TM 5-811-1/AFJMAN 32-1080
of high-resistance grounded systems must be justi-
nection at the transformer station provides a
fied on the basis of a paramount necessity for
second path of fault current which is not sensed.
service continuity.
(b) Multiple source electric systems. Errone-
(b) Generator neutral grounding. Generating
ous ground fault response can occur in multiple-
units will be provided with reactor grounding only
source, three-phase, four-wire distribution systems.
when solid grounding would cause ground fault
The common neutral conductors have multiple
current to exceed the short-circuit current for
ground points providing alternate paths for fault
which the unit is braced and when harmonic
currents, which if not properly monitored, can
current circulation needs to be minimized.
cause nuisance tripping or failure to trip. For such
(2) Equipment grounding. Intentional equip-
systems, a detailed analysis will be necessary to
ment grounding maintains metallic surfaces at low
ensure ground fault protection that will trip appro-
potentials above ground, thereby decreasing possi-
priate circuit breakers dependent upon the fault
bility of electric shocks. System grounds and
location.
equipment grounds will be interconnected in ac-
9-3. Ground Electrodes.
cordance with ANSI C2 and the NEC. Some state
safety orders do not permit grounding of enclosure
The most elaborate grounding system that can be
cases supported on wood poles, when accidental
designed may prove ineffective unless the connec-
contact with bare aerial lines might occur.
tion of the system to earth is adequate and has a
g. Ground fault relaying. Two types of ground
sufficiently low resistance. Since the desired resis-
fault relays are in general use. Ground overcur-
tance varies inversely with the fault current to
rent relays are used on medium-and high-voltage
ground, the larger the fault current the lower the
systems, and the less expensive ground fault pro-
resistance must be. For main electric supply sta-
tection device is used on low-voltage systems.
tions and plants generating at medium voltages,
Since no current or voltage is present in the
the earth resistance will not exceed one ohm, and
ground conductor under normal system operation,
the ground grid will be designed in accordance
ground relays can be made very sensitive. Ground
with IEEE Std 80. For secondary unit substations
relays can also be set to operate very quickly since
and pad-mounted compartmental transformers
coordination between voltage levels is not a con-
larger than 500 kVA, earth resistance will not
straint. Their use permits isolation of faulty equip-
exceed five ohms, and the grounding system will
ment before short-circuits can cause damage.
be designed in accordance with IEEE Std 142 and
(1) Medium- and high-voltage sytems. The
IEEE Std 80. For electrical installations other
ground fault relay used will have the same time
than those listed above, the NEC requirement of
overcurrent characteristics as the overcurrent re-
25 ohms maximum is acceptable.
a. Resistivity of the soil. The resistivity of the
lays used for phase protection. The ground fault
relay is interposed in the residual connection
earth varies dependent upon its composition, as
between the current transformers in each of the
indicated in table 9-2. More moisture in the soil or
three phases and senses the fault current of a
a higher soil temperature decreases soil resistivity.
grounded wye connection. For further information
The methods of providing earth connections given
in this manual will be used to provide the required
on ground fault relaying see IEEE 141, IEEE 242
and TM 5-811-14.
resistance to ground, except when the installation
(2) Low-voltage sytems. Where low-voltage
or the local utility indicates that special tech-
niques are necessary. In that case, local practice
ground fault protection is required by the NEC,
protection will be installed as a part of the
will be followed.
b. Elements of the system. Ground cables will be
switchboard, but some instances make installation
copper. Other metals or metal combinations will
advisable at the exterior transformer station. Al-
be used only in those cases where the mechanical
though overcurrent relays can be used to meet the
strength of copper is inadequate. Driven copper-
NEC requirements, the less expensive ground
clad steel ground rods will be specified as ground
fault protection device is satisfactory.
(a) Single electric source systems. Ground
electrodes since such rods have a higher conductiv-
ity than most other types. Where low soil resistivi-
fault protectors will utilize sensors of the vectorial
ties are encountered and galvanic corrosion may
summation type which either requires one sensor
occur between adjacent ferrous metallic masses
for each phase and the neutral (residual sensing)
or one window type sensor around all three phases
and the copper clad rods, consider the use of zinc
coated steel or stainless steel rods. Stainless rods
and the neutral (zero sequence sensing). Use of a
have a much longer life than zinc coated steel rods,
single sensor on the main bonding jumper is not
especially in soils with high lime content, but
acceptable, because the additional grounding con-
9-5
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