What is the role of system grounding? System grounding is to provide a reference potential to the power distribution system and to operate the power distribution system properly and safely. After the star node of a 220/380 volt distribution system is grounded, the phase-to-ground potential is generally "clamped" at a voltage of 220 volts, thereby reducing the system's requirements for ground insulation. When a lightning strike occurs, the distribution line induces a large amount of electric charge. The system grounding can discharge the lightning charge into the ground, reduce the lightning transient overvoltage of the line to the ground, and avoid the breakdown of the insulation of the line and equipment, such as high and low voltage. A common pole overhead line, if the high voltage line falls on the low voltage line, will cause danger to the low voltage line and equipment. With the system grounded, the high-voltage line fault current can be made to return to the high-voltage power supply through the ground, so that the high-voltage side relay protection detects the fault current and acts to eliminate the danger. When a ground fault occurs in the low-voltage distribution line, the system ground also provides a path for the fault current to return to the power source through the earth, so that the protective device on the low-voltage line operates. It has both functional and protective effects.
When a phase fault occurs in one phase of the system, the other two opposing ground voltages will be as high as 380 volts. Since there is no conductor path returning to the power supply, the fault current is only the phasor sum of the two non-faulty relative ground currents, and its value is very small. The normal overcurrent protection device cannot operate, and the fault voltage will continue to exist, and the human body touches without failure. Phase line, the contact voltage will be 380 volts line voltage, and the risk of electric shock is very high. In addition, electrical equipment and wiring will continue to withstand 380 volts to ground voltage, which is also unfavorable for equipment insulation safety. Sometimes it is necessary to increase the insulation level to ground.
There are also power distribution systems that do not have a system grounding. This is for special needs. It needs to be supplemented with some safety measures, which will be explained in the following description of the IT system.
What is the role of protective grounding? The protective grounding is to reduce the ground voltage or contact voltage of the exposed conductive part of the electrical device in the event of a fault.