What is a three phase four wire system and what is a three phase five wire system

Three-phase four-wire system: three phase wires (live wire), one neutral wire (N) three-phase five-wire system: three phase wires (live wire), one neutral wire (N), one ground wire (E) protective grounding:

It is a safety measure for the grounding of the metal shell of the electrical equipment. It can prevent strong current from passing through the human body when the metal shell is energized in the case of insulation damage or accidents, so as to ensure personal safety. If there is no protective grounding:

In the case of leakage of the metal shell, if the human hand touches the metal shell, it will cause an electric shock accident, if there is a protective grounding: the live shell will be connected to the ground through the grounding wire, and when the person touches the equipment shell, there will be no danger!

Protection zero: a way to connect the metal shell of the electrical equipment with the zero line of the power grid, when the three phase wires (live wire) leakage, the leakage protection switch can detect the leakage current and can trip in time, without hurting people, the so-called protection zero line.

But one thing to remember: the neutral line is not absolutely uncharged!

The so-called three-phase four-wire system is three live wires and one neutral wire, that is, the neutral wire. The three-phase wireless system is the so-called three live wires and one neutral wire and one grounding wire! Is this satisfying?

First, the role is different:

Three-phase four-wire system.

Phase line A, B, C, protect the neutral line PEN, there is a working current on the PEN line, and the PEN should be repeatedly grounded when entering the power building.

It belongs to the TN-C grounding system.

Three-phase five-wire. System: phase wire A, B, C, neutral line N, protective grounding wire.

PE, N wire has a working current passing through, PE line.

There is no current at normal time (there is only a fault current in the event of leakage or short circuit to the ground); The power distribution mode of civil buildings in China adopts TN-S grounding system.

Second, the meaning is different:

The three-phase four-wire system is three sine waves that derive a difference of 120 degrees.

voltage of the live wire, plus a neutral point.

The zero line that is drawn out. Residential electricity is taken from a line of fire and flows back to the generator from the zero line. The three sine waves at the neutral point cancel each other out, and the voltage is 0, so it is called the zero line (n).

The three-phase five-wire system is based on the three-phase four-wire system and adds an additional grounding ground wire (PE), which is connected to the metal surface of the electrical appliance and uses the zero potential of the earth to guide away the leaked electricity.

Note that the "three-phase four-wire" system is one of the classifications of live conductor systems

It has nothing to do with the classification of the grounding system, and care should be taken to avoid the erroneous name of "three-phase five-wire system", and the TN-S system is not "three-phase five-wire system". Any grounding system can be used with any electrical conductor system. For example, a three-phase four-wire live conductor system can use a TN-S grounding system, a TN-C-S or TT grounding system.

All three types of grounding systems end with five wires, all of which can be called "three-phase five-wire systems".

The above content reference: Encyclopedia - three-phase four-wire system.

Today, I learned that there are three forms of three-phase electricity, and many electricians are confused and never understand.

1. Three-phase four-wire system: refers to R yellow, S green, T red, N blue or black line or R yellow, S green, T red, rehmannia plus green two-color line.

In the low-voltage distribution network, the transmission line generally adopts a three-phase four-wire system, in which three lines represent A, B, C three phases, and the other is a neutral line N or Pen. In the single-phase transmission line that enters the user, one is called the phase line L, and the other is called the middle line N, and in the three-phase system, when the three-phase is balanced, the neutral line (neutral line) is current-free, so it is called the three-phase four-wire system.

2. Three-phase five-wire system: refers to R yellow, S green, T red, N blue or black, rehmannia and green two-color lines.

3 phase wires, plus a ground wire, and a neutral wire. The most widely used low-voltage transmission mode is the three-phase four-wire system, which adopts three phase wires plus the neutral line for power supply, the neutral line is drawn out and grounded by the neutral point of the transformer, the voltage is 380 220V, and any phase wire plus the neutral line is taken to form a 220V power supply line for general household use.

1. The three-phase four-wire system is the phase line A, B, C, to protect the zero line Pen, there is a working rush current on the Pen line, and the Pen should be repeatedly grounded when entering the electricity building; It belongs to the TN-C grounding system.

2. The three-phase five-state judgment line system is phase line A, B, C, neutral line N, protective grounding wire PE, N line has working current through, PE line usually has no current; The power distribution mode of civil buildings in China adopts TN-S grounding system.

3. In the TN-C system, the protection line and the neutral sail elimination line are combined into the PEN line, which has the advantages of simplicity and economy. When a grounding fault occurs, the fault current is large, and the general overcurrent protection electrical appliance can be used to cut off the power supply to ensure safety. At the same time, because the PEN wires are often electrically connected to each other in the same building, when the PEN wire is broken or the phase wire is directly short-circuited to the earth, it will present a fairly high fault voltage to the ground.

4. In the TN-S system, the protective wire is separated from the neutral wire, which has the advantages of TN-C system, because the PE wire does not pass the load current under normal circumstances, and the metal shell of the electrical equipment connected to the PE wire does not have potential, so it is suitable for data processing and power supply of precision electronic equipment, and can also be used in dangerous environments.

At present, most of the power supply systems are three-phase five-wire system and three-phase four-wire system, which is the lack of ground wire.

The three-phase five-wire system includes three phase wires (A, B, C lines) and neutral wires (N lines) of three-phase electricity; and ground wire (PE line).

The neutral line (n-line) is the zero line. When the three-phase load is symmetrical, the sum of the current vectors flowing into the neutral line of the three-phase line is zero, but for a single phase, the current is not zero. When the three-phase load is asymmetrical, the current vector sum of the neutral wire is not zero, which will produce a voltage to the ground.

