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Page 245

| Figure 13-7 Arrangement of auxiliary circuit-opening contacts of induction relay. |

| Directional Relays |

| This type of protective relay operates on the principle of an abnormal direction of current flow. Both sensitivity and selectivity may be obtained by using the directional element, since a relatively low-current setting of the relay may be made in the direction opposite to normal. To guard against operation caused by transients that result in a momentary reversal of power, a sensitive instantaneous directional element is combined with an overcurrent element having a time characteristic (Fig. 13-9 on page 248). |

| All relays dependent on current alone for their operation function when sufficient current flows in their coil, regardless of the direction of the power flow in the line. To make such relays sensitive to the direction of power flow, a second element must be introduced. This second element operates essentially as a wattmeter that tends to turn the relay disc or cylinder in one direction when power flows normally and in the other direction when power flows in the opposite direction. The contacts of the overcurrent element are connected in series with the contacts of the wattmeter element. Both sets of contacts, therefore, have to be closed and the power flowing in a given direction before the relay will operate as a unit. |

| Since this relay has all the characteristics of an overcurrent relay, three conditions must exist before the tripping circuit is completed: |

| 1. Excess current must be flowing |

| 2. Power must be flowing in the opposite direction to its normal flow |

| 3. The excess current must be flowing for a sufficient length of time. |

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