We know that in the installation of electrical appliances, first use the pen to distinguish the fire line from the zero line. If there are only two entry lines, one is the fire line and the other is the zero line, when distinguishing, the human body should not have any direct contact with the earth, hold the pen in one hand, and cannot touch the metal part of the pen tip, but the finger should contact the metal part of the pen tip, and contact the metal part of the two wires separately, if only one can make the pen tip contact with the metal part of the two wires. The neon bubbles of the pen glow, and this one is the fire line. If neither of them can make the neon bubbles of the pen glow, it may be that it has not touched the metal part or connected the power supply, or that the pen is broken, or that the finger has not touched the metal part of the pen tail. If both can make neon bubbles glow, the zero line is disconnected.

Detailed Differences Between Ground Line and Zero Line            
1. Zero line and ground line are two different concepts. They are not the same thing. Never exchange or mix them.            
2. Ground-to-ground potential of the ground wire is zero, and it is grounded close to the ground.            
3. The ground potential of the zero line is not necessarily zero. Zero line is grounded in the nearest substation, and there may be a potential difference between the local grounding and zero line grounding.            
4. Zero line sometimes calls. When the fire line is electrified, but the equipment is not working, the zero line may be broken. The zero line near the end of the equipment from the breakpoint is 220V, just like the fire line.

How to Distinguish Zero Line from Ground Line            
Wiring Standard: Red, Yellow and Green Zero Line (N) colour, Black and Blue Ground Line (PE) colour, yellow and green double colour line facing 3 sockets, left zero, right fire, middle ground.            
Install a leakage circuit breaker on the bus and connect it with a light bulb to the fire line and zero line or ground line. If the action of the leakage circuit breaker is ground line, otherwise it is zero line. When testing, pay attention to safety, there may be small sparks, be prepared in mind, don't be frightened!

If you are at home: 1. Turn on the electricity and measure it with a pen, all the lights will be on the fire line. 2. Disconnect the zero line at the main switch, only turn on the fire line, turn on the position of the light in the house, and measure it with a pen. It just didn't turn on, but now it's all zero line. 3. All that remains is the ground line.            
The simplest way is to take a 220 V light bulb and fix the line of fire with a pen. Two lines and two wires are connected to the lamp head respectively. The brightness can distinguish zero from ground. The brightness is zero and the lightness is slightly dim.

Put the multimeter in the AC gear of 500V with the multimeter. Hold a pen by hand. The other pen touches the power line separately. The high voltage is the fire line, the low voltage is the zero line, and the ground line is the zero line. Zero line to ground resistance less than 4 ohms is reliable grounding. The pressure difference between 250V fire line and zero line, between fire line and ground line is reliable grounding when the multimeter is placed in AC gear.

Consequences of Ground and Zero Connection Errors            
1. Because it is an alternating current, there is no effect of fire and zero exchange on electrical appliances. Faced with 3-hole socket, left zero, right fire, mainly for maintenance. Better not change.           
2. Zero and ground connection are reversed or mixed. This is normal, but it is dangerous. Ground connection cannot be connected to zero line. Otherwise, the equipment housing will be charged. (Normally, the housing is barely electrified when the equipment is in normal condition)            
3. Ground wires can be temporarily used as zero wires (so that the meter does not turn, belongs to stealing electricity), but if you want to use them for a long time, there are many unfavorable factors, such as: voltage instability due to changes in the environment of the grounding point (thunderstorms, humidity, etc.); substation factors make the voltage of the fire wires to the ground reach 380V, which can lead to the easy use of electrical equipment. Affected and seriously damaged.

Ground Line Color            
In order to make alternating current have a convenient power conversion function, power transmission is usually in the form of three-phase four-wire, and the three heads of three-phase electricity are called phase lines.            
The three tails of three-phase electricity are connected together and called neutral line, which is also called "zero line". The reason for calling zero line is that there is no current passing through the neutral line in the three-phase equilibrium time. Moreover, it directly or indirectly connects to the earth and is close to zero with the earth voltage.            
Ground wire is a reliable connection between the shell of equipment or electrical appliances and the earth. It is a good scheme to prevent electric shock accidents. Generally speaking, three colors of red, yellow and blue are used to represent three lines of fire in three-phase circuit, and black is used for zero line. In single-phase illumination circuit, generally yellow means that the fire line, blue is zero line, yellow and green is ground line. In some places, red is used to denote the line of fire, black is used to denote the zero line, and yellow and green is used to denote the ground line.

