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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
important at these points. Where recabling is re-
cable. The deterioration usually results in increased
quired do not use "T" splices in manholes, except
dielectric losses and decreased dielectric strength.
Large variations in daily temperatures accelerate
where the facility's engineering staff concur that
the possibilities of cable sheaths cracking and
avoiding their use is uneconomical.
e. Lightning protection and grounding. Lightning
bolted or clamped connections loosening. A, short-
protection for aerial to underground primary cable
time large overload, and accompanying high tem-
connections, and grounding and bonding of under-
perature, can produce aging of insulation equiva-
ground cables, contribute to the protection of the
lent to operation for a longer time at a smaller
cables and to the safety of the system.
overload. Since power surges contribute to cable
(1) Surge arresters. When a transition is made
aging, a cable serving large motors with full-voltage
between overhead conductors and underground or
starters having intermittent loads, or a cable sub-
submarine primary cables, facility practice requires
jected to a higher level of lightning or switching
that a surge arrester be installed at the termination
surges, will probably have a shorter life than an
connecting insulated underground cables to aerial
identical cable with a constant load and infrequent
bare conductors. A ground rod should be installed
low-level switching surges.
(b) Indirectly. The temperature of the soil ad-
and the metallic sheath or armor of the cable
bonded to that ground installation. The surge ar-
jacent to a buried cable or conduit system must also
rester then protects the primary cable from switch-
be considered as affecting cable life. If cable tem-
ing or lightning surge overvoltages which could
peratures become high enough, the moisture in the
overstress the cable insulation. Secondary cables
soil will migrate away from the cable causing a
are usually protected from these over-voltages by
considerable increase in the soil thermal resistivity.
primary surge arresters located at pole or ground-
This may lead to thermal instability of the soil and
mounted transformer installations.
further increase its thermal resistivity which, in
(2) Grounding and bonding. All noncurrent-
turn, may cause excessive cable temperatures and,
carrying conductive materials in the structure and
perhaps, even cable failure.
(2) Cable insulation failure. Underground pri-
any neutrals must be grounded. Most standard
structures are provided with a driven ground rod.
mary distribution cables with solid-dielectric insu-
Bonding includes the metallic sheath or armor of all
lation have experienced a high rate of electrical
---
cables, cable shields, manhole hardware, the tanks
failure after several years of operation as the result
of all equipment and apparatus, and the secondary
of carbonized paths (electrochemical tree design
neutral of transformer installations. Where
markings) usually caused by the presence of water
nonmetallic-sheathed cable having a ground wire is
in the conductor.
d. Termination and splicing impacts. Termina-
used, the ground wire is usually brought out at the
joint. These ground wires should be grounded to the
tions and splices are usually the weakest point in a
neutral and the driven ground The resistance of
cable system, and the cable system is usually the
weakest link in an electrical system. Therefore, in-
ground connections must meet the requirements
given in chapter 10, section III.
spection, including riser pole inspections, is doubly
Section II - SAFETY PRECAUTIONS
5-3. Cable safety.
5-4. Structure safety.
The compact spacing of conductors and nearness to
Subsurface structures such as manholes, hand-
any grounded sheaths is the reason that working on
holes, equipment vaults, and splicing boxes are sub-
energized conductors even at low voltages is prohib-
ject to accumulation of dangerous gases that may be
combustible and/or explosive, toxic, or deficient in
ited. A voltage detection tester should be used to
oxygen. Before entering any manhole or vault, it
ensure that the cable is not energized. Materials
must be checked for these conditions.
such as a lead sheath, which will act as a shield,
a. Combustible gases. Combustible gases may be
must not be between the tester and the conductors
detected by means of a test instrument or safety
of the circuit being tested. To prevent a de-energized
lamp. When using this equipment, the precautions
circuit from being energized while it is being
and instructions provided by the manufacturer
worked on, good safety practice requires that the
should be followed. If it is determined that combus-
disconnecting means at each end be tagged and
tible gases are present, it will be necessary to ven-
locked in the open position, and ground clamps ap-
tilate the manhole or vault before any work is done.
plied.
5-2
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