What is the principle of shielded wire?

Update:Dec 16,2022
Summary:The principle of shielded cable: The shielded wiring system originated from Europe. It adds a metal shielding layer on the outside of the ordinary uns...
The principle of shielded cable:
The shielded wiring system originated from Europe. It adds a metal shielding layer on the outside of the ordinary unshielded wiring system, and uses the reflection, absorption and skin effect of the metal shielding layer to realize the function of preventing electromagnetic interference and electromagnetic radiation. The shielding system comprehensively utilizes The balance principle of the twisted pair and the shielding effect of the shielding layer have very good electromagnetic compatibility (EMC) characteristics.
Electromagnetic compatibility (EMC) means that electronic equipment or network systems have a certain ability to resist electromagnetic interference, and at the same time cannot generate excessive electromagnetic radiation. That is to say, it is required that the equipment or network system can work normally in a relatively harsh electromagnetic environment, and at the same time, it cannot radiate excessive electromagnetic waves to interfere with the normal operation of other surrounding equipment and networks.
The balance characteristics of U/UTP (unshielded) cables do not only depend on the quality of the components themselves (such as twisted pairs), but are also affected by the surrounding environment. Because the metal around U/UTP (unshielded), concealed "ground", pulling, bending, etc. during construction will destroy its balance characteristics, thereby reducing EMC performance.
Therefore, there is only one solution to obtain long-lasting balance characteristics: add an extra layer of aluminum foil to the ground for all core wires. Aluminum foil adds protection to the fragile twisted-pair cores while creating an artificially balanced environment for U/UTP (unshielded) cables. Thus forming what we now call a shielded cable.
The shielding principle of the shielded cable is different from the balanced offset principle of the twisted pair. The shielded cable is to add one or two layers of aluminum foil outside the four pairs of twisted pairs, and use the principle of metal reflection, absorption and skin effect on electromagnetic waves (the so-called trend Skin effect means that the distribution of current in the cross section of the conductor tends to be distributed on the surface of the conductor as the frequency increases. The higher the frequency, the smaller the skin depth, that is, the higher the frequency, the weaker the penetration ability of electromagnetic waves), effectively preventing external Electromagnetic interference enters the cable, and at the same time prevents internal signals from radiating out, interfering with the work of other equipment.