The principle and connection method of shielded wires

What is a shielded wire?

Definition: A wire with a conductor wrapped around the outside is called a shielded wire, and 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 ground through this layer.

Function: To prevent interference signals from entering the inner layer and to reduce the loss of transmitted signals while avoiding conductor interference.

Structure: (ordinary) insulating layer + shielding layer + wire; (Advanced) Insulation layer + shielding layer + signal wire + shielding layer grounding wire

Note: When choosing shielded wires, the insulation layer of the shielding layer grounding wire has a conductive function and can conduct 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. It uses the reflection, absorption and skin effect of the metal shielding layer to prevent 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.

Electromagnetic compatibility (EMC) refers to the ability of electronic devices or network systems to resist electromagnetic interference to a certain extent, while not generating excessive electromagnetic radiation. That is to say, it is required that the device or network system can operate normally in a relatively harsh electromagnetic environment, while not radiating excessive electromagnetic waves to interfere with the normal operation of other surrounding devices 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 the U/UTP (unshielded), concealed "ground", pulling during construction, bending and other conditions can all disrupt its balance characteristics, thereby reducing its EMC performance.

Therefore, to achieve a lasting and unchanging balanced characteristic, there is only one solution: add an extra layer of aluminum foil outside all the core wires for grounding. Aluminum foil adds protection to the fragile twisted-pair core wire and artificially creates a balanced environment for U/UTP (unshielded) cables. This thus forms what we now call shielded cables.

The shielding principle of shielded cables is different from the balance cancellation principle of twisted-pair cables. Shielded cables are made by adding one or two layers of aluminum foil outside four pairs of twisted-pair wires. They utilize the reflection, absorption and skin effect of metals on electromagnetic waves (the skin effect refers to the fact that the distribution of current across the cross-section of a 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, 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 38μm thick aluminum foil. 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 balancing 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 companies, Many different types of shielding have been derived: 1. F/UTP Foil Screened Cable single-layer aluminum Foil shielding structure 2. Foil and Braid Screened Cable double-layer shielding structure of aluminum foil and copper braided mesh a) SF/UTP Aluminum Foil and copper braided mesh are simultaneously wrapped around the outer layer of four pairs of wires b) S/FTP (PIMF) wire pairs. Single Pair of aluminum foil shielding plus the outer layer of copper braided mesh wrapped around the four pairs of wires PIMF =Pair in Metal Foil.

The resistance of shielded cables to external interference is mainly reflected in: the integrity of signal transmission can be guaranteed to a certain extent through the shielding system. The shielded cabling 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 lines are common sources of electromagnetic interference. 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, choosing braided layer shielding is the most effective, that is, metal mesh shielding, because it has a relatively low critical resistance. For radio frequency interference, metal foil shielding is the most effective because the gaps created by metal mesh shielding allow high-frequency signals to freely enter and exit. 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 interference in the low-frequency range and the metal foil shielding to be suitable for interference in the high-frequency range.

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 single-layer 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 fabric can withstand greater destructive force. Thus, it can provide users with higher quality transmission performance.

Shielded wire connection method:

One end of the shielded wire is grounded while the other end is suspended.

When the transmission distance of the signal line is relatively long, due to the different grounding resistances at both ends or the presence of current in the PEN line, the potentials at the two grounding points may be different. At this time, if both ends are grounded, the shielding layer will generate current, which will instead interfere with the signal. Therefore, in such cases, it is generally advisable to ground one point and leave the other end unconnected to avoid the formation of such interference.

The shielding effect of grounding at both ends is better, but the signal distortion will increase.

Please note: The two layers of shielding should be mutually insulated and isolated shielding! If they are not insulated from each other, they should still be regarded as single-layer shielding!

The grounding at both ends of the outermost shielding layer is due to the introduced potential difference, which induces a current and thus generates a magnetic flux that reduces the intensity of the source magnetic field, thereby basically canceling out the voltage induced without the outer shielding layer.

The innermost shielding end is grounded. As there is no potential difference, it is only used for general anti-static induction. The following norms are the best evidence!

"GB 50217-1994 Code for Design of Power Engineering Cables" - 3.6.8 The grounding method of metal shielding of control cables shall comply with the following provisions:

(1) The shielding layer of the control cable in the analog signal loop of the computer monitoring system shall not form two-point or multi-point grounding. Centralized single-point grounding is preferred.

(2) For the shielding layer of control cables other than those in item (1) that require single-point grounding, when the electromagnetic induction interference is relatively large, it is advisable to adopt two-point grounding. The interference of electrostatic induction is relatively large, so it can be grounded at one point.

For double shielding or composite overall shielding, it is advisable to use one point for the inner and one point for the outer shielding, and ground both points

(3) When choosing two-point grounding, it is also advisable to consider that the shielding layer will not be melted under the action of transient current.

