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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
CHAPTER 5
UNDERGROUND AND SUBMARINE CABLES
Section 1 - ASSOCIATED GUIDANCE
5-1. Relevant cable guidance.
the United States, permission must be obtained
from the nearest District Engineer of the U. S.
Maintenance work involving underground or sub-
Army Corps of Engineers, who will specify depth
marine cable changes requires an understanding of
requirements and any other pertinent conditions.
the basic design premises of such cables.
When crossings are made in waterways under the
a. Types of installations. Underground cables
jurisdiction of other authorities, those authorities
may be installed in conduit, in duct banks, or by
should also be consulted.
direct burial in the earth; submarine cables are
b. Identification requirements. Because under-
usually submerged directly in the water and lie on
ground and submarine cables cannot be visually
the bed of the waterway. The terminal ends of both
traced between structure access points, it is impor-
underground and submarine cables are often above-
tant that they be marked at all points at which they
ground. The burial depth of raceways or cables
are accessible. Any such cables will be identified by
should never be less than the depths permitted by
plastic or corrosion-resistant tags wherever they
the NEC or the NESC and, in most cases, will be
can be worked on and wherever they can possibly be
more to conform to facility design practice.
mistaken for another cable. Identification tags will
(1) Cable in conduit removal freplacement. Al-
be located at terminations and at least in every
though it is easy enough to install several cables in
structure. If tags become missing or illegible, they
one conduit and mechanically easy to withdraw
will be replaced as part of the maintenance pro-
them, the removal usually ruins the cable. Cables
gram.
become impacted in a conduit, and, when one is
c. Cable impacts. The major cause of electric fail-
drawn out, the sheath may be stripped either from
ure is the breakdown of insulation. Even under nor-
the withdrawn cable or from one of the other cables.
mal conditions, an electrical cable experiences
Therefore, when one cable of a set in a conduit fails,
stress that will gradually weaken it, leading to fail-
all cables must be replaced.
ure. Cable tests provide data which permits the
(2) Direct-burial cable reinstallation. Direct-
anticipation of cable failures. An understanding of
burial cables being replaced must be installed below
items which can accelerate insulation deterioration
the frost line.
is of help in determining inspection and testing in-
b. Joint electric supply and communication cir-
tervals.
cuits. Unlike aerial lines, joint structure use is not
(1) Cable Loading. The current-capacities or al-
allowed for electric supply and communication cir-
lowable loading of underground cables is based on
cuits. Communication cables are installed to be
the conductor size, material, and assumed ambient
completely isolated from electric power cables and
temperatures. Complex calculations are required to
require separate ducts and structures. Economy
take all these effects into account. IEEE S-135-1
may dictate contiguous structures and duct lines
and IEEE S-135-2 are used as the basis for ampaci-
having a common trench excavation. Direct-burial
ties given by the NEC. The factors used by the NEC
power and communication lines should be separated
represent a theoretical average value and may be
at least the minimum required distance, usually set
considered to be safe factors, especially if the load-
by the local communication agency. Control, alarm
ing is based on a loo-percent load factor. The load
signalling, and other low-current and low-voltage
factor for primary circuits on most facilities will
circuits may be installed in electric manholes, de-
probably range from 45 to 65 percent at the time of
pendent upon facility requirements, but require
initial design. Voltage drop, especially at lower volt-
special shielding or increased insulation levels.
ages, may also have been a factor in determining
5-2. General construction guidance.
the cable sizing. The actual temperature conditions
Rights-of-way for navigable waters and identifica-
affecting the cable become an important consider-
tion must meet the following requirements. The in-
ation. Added loads and variable loads affect cable
fluence of conditions which can generate cable fail-
temperatures both directly and indirectly.
ures in the following discussion should be checked
(a) Directly. In general, the higher the tem-
for their impacts.
perature, the faster the rate of deterioration in the
a. Rights-of-way requirements. When the system
physical properties of the insulation, including the
is being extended across navigable waters within
formation of voids in solid-type or paper-insulated
5-l
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