In recent years, light steel structure buildings are widely used in various industrial workshops, warehouses, gymnasiums and other places due to their beautiful appearance, short construction period and low engineering cost.The metal sheet roof is easy to be struck by lightning, which is different from the common brick concrete structure and frame structure buildings.Combined with the lightning protection design of a steel structure workshop, this paper discusses the related problems of lightning protection design of metal roof, and provides reference for effectively solving the lightning protection problem of metal roof in the future.

Project overview

A steel structure workshop in Henan Province, in which the melting and casting workshop is mainly responsible for processing aluminum poles 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 roof ventilator).According to the thunderstorm days in Zhengzhou, the annual average thunderstorm days are 22.0d/a according to table 16-32 of the industrial and civil power distribution design manual (Second Edition). Since the workshop is located in the plant area, there are many building groups around, which are not isolated buildings, so the correction factor is taken as 1 according to the calculation of general buildings.

Determination of lightning protection grade of buildings

According to appendix 1.2 of GB 50057-94, the annual average density of the ground struck by lightning 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 in the appendix of GB 50057-94, the equivalent area of the same number of lightning strokes intercepted by buildings is obtained  

Finally, the annual estimated number of lightning strikes is obtained

N=KNgAe

=1×1.334×0.05934

=0.0791 times / A

Where: TD – annual mean Thunderstorm Day (D / a);

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

AE – equivalent area of the same number of lightning strokes intercepted by buildings (M2);

50. W and H are the length, width and height of the building (m);

According to Article 3 of article 2.0.4 of GB 50057-94, it is estimated that the number of lightning strokes is 0.3 times / a ≥ n ≥ 0.06 times / A, so it is designed according to category III lightning protection.

Lightning arrester

According to the light steel structure of buildings, the use of lightning rod is cumbersome and incompatible with the appearance of the whole building. However, the use of metal roof as lightning arrester should meet the four requirements given in article 4.1.4 of code for design of lightning protection of buildings (GB 50057-942000 Edition).

According to the requirements of the specification, “when there is no combustible material under the metal plate, the thickness should not be less than 0.5mm”.The enclosure structure (roof) of the main workshop of the workshop is made of hot-dip aluminized zinc steel plate. The thickness of the base plate is 0.53mm (including the aluminized zinc layer). The 75mm thick glass wool pad is added under the roof panel, which is wrapped with aluminum foil and supported with the upper edge of purlin with φ 1.5 stainless steel wire mesh.Glass fiber cotton is a class a building material with small thermal conductivity, flame retardant, non-toxic and stable chemical properties, which fully meets the requirements of the code for non combustible.At the same time, according to the introduction of civil engineering, the connection mode of enclosure system of light steel structure building is occluded and overlapped. The tower connection length should reach at least one wave peak or trough, and more than 100m is fully meeting the requirements of the code.

In addition, the “metal plate without insulation coating” mentioned in article 4.1.4, the hot-dip aluminum zinc steel plate used for the roof is a good conductor, and the thin paint protective layer on the surface is not considered as the insulation coating according to the specification notes, so it meets the requirements of the specification.To sum up, in this project, the metal panel of roof light steel structure can be used as lightning arrester, which is not only beautiful and elegant, but also reduces the project cost and is convenient to install, but also does not affect the effect of the whole building.In order to do a good job in the design of lightning protection system for metal sheet roof, we should not only select a good lightning protection form and a good lightning arrester, but also do a good job in down lead and grounding.

Downlead

For light steel structure buildings, there are many kinds of color profiled plate, which can be divided into single plate and composite plate. Composite plate is used in this workshop.In the construction, we require the cooperation of civil engineering and electrical specialty to ensure the reliable connection of roof metal profiled plate, roof truss, purlin and steel column, so steel column is used as downlead in construction.Metal structures such as fire ladders and steel columns of buildings should be used as downlead.

Grounding device

At present, we often use the foundation reinforcement as the natural grounding body, connect it with 40×4 galvanized flat steel, and implement the total equipotential bonding.In this design, we did not strictly require the use of 40×4 galvanized flat steel, but the use of steel column strip foundation and steel column independent foundation reliable welding to form electrical access.For the buildings with light steel structure, the foundation bolts should be embedded in the civil engineering when the steel column independent foundation is constructed, so that when the steel column is in place, the anchor bolt, nut and steel column are connected together.Attention should be paid to the fact that the foundation bolts and steel column foundation reinforcement are not connected underground!Therefore, the electrical designer shall require the civil engineering specialty to indicate in the design specification that the foundation reinforcement and grounding bolt shall be reliably welded with no less than φ 10 steel bar or round steel bar in the construction, and the anti-corrosion treatment shall be made 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, do not forget to reserve the grounding connection plate in the appropriate position outdoors to prevent the construction unit from connecting the manual grounding device and testing the grounding resistance when the grounding resistance fails to meet the requirements in the construction.

Conclusion

Lightning protection of metal roof buildings is a difficult project to deal with, but in this kind of buildings, as long as the thickness of 0.5mm light steel structure building roof sandwich plate (or profiled steel plate) can be used as lightning arrester, using steel column as downlead, foundation reinforcement as natural grounding body, and reliable connection through galvanized flat steel or ring beam and equipotential connection.

Questions and answers on lightning protection knowledge of steel structure

Q: the section steel is used as the lightning protection downlead, and the section steel is exposed. In this way, if the building falls into lightning, will the lightning current counterattack the surrounding equipment or people and cause damage.How to avoid it?

