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Different Types of Earthing Devices
1-Solid Earthing In this case, the neutral neutral of a power transformer transformer is earthed solidly with wi th a copper conductor (see Figure 1). Advantages -Neutral held effectively at earth potential. -Phase-to-ground faults of same magnitude as phase-to-phase faults; so no -Phase-to-ground need for special sensitive relays. -Cost of current current limiting li miting device is eliminated. elim inated. -Grading insulation towards neutral point N reduces reduces size si ze and cost of transformers.
Disadvantages -As most system faults are phase-to-ground, severe shocks are more considerable than with resistance earthing.
-Third harmonics tend to circulate between neutrals.
2-Resistance A resistor is connected between the transformer neutral and earth (see Figure 2).
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-Mainly used below 33 kV.
Apply for Job Now -Value is such as to limit an earth fault current to between 1 and 2 times full load rating of the transformer. Alternatively, to twice the normal rating of the largest feeder, whichever is greater. Advantages -Limits electrical and mechanical stress on system when an earth fault occurs,but at the same time, current is sufficient to operate normal protection equipment.
-Full line-to-line insulation required between phase and earth.
3-Reactance earthing A reactor is connected between the transformer neutral and earth (see Figure 3). -Values of reactance are approximately the same as used for resistance earthing. -To achieve the same value as the resistor, the design of the reactor is
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