What will be the value of ground fault current?
Say, I have a substation which is fed from a 630kVA, 11/0.433kV, 4% impedance, Dyn11, solidly grounded neutral transformer.
For grounding design we presume that the SLG fault current = 3Phase fault current. Thus while designing the grounding system we calculate the ground electrode size, grounding conductor size considering the SLG fault current (=21.5kA).
Now, it is a practice to restrict the ground(earth) resistance to 1ohm in industrial plants. And we design the grounding system accordingly.
Therefore, if a ground fault occurs in a 415V system, the ground fault current will be restricted by the ground resistance.Considering the ground resistance to be 1ohm, the ground fault current shall be (415/(1.732*1)=240Amp.
My question why we consider the ground fault current to be equal to 3phase fault current? Where am I wrong?