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.
Buried Horizontal Conductors.
Where bedrock is near the surface of
the earth, the use of driven rods is unpractical. 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.
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 or soil conditions prohibit installation of
other ground systems.
Antenna counterpoise systems shall be
installed in accordance with guidance requirements of the
manufacturer.
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.
Metal Frameworks of Buildings.
The metal frameworks of buildings may
exhibit less than 10 ohms, depending upon the size of the building,
the type of footing, and particular location. Buildings that rest on
steel pilings in particular may exhibit connection to earth.
For this low resistance to be used
advantageously, it is necessary framework be bonded together. a
resistance to earth of the type of subsoil at a a very low resistance
that all elements of the framework bonded together.
Water Pipes.
Metal underground pipes have
traditionally been relied upon for grounding electrodes. The
resistance to earth provided by piping systems is usually quite low
because of the extensive contact made with soil.
Municipal water systems in particular
establish contact with the soil over wide areas. For water pipes to
be effective, any possible discontinuities must be bridged with
bonding jumpers. The NEC requires that any water metering equipment
and service unions be bypassed with a jumper not less than that
required for the grounding connector.
However, stray or fault currents
flowing through the piping network into the earth can present a
hazard to workmen making repairs or modifications to the water
system. For example, if the pipes supplying a building are
disconnected from the utility system for any reason, that portion
connected to the building can rise to a hazardous voltage level
relative to the rest of the piping system and possibly with respect
to the earth.
In particular, if the resistance that
is in contact with the soil near the building happens to be high, a
break in the pipe at even some distance from the building may pose a
hazardous condition to unsuspecting workmen.
Some water utilities are inserting
non-conductive couplings in the water mains at the point of entrance
to buildings to prevent such possibilities.
For these reasons, the water system
should not be relied upon as a safe and dependable earth electrode
for a facility and should be supplemented with at least one other
ground system.
Incidental Metals.
There may be a number of incidental,
buried , metallic objects in the vicinity of the earth electrode
subsystem. These objects should be connected to the system to reduce
the danger of potential differences during lightning or power fault
conditions: their connection will also reduce the resistance to earth
of the earth electrode subsystem. Such additions to the earth
electrode subsystem should include the rebar in concrete footings,
buried tanks, and piping.
Well Casings.
Well casing can offer a low resistance
contact with the earth. In some areas, steel pipe used for casing in
wells can be used as a ground electrode. Where wells are located on
or near a site, the resistance to earth of the casing should be
measured and, if below 10 ohms, the well casing can be considered for
use as a ground electrode.
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