Zero Ohms… Not Really!
The goal in ground resistance is to achieve the lowest ground resistance value possible, that makes sense economically and physically, when contacting the earth, also known as the soil/ground rod interface.
Ideally, a ground should be zero ohms of resistance, but…
Unfortunately, there is not one standard ground resistance threshold recognized by all certifying agencies.
The NFPA and IEEE recommend a ground resistance value of 5 ohms or less while the NEC has stated to “Make sure that system impedance to ground is less than 5 ohms specified in NEC 50.56. In facilities with sensitive equipment it should be 5ohms or less.”
The telecommunications industry has often used 5 ohms or less as their value for grounding and bonding while electric utilities construct their ground systems so that the resistance at a large station will be no more than a few tenths of one ohm.
Reference: Fluke Earth Ground Testing Fact – Distributor Training Program