In recent years, due to the advantages of light steel structure buildings, such as beautiful appearance, short construction period and low project cost, it is widely used in various industrial plants, warehouses, gymnasiums and other places. The metal plate roof is vulnerable to lightning strikes, which is different from the lightning protection design of ordinary brick-concrete structure and frame structure buildings. Combined with the lightning protection design of a steel structure workshop, this paper makes a superficial discussion on the related problems of lightning protection design of metal roof, which provides reference for effectively solving the problem of lightning protection of metal roof in the future.

Project Overview

A steel structure workshop in Henan, in which the casting workshop is mainly responsible for processing aluminum wire rods and aluminum ingots. The whole building adopts light steel structure, the actual size of the building is rectangular, L = 182m,W = 93m,H = 18.5m (floor to the bottom of the roof ventilator). According to the calculation of thunderstorm days in Zhengzhou, the annual average thunderstorm days in Table 16-32 of the Industrial and Civil Power Distribution Design Manual (Second Edition) are 22.0d/a. As the workshop is located in the factory area and there are many buildings around it, it is not isolated buildings, so the correction coefficient is 1 according to the calculation of general buildings.

Determination of Lightning Protection Grade of Buildings  

According to the attached 1.2 of the GB50057-94, the average density of the annual lightning strike ground is calculated according to the known conditions:

Ng = 0.024td1.3  

= 0.024 × 221.3

= 1.334 times/(km2.a）

According to the formula 1.4 attached to the GB50057-94, the equivalent area of the same number of lightning strikes intercepted by the building is obtained: 

Estimated number of lightning strikes in the last building year:

N=KNgAe  

= 1 × 1.334 × 0.05934

= 0.0791 times/a

Where: Td-Annual average thunderstorm day (d/a);

Ng-Annual average density of lightning strike land in the area where the building is located (times/km2.a）；  

         Ae-the equivalent area of the same number of lightning strikes intercepted by the building (m2）；  

L, W, H-length, width and height (m) of the building respectively;

According to Article 3 of Article 2.0.4 of the GB50057-94, the estimated number of lightning strikes is 0.3/a≥N≥ 0.06/a, so it is set according to the third category of lightning protection.

lightning receptor  

According to the light steel structure of the building, the use of lightning rod is heavy, and the whole building appearance is not coordinated, and the use of metal roof as a lightning receptor, need to meet the "building lightning protection design code" (GB50057-94,2000 edition) article 4.1.4 given the four requirements.

According to the specification requirement "when there is no flammable material under the metal plate, its thickness shall not be less than 0.5mm". The enclosure structure (roof) of the main workshop adopts hot-dip aluminum-zinc plated steel plate, with a substrate thickness of 0.53mm (including aluminum-zinc plated layer). Under the roof plate, a glass cotton pad with a thickness of 75mm is added, wrapped with aluminum foil, and the upper edge of purlin is braced with Φ 1.5 stainless steel wire mesh. Glass wool is a class a building material, thermal conductivity is small, flame retardant, non-toxic, stable chemical properties, fully meet the requirements of the specification is not easy to burn. At the same time, according to the introduction of civil engineering specialty, bite and lap joint are adopted in the construction of the enclosure system connection mode of light steel structure buildings. The tower connection length should reach at least one wave crest or trough, and more than 100m is fully meeting the specification requirements.

In addition, the "metal plate without insulation coating" mentioned in Article 4.1.4, the hot-dip aluminum-zinc plated steel plate used for the roof is a good conductor, and the thin paint protective layer coated on the surface is not an insulation coating according to the specification notes, so it meets the specification requirements. To sum up, in this project, the metal panel of the roof light steel structure can be used as a lightning receptor, which is not only beautiful and generous, the project cost is reduced, the installation is convenient, but also does not affect the effect of the whole building. To do a good job in the design of lightning protection system for metal plate roof, in addition to choosing a good lightning protection form and a good lightning receptor, it is also necessary to do a good job in down lead and grounding.

Downlead  

For light steel structure buildings, there are many types of color profiled panels, which are divided into single panels and composite panels. Composite panels are used in this workshop. During the construction, we require the civil engineering specialty to cooperate with the electrical specialty to ensure the reliable connection between the roof metal pressure plate, roof truss, purlin and steel column, so the steel column is used as the down lead in the construction. Metal structures such as fire ladders and steel columns of buildings should be used as downleads.

