(a) to prevent electric shock.
The impedance of the human body has a great relationship with the environment. The wetter the environment, the lower the human impedance and the more vulnerable to electric shock. For example, almost everyone who has installed an AC radio has been electrocuted, but almost all of them can get rid of the power supply, because the environment in which people live at this time is dry and their skin is dry. Grounding is an effective way to prevent electric shock. After the electrical equipment is grounded through the grounding device, the potential of the electrical equipment is close to the ground potential. Because of the existence of grounding resistance, the grounding potential of electrical equipment always exists. The greater the grounding resistance of electrical equipment, the greater the grounding potential of electrical equipment when a fault occurs, and the greater the danger of human contact. However, if there is no grounding device, the voltage of the shell of the faulty equipment will be the same as the voltage of the phase line to the ground, which is much higher than the grounding voltage, so the danger will increase accordingly.
? (two) to ensure the normal operation of the power system.
Power system grounding, also known as working grounding, is generally used for neutral grounding of substations or substations. Working grounding has little requirement for grounding resistance, so large substations need a grounding grid to ensure small and reliable grounding resistance. The purpose of working grounding is to make the potential between the neutral point of the power grid and the ground close to zero. Low-voltage distribution system can't avoid the phase line contacting the shell or the ground after the phase line is disconnected. If the neutral point is insulated from the ground, the other two relative ground voltages will rise to three times the phase voltage, which may burn out the electrical equipment with the working voltage of 220. In neutral grounding system, even if one phase is relatively short-circuited, the other two phases can still approach the phase voltage, so the electrical equipment connected to the other two phases will not be damaged. In addition, it can prevent system oscillation, and the insulation level of electrical equipment and lines only needs to be considered according to the phase voltage.
(three) to prevent lightning and electrostatic hazards.
When lightning happens, in addition to direct lightning, it will also produce induction lightning, which can be divided into electrostatic induction lightning and electromagnetic induction lightning. The most important method of all lightning protection measures is grounding.