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# Grounding Riddle No.35 – Magnitude of Gound Fault Current

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• #923
Bishu

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. …

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?

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• #2974
Hamid

In low voltage network which ordinary grounding standard systems (TN-S,TN-C-S,TT) is applied, ground loop resistance is used for ground fault current determination. Indeed total grounding loop impedance is determined when we know groundmass resistance ( Zg as you say 1 ohm) and artifficial grounding path impedance (Zc). Zt=Zgװ Zc = Zg.Zc /(Zg+Zc) In industrial low voltage network, Zt is negligible against source zero sequence impedance, concecontly as you say we can presume that the SLG fault current = 3Phase fault current.

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