The principle of shielded wire and connection method What is a shielded wire?

2022-07-12

Role: to avoid interference signals into the inner layer, and conductor interference while reducing the loss of transmission signal.


Structure: (ordinary) insulation layer + shielding layer + wire; (advanced) insulation layer + shielding layer + signal conductor + shielding layer grounding conductor

Note: in the selection of shielded wire, the shielding layer grounding conductor insulation layer has a conductive function, and can be conducted with the shielding layer (with a certain resistance)

 

The principle of shielded cable

The shielded wiring system originated in Europe, it is outside the ordinary non-shielded wiring system with a metal shield, the use of metal shield reflection, absorption, and skin effect to achieve the function of preventing electromagnetic interference and electromagnetic radiation, shielding system uses a combination of the principle of balance of twisted pair and shielding shield and thus has very good electromagnetic compatibility (EMC) characteristics.

Electromagnetic compatibility (EMC) means that the electronic equipment or network system has a certain ability to resist electromagnetic interference, while not generating excessive electromagnetic radiation. In other words, the device or network system is required to be able to work normally in a relatively harsh electromagnetic environment, while not radiating excessive electromagnetic waves to interfere with the normal work of other devices and networks around.

The balance characteristics of U/UTP (unshielded) cables do not depend only on the quality of the components themselves (such as stranded pairs) but can be influenced by the surrounding environment. The metal around the U/UTP (unshielded), the hidden "ground", the pulling and bending during the construction, etc. can destroy the balance characteristics and thus reduce the EMC performance.

Therefore, there is only one solution to obtain long-lasting balanced characteristics: grounding all cores with an additional layer of aluminum foil. The aluminum foil adds protection to the fragile twisted cores and creates an artificially balanced environment for U/UTP (unshielded) cables. This results in what we now call a shielded cable.

The shielded cable shielding principle is different from the twisted balance offset principle, shielded cable is in the four pairs of twisted wire outside the addition of an extra layer or two layers of aluminum foil, the use of metal reflection of electromagnetic waves, absorption, and skin effect principle (the so-called skin effect refers to the distribution of current in the conductor cross-section with the increase in frequency and tend to conductor surface distribution, the higher the frequency, the smaller the skin depth, that is, the higher the frequency, the weaker the penetration ability of electromagnetic waves), effectively preventing external electromagnetic interference. ), effectively prevent external electromagnetic interference into the cable, but also prevent the internal signal radiation out, interfering with the work of other equipment.

Experiments show that electromagnetic waves with a frequency of more than 5MHz can only pass through 38μm thick aluminum foil. If the thickness of the shield is allowed to exceed 38μm, it makes the frequency of electromagnetic interference that can enter the inside of the cable through the shield mainly below 5MHz. The low-frequency interference below 5MHz can be effectively offset by applying the balancing principle of twisted-pair cables.

According to the earliest definition of cabling, there are two types of unshielded cables - UTP and shielded cables - STP. Later, with the development of technology and different processes, many different types of shielding have been derived 1. b) S/FTP (PIMF) Single pair of aluminum foil shielding plus copper braid wrapped in the outer layer of four pairs of wires PIMF =Pair in Metal Foil.

Shielded cables resist external interference mainly because the integrity of the signal transmission can be guaranteed by the shielding system. A shielded cabling system can prevent the transmission of data from external electromagnetic interference and radio frequency interference. Electromagnetic interference (EMI) is mainly low-frequency interference, motors, fluorescent lamps, and power lines are the usual sources of electromagnetic interference. Radio Frequency Interference (RFI) is high-frequency interference, mainly radio frequency interference, including radio, television transmissions, radar, and other wireless communications.

For resistance to electromagnetic interference, the choice of woven layer shielding is most effective, that is, metal mesh shielding because it has a low critical resistance. And for RF interference, metal foil layer shielding is most effective because the gaps created by the metal mesh shielding allow high-frequency signals to move freely in and out. For mixed high and low-frequency interference fields, it is necessary to use a combination of metal foil layers plus metal mesh shielding, that is, S/FTP form of double-shielded cable, which can make the metal mesh shielding for low-frequency range of interference, metal foil shielding for a high-frequency range of interference.


IBM ACS shielding cable in the aluminum foil shielding layer thickness of a single layer that is 50-62μm, to play a more complete shielding effect. At the same time, due to the use of only a single layer of shielding, for the construction will be simpler, easier to install, less likely to cause human damage during construction, and the thickness of aluminum foil can withstand greater destructive forces. Thus can provide users with higher quality transmission performance.

 

Shielded wire connection method


One end of the shielded wire is grounded, and the other end is suspended.

When the signal line transmission distance is relatively far, due to the grounding resistance of the two ends of the different or PEN line current, may lead to two grounding potential is different, then if the two ends of the ground, the shield layer is electrically popular into, but the formation of interference with the signal, so this case generally take a point of grounding, the other end of the overhang approach to avoid the formation of such interference.

Both ends of the grounded shielding effect is better, but the signal distortion will increase.

Please note: two layers of shielding should be mutually insulated isolated shielding! If there is no mutual insulation should still be regarded as a single layer of shielding!

The outermost shield is grounded at both ends due to the potential difference introduced and the induction of current, thus generating a magnetic flux that reduces the strength of the source magnetic field, thereby essentially canceling out the voltage induced when there is no outer shield.

