Why do neutral conductors need to be grounded repeatedly? What does it do? Presumably, most of the electrical personnel are in the clouds.
In fact, in our electrical design generally do not need to repeat the grounding, but if it is civil electricity, the distribution line is too long, in order to avoid the unbalanced current in the neutral line to form too high a voltage, in the distribution box where the neutral line is re grounded, to ensure the safety of electricity, even if the zero line is broken will not occur dangerously. That's what it does, and in general, it's about the safety and reliability of electricity.
1. Introduction
When the power side (transformer or generator) or load side is a star wiring, the common contact at the head (or tail end) of the three-phase coil is called the neutral point. Neutral points are divided into power supply neutral points and load neutral points. The wires that are led out of the neutral point are called neutral lines, or the center line for short.
If the neutral point is directly connected to the grounding device to obtain a reference zero potential of the earth, the neutral point is called the zero point, and the wire leading out of the zero point is called the zero line. Usually one of the two lines of a 220-volt single-phase loop is called the "phase line" or "fire line", while the other line is called the "zero line" or "ground line".
The names "fire line" and "ground line" are only a common name in practice, especially the name of "ground line" is not accurate. Strictly speaking, it should be: if the power side of the circuit (the neutral point of the three-phase distribution transformer) is grounded, it is called "zero line"; if it is not grounded, it should be called "middle line" so as not to be mixed with the "ground wire" in the grounding device.
When it is a three-phase line, in addition to three phase lines, a wire can be led from the neutral point, that is, the neutral point, thus constituting a three-phase four-wire line. The voltage between the phase lines in this line is called the line voltage, and the voltage between the phase line and the zero line is called the phase voltage. Whether the neutral point is grounded is also known as the neutral point system.
The neutral point system can be roughly divided into two categories, namely neutral point grounding systems and neutral point insulation systems. According to the provisions of the International Electrotechnical Commission (IEC), the low-voltage distribution system is divided into IT, TT, TN three types, of which the TN system is divided into TN-C, TN-S, TN-C-S three categories.
2. Overview of it, TT, TN three kinds of low-voltage power supply system
3. It should be done under the neutral point insulation operation mode
(1) All electrical equipment must adopt protective grounding, and it is not allowed to use protective grounding;
(2) The mechanical strength of the neutral line should be the same as the phase line, and the neutral line should not be allowed to be disconnected;
(3) The neutral line current should not exceed 25% of the rated power supply of the secondary coil of the transformer, and the three-phase load current should not be too different, so as not to affect the balance of the three-phase voltage;
(4) Eliminate the direct grounding of the neutral line, and the low-voltage distribution panel must be provided with a three-phase insulation monitoring device in order to timely find and eliminate the grounding fault in the low-voltage power grid;
(5) Four lightning arresters should be installed on the secondary side of the distribution transformer to prevent lightning overvoltage.
4. The neutral point should be directly grounded under the operation mode
(1) All metal parts of the electrical equipment that are not charged under normal circumstances must adopt protective zeroing or protective grounding;
(2) In the same low-voltage distribution system of three-phase four-wire system, the protection of zero and the protection of the ground can not be mixed, that is, part of the use of protection of the zero, while the other part of the use of protection ground, but if the same equipment at the same time to use the protection of the zero and protect the ground is allowed, because its safety effect is better;
(3) The neutral conductor must be repeatedly grounded, because in the case of the neutral wire being disconnected, the zeroing equipment shell has a ground voltage of 220V, which is absolutely not allowed.
5. The N line of the power supply line is repeatedly grounded
The difference between neutral conductor (zero line) and ground wire is in the power frequency low-voltage circuit, which simply speaks of their structural and principle differences.
(1) The difference in structure: the zero line (N) is the trunk line after the neutral point of the transformer is grounded; the ground wire (PE) is the trunk line after the neutral point of the transformer is grounded. According to the standard, the grounding is repeated every interval of 20-30 meters.
(2) The difference in principle: the zero line (N) is mainly used in the working circuit, and the voltage generated by the zero line is equal to the line resistance multiplied by the current of the working circuit. Due to the long-distance transmission, the voltage generated by the zero line cannot be ignored, and it becomes unreliable as a measure to protect personal safety; the ground wire (PE) is not used for the working circuit, only as a protective line. Using the absolute "0" voltage of the earth, when the equipment shell leaks, the current will quickly flow into the earth, and even if the PE line has an open circuit, it will flow from the nearby grounding body to the earth.
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