How to do well in lightning protection of coaxial

2022-09-21
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How to do a good job in lightning protection of coaxial cable system detailed explanation of lightning protection of coaxial cable system

in recent years, cable has been booming, and how to meet people's desire to see stable TV programs at any time. Lightning protection of coaxial cable system has become an urgent problem for cable TV workers to solve, but there are many lightning protection methods, which need to be summarized for selection. Then, how to do a good job in lightning protection of coaxial cable system

how to do a good job in lightning protection of coaxial cable system

the lightning protection methods of coaxial cable system include grounding lightning protection, intraocular pressure lightning protection, isolation lightning protection, etc

grounding method is to install a grounding wire separately in each amplifier or other devices prone to lightning strike, so that the energy generated by lightning can be released to the earth to protect the devices. The main points are: first, the grounding resistance should be as small as possible; second, it must be separated from the power grounding wire, otherwise it will not play the role of lightning protection. However, if the system is large, and there are many and scattered devices that need lightning protection, installing a good grounding wire on each device is bound to increase the engineering quantity

voltage limiting method is to press a voltage limiting lightning protection device at the port of the equipment to limit the induced lightning voltage of the cable within a certain range, so as to protect the equipment. Its disadvantage is that the lightning protection device itself is easy to fail or burn out after discharge

the isolation method is to shield the cable and its connected devices outside the lightning induction area. In practice, the buried laying method is generally adopted, that is, laying along the underground pipeline. Due to the shielding protection of the earth, lightning will not be induced. However, the buried laying works are large, the capital investment is large, and in the residential area of the building, it is impossible to use this method, and other methods must be used for lightning protection to maintain the stability of the system

the above lightning protection methods are similar to those of ordinary parallel cable or twisted pair cable system. In fact, due to its special structure and mainly used for transmitting high-frequency signals, some special structures can also be adopted. Low cost lightning protection method. The following will introduce the parallel lightning protection method.

coaxial cable is an unbalanced cable. When there is lightning, the outer conductor generates induced electromotive force; The inner conductor will not be affected by the shielding of the outer conductor. On a certain length of cable, the induced pressure difference between the inner and outer conductors is likely to accumulate from one end to the other, which is enough to damage the connected equipment. However, when we connect the balance conversion device in series in the cable to make the internal and external conductors have the same chance of induction, it can effectively eliminate the induced voltage difference caused by lightning and achieve the purpose of lightning protection

it is known from practice that the spectrum of lightning wave is between tens of Hertz and one megahertz, and its energy is mainly concentrated in the tens of kilohertz frequency band. According to national regulations, the spectrum of TV modulation signal is more than 40 megahertz, which has a large interval from the spectrum of lightning wave. It is easy to distinguish the two with a simple filter, which is convenient for us to manufacture balanced conversion devices. Figure 1 shows the parallel conversion lightning arrester with circuit structure

c1, C2 and C3 are high pass filters, which are used to let TV signals pass; Second, keep the non parallel structure of signal transmission (C1 and C2 represent the multipoint coupling of outer conductors) and match the cables connected at both ends; Third, if the lightning wave is blocked and continues to accumulate along the outer conductor, activators (zinc oxide, urea, etc.) can be used to reduce the decomposition temperature of the foaming agent. L1 and L2 are low-pass filters. The first function is to prevent the signal from "short circuit"; Second, the induced electromotive force on the outer conductor that has induced lightning is transferred to the inner conductor at the other end to prevent it from being induced again. The inner conductor that has not induced lightning is connected to the outer conductor at the other end, so that the outer conductors of the cables at both ends have equal opportunities to be induced and accumulate to the cable port. The induced intensity of the inner and outer conductors is the same, effectively eliminating the induced voltage difference between the inner and outer conductors to achieve the purpose of lightning protection

in the CATV system, the amplifier has two power supply modes, independent mains power supply and centralized low-voltage overcurrent power supply (also divided into DC power supply and AC power supply). Among them, the amplifier with independent mains power supply, each stage may introduce sound current from the mains power, and the urban self-defense of the power supply must be considered additionally. When the power failure of the area where the first stage is located will affect the transmission of the next stage of the signal, so it is less used for multi-stage transmission. In the centralized low-voltage overcurrent amplifier transmission system, only one power supply is required, as long as it works normally, and the cables at the same level act as both signal transmission lines and transmission lines, Even if the warehousing inspection of rare earth storage in one of the amplifiers will be carried out in the near future, the signal transmission will not be affected. Moreover, the amplifier has no power line and does not need to consider the lightning problem of the power line. Therefore, it is often used in multi-level transmission systems, but it has its advantages and disadvantages, because in the overcurrent amplifier, the core wire at the input end and the output end is short circuited at low frequency to make the power supply current flow, It is equivalent to extending and connecting the coaxial cable connected with the input and output terminals. When lightning occurs, the outer conductor is induced, and the inner conductor is not induced and has been connected together, which increases the inner and outer layers. The new external wall insulation material with good fire resistance, environmental protection and energy conservation will have a good opportunity to show its skill, and the intensity of the induced differential pressure of the conductor will be increased. For the amplifier, it also increases the difficulty of preventing inductive lightning strike from the coaxial cable, but the use of balanced lightning arrester will solve this problem

the specific usage is as follows:

1. In the low-voltage AC power supply system, if the grounding lightning protection method is adopted on the power supply (because the power supply has another power supply lead, the grounding lightning protection is not redundant), the bit output voltage is not affected by lightning, and a balanced lightning protection device is installed at half the length of the coaxial cable connecting the power supply and the first amplifier, then, Unless the lightning discharge point is very close to one end of the cable (this opportunity is very small), it can be considered that the distance between the discharge point and the cable at both ends of the lightning arrester is equal or close, and the cable at both ends of the lightning arrester has strong flexibility. The lightning induction intensity of the cable at both ends of the lightning arrester is close to or equal, and the induced differential pressure of the inner and outer conductors of the wire at the amplifier port is eliminated, which protects the amplifier. Similarly, A lightning arrester is also installed at half of the cable connected between the first amplifier and the second amplifier, and the second amplifier is also protected. By analogy, each amplifier powered by this power supply can also achieve effective lightning protection without using grounding or other lightning protection methods, greatly reducing construction intensity and saving costs. Moreover, for the amplifier with single tube half wave rectification, due to the "commutation" effect of lightning arrester, the upper and lower half cycles of AC can be used, which improves the utilization rate of power supply. Schematic diagram of balanced lightning protection method for low-voltage AC power supply overcurrent system

the independent mains power supply amplifier system can also adopt the above method to prevent induction lightning from being introduced from the coaxial line, but other methods must be taken to prevent lightning from being introduced from the power lead

2. In the low-voltage DC overcurrent power supply system, due to the "commutation" effect of the lightning arrester, it must be connected in two, so that the inner and outer conductors with opposite polarity can be used to restore the polarity of the output of the power supply, so that the amplifier of the same model can be powered, and its lightning protection effect is the same as that of the AC power supply. The schematic diagram of the balanced lightning protection method of the DC power supply overcurrent system is added and left as 3.

the above is the lightning protection method to achieve balance by means of "frequency division", It is not only suitable for CATV system, but also for other coaxial transmission systems whose signal frequency can be separated from lightning frequency

if the signal frequency is the same or similar to the lightning wave frequency, the "time division" method can be used to achieve balance, that is, when the cable is in the "easy to connect" state at ordinary times, when the lightning is salty, the inner and outer conductors at both ends are connected with each other, so that the outer conductors at both ends have the same chance of induction, and the induced voltage base balance of the inner and outer conductors can also be maintained to avoid the delivery of induced lightning

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