A set of general weak current lightning protection and grounding system design materials

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1.1 Weak current lightning protection and grounding system

1.1.1 System Overview

Thunder and lightning is a special and extremely spectacular sound, light and electricity phenomenon in nature. This is not only because of its dazzling lightning and deafening thunder that pierced the sky, but more importantly, the lightning has brought great harm to human production and life. The weak current lightning protection grounding system inside the building is set up to protect the safety of the equipment and personnel inside the building. By installing a suitable surge arrester at the front end that needs to be protected, the equipment, the line and the ground form a conditional equipotential body. The lightning current that may enter is blocked, and the lightning current sensed by the internal facility due to lightning strikes can be safely discharged to the ground, ensuring the safety of the equipment. In order to ensure the stable and reliable operation of the equipment and network system in the computer room and ensure the safe working environment of the staff in the computer room, in addition to erecting a good lightning rod and lightning belt, it is also necessary to carry out reliable and effective protection for all the computer rooms in the building such as power supply equipment (all power supply equipment, electrical equipment, standby power generation equipment), antenna feeder system, signal acquisition and transmission system, program-controlled switching system, etc., and make complete multi-level protection in six aspects, such as interception, diversion, balancing, shielding, grounding, and wiring.

1.1.2 Demand analysis

The weak current lightning protection and grounding system of XXXX hospital mainly includes: intelligent system monitoring center computer room, elimination control room computer room, and lightning protection and grounding of weak current equipment in the weak current room (well) on the floor. The purpose of the system design is to achieve the safety of all kinds of weak current system equipment, and different protective measures are adopted for the distribution box, UPS box and floor weak current distribution box that enter the computer room of the building. Protect outdoor broadcasting, cameras and other equipment. Prevent lightning damage to equipment and lines.

1.1.3 System design

(1) Direct lightning protection

Traditional lightning belts and lightning rods have their applicability in protecting buildings, but with the extensive use of electronic computers, it is not uncommon for electronic equipment to be damaged due to lightning strikes caused by lightning rods. There are three reasons for this:

1. Because the lightning rod itself is the tip shape, under the action of the thundercloud electric field, the tip electric field is distorted, which will produce a strong head-on leader, which often preferentially triggers lightning strikes and causes lightning strikes to occur;

2. In the process of lightning rod leading lightning into the ground, an instantaneous strong electromagnetic field will be generated around the down conductor (steel bar), which will directly induce overvoltage and overcurrent on the internal signal and power supply line, thereby causing damage to the equipment;

3. After the lightning current is not attenuated into the ground, the ground potential rises, and the lightning protection ground network and other ground networks, especially when the computer working place is less than 20 meters away, the current will be reversed into the computer room through the earth, causing damage to the equipment.

For the protection of direct lightning, in addition to using lightning protection tape as a lightning receiver, it is recommended to use an optimized lightning rod, which has the ability to connect lightning, reduce the radiation value of lightning current and prolong the discharge time, and can withstand direct lightning with very high flux, thereby reducing the probability of induced lightning strike. The protection of direct lightning is implemented by the electrical major, and this scheme is not designed.

(2) Anti-induction lightning strike

At present, it has been verified by actual operation experience that the damage caused by induced lightning strikes coupled into the power supply system accounts for more than 60% of the probability of lightning disaster losses. Therefore, lightning protection measures for the power supply system are an indispensable part of the whole lightning protection project. It is necessary to prevent the intrusion of induced lightning waves and lightning electromagnetic pulses from external transmission lines, so that they can be leaked into the ground before entering the power supply system of the building.

Since the power supply of the computer room is introduced by the building substation and distribution room of the building, the lightning protection of the high-voltage end of the power supply has been implemented by the power supply department. Therefore, for the lightning protection of the power supply system in the computer room, we adopt the following lightning protection schemes:

The building's low-voltage main power distribution system is provided with first-class lightning protection (three-phase power supply) before the UPS;

The second level of lightning protection is taken from the rear end of the UPS to the front of each motor room (one set of each of the three sets of UPS is installed);

Install the third-level lightning protection device on the power supply line of the integrated computer room (completed by strong electricity);

Install Class D lightning protectors in front of important equipment room equipment, such as central switches, servers, and other equipment.

