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Page 237

| Figure 13-2 Protective devices on a circuit to be coordinated. |

| trolling the breaker or breakers in a faulted circuit or piece of equipment operate before, after, or simultaneously with other protective devices. |

| Since some electrical quantities may change during fault conditionsfor example, current, voltage, their relationship or phase-angle (in an ac system), and frequencyit is usually necessary to provide relays that are responsive to more than one of these conditions. In practically all ac circuits, the relays receive their operating energy through current transformers or potential transformers, or both. |

| In the design and application of protective relays, the principal considerations for achieving positive operation and selectivity are the magnitude, duration, and direction of the relevant electric currents. All electromagnetic relays operate on the basis of the currents flowing through them since these currents set up magnetic fields that actuate mechanisms to close or open contacts and thus cause a circuit breaker or other device to operate. A relay, therefore, is essentially a switch. |

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

| Any protective system employs one or more protective principles, depending upon the nature of the protection desired. Relays of many kinds and forms are used for the protection of electrical systems. Often, two or more relayseach sensitive to a different conditionare employed together. In this chapter, discussion will include overcurrent, directional, and differential relays, which are among the most common types used and, together with reclosing relays, are almost standard equipment. The principles and practices associated with these relays apply for the most part to the many other types of relays employed in specialized applications. |

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