The connection between a certain point of the electrified part of an electrical device or electric line and the earth, or the artificial connection between a certain point of the electrified part of an electrical device or other device and the earth when it is normal, is called grounding; it can also be said that the grounding wire for electric equipment, tower or overvoltage protection device is connected with the earth through the metal conductor embedded in the earth and directly in contact with the earth. In the power system, the grounding part is generally the neutral point, or a point on the phase line. The grounding part of the electrical equipment is normally uncharged metal conductor, generally metal shell.

1、 Function of grounding

We often only know that grounding can prevent people from electric shock. In fact, in addition to this function, grounding can also prevent equipment and lines from being damaged, prevent fire, prevent lightning, prevent electrostatic damage and ensure the normal operation of the power system.

（1） Prevent electric shock.

The more humid the environment is, the lower the impedance of the human body is, and the more vulnerable it is to electric shock. For example, almost all people who have installed their own AC radio have been shocked, but almost all of them can get rid of the power supply, because the environment they are in 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. Due to the existence of grounding resistance, the potential to ground of electrical equipment always exists. The greater the grounding resistance of electrical equipment, the greater the potential to ground of electrical equipment in case of failure, and the greater the risk of timely human contact. However, if the grounding device is not set, the voltage of the fault equipment shell is the same as that of the phase line to ground, which is much higher than that of the grounding voltage, so the risk increases accordingly.

（2） Ensure the normal operation of the power system.

Power system grounding, also known as working grounding, is generally used to ground the neutral point in substation or substation. The requirement of working grounding resistance is very small, and a grounding grid is required for large substations to ensure small and reliable grounding resistance. The purpose of working grounding is to make the potential between neutral point and ground close to zero. The low-voltage distribution system can not avoid the phase line touching the shell or the phase line breaking and then touching the ground. If the neutral point is insulated from the ground, the voltage of the other two phases to the ground will rise to three times of the phase voltage, which may burn out the electrical equipment with the working voltage of 220. For the system with neutral grounding, even if one phase is short circuited to the ground, 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, so long as the insulation level of electrical equipment and lines is considered according to the phase voltage.

（3） Prevent the harm of lightning and static electricity.

When lightning occurs, 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.

2、 Types of grounding

1. Working grounding. In order to meet the operation requirements of power system or electrical equipment, grounding a certain point of power system is called working grounding, such as neutral grounding of power system.

2. Lightning protection and grounding. In order to prevent the harm of lightning overvoltage to people or equipment, the grounding of overvoltage protection equipment is called lightning protection grounding, such as the grounding of lightning rod and arrester.

3. Protective grounding. In order to prevent the insulation damage of the electrical equipment, the voltage of the metal shell to the ground is limited within the safe voltage, so as to avoid the personal electric shock accident

(1) Metal bases and enclosures for motors, transformers, lighting appliances, hand-held or mobile electrical appliances and other electrical appliances;

(2) Transmission device of electrical equipment;

(3) Frame of panel (table, box) for power distribution, control and protection;

3、 Technical principle of electrical equipment grounding

1. In order to ensure personal and equipment safety, all kinds of electrical equipment should be grounded according to the national standard gb14050 "forms and safety technical requirements of system grounding". The protective grounding wire shall not be used for other purposes except for the specified working grounding or protective grounding.

2. For electrical equipment with different purposes and different voltages, except for special requirements, a general grounding body shall be used. According to the equipotential connection requirements, the building metal components and metal pipes (except the metal pipes conveying inflammable and explosive materials) shall be connected with the general grounding body.

3. The artificial main grounding body should not be set in the building, and the grounding resistance of the main grounding body should meet the requirements of the minimum grounding resistance in all kinds of grounding.