How to identify the quality of weak electric cables?

[Guide] How to identify the quality of the weak current cable? This is an engineering problem that seems to be "not a big problem" but is very important. Today, I will share the identification method of the quality of the low-voltage cable.

How to identify the quality of weak electric cables? This is an engineering problem that seems to be "not a big problem" but is very important. Today, I will share the identification method of the quality of the low-voltage cable.

I. Visual appearance identification:

1. PVC sheath: It can be seen on the surface that the regular "unevenness" of the braided net inside is compacted, which indicates that the processing technology is good, and it will not cause relative sliding, which is a good cable. The appearance is smooth, the "unevenness" of the braided net cannot be seen, and the sheath is loose when pinched by hand, which is a bad cable;

2. Check the shielding wire netting: whether the number of wires is sufficient for copper wire netting, check solderability, tinned copper wire is scraped to see if it is copper wire, the hardness of aluminum-magnesium alloy wire is significantly greater than that of copper wire; wire netting is sparse and distributed Non-uniform, not tightly wrapped with insulation, etc. are poor cables;

3. Check the core wire: diameter-SYV cable is 0.78-0.8mm, SYWV cable is 1.0mm; recently there is a SYV75-5 cable with a core diameter of 1.0mm. The characteristic impedance of this cable is definitely not 75 Ohm, should not be used in 75 ohm transmission systems;

4. Check the adhesion between the core wire and the insulation layer: Cut the insulation layer obliquely and pull the core wire in the direction of peeling to see if the core wire and the insulation layer adhere to the process materials; a good cable has a large adhesion force. The cable is not stuck;

5. Longitudinal tensile test: Take a one-meter cable, peel off the core wire, insulation layer, shielding layer, and outer cover layer by layer, and leave each 10 cm long. The method is: two hands hold the two adjacent layers of the cable and pull them in opposite directions; a good cable cannot be pulled with general strength, and a poor cable can be easily pulled out without much effort—the elevator cable is very important "Elevator special cables" all have problems in this regard.

Second, test with fire

It is a difficult problem for consumers to tell whether a cable product has fake copper wire. There are many types of fake copper wires on the market today, and the degree of simulation is high, and the average consumer cannot distinguish between good and bad. The three types of fake copper wires are collected and sorted out below. Consumers can avoid these three types of cable products when purchasing.

General fake copper wire: copper-clad aluminum, copper-clad aluminum-magnesium alloy, copper-clad steel, copper-clad iron,

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

Unqualified copper wire: the material is not drawn wire made of oxygen-free copper, the transmission performance is not good, or the number of cores is not enough, etc.

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

As the saying goes, real gold is not afraid of fire, and true and false copper wires such as SYV and RVV can also be used to distinguish authenticity. First, open a section of the wire to expose a section of copper wire and divide them into a thinner (that is better) Generally, once a windproof lighter is burned, it will appear. The real copper wire is very resistant to high temperature, and it is absolutely no problem for more than one minute. At most, the color will change a little. If it is copper-clad aluminum, copper-clad aluminum-magnesium alloy, it will bend down quickly. After the kind of iron was burnt, it became visible with a knife.

Three, transmission performance test:

Video cable, as its name implies, is a transmission line used to transmit video signals. Since it is transmitting video signals, it is necessary to understand at least the transmission characteristics of transmission lines in the 0-6M frequency band range, or transmission performance. Here I mainly talk about the "oscilloscope measurement method" for reference. Because the oscilloscope is an indispensable "weapon" for engineering companies and one of the necessary equipment for qualification inspection, the following description is based on the ability to use the oscilloscope.

1. The color camera video signal can be used as a "standard video signal source": the camera video output for test engineering should be 1Vp-p on a 75 ohm load, that is, the bottom of the line sync head to the highest white level of the video signal "Peak-peak"; Note that the amplitude of the line sync head is "-0.3V", and the amplitude of the color sync head (4.43M sine wave pulse) is 0.3Vp-p; select the oscilloscope sensitivity and hit the amplitude calibration state. Select a camera with better indicators as the "video source";

2. The test cable should be as long as possible to reduce the measurement error. For example, the cable's middle connector must use "F-type connector" and coaxial double-pass (cable TV equipment). Do not use welding methods, because the welding method destroys the cable. Coaxiality and continuity of characteristic impedance.

3. Measure the DC resistance data of the cable: for example, SYV75-5 cable 1000 meters, the DC resistance core wire is 35-40 ohms, and the outer shield resistance 1000 meters is 24-36 ohms (different shield layer numbers, the resistance is very different) ; SYWV75-5 cable 1000 meters, DC resistance core wire is 18-22 ohms, external shield resistance 1000 meters is 24-36 ohms; accumulating information in this area is useful, not only can judge the quality of cable materials, but also used for engineering Check the quality of wiring and pipe running. For example, when you wear a pipe, you break the wire, the resistance value becomes larger, and the video signal becomes weaker. Undesirable interference also occurs. The probability of such "accidents" is very high, but Often overlooked;

4. Measure the high and low frequency attenuation characteristics of the cable: Measure the amplitude of the line head and the color burst at the end, and calculate the attenuation with 0.3V as the 0db reference. The line header represents the low frequency attenuation, and the color burst represents 4.43M high frequency attenuation. —— For example: Measure the 1000m line sync head as 0.15V, and calculate the db number of attenuation multiple according to 20log as “-6db / 1000m”. Measure the 4.43M color sync head after the attenuation of 1000m is 30mv, which is 1 / 10 times, attenuation is -20db / 1000m; this method can accurately grasp the transmission quality of different cables, and has an intuitive concept of "frequency distortion (high and low frequency attenuation difference)", you can more accurately measure The difference and performance of SYV and SYWV cables of the same model and structure, compare the differences and performance of products from different manufacturers, and also compare the changes of different batches of products from the same manufacturer;

5. The above methods can also test the performance of video transmission systems and equipment: such as the transmission characteristics of each coaxial video cable in the project, the transmission characteristics of the optical terminal (can be measured for good or bad, don't think they are all ideal), radio frequency transmission, microwave Transmission characteristics, transmission characteristics of twisted pairs, distribution characteristics of video distributors, and switching characteristics of matrix hosts. Pay special attention when multiple outputs switch the same input signal at the same time. Wrong, it should be unchanged. After testing, you will have a lot of unqualified products;

6. Observe the field signal to see if the field sync position distortion is large (flat or flat)-it should be very flat; at the same time, you can also use the oscilloscope to check the low-frequency interference: if the field signal has slow fluctuations, it is 50/100 cycle interference There are a lot of "thatched" beats, most of which are frequency-conversion harmonic interference. Disconnect the far-end camera and short-circuit the inner and outer conductors at the far end of the cable. At the end, you can directly observe the interference waveform and intensity with an oscilloscope. The true performance of the device.