Principle and Structure of Shielded Control Wire

1. About the shielded control wire

The shielded control wire is a transmission line that uses a metal mesh braid to wrap the signal line, and the braid is generally red copper or tinned copper. The shielded control wire is a wire with a metal braided shell specially used to reduce the influence of external electromagnetic fields on the power supply or communication lines. This shielded control wire also has the effect of preventing the line from radiating electromagnetic energy to the outside. The shielding layer needs to be grounded, and external interference signals can be guided to the ground by this layer.

The shielding layer of the shielded control wire is mainly made of non-magnetic materials such as copper and aluminum, and the thickness is very thin, which is much smaller than the skin depth of the metal material at the frequency of use (the so-called skin effect refers to the distribution of the current in the conductor cross-section with the frequency. The higher the frequency, the smaller the skin depth, that is, the higher the frequency, the weaker the penetration ability of electromagnetic waves), the effect of the shielding layer is not mainly due to the reflection of the electric field and magnetic field by the metal body itself, It is caused by absorption, but is caused by the grounding of the shielding layer. Different grounding forms will directly affect the shielding effect.

The shielding principle of the shielded control wire is different from the balanced cancellation principle of the twisted pair. The shielded control wire is to add one or two layers of aluminum foil to the outside of the four pairs of twisted pairs, using the reflection, absorption and skin effect principle of metal on electromagnetic waves. , effectively prevent external electromagnetic interference from entering the cable, and also prevent the internal signal from radiating out, interfering with the work of other equipment.

2. The structure of the shielded control wire

The conductor wrapped by the shielded control wire is called the shielding layer, which is generally conductive cloth, braided copper mesh or copper (aluminum) foil. Ordinary: insulating layer + shielding layer + wire. Advanced: Insulation layer + shield layer + signal wire + shield layer ground wire. The shield layer of the shielded control wire generally needs to be grounded. The function of the shielded control wire is to isolate the electromagnetic field noise source from the sensitive equipment and cut off the propagation path of the noise source.

Shielding is divided into active shielding and passive shielding. Active shielding is to prevent noise sources from radiating outward, and is shielding noise sources; passive shielding is to prevent sensitive equipment from being interfered by noise sources, and is shielding sensitive equipment. The shielding layer of the shielded control wire is not allowed to be grounded at multiple points, because different grounding points are always different, and there is a potential difference at each point. Such as multi-point grounding, the formation of current in the shielding layer, not only does not play the role of shielding, but introduces interference, especially in the occasions where the frequency converter is used a lot, the interference contains various high-order harmonic components, which has a greater impact, and should be specially noticed.