Quantcast Chapter 10 Grounding

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
10-1. Basic principles of grounding.
(1) Water tab1 es are gradually falling in many
Grounding is provided to limit potential (voltage)
(2) There are more underground installations
differences to values that will not cause undue haz-
of nonmetallic pipes and conduits, which do not
ards to personnel and equipment. A ground system
which provides adequate current-carrying capacity
provide low-resistance ground connections.
and a low-resistance path to an earthing connection
(3) Electric systems are continually expanding
will dissipate, isolate, or disconnect overpotential
with an associated fault current increase which may
areas resulting from fault overcurrents or surge
require a decrease in grounding resistance.
overvoltages. A ground path can consist of single or
(4) Corroded connections may increase the re-
multiple conductors whose connection provides ad-
equate thermal and conductance capacities. The
earthing connection is generally a metallic electrode
1O-2. Grounding provisions.
such as a rod, a water pipe, a counterpoise, or a
Maintenance personnel deal with two types of
ground grid system installed below grade.
grounding systems: permanent and temporary.
a. Electrode resistance. The resistance of a
ground electrode is primarily determined by the
a. Permanent grounding systems. Permanent
earth surrounding the electrode. Test data given in
grounding is provided for the efficient, effective,
IEEE 142 indicate that about 90 percent of the total
safe operation of electrical power systems.
resistance of a ground lies within 6 to 10 feet (1.8 to
(1) Safety. Equipment grounding, which is the
3 meters) from the electrode. The diameter of the
grounding of all exposed or accessible noncurrent-
rod has only a negligible effect on the resistance of a
carrying parts of electrical devices and equipment,
ground. The resistance of the soil is dependent upon
reduces the hazards of contact by personnel.
the type of soil and its moisture content. Electrodes
(2) System operation. System grounding, which
should be long enough to penetrate a relatively per-
is the grounding of one conductor point on an elec-
manent moisture level and should extend well be-
trical circuit, stabilizes the voltages to protect the
low the frost line.
equipment and provides a basis for adequate protec-
b. Factors which can degrade initial good
tive relaying.
grounds. Tests should always be made at times
b. Temporary grounding. Temporary grounding is
when the surrounding soil can be expected to have
the least moisture. The following factors indicate
the personal protective grounding, which is pro-
the importance of continuous periodic testing of
vided to protect persons engaged in de-energized
electric line maintenance.
grounding systems.
1O-3. Grounding maintenance safety.
10-4. Visual inspection of grounds.
Extreme care must be exercised in inspecting,
Visual inspection of ground connections to equip-
maintaining and testing grounds and ground sys-
ment, equipment enclosures, structural members,
terns. Never open a grounding connection unless the
fencing, and system neutrals should be made at
least every 2 years. More frequent inspections
connected equipment is deenergized, or an adequate
safety bypass is provided. Always wear rubber
should be made where appropriate to the system's
gloves and follow facility safety manual procedures.
size and importance. Loose, broken, or missing con-
This applies equally to grounds installed on struc-
nections should be repaired or replaced as required.
Connections or connectors showing signs of over-
tural or supporting members, ground connections to
equipment enclosures, and neutral grounds of pri-
heating, as evidenced by discolorations, should be
mary or secondary systems. The life and safety of
reported, as this may be the result of an improper
those in the vicinity of electrical facilities depend on
application or installation. If connections are found
how carefully and completely inspections and main-
to be corroded or rusted, they should be cleaned and
tenance of grounds and grounding systems are per-
corrective measures should be taken to prevent a
recurrence of this situation. Excessive amounts of


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