Solution and Case

This scheme is applicable to the lightning protection design and model selection of safety (urban) projects, urban video monitoring system lightning protection, rural video monitoring system lightning protection, public security video monitoring system lightning protection, outdoor monitoring system, monitoring center machine room lightning protection, factory enterprise perimeter protection system lightning protection, bank securities hospital monitoring system lightning protection, schools, mines, airports, transportation and other types of monitoring system networks.

At present, the video monitoring system is widely used in various fields of security. Because most of the equipment composed of the television monitoring system has low overvoltage resistance, the overvoltage generated by lightning electromagnetic induction and lightning electromagnetic pulse will interfere with the equipment and even cause permanent damage. The consequences may cause the operation of the entire television monitoring system to malfunction and cause incalculable economic losses. Therefore, it is necessary to strengthen and improve the lightning protection of video monitoring system.

In order to effectively formulate lightning protection measures for video surveillance and ensure the normal operation of the system, first of all, it is necessary to identify the ways in which the system may be subject to lightning intrusion, correctly select and use the lightning protection devices for the monitoring system equipment, and reasonably lay, shield and ground the signal and power lines, which can effectively improve the lightning resistance of the monitoring system and optimize the lightning protection effect of the system.

Composition of video monitoring system and lightning hazard analysis

1. Components

Front end part: mainly composed of camera, lens, PTZ, decoder, protective cover, support, etc.

Transmission part: coaxial cable, wire, multi-core wire, twisted pair, optical fiber, etc. are used to transmit video, audio or control signals by means of overhead, buried or laid along the wall.

Terminal part: mainly composed of picture splitter, monitor, control equipment, TV curtain wall, console, etc.

2. Lightning invasion route

Direct lightning strike: the lightning directly strikes the camera in the open air, causing damage to the camera due to electrical, thermal and mechanical effects; Lightning directly strikes the transmission cable, causing damage or fusing of cable insulation.

Lightning wave invasion: when the power line, video line, control line of the monitoring system or the metal pipeline entering the monitoring room are struck by lightning or lightning induced over-voltage, the lightning wave invades the equipment along these metal wires, causing damage to the equipment.

Electromagnetic induction: when lightning strikes the lightning rod, strong transient electromagnetic field will be generated around the downlead. Monitoring equipment and transmission lines in the electromagnetic field will induce large electromotive force. This phenomenon is called electromagnetic induction. When there is a charged thunderstorm cloud, the buildings and transmission lines under the thunderstorm cloud will sense the opposite charge to the thunderstorm cloud. This induced charge can reach 100kv on low-voltage overhead lines and 40-60kv on signal lines. This phenomenon is called electrostatic induction. Electromagnetic induction and electrostatic induction are called induction lightning, also called secondary lightning. Its damage to equipment is not as severe as direct lightning strike, but it is much more likely than direct lightning strike.

3. Lightning protection design scheme

design basis

IEC61024 Lightning Protection of Buildings
IEC61312 Protection against Lightning Electromagnetic Pulse

JGJ/T16-92 Code for Electrical Design of Civil Buildings

GB50343-2004 Technical Code for Lightning Protection of Building Electronic Information System

GB50057-94 Code for Design of Lightning Protection of Buildings

GB50174-93 Code for Design of Electronic Computer Room

GB50198-94 Technical Code for Civil Closed Circuit Monitoring Television System Engineering

GB/T50311-2000 Code for Engineering Design of Generic Cabling System in Buildings and Building Groups

3.1 Scheme design

1. Lightning protection of front-end equipment

Front end equipment can be installed outdoors or indoors. Equipment installed indoors will not suffer from direct lightning stroke generally, but it is necessary to consider preventing lightning overvoltage from damaging equipment, while outdoor equipment should also consider preventing direct lightning stroke.

Front end equipment such as cameras shall be placed within the effective protection range of air terminals (lightning rods or other lightning conductors). When the camera is erected independently, the lightning rod should be 3-4 meters away from the camera. If it is difficult, the lightning rod can also be erected on the support rod of the camera, and the downlead can directly use the metal rod itself or select Φ 8 or more galvanized round steel. To prevent electromagnetic induction, the power line and signal line of the camera led up along the pole shall be shielded through metal tubes.

To prevent the lightning wave from invading the front-end equipment along the line, each line in front of the equipment shall be equipped with a surge protector corresponding to the lightning protection energy, such as power line (AC220V or DC12V), video line, signal line and PTZ control line.

The power supply of the camera is generally AC220V or DC12V. If the camera is powered by a transformer, the single-phase surge protector shall be connected in series or parallel at the front end of the transformer. If the DC power transmission distance is greater than 15 meters, the camera end shall also be connected in series with the low-voltage DC surge protector.

The signal line has a long transmission distance, low withstand voltage level, and is very easy to induce lightning current to damage the equipment. In order to transmit lightning current from the signal transmission line to the ground, the signal surge protector must respond quickly. When designing the protection of the signal transmission line, the transmission rate, signal level, starting voltage, lightning flux and other parameters must be considered.

Thoutdoor front-end equipment shall be well grounded, with the grounding resistance less than 4 Ω, and the area with high soil resistivity can be extended to less than 10 Ω.

3.2 Lightning protection of transmission lines

Transmission lines mainly include transmission signal lines (video lines, PTZ control lines) and power lines. The power supply of outdoor camera can be introduced from terminal equipment or from the power supply near the monitoring point.

Control signal transmission lines and alarm signal transmission lines generally use multi-core shielded flexible cables, video signal lines generally use coaxial cables, and a few use twisted pairs, which are erected (or laid) between the front end and the terminal.

