should be used. For shielding against lightning-produced fields, steel conduit or cast iron pipe
are much more effective than aluminum or other non-ferrous materials.
3.6.3.3 Capacitively-Coupled Voltage.
Prior to the lightning discharge, an electric charge slowly accumulates on earth-based objects in the vicinity of
the electrified clouds. This increase in charge occurs slowly enough so that the potential of grounded
conductors does not change appreciably with respect to the earth, even when the impedance to ground is high.
When the lightning stroke terminates on a structure or other point having contact with the earth as illustrated
in Figure 3-11, the charge on all grounded objects nearby suddenly becomes redistributed. The redistribution of
charge produces a current flow through the grounding impedance of the grounded objects and produces a voltage
across that impedance.
Referring to Figure 3-11, the voltage between the conducting objects and the ground can be expressed as
(3-11)
where Q is the stored charge in coulombs, C is the total capacitance to ground in farads, R is the effective
resistance to ground in ohms, and t is the elapsed time in seconds from the occurrence of the stroke.
Equation 3-11 shows that if the product RC is small, the exponential term will be large (for a time t on the
order of 10
thus making the voltage capacitively induced on any reasonably well-grounded object quite
small for a typical lightning stroke.
3.6.3.4 Earth Resistance.
Consider a facility such as the one illustrated in Figure 3-12, that has more than one possible electrical path to
earth. For example, a ground rod is driven into the earth at the transformer pole or at the service entrance to
Building 1. The resistance,
of this rod could be 25 ohms or higher and still conform to NEC requirements.
Metal utility pipes such as water lines generally offer a relatively low resistance (labeled
to earth. (In
soils of high resistivity the point of effective contact between utility pipes may be an appreciable distance from
the facility.) Empirical data indicates that the grounding resistance offered by water pipes is on the order of 1
to 3 ohms. If the electrical ground is not connected to the water pipe, a lightning strike to the ground wire of
the electrical distribution system could produce a potential difference high enough to possibly produce an arc
between the electrical ground (including the equipment cabinet and the building's structure, if connected) and
the utility piping. A definite personnel hazard would then exist because of the high voltage that would be
developed between the equipment and building ground and pipes. Because of this reason as well as the
requirement to prevent analogous hazards from existing during power system faults, MIL-STD-188-124A
requires electrical safety grounds be connected to the metallic water system in the building and recommends
they also be directly connected to the ground rod at the transformer.
3-21
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