TM
5-811-1/AFJMAN
32-1080
Resistance in ohms
Maximum
Average
Minimum
Type of Soil
22
7.6 . . . . . . . . . .
Ashes, cinders, brine waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.9 . . . . . . . . . . .
53
1.1 . . . . . . . . . . . . . . . 1 3 . . . . . . . . . . . . . . . .
Clay, shale, gumbo, loam . . . . . . . . . . . . . . . . . . . . . . . .
433
3.3 . . . . . . . . . 50 . . . . . . . .
Same, but with sand and gravel . . . . . . . . . . . . . . . . . . .
1,460
19.2 . . . . . . . . . 300 . . . . . .
Gravel, sand, stones, little clay or loam . . . . . . . . . . . . . . . . .
a
Based on the resistivity of soils and formulas for calculation of resistance given in IEEE Std 142-1982. Use of a 3/4-inch
diameter rod of the same length decreases resistance less than five percent.
9-4. Grounding Details and Requirements.
stainless steel will corrode in certain soils and the
higher cost of stainless steel must be justified.
Grounding will be provided in accordance with
Stainless steel rods will not be used for Air Force
NESC, NEC, and IEEE Std 80.
installations. Use of pure copper rods must be
a. Main electric supply stations. Because of the
justified because of the high cost and susceptibility
equipment layout within a station, steep voltage
of copper to damage during installation. The de-
gradients might occur if each apparatus "island"
signer will coordinate and standardize grounding
were separately grounded.
materials selection for each facility based on corro-
(1) Ground grid. In order to prevent steep-
sive conditions, grounding materials requirements,
voltage gradients and also to design for maximum
cathodic protection, and lightning protection. Note
voltage excursions at the station, without the use
that zinc coated rods do not conform to require-
of an excessive conductor size, a grid system
ments of NFPA 78. While connection to an exist-
designed in accordance with IEEE Std 80 will be
ing metallic water system provides low ground
installed below grade enveloping the fenced area.
resistance, there is the possibility that water main
Figure 9-2 shows a typical substation grounding
maintenance or other work might result in acci-
grid, Ground wire spacings of approximately 10 to
dental disconnection of grounds and create a haz-
12 feet are commonly used. Exact spacing may be
ardous condition. Therefore, such connections will
slightly more or less to suit station configurations.
only be provided as a secondary backup to made
The perimeter ground wire will be installed not
electrodes.
less than two feet outside the station fence to
( 1 ) Additional electrodes. Whenever the
protect approaching personnel from step-and-touch
ground rod does not provide the required ground
potential exposure.
resistance, either longer or additional ground rods
(2) Special danger points. Equipment operat-
will be necessary. Since ground resistance de-
ing handles are a special danger point because of
creases with an increase in ground rod depth, the
the higher probability for coincidence of adverse
use of longer ground rods is the most economical
factors, namely, the presence of a person contact-
method. However, where rock is encountered, use
ing grounded equipment and performing an opera-
tion that can lead to electrical breakdown. If the
of more rods, a ground mat, or a ground grid may
grounding system is designed conservatively for
be necessary. The space between rods will not be
safe mesh potentials, then the operator is not
less than the length of a rod, and never less than
exposed to unsafe voltages. However, due to the
six feet. The optimum spacing of ground rods is
uncertainty inherent in substation grounding de-
twice the length of the rod. Where the above
sign, a metal grounding platform, connected to the
methods do not result in the required resistance,
operating handle and to the grid in at least two
electrolytic ground rods filled with nonhazardous
places, will be placed so the operator must stand
metallic salts or bentonite slurry grounding wells
on the platform to operate the device (see fig. 9-2).
may be used. Bentonite wells will be designed to
This arrangement will be provided regardless of
meet criteria in IEEE 80.
whether the operating handle is insulated.
(2) Other made electrodes. Butt grounds or
b. Transformer installations.
ground plates may be provided on poles as an
(1) Multigrounded systems. Primary and sec-
economical method of grounding the overhead
ondary grounding conductors will be intercon-
ground wire, but their use is permitted only in
nected as required by the NESC. Spark gaps will
areas where such an installation is local practice.
not be used. Grounding conductors will be sized in
These made electrodes will not be used as the sole
accordance with NEC and NESC standards.
grounding electrode for apparatus or neutral
(a) For aerial transformer installations
grounds, which require a ground rod installation
(multi-grounded, c o m m o n - n e u t r a l systems),
as a minimum.
9-6
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