The grounding of electronic equipment and its anti-interference technology, the ground connection may constitute a large number of annoying interference sources, especially the flow of the earth of the fire-type electronic equipment, which will cause the potential difference between the ground connections and form strong interference. The continuous improvement of the accuracy and resolution of electronic equipment, small external disturbances will produce large errors, and even the entire electronic system will not work properly. Grounding is a necessary technology to ensure the stability and reliability of the system and isolate the mutual influence of internal and external electric fields. It plays an important role in avoiding the interference of environmental noise on the system and ensuring the safety of equipment and personnel.
2 Grounding body and grounding resistance Grounding body is set to establish a low-resistance contact with the earth, usually a kind of network formed by connecting some metal grounding rods, wires, pipes or other metal structures, and then connected to the equipment through a grounding connection The effectiveness of the grounding body and the grounding body depends on the grounding resistance requirements of devices with different resistance values ​​to the ground. For electrical circuit and equipment fault protection systems, the grounding resistance requirement is low. A single electrode grounding rod can be formed by a grounding rod or a grounding plate. It is the most common artificial electrode. It is a metal rod or pipe directly driven into the ground, usually This method is also often used when the soil thickness exceeds 31. For the bottom rock close to the ground, because the ground rod cannot be used, a horizontal conductor can be buried, that is, a horizontal metal strip solid wire, etc., buried underground, 5 for communication For electronic equipment and some precision large-scale electronic systems, where the grounding resistance is relatively high, a grid system is required to meet the requirements. The grid system consists of metal rods buried about 15 deep underground, and forms a rectangular network, usually metal Rod spacing For some devices that require higher grounding resistance, a plate electrode can be used. The plate electrode is composed of a rectangular or round iron or copper plate. The thickness should not be less than 6. It should be buried 1.525 below the ground plane. Of course, ground The smaller the resistance value, the better its protective effect and the correspondingly higher cost. Therefore, the appropriate grounding method should be selected according to the small side of the equipment. The grounding resistance value of the grounding body is difficult. The shape and season of the grounding body you use have a great relationship. The following are some common calculation methods for readers ’reference but the most accurate. It is the result of on-site testing with a ground resistance tester. Ground plate = single pipe and floor thickness. 0; The buried depth is under the better soil conditions, using a fixed size copper plate, or a square tube of zinc, can make its grounding resistance reach about 12.
3 Protective grounding Grounding is a kind of protective measure originated from strong electric technology. The main purpose is to prevent the high voltage and large capacity of strong electric power from endangering personal safety. Therefore, from a safety point of view, the chassis chassis of all electronic equipment used in the test site should be grounded to maintain an equipotential with the ground. When a strong fault current passes, the circuit breaker can be tripped or the fuse can be fused, thereby ensuring The safety of the person and equipment in the fault state.
3.1 Fault protection grounding in the use of equipment Whether it is a power distribution system or a direct short circuit or arc in electrical equipment related to this system, it will directly threaten the safety of workers and equipment. Incorrect assembly and fault conditions will cause personal injury. When the equipment is not grounded, the voltage of the shell will rise to 22, which will give The risk of fatal injury to equipment staff If the chassis is grounded through the wire at this time, the fault current can flow back to the power supply through the fault channel resistance, and the low resistance channel will quickly increase the fault current, thereby protecting the circuit from overload The device trips and plays a role in fault protection. However, it should be noted that the overload protector must be connected to the phase line of the power grid. If it is only connected to the neutral line, although the overload protector trips, the chassis still carries the phase voltage, and the risk of electric shock cannot be ruled out. 3.2 Grounding of the power line In order to ensure the safety of equipment and personnel, the grounding of power lines is necessary. In the single-phase 220 AC power supply system, especially when the oil machine is used for field testing, the wire system should be used, that is, there should be a ground wire in the power supply wire. Under normal circumstances, the load current flows through the phase line live wire After returning from the neutral line of the neutral line, if there is no current flowing through the ground line, if a fault occurs, a large current flows through the ground line, causing the fuse to blow.
