Why is shielded wire so important? But remember about one-way grounding!

PART 01

The role and use of shielded wire

Shielded cable

Shield wire is a transmission line that uses a metal mesh braid to wind signal wire. The braid is usually red or tinned copper.

Shielded wire is a type of wire with a metal braided sheath, specially used to reduce influence of external electromagnetic fields on power supply or communication lines. This shielded wire also prevents line from radiating electromagnetic energy.

The shielding layer must be grounded, and external noise can be directed to ground by this layer.

The protective layer of a shielded cable is mainly made of non-magnetic materials such as copper and aluminum, and has a very thin thickness, which is much smaller than skin depth of a metal material at frequency of use (the so-called skin effect means that distribution current in cross section of conductor changes with increasing frequency, it tends to be distributed over surface of conductor. The higher frequency, shallower depth of skin layer, i.e., higher frequency, weaker penetrating power of electromagnetic waves). The influence of protective layer is basically unrelated with metal case itself. Reflection, absorption, but due to grounding of protective layer, various forms of grounding will directly affect shielding effect.

PART 02

Structure

The wrapped conductor is called shield layer, which is usually a conductive cloth, woven copper mesh, or copper (aluminum) platinum.

General: insulating layer + shielding layer + wire;

Optional: insulation + shield + signal conductor + shield ground wire.

Shielded wire

Usually, shield needs to be grounded. The function of shielded wire is to isolate source of electromagnetic field noise from sensitive equipment and cut off path of noise source.

Shielding is divided into active shielding and passive shielding. The purpose of active shielding is to prevent noise sources from emitting to outside, i.e. shielding noise sources; purpose of passive shielding is to prevent sensitive equipment from being affected by noise sources Interference is shielding of sensitive equipment.

The protective layer of shielded wire does not allow multiple grounding points because different grounding points are always different and there is a potential difference at each point.

If several points are grounded, a current will be generated in shielding layer, which not only will not provide a shielding effect, but will also create interference, especially in cases where frequency converters are often used, interference contains various high-order harmonic components that have a greater influence should be given special attention.

The shielded wiring system originated in Europe. It adds a metallic shielding layer on outside of a conventional unshielded wiring system. It uses reflection, absorption and skin effect of metal shielding layer to prevent electromagnetic interference and electromagnetic radiation. The system comprehensively uses twisted pair balance principle and shielding effect of protective layer, so it has very good electromagnetic compatibility (EMC) performance.

Electromagnetic compatibility (EMC) means that electronic equipment or networked systems have a certain ability to withstand electromagnetic interference and at same time cannot generate excessive electromagnetic radiation.

That is, it is required that equipment or network system can operate normally in a relatively harsh electromagnetic environment, and at same time cannot emit excessive electromagnetic waves that interfere with normal operation of other surrounding equipment and network.

The shielding principle of a shielded cable is different from that of a twisted pair balance shift. The shielded cable consists of adding one or two layers of aluminum foil on outside of four pairs of twisted pairs, and using principle of metallic reflection, absorption and skin effect of electromagnetic waves., Effectively prevent external electromagnetic interference from entering cable, and also prevent internal radiation of signal from interfering operation of other equipment.

Experiments have shown that electromagnetic waves with a frequency of more than 5 MHz can only pass through aluminum foil 38 microns thick. If thickness of shielding layer exceeds 38 µm, frequency of electromagnetic interference that can enter cable through shielding layer willmostly below 5 MHz.

To compensate for low frequency interference below 5 MHz, balanced twisted pair principle can be applied.

One end of shield wire is grounded and other end is left floating.

When transmission distance of signal line is relatively long, potential of two ground points may be different due to different ground resistance at both ends or current of PEN line. At this time, if both ends are grounded, shielding layer will carry current. On contrary, it will interfere with signal. Therefore, in this case, generally one point is grounded and other end is suspended, thus avoiding such interference.

Grounding the shield at both ends is better, but signal distortion will increase.

Shield wire connection method

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

Grounding at both ends of farthest shield is due to an induced potential difference that induces a current, thus creating a magnetic flux that reduces strength of source's magnetic field, thereby basically eliminating induced voltage without external shielding. layer and innermost layer One end of shield is grounded. Since there is no potential difference, it is only used for general antistatic induction. The following specification is best proof!

"GB50217-1994 Electrical Power Cable Design Code" - 3.6.8 The grounding method of metal shield of control cables should meet following requirements:

(1) The shield layer of analog signal circuit control cable of computer monitoring system must not form two or more ground points, and a centralized single ground point should be used.

(2) For shielding layer of control cable, except for item (1), which requires a single-point grounding, when there is a large amount of electromagnetic induction noise, a two-point grounding should be used; if interference of electrostatic induction is large, a single-point grounding can be used. For double shielding or composite overall shielding, one point must be used for internal and external shielding, and two points must be earthed.

(3) When choosing a two-point grounding, it should also be taken into account that shielding layer will not melt under influence of transient current.

GB50057-2000 Code for Design of Lightning Protection for Buildings - Article 6.3.1 stipulates that when shielded cables are used, their shield layers must be equipotentially connected at least at both ends. When a system requires equipotential bonding at one end only, two layers of shielding should be used, and outer shielding should be treated in accordance with above requirements. The principle is:

(1) One end of single-layer shield is grounded, no potential difference is formed, and it is usually used for antistatic induction.

(2) Double-layer shielding, both ends of outer shield are grounded and one end of inner shield is equipotentially grounded. At this point, outer shield induces a current due to potential difference, thereby creating a magnetic flux that reduces strength of source's magnetic field, thereby essentially canceling out voltage induced without outer shield. If it is necessary to prevent electrostatic interference, it must be grounded at one point, whether it be a layer or a second layer of shielding. Because electrostatic discharge rate of single-point grounding is highest.

However, there are two exceptions:

(1) There is strong external current interference, and single-point grounding cannot satisfy fastest discharge of static electricity. If cross-sectional area of ​​the ground wire is large, it must also be grounded at one point to ensure that static electricity is discharged as quickly as possible. Of course, if this is case, there is no need to choose two shielding layers. Otherwise, two layers of shielding are required. External shielding is mainly to reduce intensity of interference, not eliminate it. Multi-point grounding is needed at present. Although it cannot be exhausted, it must be weakened as soon as multi-point grounding is best used to attenuate grounding. For example, a cable tray in an enterprise is actually an outer shielding layer that must be grounded at several points, first line of defense and reduce power of interferer. The inner shield layer (actually people will not buy double layer cables, generally outer layer is cable tray and inner layer is shield layer of shielded cable) must be grounded at one point, because external strength has been reduced, discharge as as soon as possible to eliminate interference. This is purpose of inner layer.

(2) Safety requirements such as protection against electric shock and lightning. In this case, two layers of protection are required. The outer layer is not used to eliminate interference, but for safety reasons. To ensure safety of people and equipment, several points must be grounded. The inner layer must prevent interference, so it must be grounded at one point.