permanent moisture level and should extend well below
the frost line. Periodic earth resistance measurements
should be made, preferably at times when the soil would
be expected to have the least moisture.
You need to test the ground rod installation to be
sure it meets the requirement for minimum earth
resistance. The earth resistance tester may be used to
perform the test. You should make this test before you
connect the ground cable to the ground rod.
When ground resistances are too high, they may be
reduced by one of the following methods:
1. Using additional ground rods is one of the best
means of reducing the resistance to ground; for
example, the combined resistance of two rods properly
spaced and connected in parallel should be 60 percent of
the resistance of one rod; the combined resistance of
three rods should be 40 percent of that of a single rod.
2. Longer rods are particularly effective where
low-resistance soils are too far below the surface to be
reached with the ordinary length rods. The amount of
improvement from the additional length on the rods
depends on the depth of the low-resistance soils.
Usually, a rather sharp decrease in the resistance
measurements is noticeable when the rod has been
driven to a low-resistance level.
3. Treating the soil around ground rods is a reliable
and effective method for reducing ground resistance
and is particularly suitable for improving high-
The treatment method is
advantageous where long rods are impractical because
of rock strata or other obstructions to deep driving.
There are two practical ways of accomplishing this
result, as shown in figure 3-7. Where space is limited, a
length of tile pipe is sunk in the ground a few inches
from the ground rod (fig. 3-7, view A) and tilled to
within 1 foot or so of the ground level with the treatment
chemical. The best treatment chemicals for all
situations cannot be covered within the scope of this
manual. You may work with your engineering office to
determine the possible corrosive effect on the electrode.
Examples of suitable noncorrosive materials are
magnesium sulfate, copper sulfate, and ordinary rock
salt. The least corrosive is magnesium sulfate, but rock
salt is cheaper and does the job.
The second method is applicable where a circular or
semicircular trench can be dug around the ground rod to
hold the chemical (fig. 3-7, view B). The chemical must
be kept several inches away from coming into direct
contact with the ground rod to avoid corrosion of the
rod. if you wish to start the chemical action promptly,
you should go ahead and flood the treatment material.
The first treatment usually contains 50 to 100 pounds of
material. The chemical will retain its effectiveness for 2
to 3 years. Each replenishment of the chemical extends
the effectiveness for a longer period so that the necessity
for future retreating becomes less and less frequent.
Figure 3-7.Methods of soil treatment for lowering of ground resistance.