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Page 235

| 13 Protective Relays: Design and Construction.. |

| Function |

| In order for an electrical system to function properly and with a minimum of disturbance under abnormal conditions, it is essential that faulty circuits and equipment be quickly disconnected from sources of energy. Protective relays are used as a means to accomplish this end by electrically tripping the proper circuit breaker. |

| The protective relay is an electrical device so placed between the circuit or equipment and the associated circuit breaker that any abnormal condition in the circuit or equipment acts upon it. The relay, in turn, after proper discrimination as to the magnitude and character of the fault or abnormal conditions, causes the circuit breaker, or breakers, to operate and relieve or protect the circuit or equipment. |

| A relay, then, is a low-powered device used to activate a high-powered device (Fig. 13-1). It may therefore, be viewed as a sensing device that triggers an action on or by the equipment with which it is associated. It may be likened to the human nervous system, including the senses. In this regard, it receives certain data, examines it, and then initiates action or actions on the part of the equipment (organs) that it controls. The actions may be positive (starting), negative (stopping), balancing (a combination of the two), or informative (reporting the status of specified conditions). |

| For electric power systems or equipment, such sensing commonly takes note of current, voltage, and frequency as well as temperature, pressure, and speed; also included may be sensitivity to sound, light, and other indicators. It |

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