The protective wiring method in which the metal part of the electrical appliance (that is, the metal structural part insulated from the live part) which may be charged after the insulation material is damaged or in other cases is reliably connected by the conductor and the grounding body. The grounding protection system has only phase and neutral lines. The three-phase power load can be used without a neutral line. As long as the equipment is well grounded, the neutral line in the system must have no ground connection except the neutral point of the power supply. The zero-connection protection system requires that the neutral line is protected in any case. If necessary, the protection neutral line and the zero-connection protection line can be erected separately. At the same time, the protection neutral line in the system must have multiple repeated grounding.
Introduction / Grounding Protection
Measures to ground the metal casing of electrical equipment. It can prevent strong current from passing through the human body when the metal casing is charged under the condition of insulation damage or accident, so as to ensure personal safety.
It is a kind of protective wiring method that connects the metal part of the electrical appliance (that is, the metal structure part insulated from the live part) which may be charged after the insulation material is damaged or in other cases, and the conductor is reliably connected with the grounding body. Grounding protection is generally used in the power supply system where the neutral point of the distribution transformer is not directly grounded (three-phase three-wire system) to ensure that the ground voltage generated when the electrical equipment leaks due to insulation damage do not exceed the safe range. If the household appliance is not protected by grounding, when the insulation of a certain part is damaged or a certain phase line touches the outer casing, the outer casing of the household appliance will be charged, and if the human body touches the outer casing (framework) of the electrical equipment damaged by the insulation, it will There is a danger of electric shock. On the contrary, if the electrical equipment is grounded, the single-phase grounding short-circuit current will flow through the two parallel branches of the grounding device and the human body. Generally speaking, the resistance of the human body is greater than 1000 ohms, and the resistance of the grounding body cannot be greater than 4 ohms according to regulations, so the current flowing through the human body is small, and the current flowing through the grounding device is large. This reduces the risk of electric shock to the human body after leakage of electrical equipment.
Protective earthing operation and precautions /Grounding Protection
The practice has proved that the use of protective grounding is an effective safety protection measure in China’s low-voltage power network. Since the protective grounding is divided into grounding protection and zero-connection protection, the objective environment used by the two different protection methods is different. Therefore, if improperly selected, it will not only affect the protection performance of the customer but also affect the power supply reliability of the power grid. Then, as a power customer in a public distribution network, how can we properly and reasonably select and use the protective ground?
Grounding protection and zero-connection protection
To understand and understand grounding protection and zero-connection protection, master the differences and scope of use of these two protection methods.
Grounding protection and zero-connection protection are collectively referred to as protective earthing. It is an important technical measure taken to prevent personal electric shock and ensure the normal operation of electrical equipment. The difference between these two protections is mainly manifested in three aspects: First, the protection principle is different. The basic principle of grounding protection is to limit the leakage current of the leakage device to the ground so that it does not exceed a certain safety range. Once the protection device exceeds a certain set value, the power supply can be automatically cut off. The principle of zero-connection protection is to use the zero-connecting line. When the device is damaged by the insulation and forms a single-phase metallic short circuit, the short-circuit current is used to prompt the protection device on the line to operate quickly. Second, the scope of application is different. According to relevant factors such as load distribution, load density, and load nature, the Rural Low Voltage Power Technical Regulations divides the scope of use of the above two power grid operating systems. TT system is generally applicable to rural public low-voltage power network, which belongs to the grounding protection mode in protective earthing; TN system (TN system can be divided into TN-C, TN-CS, TN-S) is mainly suitable for urban public low voltage A dedicated low-voltage power network for power customers such as power grids and factories and mines. This system is a zero-connection protection method in protective earthing. At present, China’s current low-voltage public power distribution network usually adopts TT or TN-C system, and implements single-phase and three-phase hybrid power supply modes. That is, three-phase four-wire 380/220V power distribution while supplying power to the lighting load and the power load. Third, the line structure is different. The grounding protection system has only phase and neutral lines. The three-phase power load can be used without a neutral line. As long as the equipment is well grounded, the neutral line in the system must have no ground connection except the neutral point of the power supply. The zero-connection protection system requires that the neutral line is protected in any case. If necessary, the protection neutral line and the zero-connection protection line can be erected separately. At the same time, the protection neutral line in the system must have multiple repeated grounding.
