Definition: A conductor wrapped around a conductor is called a shielded wire, and the wrapped conductor is called a shielding layer. Generally, it is woven copper net or copper (aluminum). The shielding layer needs to be grounded, and external interference signals can be guided to the earth by this layer.
Function: Avoid interference signal from entering the inner layer, conductor interference and reduce the loss of transmission signal.
Structure:
(ordinary)
Insulating layer+shielding layer+conducting wire
(advanced)
Insulating layer+shielding layer+signal conductor+shielding layer grounding conductor
Note: When selecting shielding wire, the insulation layer of shielding layer grounding wire has a conductive function, which can conduct with the shielding layer (with a certain resistance).
Principle of shielding cable:
The shielded wiring system originated in Europe. It adds a metal shielding layer on the outside of the common 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 radiation. The shielded system comprehensively utilizes the balance principle of twisted pairs and the shielding effect of the shielding layer, so it has very good electromagnetic compatibility (EMC) characteristics.
Electromagnetic compatibility (EMC) means that electronic equipment or network system has certain ability to resist electromagnetic interference, and at the same time can't generate excessive electromagnetic radiation. That is to say, it is required that the equipment or network system can work normally in a harsh electromagnetic environment, and at the same time, it can't radiate excessive electromagnetic waves to interfere with the normal work of other surrounding equipment and networks.
The balance characteristic of U/UTP (unshielded) cable depends not only on the quality of the components (such as twisted pairs), but also on the surrounding environment. Because the metal around U/UTP (unshielded), hidden "ground", pulling and bending during construction, etc. will destroy its balance characteristics, thus reducing EMC performance.
Therefore, there is only one solution to obtain lasting balance characteristics: add an extra layer of aluminum foil to all core wires for grounding. It increases the protection for the fragile twisted-pair wires of aluminum foil, and artificially creates a balanced environment for U/UTP (unshielded) cables. Thereby forming what we now call a shielded cable.
The shielding principle of shielded cable is different from the balanced offset principle of twisted pairs. One or two layers of aluminum foil are added on the outside of the four twisted pairs of cables. By using the principle of reflection, absorption and skin effect of metal on electromagnetic waves (the so-called skin effect means that the distribution of current in the conductor section tends to the conductor surface with the increase of frequency, and the higher the frequency, the smaller the skin depth, that is, the higher the frequency, the weaker the penetration ability of electromagnetic waves), the shielded cable can effectively prevent external electromagnetic interference from entering the cable, and at the same time prevent internal signals from radiating out.
Experiments show that electromagnetic waves with frequency over 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 enter the cable through the shielding layer is mainly below 5MHz. However, the low-frequency interference below 5MHz can be effectively counteracted by the balanced principle of twisted pair.
According to the earliest definition of wiring, it can be divided into unshielded cable -UTP and shielded cable -STP. With the development of technology and different technologies, 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 a) SF/UTP aluminum foil and copper woven net are simultaneously wrapped on the outer layers of four pairs of wires b) S/FTP (PIMF) wire shields a single pair of aluminum foil plus copper woven net PIMF =Pair in Metal Foil wrapped on the outer layers of four pairs of wires.
Shielded cables resist external interference mainly in that the integrity of signal transmission can be guaranteed by shielding system. The shielding 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, and motors, fluorescent lamps and power lines are the common sources of EMI. Radio frequency interference (RFI) is high frequency interference, mainly radio frequency interference, including radio, television relay, radar and other wireless communications.
For resisting electromagnetic interference, the most effective choice is braided shielding, that is, metal mesh shielding, because of its low critical resistance. For radio frequency interference, metal foil shielding is the most effective, because the gap produced by metal mesh shielding can make high-frequency signals enter and exit freely. For the interference field with high and low frequencies, the combined shielding method of metal foil and metal mesh should be adopted, that is, the double-layer shielded cable in the form of S/FTP, which can make the metal mesh shielding suitable for low-frequency interference and the metal foil shielding suitable for high-frequency interference.
The single layer thickness of aluminum foil shielding layer in IBM shielded cable is 50-62μm, which plays a more complete shielding effect. At the same time, because only single-layer shielding is adopted, it will be simpler for construction, convenient for installation, not easy to cause artificial damage in the construction process, and the thickness of aluminum silks can bear greater destructive power. Thereby providing users with higher quality transmission performance.
Wire connection method:
One end of the shielded wire is grounded and the other end is suspended.
When the transmission distance of the signal line is relatively long, the potential of the two grounding points may be different due to the difference of grounding resistance at both ends or the current in the PEN line. At this time, if both ends are grounded, the shielding layer will have electric current, which will interfere with the signal instead. Therefore, in this case, the method of grounding one point and suspending the other end is generally adopted to avoid this interference.
The grounding shielding effect at both ends is better, but the signal distortion will increase.
Please note: the two layers of shielding should be insulated from each other! If they are not insulated from each other, they should still be regarded as single-layer shielding!
