TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
with the seasons, and one earth resistance reading
relay operates within acceptable limits. Tests
alone with not guarantee a safe earth ground.
should be made with the relay in its panel and case,
c. Grounding system. As the grounding electrode
and the time tests run at the calibrated setting. For
is placed further into the earth the ground resis-
precise testing procedures, manufacturer's instruc-
tance decreases (fig 14-7), and there is less resis-
tions should be consulted. Some protective relays
tance change due to temperature and moisture
operate instantaneously; that is, with no intentional
variations. Changing the diameter of the electrode
time delay. They should be set by test. Most types of
has little effect on ground resistance. An electrode
protective relays have a combination target and
(fig 148) is pictured surrounded by hemispheres of
seal-in unit. The target indicates that the relay has
equal thickness and composed of the same type of
operated; the seal-in holds the relay contacts closed.
soil. Each additional hemisphere away from the
It should be verified that the target is functional
electrode increases in area. As the hemisphere's
and that the relay will seal-in with the minimum
area increases, the resistance decreases- In effect,
specified DC current applied to the seal-in unit.
the earth resistance is the sum of all the hemis-
phere resistances. A point will be reached where
14-4. Equipment ground resistance testing.
the addition of new hemispheres will not effectively
An equipment ground is a connection to ground
change the total resistance. This will be the value of
from one or more noncurrent-carrying m e t a l p a r t s
the earth resistance.
of the equipment (para 8-2b). Instrument are avail-
(1) Precautions. All earth resistance testing
able to determine if the grounding path is continu-
methods can involve hazards to the operator. Pre-
ous and has sufficiently low resistance. When using
cautions should be taken as follows:
these instruments, one should remember that al-
though a high resistance value is an indication of a
Electrode to earth
problem, for example a loose connection or excessive
conductor length, a low resistance reading does not
necessarily indicate the adequacy of the grounding
path. A grounding path that is found to have a low
resistance may not have sufficient capacity to
handle large ground faults. Visual examinations
and torquing connections are still needed to deter-
mine that adequacy of the grounding path.
14-5. System ground resistance testing.
A system ground is a connection to ground from one
Figure 14-4. Resistive components of a made electrode.
of the current-carrying conductors (para 8-2c). An
adequately grounded system is necessary to provide
for ground fault protections and to reduce the haz-
ards of fire and shock to personnel. A system ground
or earth resistance test has been developed to deter-
mine the effectiveness and integrity of the grounded
system. Periodic testing is recommended based
upon the importance of the ground system. The
current flowing through an earth electrode encoun-
ters three basic resistive components: electrode;
electrode-to-earth; and earth (fig 144). The earth
resistance is the largest of the three resistance com-
ponents. The earth resistance depends on the fol-
a. Type of soil. As the soils composition varies so
does the corresponding resistance values. Also as
the soil becomes more closely packed, the resistance
b. Moisture and temperature of soil. When a soil
dries out, or its temperature is lowered, the soils
Percent moisture in soil
resistance value increases (figs 14-5 and 14-6).
Therefore, resistance values measured will vary
Figure 14-5. Soil resistivity vs. moisture content of red clay soil.