Calculation of Earth Fault Current

The magnitude of earth fault current in the line to earth fault on an earthed system is determined by 
  • Impedance from source to fault
  • Impedance of earth return paths
  • Impedance of Earthing transformer
  • Resistors
  • Reactors

The voltage across resistor is nearly is equal to line to neutral voltage for single line to earth fault on a resistance grounding system.
The resistor current is equal to fault current for low resistance grounded system therefore the current is practically equal to line to neutral voltage divided by the resistance
  • This method is application only when earth fault current is small as compared to three phase fault current.
  • The earth fault current is computed for resistance earthed system for line to earth fault.

        IG = 3E / [ X1 * X2 + X0 + 3 ( Xn + XGP )]
        E = Line to earth voltage  ( V )
       IG = Earth fault current in ( Amp )
      X1 = Positive sequence reactance in ohm / phase including sub – transient reactance of the rotating machines
      X2 = Negative sequence reactance
     X0 = Zero sequence reactance
     Xn = Reactance of neutral grounding reactor
   XGP = Reactance of ground return circuits
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