Answer: using section steel as lightning protection downlead, when the lightning current passes through the down lead, the surrounding moving and changing magnetic field will inevitably pollute the nearby electronic equipment, and different potential should be sensed on the surrounding conductive body. If indoor personnel touch the conductor with high potential difference at the same time, it will cause personal safety accident.

resolvent:

1）Personal safety: the equipotential electrical connection with the purpose of eliminating potential difference is directly implemented in all metal conductors that can be touched by people and have direct and indirect contact with the earth, so as to protect personal safety to the maximum extent. 

2）According to GB 50057, GB 50343 and other national standards, SPD protection is implemented for equipment and microelectronic equipment. 

The concept of counterattack: the so-called lightning counterattack phenomenon is that the lightning strikes the lightning arrester [including various steel sections], and the down lead changes the lightning high voltage into current and releases it to the earth. After arriving at the ground, it appears in the form of electric charge on the main grounding system of buildings. The ground is the zero potential body that can balance out 500000 Coulomb negative charge — the basic foundation calibration value of modern physics]However, the soil has many parameters such as resistance, capacitance, etc., so it is impossible to have the opposite equal charge and its sum immediately [taking a certain time]. At this time, most of the charges generated by lightning current are shown on the ground near the building in the form of voltage. At this time, there are heating pipes, gas pipes, water pipes and equipment under the ground near the buildingsThe ground wire, medium-sized point grounding and other conductors that are connected with the interior of the building. There is a certain distance between these conductors underground. Because the ground surface has a high potential [the highest potential point is near the main grounding electrode], the distance difference produces potential difference, and this potential difference will counterattack into the building, causing an accident – this is the lightning high-voltage counterattack phenomenon.

Hypothesis 1: in counterattack, if someone touches the water pipe with his left hand and touches the shell of electronic equipment with his right hand, then the potential difference exceeds a certain safety limit and personal safety accident will occur.

Hypothesis 2: if the electronic equipment shell is independently grounded, the counterattack potential difference between the electronic equipment neutral line grounding and the shell grounding [there is a grounding distance underground] will generate a potential difference with the live wire 220 / 380. If the potential difference is large, the equipment will be burned down.

Q: for steel structure buildings, if the exposed section steel is used as the downlead, even if a comprehensive equipotential is done, will the strong lightning current break through the surrounding air when the lightning falls?Damage to people near or in contact with down lead?I have been very worried about this problem.

A: when a thunder falls, a strong lightning current can transmit potential to the earth along the down lead. However, we must know that the down lead has an inductance. The inductance can block the normal discharge of the lightning current at a certain time, so as to store the potential. When the potential is large enough, there will be enough between the potential and a small potential to break through air or other media at a certain distanceDischarge phenomenon.There is a standard that the phenomenon of side flash also occurs.

This point must be calculated down lead inductance.Another phenomenon is that the fast moving lightning current on the down lead wire will inevitably produce a moving magnetic field. This moving magnetic field provides the basic conditions for cutting the magnetic line of force to produce overvoltage. The overvoltage generated by cutting the magnetic line of force can also break through air, electrical medium (including semi-conductor medium), and all direct and indirect conductors (semiconductors) that can provide potential differenceOur work and life.It is reported that in a factory in Tianjin [Hangu area], a female worker turned on the power supply air switch when working, causing the overvoltage generated by the power line to break through the plastic switch shell, enter the palm, and then short circuit the body to the ground.In a village in Hunan Province, a lightning electromagnetic pulse induced overvoltage occurred in the power line of the bulb, which broke through the air and formed a path [loop] death on the top of the head.The cause of injury to the person near or in contact with the down lead is whether there is a potential difference between two parts of the human body, which is the fatal factor.For example, one hand touches the mold downlead, and the other hand touches the heating pipe. If the heating pipe grounding point is far away from the main ground grid of the building, the distance difference will produce potential difference. When the potential difference reaches a certain value, it can cause casualties.Don’t always consider personal safety voltage, but also consider personal safety current.I wonder if my advice will help you.

Q: can we draw such a conclusion: the roof of a large workshop (50m x 80m) in an open area is made of iron sheet, and it is quite dangerous to use bare steel as column and lightning protection downlead?

A: it’s very dangerous. You should consider step voltage as a whole. You should study and use the standard according to local conditions. Don’t consider the cost too singly. Life matters!

Q: I also want to ask a question. Since the closer the lightning is to the down lead, the greater the induced magnetic field is. Should communication equipment, fire engine and distribution box avoid downlead as much as possible?

A: if you have the conditions, you must stay away from it! If a 10 / 350-100ka radius of 83 meters is exposed, the intensity of interference electromagnetic pulse can be up to 2.4gs, and 2.4gs can make unprotected microelectronic equipment permanently damaged

Q: I am working on the construction of a steel structure workshop at hand. Because the design unit has not considered lightning protection, I need to reconsider the remedial measures.The steel structure workshop is 35 meters high and 9 * 17 meters long and wide. The roof and walls are covered with 0.5 mm double-layer color steel tile (with non combustible insulation cotton inside). There are three types of lightning protection structures.The measures I am going to take are:

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

2.Considering the difficulty and reliability of the roof color steel tile as lightning arrester, I will use 10 mm round steel to install a lightning strip around the roof, and the round steel is welded with the steel column;

3.Steel column is used as downlead.How can I improve the lightning protection measures?

A: 1. Remedy is OK, but multi-point grounding welding is required. 1. If possible, electrical connection can be carried out at the upper end, which is safe! In principle, measuring the earth resistance at the exposed place can be equal everywhere, but this is only a remedy!

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

3. The grounding resistance is less than 4 ohm, and the grounding resistance of all steel beams / columns is equal everywhere. The main grounding system shall refer to the requirements of GB 50057!