Grounding device  

In the current design, we often use the foundation steel as a natural grounding body, with 40X4 galvanized flat steel to connect it, and the implementation of the total equipotential bonding. In this design, we did not strictly require the use of 40X4 galvanized flat steel, but the use of steel column between the strip foundation and steel column independent foundation reliable welding to form an electrical path. For buildings with light steel structure, the anchor bolts shall be embedded in civil engineering during the construction of steel column independent foundation, so that when the steel column is in place, the anchor bolts, nuts and steel columns are connected together. It is important to note that the anchor bolts and the steel column foundation reinforcement are not connected underground! Therefore, the electrical designer shall require the civil engineering specialty to indicate in the design instructions that the foundation steel bar and grounding bolt shall be reliably welded with no less than Φ 10 steel bar or round steel bar during construction, and anti-corrosion treatment shall be done at the welding place for grounding. In this way, a reliable connection is formed from the roof to the steel column and then to the foundation to ensure that the lightning current has a complete discharge channel.

In addition, don't forget to reserve a grounding connection board in an appropriate location outdoors to prevent the construction unit from connecting artificial grounding devices and testing grounding resistance when the grounding resistance fails to meet the requirements during construction.

Concluding remarks

Lightning protection for metal roof buildings is a difficult project to deal with. However, in such buildings, as long as the roof sandwich panel (or profiled steel plate) of light steel structure buildings with a thickness of 0.5mm can be used as a lightning receptor, steel columns are used as downleads, foundation steel bars are used as natural grounding bodies, and galvanized flat steel or ground ring beams are reliably connected and equipotential connected.

★ Steel structure lightning protection knowledge Q & A

Q:The section steel is used as the lightning protection downlead, and the section steel is exposed, so if the building falls under the lightning, the lightning current will not counterattack the surrounding equipment or people, causing damage. How to avoid?

Answer:Using section steel as lightning protection down lead, when lightning current passes through the down lead, the surrounding movement and change magnetic field will inevitably pollute the nearby electronic equipment and induce different potentials on the surrounding conductors. If indoor personnel touch conductors with high potential difference at the same time, personal safety accidents will be caused.

Solution:  

1) Personal safety: All metal conductors that can be touched by people and the earth have direct contact and indirect contact are directly implemented to eliminate the potential difference for the purpose of equipotential electrical connection to maximize the protection of personal safety.

2) Implement SPD protection for equipment and microelectronic equipment according to national standards such as GB50057 and GB50343.

The concept of counterattack: the so-called lightning counterattack phenomenon is that lightning strikes the lightning receptor [including various types of steel], and the down lead changes the lightning high voltage into the form of current and discharges it to the earth. After reaching the earth, it appears on the main grounding system of the building in the form of electric charge. [the earth is a zero potential body that can balance the negative charge of the 500000 coulomb-the basic foundation calibration value of modern physics], however, the soil has many parameters such as resistance and capacitance, so it is impossible to have opposite and equal amount of charge and neutralization [it takes some time]. At this time, most of the charge generated by lightning current is expressed on the ground near the building in the form of voltage. At this time, there are underground conductors near the building: heating pipe, gas pipe, water pipe, equipment grounding wire, medium point grounding and so on, which are connected with the inside of the building, there is a certain distance between these conductors underground. Because the surface has a high potential [the highest potential point is mainly near the grounding electrode], the potential difference is generated by the distance difference, and this potential difference will fight back into the building, causing an accident-this is the phenomenon of lightning high-voltage counterattack.

Assumption 1: When fighting back, if someone touches the water pipe with his left hand and the shell of the electronic device with his right hand, the potential difference exceeds a certain safety limit, and a personal safety accident occurs.

Assumption 2: If the electronic equipment shell is grounded independently, the counterattack potential difference between the neutral grounding of the electronic equipment and the shell grounding [there is a grounding distance under the ground] will generate a potential difference with the live wire 220/380. If the potential difference is large, the equipment will be burned.