When the transmission lines are laid in the suburbs and villages, the direct burial method can be used. When conditions are not sufficient, communication pipeline or overhead mode can be used. At this time, the minimum distance between the transmission cable and other lines in the trench and the minimum vertical distance between the transmission cable and other lines in the same pole can be laid according to GB50198-94 Technical Code for Civil Closed Circuit Monitoring Television System Engineering. For example, the minimum distance between transmission cables and 220V AC lines in the same trench (tunnel) is 0.5 m, and the minimum distance between transmission cables and communication cables is 0.1 m; The minimum vertical distance between transmission cables and 1-10KV power lines is 2.5m, the minimum vertical distance between power lines below 1KV is 1.5m, the minimum vertical distance between transmission cables and broadcast lines is 1.0m, and the minimum vertical distance between transmission cables and communication lines is 0.6m.

When the transmission line is laid inside the building, the minimum distance between it and other cables shall refer to GB50343-2004 Technical Code for Protection of Building Electronic Information System against Lightning. the perspective of lightning protection, the direct buried laying method has the best lightning protection effect. Overhead lines are most vulnerable to lightning strikes, which are destructive and have a wide range of impacts. In order to avoid damage to the equipment at the head and tail ends, the overhead lines should be grounded on each pole during transmission. The suspension wires of overhead cables and the metal pipes in the overhead cable lines should be grounded. The signal source and power supply at the input end of intermediate amplifier shall be connected to appropriate lightning arrester respectively.

The buried laying of transmission line cannot prevent the occurrence of lightning strike equipment. A large number of facts show that lightning strike causes the fault of buried cable, accounting for about 30% of the total fault. Even if the lightning strike is far away, some lightning current will still flow into the cable. Therefore, cables with shielding layer or cables are buried through steel pipes to keep the electrical connection of steel pipes. It is very effective to prevent electromagnetic interference and electromagnetic induction, mainly due to the shielding effect of metal tubes and the skin effect of lightning current. If it is difficult for the cable to pass through the metal pipe in the whole process, it can be buried in the metal pipe before the cable enters the terminal and front-end equipment, but the buried length shall not be less than 15m. The metal sheath and steel pipe of the cable shall be connected with the lightning protection grounding device at the entrance end.

3.3 Lightning protection of terminal equipment

In the monitoring system, the lightning protection of the monitoring room is the most important, which should be carried out from direct lightning protection, lightning intrusion, equipotential bonding, surge protector and other aspects.

The building where the monitoring room is located shall be equipped with lightning rod, lightning strip or lightning network to prevent direct lightning strike. The direct lightning protection measures shall comply with the provisions on direct lightning protection in GB50057-94.

All kinds of metal pipelines entering the monitoring room shall be connected to the grounding device against inductive lightning. When overhead cables are directly led in, surge protectors shall be installed at the entrance, and the metal outer sheath of cables and self supporting steel cables shall be connected to the grounding device.

The monitoring room shall be equipped with an equipotential bonding bus (or metal plate), which shall be connected with the building lightning protection grounding, PE line, equipment protection grounding, anti-static grounding, etc. to prevent dangerous potential difference. The grounding wires of various surge protective devices shall be electrically connected with the equipotential bonding busbar at the straightest and shortest distance.

Good grounding is a key link in lightning protection. The smaller the grounding resistance is, the lower the overvoltage is. When the monitoring center adopts special grounding device, its grounding resistance shall not be greater than 4 Ω. When integrated grounding grid is adopted, its grounding resistance shall not be greater than 1 Ω.

3.4 Selection of surge protector

3.4.1 Front end

A set of LM-SN3 surge protector is installed in front of the power supply, video and PTZ control lines of each PTZ camera gun for lightning protection of PTZ camera gun.

A set of LM-SN2 two in one camera gun surge protector is installed in front of the power supply and video line of each fixed camera for lightning protection of fixed camera gun.

Or implement the following methods:

A set of LM-XD275-20/2P or LM-DC20-24, LM-DC20-12 power surge protector shall be installed in front of the power line of each camera gun for lightning protection of the camera gun power line.

A set of LM-RJ45E100 video signal surge protector is installed in front of the video line of each camera gun for lightning protection of the camera gun video line.

A set of LM-CSV control signal surge protector is installed in front of the PTZ control line of each camera gun for lightning protection of the camera gun control signal line.

3.4.2 Transmission part

A set of LM-BNC/TV coaxial video signal surge protector is installed on the signal line at the input and output ends of the intermediate amplifier.

A set of LM-DC110 power supply surge protector is installed on the power line at the input end of the intermediate amplifier to protect the intermediate amplifier.

3.5 Terminal

Machine room power system

Since most lightning strike high potentials are intruded from the power line, in order to improve and ensure the safety of the equipment, generally the power supply should be provided with lightning protection of level 3 or above.

a. Considering the space limitation of the monitoring room, B+C composite surge protector (LM-KB255-25BC40) can be installed in the distribution box of the monitoring room. It can solve the limitation of small installation distance of the first and second stages. It is equipped with T1 and T2 combined installation, and can automatically match the energy to meet the requirements of the coupler; Large flow capacity; Low residual voltage (≤ 2KV); 35mm guide rail installation and other advantages.

b. Install single-phase surge protector (LM-XD275-20) in front of UPS power supply or monitoring equipment in the monitoring room, with ultra-low residual voltage output and nanosecond response speed as the third level of fine protection for power lines. Or install the power lightning protection socket (LM-PDU16) on the monitor, video matrix, control equipment and other power lines, plug in installation, convenient and flexible, ultra-low residual voltage output, as the third level of fine protection of power lines.

Signal part

Install a video signal surge protector (LM-RJ45E100) before the video transmission cable enters the video matrix or other equipment in the computer room to protect the video matrix.

Install the control signal surge protector (LM-CSV) before the PTZ control line enters the control equipment of the machine room to protect the control equipment.