Shielding is the only way to kill lightning in the decent martial arts of lightning protection. Unfortunately, many people can't use it well.

The shielding we are talking about here has two meanings: 1, shielding buildings; 2. Shielding of various lines in the building.

I have a paper "Calculation and Analysis of Electromagnetic Environment in the Top Computer Room of Intelligent Building", which is about the electromagnetic coupling strength and mechanism of lines in the building. /cgi-bin/2 1/topic.cgi？ Forum = 2 & amptopic=360

Theoretical things are shown in the paper for two reasons: 1. Understand why buildings should be shielded; 2. What is the theoretical basis and calculation of shielding building?

Conclusion on Building Shielding

A for the electronic information equipment room located at the top of the building, it is necessary to shield the lightning electromagnetic pulse with a metal grid at the top of the building, instead of relying on the metal frame of the equipment itself as a shielding grid for electromagnetic field attenuation. In engineering design, we should not choose 2.4Gs as the design basis of electromagnetic field intensity in computer room to design shielding grid and safety distance, but should design shielding grid and safety distance according to GB 50 174-93 Code for Design of Electronic Computer Room and 10Gs as the design basis of electromagnetic field intensity in computer room.

B, in the choice of safe distance, we always choose the largest safe distance to ensure the safety of electronic information equipment in the computer room. Usually, for the electronic information equipment room located at the top of the building, we only consider the electromagnetic field environment when the direct lightning protection device at the top of the building is connected with lightning. At this time, the magnetic field intensity in the room (LPZ 1 area) is greater than that when the building is struck by lightning, and the width of the shielding grid used to shield the building is much less than 2.65 m, so the value of dS/ 1 is generally chosen as the safe distance in engineering.

C. In order to meet the requirement that the magnetic induction intensity in the machine room at the top of the building is less than 10Gs, that is, less than 800 A/m, the width of the metal grid should not be less than 1m× 1m for the electronic information equipment room with an area greater than 140 m2. For a small electronic information room with an area of only tens of square meters, in order to effectively use the room area, it is necessary to use a metal grid of 0.4 m×0.4 m for electromagnetic shielding of buildings.

D. When arranging the down lead of a building, it is advisable to use the steel bar group in the column of the building as the down lead, and it is not advisable to set the down lead separately. When the steel bar group in the inner column of the building is selected as the electrical grounding trunk line of the building, the overvoltage generated on the grounding trunk line during lightning stroke can be greatly reduced.

Let me sum up my point: 1, for a small non-important computer room, it is enough to make a grid of 1m× 1m with φ 8 mm galvanized round steel to shield and ground the six sides of the building. 2. When the grid is less than 1m× 1m, and the area is greater than 140m2, the shield shall be made according to the above method. 3. Only important computer rooms larger than140m2 need to be shielded with a grid of 0.4m×0.4m.

Here, only the computer room located on the top three floors of the building or independent on the ground needs to do this. What I have summarized is the general situation. According to the size and importance, if the computer room is small, but there are many devices in it, and the available space is limited, so it is impossible to reach a safe distance, then a small shielding grid should be used to increase the utilization space of the computer room.

Did you talk about building shielding yesterday? Now let's talk about wire shielding. Before talking about the shielding of lines, we must know what is the "skin effect".

Suppose that the cable core and sheath are connected at the beginning, and a voltage wave flows in from the beginning. What current flows in the core wire and the sheath respectively? Which is bigger? Why? Understand this, naturally understand the "skin effect".

The core wire and the sheath are two parallel wires, and the magnetic flux generated by current flowing through the sheath is completely interlocked with the core wire, so the potential on the cable sheath will be completely transferred to the core wire, so the self-wave impedance of the cable sheath is equal to the mutual wave impedance between the core wire and the cable sheath, while the magnetic flux generated by current flowing through the cable core is only partially interlocked with the cable sheath, so the self-wave impedance of the cable core will be greater than the mutual wave impedance between the cable core and the cable sheath.

If you do a set of equations, it is:

U=Z core I core +Z core to skin I skin

U=Z skin I skin +Z skin to core I core

So, at this time:

Z core I core +Z core to skin I skin = Z skin I skin +Z skin to core I core.

However, at this time: z skin = Z core to skin; Z core > z skin to core; The only condition that satisfies the equation is I core =0.

This "only condition" is the "skin effect":)

Its physical significance is that when the current propagates on the cable sheath, the electromotive force equal to the cable sheath voltage is induced on the core wire, which prevents the current from flowing in the core wire, similar to the skin effect of the conductor.

So ... use the "skin effect" of lightning to shield the line. It forces most of the lightning current to flow through the shielding layer and merge into the earth, rather than through the conductor.

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Supplement: The original title is as follows

Lightning protection, the theoretical design sequence is:

1 Avoidance: Avoid, and don't build in a place vulnerable to lightning;

2. Shielding: Shielding, shielding lines susceptible to lightning overvoltage;

3 isolation: isolation, which isolates the lines that are easy to interfere with each other and the voltage difference is easy to counterattack each other;

4 grounding: grounding: according to the grounding function, all the devices that need to be grounded are grounded in turn to keep a sufficient grounding distance;

Class 5: equipotential bonding, which can only be carried out without shielding, isolation and sufficient grounding distance. Among them, installing surge protector is a way of equipotential bonding.

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Reference: Baidu Encyclopedia faraday cage

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