Comprehensive lightning protection measures
Modern lightning protection includes external lightning protection (direct lightning protection of buildings or facilities) and internal lightning protection (lightning electromagnetic pulse protection). The external lightning protection system is mainly to protect buildings from fire accidents and personal safety accidents caused by direct lightning strike, while the internal lightning protection system is to prevent lightning wave intrusion, lightning induced overvoltage and system operation overvoltage from invading equipment, which cannot be guaranteed by the external lightning protection system.
lightning protection is a very complicated problem. It is impossible to completely eliminate the impact of lightning overvoltage and induced overvoltage by one or two advanced lightning protection equipment and measures. We must eliminate all kinds of factors that may cause lightning strike according to the invasion route of lightning damage, and adopt comprehensive prevention and control-lightning, voltage sharing, shielding, grounding and shunt (protection) to minimize lightning damage.
1. Lightning connection
The lightning connection device is the lightning rod, lightning belt, lightning wire or lightning protection network that we often say. Lightning connection means that the lightning discharge in a certain range cannot be arbitrarily selected, but the lightning energy can only be discharged into the earth according to the specified channel of the lightning protection system designed by people in advance.
2. Voltage-sharing
When lightning strikes, the down conductor immediately generates a high potential, which will cause side flashover to the conductors around the lightning protection system that are still at ground potential, and make their potential rise, thus posing a hazard to personnel and equipment. In order to reduce the risk of flashover, the simplest way is to use the equalizing ring to connect the conductors at ground potential to the grounding device. If the distance between indoor metal facilities, electrical devices and electronic equipment and the conductor of lightning protection system, especially the lightning arrester, cannot meet the specified safety requirements, they should be equipotentially connected with lightning protection system with thick wires. In this way, when lightning current passes through, all indoor facilities immediately form an "equipotential island" to ensure that no harmful potential difference occurs between conductive parts and no side flashover discharge occurs. Perfect equipotential bonding can also prevent the counterattack caused by the rise of ground potential caused by lightning current entering the ground.
in order to completely eliminate the destructive potential difference caused by lightning, it is particularly necessary to carry out equipotential connection. Power lines, signal lines, metal pipes, etc. should be equipotentially connected through overvoltage protectors, and the interface of each inner protection zone should also be partially equipotentially connected accordingly, and finally connected to the equipotential connection bus.
3. Shielding
Shielding is to surround the object to be protected with conductors such as metal mesh, foil, shell or pipe, so that all the channels invaded by lightning electromagnetic pulse waves are cut off. All shields, shells, etc. need to be grounded.
shielding is the most effective method to prevent the influence of lightning electromagnetic pulse radiation on electronic equipment.
4. Grounding
Grounding means that the lightning current that has entered the lightning protection system can flow into the earth smoothly, but the lightning energy cannot be concentrated in a certain part of the lightning protection system to cause damage to the protected object. Only by good grounding can the lightning energy be effectively discharged, the voltage on the down lead can be reduced, and counterattack can be avoided.
in the past, some specifications required electronic equipment to be grounded separately, in order to prevent stray current or transient current in the power grid from interfering with the normal operation of the equipment. Before the 199s, the communication and navigation equipment of the army was mainly electronic tube devices, and analog communication was adopted. Analog communication was particularly sensitive to interference. In order to prevent interference, the power supply and communication grounding were separated. At present, the field of lightning protection engineering does not advocate separate grounding. Independent grounding is not advocated in IEC standards and ITU related standards. American standard IEEEStd11-1992 points out more pointedly that it is not recommended to use any so-called separated, independent, computer, electronic or other incorrect grounding body as a connection point of equipment grounding conductor. Lightning protection grounding is the most basic link in the lightning protection system, and it is also the most basic safety requirement in the lightning protection installation acceptance specification. If the grounding is not good, the lightning protection effect of all lightning protection measures cannot be exerted.
5. Shunt (protection)
This is the focus of the rapid development of modern lightning protection technology and the key measure to protect various electronic equipment or electrical systems.
the so-called shunt is to connect an appropriate lightning arrester SPD in parallel between all conductors coming from the outdoor (including power supply lines, data lines, telephone lines or antenna feeders, etc.) and lightning protection grounding devices or grounding wires. When the overvoltage wave generated by direct lightning or lightning strike on the lines enters the room or equipment along these wires, the resistance of the lightning arrester suddenly drops to a low value, which is close to a short circuit state, and the lightning current flows into the ground. After shunting, a small part of lightning current will still enter the equipment along the conductor, which is very dangerous for some microelectronic devices that are not resistant to high voltage. Therefore, for such devices, multi-stage shunting (that is, no less than three levels of lightning protection) should be carried out before the conductor enters the casing.
now the research and development of lightning arrester is beyond the scope of shunt. Some lightning arresters can be directly connected in series to the signal line or the feeder of the antenna, which can let the useful signals pass smoothly and block the lightning overvoltage wave.
when shunt lightning protection measures are adopted, special attention should be paid to the selection of lightning arrester performance parameters, because the installation of additional facilities will affect the performance of the system more or less. For example, the access of signal arrester should not affect the transmission rate of the system; The loss of the feeder arrester in the passband should be as small as possible; If it is used in directional equipment, it cannot lead to positioning error.
