Lightning-proof in Surveillance Equipments
1.Introduction
As CCTV system is more and more popular in bank, traffic, residential district and store management, it is more probable for surveillance equipments to be stricken by thunder than before. If the probability becomes true, whole system maybe out of operation and loses seriously.
In order to protect CCTV system efficiently and guarantee that system normal works, first, we should know why system is stricken by thunder and the method of thunder invading, especially for outdoor surveillance devices which is destroyed by thunder most greatly; we must analyze the reason, and then choose right lightning-proof kits for system. We also should research configuration of signal channel as well as power lines. Thus Security Company can improve skill in anti-lightning ability greatly.
2.Construction of CCTV system and reason for lightning disturbance
CCTV is fully called Closed Circuit Television. It is commonly composed by the below parts:
Forehead section: include BW (color) camera, lens, pan / tilt, housing, bracket etc.
Transmission section: transmitting signal by coaxial-cable, wire, multi-core wire, which are set in air, underground and wall.
Terminal section: include image quads, monitor, controlling devices etc.
The reason for lighting disturbance in CCTV system
Direct lightning strike: lightning directly strikes camera destroyed; or lightning directly strikes air-cable burnt.
Lightning wave disturbance: power wires as well as coaxial or metal tube inside monitor room, can be stricken by lightning wave as to destroy equipments.
Lightning inductance: when lightning rod is stricken, the down-lead will be around by Transient Electromagnetic Field. The monitoring equipments and coaxial will appear electric motion. This is the called electromagnetic induction. When lightning cloud appears with electricity, the bellow buildings and coaxial will create opposite electron. This kind electron on the air wire can reach 100kv and on coaxial can reach 40-60kv. This phenomenon is called static-induction. Electromagnetic induction and static induction are called inductive lighting or secondary lighting which takes place more probably than direct lighting, though it results less destruction relatively.
3.Cooperative anti-lightning in CCTV system
Forehead equipments anti-lightning
Forehead equipments can be indoor or outdoor. Indoor equipments may not be stricken by lightning, but we should avoid; outdoor equipments should avoid direct lightning strike as well as lightning wave destruction.
Forehead equipments such as camera should be set under the protection of lighting air-termination system (lighting rod or other lighting receiver). When camera is installed alone, lightning rod should be away 3-4m distance from camera. If there are troubles, lightning rod can be installed on the bracket of camera. Besides, the down-lead can be replaced by metal bracket or adoptΦ 8 steel. In order to avoid creating electromagnetic induction, electric wire and coaxial-cable should be covered by metal tube.
In order to prevent lightning wave from invading foreside equipments, lightning rod should be installed in every wire for each equipments including electric wires (220V or DC12V), video wire, signal wire and pan / tilt controlling wire.
Camera power supplies usually adopt AC220V or DC12V. For camera is driven by direct-current-transformer, mono-phase current lightning rod should be lined to foreside with series connection or parallel connection. If transmission distance is beyond 15 meters, camera must connect to low voltage direct lightning rod in series.
Long distance transmission and weak ability for bearing pressure make it much possible for equipments to be destroyed by lighting current. In order to transmit lightning current from coaxial cable to ground, over-voltage protector must quickly react, so when designing the protection of signal-transmission-cable, we must think over signal’s transmitting speed, signal PWL, start voltage and lightning electricity quantity.
Outdoor foreside equipments should have good down-lead devices. Earth-resistance is less than 4Ω, and in high earth-resistance areas can be less than 10Ω.
Transmission lines anti-lightning
In CCTV system, mainly there are transmitting cable and power supplies wire. The power supplies wires can be linked from terminal devices or connected to power supplies near by surveillance site.
Signal-controlling cable and alarm cable generally adopt soft cable with screened core. It is usually span between the forehead and the terminal.
GB50198－1994 orders: When transmitted in outskirts or rural areas, transmission cables can be buried under ground. But when this method has difficulties, it is optional to communication tube or air-span. Now the minimum distance during cables is ordered.
Thinking by anti-lightning, buried cable is the optimum method for anti-lightning. It is most possible for air-span cable to be stroke by lightning, even with serious destruction and broad affection. In order to prevent forehead-and-terminal equipments from being destroyed, airing-cable should connect to down-lead on each electric pole. Besides, in airing-cables, both plumb-line and metal tube should have earth-leads. Signal and current which come from the central amplifier interface should connect to proper lightning rod.
In fact, buried cables can not prevent lightning from hitting equipments. According to lots of data, the accidents quantity of buried cables which are destroyed by lightning places 30% in all accidents. Even though the sections far from hitting center have lightning current of putting into cable. So in the screened cable or tube-covering cable, it is greatly effective to keep connecting between steel tube and electrics for avoiding electromagnetic interference and electromagnetic induction. Why the affection is so great? The reason lies in screened function of metal tube and skin effect of lightning current. If it is hard for cable to be covered by metal tube in whole distance, cable can be buried under ground with tube covering before connecting to terminal device and front-end equipment. But buried distance must be no less than 15m and cable skin and tube must connect to the grounding for anti lightning.
Terminal device anti-lightning
In CCTV system, the most important place for anti-lightning is monitoring room which should be protected from direct lightning protection, lightning wave interfere, equipotential connection and surge protection.
Monitoring building should have anti-direct lightning devices including lightning rod, lightning belt and lightning conduction. Anti-direct lightning method should meet GB50057-94 orders.
Metal cables in monitoring room should be connected to groundings for induction lightning. When air cable directly connects into room, room interface should add lightning arrester, even connects cable’s metal skin and tight-wire to grounding device.
Monitoring room should set an equipotential connection generatrix (or metal board). This equipotential connection generatrix should connect to building grounding, PE lines, device protection grounding and anti-static grounding in order to avoid dangerous potential difference. All surge arresters (lightning arrester) groundings should connect to equipotential connection generatrix as the max distance.
As for 80% lightning high potential is interfered from power lines, in order to ensure device safe, it is usually set three levels protection against lightning stroke. Before video transmission line, signal control line and interfere alarm line connecting to front-end equipment or central console, we should add related lightning arresters.
Good grounding is an important section in anti-lightning. The lower the grounding resistance is, the lower the over-voltage is. When control center adopts dedicated grounding, resistance should be no higher than 4Ω; adopts integrated grounding, resistance should be no higher than 1Ω.
4.Conclusion
Lightning destroys CCTV system can be many ways. This article mainly introduces major reason and interfere methods of CCTV system being destroyed. It is should be noticed that lightning protection is a complicated problem. Lightning protection design is far from enough determined by lightning kit performance, the more important is that before designing & constructing, we should think over CCTV environment, adopt suitable cable composition mode and screen & grounding mode. In one words, lightning protection should be considered from all factors in order to get remarkable effect.