Distinguishing colour and function of fire line, zero line and ground line            
Power cord from the national logo, the general phase line - A phase yellow, B phase green, C phase red. Zero line - light blue. The ground line is yellow and green. If it is a three-phase socket, the left side is zero line, the middle (above) is ground line, and the right side is fire line.           
Usually from the use of the middle line for yellow, green, red, white, grey, brown, zero line for light blue and blue, ground line for double yellow and black.            
Electricity is divided into power and household electricity. Power is commonly referred to as 380 volts of electricity, mostly used in factories and some large electrical equipment such as 5P air conditioning and hotel smokers. This kind of electricity is mostly three-phase four-wire. In four-wire, there are three fire wires and one zero wire. Fire wire refers to any phase in A B C of three-phase four-wire network. Zero wire refers to the one wire without voltage and current to the ground. After three fire wires pass through load such as electric motor and other electrical equipment, they all pass through the zero wire to form a circuit so that the equipment can work normally. Zero wires are grounded in power plants. Generally speaking, three colors of red, yellow and blue are used to represent three wires in three-phase circuit, and black or blue is used for zero wires.           
 Household electricity use refers to what we often call 220 volts or single-phase electricity. There are two wires, one fire line and one zero line. Fire wires pass through electrical appliances, such as light bulbs, and then form a circuit through zero wires, so that electrical appliances can work normally. The zero line here is also grounded in the power plant. In single-phase illumination circuit, yellow, green, red, white, grey and brown indicate that the line of fire, light blue and blue are zero, yellow-green and black are ground lines.

Although the zero line of power and household appliances is grounded in power plants, the ground line and zero line we usually refer to are not a concept. Look at the three-hole power outlet in our house. If it is constructed regularly, one of the holes is the fire line, one is the zero line, and the other is the ground line. The grounding line here is the grounding line after the whole building is assembled, which is often said to be the grounding line. Most household appliances require a grounding wire, which is connected to the grounding wire. Generally, the zero line is through the leakage switch, but the ground line can not. Ground-to-ground potential is zero, the nearest point of the electrical appliances used is grounded; zero-to-ground potential is not necessarily zero, the nearest grounding point of the zero-to-ground line is at the substation or the transformer of the power supply; sometimes the zero-line will be electrified, such as the furnace does not heat, do not think that there is no electricity, no electrification, there may be such a possibility. Yes, the N-wire is disconnected far away from your electric appliance. If you measure it with a voltmeter, you will find that the LAN-wire of the electric appliance is the voltage of the city electricity; the ground wire will not be electrified unless it is in bad condition or the installer does not understand or mess up. In some circuits, there are zero and ground wires, and you will find a high-voltage capacitor in the middle of them.  

Fire wires are live, ground wires and zero wires are not. There is a fire wire and a zero wire in the socket of two household sockets. It can be detected with a pen that the live wire comes from the fire wire, but not from the zero wire. There is only ground wire in the socket of three sockets. Ground wires should be connected to the shell of the electrical appliances to prevent electric leakage from causing electric shock casualties. The point marked with L is connected to the fire line, the point marked with N is connected to the zero line, and the ground line has a special grounding symbol.       
     
At present, most of the power sockets we use are single-phase three-wire sockets or single-phase two-wire sockets. In the single-phase three-wire socket, the ground wire in the middle also plays a role in positioning; the connection sequence is left zero and right fire. All household appliances with metal housing are equipped with single-phase three-wire power plugs. The three plugs are arranged in regular triangle, and the longest and thickest copper plug above is the ground wire. The two below the ground line are the line of fire (L) and the line of zero (N), in the order of left zero and right fire (when the plug is facing itself on the back).

