Quantcast Section IX -Overhead Open Wire Conductors

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
4-32. Overhead conductor construction.
nealed or soft-drawn copper wire where it is neces-
sary to bend and shape the conductor, such as for
Overhead electrical distribution at all voltages most
ground wires. Medium-hard-drawn copper is used
often uses open wire construction, although aerial
for distribution, especially where wire sizes smaller
cables of various types are employed to some extent.
than No. 2 AWG are needed.
a. Open wire construction. The basic features of
b. Aluminum. An aluminum conductor has about
open wire construction are single conductors, insu-
61 percent of the conductivity of copper of the same
lated supports, and wide separation, with little or
cross section but is lighter. Aluminum is relatively
no conductor covering on the conductors. The air
soft and, although low in tensile strength, is very
space around the conductors must be large enough
to allow relative conductor movement without a
durable. Some alloys are available with greater
flashover. Open wire construction is mounted on
strength but less conductivity. Various combinations
insulators, either as armless or crossarm construc-
of steel and aluminum strands are available for use
where both strength and good conductivity are re-
(1) Armless construction. Armless construction
quired. Standard aluminum conductor steel-
consists of insulators on supporting brackets
reinforced (ACSR) conductors should not be used in
mounted directly on the pole. When possible, this
areas of severe corrosion. There are a variety of
construction is preferred for use on pole replace-
special aluminum alloy conductors some with spe-
ments because of its more attractive appearance
cial steel reinforcing, for use under conditions of
and lower maintenance cost. Triangular tangent
corrosion, for greater strength requirements, and
construction is preferred over vertical tangent con-
for self-damping to limit aeolian vibration. When
struction, as it requires the least conductor space
replacing aluminum conductors, check to be sure
and is more economical. The difference between the
the selection meets the requirements of the original
two is shown in figure 4-l. Triangular construction
design. Connectors used will conform to section XI.
is not suitable for configurations which require an
c. Copper-clad steel. High-strength steel may be
overhead ground wire. It is not recommended ex-
covered with copper to yield a conductor having 30
cept for the tangent and minor angle construction
to 40 percent of the conductivity of pure copper. It is
shown in figures 4-l and 4-2.
corrosion resistant and may be stranded in various
(2) Crossarm construction. Unless it conflicts
combinations with copper to give various combina-
with facility practice, crossarm construction should
tions of strength and conductivity. Its chief applica-
be phased out whenever possible, but may be neces-
tion is for use as an overhead ground wire.
sary where equipment or line installations, utilizing
4-34. Overhead conductor covering.
armless construction, would result in excessive pole
heights. Facility practice usually matches the local
No covering is provided on open-wire primary cir-
utility company's open wire construction.
cuit conductors. For open-wire secondary circuit
b. Cables. Cables utilize conductors with covering
conductors, a triple-braid weatherproof covering of
which is sufficient to withstand the voltage at which
impregnated cotton or layers of neoprene or poly-
the line is operating and, therefore, do not need
ethylene covering are provided. This covering is not
insulators. Cables are discussed in section X.
sufficient to withstand the operating voltage and
conductors must be mounted on secondary rack in-
4-33. Overhead conductor material.
sulators. The covering is not to be considered as
Conductors used in open wire construction are usu-
insulation, although when dry it will help prevent
ally copper, aluminum, or combinations of copper
breakdowns at lower voltages if conductors swing
and steel or aluminum and steel. Specially designed
together. The wires should always be treated as
connectors are required for splicing or otherwise
though they are bare. Because of both space re-
connecting conductors of dissimilar metals.
quirements and unattractive appearance, this type
of installation is being phased out in favor of insu-
a. Copper. Copper has high conductivity and is
lated cable.
easily handled. Hard-drawn copper is desirable for
distribution conductors because of its strength. Sol-
4-35. Overhead conductor sag.
dering will anneal copper and reduce a hard-drawn
Sag is the maximum droop of a wire in a given span,
copper wire's tensile strength from 50,000 to 35,000
measured vertically from a straight line between
pounds per square inch (345,000 to 241,000
the' two points of support. The amount of sag de-
kilopascals). Splices and taps, therefore, should be
pends on the characteristics of the conductor, the
made with connectors, clamps, or sleeves suitable
temperature, and the tension. A properly sagged
for copper. Never make soldered splices. Use an-


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