Saturday, October 8, 2011

Reactance Earthing

       In this system, instead of resistance, a reactance is connected between neutral and earth with ratio of reactance more than 3. The system is represented in the Fig. 1. Let the earth fault occurring on phase B.
Fig. 1

       In addition to zero sequence impedance of the source and faulty phase upto point of fault, the fault current is dependent on reactance. By changing the value of reactance, the fault current can be varied. In practice this method is employed to give characteristics similar to solidly earth system.
       There is drawback of this system. With increase in reactance, there is increase in transient voltages resulting from arcing. Hence it is not commonly employed though it ensures satisfactory relaying, partial grading of equipment insulation, less interference with neighbouring circuits, and intermediate cost.
       For reactance earthing, it is necessary that the magnitude of fault current should be at least 25% of the three phase fault current. This is higher than the requirement on resistance earthing and thus it can be seen that the resistance earthing and the reactance earthing are not similar.

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