Quantcast Figure 10-4. Field setup and curve for fall-of-potential method

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
Station f e n c e p e r i m e t e r
Fixed C u r r e n t
Probe C
P r o b e :j
--ANSI/IEEE 8 1 Distances
Figure 10-4. Field setup and curve for fall-of-potential method
obstructions to deep driving. There are two practi-
grounding system that uses a 2-inch (50-millime-
cal ways of accomplishing this as shown in figure
ter) copper tube filled with metallic salts and avail-
10-5. Where space is limited, a length of tile pipe is
able in various lengths. Since this method uses me-
sunk into the ground a few inches (millimeters)
tallic salts it is not recommended except as a last
from the ground rod and filled to within approxi-
resort. The tube is also available as a straight unit,
mately 1 foot (0.3 meters) of the ground level with
or in an L-shaped configuration which allows the
the treating chemical. The second method is appli-
tube to be installed on its side in a shallow trench.
cable where a circular or semicircular trench can be
Changes in atmospheric pressure "pump" air
dug around the ground rod to hold the chemical.
through the breather holes at the top of the tube.
The chemical must be kept several inches (millime-
Moisture in the air condenses inside the tube to
ters) away from direct contact with the ground rod
move slowly down through the bed of metallic salts,
to avoid corrosion of the rod. The first treatment
providing a self-maintaining low-resistance system
usually requires 50 to 100 pounds (22 to 45 kilo-
with a much greater life expectancy than conven-
grams) of material and will retain its effectiveness
tional ground rods.
for 2 to 3 years. Each replenishment of the chemical
e. Combination methods. A combination of meth-
extends its effectiveness for a longer period, thus
ods may be advantageous and necessary to provide
increasing treatment intervals. To start the action
the desired ground resistance. Adding specialized
promptly, the first treatment of chemical should be
rods or a combination of multiple rods and soil
d. Specialized rods. In lieu of adding additional
treatment may be effective. Multiple of longer rods
rods or lengthening rods, a copper tubing grounding
are effective where conditions permit this type of
system can be used. There is an Underwriters-listed


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