1. Under the same power supply system (such as transformer), some equipment cannot be grounded by protection, and some equipment can be grounded by protection.
2. The risk of protective zero connection is greater, because if the zero line is broken, the equipment shell of protective zero connection will be charged by single-phase equipment, so the protective zero connection should be led from the main line, and must not be led from the branch line. In addition, it will be safer if the protection is grounded repeatedly.
3. Protective grounding generally refers to TT grounding system, which is characterized in that the grounding of equipment (protective grounding) is separated from the working grounding of power supply, so the protective grounding of power supply and working grounding will have grounding resistance, so once the equipment leaks, the working grounding resistance of power supply will generate voltage drop, and the voltage level is directly proportional to the protective grounding resistance and working grounding resistance of power supply. The larger the resistance value, the higher the voltage. Because the neutral point of the power supply is grounded, the zero line will be charged due to the voltage drop of the working grounding resistor, so the equipment shell connected with the protective zero line will be charged through the zero line, so it has little to do with the distance.
4. The pe line is a TN-s system (the PE line is directly connected to the equipment shell from the neutral point of the power supply with wires, so the current directly returns to the neutral point of the power supply through the PE line, forming a strong short-circuit current, and the switch will trip quickly, thus cutting off the fault current and ensuring safety.
5. If the earth is regarded as the PE line, it is actually equivalent to returning to TT system. The characteristic of TT grounding system is that the grounding of equipment (protective grounding) and power supply are separated, and there is no wire connection between them. Because both the protective grounding and working grounding have grounding resistances, after the equipment leaks, the current will return to the neutral point through the protective grounding resistance and the working grounding resistance, so the grounding current will not be very large, so the main switch will not trip, so that the fault current will always exist, resulting in a voltage drop on the protective grounding, which will make the equipment shell charged for a long time. It will be safer if a leakage switch is installed.
6. The earth is conductive, but equipment grounding must have grounding resistance, so it can't be simply regarded as a point, but the grounding resistance should be taken into account, so it can't be regarded as a point, and it can be regarded as an electrical circuit with series resistance. The resistance of wires is very low and can be ignored.
When do you need grounding?
Ensure equipment and personal safety, and grounding is required when lightning protection is carried out.
The grounding wire in the power system is a lifeline to some extent. As long as electrical facilities and equipment endanger people's life safety, grounding wires must be installed.
What are the requirements for protecting ground wire and zero wire?
Safety technical requirements for grounding and neutral protection system. TN-S protection is adopted in the power distribution system of all construction sites equipped with special transformers, and the power supply of neutral point protection system adopts three-machine five-wire system. Second, in the power supply system, it is forbidden to connect part of the equipment protection to zero, while grounding the other part of the equipment protection. Three, protect the zero line shall not be installed switch or fuse. The protective zero line shall be laid separately and shall not be used for other purposes. Repeated grounding wire should be connected with protective zero line. Four, the metal shell of electrical equipment must be connected to the special protective zero line, and the protective zero line of electrical equipment should be connected in parallel to the protective zero line terminal board. The cross section of the protective zero line connected with electrical equipment shall not be less than 2.5 square centimeters, and the protective zero line must adopt green/yellow double-color lines. Under no circumstances are green/yellow double-color lines allowed as load lines. Five, the same electrical equipment repeated grounding and lightning protection grounding can use the same grounding body, grounding resistance should meet the requirements of repeated grounding resistance. Six, power transformer neutral grounding resistance shall not be greater than 4 Ω, repeated grounding resistance shall not be greater than10 Ω, the total distribution box and power transformer distance more than 50M and above, the protection of the zero line should be repeated grounding. Seven, ground wire to the ground, should be set up to test the bolt clamp. The grounding wire of each grounding device should use more than two conductors, each with a length of 2.5M and a horizontal buried depth of not less than 0.6M The minimum cross section of the conductors of the grounding device:
Model, specification, unit, diameter of round steel connected to horizontal grounding body, MM 6 8, flat steel section, MM2 24 48, angle steel thickness, mm 3, long steel pipe thickness, mm 3.5.
Please specify when the equipment adopts neutral protection and when it adopts grounding protection.
Finally, you have to be grounded, so you don't have to struggle.
Under what circumstances are protective grounding and protective zero connection used respectively? Can they be used in the same system? Why? (detailed explanation of reasons)
The main differences between protective grounding and protective grounding are:
(1) protection principle is different. Protective grounding is to limit the voltage to the ground after the equipment leaks, so that it does not exceed the safe range. In high voltage system, protective grounding not only limits the voltage to the ground, but also promotes the action of power grid protection devices in some cases. The zero connection of protection is to make the equipment leak to form a single-phase short circuit with the help of zero line, so as to prompt the protection device on the line to act and cut off the power supply of the faulty equipment. In addition, in the protective neutral network, the protective neutral line and repeated grounding can also limit the voltage to ground when the equipment leaks electricity.
(2) Different scope of application Protective grounding is suitable for high and low voltage power grids that are generally ungrounded, and also for low voltage power grids that have taken other safety measures (such as installing leakage protectors); Protective zero connection is only applicable to low-voltage power grid with neutral point directly grounded.
(3) If protective grounding measures are taken for different line structures, there can be no working zero line in the power grid, and only protective grounding lines can be set; If protective zero connection measures are taken, the working zero line must be set for zero connection protection. The protective neutral wire should not be connected with the switch and fuse. When the circuit breaker such as fuse is installed on the neutral line, the protective grounding line or neutral line must be installed separately.
Five wires in the three-phase five-wire system refer to: three-phase wires plus a grounding wire and a zero wire. The most widely used low-voltage transmission mode is three-phase four-wire system, which uses three-phase wire and a zero line to supply power. The neutral line leads from the neutral point of the transformer and is grounded, with a voltage of 380/220V. Any one phase line and the neutral line form a general household 220 V power supply line, and the voltage between three-phase lines is 380V, which is generally used by power suppliers.
The three-phase five-wire system has one more grounding wire than the three-phase four-wire system, which is used in places with high safety requirements and unified grounding of equipment.
The knowledge of three-phase five-wire system lies in these two "zero lines". When used in the power grid of more precise electronic instruments, if the zero line and the ground line are used in the same line, it will have an impact on the working zero point in the circuit, although theoretically they are both zero potential points. If there are occasional surge pulses that impact the working zero line, the zero line and the ground line are not separated. For example, in some electronic circuits, this pulse is caused by phase line leakage.
The fundamental difference between neutral wire and ground wire is that they form a working loop, one of which is called protective grounding, the other is connected to the power grid and the other is grounded. In electronic circuits, these two concepts should be distinguished. In a regular company, these two wires are required to be connected separately.
At present, there is actually a three-phase six-wire connection mode, in addition to the working zero line and protective grounding, there is also a special grounding wire. This wire is connected with the equipment grounding wire separately, and is not connected with any other wire, which is used to protect instruments and equipment. Because the damage of electrical components is often only a few microseconds, it is necessary to guide the misoperation current back to the earth more quickly, and the instrument needs to be directly grounded.