Selection of protection methods
According to the power supply system where the customer is located, the grounding protection and zero-connection protection method should be correctly selected.
What kind of protection should a power customer take? First, it must depend on what kind of power distribution system the power supply system is in. If the public distribution network where the customer is located is the TT system, the customer should adopt grounding protection in a unified manner; if the public distribution network where the customer is located in the TN-C system, zero-connection protection should be adopted uniformly.
The TT system and the TN-C system are two systems with their own independent characteristics. Although both systems can provide customers with the 220/380V single and three-phase hybrid power supplies, they can not only replace each other but also protect them. The requirements above are completely different. This is because, in the same power distribution system, if the two protection modes exist at the same time, the phase-to-ground voltage of the neutral line will rise to half or higher of the phase voltage in the case of a ground-protected device. At this time, all devices on the zero-protection (because the metal casing of the device is directly connected to the neutral line) will carry the same high potential, so that the metal parts such as the device casing exhibit a high voltage to the ground, thereby endangering the user. Security. Therefore, the same distribution system can only use the same protection method, and the two protection methods must not be mixed. Secondly, the customer must understand what is called protective grounding, and correctly distinguish the difference between grounding and zeroing protection. Protective grounding refers to the fact that household appliances, electrical equipment, etc. may be charged with metal casing due to insulation damage. The grounding provided to prevent such voltage from endangering personal safety is called protective grounding. The grounding protection of the metal casing with the protective earthing wire (PEE) directly connected to the grounding pole is called grounding protection. When the metal casing is connected to the protective conductor (PE) and the protective neutral conductor (PEN), it is called zero-connection protection.
Standard design, process standard
According to the different setting requirements of the two protection methods, standard design and construction process standards.
Standardize the design and construction process standards and requirements of the distribution lines in the customer’s power receiving buildings, and replace the indoor power distribution part of the newly built or renovated customer buildings with a local three-phase five-wire system or a single-phase three-wire system. The three-phase four-wire or single-phase two-wire power distribution mode in the TT or TN-C system can effectively realize the protection grounding of the client. The so-called “local three-phase five-wire system or single-phase three-wire system” means that after the low-voltage line is connected to the customer, the customer has to change the original traditional wiring mode, based on the original three-phase four-wire system and single-phase two-wire system wiring. On the top, each additional protection line is connected to each of the customer’s grounding wire terminals that need to implement the grounding protection electrical socket. In order to facilitate maintenance and management, the intersection of the indoor lead-out and the outdoor lead-in end of the protection line shall be installed on the power distribution board that the power supply is introduced, and then the access method of the protection line shall be separately set according to the power distribution system where the customer is located.
1, Setting requirements for TT system grounding protection line (PEE)
When the customer’s power distribution system is a TT system, the system requires the customer to take a grounding protection method. Therefore, in order to meet the grounding resistance value of the grounding protection, the customer should bury the artificial grounding device in the outdoor according to the requirements of the “Technical Regulations for Rural Low Voltage Power”. The grounding resistance should meet the following requirements:
Re grounding resistance (Ω)
Ulom is called the voltage limit (V). Under normal circumstances, it can be considered as the AC RMS value of 50V.
Operating current of residual current (leakage) protector adjacent to Iop (I)
For the average customer, as long as 40×40×4×2500 mm angle steel is used, it can be driven into the underground 0.6 m vertically by mechanical driving, which can meet the resistance requirement of the grounding resistance. Then, it is welded with a round steel with a diameter of ≥ φ8 and then led out to the ground for 0.6 m, and then connected to the protection wire (PEE) of the switchboard with the same material and type of wire as the imported power supply phase.
