State grid on the secondary line control cabinets, end solenoid box internal grounding, zeroing and equipotential grounding requirements specification

To protect personal safety for the purpose of electrical equipment is not electrically charged metal enclosure grounded or zeroed, called protective grounding and protective grounding. First, protective grounding in the neutral point is not grounded in the three-phase power system, when the electrical equipment connected to this system due to insulation damage and make the shell charged, if a person stands on the ground and touches the shell, due to the transmission line and the ground between the existence of distributed capacitance, there will be a current through the human body and the distributed capacitance back to the power supply, so that the person is electrocuted, as shown in Figure 6-7-13. In general this current is not large. However, if the grid is widely distributed, or the grid insulation strength is significantly reduced, this current may reach a dangerous level, which necessitates safety measures. Figure 6-7-13 One phase of a motor without protective earthing touching the shell Protective earthing is to connect the metal casing of electrical equipment reliably to the earth with a sufficiently thick metal conductor. Electrical equipment using protective grounding measures, the equipment shell has been through the conductor and the earth has good contact, then when the human body touches the charged shell, the human body is equivalent to a parallel branch of the grounding resistance, as shown in Figure 6-7-14. As the human body resistance is much greater than the grounding resistance, so the current through the human body is very small, to avoid electrocution. Figure 6-7-14 is equipped with a protective grounding of the motor a phase of the shell touching the case of protective grounding applied to the neutral point is not grounded in the power distribution system. Second, the protection of zero (a) the concept of the so-called protection of zero (also known as zero protection) is in the neutral grounding system, the electrical equipment under normal circumstances is not charged metal parts and zero line for a good metal connection. Figure 6-7-15 is a schematic diagram of the fault current in the case of protective zeroing. When a phase of insulation damage to the phase line touching the shell, the shell charged, because the shell using the protection of zero measures, so the phase line and the zero line constitutes a circuit, a single-phase short-circuit current is large enough to make the line on the protective device (such as fuses) to fuse quickly, so that leakage of equipment and the power supply is disconnected, thus avoiding the possibility of personal electric shock. Figure 6-7-15 Protective Zeroing Protective zeroing is used in 380/220V, three-phase, four-wire system, the neutral point of the power supply is directly grounded distribution system. In the power distribution system where the neutral point of the power supply is grounded, only protection zero can be used, and if protection grounding is used, it is not effective in preventing personal electric shock accidents. As shown in Figure 6-7-16, if the use of protective grounding, power neutral grounding resistance and electrical equipment grounding resistance are considered according to 4Ω, and the supply voltage is 220V, then when the insulation of the electrical equipment is damaged so that the electrical equipment shell charged, then the current between the two grounding resistors will be: Figure 6-7-16 Neutral grounding system to adopt the consequences of the protective grounding of the fuse rated current is based on the requirements of the protected equipment. The requirements of the protected equipment selected, if the equipment is easy to larger, in order to ensure that the equipment works under normal conditions, the rated current of the selected fuse will also be larger, in the 27.5A grounding short-circuit current under the action of the fuse will continue to fuse, the shell of the electrically charged electrical equipment can not be immediately detached from the power supply, so in the shell of the equipment for a long time on the ground voltage Ud, the value of which is: Ud = 27.5 × 4 = 110V Obviously, this is very dangerous. If the protective grounding resistance is greater than the neutral grounding resistance of the power supply, the equipment shell of the ground voltage is also high, which is even more dangerous. (B) the system using protective zero need to pay attention to the problem 1. In the protective zero system, the zero line plays a very important role. Once the zero line breaks, connected to the broken line behind a section of the line on the electrical equipment, equivalent to no protection zero or protection grounding. If there is a break in the zero line behind the shell of the electrical equipment leakage, can not constitute a short circuit circuit, so that the fuse is blown, not only the shell of this equipment is charged for a long time, but also connected to the break in the back of all the equipment for the protection of the shell of zero voltage close to the power supply phase voltage of the ground voltage, the risk of electrocution will be enlarged, as shown in Figure 6-7-17 (a). For single-phase power-using equipment, even if there is no leakage from the enclosure, with the zero wire disconnected, the phase voltage will appear on the enclosure of the power-using equipment through the load and the section of the zero wire behind the disconnected place, as shown in Figure 6-7-17(b). Figure 6-7-17 Consequences of Zero Wire Disconnection When Protective Zeroing is Used The connection of the zero wire should be firm, reliable, and in good contact. The connection of the zero wire to the equipment should be crimped with bolts. The zero wire of all electrical equipment should be connected to the zero wire in parallel, and series connection is not allowed. Installation of fuses or separate disconnect switches on the zero line is prohibited. In the environment of corrosive substances, in order to prevent the corrosion of the zero line, the surface should be coated with the necessary anticorrosive coating. 