By hvhipot 
When most facilities are struck by lightning or overload voltage occurs during use, the current will soar. If there is no safe path to conduct the current, it is easy to cause accidents such as personal electric shock, fire, and explosion. The ground electrode is this current evacuation channel.
So the question is, how do you know that such an important passage is invisible to the naked eye? Let’s start with the grounding resistance. An important parameter to measure whether the grounding state is good is the grounding resistance, which is the sum of resistance encountered in the process of current flowing from the grounding device to the ground and then flowing through the ground to another grounding body or spreading to a distance.
How large is the grounding resistance value to ensure a good grounding state? There is no fixed standard, but there is usually the following relationship
1. Ideally, the grounding resistance value should be zero ohms.
2. NFPA and IEEE recommend grounding resistance value ≤5.0 ohm.
3. NEC stipulates “Ensure that the impedance of the system to ground is less than 25 ohms specified in EC 250.56. In facilities containing sensitive equipment, it should be less than 0.5 ohms or less.”
4. The telecommunications industry often uses 5.0 ohms or lower as the limit for grounding and pressure welding.
5. Why test the grounding resistance At the initial installation, the grounding resistance value of the grounding system is very small, but long-term corrosion by corrosive soil, its resistance value will increase, so the grounding and grounding connection should be checked regularly. The tool for measuring grounding resistance and grounding safety detection is called a grounding resistance tester.