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Page 236

| Figure 13-1 Features of the relay. |

| has the function of detecting actual or incipient faults or undesirable conditions and to initiate action that will result in switching or in giving adequate warning. |

| Principles of Operation |

| Modern protective systems are based on three fundamental requirements: accuracy, sensitivity, and selectivity. A protective system must accurately distinguish between normal and abnormal conditions. It may be required to operate at a current, voltage, or other quantity that is very small when compared with the capacity of the circuit or equipment to be protected; that is, it must be sensitive to small quantities or changes in value of the actuating forces. Under abnormal conditions, it must be able to distinguish in a positive fashion, between a fault in the circuit or equipment that is being protected and a fault in the rest of the system; that is, it must be selective. |

| Abnormal conditions include not only those involving the upper (or lower) tolerance limits of allowable continuous overcurrent or overvoltage, but also those transient conditions that, even if they involve values of current or voltage in excess of normal, are not of sufficiently large magnitude and long duration to have harmful effects. Such transient conditions include motor starting currents, switching of transformer or capacitor banks, synchronizing of machines, and so forth. |

| The several elements of an electrical system may include various circuits or lines and pieces of equipment connected together by circuit breakers. The opening of one or more breakers isolates an element from the rest of the system. Sometimes, however, two or more elements may be joined together as a unit by means of fuses or switchesall supplied through one circuit breaker. In such cases, protection may be achieved by blowing a fuse or by the operation of a switch. Relays, therefore, when called upon to act jointly with other forms of protection, must distinguish between conditions requiring the operation of a circuit breaker and conditions where other protective devices should function (Fig. 13-2). |

| Protective relays vary widely in their operating principles and design. They must be responsive to electrical quantities that are different during normal and abnormal conditions. They must be so arranged that those con- |

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