The role of the ground is to prevent the metal casing of the electrical equipment is electrified to endanger the safety of persons and equipment and grounding protection. In order to prevent personal electric shock accidents, to ensure the normal operation of electrical equipment to take an important technical measures
Working grounding is the power system operation needs to set up (such as neutral grounding), so under normal circumstances there will be a current flowing through the grounding electrode for a long time, but only a few amperes to dozens of amperes of the imbalance of the current. In the event of a ground fault in the system, there will be thousands of amperes of operating current flowing through the grounding electrode, however, the current will be removed by the relay protection device within 0.05 ~ 0.1s, even if the backup protection, the action is generally within 1s.
Lightning protection grounding is in order to eliminate the dangerous effects of overvoltage and grounding, such as lightning rods, lightning lines and lightning arrester grounding. Lightning protection grounding is only in the role of lightning impact will have current flow through, flow through the lightning protection grounding electrode of lightning current amplitude up to tens to hundreds of kiloamperes, but the duration is very short.
Protective grounding is to prevent the equipment due to insulation damage charged and endangered personal safety of the grounding, such as the metal casing of power equipment, reinforced concrete poles and metal towers. Protective grounding is only in the case of equipment insulation damage to the current flow, the value can be varied within a wide range.
Current flow through the above three grounding electrode will cause the grounding electrode potential increases, affecting the safety of people and equipment. For this reason, the grounding electrode potential rise must be limited, or take appropriate safety measures to ensure equipment and personal safety.