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Page 266

| 1. Sources of generation and their outgoing circuits (likened to a manufacturing plant) |

| 2. A subsource or station handling relatively large amounts of power (likened to a wholesale warehouse) |

| 3. Local sources utilizing equipment and circuits for distribution to the ultimate users (likened to retail stores serving local areas). |

| Each part, moreover, can be further subdivided into various zones of protection whose requirements can be considered more or less independently. In practice, however, coordination can best be achieved by analyzing conditions in reverse order. First the local distribution should be analyzed, then the intermediate subtransmission-substation area, and last the generation with its associated high-voltage transmission system. |

| Distribution Circuits |

| Distribution circuits are most frequently operated as radial circuits, that is, they are supplied from only one point. As shown in Fig. 14-1, each transformer is fused on the high-voltage side. The circuit is sectionalized with fuses and reclosers that provide points for clearing faults without interrupting the entire circuit. |

| The time-current characteristics of the overcurrent relays associated with the circuit breakers in this type of circuit must coordinate with those of the reclosers and fuses. Several characteristic curve shapes are available for different induction-type overcurrent relays, as previously indicated in Fig. 13-3. These provide some margin in the selection of the relay settings that coordinate best with the characteristic curves of the reclosers and fuses for the current values involved. The curves are shown in Fig. 14-2. |

| Figure 14-1 Radial distribution protection zone. |

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