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Page 273

| Figure 14-6 Protective zones for generators and outgoing high-voltage transmission lines. (Courtesy, Westinghouse Electric Co.) |

| relaying, somewhat similar to the coordination achieved by the subtransmission-substation schemes described above. Radial feeders supplied from these sources require only overcurrent relaying. |

| In schemes utilizing differential relaying where protective zones overlap, the protection of the overlapped equipment is accommodated by the cross-connecting of its associated current transformers (Fig. 14-7). In addition to their protective function in the schemes described above, the circuit breakers are also used for switching of circuits under normal conditions. |

| Whatever protective scheme is selected, the incorrect tripping of circuit breakers not directly affected by a fault can result in unforeseen, widespread interruption. Failure to trip, however, is also serious and more apt to occur, and backup protection has to be relied upon to clear a fault, usually resulting in interruptions to more than one section. When any relay fails to function, the next relay to operate should be the one that is immediately in series toward the source of supply. |

| Figure 14-7 Principle of overlapping protection around a circuit breaker. |

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