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 chance of damage. There are modifications of both systems. In one, single-ended grounding
may be attained by insulating the shields at each splice or sectionalizing point, and grounding
only the source end of each section. This limits possible shield damage to only the faulted
section. Multiple grounding, rather than just double-ended grounding, is simply the grounding of
the cable shield or sheath at all access points, such as manholes or pull boxes. This also limits
possible shield damage to only the faulted section.
2.5 POWER SYSTEM APPLICATIONS. A power system consists of transmission and
distribution systems. The transmission system is typically of higher voltage and is usually
referred to as a high side of the system. The distribution system is called a low side of the
system. The two systems are usually connected by means of a transformer to transform the high
voltage to low voltage.
2.5.1 Transmission System. Three-phase 3-conductor circuits are universally used in
transmission systems. Transmission circuits consist of overhead or underground cables, or some
combination of both.
2.5.1.1 Overhead Systems. Overhead circuits consist of aerial cables or open-wire
conductors carried on poles or towers. Aluminum conductors have, to a large degree, replaced
copper in overhead installations. Conductor sizes most commonly used vary from No. 2 AWG to
556 kcmil aluminum cable steel reinforced (ACSR) or stranded aluminum alloy.
An aluminum cable stranded around a steel core sized to give the required strength is known as
ACSR. Other cables are aluminum conductor alloy-reinforced (ACAR) and all aluminum-alloy
conductor (AAAC).
2.5.1.2 Underground Systems. In older areas, transmission circuits are often 3-conductor
lead-covered cables. Cross-linked polyethylene and ethylene-propylene rubber are replacing
lead. Ordinarily, a 3-conductor copper cable varying in size from 1/0 AWG to 500 kcmil is
installed in underground conduits. Aluminum cable is seldom used in underground installations
because these cables are larger than copper cables of equal ampacity.
2.5.2 Primary Distribution System. A 3-phase 3-wire system is commonly used in primary
distribution.
2.5.2.1 Overhead Primaries. The conductor sizes most commonly used in overhead
primaries range from No. 4 AWG ACSR to 336.4 kcmil stranded aluminum. Laterals, or
branches, are often No. 4 or No. 2 AWG ACSR, but feeders run to the larger sizes. Aluminum,
ACSR, and alloys of aluminum have largely displaced copper from primary circuit construction,
although they may be specified on a copper-equivalent basis. Aerial cable is often used for
primary conductors where clearances are too close for open wire or tree trimming is not
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