Article 6.3.1 of the "Code for Design of Lightning Protection of Buildings GB50057-2000" stipulates:... When shielded cables are used, the shielding layer should be at least equipotential connected at both ends. When the system requires equipotential connection at only one end, two layers of shielding should be adopted, and the outer layer of shielding should be treated as per the aforementioned requirements.

The principle is as follows: 1. One end of the single-layer shield is grounded, which does not form a potential difference and is generally used for anti-static induction. 2. Double-layer shielding: The outermost shielding is grounded at both ends, and the inner shielding is grounded at the same potential at one end. At this point, the outer shielding layer induces a current due to the potential difference, thus generating a magnetic flux that reduces the intensity of the source magnetic field, which basically offsets the voltage induced without the outer shielding layer.

If it is to prevent static interference, single-point grounding must be carried out, whether it is one layer or two layers of shielding. Because the electrostatic discharge speed of single-point grounding is the fastest.

However, the following two situations are excluded:

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

If the cross-sectional area of the grounding wire is large and can ensure the fastest discharge of static electricity, it should also be grounded at a single point. Of course, if that were the case, there would be no need to choose two layers of shielding.

Otherwise, two layers of shielding must be used. The outer layer of shielding mainly reduces the interference intensity rather than eliminates it. At this time, multi-point grounding is necessary. Although it cannot be completely discharged, it must be weakened as soon as possible. To weaken it, multi-point grounding is the best choice.

For instance, the cable tray in an enterprise is actually an outer shielding layer. It must be grounded at multiple points as the first line of defense to reduce the intensity of the interference source.

The inner shielding layer (in fact, people usually don't buy double-layer cables; generally, the outer layer is the cable tray and the inner layer is the shielding layer of the shielded cable) must be grounded at a single point because the external strength has decreased. The purpose of the inner layer is to discharge as soon as possible and eliminate interference.

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

This situation requires two layers of protection. The outer layer is not for eliminating interference but for safety considerations to ensure the safety of personnel and equipment. It must be grounded at multiple points. The inner layer is for preventing interference, so it must be grounded at a single point.

The function of shielded wire:

The function of the shielding wire is to isolate the electromagnetic field noise source from sensitive equipment and cut off the transmission path of the noise source. Shielding is divided into active shielding and passive shielding. The purpose of active shielding is to prevent noise sources from radiating outward, which is a form of shielding for noise sources. The purpose of passive shielding is to prevent sensitive equipment from being disturbed by noise sources and it is a form of shielding for sensitive equipment.

The shielding layer of shielded cables is mainly made of non-magnetic materials such as copper and aluminum, and is very thin, far less than the skin depth of the metal material at the usage frequency. The effect of the shielding layer is mainly not caused by the reflection and absorption of the electric and magnetic fields by the metal body itself, but by the grounding of the shielding layer. Different grounding forms will directly affect the shielding effect. The grounding methods for the shielding layers of the electric field and magnetic field are different. Ungrounded, single-ended grounded or double-ended grounded methods can be adopted

Summary

Single-ended grounding

Single-ended grounding of shielded cables is helpful in avoiding interference from low-frequency electric fields. Or rather, it can avoid frequency interference where the wavelength λ is much greater than the cable length L. L<λ /20

Single-ended grounding of the cable shielding layer can prevent low-frequency current noise on the shielding layer. This current causes common-mode interference voltage internally and may interfere with analog devices.

3) Single-ended grounding of the shielding layer is advisable for circuits that are sensitive to low-frequency interference (analog circuits).

4) The up and down fluctuations and permanent deviations of continuous measurement values indicate the presence of low-frequency interference. Double-ended grounding: A. Ensure that the connection to the electrical control cabinet or plug (circular contact) passes through a large conductive area (low induction coefficient). It is better to choose metals on metals than non-metals on non-metals. b. Since some analog modules use pulse technology (for example, the processor and A/D converter are integrated in the same module), it is recommended to shield the analog signals from each other to ensure correct equipotential bonding. Only in this case should double-ended grounding be carried out. c. Generally, the transmission impedance of a metal foil shielding layer is much greater than that of a copper braided wire shielding layer, with a difference of 5 to 10 times in effect. Therefore, it cannot be used as a digital signal cable. Occasional functional failures indicate high-frequency interference. This is something that cannot be eliminated by equipotential bonding of wires.

5) It is beneficial for the shielding layer to be grounded at multiple points except for the endpoints of the cable.

6) Do not attach the shielding layer to the pins to avoid the "pigtail" phenomenon.

7) Always pay attention that the parallel impedance of the shielding layer should be less than 1/10 of its own impedance. Cable trays, mechanical frames, other shielding layers or other parallel cables can all enable the system to achieve equipotential bonding.

8) If the shielding layer of the cable gets hot when both ends of the shielding layer are grounded, or if the shielding layer sparks when it touches the casing of the electrical control cabinet or the shielding bus, it indicates that the equipotential bonding is unreliable.