The three-phase five-wire system is divided into TT grounding mode and TN grounding mode, of which TN is specifically divided into three modes: TN-S, TN-C, TN-C-S.

TT Grounding Method:

The first letter T represents the neutral point grounding of the power supply, and the second T is the grounding of the metal shell of the equipment, which is commonly used in high-voltage systems, and should not be used when there are large-capacity electrical appliances in low-voltage systems.

TN-S Grounding Method:

The letter S stands for N to be separated from PE, the metal shell of the device is connected to PE, and the neutral point of the device is connected to N.

The advantage is that there is no current in the PE, so the potential of the metal housing of the device to ground is zero. It is mainly used for data processing, precision detection, and power supply system of high-rise buildings.

TN-C grounding mode:

The letter C indicates that N is merged with PE to form PEN, which is actually a four-wire power supply system. Both the neutral point and the metal housing of the device are connected to N. Due to the three-phase unbalanced current and harmonic current circulating when n is normal, the metal shell of the equipment has a certain voltage to the ground normally, which is usually used in general power supply places.

TN-C-S Grounding Method:

Part of N is separated from PE, and it is a four-wire and a half-system power supply mode. It should be used in places with poor environment.

When N and PE are separated, they are not allowed to be merged.

China stipulates that the voltage between the phase wires (i.e. line voltage) of civil power supply lines is 380V, and the voltage between the phase line and the ground wire or neutral wire (i.e. phase voltage) is 220VThe entrance line generally adopts a single-phase two-wire system, that is, any one of the three phase lines and a neutral line (as a neutral line).In case of high-power electrical appliances, you need to set the grounding wire by yourself.

Three-phase five-wire system", people usually refer to a certain electrical equipment has five wires leading out. Specifically, the function of the five wires is usually described in the following two ways: the first statement, the five wires are three phase wires, a neutral wire, and a ground wire connected to the equipment housing; In the second statement, five wires refer to three phase wires, a working neutral wire (or system neutral wire), and a protective neutral wire connected to the equipment housing.

Both of these statements have something in common, when the three phases are unbalanced, the neutral line or the working neutral line (system neutral line) flows through the unbalanced load current, while the wires connected to the equipment have no current through. Three-phase power supply, five-wire connection, at first glance, reasonable.

What is the difference between three-phase four-wire and three-phase three-wire?

1. In general practical application, there is only a three-phase four-wire system (not counting the ground wire, counting the ground wire is called a three-phase five-wire system), and the three-phase three-wire system will not be used.

2. In addition to the three live wires of the three-phase four-wire system, the fourth one is called the middle line (and the disadvantage note: there is only a zero line in the single phase, and there is no zero line in the three-phase system), and the middle line plays a role in ensuring that the load phase voltage is unchanged.

3. If the load connected to each of the three-phase circuits is the same, and the sum of the currents flowing through the center line at each moment is zero, remove the center line and use the three-phase three-wire system to supply power. However, this is an ideal situation, and in practice it is impossible for a three-phase load to be perfectly symmetrical in a three-phase circuit with multiple single-phase loads connected to a three-phase circuit. In this case, the midline is particularly important and not optional.

4. If there is no center line in the case of asymmetrical load, the three-phase three-wire power supply of asymmetrical load will be formed. Due to the asymmetry of the load impedance, the phase current is also asymmetrical, and the load phase voltage is naturally not symmetrical. Some phase voltages may exceed the rated voltage of the load, and the load may be damaged (the bulb of the phase is too bright and burned); Some phase voltages may be lower and the load may not work properly (the bulb is dim).

With the light on, the light off and other reasons cause the change of the load impedance of each phase. The phase current and phase voltage all change accordingly, the lights flicker and flicker, and other electrical appliances cannot work properly, or even be damaged. So there must be a center line, that is, a three-phase four-wire system.

In the three-phase five-wire system, the five wires refer to: 3 phase wires plus a ground wire and a neutral wire. The most widely used low-voltage transmission mode is the three-phase four-wire system, using three phase wires plus zero line power supply, the zero line is drawn out from the neutral point of the transformer and grounded, the voltage is 380 220V, take any one phase wire plus the neutral line to form a 220V power supply line for general household use, the voltage between the three phase wires is 380V, and the general power supply machine is used.

In the three-phase four-wire power supply system, the two functions of the zero trunk line are separated, that is, one line is used to work the zero line (N), and the other line is used to protect the zero line (PE), and the power supply junction method is called the three-phase five-wire power supply mode.

The principle of the three-phase five-wire system:

It is well known that in the three-phase four-wire power supply, due to the three-phase load imbalance and the zero line of the low-voltage power grid is too long and the impedance is too large, the zero line will have zero sequence current to pass through, the low-voltage power grid that is too long, due to factors such as environmental deterioration, wire aging, moisture, etc., the leakage current of the wire passes through the zero line to form a closed loop, so that the zero line also has a certain potential, which is very unfavorable to the safe operation.

In the special case of the zero trunk disconnection, the single-phase equipment after the disconnection and all the equipment that protects the zero connection generate dangerous voltage, which is not allowed.

If the three-phase five-wire power supply mode is adopted, the working zero line N and the protection zero line PE that are connected on the electrical equipment are laid separately, and the potential on the working zero line can not be transmitted to the shell of the electrical equipment, so that the dangerous voltage caused by the three-phase four-wire power supply mode can be effectively isolated, and the potential on the shell of the electrical equipment is always in"land"potential, thus eliminating the hidden danger of the equipment generating dangerous voltages.