One end of the shielded wire is grounded and the other end is suspended.
When the transmission distance of the signal line is relatively long, the grounding resistance at both ends is different or there is current on the PEN line, which may cause the potentials of the two grounding points to be different. At this time, if the two ends are grounded, the shielding layer has current flowing, but the signal is opposite. Interference is formed, so in this case, one grounding is generally used, and the other end is suspended to avoid such interference.
Grounding shielding at both ends is better, but signal distortion will increase
Please note: the two-layer shield should be insulated and shielded from each other! If not insulated from each other, it should be treated as a single layer shield!
The grounding of the outermost shield is caused by the induced potential difference, thereby generating a magnetic flux that reduces the strength of the source magnetic field, thereby substantially canceling the voltage induced when there is no outer shield;
The innermost shield is grounded at one end. Because there is no potential difference, it is only used for general anti-static sensing. The following specifications are the best proof!
"GB 50217-1994 Power Engineering Cable Design Specification" - 3.6.8 The grounding method of the control cable metal shield shall comply with the following provisions:
(1) The analog signal loop control cable shielding layer of the computer monitoring system shall not constitute two or more points of grounding, and it is recommended to use a centralized one-point grounding.
(2) In addition to the (1) item and other control cable shielding layers that require a little grounding, when the electromagnetic induction interference is large, two points of grounding should be used; the electrostatic induction interference is large, and one point can be grounded.
For double shield or composite general shield, it is better to use one point for the inner and outer shields and two points for 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 Building Lightning Protection Design Code" - Article 6.3.1 stipulates: ... When shielded cable is used, its shielding layer should be equipotentially connected at least at both ends. When the system requires only equipotential bonding at one end. Two layers of shielding shall be used, and the outer shielding shall be treated as described above.
The principle is as follows: 1. The single-layer shield is grounded at one end, and no potential difference is formed, which is generally used for anti-static induction. 2. Double-layer shielding, the outermost shield is grounded at both ends, and the inner shield is equipotentially grounded at one end. At this time, the outer shield induces a current due to the potential difference, thereby generating a magnetic flux that reduces the strength of the source magnetic field, thereby substantially canceling the voltage induced when there is no outer shield.
If it is to prevent static interference, it must be grounded at a single point, whether it is one layer or two layers. Because the electrostatic discharge rate of single point grounding is the fastest.
However, the following two cases are excluded:
1. There is strong current interference outside, and single point grounding cannot meet the fastest discharge of static electricity.
If the grounding wire has a large cross-sectional area and can ensure the fastest discharge of static electricity, it must also be grounded at a single point. Of course, that's true, there is no need to choose two layers of shielding.
Otherwise, two layers of shielding must be used. The outer layer shielding is mainly to reduce the interference intensity. It is not to eliminate the interference. At this time, it must be grounded at multiple points. Although it cannot be finished, it must be weakened as soon as possible. To be weakened, multi-point grounding is the best choice.
For example, the cable tray in an enterprise is actually an outer shield. It must be grounded at multiple points, and the first line of defense reduces the intensity of the interference source.
Inner shield (in fact, we don't buy double-layer cables, usually the outer layer is the cable tray, the inner layer is the shield of the shielded cable). It must be grounded at a single point, because the external strength has been reduced, discharge as soon as possible, and eliminate interference. It is the purpose of the inner layer.
2. Safety requirements such as external electric shock and lightning protection.
This situation requires two layers of protection. The outer layer is not used to eliminate interference. It is for safety reasons. To ensure personal and equipment safety, it must be grounded at multiple points. The inner layer is to prevent interference, so it must be grounded at a single point.
In summary, the summary is as follows:
Most fans talk about the signal line and say that the shield line is grounded at one end is actually one-sided. For double-layer shielded wires, the outermost layer should be grounded at both ends; in places with large electromagnetic interference, grounding at both ends should be used.
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