In the process of selling low-voltage cables, whether online or offline, many customers ask how to identify the quality of low-voltage cables.
1. Sheath: A good cable will have a surface with visible, regularly uneven, pressed braided mesh, indicating good manufacturing quality and preventing relative slippage. A cable with a smooth surface, lacking visible unevenness, and feeling loose when squeezed is of poor quality.
2. Inspect the shielding mesh: Check if the braid count is sufficient for copper mesh, and inspect its solderability. For tinned copper wire, scrape to check if the inside is genuine copper wire. Aluminum-magnesium alloy wire is significantly harder than copper wire. Sparse, unevenly distributed mesh, and poor wrapping with the insulation layer indicate a poor-quality cable.
3. Check the core wire: Diameter—SYV cable is 0.78-0.8mm, SYWV cable is 1.0mm; recently, a type of SYV75-5 cable with a core wire diameter of 1.0mm has appeared. The characteristic impedance of this cable is definitely not 75 ohms and should not be used in a 75-ohm transmission system.
4. Check the adhesion between the core wire and the insulation layer: Cut the insulation layer diagonally, pull the core wire apart in the peeling direction, and check if there is any adhesive material between the core wire and the insulation layer; good cables have strong adhesion, while poor cables have no adhesion.
5. Longitudinal tensile test: Take one meter of cable and strip it layer by layer, separating the core wire, insulation layer, shielding layer, and outer sheath, leaving 10 cm of each. The method is: hold two adjacent layers of the cable with both hands and pull in opposite directions; a good cable is generally difficult to pull with force, while a poor cable can be easily pulled out with little effort—this is very important for elevator cables, and many so-called "elevator-specific cables" have problems in this regard;
Fire test:
There are so many types of fake copper wire on the market now, and the simulation is so high that it's difficult for ordinary people to distinguish them. The worst part is that some sellers even deceive merchants by claiming it's all copper, leaving ordinary merchants with no choice but to be fooled. Now I'll share my limited experience with you, hoping it will be helpful. The fake copper wires currently on the market fall into the following categories:
1) Common fake copper wires: copper-clad aluminum, copper-clad aluminum-magnesium alloy, copper-clad steel, copper-clad iron, etc.
2) High-quality imitation copper wire: Copper-clad aluminum-magnesium alloy is added to multi-strand copper wire in a certain proportion, and other materials are electroplated with copper. The price is generally much higher than the previous type because it contains real copper.
3) Substandard copper wire: The wire is not made of oxygen-free copper, has poor transmission performance, or has insufficient core count, etc.
As the saying goes, true gold fears no fire, and the same applies to genuine SYV, RVV, and other copper wires; you can use fire to distinguish between them. First, peel back a section of the wire to expose a section of copper wire, and separate them slightly (this will improve the effect). Generally, you can use a windproof lighter to burn it, and it will become apparent. Real copper wire is very heat-resistant and will withstand more than a minute without any problems; at most, it will change color slightly. If it is copper-clad aluminum or copper-clad aluminum-magnesium alloy, it will bend quickly. If it is copper-clad steel or copper-clad iron, you can also scratch it with a knife after burning it;
Transmission performance test:
Video cables, as the name suggests, are transmission lines used to transmit video signals. Since they transmit video signals, it's essential to understand the transmission characteristics, or transmission performance, of these cables in the 0-6MHz frequency band. This section primarily discusses "oscilloscope measurement methods" for reference, as oscilloscopes are indispensable tools for engineers and are among the mandatory equipment checked in qualification reviews. The following description assumes prior proficiency in using an oscilloscope.
1. A color camera video signal can be used as a "standard video signal source": The video output of the camera used in the test project should have an amplitude of 1Vp-p on a 75-ohm load, that is, the peak-to-peak value from the bottom of the horizontal sync header to the highest white level of the video signal; note that the horizontal sync header amplitude is -0.3V, and the color sync header (4.43M sine wave pulse) amplitude is 0.3Vp-p; select the appropriate oscilloscope sensitivity and set it to amplitude calibration mode. Select a camera with good specifications as the "video source";
2. Use a longer test cable to reduce measurement errors, such as 1000 meters. The cable joint must be an "F-type connector" and a coaxial double connector (for cable TV equipment). Do not use welding, as welding will destroy the coaxiality and characteristic impedance continuity of the cable.
3. Measure the DC resistance of the cable: For example, for 1000 meters of SYV75-5 cable, the DC resistance of the core wire is 35-40 ohms, and the resistance of the outer shield layer is 24-36 ohms (the resistance varies greatly depending on the number of braids in the shield layer); for 1000 meters of SYWV75-5 cable, the DC resistance of the core wire is 18-22 ohms, and the resistance of the outer shield layer is 24-36 ohms. Accumulating this information is very useful, not only for judging the quality of cable materials but also for checking the quality of engineering wiring and conduit installation. For example, if someone pulls the cable through a conduit, the resistance will increase, the video signal will weaken, and unwanted interference will occur. These kinds of "accidents" occur very frequently but are often overlooked.
4. Measuring the high and low frequency attenuation characteristics of cables: Measure the amplitude of the horizontal sync connector and the color sync connector at the end. Use 0.3V as the 0dB reference to calculate the attenuation. The horizontal sync connector represents low-frequency attenuation, and the color sync connector represents 4.43M high-frequency attenuation. For example, if the horizontal sync connector measured at 0.15V for 1000 meters, the attenuation factor (dB) calculated using 20log is -6dB/1000m. The amplitude of the 4.43M color sync connector after attenuation for 1000 meters is 30mV, or 1/10, resulting in an attenuation of -20dB/1000m. This method allows for accurate assessment of the transmission quality of different cables and provides an intuitive understanding of "frequency distortion (high and low frequency attenuation difference)." You can accurately measure the differences and performance of SYV and SYWV cables of the same model and structure, compare the differences and performance of products from different manufacturers, and even compare the variations between different batches of products from the same manufacturer.
5. The above methods can also be used to 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 transceiver (it can determine whether it is good or bad, don't assume that they are all ideal), the characteristics of radio frequency transmission and microwave transmission, the transmission characteristics of twisted pair cables, the distribution characteristics of video distributors, and the switching characteristics of matrix hosts. Pay special attention when multiple outputs switch to the same input signal at the same time. If you find that the more channels are switched, the greater the attenuation, then it is wrong. It should remain unchanged. After testing, you will be able to identify many defective products.
6. Observe the field signal and check if the field synchronization position distortion is significant (whether it's flat) – it should be very flat;
7. At the same time, an oscilloscope can be used to check the low-frequency interference: if the field signal has slow fluctuations, it is 50/100 cycle interference; if there are many "grass" jumping, it is mostly frequency conversion harmonic interference. Disconnect the remote camera and short-circuit the inner and outer conductors of the cable at the far end. At the end, you can directly observe the interference waveform and intensity with an oscilloscope; this method can also check and test the anti-interference.
