What Is a Shielded Wire?

Shielded wire is a transmission line wrapped with a metal mesh braided layer. The braided layer is usually copper or tinned copper. The shielding layer of the shielded wire needs to be grounded, and external interference signals can be led into the ground by this layer.

A conductor wrapped by a conductor outside the conductor is called a shielded conductor.

The conductor wrapped with a conductor outside the conductor is called a shielded wire, and the wrapped conductor is called a shield layer, which is generally a braided copper mesh or copper podium (aluminum). The shield layer needs to be grounded, and external interference signals can be introduced into the ground by this layer.

Role: Avoid interference signals entering the inner layer, conductor interference and reduce the loss of transmission signals.

Note: When selecting a shielded wire, the insulation layer of the shielded ground wire has a conductive function and can be connected to the shielded layer (with a certain resistance).

The shielding layer of shielded cable is mainly made of non-magnetic materials such as copper and aluminum, and its thickness is very thin, which is far less than the skin depth of the metal material at the frequency of use. The effect of the shielding layer is not mainly due to the reflection of electric and magnetic fields by the metal body itself. The absorption is caused by the grounding of the shielding layer. The difference in the form of grounding will directly affect the shielding effect. There are different grounding methods for the electric and magnetic shielding layers. Can be ungrounded, single-ended or double-ended.

The shielding layer of the shielded wire cannot be grounded at multiple points, because different grounding points are always different, and there is a potential difference at each point. If multiple points are grounded, a current is formed on the shielding layer, a current is induced on the conductor, and interference is induced on the signal line. Not only does it not function as a shield, but it also introduces interference. Especially in many occasions where the inverter is used, the interference contains All kinds of higher-order harmonic components have a greater impact, so special attention should be paid.

The shielded wiring system originates from Europe. It adds a metal shielding layer to the outside of an ordinary unshielded wiring system, and uses the reflection, absorption, and skin effects of the metal shielding layer to prevent electromagnetic interference and electromagnetic radiation. The shielding system makes comprehensive use of The balanced principle of the twisted pair and the shielding effect of the shielding layer have very good electromagnetic compatibility (EMC) characteristics.

Electromagnetic compatibility (EMC) refers to the ability of electronic equipment or network systems to resist electromagnetic interference, while not generating excessive electromagnetic radiation. In other words, it is required that the device 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 work of other devices and networks around it.

The balancing characteristics of U / UTP (unshielded) cables do not depend only on the quality of the components themselves (such as twisted pairs), but are affected by the surrounding environment. Because the metal around U / UTP (unshielded), hidden "ground", tension during construction, bending, etc. will destroy its balance characteristics and reduce EMC performance.

Therefore, to obtain a permanent balance, there is only one solution: add an extra layer of aluminum foil to the ground for all cores. Aluminum foil adds protection to fragile twisted-pair wires, while creating a balanced environment for U / UTP (unshielded) cables. This forms what we now call a shielded cable.

The shielding principle of shielded cable is different from the principle of balanced cancellation of twisted pair. Shielded cable is added with one or two layers of aluminum foil on the outside of four pairs of twisted pairs. The principle of reflection, absorption and skin effect of electromagnetic waves by metals (the so-called Skin effect refers to the distribution of current on the cross section of the conductor tends 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 weaker the electromagnetic wave penetration ability), effectively preventing external Electromagnetic interference enters the cable, and also prevents internal signals from radiating out, interfering with the work of other equipment.

Experiments show that electromagnetic waves with a frequency exceeding 5 MHz can only pass through 38 m-thick aluminum foil. If the thickness of the shielding layer is more than 38 m, the frequency of electromagnetic interference that can enter the cable through the shielding layer is mainly below 5 MHz. For low frequency interference below 5MHz, the balanced principle of twisted pair can be used to effectively cancel it.

According to the earliest definition of wiring, it is divided into two types: unshielded cable-UTP and shielded cable-STP. Later, with the development of technology and different processes, many different shielding types have been derived: 1. F / UTPFoilScreenedCable single-layer aluminum foil shielding structure; 2. FoilandBraidScreenedCable aluminum foil and copper braided mesh double-layer shielding structure, a) SF / UTP aluminum foil and copper woven mesh are wrapped on the outer layer of four pairs of wires at the same time; b) S / FTP (PIMF) wire pair single-pair aluminum foil shield plus copper woven mesh wrapped on the outer layer of four pairs of wires PIMF = PairinMetalFoil.

The shielded cable's resistance to external interference is mainly reflected in: the integrity of the signal transmission can be guaranteed to a certain extent through the shielding system. The shielded wiring system can prevent the transmission 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 cords are common sources of electromagnetic interference. Radio frequency interference (RFI) is high frequency interference, mainly radio frequency interference, including radio, television broadcasting, radar and other wireless communications.

For resisting electromagnetic interference, the braided shield is the most effective, that is, the metal mesh shield, because it has a lower critical resistance. For radio frequency interference, metal foil shielding is the most effective, because the gap created by the metal mesh shielding allows high-frequency signals to enter and exit freely. For the interference field of mixed high and low frequencies, a combination of metal foil layer and metal mesh shielding method, that is, a double-layer shielded cable in the form of S / FTP, can be used to make metal mesh shielding suitable for low frequency interference, and metal foil shielding Suitable for interference in the high frequency range.

