Surge protectorsspdIt is a device used to prevent damage to electrical equipment caused by lightning and other external interference. The selection and installation of surge protectors need to be carried out according to different grounding methods to achieve the best protection effect. Dikai TechnologyIntroducing the common onesThere are three grounding methods: IT system, TT system and TN systemas well as their characteristics and scope of application. Then, according to different grounding methods, the key points such as wiring form, parameter selection and installation position of the surge protector were analyzed. Finally, some precautions and suggestions for the selection and installation of surge protectors are given.
1. The concept and classification of grounding methods
The grounding method refers to the connection between the neutral point and the ground in the power supply system, and the connection between the exposed conductive part of the electrical device and the ground. The difference in grounding methods will affect the operational safety, reliability and economy of electrical equipment, as well as the effect of lightning protection. According to GB50054 "Code for Design of Low-Voltage Power Distribution", there are three grounding forms for low-voltage power distribution systems, namely IT system, TT system and TN system.
An IT system is one in which the neutral point of the power transformer is not grounded, or it is grounded by high impedance, and the exposed conductive part of the electrical installation is directly grounded, and this grounding point is electrically independent of the grounding point at the power supply end. The IT system is characterized by the fact that when a single-phase ground fault occurs, the system will not trip and can continue to operate, which is suitable for occasions with high requirements for power supply reliability, such as hospitals, aviation, navigation, etc. However, the disadvantage of the IT system is that the fault is difficult to locate, and when a second ground fault occurs, it will cause a short circuit, which may cause a disaster or a disaster.
The TT system refers to the direct grounding of the neutral point of the power transformer, and the direct grounding point of the exposed conductive part of the electrical installation, which is electrically independent of the grounding point of the power supply end. The TT system is characterized by small grounding resistance, and when the grounding is faulty, the fault circuit can be quickly cut off to protect personal safety, and it is suitable for occasions with high requirements for personal safety, such as residential buildings, public buildings, etc. However, the disadvantage of the TT system is that the ground current is large, which may cause overvoltage and damage to electrical equipment.
The TN system refers to the direct grounding of the neutral point of the power transformer, and the exposed conductive part of the electrical installation has a direct electrical connection to the power termination grounding point. TN systems are further divided into three types, namely TN-C, TN-S, and TN-C-S. TN-C refers to the merging of the neutral and protective lines into a single line, which is called the PEN line. TN-s refers to the separation of the neutral and protective lines. TN-C-S means that the front part supplies power to the TN-C mode, and the rear part separates the N line and PE line from the PEN line. The characteristic of the TN system is that when the ground fault is faulted, the neutral wire or PEN wire can be used as a loop to achieve fast circuit breaking, which is suitable for occasions with high requirements for power supply reliability, such as industrial and commercial. However, the disadvantage of TN systems is that the ground resistance is affected by the length and cross-section of the line, which can cause protection failure or malfunction.
IIDikai TechnologySelection and installation of surge protectors under different grounding methods
Dikai Technology DKSurge protectorsIt is a device used to prevent damage to electrical equipment caused by lightning and other external interference. The selection and installation of surge protectors need to be carried out according to different grounding methods to achieve the best protection effect. The following are the key points for the selection and installation of surge protectors under different grounding methods.
1.IT systems
Since there is no neutral wire in the IT system, or the neutral wire is grounded by high impedance, only a surge protector needs to be installed between the phase wire and the PE wire to achieve common-mode protection. The wiring form of the surge protector is 3P, that is, the three phase pairs of PE wires are each connected to a surge protector. The parameter selection of the surge protector should be determined according to the maximum permissible overvoltage and the maximum discharge current of the protected equipment. The installation position of the surge protector should be as close as possible to the protected equipment to reduce the lead length and impedance and improve the protection effect. The grounding wire of the surge protector should be connected with the PE wire, and the length of the grounding wire should be as short as possible, and the cross-sectional area should be as large as possible to ensure a good grounding effect.
2.TT system
Since the grounding points of the power supply and load sides of the TT system are independent, it is necessary to install surge protectors between the phase line and the PE line, and between the N line and the PE line respectively to achieve common mode and differential mode protection. There are two wiring forms of surge protectors, one is 4P, that is, the four phase pairs of PE lines are each connected to a surge protector, and the other is 3P+N, that is, the three phase pairs of N wires are each connected to a surge protector, and the N line is connected to the PE line and then connected to a surge protector. The choice of the two wiring forms depends mainly on the installation location of the surge protector, i.e. whether it is installed on the power side or the load side of the leakage protector. If it is installed on the power supply side of the leakage protector, the wiring form of 3P+N should be adopted to avoid the malfunction of the leakage protector caused by the direct connection between the phase wire and the PE wire. If it is installed on the load side of the leakage protector, it can be wired in the form of 4P to improve the protection effect. The parameter selection of the surge protector should be determined according to the maximum permissible overvoltage and the maximum discharge current of the protected equipment. The installation position of the surge protector should be as close as possible to the protected equipment to reduce the lead length and impedance and improve the protection effect. The grounding wire of the surge protector should be connected with the PE wire, and the length of the grounding wire should be as short as possible, and the cross-sectional area should be as large as possible to ensure a good grounding effect.