Q:For steel structures, the use of exposed steel as the down lead, even if a comprehensive equipotential is made, but when the thunder falls, the strong lightning current, will not break down the surrounding air? Cause harm to people near or in contact with the down lead? I have been very worried about this problem.

Answer:When a thunder falls, a strong lightning current can transmit potential to the ground along the downlead, but it must be known that the downlead has an inductance. The size of this inductance can block the normal discharge of lightning current at a certain time, thus registering the potential. When this potential is large, it will cause enough breakdown of air or other media at a certain distance from a certain small potential to cause discharge. There is a standard said side flash phenomenon also occurs.

It is 1.1 necessary to calculate the down conductor inductance. Another phenomenon is that the rapid movement of lightning current on the down line will inevitably generate a moving magnetic field. This moving magnetic field provides the basic conditions for cutting the magnetic lines of force to generate overvoltage. The overvoltage generated by cutting the magnetic lines of force can also breakdown air, electrical media [including semiconductor media], and all direct and indirect conductors [semiconductors] that can provide potential difference, thus affecting our work and life. It is reported that a female worker turned on the power air switch at work in a certain factory in Tianjin [in Hangu area], causing the overvoltage generated on the power cord to break down the plastic switch shell and enter the palm of her hand and then short-circuit with the earth through her body. A resident of a village in Hunan Province has a phenomenon of death of lightning electromagnetic pulse induced overvoltage on the power line of a light bulb in the room, which breaks down the air and forms a path [circuit] on the top of a person's head. The cause of injury caused by people near or in contact with the downlead is whether there is a potential difference between two parts of the human body, which is his fatal element. For example, if one hand touches the mold down lead and the other hand touches the mold heating pipe, the connection point of the heating pipe is assumed to be far away from the building owner's ground network, then the distance difference generates a potential difference, which can cause casualties when the potential difference reaches a certain value. Don't always consider personal safety voltage but also personal safety current. I don't know if my suggestion will help you.

Q:Can we come to the conclusion that it is quite dangerous to use bare steel as pillars and lightning protection downlead for large-area factories (50 m x 80 m) in open areas with iron roofs?  

Answer:Very dangerous, you consider step voltage ah! Consider it as a whole. We should study and use standards according to local conditions. We should not consider the cost too single. Life is a matter of life!

Q:I would also like to ask a question, since the induction magnetic field is greater the closer to the downlead when the thunder falls, should the communication equipment, fire fighting host and distribution box avoid the downlead as much as possible?

Answer:If you have the conditions, be sure to stay away! If a 10/350-100KA [seems to be 200KA can't remember] is exposed to a radius of 83 meters, the interference electromagnetic pulse intensity of 2.4GS can be reached, and 2.4GS can permanently damage unprotected microelectronic equipment.  

Q:I am carrying out the construction of a steel structure factory building, because the design unit did not consider lightning protection, I need to reconsider the remedial measures. The steel structure workshop is 35 meters high and 9*17 meters long and wide. It is all steel structure. The roof and walls are covered with 0.5mm double-layer color steel tiles (with incombustible insulation cotton inside) and three types of lightning protection structure. The steps I intend to take are:

1. Because all steel column foundations have been covered by concrete, it is impossible to check the quality of reinforcement binding and welding, so I use 40*4mm galvanized flat steel to connect the lowest end of all steel columns (near the foundation) and connect it with the grounding grid of the whole plant;

2. Considering the operation difficulty and reliability of the roof color steel tile as the lightning receptor, I will use 10mm round steel to install a lightning protection belt around the roof, and the round steel will be welded with the steel column;

3. Use steel column as down lead. May I ask how to improve its lightning protection measures?

Answer:1. Remediation is possible, but multi-point grounding welding is required. 1 If conditions permit, electrical connection can also be made at the upper end, which is safe! In principle, it is OK to measure the ground resistance in bare places, but this is only a remedy!

2. The roof color steel tiles should be electrically connected with steel balls at multiple points, and electrical connections between multiple metal tiles should also be made to avoid arc generation.

3. The grounding resistance is less than 4 ohms, and the grounding resistance of all steel beams and steel pillars is equal everywhere. The main grounding system shall refer to the GB50057 requirements!