2. The power supply electrical point is not grounded in the three-phase four-wire system of power distribution system, is not permitted to use the protection of the zero, and can only be used with the protection of the grounding. In the power neutral point grounding power distribution system, when a phase line and the earth contact, through the grounded phase line and the power neutral point grounding device of the short-circuit current, can make the fuse fuse, immediately cut off the fault line. But in the neutral point is not grounded in the distribution system, any phase of grounding, although the system can still operate as usual, but then the earth and grounded phase pin equipotential, then connected to the zero line on the shell of the electrical equipment to the ground voltage will be equal to the grounded phase from the grounding point to the neutral point of the voltage value of the power supply, it is very dangerous, as shown in Figure 6-7-18. Figure 6-7-18 neutral point not grounded system using the consequences of protective zero 3. In the use of protective measures, care must be taken not to allow a part of the equipment on the same system to zero, another part of the electrical equipment grounded. In Figure 6-7-19, when the shell of the grounded equipment leakage occurs touching the shell, and the accident caused by the current burns out the fuse, the equipment shell is charged 110V, and make the entire zero line to ground potential rises to 110V, so the other zero equipment shell to ground are 110V potential, which is very dangerous. It can be seen that on the same system is not allowed to use part of the equipment zero, part of the equipment grounding mixed practices. Even if the fuse meets the requirements can be blown, but also does not allow mixed connection. Because fuses are often switched in use, it is difficult to ensure that no errors are made. Figure 6-7-19 incorrect zero protection 4. In the use of protective zero system, but also in the power supply neutral point for the work of grounding and zero line at certain intervals and terminals for repeated grounding. In the three-phase four-wire power distribution system, the distribution transformer vice neutral point through the grounding device and the earth directly connected to the ground is called work grounding. The neutral point of the power supply to ground, can reduce the voltage of each phase of the power supply to ground, when a person touches a phase of the power supply, the human body is subjected to phase voltage. In the neutral ungrounded system, when a phase line grounded, the human body touches another phase line, the role of the human body is the line voltage of the power supply, the danger is great. At the same time, the neutral point of the distribution transformer grounding, for the use of protection zero way to provide the necessary conditions. Work grounding resistance shall not be greater than 4Ω, as shown in Figure 6-7-20. Figure 6-7-20 work grounding schematic (a) power neutral ungrounded system (b) power neutral grounding system in the neutral grounding system, in addition to the neutral point of the distribution transformer for the work of grounding, along the zero line towards the zero line again in one or more places to be grounded, called repeat grounding. Repeat the role of grounding is when the shell of the electrical equipment leakage can reduce the zero line voltage to ground; when the zero line is broken, can also reduce the risk of electric shock. When the equipment shell leakage, as mentioned above, after the phase line, the zero line constitutes a short-circuit circuit, the short-circuit current can quickly fuse, cut off the circuit, the metal casing is also followed by no electricity, to avoid the risk of electric shock. But from the equipment shell leakage to the fuse to fuse after a short period of time, in this short period of time, the equipment shell there is a ground voltage, the value of the short-circuit current in the zero line voltage drop. In this short period of time, if someone touches the equipment shell, it is still very dangerous. If in close proximity to the equipment, and then add a grounding device, that is, the implementation of repeated grounding, as shown in Figure 6-7-21, the equipment shell of the voltage to ground can be reduced. Figure 6-7-21 repeat grounding In addition, if there is no repeat grounding, when the zero line somewhere in the event of disconnection, in the disconnection at the back of all the electrical equipment in neither protective zero, and no protective grounding state. Once a phase of the power supply touch shell, disconnect the zero line behind and connected to the shell of the electrical equipment will be equal to the phase voltage on the ground voltage, is very dangerous, as shown in Figure 6-7-22. Figure 6-7-22 without repeated grounding when the zero line breaks in the case of repeated grounding, when the zero line is occasionally broken, the occurrence of electrical equipment shell charged, the phase voltage through the leakage of the equipment shell, and repeated grounding resistance, work grounding resistance constitutes a circuit, the flow of current, as shown in Figure 6-7-23. Leakage equipment shell of the voltage to ground for the phase voltage in the repetition of the grounding resistance of the voltage drop, so that the degree of danger of the accident has been reduced, but still dangerous to people, therefore, the zero line break accident should be avoided as far as possible. Figure 6-7-23 with repeated grounding when the zero line breaks in the line for zero protection, overhead line trunk and branch line terminals and every kilometer along the line, the zero line should be repeated grounding. Cable lines and overhead lines in the introduction of the building, the zero line should also be repeated grounding, but if there are no special requirements, not more than 50m from the grounding point of the building can not be repeated grounding. Third, the scope of application of protective zero and protective grounding for the following metal parts of electrical equipment should be taken to protect the zero or protective grounding measures. (1) motors, transformers, electrical appliances, lighting equipment, portable and mobile electrical appliances, etc., the base and shell; (2) the transmission of electrical equipment; (3) voltage and current transformers, the secondary winding resistance; (4) distribution panels and control panels of the frame; (5) indoor and outdoor power distribution devices, metal frame, reinforced concrete and metal reinforcement of the main enclosure; (6) threading of the steel pipe, metal junction box and cable head, The shell of the box; (7) the tower of the power line equipped with lightning conductors and switchgear installed on the distribution line poles and the shell of the capacitor.