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 MIL-HDBK-419A
1.2 EARTH ELECTRODE SUBSYSTEM. The earth electrode subsystem establishes the electrical connection
between the facility and earth. This connection is necessary for lightning protection, useful in power fault
protection, and aids in the minimization of noise. The system should be tailored to reflect the characteristics
of the site and the requirements of the facility.* It must be properly installed and steps must be taken to
assure that it continues to provide a low resistance connection throughout the life of the structure. To achieve
these objectives, first determine the electrical and physical properties of the site, design an earth electrode
subsystem appropriate for the site, install the subsystem in accordance with the recommended procedures, and
finally, measure the earth resistance of the subsystem to verify that it meets the recommended goals or design
specifications.
1.2.1 Determination of Site Parameters (Site Survey). Before beginning the design, conduct a survey of the
site where the earth electrode subsystem is to be installed. Through this survey, determine the resistivity of
the soil, identify significant geological features, gather information on architectural and landscape features
which may influence the design of the subsystem, and review local climate effects. (If possible, conduct this
survey in advance of the final site selection in order to avoid particularly troublesome locations.)
1.2.1.1 Soil Resistivity. As the first step of the site survey, measure the resistivity of the soil at several
points over the area of the planned facility. For even the smallest facility, the effective facility area in so far
as the electrode subsystem is concerned is assumed to be at least 15 meters by 15 meters (50 feet by 50 feet).
For larger facilities, the facility areas are assumed to extend at least 6 meters (20 feet) beyond the basic
building or structural outline, i.e., the ground floor plan, substation grid, tower footing, transformer housing,
etc. It is necessary that the soil resistivity be known over the area encircled or covered by the earth electrode
subsystem.
a..
A single soil resistivity measurement is made using the four-probe method (see Volume I, Section
2.4) in the following manner:
(1) At a location near the center of the site, insert the four short probes supplied with the earth
resistance test set into the soil in a straight line as illustrated in Figure l-l. A convenient probe spacing of 6
to 9 meters (20 to 30 feet) is recommended as a start. If probes are not supplied with the test set or if they
have been lost or misplaced, four metal (steel, copper, or aluminum) rods, 1/4 to 3/8 inch in diameter and 12 to
18 inches in length, may be used. Drill and tap No. 6-32, 8-32, or 10-24 screws, according to rod size and
securely fasten the test set leads to the rock. Clamps may also be used for connecting the leads to the probes.
*The relationship between the performance of an electronic system and the resistance of the earth ground is
unclear. The value of 10 ohms earth electrode resistance recommended in Section 1.2.2.1a represents a
carefully considered compromise between overall fault and lightning protection requirements and the estimated
relative cost of achieving the resistance in typical situations. In locations characterized by high soil
resistivities, to achieve 10 ohms could be very expensive, In such locations, examine all elements of the site,
consider the requirements of the planned facility, and then choose the best compromise based on soil conditions,
relative costs, etc.
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