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Page 264

| 4. Electromagnetically operated relays are usually composed of three elements: an actuating device (relay coil or coils), a movable element (core or armature, disc, cylinder), and one or more sets of contacts. |

| 5. Over-current relays operate when the current flowing through them exceeds a predetermined value. Their action may be instantaneous, or may be made to delay deliberately to avoid too frequent operation caused by transient nonpersistent conditions (Fig. 13-3). They may operate on an inverse-time principle in which the greater the amount, the shorter the time for operation. The delay and inverse-time features may also be combined into an inverse definite-minimum time method of operation. |

| 6. Directional type relays respond to abnormalities in the direction of the current flow. These are often combined with an overcurrent element to guard against momentary transient reversals of power (Fig. 13-9). |

| 7. Differential relays operate on the difference between currents entering and leaving the protected line or equipment (Fig. 13-10). |

| 8. Other type relays are used for automatically reclosing circuit breakers after abnormal conditions no longer exist (Fig. 13-11) and for miscellaneous auxiliary purposes. |

| 9. Electronic or solid-state type relays may accomplish the same purposes as the electromagnetic or induction types, and are generally faster in their operation (Figs. 13-15 and 13-16). |

| 10. Transducers are devices that convert nonelectric quantities into electric ones, and vice versa (Fig. 13-18). |

| STUDY QUESTIONS |

| 1. What is the function of a protective relay? |

| 2. What three things must a protective relay be capable of doing? |

| 3. What is meant by normal conditions in a relaying system? |

| 4. What three factors concerning electric currents are utilized in the design and application of protective relays? |

| 5. What are three essential elements of a relay? |

| 6. What are the more common types of protective relays? How do they operate? |

| 7. What two general methods of operation are employed in the design and construction of protective relays? |

| 8. Why and how are overcurrent relays modified to increase their sensitivity and selectivity? |

| 9. What are reclosing relays? How do they function? |

| 10. What are auxiliary relays? Where and how are they employed? |

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