The grounding of both ends of the outermost shield induces current due to the introduced potential difference, thus generating magnetic flux that reduces the intensity of the source magnetic field, thus basically offsetting the voltage induced when there is no outer shield;
While one end of the innermost shield is grounded, because there is no potential difference, it is only used for general anti-static induction. The following specifications are the best evidence!
GB 50217-1994 Code for Design of Electric Power Engineering Cables-3.6.8 Grounding mode of metal shield of control cable shall meet the following requirements:
(1) The shielding layer of the analog signal loop control cable of the computer monitoring system shall not be grounded at two or more points, and centralized one-point grounding is preferred.
(2) Except for the control cable shielding layer under the condition that one point grounding is required in item (1), when the interference of electromagnetic induction is large, two points grounding should be adopted; The induced interference is large, so it can be grounded at one point.
Double shield or compound total shield, it is advisable to use one point for internal and external shield and two points for grounding.
(3) In the selection of two-point grounding, it should also be considered that the shielding layer will not be melted under the action of transient current.
Article 6.3.1 of Code for Design of Lightning Protection of Buildings (GB50057-2000) stipulates: …… When shielded cables are used, their shielding layers should be at least equipotentially connected at both ends. When the system requires equipotential connection at only one end, two layers of shielding should be used, and the outer shielding should be treated according to the aforementioned requirements.
Its principle is as follows: 1. One end of the single-layer shield is grounded without potential difference, and it is generally used for anti-static induction. 2. Double shield, both ends of the outermost shield are grounded, and one end of the inner shield is equipotentially grounded. At this time, the outer shield induces a current due to the potential difference, thus generating a magnetic flux that reduces the intensity of the source magnetic field, thus basically offsetting 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 the electrostatic discharge speed of single point grounding is the fastest.
However, there are two exceptions:
1. There is strong external current interference, and single-point grounding cannot meet the fastest discharge of static electricity.
If the grounding wire has a large cross-sectional area and can ensure the fastest electrostatic discharge, it should also be grounded at a single point. Of course, in that case, there is no need to choose two layers of shielding.
Otherwise, it is necessary to have two layers of shielding. The outer shielding is mainly to reduce the interference intensity, not to eliminate it. At this time, it must be grounded at multiple points. Although it can't be finished, it must be weakened as soon as possible. To be weakened, multi-point grounding is the best choice.
For example, the cable tray in an enterprise is actually the outer shielding layer, which must be grounded at multiple points as the first line of defense to reduce the intensity of interference sources.
The inner shielding layer (in fact, people won't buy a double-layer cable, usually 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 been reduced, and 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.
In this case, two layers of protection are required. The outer layer is not used to eliminate interference, but for safety reasons. To ensure the safety of people and equipment, multi-point grounding is required. The inner layer is to prevent interference, so it must be grounded at a single point.
The function of shielding wire
The function of shielding wire is to isolate the noise source of electromagnetic field from sensitive equipment and cut off the propagation path of noise source. Shielding is divided into active shielding and passive shielding. The purpose of active shielding is to prevent the noise source from radiating outward, and it is to shield the noise source. The purpose of passive shielding is to prevent sensitive equipment from being interfered by noise sources, and it is to shield sensitive equipment.
The shielding layer of the 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 materials on the frequency of use. The effect of the shielding layer is not mainly due to the reflection and absorption of the electric field and magnetic field by the metal body itself, but due to the grounding of the shielding layer. Different grounding forms will directly affect the shielding effect. There are different grounding modes for electric field and magnetic field shielding layers. It can be ungrounded, single-ended grounded or double-ended grounded.
Summary:
Single-ended grounding:
1) Single-end grounding of shielded cable is helpful to avoid the interference of low-frequency electric field. Or it can avoid frequency interference whose wavelength λ is much larger than the cable length L .. L<λ /20
2) Single-ended grounding of cable shielding layer can avoid low-frequency current noise on the shielding layer. This current internally causes common-mode interference voltage and may interfere with analog devices.
3) Single-ended grounding of shielding layer is advisable for those circuits (analog circuits) that are sensitive to low-frequency interference.
4) The fluctuation and permanent deviation of continuous measured values indicate low-frequency interference. Two-terminal grounding:
(1) Make sure that the connection to the electric control cabinet or plug (circular contact) passes through a large conductive area (low inductance). It is better to choose metal on metal than nonmetal on nonmetal.
(2) As some analog modules use pulse technology (for example, the processor and the A/D converter are integrated in the same module), it is recommended to shield the analog signals from each other to ensure the correct equipotential connection. Only in this case, double-ended grounding should be carried out.
(3) Generally, the transmission impedance of metal foil shielding layer is far greater than that of copper braided wire shielding layer, and its effect is 5-10 times different, so it cannot be used as digital signal cable.
(4) Occasional functional failure indicates high frequency interference. This can't be eliminated by equipotential bonding of wires.
5) It is advantageous to ground the shielding layer at multiple points except the end points of the cable.
6) Do not connect the shielding layer to the pin to avoid the phenomenon of "pigtail".
7) Always pay attention to that the parallel impedance of the shielding layer should be less than 1/10 of its own impedance. Cable bridge, mechanical frame, other shielding layers or other parallel cables can make the system equipotential.