6. Avoid
When choosing the location of building infrastructure, you should avoid the places with many minefields or lightning strikes, so as not to increase the expenses and expenses of lightning protection projects in the future.
when lightning occurs, turn off the equipment and unplug the power supply.
lightning protection design scheme of network computer room 27-5-1 7:7 At present, with the rapid development of computer and network communication technology, the computer network system has higher and higher requirements for lightning protection. Due to the weak protective measures or cognitive deviation, the due protective effect is often not achieved, and the computer room suffers from lightning strikes frequently. Especially in thunderstorm season, some electronic and electrical equipment of computer network system is disturbed by lightning strike, and some are burned by lightning strike, resulting in direct economic losses. The lightning protection of computer network system should be paid enough attention to, so as to be prepared, and the lightning protection facilities should be rectified and the overall protection measures should be taken to better maintain the safe operation of the computer room.
ii. solutions
1.1 direct lightning protection for buildings
according to the requirements of the national standard GB 557-94 "code for lightning protection design of buildings", the building where the computer rooms of important computer network systems are located is the second or third type of lightning protection building, and generally lightning protection facilities are built as required, such as lightning protection network (belt) on the top of the building, lightning rods or mixed lightning receptors, etc. These lightning receptors, The computer system is located in the building and protected by the lightning protection system of the building, so the possibility of direct lightning hitting the computer network system is very small, so it is usually unnecessary to install equipment to protect the direct lightning.
1.2 computer network system induced lightning protection
induced lightning is generated by electrostatic induction or electromagnetic induction, which has a high probability of forming induced lightning voltage and poses a great threat to low-voltage electronic equipment in buildings. The lightning protection work of computer network system focuses on preventing the intrusion of induced lightning. There are three main ways of lightning overvoltage and overcurrent invading the computer system:
(1) invading from AC power supply line
The power supply of the computer system is input into the room from outdoor overhead power lines, which may suffer from direct lightning and induced lightning; Direct lightning strikes the high-voltage power line and is coupled to the 38V low-voltage side through the transformer, invading the computer power supply equipment; In addition, low-voltage lines may also be hit by direct lightning or induce lightning overvoltage. The lightning overvoltage on the 22V power line can reach 1V on average, which can cause a devastating blow to the computer network system.
(2) intrusion by computer communication lines
intrusion by computer communication lines can be divided into three situations.
case 1: when the ground protrusion is struck by direct lightning, the strong lightning voltage will break through the adjacent soil, and the lightning current will directly invade the cable sheath, and then break through the sheath, so that the high voltage will invade the line.
case 2: when Lei Yun discharges to the ground, it induces an overvoltage of thousands of volts on the line, which damages the electrical equipment connected to the line and invades the communication line through the equipment connection. This kind of invasion spreads along communication lines, involving a wide range and causing great harm.
case 3: if wires from different sources are connected by a multi-core cable or multiple cables are laid in parallel, when a wire is struck by lightning, an overvoltage will be induced in the adjacent wires, which will damage the low-voltage electronic equipment.
(3) Ground potential counterattack voltage invades through the grounding body
When lightning strikes, powerful lightning current leaks into the earth through the down conductor and the grounding body, and there is a radial potential distribution near the grounding body. If other grounding bodies connected with electronic equipment approach, a high-voltage ground potential counterattack is generated, and the invasion voltage can reach tens of thousands of volts. The lightning protection of buildings against direct lightning introduces a strong lightning current into the ground through the down conductor, which produces a strong electromagnetic field change in the nearby space and induces lightning overvoltage on adjacent wires (including power lines and signal lines). Therefore, the lightning protection system of buildings can not protect the computer system, but may introduce lightning current. The integrated circuit chip of computer network system and other equipment has weak withstand voltage, usually below 1V V. Therefore, it is necessary to establish a multi-level computer lightning protection system to ensure the safety of computer network system.
