0195136047.pdf

10.3 POWER TRANSMISSION AND DISTRIBUTION 461

feeders, distribution transformers, secondary distribution circuits, and customers’ connections
with metering. Depending on the size of their power demand, customers may be connected to
the transmission system, the subtransmission system, or the primary or secondary distribution
circuit.
In central business districts of large urban areas, the primary distribution circuits consist of
underground cables which are used to interconnect the distribution transformers in an electric
network. With this exception, the primary system is most oftenradial. However, for additional
reliability and backup capability, aloop-radialconfiguration is frequently used. The main feeder
is looped through the load area and brought back to the substation, and the two ends of the
loop are connected to the substation by two separate circuit breakers. For normal operation,
selected sectionalizing switches are opened so as to form a radial configuration. Under fault
conditions, the faulted section is isolated and the rest of the loop is used to supply the unaffected
customers.
Most residences and small buildings are supplied with power by means ofsingle-phase,
three-wireservice, as illustrated in Figure 10.3.3. A distribution transformer is located on a power
pole or underground, near the residential customer. Inside residences, the 220-V supply, being
available between the two “hot” wires, is used for major appliances such as dryers, ranges, and
ovens. The 110-V loads up to 20 to 40 A are connected between the ground wire and either “hot”
wire, while nearly balancing the loads on the two “hot” wires. Each of these circuits, protected by
its own fuse or circuit breaker, supplies lighting loads and/or convenience outlets. In the wiring
of residential and commercial buildings, safety considerations are of paramount importance, the
principal hazards being fire and electric shock. Residential wiring is also discussed in Section 4.4.

Circuit

breaker Substation

transformers

Circuit breakers

Primary

circuits

or

main feeders

Laterals

Distribution substation

Distribution

transformers

Secondary

circuits

Service entry

(1 to 10 residential customers)

Figure 10.3.2Typical electric power distribution system.