Principle and connection method of shielded wire

Principle and connection method of shielded wire
The shielded wire adds a metal shielding layer outside 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 twisted pair and the shielding function of the shielding layer, so it has very good electromagnetic compatibility (EMC) characteristics.

Principle of shielded wire

The shielding principle of shielded cable is different from the balanced cancellation principle of twisted pair. The shielded cable adds one or two layers of aluminum foil on the outside of four pairs of twisted pair, using the reflection of metal to electromagnetic wave The principle of absorption and skin effect (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. The higher the frequency, the smaller the skin depth, that is, the higher the frequency, the weaker the penetration ability of electromagnetic wave). It can effectively prevent external electromagnetic interference from entering the cable and prevent internal signal from radiating out to interfere with the work of other equipment.

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

According to the earliest definition of wiring, it is divided into unshielded cable UTP and shielded cable STP. Later, with the development of technology and different processes, 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 aluminum foil and copper braided mesh double-layer shielding structure a) SF / UTP aluminum foil and copper braided mesh are wrapped in the outer layer of four pairs of wires at the same time B) s / FTP (pimf) Wire pair single pair aluminum foil shielding plus copper woven mesh wrapped in the outer layer of four pairs of wires pimf = pair in metal foil.

Shielding cable resistance to external interference is mainly reflected in: the integrity of signal transmission can be guaranteed through the shielding system. 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. Motor, fluorescent lamp and power line are common electromagnetic interference sources. 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, that is, metal mesh shielding, because it has low critical resistance. For RF interference, metal foil shielding is the most effective, because the gap generated by metal mesh shielding can make high-frequency signals enter and leave freely. For the mixed interference field of high and low frequency, the combined shielding mode of metal foil layer and metal mesh shall be adopted, that is, the double-layer shielded cable in the form of S / FTP, which can make the metal mesh shielding suitable for the interference in the low frequency range and the metal foil shielding suitable for the interference in the high frequency range.



The single layer thickness of aluminum foil shielding layer in IBM ACS shielded cable is 50-62 μ m. Play 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 man-made damage during construction, and the thickness of aluminum silk can bear greater destructive force. Thus, it can provide users with higher quality transmission performance.

Shield wire connection method:
One end of the shield wire is grounded and the other end is suspended.
When the signal transmission distance is far, due to the different grounding resistance or PEN at the ends of the line current, may lead to two different ground potential, as if on both ends of grounding, shielding layer there is electricity pop into, instead of to form interference signal, so this kind of circumstance commonly take one-point earthing, the other end dangling, can avoid this kind of interference.  
 
The shielding effect of grounding is better, but the signal distortion will increase.  
 
Please note: the two layers of shielding should be mutually insulated!  If there is no mutual insulation should still be regarded as a single layer shielding!  
 
The grounding of both ends of the outermost shield is due to the induction of current due to the potential difference introduced, so the magnetic flux reduces the strength of the source magnetic field, which basically cancels out the voltage induced when there is no outer shield layer;  
 
And the innermost shielding end grounding, because there is no potential difference, only used for general anti-static induction.  The following specification is the best evidence!  
 
GB 50217-1994 Power Engineering Cable Design Code -- 3.6.8 Control cable metal shield grounding mode, shall comply with the following provisions:  
(1) the shielding layer of the analog signal loop control cable of the computer monitoring system shall not constitute two points or multi-point grounding, and should be used for centralized one-point grounding.  
(2) Except for item (1), when the interference of electromagnetic induction is large, the shielding layer of control cable should adopt two-point grounding;  Electrostatic induction interference is larger, can use a point grounding.  Double shielding or compound total shielding, appropriate internal and external shielding points, two points grounding.  
 
(3) The choice of two-point grounding should also consider that the shielding layer will not be melted under the action of transient current.  
 
Gb50057-2000 Lightning Protection Design Code for Buildings -- Article 6.3.1 provides:......  When a shielded cable is used, its shielding layer should be at least equipotential connection at both ends. When the system requires equipotential connection at one end only, two layers of shielding should be used, and the outer shielding should be processed according to the above requirements.  
 
Its principle is: 1. One end of single-layer shielding is grounded, no potential difference is formed, generally used for anti-static induction.  2. Double layer shielding, the outermost shielding ends grounding, one end of the inner shielding equipotential grounding.  At this point, the outer shield induces a current due to a potential difference, thus creating a flux that reduces the strength of the source field, essentially cancelling out the voltage induced in the absence of the outer shield.  
 
To prevent electrostatic interference, a single point of grounding must be used, either with a layer one or layer two shielding.  Because single point grounding is the fastest electrostatic discharge.  
 
Except, however, in the following cases:  
 
1, external strong current interference, single point grounding can not meet the fastest electrostatic discharge.  
If the cross-sectional area of the ground cable is large enough to ensure the fastest discharge of static electricity, single point grounding is also required.  Of course, in that case, there is no need to choose two layers of shielding.  
 
Otherwise, there must be two layers of shielding, the outer shielding is mainly to reduce the intensity of interference, not to eliminate interference, then must be multi-point grounding, although not finished, but must be weakened as soon as possible, to weaken, multi-point grounding is the best choice.  
For example, the cable tray in the enterprise is actually the outer shield layer, it must be multi-point grounding, the first line of defense, reduce the intensity of the interference source.  
 
The inner shield layer (in fact, we will not buy a double layer of cable, generally the outer layer is the cable bridge, the inner layer is the shielding layer of the shielded cable) must be single point grounding, because the external strength has been reduced, discharge as soon as possible, eliminate interference is the purpose of the inner layer.  
 
2, external shock and lightning protection and other safety requirements.  
In this case, two layers of protection are required. The outer layer is not used to eliminate interference but to ensure the safety of the human body and equipment. Multiple layers of protection must be grounded.  The inner layer is interference proof, so it has to be single grounded.