One．The Role of Ground Line            
The main function of the ground wire is that when the electrical appliance fails, the power supply may break down (or: destroy) some components and make the shell of the electrical appliance live. Grounding the outer shell of the electrical appliance can make the leakage protection device            
1）Signal "ground". Signal "ground", also known as reference "ground", is the reference point of zero potential, but also constitutes the common section of circuit signal circuit, graphical symbol "".             
2) DC ground: DC circuit "ground", zero potential reference point.            
3）AC ground: AC zero line. It should be distinguished from the ground line.            
4）Power ground: zero potential reference point of high current network devices and power amplifier devices.             
5）Analogously: Zero Potential Reference Points of Amplifiers, Sample Holders, A/D Converters and Comparators.            
6）Digitally: also called logically, it is the zero potential reference point of digital circuit.            
7）"hot ground": switching power supply does not need to use transformers, the "ground" of its switching circuit is related to the municipal power grid, which is called "hot ground". It is charged, and the graphic symbol is "hot ground".            
8）"Cold ground": Because the high-frequency transformer of switching power supply isolates the input and output terminals, and because its feedback circuit is usually photoelectric coupling, which can transmit feedback signals and isolate the "ground" of both sides, the output terminal is called "cold ground", which is not charged. The graphic symbol is“⊥”.
Two. Protecting the "land";           
Protecting the "ground" is a way of wiring designed to protect the safety of personnel. One end of the earth wire is connected with electrical appliances, and the other end is reliably connected with the earth.            
Three. Ground in Sound            
Shielding wire grounding: In order to prevent interference, the metal housing of the audio system is connected with the signal "ground" by wire, which is called shielding grounding.            
Audio special "ground": In order to prevent interference, professional audio needs to be connected with audio special "ground" besides shielding "ground". This grounding device should be specially buried, and connected with the corresponding grounding terminal of isolation transformer and shielded voltage stabilized power supply as the special audio grounding point in the sound control room.
Processing methods of different ground lines            
1. Digital and analog should be separated           
In high-demand circuits, digital and analog must be separated. Even for A/D and D/A converters, it is better to separate the two "ground" on the same chip, and only connect the two "ground" on the system point.            
2. Floating and Grounding            
Floating ground of a system means that the ground wires of all parts of the system circuit are floated and not connected to the earth. This connection has a certain anti-interference ability. However, the insulation resistance between the system and the ground should not be less than 50M. Once the insulation performance decreases, it will cause interference. Usually the system is floating and the chassis is grounded, which can enhance the anti-interference ability and make it safe and reliable.
3. One point grounding            
In low-frequency circuits, wiring and components do not have much impact. Usually the frequency of the circuit is less than 1 MHz, using one-point grounding.            
4. Multipoint Grounding            
In high frequency circuit, parasitic capacitance and inductance have great influence. Usually the frequency of the circuit is more than 10MHz. In addition to correct grounding design and installation, multi-point grounding should also correctly handle the grounding of different signals. In the control system, there are roughly the following ground wires:            
(1) Digital: also known as logic, is the zero potential of various switching (digital) signals.            
(2) Simulated: Zero potential of various analog signals.            
(3) Signal ground: usually sensor ground.            
(4) Exchange place: The ground of AC power supply, which usually produces noise.            
(5) DC ground: DC power supply ground.            
(6) Shielding ground: also known as shell ground, to prevent electrostatic induction and magnetic induction.
The above ground wire processing is an important problem in system design, installation and debugging. Here are some opinions on grounding problem:            
(1) The control system should be grounded at one point. Generally speaking, the high frequency circuit should be grounded near multiple points, and the low frequency circuit should be grounded at one point. In low frequency circuit, the inductance between wiring and components is not a big problem, but the interference of the loop formed by grounding has a great influence, so a point is often used as the grounding point; but a point grounding is not suitable for high frequency, because the inductance on the ground increases the impedance of the ground wire at high frequency, and at the same time, there is a generation between the lines. Inductive coupling. Generally speaking, when the frequency is below 1MHz, one-point grounding can be used; when the frequency is above 10MHz, multi-point grounding can be used; between 1 and 10MHz, one-point grounding can be used, or multi-point grounding can be used.            
(2) Exchange ground and signal ground cannot be shared. Because there will be several mV or even several V voltage between two points of a power supply ground wire, this is a very important interference for low-level signal circuit, so it must be isolated and prevented.            
(3) The comparison between floating ground and grounding. This method is simple, but the insulation resistance between the whole system and the earth can not be less than 50M. This method has a certain anti-interference ability, but once the insulation decreases, it will bring interference. Another way is to ground the chassis and float the rest. This method has strong anti-jamming ability and is safe and reliable, but its implementation is complex.            
(4) Simulated. The connection of simulated ground is very important. In order to improve the anti-common-mode interference ability, the shielded floating technology can be used for analog signals. The grounding processing of specific analog signals should be strictly designed in accordance with the requirements of the operation manual.            
(5) Shielding ground. In order to reduce the capacitive coupling noise and detect and control accurately in the control system, it is necessary to shield the signal. According to the purpose of shielding, the connection of shielding ground is also different. Electric field shielding solves the problem of distributed capacitance, which generally connects with the earth. Electromagnetic field shielding mainly avoids the radiation interference of high frequency electromagnetic fields such as radar and radio. Made of low resistance metal material with high conductivity, it can connect with the earth. Magnetic shielding is used to prevent magnets, motors, transformers, coils and other magnetic induction. Its shielding method is to use high permeability materials to make the magnetic circuit closed, generally connecting the earth is better. When the signal circuit is grounded at one point, the shielding layer of low frequency cable should also be grounded at one point. If there is more than one shielding layer in the cable, noise current will be generated and noise interference source will be formed. When a circuit has an ungrounded signal source connected to the grounded amplifier in the system, the shielding of the input end should be connected to the common end of the amplifier; on the contrary, when the grounded signal source is connected to the ungrounded amplifier in the system, the input end of the amplifier should also be connected to the common end of the signal source.            
For grounding of electrical system, it is necessary to classify grounding according to the requirements and purposes of grounding. Instead of simply and arbitrarily connecting different grounding systems, it is necessary to divide them into several independent grounding subsystems. Each subsystem has its common grounding point or main grounding line. Finally, the grounding system is connected together and the total grounding system is implemented.