Overvoltage and overcurrent of the hottest communi

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Overvoltage and overcurrent of communication equipment and its protective measures

on the communication and data lines, overvoltage and the over-current generated by the new high-efficiency building insulation materials will endanger and interfere with the normal operation of communication and computer systems, and may be harmful to the operation and maintenance personnel. Damage and loss caused by equipment and property. This kind of harm can also affect the client, which will also pose a threat to the user's personal and property

apart from the direct damage of components, the damage of communication equipment will not cause economic losses caused by communication interruption, which may be several times the direct loss of equipment. Coupled with the fierce competition of communication enterprises, users' requirements for service quality are increasing. Therefore, appropriate protective measures must be taken to avoid harm to communication equipment, lines and relevant personnel due to overvoltage and overcurrent. Communication buildings are generally equipped with lightning rods, lightning strips or lightning protection, and the joint grounding method is adopted. It seems that it has good lightning protection and anti external electromagnetic interference performance. Why do communication equipment sometimes suffer from over-voltage and over-current damage? Z also poses a threat to the personnel of operation and maintenance personnel

I. causes of equipment overvoltage

1. From the analysis of faults caused by lightning strike communication equipment at home and abroad, most of them are caused by lightning traveling waves invading from outdoor power lines, transmission lines and antenna feeders. At this time, the machine room itself was not struck by lightning, but the lightning directly struck the power line, transmission line, tiandebt line, or there was lightning flash at a certain distance from the power line, transmission line or antenna feeder. Assuming that there is a lightning current of 80kA at 400m away from the power line, and the average height of the overhead power line from the ground is 5m, it can be estimated that the instantaneous Overvoltage on the power line is 25kV. Such a high induced voltage extends to both ends of the power line. Although the lightning induced voltage can be gradually attenuated through the line, if the communication machine room is not far from the place where the lightning falls, the residual induced voltage may still have a certain strength, and the communication equipment may be damaged after invading the machine room

2. When the lightning rod or lightning strip on the top of the communication building is struck by lightning, the lightning current passes through the lightning rod, lightning strip or lightning, and the downlead and joint ground enter the ground, and the ground potential rises. Assuming that the lightning current is 40ka and the grounding resistance of the joint ground wire is 1 ohm, at this time, its potential increases by 40kV. Since the metal shell of the communication equipment in the machine room is also grounded, the potential of the metal shell of the communication equipment also increases by 40kV. Since the power line and transmission line entering the machine room from the outdoors are introduced from a distance, the remote potential can be seen as zero volts, so there is also a 40kV counterattack voltage between the communication equipment connected to the power line and transmission line and its metal shell, which can cause equipment damage. If the lightning current or the grounding resistance of the joint ground wire is larger, the equipment damage will be more serious

3. When the user line or transmission line is in contact with 220V/50Hz AC, a large current can be generated. This kind of current can damage equipment and even cause serious fire, affecting other equipment in the building. This situation is mainly caused by bad weather such as lightning storms, damage to cable insulation in infrastructure projects, or intentional damage or lack of common sense

4. even if the communication building or external lines are not affected by lightning or strong current at all, the ideal sine wave voltage is not always present in the AC power supply line. In fact, the switching operation process of the power supply circuit in the electricity, the automatic switching process of the municipal power supply and the self-contained power supply, and the input and removal of the inductive load in the power supply system will cause peak pulse interference to the power supply system, causing the distortion of the sine wave voltage waveform. Although the duration of this distorted voltage is not long, the harm to modern communication equipment using VLSI can not be ignored. In the power supply of equipment, as long as there is a certain amplitude of pulse impact, serious damage may be caused to devices in the equipment. Slightly, due to repeated influence, the performance of components in the equipment may be reduced and the service life may be shortened

II. How to prevent overvoltage and overcurrent of equipment? Lightning rods and other lightning protection facilities on the roof of the communication building cannot prevent overvoltage and overcurrent of equipment caused by the above three reasons. The effective way is that in addition to taking lightning protection measures for high-voltage lines, overvoltage protectors must also be installed at the high and low voltage sides of the transformer to reduce the overvoltage value and discharge a large amount of lightning current. Cables entering and leaving the communication building shall be laid with metal outer sheath to reduce the probability of lightning stroke, and the metal outer sheath of the cable shall be grounded at both ends to reduce the transverse potential difference between the metal outer sheath of the cable and the core wire when being struck by lightning. At the same time, the method of "blocking" should be adopted, that is, the lightning current and various interferences outside the machine room should be blocked outside as far as possible, which requires all cables entering and leaving the communication building, such as power lines, transmission lines, user lines, antenna feeders, etc., to be equipped with overvoltage protectors. The metal pipes entering and leaving the building are equipotentially connected with the joint ground wire. Some equipment installed in the machine room may not be able to completely bypass the overvoltage (such as AC distribution panel, main distribution frame, etc.), and it is also required to install hierarchical overvoltage protectors (such as rectifier, DC distribution panel, switchboard user board, etc.) at the second incoming line end and the following levels of equipment, so as to further suppress the residual voltage to the allowable safety range of the equipment. On the other hand, if the first level protection device is damaged due to lightning, the second level protection device can provide protection immediately