For the shape power supply system, the line power supply should be used, that is, in addition to the conventional root phase line and root center line, there should be a separate Vi9942014 fault point fault channel resistance separate safety ground wire, square kilogram. The neutral line should be grounded not only at the power inlet of the power supply circuit breaker, but also at the distribution transformer to prevent possible electric shock to the shell. Transformer neutral grounding earth electrode system, 1 equipment, safety grounding wire 4 signal ground signal Grounding is usually defined as an equipotential point or equipotential surface to be used as a reference potential for equipment or system operation. This grounding reference system will provide the necessary return path for fault protection electrostatic discharge and signal current at the same time, the purpose is to use the above grounding Efficacy, reduce interference and noise.
Whether the signal grounding system is used for all circuits in a piece of equipment or many pieces of equipment in a set system, the commonly used configuration form is a combination of single-point grounding and multi-point grounding.
4.1 Suspension grounding The suspension grounding system is torn apart in a system. This signal grounding system is electrically isolated from other grounding systems and other conductive systems; similarly, its signal return channel is also isolated from the equipment cabinet. In this way, the noise current of other grounding systems and cabinets will not be directly coupled to the signal circuit.
The effectiveness of the floating grounding system depends on the isolation of the floating system from adjacent conductors, but it is difficult to obtain real isolation in the equipment. Even if the isolation is made, it is difficult to maintain it in the use of the equipment. At the same time, it is generated in the isolated signal circuit There is no discharge circuit of static electricity, which is extremely susceptible to the risk of electric shock and spark power generation. In addition, in most electronic equipment, the external power supply is based on the ground as the reference point of the power supply disconnect device. The short circuit fault between the signal system and the power supply will increase the voltage of the entire signal system relative to other conductive systems to a dangerous voltage; Especially when lightning strikes, insulation breakdown or flashover will occur between the signal system and the structure due to high voltage, so it is generally not recommended to use this grounding system 4.2 single-point grounding ideal single-point grounding system, each The ground conductor starts from the same point of the grounding system of the equipment and extends to the return channels of various circuits of the entire equipment. Usually this grounding method requires a large number of wires, which is not economical enough, so an approximate single-point grounding system of 9 is often used, that is, a single-point grounding system using a common bus.
The system uses multiple ground busbars to extend from the grounding system to each independent electronic device. In each electronic device, various electronic subsystems are individually connected to the same point of the grounding busbar.
The advantage of single-point grounding is that it helps to control the interference of conductive coupling. Not only can it avoid the formation of a closed loop of interference current in the signal grounding network, but also reduce the low-frequency noise current that may occur in the equipment grounding system Multi-point grounding system The single-point grounding system described above is generally only applicable to equipment systems with operating frequencies lower than 1 centimetre. When the operating frequency is higher than centimetres or the signal ground length in the system exceeds 120 wavelengths, that is The so-called high-frequency equipment system, if you continue to use single-point grounding at this time, not only is it easy to form a long ground line and increase the impedance of the ground line, but also the stray inductance and distributed capacitance between the signal lines will cause the signal between Coupling, resulting in unstable system operation. When the frequency progress increases, the length of the ground wire reaches the length of signal 1 or its odd multiple, the ground wire will quickly reduce the impedance due to resonance, thereby radiating noise signals outward, and the interference will be more serious at this time. Reduce the impedance of the ground wire. Generally speaking, the ground impedance of high-frequency system equipment can be point grounded when the operating frequency of the system is below 1%; when the frequency is higher than 12, multi-point grounding is the only practical method; the operating frequency is at If the point grounding is used between 10 degrees 2, the length of the ground wire must not exceed l20 of the signal wavelength, such as 10MHz, and it should not exceed l, 5m, otherwise multi-point grounding should be used.
