Jiangsu QiSheng Cable Co., Ltd.
1: It is difficult to ensure that there is no potential difference between the grounding points at both ends. If there is a potential difference, there will be a weak current, so that the shielding layer actually becomes the grounding line;
2: The shielded wire with grounding at both ends works at the working site where the high-frequency interference is more serious. The line capacitance formed between the shielding layer and the internal signal line is coupled to the signal loop, which will seriously affect the signal misjudgment.
3: I have seen all kinds of governors and plc instructions that the signal line shield must be grounded at one end and the ground should be on the controller side.
There are two main types of grounding:
1. Earth grounding, first of all to ensure the reliability of the total grounding body, the grounding resistance should be small, and at the same time try to stay away from the building lightning protection grounding and high grounding system. Ensure the equipotential of each ground point. Used for grounding the device case and grounding the double shielded outer layer. If it is determined that the grounding points are reliable and absolutely equipotential, multi-point grounding is certainly good. (This is my personal opinion, in my actual electrical operation, it has a certain effect on the occasion of large interference)
2. Technical grounding (signal ground), mainly used for the second layer of shield grounding, so that the signal distortion is reduced, because the signal ground is actually the zero potential of the working power of the electronic printing board, which is the zero point of the system signal power supply.
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.
To prevent static interference, it must be grounded at a single point, whether it is a layer or a second layer. 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 a single point grounding is like a drainage pipe in a city. The static generated is like the water in the drain.
How the pipeline allows water to be removed as quickly as possible requires the following:
1. The pipe diameter of the pipe should be large, just like the grounding pole and the grounding wire should be thick.
2. The pipe network reduces the nodes as much as possible, allowing the water to drain directly into the ground. If there are many nodes in the drainage pipe, the water flow will naturally be slow, as the electrostatic discharge of the grounding electrode is slow, and the interference cannot be quickly eliminated. Therefore, the grounding of all equipment You must use a single point to access the ground. That is, single point grounding.
3. The water in the pipeline is not uniform. In some places, there is more water. In some places, there is less water. The entrance to the ground may be placed in a place with plenty of water. In this way, the water in the pipeline is placed fast. The same is true for static electricity. Grounding points should be placed as close as possible to equipment that is prone to static electricity, such as inverters and thyristors.
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