What the earthing system has to do

Provide a low-impedance path to earth so that fault current operates the protective device, so that surge and lightning current is dissipated, and so that touch voltages stay within safe limits. If the earth resistance is too high, none of those happen reliably.

What determines earth resistance

Soil resistivity at the site, which varies enormously with soil type and moisture

The surface area of the electrode in contact with soil — not its length alone

Depth, and whether the electrode reaches permanently moist strata

The number and spacing of electrodes where more than one is used

Seasonal variation: a reading taken in the dry season must still be acceptable

Materials

Earthing electrodes — copper-bonded rods, maintenance-free chemical electrodes, pipe and plate electrodes

Backfill compounds and ground-enhancing materials, which lower the resistance around the electrode

Conductors and clamps rated for the fault current and for burial

Bonding conductors for services and structural steel

The mistake that undoes the design

Installing a good electrode and then connecting it with an undersized or corroded conductor. The conductor has to survive the fault current and the burial environment for the life of the installation.

Measurement, not assumption

Earth resistance is measured after installation and recorded. If the target is not met, the fix is more or deeper electrode, or a lower-resistivity backfill — not a lower target on the certificate.