(3) Signal lightning protection

According to the requirements of the construction code of lightning protection and grounding system, lightning protection must be carried out on the outdoor lines entering the building, and the lines entering the building are as follows:

public telephone network incoming -------- fiber optic cables;

cable TV trunk -------- coaxial cable;

computer network ---------------- optical fiber + coaxial cable;

Outdoor CCTV lines----------- video cables, signal cables, PTZ control cables;

Public Address Cables ----------- Public Address Power Lines.

(4) Joint grounding mode

Joint grounding is also known as single-point grounding, that is, all grounding systems share a common "ground". Joint grounding has the following characteristics:

(1) The grounding system of the whole building forms a cage pressure equalizer, and for direct lightning, each point on the same floor in the building is relatively uniform; For induction mines, the cage equalizer and the frame structure of the building can also provide a shielding effect of 10-40dB against external electromagnetic field interference;

(2) The grounding resistance of the general joint grounding method is very small, and there is no coupling between various grounding bodies (3) It can save metal materials and occupy less space.

(5) Advantages of joint grounding

The advantages of using joint grounding in intelligent buildings are obvious, and the specific points are as follows.

The total grounding resistance of the joint grounding is relatively small (generally below 1Ω, and some are only 0.2Ω or even less), because the grounding electrodes are connected in parallel.

Under the same grounding resistance value requirements, the use of joint grounding can reduce the number of grounding electrodes, simplify the grounding system, and save investment.

When using joint grounding, even if one grounding body fails and does not work, the impact on grounding is not large, and the grounding work can be completed by other grounding bodies, which improves the reliability of grounding.

When the foundation steel bar of the intelligent building is used as the grounding electrode, the grounding resistance can be much smaller than that of the artificial grounding pole, which is conducive to reducing the contact voltage and facilitating the scattering of the grounding current.

(6) Joint grounding system setting

All kinds of grounding in intelligent buildings are connected according to the following principles, and then all are connected to the joint grounding network made of base reinforcement.

Protective grounding and system grounding can be combined grounding.

The grounding of computer equipment and the grounding of electrical and medical gas equipment can be grounded with electronic information equipment, computer line filter grounding, and computer case grounding.

The lightning protection grounding must be connected separately to the grounding grid.

Protective grounding and grounding of electronic information equipment can be combined grounding.

When the joint grounding, the grounding resistance does not exceed lΩ under normal circumstances; The total ground resistance can also exceed lΩ when the contact voltage and step voltage do not exceed the safe value.

A grounding copper bar (commonly known as flat steel) is installed in each equipment room (weak current shaft), and the distribution frame and other equipment in the weak motor cabinet are connected to the cabinet through copper wires, and the cabinet is connected to the grounding copper bar.

(7) Technical requirements

1. DC working grounding ≤ 1Ω

2. AC working grounding ≤ 4Ω

3. Safety protection grounding ≤ 4Ω

4. Anti-static grounding ≤ 10Ω

5. Lightning protection grounding ≤ 10Ω

In order to make the system scientific and reasonable, practicable and convenient construction, and make full use of the various structural conditions of the building, the grounding system adopts the comprehensive grounding method of the building, and the grounding resistance is ≤ 1Ω.

1.1.4 System Functions

(1) Weak current lightning protection function

The lightning protection equipotential design of the weak current system includes: electromagnetic protection and shielding measures, reasonable wiring measures, lightning protection and grounding facilities, etc.

The communication lines or power lines connected to the external network of each weak current equipment in the computer room shall be equipped with lightning protection isolators (protectors) (action time less than 25ns).

Outdoor networks and communication cables should be installed with lightning protection isolators (protectors) at the front end of the connection with indoor electrical equipment (action time less than 25ns).

(2) Weak current grounding function

The grounding of the weak current equipment in each computer room should be concentrated at one point and then connected to the grounding terminal of the hospital building to avoid the generation of non-equipotential, and the anti-static grounding in the computer room should be considered.

The metal shell of the cabinet and the weak current distribution box set up in the equipment room (weak current well) should be connected to the building grounding; The power distribution system in the hospital should comply with the TN-S system.

The ground grid resistance of the grounding system should be less than 1 ohm. At the same time, it should also be ensured that the grounding point of the electrical equipment is equipotential with the grounding point of the building (the reinforcement mesh of the building foundation is used as the grounding body).

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