2, Setting requirements for the zero-protection line (PE) of TN-C system
Since the system requires the customer to adopt the zero-connection protection mode, it is necessary to add a special protection line (PE) on the basis of the original three-phase four-wire system or single-phase two-wire system, which is protected by the customer’s power receiving end. The protective neutral line (PEN) of the switchboard is taken out and connected with the original three-phase four-wire system or single-phase two-wire system. In order to ensure the safety and reliability of the whole system, special attention should be paid for the use. After the protection line (PE) is taken out from the protection neutral line (PEN), the neutral line N and the protection line (PE) are formed on the client side. The two wires cannot be combined into a (PEN) line during use. In order to ensure the reliability of the repeated grounding of the protection neutral line (PEN), the first and the end of the TN-C system mainline, all the branch T terminal rods, the branch end rods, etc. should be equipped with repeated grounding lines, and three-phase The four-wire system should also be grounded repeatedly at the entrance bracket of the subscriber line, before the (PEN) line is divided into the neutral line (N) and the protection line (PE). The wire cross-section of the protective neutral (PEN), neutral (N) or protective wire (PE) is always selected according to the wire type and section standard of the phase line.
Protective earthing and shield grounding/Grounding Protection
1, Protected area:
The cabinets are all inside. For example, there is usually no place in the cabinet where there is no paint, and then the wires are connected. This is the grounding of the cabinet body. The ground wire inside the power supply (that is, the yellow-green phase) is also the role. Its purpose is to prevent the cabinet from being charged.
2, the protection area is generally done by electrical appliances
3 power ground:
This line, usually through the power supply, returns to the transformer center line and then enters the ground. In some places, this and the protected area are one, and some places are not one.
1, Also called the instrument ground:
It should be noted that the instrument ground wire should be prevented from contacting the electrical/protective ground during the connection process, otherwise it will lose its meaning.
2, Shielding attention:
When using the shielded cable, use single-ended grounding. Do not ground the shielded wire in the field. Pay attention to clean up. In the main control room, braid the shield wires of several cables and connect them to the shield grounding terminal of the cabinet. (Good cabinets have grounded copper strips and are insulated from the cabinet)
3, Specific analysis
The shield grounding terminal of the cabinet is connected with the instrument shield grounding. This makes it possible to connect the grounding of the instrument in general. It has analog ground, digital ground, low voltage power ground, high voltage power supply (220v), and several types of protection. In the control center, the point grounding is carried out, the grounding resistance is 1 ohm, and if it is not 4 ohms, the grounding wires of various different lines are first collected to a special grounding point. Then connect all the grounding points to the summary location, the grounding regulations for each site, the analog ground, the digital ground low-voltage power ground wires are respectively concentrated, and then connected with the ground signal grounding point, and finally connected to the cable shield, high-voltage power ground and protection After ground connection, the grounding resistance is 4 ohms, and the two field grounding points are insulated. The insulation resistance should be specified according to the requirements of the sensor, but it must be greater than 0.5 megohms. That is to say, the signal loop is grounded at one end, and the field protection ground has a front grounding protection as a signal ground to prevent the ground breakdown due to the induced voltage. If the two ends are grounded, an inductive loop will be formed, which will induce an interference signal and be self-defeating. If you feel uneasy, you can use the indirect zinc oxide varistor surge absorber on site and on-site protection. The voltage level is less than the maximum voltage that the sensor can withstand. Generally, do not exceed the supply voltage of 24 volts. Shielding has two meanings, electromagnetic shielding, and electrostatic shielding, which refer to the shielding of magnetic circuits and circuits, respectively. The usual copper mesh shielding wire has no effect on the magnetic circuit, so only the shielding of electrical interference, that is, electrostatic shielding, is considered. At this time, the shielding layer must be grounded (the magnetic circuit is shielded without grounding). The principle is basically the same: the interference source and the receiving end are equivalent to the two poles of the capacitor. One side of the voltage fluctuation will sense the other end through the capacitor. The intermediate layer (that is, the shield) that is inserted into the ground destroys this equivalent capacitance, thereby cutting off the interference path. Be careful to connect to the ground of the signal you want to protect when grounding, and only connect at one end of the shield. Otherwise, there will be a large current (ground current loop) causing damage when the potentials on both sides are not equal.