Design inputs
Soil resistivity measured on site (commonly by the Wenner method to IEEE 81) and interpreted as a layered model; earth-fault current at the site and its split between grid, overhead earth wires and cable sheaths; protection clearing times (including backup); and the substation layout, fence and nearby third-party assets.
Safety criteria
IEEE 80 calculates tolerable touch and step voltages from body weight, fault duration and surface layer resistivity. In Australia, AS 2067 and the risk-based ENA EG-0 approach are widely used to set acceptable limits based on the probability of fault coincident with contact.
The design objective is not a single resistance value: it is keeping earth potential rise (EPR), touch and step voltages, and transferred voltages within the applicable criteria.
Grid design
Conductor size is set by fault current and duration (adiabatic sizing). Mesh spacing, burial depth, perimeter and corner treatment, earth rods and a high-resistivity surface layer such as crushed rock are adjusted until computed touch and step voltages comply. Fence earthing and transferred potentials through communications and LV services must be addressed.
Verification
After construction, verify with an earth impedance (fall-of-potential or current-injection) test and touch/step voltage measurements at representative locations. Results that differ materially from design should be investigated before energisation.