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Page 238

| Overcurrent Relays |

| Overcurrent relays, perhaps the simplest of the many protective relays, close their contacts when the current flowing in the circuit or equipment reaches a predetermined value. The principle of operation of each of the several types is described below; the variations in their characteristics are illustrated by the curves in Fig. 13-3. |

| Instantaneous Principle |

| In the basic overcurrent relay, the circuit is designed to open instantaneously, that is, as quickly as the device can be made to operate with no time delay deliberately added. As a normal result, the relay closes its contacts within one-half cycle to twenty cycles. Although great accuracy can be obtained in the setting of the relay, the instantaneous feature often causes the settings to be made at values that are higher than desirable in order to prevent too frequent operation of the relay as a result of transient, nonpersistent conditions. The time-current characteristic of this type of relay is shown by curve (a) in Fig. 13-3. |

| Inverse-Time Principle... |

| The inverse-time principle was developed as a refinement of the instantaneous (overcurrent) principle. With it, the time of operation of the protective device is made to vary approximately inversely with the magnitude of the current; that is, the greater the current, the shorter the time for operation. Current settings may be made lower and sensitivity increased because |

| Figure 13-3 Representative time curves for overcurrent relays. (Courtesy, Westinghouse Electric Co.) |

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