Commonly used for shielded wires in electronic equipment testing, there are single-core two-core and multi-core common shielded wires for low-frequency equipment, twisted-pair shielded wires and coaxial cables for high-frequency equipment, etc. Due to their use of environmental conditions and transmission signals Different, so the grounding method when shielding is also different 5.1 Grounding of the low-frequency equipment shielding wire The shielding layer of the shielding wire used for low-frequency equipment test connection usually adopts a single-point grounding method. If the shielding layer of the shielded wire used at this time has more than two grounding points, it is possible to form a noise ground loop through the shielding layer and generate a noise current, thereby generating a noise voltage on the shielding core wire, so it should be avoided as much as possible. However, even if point grounding is used, the distributed capacitance between the core wire and the shielding layer will also generate noise. In actual test work, due to insufficient knowledge of shielding grounding or the specific implementation method is inappropriate, it may be intentional or Inadvertently cause improper shield grounding, thereby forming unexpected interference or reducing the shielding protection. Therefore, you should pay attention to the following questions when connecting the signal shielding wire during the test. 3. In low-frequency circuits, in order to obtain the best noise suppression effect, the shielding layer cannot be used as a signal wire. The shielding layer of the shielding wire must be grounded. The floating shielding layer will often form more serious interference; the grounding point of the shielding layer is preferably the ground connected to the signal source, and the ground connected to the measurement equipment is likely to form noise interference. 1 When the signal source is floating, the shielding layer can only Connect to the common ground of the measuring equipment; 6. When both the signal source and the measuring equipment are not grounded, the shielding layer should be connected to the low impedance end of the signal source to ground.
5.2 The grounding of the shielding wire of high-frequency equipment For high-frequency signals, it is not appropriate to continue to use the point grounding method of the shielding layer at low frequencies because the effect of the stray capacitance between the conductors at this time cannot be ignored, especially the longer the transmission cable The more significant the impact. The grounding at both ends of the cable shielding layer is a practical grounding method for connecting high-frequency equipment to the shielding wire. For the case of using long cables to transmit signals, the cable shielding layer should be grounded at multiple points to ensure that the cable length between the two grounding points does not exceed the signal wavelength 14. To reduce grounding impedance and prevent reflection interference in cable transmission.
For systems that require wide-band operation, the thorn grounding method can be used, and small capacitors are used to replace stray capacitors at the shielding layer to take into account the multi-point grounding of high-frequency equipment and the single-point grounding requirements of low-frequency equipment. The impedance of the capacitor is very small, which can be regarded as a short circuit, and the shield layer realizes multi-point grounding; while at low frequencies, the impedance of the small capacitor is large, which can be regarded as an open circuit, and the shield layer realizes point grounding. This method is very convenient in practical use.
There are many types of electronic equipment used in modern testing. The frequency ranges from DC to microwave energy, from weak signal to high power. Their grounding systems should also be based on different types of equipment and test requirements. The factory should consider the grounding of electronic equipment. It is only to ensure the safety of personnel and equipment, and more importantly, to improve the anti-interference performance of the equipment and improve the accuracy and reliability of testing and control because in electronic equipment, even its insulation corrosion can be very good. It is also unavoidable that the stray current caused by distributed capacitors is very small for these capacitor currents when working at the mains frequency, but it can be ignored. However, when the frequency is higher than 5 rivers, this capacitor current is about larger than the mains frequency 100,000 times, it will have a great impact on the work of electronic instruments. The most effective method is to use a metal plate shell to shield the entire instrument. At this time, such as electrical equipment, the external is not grounded. Between it and the ground will be 60% of the distribution of the electric world. When the frequency increases, the capacitive reactance decreases; when the frequency is very high, the instrument shell is equal to ground but for reliability, it is generally connected to the ground with a short and thick wire. The length of the ground wire must consider the wavelength of the electronic signal. For example, when When the frequency is 3012, its 1 break length is only 25 meters; if the length of the ground wire is 2.5 times its odd multiple, a standing wave is generated on the ground wire, close to the ground. The end is at ground potential, while the other end has a higher voltage to ground, but personal accidents are more likely to occur. Therefore, the ground wire should not exceed 14 of the wavelength. If it is unavoidable in the project, it must not be at least an odd multiple of 1 long. Therefore , A special grounding body should be set at the test site, and then connected to the main grounding wire, so as to shorten the grounding distance of the field equipment. In addition, no natural grounding body such as underground water pipes or heating is used as the grounding wire, which is mostly distributed in the building. Each part of the object is easily close to the mains power supply line, and these transmission lines are sources of interference, but will cause interference to the receiving equipment.
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