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| the normal transient currents (small in comparison to fault currents) that may start the operation of the relay will not last long enough to complete it. Furthermore, by varying the current setting and the restraint on the movable element, the time-current characteristics of this type of relay may be varied, as indicated by curves (b) and (c) in Fig. 13-3. Greater selectivity between relays and fuses in the circuit may thus be obtained, as will be discussed subsequently. |

| Definite-Time Principle |

| For still greater selectivity, the principle of definite time was introduced. It prevents the abnormal currents of any value from operating the relay until a definite time has elapsed after the relay is set in operation. This principle is often combined with the inverse-time principle to give inverse-time characteristics to a certain value of current, beyond which operation can be completed only after the definite time for which the device is set has elapsed. This combination is called the inverse definite-minimum time principle. The selection of a minimum time common to all such relays in series simplifies their coordination. The large flat portion of its characteristic, shown as curve (d) in Fig. 13-3 results in only a small increase in the relay time for smaller values of fault current. Relays of this type are used in the majority of overcurrent relay applications. |

| Overcurrent Relay Design and Construction |

| The design and construction of overcurrent relays may be divided into two general classes based on their method of operation: (1) the electromechanical or plunger type, and (2) the induction or disc type. Since both methods, particularly the second, are employed in many other types of relays, a description here will prove useful in discussing the latter. |

| Electromechanical or Plunger Type |

| The plunger-type (overcurrent) relay consists of a core or plunger, sometimes also called an armature, that is movable within a solenoid or electromagnetic coil. When a sufficient amount of current is passed through the coil, the core is pulled up, thus causing the cone-shaped discthe movable contactto bridge the gap between the stationary contacts. The bridging of the contacts completes the tripping circuit, energizing the associated circuit breaker trip coil that will open the breaker. The main parts of this relay are shown in Fig. 13-4A. Single-phase and three-phase circuits are shown in Figs. 13-4B and 13-4C, respectively. |

| By adjusting the position of the plunger with respect to the solenoid, the |

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