1. Electric power, lighting, fans, control and communication cables in the elevator shaft will generate electromagnetic radiation. Like the antenna receiving principle, the coaxial cable will also "receive" these interferences, that is, the interference electromagnetic field will produce interference induced current on the cable, and this interference induced current will also produce interference induced voltage (electromotive force) on the longitudinal resistance of the cable outer conductor (braided net), and the cable outer conductor is just connected in series on the "long ground wire" of the video signal transmission loop to form interference;
2. More importantly, these accompanying cables are parallel to the video cable and tied together at close range. This forms an interference coupling relationship close to the "best and most effective". In general engineering, metal pipes or metal grooves can be used to shield interference, but in the environment of elevator follow-up, this method is powerless. Therefore, the anti-interference of elevator environment is very difficult, so we can only choose better design and construction methods;
3. Understanding the basic principle of interference is very important for improving anti-interference design and construction;
Second, the common coaxial cable transmission scheme of anti-interference measures:
1. Common copper shaft cables: whether it is a multi-layer high-woven copper mesh cable, a double-shielded cable of aluminum foil-woven mesh or a four-shielded cable of aluminum foil-woven mesh, they all belong to the same shielding layer. The interference induced voltage is directly connected in series in the video signal transmission loop. Only the outer conductor resistance of multilayer high braided cable is small, and the interference induced voltage is relatively low. This has a certain effect on low frequency power supply interference and motor spark interference (interference below tens of Khz). But for high frequency interference, due to the "skin effect", the high frequency impedance is the same as that of low braided cable, and the anti-interference effect is basically the same; Therefore, we should be soberly aware that high braided cables can only appropriately weaken the effect of low-frequency interference, and nothing can prevent strong interference;
2. Anti-interference measures for elevator wiring mode:
① Selection of the location where the video cable exits the elevator shaft: the ideal choice should be in the middle of the shaft, because the length of the video cable accompanying the shaft at this time is only a little more than half of the depth of the shaft, which is the shortest and naturally introduces the least interference; However, this kind of qualification requirement in engineering can only be won according to the situation, and it is not necessarily allowed in actual engineering;
(2) In the past, without understanding the principle, the outlet position mostly came out from the top or bottom of the cable shaft together with other accompanying cables. In this case, considering that only half of the cable moves with it and the other half is just a fixed extension connection, we call this part "fixed cable"; This provides a possibility: the semi-accompanying cable can only be tied with other accompanying cables; The other half of the fixed cable can be routed separately away from the accompanying cable, and the video cable can be routed vertically near the shaft wall in the elevator shaft. This part of the cable can be shielded from interference through metal pipes or metal grooves, which is more effective;
(3) When the video cable of the walking part is bundled with other walking cables, the designer should fully understand the structure and distribution of other walking cables. When bundling, video cables should be bundled as far away from high-current and high-frequency cables as possible and close to low-current and low-frequency cables; Here, even if there is a choice of 1 cm, we may have to fight for it, because the interference influence is at least inversely proportional to the square of the distance;
④ Video signals such as the metal shell of the camera, the BNC head shell and the outer conductor of the coaxial cable should be insulated from the elevator car and guide rail, and special attention should be paid when installing the camera;
⑤ Centralized DC power supply should be the first choice for camera power supply, followed by automobile lighting power supply, not power supply.
⑥ Use shielded cables as far as possible for monitoring cables such as power supply and control to prevent interference signals from leaking out;
⑦ For the video cable from the elevator shaft exit to the control center, metal pipes or metal grooves should be used to shield the influence of environmental interference along the way on this part of the cable, and pay attention to this part of the shielding and the shielding in the elevator shaft, and make electrical connections;
Third, the application of anti-interference coaxial cable
1. Anti-interference coaxial cable is a kind of "double-insulated and double-shielded coaxial cable", and there is no difference between the "core-insulation-shielding layer" and the standard 75 ohm cable. The difference is that a second insulating layer and a second shielding layer are added outside the original shielding layer, and a sheath is added outside. From the above analysis of the interference generation principle, it can be seen that the induced voltage generated by the external interference of the traditional coaxial cable is connected in series in the "long ground wire" of the video signal transmission loop, thus forming interference. However, after using the anti-interference coaxial cable, the situation has changed qualitatively: the interference induced voltage can only be formed on the "second shielding layer", which is insulated from the "long ground wire" of the video signal transmission loop through the internal "second insulating layer", so that the interference is excluded from the video signal transmission loop and the anti-interference purpose is achieved.
2. This kind of anti-interference cable has outstanding anti-interference performance for ultra-low frequency power supply interference, motor spark interference, variable frequency motor interference and control signal interference below tens of kilohertz in elevator environment.
3. In the engineering design of long transmission lines, adopting "double insulated and double shielded coaxial cable" can greatly simplify some anti-interference measures in traditional projects and effectively reduce the total project cost.
4. We openly introduce the core technical know-how of anti-interference cable in order to make this patented technology with our own intellectual property rights play a role in security engineering as soon as possible. If infringement is found, please help your friends to pay attention and inform them in time. Contact information can be found on the website ().