Lanzhou Xining Jinan Elevator video surveillance interference causes

First, the principle of interference generation:

Power cables, lighting, fans, control, communications, and other cables are usually placed in elevator shafts, and various cables generate electromagnetic radiation. As with the principle of antenna reception, the coaxial cable will “receive” these interferences, ie, it will interfere with the electromagnetic field and cause interference induced currents on the cable. This interference induced current will also cause interference induced voltage on the longitudinal resistance of the cable outer conductor (woven mesh). (Electromotive force), this disturbance induced voltage is just connected in series in the "long ground" of the video signal transmission circuit, forming interference.

What's more important is that these accompanying cables are parallel with the video cable and bundled together in close quarters. This results in a near "effective" interference coupling relationship. In general engineering, the method of shielding and interfering with metal pipes or metal channels can be used. However, this method cannot do anything in the elevator follow-up environment. Therefore, the anti-jamming in the elevator environment is very difficult, and only good design and construction methods can be selected.

Second, anti-jamming measures:

1, common copper shaft cable anti-jamming:

Whether it is a multi-layered high-copper braided cable, an "aluminum foil-woven net" double-shielded cable, or an "aluminum foil-braided net-aluminum foil-braided net" quadruple shielded cable, it is electrically a shield layer. Interference induced voltages are directly connected in series in the video signal transmission loop. However, the resistance of the outer conductor of the multi-layer high-knit cable is small, and the interference induced voltage is relatively low. This has a certain effect (interference below tens of kHz) against low-frequency power disturbances, motor spark disturbances, and the like. However, for high-frequency interference, due to the “skin effect”, the high-frequency impedance is the same as that of the low-profile cable, and the anti-jamming effect is basically the same; therefore, it should be clear to see that the cable of Gaobian has only a proper effect of reducing low-frequency interference, and prevents strong interference. High-frequency interference is still powerless.

2. Anti-jamming measures for elevator wiring:

1 The position of the video monitoring cable to get out of the elevator shaft: The ideal choice should be in the middle of the well, because the length of the video cable accompanying the well at this time is only about half the depth of the well, and the shortest, the interference introduced naturally is also small; but on the engineering These kinds of qualifying requirements can only be fought for and the actual project may not be allowed.

2 In the past, without knowing the principle, most of the outlets were taken along with other accompanying cables and they exited from the top or bottom of the cable well. In this case, considering that only half of the cable is accompanied by the movement, the other half is only a fixed extension connection and does not move. We call this part “immovable cable”; this provides a possibility: that half of the accompanying cable is only The cable can be bundled with other accompanying cables; the other half of the fixed cable can be routed separately from the accompanying cable. In the elevator shaft, the video cable is placed close to the vertical line of the well and the cable is passed through the metal pipe or Take the metal slot to shield the influence of interference on this part of the cable, more effective.

3 When accompanied by video cables and other accompanying cables, designers should fully understand the structure and distribution of other accompanying cables. When tying, video cables should be kept away from cables with high current and high frequency, cables with low current and low frequency. Bundling; Here, even a 1 cm selection may have to fight, because the impact of interference is at least inversely proportional to the square of the distance.

4 The “ground” of the video signal of the metal shell of the camera, the casing of the NC head, and the outer conductor of the coaxial cable must be insulated from the elevator car and guide rails. This should be done with special care when installing the camera.

5 The camera power supply should be preferably centralized DC power supply mode, followed by the choice of car lighting power, can not use power.

6 For monitoring and control cables such as power supply and control, use shielded cables as much as possible to prevent external signals from leaking.

7 The video cable from the elevator hoistway exit to the control center should take the metal pipe or take the metal slot to shield the impact of environmental interference on this part of the cable, and pay attention to this part of shielding and shielding in the elevator shaft, and should make electrical connection.

Third, the application of anti-interference coaxial cable:

　 1. The anti-jamming coaxial cable is a kind of "double insulated and double shielded coaxial cable". The core, insulation and shielding layers inside it are still standard 75 ohm cables. There is no difference. The difference is that in addition to the original shielding layer, a second insulating layer and a second shielding layer are added, and a sheath is added on the outside. From the above analysis of interference generation principle, it has been known that the induced voltage generated by the interference on the outer layer of the conventional coaxial cable is connected in series with the “long ground line” of the video signal transmission loop, thereby forming interference. However, after the use of anti-interference coaxial cable, the situation has undergone a qualitative change: the interference induced voltage can only be formed on the “second shield layer”, and the “second insulation layer” inside it is used to “long” it with the video signal transmission circuit. The long ground wire is insulated and isolated from the video signal transmission circuit to achieve anti-interference.

2. The characteristics of this anti-jamming cable are very prominent for the interference of ultra-low frequency power supply in the elevator environment, electrical spark interference of motor, frequency motor interference, control signal interference, etc., which are below tens of kilohertz.

3. In the long-term engineering design of video surveillance transmission lines, the use of "double-insulated double-shielded coaxial cable" will allow some of the anti-jamming measures in traditional projects to be substantially simplified and effectively reduce the total project cost.