The innermost shield is grounded at one end and is only used for general anti-static induction because there is no potential difference. The following specification is the best proof!

GB 50217-1994 Electric Power Engineering Cable Design Specification" - 3.6.8 Control cable metal shield grounding method, should comply with the following provisions.

(1) computer monitoring system of the analog signal circuit control cable shield, shall not constitute two or more points of grounding, it is appropriate to use a centralized point of grounding.

(2) In addition to (1) and other situations requiring a point of grounding control cable shield, when the interference of electromagnetic induction is large, it is appropriate to use two points of grounding; electrostatic induction of interference is large, available a point of grounding.

Double shielding or composite total shielding, it is appropriate to use one point for internal and external shielding, two points of grounding.

(3) the choice of two points of grounding, it is also appropriate to consider the role of transient currents in the shielding layer is not burned melt.

GB50057-2000 Building Lightning Protection Design Code - Article 6.3.1 states: ...... When using shielded cables, the shield should be at least two ends of the equipotential connection, when the system requires only one end of the equipotential connection, should be used Two layers of shielding, the outer shield is handled according to the previous requirements.

The principle is: 1. single shield grounded at one end, no potential difference, generally used for anti-static induction. 2. double shielding, the outermost shield two ends grounded, the inner shield one end equipotential grounding. At this point, the outer shield due to potential differences and induction of current, so the magnetic flux to reduce the strength of the source magnetic field, thus basically offset the voltage induced when there is no outer shield.

If it is to prevent electrostatic interference, it must be grounded at a single point, whether it is a layer or a two-layer shield. Because the single point of grounding the electrostatic discharge rate is the fastest.

However, except for the following two cases.

1, there is strong external current interference, single-point grounding can not meet the fastest discharge of static electricity.

If the cross-sectional area of the grounding wire is very large, to ensure that the fastest discharge of static electricity, the same single-point grounding. Of course, it is really that way, there is no need to choose two layers of shielding.

Otherwise, there must be two layers of shielding, the outer layer of shielding is mainly to reduce the intensity of interference, not to eliminate interference, which must be multi-point grounding, although the discharge is not finished, but 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 shield, it is necessary to multi-point grounding, the first line of defense to reduce the intensity of the source of interference.

The inner shield layer (in fact, people will not buy double-layer cable, generally the outer layer is the cable bridge, the inner layer is the shield of the shielded cable) must be single-point grounding, because the external strength has been reduced, as soon as possible to discharge, eliminate interference is the purpose of the inner layer.

2, external electric shock and lightning protection and other safety requirements.

This situation must be two layers of protection, the outer layer is not used to eliminate interference, is for safety reasons, to ensure the safety of personal and equipment, must be multi-point grounding. Click here to learn more about cable technology The inner layer is what prevents interference, so it must be grounded at a single point.
 

The role of shielded wire

The role of shielded wire is to isolate the electromagnetic field noise source from sensitive equipment and cut off the propagation path of the noise source. Shielding is divided into active shielding and passive shielding, active shielding purpose is to prevent noise sources from radiating outward, is the shielding of noise sources; passive shielding purpose is to prevent sensitive equipment from being interfered with by noise sources, is the shielding of sensitive equipment.

The shielding layer of shielded cable is mainly made of copper, aluminum and other non-magnetic materials, and the thickness is very thin, much smaller than the skin depth of metal materials on the frequency of use, the effect of the shielding layer is not mainly due to the reflection and absorption of the metal body itself on the electric and magnetic fields, but due to the grounding of the shielding layer, the grounding of different forms will directly affect the shielding effect. For electric field, magnetic field shield grounding way is different. Can be used without grounding, single-ended grounding or double-ended grounding.

To summarize.

Single-ended grounding


1) The single-end grounding of shielded cable is helpful to avoid the interference of low frequency electric field. Or it can avoid interference at frequencies where the wavelength λ is much larger than the cable length L. L

2) Single-ended grounding of the cable shield can avoid low-frequency current noise on the shield. Such currents cause common mode interference voltages internally and may interfere with analog devices.

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

4) Up and down fluctuations and permanent deviations of continuously measured values indicate low-frequency disturbances.
 

Double-ended grounding


1) Make sure that the connection to the electrical cabinet or plug (circular contact) passes through a large conductive area (low induction coefficient). It is better to choose metal on metal than non-metal on non-metal.

2) Since some analog modules use pulse technology (e.g. processor and A/D converter integrated in the same module), it is recommended to shield the analog signals from each other to ensure a correct equipotential connection, and only in this case make a double-ended grounding.

3) Usually the transmission impedance of a metal foil shield is much higher than that of a copper braid shield, with a difference of 5-10 times, and cannot be used as a digital signal cable.

4) Occasional malfunction indicates high frequency interference. This is the wire isotropic connection can not be eliminated.

5) It is advantageous to ground the shield at multiple points other than the end of the cable.

6) Do not connect the shield to the pins to avoid the "pig tail" phenomenon.

7) Always pay attention to the parallel impedance of the shield should be less than 1/10 of its own impedance. cable bridges, mechanical frames, other shields or other parallel cables can make the system to equipotential.

8) If the cable shield is hot when the shield is grounded at both ends, or if the shield fires when it touches the electrical control cabinet shell or shield bus, it means the equipotential connection is not reliable.