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How to solve the elevator monitoring interference? Elevator surveillance video interference causes and solutions

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Author : Cobain
Update time : 2021-05-08 16:40:33

Elevator surveillance video interference has always been one of the most common, difficult and concerned problems in closed-circuit monitoring projects. In order to solve the problem of interference of elevator video surveillance, we must first make clear the principle of interference generation, and the problem of interference can be solved.
First, interference generation principle introduction:
    Elevator wells are usually equipped with power, lighting, fans, control, communications and other cables, all kinds of cables will produce electromagnetic radiation. In the same way as antenna reception, coaxial cables also "receive" these interferences, i.e. interference electromagnetic fields generate interference induction current on the cable, which also produces interference induction voltage (electric potential) on the vertical resistance of the cable external conductor ( (  ) which is just connected in series in the )video signal transmission circuit "long ground line" to form interference.
    What's more, these accompanying cables are parallel to the video cables and are tied up at close range. This results in a close to the "best and most effective" interference coupling relationship. In general works, the shielding interference method of wearing a metal tube or walking a metal groove can be used, but in the elevator follow-up environment, this method can not do anything. Therefore, the elevator environment is very difficult to resist interference, can only choose a better design and construction methods.
    Anti-jamming measures:
1,commonly used copper shaft cable anti-jamming:
    Whether it is a multi-layer high-made copper mesh cable, "aluminum foil - netting" dual shielding cable, or "aluminum foil - netting -- aluminum foil - netting" four-shielded cable, electrically belongs to a shielding layer. Interference induction voltages are directly connected in series in the video signal transmission circuit. Only multi-layer high-fabricated cables have a small external conductor resistance and a relatively low interference sensing voltage. This has some effect ( below dozens kHz) against low-frequency power supply interference, motor spark interference, )and soon. But for high-frequency interference, because of the "skinning effect", high-frequency impedance and low-fabrication cables are the same, anti-jamming effect is basically the same;
2,elevator wiring mode of anti-jamming measures:
    (1) Video surveillance cable out of the elevator shaft location selection: the ideal choice should be in the middle of the well, because this time the length of the accompanying video cable in the well, only about half of the depth of the well, the shortest, natural introduction of the interference is also the smallest;
    (2) In the past, without understanding the principle, most of the exit positions were walked with other accompanying cables, coming out of the top or bottom of the cable well. In this case, considering that only half of the cable is moving with the line, the other half is only a fixed extension connection, not moving, we call this part of the "immobilized cable"; It's more effective.
    (3) When the video cable of the accompanying motion part is tied with other accompanying cables, the designer should fully understand the structure and distribution of the other accompanying cables, and the video cable should be as far away from the cable with high current and frequency as possible, and the cable strapping at low frequency of near current, where even the choice of 1 cm may be fought for, because the interference impact size is at least inversely proportional to the square distance.
    (4) Camera metal shell,NC head shell, coaxial cable external conductor and other video signals "ground", and elevator car, rail, etc. to be insulated, which should be particularly careful when installing the camera.
    (5) Camera power supply should be preferably concentrated DC power supply mode, followed by the choice of car lighting electricity, can not use power.
    (6) power supply, control and other monitoring cables, as far as possible to choose a shielded cable, to prevent interference signal leakage.
    (7) From the elevator shaft exit to the control center video cable, should go metal pipe or take metal groove, in order to shield the environmental interference along the way on this part of the cable impact, and pay attention to this part of the shield and elevator well shielding, should do a good job of electrical connection.
    Third, the application of anti-jamming coaxial cable:
1,anti-jamming coaxial cable is a "double insulated double shielded coaxial cable", its core wire, insulation layer, shielding layer is still the standard 75 ohm cable, no difference. The difference is that in addition to the original shield, a second insulation layer and a second shield have been added, with sheaths on the outside. The principle analysis of interference generation from above has been known that the inductive voltage generated by interference on the outer layer of a conventional coaxial cable is connected in series in the "long ground line" of the video signal transmission circuit, thus forming interference. However, after the use of anti-jamming coaxial cable, the situation has a qualitative change: interference induction voltage can only be formed on the "second shield", and by the "second insulation layer" inside it and the video signal transmission circuit "long ground line" insulation isolation, interference is excluded from the video signal transmission circuit, to achieve anti-jamming purposes.
2,the characteristics of this anti-jamming cable, for the elevator environment of ultra-strong low-frequency power supply interference, motor electric spark interference, variable frequency motor interference, control signal interference, such as dozens of thousands of girths of interference, anti-jamming performance is very prominent.
3,in the video surveillance transmission line longer engineering design, the use of "double insulated double shielded coaxial cable", the traditional engineering on some anti-jamming measures can also be greatly simplified, and can effectively reduce