overvoltage protector alone cannot completely protect equipment and lines from electrical damage. Although high voltage can cause serious damage to voltage sensitive components of equipment, the greater reason for the damage of carton compressive strength calculation formula of some equipment is caused by overcurrent. Sometimes the surge voltage is not high enough to make the overvoltage protector act. If the overvoltage protector fails to work, the surge voltage may generate enough additional current in the circuit to damage sensitive equipment or cause fire. For example, when a line touches 220V/50Hz AC at a distance of 1000m from the SPC exchange, 220V AC will generate a voltage of about 90V on the security device of the main distribution frame. This voltage is not enough to make the overvoltage protector act, but it can generate a current of about 900mA. Since the line and 220V/50Hz AC electromagnetic contact are continuous, the main distribution frame and switch user board will inevitably suffer from fire under the action of overcurrent for a long time. Therefore, protective devices combining overvoltage and overcurrent should be selected for equipment such as switchboard, such as discharge tube and PTC for protection

III. how to select the retainer protector, which is divided into overvoltage protection element and overcurrent protection element. We usually call "lightning arrester" and voltage limiting elements such as "surge suppressor" and "overvoltage limiter" introduced with foreign lightning protection devices. Discharge tubes and zener diodes also belong to voltage limiting elements. Their working principles are similar, but there are great differences between them in flow capacity, action speed, residual pressure, etc. Positive temperature coefficient PTC, inductance, resistance and fuse are overcurrent protection elements

(I) introduction and application of common protective elements

1. metal oxide variable resistance is a kind of ceramic component, which is more and more widely used. Its typical application product is zinc oxide varistor, which is a polycrystalline semiconductor overvoltage suppression device composed of zinc oxide particles. Using its good volt ampere characteristics, the impulse voltage is limited to a certain range. Its main technical parameters include current carrying capacity, impulse breakdown voltage and residual voltage. It can withstand large current impact, and it has fast response speed, which can reach the nanosecond level, and the price is relatively cheap. The disadvantages are: its volume and capacitance are large; There is a certain leakage current. If the quality is not good, the leakage current will gradually increase or even damage; The higher the residual voltage is, the worse the embedding effect is. The larger the impulse current is, the higher the embedding voltage is; The repeated impact tolerance is poor, and its character deteriorates after being impacted many times, which directly affects its use effect and life span. Zinc oxide varistor can be used as the first or second level protection. In some electronic equipment with high requirements, it is not enough to protect it only, but also combined with other protective devices such as semiconductor diodes

2. semiconductor diodes include switching diodes, zener diodes, transient diodes, etc. Their protection performance is roughly the same, with slightly different impact resistance and limiting voltage level. Under normal conditions, the tube has high resistance. When the applied voltage reaches its threshold value, the current increases rapidly. It is a protective element with very fast response time. The limiting voltage is low, and the voltage drop at both ends of the tube is basically not affected by impulse voltage and impulse current. It is necessary to protect semiconductor circuits in precision equipment. In recent years, the transient diode, which is being developed, has made a great breakthrough in the current capacity, and can be used in the second or even the first level protection

3. a gas discharge tube is a gas discharge tube formed by encapsulating a pair of discharge gaps in glass or ceramics filled with discharge medium (such as inert gas). The commonly used impulse breakdown voltage of the discharge tube ranges from more than 100 volts to several thousand volts. Once the impulse overvoltage reaches the impulse breakdown voltage of the discharge tube, the gas in the tube ionizes and the discharge tube changes from the original open circuit state to an approximate short circuit. Although the discharge tube has the ability to withstand a large current impact, and is small in size and low in price, it has a slow response speed. During the conduction period, the power supply is approximately short circuited, which may cause the tripping of the upper air switch. Discharge tubes should not be used to protect some existing graphene technologies that are not allowed to be short, and most of them are still in the laboratory stage to temporarily interrupt the power supply. However, due to its low price, it can be used as the first or second level protection element in general occasions with low requirements

4. PTC (positive temperature system) resistor is a current limiting solid-state element. It shows ohmic characteristics at normal temperature. When it exceeds a specific temperature, the resistance rises by times sharply. The temperature rise may be caused by external heating or internal heating by the current of the circuit in which it is located. During internal heating, PTC is similar to fuse, except that after troubleshooting, PTC can automatically connect the circuit. Therefore, it does not need maintenance. PTC is divided into ceramic devices and conductive polymer devices. Conductive polymer device is a relatively new technology, which overcomes the shortcomings of ceramic devices, small size, and will not show negative temperature coefficient in any case

(II) how to select a protector? The following characteristics should be considered when selecting a protector:

1. It can withstand a certain degree of surge energy. In the equipment close to the low-voltage head end and vulnerable to external impact, it is necessary to select a protector with large through current capacity, otherwise, a protector with smaller through current capacity can be selected

2. to reliably suppress the instantaneous overvoltage, the residual voltage should be lower than the lightning impulse voltage of the equipment. The lower the residual voltage is, the better the protection performance is, and the more reliable the equipment can be protected

3. response speed should be fast. That is, the normal state is high impedance, which has no impact on the normal operation of the equipment. The action is low impedance, and the process time from high impedance to low impedance is short. 4. The protector should have high stability, and it should be selected as much as possible after repeated impact

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