2. Solution
( 1) The impact on lightning magnetic field is mainly that when direct lightning strikes the computer room building, the distribution of lightning current in the building directly affects computer network system equipment, especially computers and network communication terminal equipment sensitive to electromagnetic interference. Reasonable selection of the location of the computer room and the reasonable layout of the equipment in the computer room can effectively reduce the lightning damage.
( 2) Install SPD at the interface between power supply system and computer network terminal equipment, and take measures such as shielding and grounding cables entering and leaving the computer room to realize equipotential connection, which can effectively reduce the damage of lightning overvoltage to computer network system equipment.
( 3) Using common earthing in the computer room can effectively solve the influence of rising ground potential, and qualified ground network is the key to effective lightning protection. The combined grounding network of the computer room usually consists of the foundation of the computer room building (including piles), the ring grounding device, the working (power transformer) grounding network, etc. For the lightning protection of sensitive data communication equipment, whether the grounding system is good or not is directly related to the effect and quality of lightning protection. If the grounding grid does not meet the requirements, the conditions of the grounding grid should be improved, the area of the grounding grid should be appropriately expanded and the structure of the grounding grid should be improved, so that the lightning current can be discharged as soon as possible and the holding time of the high overvoltage caused by the lightning current can be shortened to meet the lightning protection requirements.
III. Examples
1. Basic information
The computer room of a company is located on the third floor of the building where the company is located, and the building has external lightning protection facilities such as lightning rods and lightning strips; The power supply system of the computer network system is supplied by the mains three-phase low-voltage power supply, the power supply for the machine room is directly supplied to the building distribution box by the distribution cabinet in the distribution room, and the UPS power supply equipment is supplied from the building distribution box to the machine room distribution box; UTP twisted-pair cable is used for the computer network communication line in and out of the computer room, voice cable is used for the special communication line, and BNC interface coaxial cable is used for the satellite feeder. The grounding of the computer room uses the building grounding network.
2. Scheme design
The building where the computer room is located has external lightning protection facilities such as lightning rod and lightning belt, so it is no longer necessary to make supplementary design for external lightning protection. The lightning electromagnetic pulse protection of computer network system is designed according to Class A requirements, and the power supply system is protected by Class 3~4 surge protector (SPD). The network communication system adopts fine protection, and properly installs signal interface surge protector (SPD) for cables and wires entering and leaving the protection zone. Common earthing shall be implemented in the computer room, a qualified grounding system shall be established, and equipotential connection shall be implemented for pipes, wires and troughs entering and leaving the interface of the protected area. Effectively reduce the lightning overvoltage to the level that the equipment can bear. The design contents mainly include:
(1) the design of transient overvoltage protection for equipment in computer room;
(2) the design of equipotential connection in the machine room;
(3) Design of grounding grid.
3. transient overvoltage protection of power supply equipment in computer room
as an area to be protected, the computer network computer room can be divided into several protection zones from the outside to the inside from the EMC (electromagnetic compatibility) point of view. Outside the building, there is a direct mine area, and the equipment in this area is the most vulnerable and the most dangerous, and it is the exposed area, which is zone ; The location from the inside of the building to the computer room is a non-exposed area, which can be divided into area 1 and area 2. The farther inside, the lower the risk. Power line is one of the main ways of lightning overvoltage intrusion. From the low-voltage output end of the transformer in the main distribution room to the equipment end of the computer room, classified protection must be implemented to reduce the lightning overvoltage to the level that the equipment can bear.
3.1 configuration of power arrester
(1) The power output of the main distribution cabinet in the main low-voltage distribution room is equipped with a three-phase box-type power arrester as the first-level lightning protection. The nominal discharge current is 5 ~ 1 kA to prevent direct lightning.
(2) The power inlet of the main distribution box on the building floor where the network equipment is located is equipped with a box-type power arrester as the second-level lightning protection. Three-phase box arrester is equipped, and the nominal discharge current is 4kA to prevent induced lightning strike or operating overvoltage.
(3) The power inlet of the distribution box in the network equipment room is equipped with a power arrester as the third-level lightning protection. It is equipped with single-phase box arrester, and the nominal discharge current is 2kA to prevent induced lightning strike or operating overvoltage.
(4) The modular power arrester is used at the important network cabinet or equipment end as the fourth-level lightning protection. The nominal discharge current is 5kA to prevent induced lightning strike or operating overvoltage.
3.2 configuration of lightning arrester for data (signal) communication interface
according to the specific situation of communication equipment, lightning protection of data (voice) or video signal lines introduced from outdoors is mainly considered. Lightning arresters are mainly connected in series at the interface of equipment at both ends of the line.
(1) A single-port RJ45 signal arrester is installed at 1M input port of the server to protect the server.
(2)2