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| 14 Protective Relays: Operation And Maintenance |

| The operation of relays (and their associated circuit breakers) is meant to prevent or limit damage during faults or overloads and to minimize their effect on the remainder of an electric system. When the great variety of possible faults and types of relays is considered, it is soon seen that any attempt to coordinate different relays to encompass all possibilities becomes very impractical. |

| In the most popular three-phase ac systems, faults include single-phase-to-ground, phase-to-phase, phase-to-phase-to-ground, three-phase-to-ground, and fault resistance differences from very high to very low. There is also an almost infinite variation in distance of the fault from the relays. The system may be a grounded one or not, and it may be delta- or wye-connected. Other differences combine to produce a host of considerations that must be taken into account in the coordinating process. |

| When these circuit variations are set against the many variations in the types and characteristics of relays as well as those of other protective devices (fuses, reclosers, lightning arresters), it is soon realized that the practical coordination of all these factors must involve a series of compromises and approximations, and coordination becomes more of an art than a science. |

| Zones of Protection.. |

| Nevertheless, electric systems can be adequately protected if small parts of the system can be disconnected. To do so, the system is divided into protective zones, which are then subdivided further as need or desirability dictates. As a first division, the electric system can be divided logically into three parts: |

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