3.TN system
Since the grounding point of the power supply and load side of the TN system is directly electrically connected, it only needs to install a surge protector between the phase line and the PE line to achieve common-mode protection. There are two wiring forms of surge protectors, one is 3P, that is, three phase pairs of PE wires are connected to a surge protector, and the other is 3P+N, that is, three phase pairs of N wires are each connected to a surge protector, and N wires are connected to PE lines and then a surge protector. The choice of the two wiring forms depends mainly on the installation location of the surge protector, i.e. whether it is installed on the power side or the load side of the leakage protector. If it is installed on the power supply side of the leakage protector, the wiring form of 3P+N should be adopted to avoid the malfunction of the leakage protector caused by the direct connection between the phase wire and the PE wire. If it is installed on the load side of the leakage protector, it can be wired in 3P to improve the protection effect. The parameter selection of the surge protector should be determined according to the maximum permissible overvoltage and the maximum discharge current of the protected equipment. The installation position of the surge protector should be as close as possible to the protected equipment to reduce the lead length and impedance and improve the protection effect. The grounding wire of the surge protector should be connected with the PE wire, and the length of the grounding wire should be as short as possible, and the cross-sectional area should be as large as possible to ensure a good grounding effect.
IIIDikai TechnologyPrecautions and suggestions for the selection and installation of surge protectors
According to different grounding methods, there are some common precautions and suggestions for the selection and installation of surge protectors, as follows:
The selection of surge protector should be based on the electrical characteristics of the protected equipment, the voltage level of the power supply system, the intensity of lightning activity and other factors, and the appropriate rated voltage, maximum discharge current, protection voltage and other parameters, as well as the corresponding wiring form and installation location.
The installation of surge protectors should follow the principle of "graded protection", that is, different levels of surge protectors should be installed at different locations of the power supply system to form a hierarchical protection network to improve the protection effect and reliability. Generally speaking, the power supply system can be divided into three protection areas, namely the first-level protection zone, the second-level protection zone and the ** protection zone. The first-class protection zone refers to the power supply inlet end, which needs to be installed with a first-level surge protector to withstand the direct lightning strike or induction of strong impact. The secondary protection zone refers to the end of the power branch line, and the secondary surge protector needs to be installed to reduce the impact of the residual voltage of the primary surge protector on the downstream equipment. Protection zone refers to the incoming end of the electrical installation, which needs to be fitted with a surge protector to protect the electrical installation from damage from overvoltage.
Attention should be paid to the length and cross-sectional area of the lead wire in the installation of the surge protector to reduce the impedance and inductance of the lead wire and improve the protection effect. In general, the length of the lead wire should not exceed 05 meters, the cross-sectional area should not be less than 10 square millimeters. If the length of the lead is more than 0At 5 meters, an attenuator or compensator should be installed on the lead to reduce the impact of the lead.
Attention should be paid to the quality of grounding in the installation of surge protectors to ensure the normal operation of surge protectors. Generally speaking, the grounding resistance should not be greater than 4 ohms, the length of the grounding wire should be as short as possible, the cross-sectional area should be as large as possible, the grounding wire and the PE wire should be parallel as much as possible, and the grounding wire should be kept at a certain distance from other metal parts to avoid causing grounding loops or grounding interference.
Dikai TechnologySurge protectorsAttention should be paid to the coordination with other protective devices to avoid protection failure or malfunction. Generally speaking, surge protectors should be used in conjunction with leakage protectors, overcurrent protectors, overvoltage protectors, etc., to achieve comprehensive protection. The coordination between the surge protector and the leakage protector is mainly to avoid the malfunction of the leakage protector caused by the direct grounding of the surge protector, so the wiring form of 3P+N should be adopted, or an isolation switch should be installed between the surge protector and the PE wire. The coordination between the surge protector and the overcurrent protector is mainly to avoid the action of the overcurrent protector caused by the short circuit or fault of the surge protector, so the fuse or circuit breaker should be installed on the power side of the surge protector to achieve the protection of overload and short circuit. The coordination between the surge protector and the overvoltage protector is mainly to avoid the protection voltage of the surge protector being higher than the operating voltage of the overvoltage protector, resulting in the failure of the overvoltage protector, so the appropriate protection voltage should be selected, or an isolation switch should be installed between the surge protector and the overvoltage protector.