In order to ensure the safe use of low-voltage power distribution systems and electrical equipment, electrical appliances, must take appropriate measures to prevent the risk of electric shock to the user and electrical equipment, electrical appliances burned. Grounding is a common method, because the earth is a conductive stratum, the potential of any point is usually taken to zero, that is, zero potential (when the single-phase grounding, from the grounding point of 20m and beyond as zero potential).
The electrical equipment, electrical appliances, if the metal shell and the earth connection, then the metal shell is close to zero potential. Even in case of failure, such as the occurrence of electrical equipment due to insulation damage caused by touching the shell short-circuit, due to the metal casing has been made with the earth for a good electrical connection, the potential difference between the metal casing and the earth becomes low, if the person in contact with the human body through the current is also small, improving the safety of indirect electric shock.
Low-voltage power distribution system, more distribution neutral grounding (known as work grounding). From the point of view of electrical safety, under certain conditions, can be with the grounding of electrical equipment **** the same role. When the ground fault, the current generated can make the distribution system in the appropriate time to act on the protective equipment, cut off the power supply, used to ensure safety.
Because the metal casing of electrical equipment and appliances can be directly grounded, can also be connected to the distribution system through the conductor has been grounded on the neutral point, the distribution system can be directly grounded or not grounded or grounded through the impedance, the combination of these grounding is known as the grounding of low-voltage power distribution system.
Second, the basic components of the grounding method
The components of the grounding method can be divided into two parts of the electrical equipment and distribution system.
1. Electrical equipment grounding part
(1) grounding body: close contact with the earth and the earth to form an electrical connection or a group of conductors.
(2) exposed conductive part: electrical equipment can touch the conductive part. Normal non-electrical, fault may be charged, usually for the metal casing of electrical equipment.
(3) the main grounding terminal board: a building or part of a building within a variety of grounding (such as work grounding, protective grounding) of the terminals and equipotential connecting line of the combination of terminals. If arranged in rows, it is called the main grounding terminal row.
(4) protection wire (PE): the exposed conductive part of the above, the main grounding terminal block, grounding and power grounding (or artificial grounding) any part of the conductor for electrical connection. For the connection of more than one exposed conductive part of the conductor is called the protective trunk.
(5) grounding wire: the main grounding terminal board or the exposed conductive part directly to the grounding body of the protective line. For connecting more than one grounding terminal board grounding wire is called the grounding trunk.
(6) equipotential connection: refers to the exposed conductive part and the device outside the conductive part of the potential is substantially equal to the electrical connection.
2. grounding part of the distribution system
(1) phase line (L). A conductor that carries electrical energy and is not normally grounded.
(2) Neutral wire (N). A conductor connected to the neutral point of the system and capable of acting as a conveyor of electrical energy.
(3) protective neutral (PEN). A conductor that serves as both a protective and neutral wire.
(4) Power Earthing Point. Grounding the power supply at a point where it can be grounded (usually the neutral).
Third, the classification of grounding
China's power distribution system grounding has been used IEC regulations, the classification is still to the distribution system and electrical equipment grounding combinations to points, generally divided into TN, TT, IT systems, etc.. The meaning of the above letters: the first letter indicates the relationship of the power supply grounding point to the ground. Where T indicates direct grounding; I indicates no grounding or grounding through impedance. The second letter indicates that the exposed conductive part of the electrical equipment and ground relationship. Where T indicates a separate direct grounding without connection to the power supply grounding point; N indicates a direct connection to the power supply system grounding point or to a conductor leading from that point.
Based on whether the neutral and protective wires are combined, the TN system is divided into TN-C, TN-S and TN-C-S system.
TN-C system: the protective and neutral wires are combined into a PEN wire.
TN-S system: protection line and neutral line are separated.
TN-C-S system: the protective and neutral wires are combined into PEN wire in a section near the power supply side, and are divided into protective and neutral wires from a certain point onwards.
Section II, the application of various grounding
In the low-voltage power distribution system, the exposed conductive part of the electrical equipment is often grounded for protection against indirect electric shock.
A, TN system
In the TN system, all the exposed conductive part of the electrical equipment are connected to the protection line, and connected to the power source of the ground point, the ground point is usually the neutral point of the distribution system.
TN system, called protective zero. When the fault so that the metal casing of electrical equipment with when, the formation of phase and zero line short circuit, circuit resistance is small, the current is large, can make the fuse quickly fuse or protective device action to cut off the power supply.
1.TN a C system
The system of protective wires and neutral wires combined for PEN line, has the advantages of simple, economical. When a short-circuit fault occurs, the fault current is large enough to make the current protection device operate and cut off the power supply.
This system for single-phase loads and three-phase unbalanced loads of the line, the PEN line always has a current flow, which produces a voltage drop that will be presented in the metal casing of the electrical equipment, which is detrimental to sensitive electronic equipment. In addition, the weak current on the PEN line may cause an explosion in a hazardous environment. Therefore, TN-C system cannot be used in explosion hazardous environment.
2. TN-S system
The system in the protective line and the neutral line separately, the system is slightly more expensive. In addition to the advantages of the TN-C system, due to the normal PE line does not pass the load current, so the PE line connected to the metal casing of electrical equipment in normal operation is not charged, so it is suitable for data processing and precision electronic equipment power supply, but also can be used for explosion-hazardous environments. Inside civil buildings, household appliances, etc. have separate grounding contacts for plugs. The use of TN-S power supply is both convenient and safe.
3. TN - C a S system
The system PEN line from the A point separated for the protection line (PE) and neutral (N). After the separation of the N line should be insulated to ground. In order to prevent confusion between PE and N wires, PE and PEN wires should be color-coded yellow and green, and N wires should be color-coded light blue. In addition, since the separation, PE wire can not be merged with N wire again.
TN-C-S system is a widely used distribution system, whether in industrial and mining enterprises or in civil buildings, its line structure is simple, but also to ensure a certain level of safety.
Two, T a T system
In the T-T system, its distribution system part of a direct grounding point, generally the transformer neutral point. The metal casing of its electrical equipment with a separate grounding cupping ground, and power in the ground without electrical connection, called protective grounding, for potential-sensitive data processing equipment and precision electronic equipment power supply.
Three, IT systems
IT system power supply is not grounded or grounded through the impedance, the exposed conductive part of the electrical equipment can be directly grounded or connected to the power supply through the protective line on the grounding body, which is also the protection of grounding.
Because of the first fault in this system when the fault current is small, the metal casing of the electrical equipment will not produce dangerous contact voltage, so you can not cut off the power supply, so that the electrical equipment to continue to operate, and can be eliminated through the alarm device and inspection of the fault.
Four, the scope of protective grounding
No matter what kind of distribution system grounding, the following electrical equipment and electrical appliances of the exposed conductive part of the ground should be through the protective wire (PE) (such as TT, IT systems) or to the neutral line (TN system).
(l) Transformers, motors, electrical appliances, hand-held and mobile appliances.
(2) Transmissions of electrical equipment.
(3) Metal frames for power distribution units, distribution cabinets and frames for protection and control panels.
(4) Metal protection tubes of distribution lines, metal junction boxes of switches, and so on.