The single-layer thickness of the aluminum foil shielding layer in the IBMACS shielded cable reaches 50-62 m, which plays a more complete shielding effect. At the same time, because only a single layer of shielding is used, it will be simpler for construction, easier to install, less likely to cause human damage during construction, and the thickness of the aluminum grate can withstand greater destructive forces. This can provide users with higher quality transmission performance.

One end of the shielded wire is grounded and the other end is left floating

When the transmission distance of the signal line is relatively long, the two ground points may have different potentials due to different ground resistances at both ends or current at the PEN line. At this time, if the two ends are grounded, a current will form on the shielding layer. Interference is formed, so in this case, one point of grounding is generally adopted, and the other end is left floating to avoid the formation of such interference.

Ground shielding at both ends is better, but signal distortion will increase

Please note: The two layers of shielding should be insulated and shielded from each other! If it is not insulated from each other, it should still be regarded as a single-layer shield!

The grounding at the two ends of the outermost shield is due to the induced potential difference to induce a current, so a magnetic flux that reduces the strength of the source magnetic field is generated, thereby substantially canceling out the voltage induced when there is no outer shield; and the innermost shield is grounded at one end Since there is no potential difference, it is only used for general antistatic induction. The following specifications are the best proof!

"GB50217-1994 Power Engineering Cable Design Code"-3.6.8 The grounding method of the metal shield of the control cable shall meet the following requirements:

(1) The shielding layer of the control cable of the analog signal loop of the computer monitoring system shall not constitute two or more points of grounding, and a centralized grounding should be used.

(2) Except for item (1), which requires a little grounding of the control cable, when the electromagnetic induction interference is large, two points of grounding should be adopted; the electrostatic induction interference is large, and one point of grounding can be used.

Double shield or composite general shield, it is better to share one point for internal and external shield, and two points to ground.

(3) For the two-point grounding, it should also be considered that the shielding layer will not be melted under the action of transient current.

"GB50057-2000 Building Lightning Protection Design Code"-Article 6.3.1 stipulates that when using shielded cables, the shielding layer should be equipotentially connected at least at both ends. When the system requires equipotential bonding at only one end, it should be Two layers of shielding are used, and the outer shielding is handled according to the aforementioned requirements.

Its principle is: 1. One end of the single-layer shield is grounded without potential difference, and is generally used for anti-static induction. 2. Double-layer shield, the outer shield is grounded at both ends, and the inner shield is grounded at the same potential at one end. At this time, the outer shield induces a current due to the potential difference, so a magnetic flux that reduces the strength of the source magnetic field is generated, thereby substantially canceling out the voltage induced without the outer shield.

If it is to prevent electrostatic interference, it must be grounded at a single point, whether it is one or two layers of shielding. Because single-point grounding has the fastest electrostatic discharge.

However, the following two cases are excluded:

1. There is strong current interference from outside, single point grounding cannot meet the fastest discharge of static electricity.

If the cross-sectional area of the ground wire is large enough to ensure the fastest electrostatic discharge, it is also necessary to ground at a single point. Of course, that's the case, there is no need to choose two layers of shielding.

Otherwise, two layers of shielding must be used. The outer shielding is mainly to reduce the intensity of interference, not to eliminate interference. At this time, multiple points must be grounded. Although it cannot be put in place, it must be weakened as soon as possible. To weaken, multi-point grounding is the best choice.

For example, the cable bridge in the enterprise is actually the outer shielding layer. It must be grounded at multiple points. The first line of defense reduces the intensity of the interference source.

The inner shielding layer (in fact, we do nt buy double-layer cables, usually the outer layer is the cable tray, the inner layer is the shielding layer of the shielded cable) must be grounded at a single point because the external strength has been reduced. That is the inner purpose.

2. Safety requirements such as external electric shock and lightning protection.

This situation requires two layers of protection. The outer layer is not used to eliminate interference. It is for safety reasons. To ensure the safety of people and equipment, multiple points must be grounded. The inner layer is to prevent interference, so it must be grounded at a single point.

Check the wires and ground wires (including coupling ground wires and shield wires) for the following defects and changes in operating conditions:

(1) Corrosion of wire, ground wire, broken strands, damage or flashover burn;

(2) Variations in sags of conductors and ground wires, and changes in phase separation conductor spacing;

(3) The wires, ground wires are lifted, vibrated, danced, de-iced and jumped, phase split wires are flogged, twisted, and stuck;

(4) Overheating, discoloration, deformation, and slippage of the fittings for the wires and ground wires;

(5) The wire slides in the wire clip, and the hull part of the release wire clip is released from the hanger;

(6) The jumper is broken, twisted and deformed, the air gap between the jumper and the tower changes, and the jumper is twisted; the jumper is dancing and swinging too much;

(7) Changes in the distance of the conductor to the ground, to the crossing and crossing facilities, and to other objects;

(8) Foreign objects are hung on the wires and ground wires ^[2] .