Shielding wire function

Shielded cable is a transmission line that uses a metal mesh braid layer to wrap the signal line, and the braid layer is generally red copper or tinned copper. Shielding wire is a wire with metal braided shell specially used to reduce the influence of external electromagnetic field on power supply or communication line. This shielding line also has the effect of preventing the line from radiating electromagnetic energy. The shield layer needs to be grounded, and external interference signals can be imported into the ground by the layer. The shielding layer of the shielded cable is mainly made of non-magnetic materials such as copper and aluminum, and the thickness is very thin, far less than the skin depth of the metal material on the frequency of use (the so-called skin effect means that the distribution of current in the conductor cross section tends to be distributed on 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). The effect of the shield layer is mainly not 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 shield layer, and the different forms of grounding will directly affect the shielding effect.

Structure of shielding line

The wrapped conductor is called the shield layer, which is generally conductive cloth, braided copper mesh or copper (aluminum) platinum.

Structure 1: insulation layer + shielding layer + wire;

Structure 2: insulation layer + shielding layer + signal conductor + shielding ground conductor.




The shielding is divided into active shielding and passive shielding. The purpose of active shielding is to prevent the noise source from radiating outward, which is the shielding of the noise source. The purpose of passive shielding is to prevent the sensitive equipment from being interfered by the noise source, and it is the shielding of the sensitive equipment.

Shielded wire connection

The shielding layer of the shielding line does not allow multi-point grounding, because different ground points are always different, and there is a potential difference at each point. Such as multi-point grounding, the formation of current in the shield layer, not only can not play a shielding role, but the introduction of interference, especially in the frequency converter used in many occasions, the interference contains a variety of high-order harmonic components, resulting in greater impact, should pay special attention to.

The shielding principle of the shielded cable is different from the balanced offset principle of the twisted pair, the shielded cable is added to the outside of the four pairs of twisted pair one or two layers of aluminum foil, the use of metal on electromagnetic wave reflection, absorption and skin effect principle, effectively prevent external electromagnetic interference into the cable, but also prevent the internal signal radiation out, interfere with the work of other equipment.

Experiments show that electromagnetic waves with frequencies greater than 5MHz can only pass through aluminum foil 38μm thick. If the thickness of the shield layer exceeds 38μm, the frequency of electromagnetic interference that can enter the cable through the shield layer is mainly below 5MHz. For low-frequency interference below 5MHz, the balance principle of twisted pair can be effectively offset. One end of the shielded cable is grounded and the other end is suspended

When the signal line transmission distance is relatively far, because the ground resistance of the two ends is different or the PEN line has current, it may lead to different ground potentials of the two ground points, at this time if the two ends are grounded, the shielding layer has electricity into, but the signal interference, so in this case, generally take a point of grounding, the other end of the suspension method, can avoid the formation of such interference.

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

Please note: The two layers of shielding should be mutually isolated shielding! If there is no mutual insulation should still be regarded as a single shield! The grounding of the two ends of the outer shield is induced by the current induced by the potential difference introduced, so as to produce a magnetic flux that reduces the strength of the source magnetic field, thus basically cancels out the voltage induced when there is no outer shield layer; The innermost shield end is grounded, because there is no potential difference, it is only used for general anti-static induction.



The above content is the content that Tianxing cable shares for you in this issue, I hope it will be helpful to you

Qingdao Tianxing Cable Co., Ltd. is a high-tech enterprise specializing in the production of wire and cable in Shandong Province, with more than 150 sets of domestic excellent cable production equipment and testing equipment, excellent production technology, modern testing equipment, to ensure stable product quality and timely supply. The main products of the company are low-smoke halogen-free cables, photovoltaic cables, aluminum alloy cables, shielding cables, radiation cross-linked cables, control cables, computer cables, high-life cables, flame retardant series wire and cable.

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