A transformer-fed low-voltage grounding study needs more than the transformer vector group. The model should include source grounding, transformer zero-sequence path, neutral-to-earth connection, PE/PEN conductors, cable impedances, electrode or grid impedance and protective-device behavior. Fault-loop impedance and touch-voltage conclusions are therefore site-specific and should not be inferred from transformer kVA alone.
The earth electrode is not the entire fault loop
In a TN arrangement, a metallic protective-conductor path is central to the fault return; in a TT arrangement, the earth-electrode path and residual-current protection require different treatment. IT arrangements introduce their own first- and second-fault conditions. These distinctions must be explicit in the model. A low measured electrode resistance does not establish the impedance of every equipment fault loop, and a transformer connection symbol does not identify how the downstream protective conductors were installed.
Use impedance at the fault location
As a purely illustrative loop calculation, 230 V divided by 0.20 ohm gives 1,150 A, while a 0.40-ohm loop gives 575 A. The factor-of-two difference can materially change the response of an overcurrent device. Neither result proves compliance: the complete circuit, conductor temperature, source minimum and device time-current behavior still matter. Evaluate remote faults as well as faults at the transformer terminals, because long conductors can dominate the return-path impedance.
Keep neutral bonding and protective paths traceable
Record the intended neutral-to-earth connection, any PEN separation point, switched-neutral arrangements and the return paths under generator or alternate-source operation. Check that drawings, cable schedules and protective-device data represent the same installation revision. Where the available information is incomplete, list the missing measurements or as-built verification instead of issuing a definitive touch-voltage or disconnection conclusion. Transformer procurement supplies part of this model; it cannot replace a site grounding assessment.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] IEC 60076-1:2011 — Power transformers, General.
[3] Schneider Electric — IT system: Principle.