When buildings are struck by direct lightning or nearby buildings are struck by direct lightning, dangerous overvoltage may occur in the lightning center 1.5 km ~ 2 km, which will damage the equipment along the line. Under its action, lines and equipment will surge due to overvoltage and overcurrent. In order to minimize the damage of lightning electromagnetic pulse to safety protection system, it is necessary to discuss the particularity and universality of lightning strike to safety protection system.
Through the analysis of the lightning-struck systems in the past, it is found that most of the lightning-struck systems were not designed and constructed according to the relevant lightning protection specifications. Therefore, students should master lightning protection knowledge and relevant specifications, strictly control design and construction, and design, construct and maintain the system in strict accordance with relevant lightning protection specifications.
1) Direct lightning strike-Lightning strikes buildings, other objects, the earth or lightning protection devices directly, resulting in electrical effect, thermal effect and mechanical force.
In the security system, lightning directly invades the outdoor camera and alarm detector, causing equipment damage; Lightning directly hits the overhead cable, causing the cable to melt. These phenomena may be caused by direct lightning strike.
2) Lightning induction-electrostatic induction and electromagnetic induction generated on nearby conductors during lightning discharge may cause sparks between metal parts.
In the security system, when the lightning rod is struck by lightning, a strong transient electromagnetic field will be generated around the down conductor. The monitoring equipment and transmission lines in the electromagnetic field will induce a large electromotive force, which is called electromagnetic induction. When there are charged thunderclouds, buildings and transmission lines under Lei Yun will induce charges opposite to those in Lei Yun. The low-voltage overhead lines can reach 100KV, and the signal lines can reach 40 ~ 60 kV. This phenomenon is called electrostatic induction. Electromagnetic induction and electrostatic induction are called induced lightning, also called secondary lightning. Its damage to equipment is not as serious as direct lightning, but the probability is much greater than direct lightning; When the transmission of the power line, signal line and control line of the security system or the metal pipeline entering the monitoring center is struck by lightning or lightning induction, lightning waves invade the equipment along these metal lines, forming a huge potential difference to damage the equipment;
3) Lightning surge-electromagnetic radiation related to lightning discharge, which can generate electric and magnetic fields that can be coupled to electrical (electronic) systems, resulting in destructive impact current or voltage.
Surge protection of safety protection system equipment: For safety protection system, once overvoltage is induced on cables, overvoltage protectors must be installed on all incoming and outgoing cables. Due to the function of overvoltage protector, the voltage of each port of the system is almost equal (i.e. equipotential), thus protecting the system from damage.
4) Classification of lightning activity area: According to the annual average thunderstorm days, lightning activity is divided into less minefield, more minefield, high minefield and strong minefield.
Less minefield: an area with an average annual thunderstorm day of less than 20 days;
Multi-minefield: an area with an average annual thunderstorm day of more than 20 days and no more than 40 days;
High minefield: the area where the annual average thunderstorm days are more than 40 days and less than 60 days;
Strong minefield: an area where the annual average thunderstorm days exceed 60 days.
5) Division of lightning protection zone (as shown in the figure below):
LPZ0A area: all objects in this area may be directly hit by lightning and take away all lightning current; The intensity of the electromagnetic field in this area is not attenuated.
LPZ0B area: It is impossible for all objects in this area to be directly hit by lightning current larger than the radius of the selected rolling ball, but the electromagnetic field intensity in this area is not attenuated.
LPZ 1 area: all objects in this area cannot be directly hit by lightning, and the current flowing through each conductor is smaller than that in LPZ0n area; According to the shielding measures, the electromagnetic field intensity in this area may be weakened.
LPZn+ 1 Follow-up lightning protection zone: When the inflow current and electromagnetic field intensity need to be further reduced, the follow-up lightning protection zone should be added, and the requirements of the follow-up lightning protection zone should be selected according to the environmental area required by the protected object.
Note: n= 1, 2, ...
6) lightning protection classification of buildings: GB 50057 Code for Lightning Protection of Buildings classifies buildings into three categories according to their importance, nature of use and possible consequences caused by lightning accidents. See chapter 2 of GB 50057 for details.
In chapter 5 of GB 50057, the protection range of lightning arrester is determined by rolling ball method. The rolling ball method is a sphere with a radius of hr, which rolls along the part that needs to be protected from direct lightning. When the sphere only touches the lightning arrester (including the metal object used as the lightning arrester), or only touches the lightning arrester and the ground (including the metal object that is in contact with the ground and can withstand lightning strike), and does not touch the part that needs protection, then this part.
Specification setting: the rolling thunder radius hr of Class I building is 30m；; The rolling thunder radius hr of the second-class building is 45m；; The rolling thunder radius hr of the third-class building is 60m.
7) Selection of lightning protection level of building electronic information system
According to the lightning protection grade of GB 50343, it is divided into four grades: A, B, C and D:
Class A: large computing centers, large communication hubs, national financial centers, banks, airports, large ports and train hubs; Video security monitoring system and alarm system of national cultural relics archives.
Class B: medium-sized computing center, medium-sized communication hub, mobile communication base station, large stadium (gymnasium) monitoring system and securities center; Video security monitoring system and alarm system of provincial cultural relics archives, radar station, microwave station and expressway monitoring and charging system; Medium-sized electronic medical equipment; Four-star hotel.
Class C: small communication hubs, large and medium-sized cable TV systems, hotels below three stars.
Class d: general electronic information system equipment other than the above-mentioned classes A, B and C. ..
8) External lightning protection device: it is composed of lightning receptor, down conductor and grounding device, which is mainly used as a protection device to prevent direct lightning strike.
The main purpose of installing lightning arresters is to protect buildings from fire accidents and personal safety accidents caused by lightning strikes, and also to avoid damage to electrical equipment in buildings.
The front-end equipment that must be installed in the non-direct lightning protection zone (LPZ0A) shall be protected by direct lightning. Install a lightning rod on the camera rod.
9) Internal lightning protection device: It consists of equipotential connection system, grounding system, shielding system, reasonable wiring system and surge protector. It is mainly used to reduce and prevent the electromagnetic effect caused by lightning current in the protected space.
The internal lightning protection system is to prevent lightning and its various forms of overvoltage from invading the equipment, which cannot be guaranteed by the external lightning protection system. In order to realize internal lightning protection, it is necessary to connect lightning protection devices and overvoltage protectors in cables and metal pipelines entering and leaving the protected area and implement equipotential grounding.
10) equipotential connection: an electrical connection in which the potential of the exposed conductive parts of equipment and devices is basically equal.
In order to eliminate the destructive potential difference caused by lightning, equipotential grounding is needed. Power line, signal line, metal tube, etc. Equipotential bonding should be carried out, and the interfaces of each internal protection zone should be partially equipotential bonded accordingly. All local equipotential joints should be connected to each other and finally connected to the general equipotential joint.
1 1) equipotential bonding belt: a metal belt that connects cables such as metal devices, foreign conductors, power lines and communication lines and equipotential bonding with lightning protection devices.
12) equipotential bonding network: a network of conductors that are equipotentially bonded in the exposed conductive part of a system.
13)*** grounding system: grounding system connecting lightning protection device, building metal components, low-voltage distribution protection line (PE), equipotential bonding belt, equipment protection grounding, shielding grounding, anti-static grounding and grounding device.
14) natural grounding body: the general name of all kinds of metal components, metal well pipes, steel bars in reinforced concrete, buried metal pipes and facilities with grounding function but not specially set for this purpose that are in good contact with the earth.
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