[S.O]

What amount of grounding impedance shall be considered in grounding system planning stage?

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[Hamid]

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 ? or less. In smaller distribution substations, the usually acceptable range is from 1 ? to 5 ?, depending on the local conditions.

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