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Introduction to shielded wire wiring system

2022-12-27
Shielding wiring system from Europe, it is in the ordinary unshielded wiring system outside the metal shielding layer, using the metal shielding layer of reflection, absorption and skin effect to prevent electromagnetic interference and electromagnetic radiation function, shielding system comprehensive use of twisted pair balance principle and shielding layer shielding effect, so it has very good electromagnetic compatibility (EMC) characteristics. 
Electromagnetic compatibility (EMC) refers to the ability of electronic equipment or network systems to resist electromagnetic interference without producing excessive electromagnetic radiation. 
That is to say, the equipment or network system is required to work normally in a harsh electromagnetic environment, while not radiating excessive electromagnetic waves to interfere with the normal work of other equipment and network around. 
The shielding principle of the shielded cable is different from the balance cancellation principle of the twisted pair. The shielded cable is to add one or two layers of aluminum foil outside the four pairs of twisted pair. By using the principle of reflection, absorption and skin effect of the metal on electromagnetic wave, it can effectively prevent the external electromagnetic interference into the cable, and also prevent the internal signal from radiating out and interfering with the work of other equipment.
Experiments show that electromagnetic waves with frequencies greater than 5MHz can only pass through aluminum foil 38μm thick. If the thickness of the shield is more than 38μm, the frequency of electromagnetic interference that can enter the cable through the shield is mainly less than 5MHz.



The low frequency interference below 5MHz can be effectively offset by the balance principle of twisted pair. 
One end of the shielded cable is grounded and the other end is suspended. 
When the signal wire is transmitted for a long distance, due to the difference in ground resistance at both ends or the current in the PEN wire, the potential of the two ground points may be different. At this time, if the two ends are grounded, the shielding layer will have electricity, but the signal interference is formed. Therefore, in this case, the method of grounding at one point and hanging at the other end is generally adopted to avoid the formation of such interference. 

The grounding shielding effect is better, but the signal distortion will increase.



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