The ground resistance of a grounding system primarily depends on four variables. These factors include: Length/depth of the ground electrode, the diameter of the ground electrode, the number of ground electrodes in system and the design of system.
While talking about ground, one of the most important question that arises in mind is What ground resistance value is considered a good ground resistance value? As a rule of thumb, the resistance should be as low as possible.
| Case | Resistance |
|---|---|
| The ideal case | Zero ohms (0 Ω) |
| NFPA and IEEE Recommendation | 5.0 ohms or less |
| NEC* | Less than 25 ohms |
| Telecommunications industry | 5.0 ohms or less |
*According to the NEC, make sure that system impedance to ground is less than 25 ohms specified in NEC 250.56. In facilities with sensitive equipment it should be 5.0 ohms or less.
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