Australia, the United States and Austria: wireless communication base station lightning protection principles and engineering design (2)

1.2 The introduction of lightning damage to the base tower and antenna feeder: The base station antenna of the mobile communication system is generally set up on a high tower. After the tower exceeds a certain height, the probability of lightning strike will increase significantly. According to the original CCITT "Lightning Protection Manual" The formula for the number of lightning strikes per year for the tower is: (times/year)

Where ng - ground lightning density; C - terrain coefficient, take 0.1 ~ 0.3 Peak Tower, take the open ground and steep slope when taking 0.3; otherwise take 0.1; in between take 0.2; non-top tower take 0.05 ~ 0.1, flat The tower takes 0; H - the average difference between the location of the tower and the surrounding ground within 1Km; h - tower height; D - average annual thunderstorm days.

When the lightning rod of the tower is directly struck by lightning, the lightning current will flow through the tower through its grounding device and flow into the ground to increase the ground potential. As shown in Figure 1, point A is the voltage across the tower, which is the tower inductance and is the lightning current. Tower resistance to ground, assuming = 5 × 10-6H, = 2Ω, lightning current = 40KA, duration 8μs, which can be obtained 105KV, such a high voltage is likely to breakdown the air, the high voltage discharge to the equipment damage. Another approach is that if the antenna feeder is a coaxial cable, a strong induced current will be induced on the inner conductor. The induced current on the entire coaxial cable is if a coaxial cable enters the base station room from the tower antenna. The total induced current entering the splitter unit CDU and the transceiver unit TRX (as shown in FIG. 2) will likely destroy the BTS device.

1.3 The introduction of lightning damage to the site machine room: If the height of the equipment room is high, although lightning rod protection is provided, sometimes direct lightning strikes may occur from the horizontal and lateral sides, and so-called bypassing of lightning rods and side-by-side strikes to the facility. In particular, sometimes it is mistaken for the site machine room to be within the lightning rod protection range, but not installing or designing lightning protection belts on the roof (or roof) of a building to form a Faraday cage will increase the probability of side lightning strikes. In addition, housing metal doors and windows, indoor cabling, etc. are also important ways for secondary lightning sensing.

2. Design of site lightning protection grounding system

According to the principle of electromagnetic theory, comprehensive protection measures such as bleed-off, peak elimination, pressure equalization, and shielding can be used to control the impact of lightning on the wireless station to the maximum. However, these measures rely on the design of an effective lightning protection grounding system. The ground is a conductor. When the grounding electrode is in contact with the ground, it will form a geoelectric field, a ball area, and a resistance with the contact point as the center of the sphere. Therefore, the farther away from the grounding point (r is larger), the smaller the current can be transmitted through the electrode. Diffused rapidly to the earth. Generally, when the distance is more than 20m from the grounding point, there will be no pressure drop between the two points and it will become a true ground potential.

(to be continued)