1. Residential electrical system
If loads are single phase, the projected system is classified as residential-commercial. A maximum ground resistance of 25 ohm is recommended.
2. Light industrial electrical system
If loads are primarily single phase on a three-phase system, the system is classified as light industrial. The maximum ground resistance recommended is 5 ohm.
3. Industrial electrical system (Substation)
If the loads are primarily three-phase, then the maximum ground resistance recommended is 1 ohm.
4. Consider Lightning Protection
When a significant lightning threat is expected, use lightning arresters coupled with a maximum ground resistance of 10 ohm or less.
According to IEEE 80-2000 article 14.1, A good grounding system provides a low resistance to remote earth in order to minimize the GPR. For most transmission and other large substations, the ground resistance is usually about 1 Ω or less. In smaller distribution substations, the usually acceptable range is from 1 Ω to 5 Ω, depending on the local conditions.