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. 