Figure 4-2.Loop, or ring, distribution system.
A fault in the primary loop is cleared by the
breakers in the loop nearest the fault, and power is
supplied the other way around the loop without
interruption to most of the connected loads. Because
the load points can be supplied from two or more
directions, it is possible to remove any section of the
loop from service for maintenance without causing an
outage at other load points. If a fault occurs in a section
adjacent to the distribution substation, the entire load
may have to be fed from one side of the loop until
repairs are made. Sufficient conductor capacity must
be provided in the loop to permit operation without
excessive voltage drop or overheating of the feeder
when either side of the loop is out of service. If a fault
occurs in the distribution transformer, it is cleared by
the breaker in the primary leads; and the loop remains
NETWORK DISTRIBUTION SYSTEM
The network and radial systems differ with respect
to the transformer secondaries. In a network system (fig.
4-3) transformer secondaries are paralleled; in a radial
system, they are not.
The network is the most flexible type of primary
system; it provides the best service reliability to the
distribution transformers or load center, particularly
when the system is supplied from two or more
distribution substations. Power can flow from any
substation to any distribution transformer or load
center in the network system. The network system is
more flexible with regard to load growth than the radial
or loop system and is adaptable to any rate of load
growth. Service readily can be extended to additional
points of usage with relatively small amounts of new
construction. The network system, however, requires
large quantities of equipment and extensive relaying;
therefore, it is more expensive than the radial system.
From the standpoint of economy, the network system
is suitable only in heavy-load-density areas where the
load center units range from 1,000 to 4,000
Figure 4-3.Network distribution system.