The word grounding gas is actually quite complicated. I hope the following passage is useful to you.
In order to ensure the safe use of low-voltage distribution system, electrical equipment and appliances, appropriate measures must be taken to prevent users from getting electric shock and electrical equipment and appliances from burning down. Grounding is a common method, because the earth is a conductive stratum, and the potential at any point is usually zero, that is, zero potential (when grounding is single-phase, it is considered as zero potential when it is 20 meters or more away from the grounding point).
For electrical equipment and appliances, if the metal shell is grounded, the metal shell will be close to zero potential. Even in the case of fault, if the electrical equipment is short-circuited due to insulation damage, the potential difference between the metal shell and the earth becomes low because the metal shell has been well electrically connected with the earth. If people contact with it, the current passing through the human body is also small, which improves the safety of indirect electric shock.
For low-voltage distribution system, the neutral point of distribution transformer is often grounded (called working grounding). From the point of view of electrical safety, it can work together with the grounding of electrical equipment under certain conditions. When a ground fault occurs, the generated current can make the protection equipment in the distribution system act at an appropriate time to cut off the power supply and ensure safety.
Because the metal shells of electrical equipment and appliances can be directly grounded or connected to the grounding neutral point of the distribution system through conductors, the distribution system can be directly grounded or ungrounded or grounded through impedance. These grounding combinations are called low-voltage distribution system grounding modes.
Second, the basic composition of grounding mode
The components of grounding mode can be divided into two parts: electrical equipment and power distribution system.
1. Grounding part of electrical equipment
(1) grounding body: a conductor or a group of conductors that are in close contact with the earth and electrically connected.
(2) Exposed conductive part: the conductive part that can be touched by electrical equipment. Under normal circumstances, it is not charged, but it may be charged when a fault occurs, usually the metal shell of electrical equipment.
(3) Main grounding terminal board: the combination of various grounding (such as working grounding and protective grounding) terminals and equipotential bonding wires in a building or part of a building. If arranged in rows, it is called the main grounding terminal row.
(4) Protective wire (PE): a conductor that electrically connects the exposed conductive part, the main grounding terminal board, the grounding body and any part of the power grounding point (or artificial grounding point). Wires connecting multiple exposed conductive parts are called protective trunks.
(5) Grounding wire: directly connect the main grounding terminal board or exposed conductive part to the protective wire of grounding body. The grounding wire connecting multiple grounding terminal boards is called grounding trunk.
(6) Equipotential connection: refers to the electrical connection where the potential of each exposed conductive part is basically equal to that of the external conductive part of the device.
2. Grounding part of power distribution system
(1) phase line (l). The conductor that transmits electric energy is not grounded under normal circumstances.
(2) neutral line (n). A conductor connected to the neutral point of the system and capable of transmitting electric energy.
(3) Protect the neutral wire (pen). Conductor with protective wire and neutral wire.
(4) Power supply grounding point. The point at which a power supply can be grounded (usually a neutral point).
Three, the classification of grounding mode
The grounding mode of distribution system in China has been stipulated by IEC, and its classification is still based on the grounding combination of distribution system and electrical equipment, which is generally divided into TN, TT, IT system and so on. The meaning of the above letters: the first letter indicates the relationship between the power supply grounding point and the ground. Where t stands for direct grounding; I means ungrounded or grounded through impedance. The second letter indicates the relationship between the exposed conductive part of electrical equipment and the ground. Where t stands for a single direct ground that is not connected to the power ground point; N means directly connected to the grounding point of the power supply system, or connected to the conductor drawn from this point.
TN system can be divided into TN-C, TN-S and TN-C-S systems according to whether neutral line and protection line are merged.
TN-C system: the protection line and neutral line are merged into PEN line.
TN-S system: the protection line is separated from the neutral line.
TN-C-S system: the protection line and neutral line near the power supply side are merged into PEN line, and after a certain point, they are divided into protection line and neutral line.
Section 2 Scope of Application of Various Grounding Methods
In low-voltage distribution system, the exposed conductive parts of electrical equipment are usually grounded to prevent indirect electric shock.
I. TN system
In TN system, the exposed conductive parts of all electrical equipment are connected to the protection line and the grounding point of the power supply, usually the neutral point of the distribution system.
TN system, called protection zero connection. When the fault causes the electrical equipment with metal shell, the phase line and zero line are short-circuited, the loop resistance is small, and the current is large, which can cause the fuse to blow quickly or the protection device to cut off the power supply.
1.TN-C system
In this system, the protection line and neutral line are merged into PEN line, which has the advantages of simplicity and economy. In case of short-circuit fault, the fault current is large, which can make the current protection device act and cut off the power supply.
In this system, for lines with single-phase load and three-phase unbalanced load, there is always current flowing in the pen line, and its voltage drop will appear on the metal shell of the electrical equipment, which is unfavorable to sensitive electronic equipment. In addition, the weak current on the pen line may cause an explosion in a dangerous environment. Therefore, TN-C system cannot be used in explosive environment.
2.TN-S system
In this system, the protective wire and neutral wire are separated, so the system cost is slightly expensive. In addition to the advantages of TN-C system, the metal shell of electrical equipment connected by PE wire is not electrified during normal operation because the PE wire does not pass through the load current, which is suitable for data processing and power supply of precision electronic instruments and equipment, and can also be used in explosive dangerous environment. There are plugs with separate grounding contacts in civil buildings and household appliances. It is convenient and safe to use TN-S power supply.
3.TN-C-S system
The PEN line of this system is divided into protection line (PE) and neutral line (N) from point A. After separation, the N line should be insulated from the ground. In order to prevent confusion between PE line and N line, PE line and PEN line should be painted with yellow-green color code and N line with light blue color code. In addition, after separation, the PE line can no longer be merged with the N line.
TN-C-S system is a widely used distribution system. No matter in industrial and mining enterprises or civil buildings, its line structure is simple, and it can ensure certain safety.
Second, T-T system.
In the T-T system, there is a direct grounding point in the distribution system, which is generally the transformer neutral point. The metal shell of its electrical equipment is grounded with a separate grounding seat and has no electrical connection with the power supply. It is called protective grounding, which is suitable for the power supply of potential-sensitive data processing equipment and precision electronic equipment.
Three. Information technology system
The power supply of IT system is not grounded or grounded through impedance, and the exposed conductive part of electrical equipment can be directly grounded or connected to the power supply grounding body through protective wires, which is also protective grounding.
Because the fault current is small when the system fails for the first time, the metal shell of electrical equipment will not produce dangerous contact voltage, so the electrical equipment can continue to run without cutting off the power supply, and the fault can be eliminated through alarm device and inspection.
Fourth, the protective grounding range
Regardless of the grounding mode of the distribution system, the exposed conductive parts of the following electrical equipment and appliances should be grounded through protective wires (PE) (such as TT and IT systems) or connected to neutral wires (TN systems).
(l) Transformers, motors, electrical appliances, hand-held and mobile appliances.
(2) Transmission device of electrical equipment.
(3) Metal frame, power distribution cabinet and protection control panel frame of power distribution device.
(4) Metal protective tubes of distribution lines and metal junction boxes of switches.