Custom Search
|
|
|
||
 MIL-HDBK-419A
2.5 TYPES OF EARTH ELECTRODE SUBSYSTEMS.
2.5.1 General. Earth electrode subsystems can be divided into two general types, the most preferable being a
ring ground with lo-foot (3-meter) minimum length ground rods every 15 feet (4.5 meters). A second and less
preferable type consists of a system of radials or grounds used when soil is rocky or has extremely high
resistivity. At sites where soil resistivity varies from high to very high and frequent electrical storms are
common, a combination of the two is recommended, i.e., a ring ground around the building (worst case-grid
under building) extending 2 to 6 feet (0.6 to 1.8 meters) outside the drip line with radials or horizontal
conductors extending to 125 feet (37.5 meters). With either system, resistance to earth and danger of arc over
can be greatly reduced by bonding any large metal objects in the immediate area to the earth electrode
subsystem. These include metal pipes, fuel tanks, grounded metal fences, and well casings.
2.5.2 Ground Rods. Vertically driven ground rods or pipes are the most common type of made electrode.
Rods or pipes are generally used where bedrock is beyond a depth of 3 meters (10 feet). Ground rods are
commercially manufactured in 1.27, 1.59, 1.90 and 2.54 cm (1/2, 5/8, 3/4 and 1 inch) diameters and in lengths
from 1.5 to 12 meters (5 to 40 feet). For most applications, ground rods of 1.90 cm (3/4 inch) diameter, and
length of 3.0 meters (10 feet), are used. Copper-clad steel ground rods are required because the steel core
provides the strength to withstand the driving force and the copper provides corrosion protection and is
compatible with copper or copper-clad interconnecting cables.
2.5.3 Buried Horizontal Conductors. Where bedrock is near the surface of the earth, the use of driven rods is
impractical. In such cases, horizontal strips of metal, solid wires, or stranded cables buried 0.48 to 0.86 meters
(18 to 36 inches) deep may be used effectively. With long strips, reactance increases as a factor of the length
with a consequent increase in impedance. A low impedance is desirable for minimizing lightning surge voltages.
Therefore, several wires, strips, or cables arranged in a star pattern, with the facility at the center, is
preferable to one long length of conductor.
2.5.4 Grids. Grid systems, consisting of copper cables buried about 15.24 cm (6 inches) in the ground and
forming a network of squares, are used to provide equipotential areas throughout the facility area. Such a
system usually extends over the entire area. The spacing of the conductors, subject to variation according to
requirements of the installation, may normally be 0.6 to 1.2 meters (2 to 4 feet) between cables. The cables
must be bonded together at each crossover.
Grids are generally required only in antenna farms or substation yards and other areas where very high fault
currents are likely to flow into the earth and hazardous step potentials may exist (see Section 2.8.1.2.3) or soil
conditions prohibit installation of other ground systems. Antenna counterpoise systems shall be installed in
accordance with guidance requirements of the manufacturer.
2.5.5 Plates. Rectangular or circular plate electrodes should present a minimum of 0.09 square meters (2
square feet) of surface contact with the soil. Iron or steel plates should be at least 0.64 cm (1/4 inch) thick and
nonferrous metals should be at least 0.15 cm (0.06 inches) thick. A burial depth of 1.5 to 2.4 meters (5 to 8
feet) below grade should be maintained. This system is considered very expensive for the value produced and
generally not recommended.
2-15
|
 
|
|
|
||
