Primary Feeders

Figure 2-4.—Loop (or ring) distribution system. connected directly to distribution centers. This eliminates the need for substations because the generator generates a usable voltage. Primary Feeders Primary feeders are those conductors in a distribu- tion system that are connected from the distribution sub- stations and that transfer power to the distribution centers (fig. 2-2). They may be arranged as radial, loop, or network systems and may be overhead or underground. RADIAL DISTRIBUTION SYSTEM.— A schematic example of a radial distribution system is shown in figure 2-3. In this system, primary feeders take power from the distribution substation to the load areas by way of subfeeders and lateral-branch circuits. This is the most common system used because it is the simplest and least expensive to build. It is not the most reliable system, however, because a fault or short circuit in a main feeder may result in a power outage to all the users served by the system. Service on this type of system can be improved by installing automatic circuit breakers that will reclose the service at predetermined intervals. If the fault continues after a predetermined number of closures, the breaker will be locked out until the fault is cleared and service is restored. PRIMARY LOOP (OR RING) DISTRIBUTION SYSTEM.— The loop (or ring) distribution system is one that starts at a distribution substation, runs through or around an area serving one or more distribution transformers or load centers, and returns to the same substation. The loop system (fig. 2-4) is more expensive to build than the radial type, but it is more reliable and may be justified in areas where continuity of service is required—at a medical center, for example. In the loop system, circuit breakers sectionalize the loop on both sides of each distribution transformer connected to the loop. 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. If a fault occurs in a section adjacent to the distribution substation, the entire load can be fed from one direction over one side of the loop until repairs are made. NETWORK SYSTEM.— The network system (fig. 2-5) is the most flexible type of primary feeder Figure 2-5.—Network distribution system. 2-3