TM
5-811-1/AFJMAN
32-1080
can be used for any angle, but are a more
6-5. Circuit Configurations.
expensive installation; therefore, their use will be
Preferred and alternate configurations for tangent
required only when the line-post or pin type is
construction are shown on figure 6-2. Also shown
unsuitable. Normal angles for armless configura-
on this figure are methods of mounting overhead
tions are shown on figure 6-3 and are in agree-
ground wires and locations for primary neutrals
ment with REA pole detail drawings. Table 6-6
when required. Insulation shown on the figures in
indicates normal angles for crossarm configura-
this chapter is for a nominal 15 kV class.
tions related to conductor size.
a. Medium-voltage circuit configurations. Arm-
b. Low-voltage circuit configurations. Low-
less and crossarm mounting, shown in figures 6-3
voltage circuits will be supported by clamping the
and 6-4 respectively, are both used for open
bare neutral conductor of a neutral-supported sec-
conductors. REA Bulletin 61-12 provides addi-
ondary aerial cable to a spool/clevis insulator
tional details on armless construction. Triangular
assembly as shown on figure 6-5 or by use of spool
tangent construction requires the least pole space
insulators on secondary racks supporting insulated
and is the most economical. Where such a configu-
phase and neutral conductors. Because of both the
ration is not suitable, because of special require-
space requirements and the unattractive appear-
ments such as the need for an overhead ground
ance, phase conductors supported on secondary
wire, vertical tangent construction will be pro-
racks will be limited to special circumstances.
vided. Requirements for overhead ground wires
6-6. Insulators.
are covered in chapter 9. Crossarm construction as
shown on figure 6-4 will be limited to equipment
The operating performance of aerial lines is depen-
installations, or where use of armless construction
dent upon the quality of the line insulators.
would result in excessive pole heights. However,
Insulators will be of the wet process porcelain
other technical considerations, such as a line with
type; the only presently acceptable alternative in
many taps, crossings, or overhead-to-underground
an appropriate situation is toughened glass, which
transitions, may impact the decision. Also, armless
is an industry standard only for suspension insula-
construction requires bucket trucks for mainte-
tors. Glass is much more susceptible to shattering
nance due to loss of climbing space.
than porcelain; so where vandalism is a problem,
(1) Angles at which guying is required. Guy-
glass will not be permitted. Polymer insulators
ing requirements for other than in-line circuits are
have some advantages such as light weight and
dependent upon the angle of deviation, the size of
resistance to vandalism; once industry standards
the conductor, and the loading district. REA pole
are issued, use of polymer units may be consid-
details show guys for angles of deviation greater
ered. Insulators need to provide ample mechanical
than two degrees on armless construction configu-
strength for the expected ice and wind loads and
rations, regardless of conductor size or loading.
must be capable of withstanding the stresses of
Local practice may permit larger angles for
lightning and switching surges without damage to
smaller conductors or vertical construction, but
the insulators. Operating stresses are increased
any pole where the angle of deviation of the line
under atmospheric conditions which causes pollu-
exceeds five degrees will be guyed. This require-
tants to build up on the insulator surface. Various
ment applies both to armless and crossarm con-
types of insulators are manufactured to meet
struction.
requirements imposed by different applications.
(2) Angles at which changes in configurations
Each type is industry rated by ANSI in classes
apply. The degree to which the more rigid line-post
which establish dimensions and minimum electri-
or pin type insulator support can be used will also
cal and mechanical performance values.
vary dependent upon the angle of deviation and
a. Types. Pin-type, line-post, or suspension insu-
the size of the conductor. Suspension insulators
lators will be used for medium-voltage circuits;
Table 6-6. Relation of Crossarm Configuration to Conductor Size."
a
Credit: US Corps of Engineers. Angles "A", "B", and "C" are shown on figure 6-3.
6-7
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