Identify The Quality Of Weak Current Cables

1、 Visual appearance identification:

1. PVC sheath: regular "unevenness" can be seen on the surface of the compressed inner mesh, indicating that the processing technology is good and will not produce relative sliding. It is a good cable. The appearance is smooth, and the "unevenness" of the compressed braided mesh can not be seen. There is looseness when the sheath is pinched by hand, which is a poor cable;

2. Check the shielding layer netting: check whether the braiding number is enough for copper netting, check the solderability, scrape the tinned copper wire to see if it is copper wire, and the hardness of aluminum magnesium alloy wire is significantly higher than that of copper wire; Sparse network, uneven distribution and loose wrapping with insulating layer are poor cables;

3. Check core wire: diameter - 0.78-0.8mm for SYV cable and 1.0mm for SYWV cable; Recently, a cable with syv75-5 core diameter of 1.0mm has emerged. The characteristic impedance of this cable is certainly not 75 ohm and is not applied to 75 ohm transmission system;

4. Check the adhesion force between the core wire and the insulating layer: cut the insulating layer obliquely, pull the core wire according to the stripping direction, and see whether the core wire and the insulating layer are bonded with process materials; Good cable has large adhesion force, but poor cable does not adhere;

5. Longitudinal tensile test: take one meter of cable, peel off the core wire, insulation layer, shielding layer and outer sleeve in layers, and leave 10 cm long for each. The method is as follows: two hands hold the adjacent two layers of the cable respectively and pull it in the opposite direction; A good cable can't be pulled by ordinary force, but a poor cable can be easily pulled out without great effort - elevator cable is very important, and many so-called "elevator special cables" have this problem.

2、 Fire test

It is difficult for consumers to distinguish whether cable products are added with fake copper wires. Now there are many kinds of fake copper wires on the market, and the simulation degree is high. Ordinary consumers can't distinguish good from bad at all. Next, three types of fake copper wires are collected and sorted out, so that consumers can avoid these three types of cable products when buying.

General pseudocopper wire: copper clad aluminum, copper clad aluminum magnesium alloy, copper clad steel, copper clad iron

High imitation fake copper wire: copper clad aluminum magnesium alloy is added in a certain proportion in multi strand copper wire. Other materials are processed by electroplating copper. Generally, the price is much more expensive than the previous one. After all, there is real copper in it.

Substandard copper wire: the material is not drawn copper wire made of oxygen free copper, the transmission performance is poor, or the number of cores is not enough

If you forget the classification of these three kinds of fake copper wires when buying, you can also try the following method:

As the saying goes, real gold is not afraid of fire. True and false SYV, RVV and other copper wires can also be distinguished by fire. First, open a section of wire to expose a section of copper wire, and divide them a little more sparsely (that will have a better effect). Generally, once burned with a windproof lighter, it will appear. The real copper wire is very resistant to high temperature. There is absolutely no problem for more than one minute. At most, the color will change a little; If it is copper-clad aluminum and copper-clad aluminum magnesium alloy, it will bend down quickly; If it is copper-clad steel or copper-clad iron, it will appear when it is scraped with a knife after burning.

3、 Transmission performance test:

Video line, as the name suggests, is a transmission line used to transmit video signals. Since you are transmitting video signals, you should at least understand the transmission characteristics of the transmission line in the 0-6m frequency band, or transmission performance. Here we mainly talk about "oscilloscope measurement method" for reference. Because oscilloscope is a necessary "weapon" for engineers and one of the necessary inspection equipment for qualification examination, the following description is based on being able to skillfully use oscilloscope.

1. The video signal of color camera can be used as "standard video signal source": the amplitude of video output of camera for test project on 75 Ohm load shall be 1Vp-p, that is, from the bottom end of line synchronization head to the "peak peak" of the highest white level of video signal; Note that the amplitude of the line synchronization head is "- 0.3V", and the amplitude of the color synchronization head (4.43M sine wave pulse) is 0.3vp-p; Select the sensitivity of the oscilloscope and turn it to the amplitude calibration state. Select a camera with good indicators as the "video source";

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2. The test cable shall be as long as possible to reduce the measurement error. For example, for 1000 meters, the middle joint of the cable must use "f-joint" and coaxial double-pass (cable TV equipment). Do not use welding method, because the welding method destroys the coaxiality and continuity of characteristic impedance of the cable.

3. Measure the DC resistance data of the cable: for example, if syv75-5 cable is 1000m, the DC resistance core wire is 35-40 ohms, and the resistance of the outer shielding layer is 24-36 ohms when it is 1000m (the resistance varies greatly with the number of shielding layers); Sywv75-5 cable is 1000m, the DC resistance core wire is 18-22 ohms, and the external shielding layer resistance is 24-36 ohms for 1000m; It is very useful to accumulate data in this regard. It can not only judge the quality of cable materials, but also check the quality of engineering wiring and pipe threading. For example, when people wear pipes, the wire is pulled off, the resistance becomes larger, the video signal becomes weaker, and the interference that should not occur also occurs. The probability of this kind of "accident" is very high, but it is often ignored;

4. Measure the high and low frequency attenuation characteristics of cable: measure the amplitude of line head and color synchronization head at the end, and calculate the attenuation based on 0.3V as 0dB. The same line head represents low frequency attenuation, and the color synchronization head represents 4.43M high frequency attenuation. - for example, the measured 1000M line synchronization head is 0.15v, and the DB number of attenuation multiple calculated according to 20log is "- 6dB / 1000m", It is measured that the attenuation amplitude of 4.43M color synchronization head after 1000m is 30mV, i.e. 1 / 10 times, and the attenuation is - 20dB / 1000m; With this method, you can accurately grasp the transmission quality of different cables, and have an intuitive concept of "frequency distortion (attenuation difference between high and low frequencies)". You can accurately measure the difference and performance of SYV and SYWV cables of the same model and structure, and compare the difference and performance of products from different manufacturers, You can also compare the changes of different batches of products from the same manufacturer;

5. The above method can also detect the performance of video transmission system and equipment: for example, the transmission characteristics of each coaxial video cable in the project, the transmission characteristics of optical transceiver (whether it is good or bad can be measured, do not think it is so ideal), RF transmission and microwave transmission characteristics, twisted pair transmission characteristics, distribution characteristics of video distributor, switching characteristics of matrix host, Special attention should be paid to when multiple outputs switch the same input signal at the same time. If it is found that the more the number of switching channels, the greater the attenuation, it is wrong and should remain unchanged. After testing, you will master many unqualified products;

6. Observe the field signal to see whether the field synchronization position distortion is large (flat and uneven) - it should be flat; At the same time, you can also use an oscilloscope to observe the low-frequency interference: if the field signal fluctuates slowly, it is 50 / 100 cycle interference, there are many "thatch" jumps, mostly frequency conversion harmonic interference, disconnect the remote camera, short circuit the internal and external conductors at the far end of the cable, and directly observe the interference waveform and intensity at the end with a oscilloscope; This method can also check and test the real performance of anti-interference equipment.