What is a shielded wire?

Definition: A wire with a conductor wrapped around the outside is called a shielded wire. The wrapped conductor is called a shielding layer, which is generally a braided copper mesh or copper foil (aluminum). The shielding layer needs to be grounded, and external interference signals can be conducted into the earth through this layer.

Function: To prevent interference signals from entering the inner layer, avoid conductor interference, and reduce the loss of the transmitted signal at the same time. Structure: (Common) Insulation layer + shielding layer + wire (advanced)

Insulation layer + shielding layer + signal conductor + shielding layer grounding conductor

Note: When choosing shielded wires, the shielding layer grounding wire. The insulating layer of the shielding layer grounding wire has a conductive function and can conduct electricity with the shielding layer (with a certain resistance).

The principle of shielded cables

The shielded cabling system originated in Europe. It is a common unshielded cabling system with a metal shielding layer added outside. By taking advantage of the reflection, absorption and skin effect of the metal shielding layer, it achieves the function of preventing electromagnetic interference and electromagnetic radiation. The shielded system comprehensively utilizes the balance principle of twisted-pair cables and the shielding effect of the shielding layer, thus having very good electromagnetic compatibility (EMC) characteristics.

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 and other conditions during construction can all disrupt its balance characteristics, thereby reducing the EMC performance. Therefore, to achieve a lasting and unchanging balance characteristic, there is only one solution: to ground all the core wires by adding an extra layer of aluminum foil. Aluminum foil adds protection to the fragile twisted-pair core wire while artificially creating a balanced environment for U/UTP (unshielded) cables. Thus, what we now call shielded cables are formed.

The shielding principle of shielded cables is different from the balance cancellation principle of twisted-pair cables. Shielded cables add one or two more layers of aluminum foil outside the four pairs of twisted-pair cables. They utilize the reflection, absorption of electromagnetic waves by metals and the skin effect principle (the so-called skin effect refers to the distribution of current across the cross-section of a conductor tending to 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 deeper the skin The weaker the penetrating power of electromagnetic waves, the more effectively it can prevent external electromagnetic interference from entering the cable, and at the same time, it can also prevent internal signals from radiating out and interfering with the operation of other equipment.

Experiments show that electromagnetic waves with frequencies exceeding 5MHz can only pass through aluminum foil 38μm thick. If the thickness of the shielding layer exceeds 38μm, the frequency of electromagnetic interference that can penetrate the shielding layer and enter the interior of the cable will mainly be below 5MHz. For low-frequency interference below 5MHz, the balance principle of twisted-pair cables can be effectively applied to cancel it out.

According to the earliest definition of wiring, it is divided into two types: unshielded cables -UTP and shielded cables -STP. Later, with the development of technology and the different processes of various manufacturers, many different types of shielding have emerged

1. F/UTP Foil Screened Cable single-layer foil shielding structure

2. Foil and Braid Screened Cable foil and copper braided mesh double-layer shielding structure

a)SF/UTP aluminum foil and copper woven mesh are simultaneously wrapped around the outer layers of the four pairs of wires

b)S/FTP (PIMF) wire pairs: Single Pair aluminum Foil shielding plus copper braided mesh wrapped around the outer layer of the four pairs of wires. PIMF =Pair in Metal Foil.

The resistance of shielded cables to external interference is mainly reflected in the fact that the integrity of signal transmission can be guaranteed to a certain extent through the shielding system. The shielded wiring system can prevent the transmitted data from being affected by external electromagnetic interference and radio frequency interference. Electromagnetic interference (EMI) is mainly low-frequency interference. Motors, fluorescent lamps and power cables are common sources of EMI. Radio Frequency Interference (RFI) is high-frequency interference, mainly wireless frequency interference, including radio, television broadcasting, radar and other wireless communications.

For resisting electromagnetic interference, braided layer shielding is the most effective choice, that is, metal mesh shielding, because it has a relatively low critical resistance. As for radio frequency interference, metal foil shielding is the most effective, because the gaps created by metal mesh shielding allow high-frequency signals to enter and exit freely. For interference fields with mixed high and low frequencies, a combined shielding method of metal foil layer and metal mesh should be adopted, that is, a double-layer shielded cable in the form of S/FTP. This enables the metal mesh shielding to be suitable for low-frequency range interference and the metal foil shielding to be suitable for high-frequency range interference.

The single-layer thickness of the aluminum foil shielding layer in the shielded cables of IBM ACS reaches 50-62μm, achieving a more complete shielding effect. At the same time, as only a single layer of shielding is adopted, it will be simpler for construction, easier to install, less likely to cause man-made damage during the construction process, and the thickness of the aluminum sheet can withstand greater destructive force. Thus, it can provide users with higher-quality transmission performance.