External lightning protection system measures and methods

External lightning protection system

The external lightning protection system is organically composed of lightning receptors (lightning conductors), deflectors, and grounding nets. Indispensable. The following describes the related technologies and installations of the above three main factors.

The main content of this section is how to cut the air outside the building and discharge the lightning current into the earth.

The content of this section is:

(1) Air-termination device: lightning rod, lightning conductor, lightning conductor, lightning protection net

(2) Structure design of lightning protection and lightning protection network

(3) Selection and arrangement of lightning receptors

Lightning receptor

Direct lightning interception, as well as the use of lightning-receiving appliances, metal components, and metal roofing, are called lightning receptors. The function is to connect the lightning current received from the ground, through the deflector and the grounding device due to the release in the earth, to protect the building from lightning.

Since Franklin invented the lightning rod in 1753, the lightning rod as the only form of lightning receptor has lasted for more than a hundred years. Since the nineteenth century, lightning protection lines, lightning protection belts, and lightning protection nets have appeared gradually. The classification is as follows: Lightning rods, lightning conductors, lightning conductors, lightning protection nets, which are introduced one by one.

lightning rod

Tip discharge

From physics we can see that usually the positive and negative charges inside an object are equal, so the charged phenomenon is not displayed as a whole. When an object's positive and negative charges are not equal, the object displays a charged characteristic. When an object has more positive charges than negative charges, the object is positively charged, and negatively charged. Since the charges have opposite characteristics of attraction and repulsion of the same charge, the same charge in a charged object is always affected by mutual repulsion of the electric field force. Take the metal ball with a sharp front as an example. If the metal ball is negatively charged (it can also be understood to be positively charged), the electrons are always distributed to the outermost layer of the metal ball due to the charge repulsion of the same charge. Surface, and easy to escape from the metal ball. In the forward part of the ball, the electrons are most strongly repelled by the same charge and are most likely to be repelled away from the metal ball. This is commonly referred to as "tip discharge."

On April 29, 1749 AD, in his letter to John Mitchel, Franklin suggested that the clouds were charged by steam and they were charged. He thought, "When charged clouds move over fields and When passing through mountains, giant trees, towering towers, pointed roofs, ship masts, chimneys, etc., dragging out electric fires, just like many sharp conductors and protrusions, the entire cloud will discharge electricity there. Therefore, he proposed the idea of ​​a lightning rod. He said: Since the sharp conductor can release the charge on a charged body far away from it, and avoid it from causing electric shock to other objects, the sharp conductor may be useful to humans. He suggested that an iron bar with a sharp upper end and a rust-proof layer be installed at the highest point of the house, and be connected to the lower end of the house with a wire, and go straight down to the ground along the wall. In the sea boat, the iron rod is fixed on the top of the mast, and it is connected to the water by wire connection. They can quietly draw electricity away from the clouds as the clouds are about to produce electric shocks, thus freeing us from the most sudden and horrifying tragedies. Franklin elaborated the device of the lightning rod and formally announced that it was in 1753.

Lightning rod's English name lightning rod, literally translated as "lightning rod" is more accurate, not to avoid the lightning strike. This noun will be misunderstood when it comes to literary righteousness. Many of our domestic physics textbooks, and even university textbooks, have described the principle of lightning rods as a way to eliminate lightning by neutralizing the charges of cloud discharges at the tip. This is wrong. In fact, when Franklin invented the lightning rod, he proposed the analysis of the working mechanism of the two lightning rods; the first analysis believes that the lightning rod emits a charge that is opposite to that of the thundercloud by the corona discharge of the needle tip, so that the charge of the lightning cloud is neutralized. , thereby exempting the building from lightning. According to the second analysis, the lightning rod guides the different types of charge brought by the thunder cloud to itself. Through a good grounding device, the lightning current leaks into the earth and the building is protected from lightning strikes. By 1753, Franklin had clearly tended to the theory of lightning rod lightning, so it was wrong to say that lightning rods could eliminate lightning by tip discharge and avoid lightning. Lightning rods could not eliminate lightning.

working principle

After the thunder cloud formation, the voltage to the earth is several million volts low and several thousand volts or even higher. The average peak current of lightning flash over the earth is more than 30 kA, and its instantaneous power is 109-1012W. the above. Due to the large instantaneous power, its destructive power is quite large.

When thunder clouds appear at high altitudes, electrostatic induction on the earth will inevitably bring charges opposite to that of thunderclouds. The lightning rod is at the highest point of the ground structure, and it is closest to the thunderclouds because of its good electrical connection. It has the same potential in the earth, so that the electric field strength in the space near the lightning rod is relatively large, it is easy to attract the lightning lead, so that the main discharge is concentrated on it, thereby protecting the nearby objects lower than the probability of a lightning strike is greatly reduced. The probability of a lightning rod being struck by lightning has greatly increased. Lightning rods not only can not avoid lightning but lead lightning, it is more than their own lightning strikes to protect the surrounding from lightning strikes.

Due to the good electrical connection between the lightning rod and the earth, the charge energy accumulated in the earth can be rapidly transferred to thunderstorm clouds; or the overcharge time caused by lightning strikes by transferring the charge energy accumulated in the thunderstorm clouds to the ground. Greatly shortened, to a great extent reduced the harm of lightning, this is the working principle of the lightning rod.

However, it needs to be explained that the lightning rod must be sufficiently reliable, and the grounding device with grounding resistance as small as possible should be matched with the grounding device. Otherwise, it will not only fail to avoid lightning, but will increase the damage degree of lightning strike.

