I chatted to one of the old electrical inspectors at the City of Cape Town Electricity department and they ideally would like to see an earth electrode resistance of < 1Ω for Installations with a supply greater than 150 Amps and < 22Ω for installations with a supply less than 150 Amps. They are prepared to accept < 5Ω for supplies > 150 Amps if certain minimum requirements are met. I have however not been able to verify these requirements in SANS 10142 or the NRS codes.
The earth fault path is normally copper conductors (called earth continuity conductors - ECC) sized to ensure a fault current of at least double the rating of the main protective device. This would not involve the earth electrode or the soil.
The earth electrode in a TNC-S system is really a backup to the Neutral conductor which is called a PEN (protective earth neutral) as it combines the neutral and earth functionality. Should the PEN conductor become damaged or open circuited then your earth electrode would come into play. Dangerous touch potentials could then exist which is dependent on your local earth electrode resistance, the combined neighborhood earth electrode resistances, and the load (higher the load the greater the current and the larger the touch voltage.)