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 provides high strength to withstand climatic rigors or mechanical shock. It may also serve as the
grounding conductor of the power circuit.
2.4.3 Above-Ground Conduits. @Rigid steel conduit systems afford the highest degree of
mechanical protection available in above-ground conduit systems. Unfortunately, this is also a
relatively high-cost system. For this reason their use is being superseded, where possible, by
other types of conduit and wiring systems. Where applicable, rigid aluminum,
intermediate-grade steel conduit, thin-wall EMT, intermediate-grade metal conduit, plastic, fiber
and asbestos-cement ducts are being used.
2.4.4 Underground Ducts. Underground ducts are used where it is necessary to provide a
high degree of safety and mechanical protection, or where above-ground conductors would be
unattractive.
2.4.4.1 Construction. Underground ducts use rigid steel, plastic, fiber, and
asbestos-cement conduits encased in concrete, or precast multi-hole concrete with close fitting
joints. Clay tile is also used to some extent. Where the added mechanical protection of concrete
is not required, heavy wall versions of fiber and asbestos-cement and rigid steel and plastic
conduits are direct buried.
2.4.4.2 Cables. Cables used in underground conduits must be suitable for use in wet areas,
and protected against abrasion during installation.
2.4.5 Direct Burial. Cables may be buried directly in the ground where permitted by codes
and only in areas that are rarely disturbed. The cables used must be suitable for this purpose, that
is, resistant to moisture, crushing, soil contaminants, and insect and rodent damage. While
direct-buried cable cannot be readily added to or maintained, the current carrying capacity is
usually greater than that of cables in ducts. Buried cable must have selected backfill. It must be
used only where the chance of disturbance is unlikely. The cable must be suitably protected,
however, if used where the chance of disturbance is more likely to occur. Relatively recent
advances in the design and operating characteristics of cable fault location equipment and
subsequent repair methods and material have diminished the maintenance problem.
2.4.6 Underwater (Submarine) Cable. Submarine cable is used only when no other cable
system can be used. It supplies circuits that must cross expanses of water or swampy terrain.
2.4.6.1 Construction. Submarine cable generally consists of a lead sheathed cable and is
usually armored. Insulation material should be XLP or EPR, except when paper insulation is
justified because of its high resistance to, and freedom from, internal discharge or corona.
Multiconductor construction should be used, unless limited by physical factors. The lead
sheathing usually consists of a copper-bearing lead material, however, other alloys may be
required when special conditions warrant nonstandard sheathing. The most common type of
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