Lightning rod protection range calculation method

At present, the formulas for the protection scope of lightning rods in various countries in the world are different in form. In general, there are several types of

Calculation method:

1. Polyline method: The protection scope of a single lightning rod is a line cone.

2. Curve method: The protection scope of a single lightning rod is a curved cone.

3. Straight line method: It is determined by taking a declination angle of the tip of the lightning rod as a vertex. A building with a risk of explosion uses a 45° angle, and a general building uses a 60° angle. The scope of protection is a straight line cone.

Since 1983, China has formally established its own lightning protection specifications. At present, China's building construction and lightning protection standard GB50057-94 also adopts the "rolling ball method" recommended by the International Electrotechnical Commissioner (IEC) as the calculation method of the lightning protection scope.

Lightning rod production specifications

A large number of simulation experiments and actual survey statistics show that the shape of the lightning rod has no obvious relationship with its lightning protection effect. Therefore, there is no need to consider using single-needle or other forms of lightning rods.

The lightning rod should be made of round steel or steel pipe, and its diameter should not be less than the following values:

Needle length 1m or less: Round steel 12mm Steel tube 20mm

Needle length 1-2m: Round steel 16mm Steel tube 25mm

Chimney on the top of the needle: 20mm round steel is 40mm (see GB50057-94. Chapter 4)

Active lightning rod

Recently, the domestic market distributes a product called active lightning rods, mainly from France and Australia. According to manufacturers, this product can absorb energy with changes in the atmospheric electric field. When the stored energy reaches a certain level, it will In the discharge of the lightning rod tip, ionization of air around the tip causes an artificial upward lightning lead above the lightning rod. It can contact the thunderbolt lightning lead earlier than the natural upward lightning channel to form the main discharge channel. In this way, on the one hand, it is possible to increase the probability of a thunderstorm cloud being discharged by the lightning rod, which corresponds to an increase in the scope of protection of the lightning rod, or an increase in the height of the lightning rod.

Lightning conductor

The initial form of the lightning receptor was just a sharpened iron rod designed by Franklin. At the beginning of the 20th century, in the power system, in order to make the transmission lines less exposed to lightning strikes, a method of erecting parallel steel wire lightning protection above the transmission line was adopted. In practice, due to its simplicity and effectiveness, it was gradually promoted. This type of steel wire erected above the transmission line is called a lightning protection line. Later, this form was also promoted in housing construction and began to be installed on square ridges, corners, and houses, etc. for lightning protection, and this method has been improved in the future.

Lightning protection zone

In the lightning protection of houses, the method of using a flat metal strip instead of a steel wire for lightning is called a lightning protection belt, which is improved by the lightning protection line. In urban high-rise buildings, the use of lightning protection strips has more advantages than lightning rods. It can be combined with the decoration of the roof of the building and can be better matched with the shape of the house, that is, beautiful and lightning-protective effects are good, especially in large areas. Buildings, its scope of protection is large and effective, which is not comparable to lightning rods. The production of lightning protection zone uses flat steel with a cross-sectional area of ​​no less than 48mm2 and its thickness should not be less than 4mm.

Lightning protection network

The lightning protection network refers to the method of using the steel mesh in the reinforced concrete structure as the lightning protection method (if necessary, it can also assist the lightning protection network), also called the concealed lightning protection network. It is based on the principle of Faraday cage in classical electricity to achieve a lightning-protected metal-conductor network.

The concealed lightning protection network uses the roof of the top floor as a lightning receiving device. According to the structure of a typical building, the steel surface is only 6-7 cm away from the surface, the thinner the surface layer, the smaller the hole in the lightning strike point. However, in some buildings, the waterproof layer and the heat insulation layer are thicker, and the thickness of the surface layer of the steel bar is greater than 20 cm. It is better to install auxiliary lightning protection nets. Auxiliary lightning protection nets are generally available with galvanized round steel with a diameter of 6 mm or more. The grid size can be made from round steel of 5m'5m or 10m'10m depending on the importance of the building. The lightning protection network has clear and dark networks, and the more reliable the grid is, the more reliable it is.

There are often many projections on the roof of a building, such as metal flagpoles, breathable pipes, steel ladders, metal chimneys, wind windows, metal gutters, etc., all of which must be welded together with the lightning protection network as a lightning receiving device. In buildings with non-concrete structures, surface-mounted lightning protection nets may be used. The approach is to first install the lightning protection main grid on the prominent edge of the roof, eaves, etc. to the top, and then add an auxiliary network on the main grid.

Structure design of lightning protection and lightning protection network

Lightning conductors and lightning protection nets are generally round steel or flat steel, and their size should not be less than the following values:

The diameter of the round steel is 8mm, the sectional area of ​​the flat steel is 48mm2, and the thickness of the flat steel is 4mm.

Lightning conductors are generally erected with galvanized steel strands with a cross-sectional area of ​​not less than 35mm2.
 

Install lightning protection and lightning protection network should pay attention to the following matters:

1. The lightning protection zone and its connecting line pass through the settling ditch (Sility Ditch: A long multi-storey building often divides the building into several sections in the horizontal direction. There is a gap between the section and the section to prevent the sections from sinking. When there is inconsistency and causes damage to the building, there should be a span line with an expansion margin of more than 10-20 cm.

2. When the width of a flat-topped house with a parapet is less than 24m, it is only necessary to lay a lightning protection strip along the wall of the daughter; when the width is greater than 24m, a surface-mounted connection must be installed between the two lightning protection strips on the surface of the house.