Table l-30. Soil Resistivity (ohm-m)
Type of Soil
5 x 104
5 x 105
3 x 104
Sandy, dry, flat, typical
2.5 x 103
7 x 103
6 x 102
Fills, ashes, cinders, brine,
4 x 103
3 x 102
2 x 104
Clay, shale, gumbo, loam
1.5 x 104
Same as above with varying
proportion of sand and gravel
5 x 104
Gravel sandstones with little
clay, loam, or granite
18.104.22.168.2.2 Single-Phase Power Distribution System. Commercially-supplied ac power is single-phase,
110/220V. Power neutral of these systems is first grounded at the transformer secondary and also at the first
service disconnect. Figure l-98 illustrates the ground connections of a single-phase power system.
22.214.171.124.2.3 DC Power System (2-Wire). A 2-wire dc power generator is grounded by connecting either the
positive or negative conductor to ground at one point only, preferably at the source. The neutral (or grounded)
conductor should not be grounded at the Power Entry Panel (PEP), Figure l-99 shows the ground connections
for the 28 V dc power system. The 3-wire dc power system requires that the neutral wire (white) or grounded
conductor be connected to the earth ground at the source (generator or transformer) only.
126.96.36.199.3 Lightning/EMP Protection Subsystem.
Low resistance earth grounds are important for
lightning/EMP ground subsystems. For these subsystems, low resistances to earth are necessary to reduce the
possibility of arcing generated by potential differences between the earth and nearby equipments or shelters
and to reduce step potentials and voltage gradients in the vicinity where a lightning discharge enters the earth.
See Sections 188.8.131.52 and 184.108.40.206 of Volume I for additional information. The need for lightning protection in a
tactical environment is determined by the frequency and intensity of lightning activity in the area and by the
type of structures needing protection. The lightning protection subsystem down conductor should be connected
to the earth electrode subsystem at a point removed from the signal reference and fault protection subsystem.
Two general ways of protecting against lightning damage are air terminals and surge protection devices.
220.127.116.11.3.1 Air Terminals. To protect a shelter from damage caused by a lightning stroke, an air terminal of
adequate mechanical strength, length, and electrical conductivity to withstand the stroke must be provided to
intercept the discharge before it penetrates the structure. A low-impedance path (cable) must be established
between the air terminal and earth electrode subsystem. The resistance of the earth electrode subsystem
should be less than 10 ohms. Detailed construction of air terminal systems are given in Section 18.104.22.168.
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