Lightning rod, lightning wire, lightning network, lightning strip and lightning arrester are lightning protection devices often used. A complete set of lightning protection devices includes lightning arresters, down conductors, and grounding devices. The above-mentioned pins, wires, nets, and belts are only lightning arresters, while lightning arresters are a specialized lightning protection device.
Lightning rod, lightning wire, lightning network and lightning strip are all lightning arresters. They use their prominent position above the protected object to lead lightning to themselves, and then discharge lightning current into the ground through downlead and grounding device to protect the protected object from lightning. The materials used for the lightning arrester should meet the requirements of mechanical strength and corrosion resistance, and should also have sufficient thermal stability to withstand the thermal damage caused by lightning current.
(1) The lightning rod is generally made of galvanized round steel or steel pipe with a diameter of about 20 mm. It is about 2500 mm long and its end is sharp. It can also be bifurcated and connected to the grounding device through the down lead. Lightning rod is mainly used to protect high-rise and isolated buildings or structures and the facilities around them, and is also commonly used to protect outdoor power transformation and distribution devices.
(2) The lightning protection network is made of galvanized round steel or flat steel, and lightning protection wires are installed along the roof eaves, and then made of the same steel pipe × 6m or 6 × 10m or 10 × A 10m square. Lightning protection nets are mainly used for buildings with flat or sloping roofs and large roof areas.
(3) The lightning protection strip is set up along the perimeter of the building using galvanized round or flat steel. Lightning strip is mainly used to protect the vertical and lateral sides of high-rise buildings from lightning strike. It forms a complete lightning protection system together with the lightning rod or lightning network on the roof.
(4) Lightning protection wires are generally installed on the same pole and tower as overhead lines using galvanized cast steel strands with a cross-sectional area of not less than 35mm2. The installation method and sag requirements are the same as those of overhead lines, and they are connected to grounding devices in the middle of the first and last sections. Lightning protection lines are mainly used to protect overhead lines and surrounding facilities erected on the same pole as them. The minimum dimensions of the lightning arrester are shown in Table 8-1. When installing the lightning arrester above the chimney, due to the corrosive effect of the smoke, the size should be appropriately increased.
Lightning arresters are connected in parallel to the protected equipment or facilities. Normally, the device is insulated from the ground. When lightning overvoltage occurs, the device changes from insulation to conduction and undergoes breakdown and discharge, leading lightning current or overvoltage to the ground for protection. After the overvoltage is terminated, the lightning arrester quickly returns to a disconnected state and resumes normal operation. Lightning arresters are mainly used to protect power equipment and power lines, and are also used as safety measures to prevent high voltage from entering the room. Lightning arresters have protective gaps, tube type lightning arresters, valve type lightning arresters, and zinc oxide lightning arresters.
(1) Protective gap
The protective gap is made by using the principle of high-voltage charged bodies breaking through air gaps. It has a simple structure, low price, and is easy to make, but its performance is poor. It is generally used on low voltage and less important lines.
(2) Tubular lightning arrester
Tubular lightning arrester is mainly composed of porcelain bushing, arc extinguishing tube and internal and external clearance. It has a complex structure and is commonly used in 10kV distribution lines as lightning protection for electrical equipment such as transformers, switches, capacitors and cable heads.
(3) Valve type lightning arrester
Valve type lightning arrester is the most commonly used lightning arrester for high-voltage lines, high-voltage electrical appliances, and the main components are porcelain bushing, spark gap, and nonlinear resistance. Its structure is complex, and it is commonly used for lightning protection of 3-550kV electrical lines, transformation and distribution equipment, motors, switches, etc. It is suitable for AC and DC power networks and is not limited by capacity, line length, short circuit current, etc.
(4) Zinc oxide lightning arrester
Zinc oxide lightning arrester is a new type of lightning arrester that uses polycrystalline semiconductor ceramic resistor sheets sintered from nonlinear high-quality zinc oxide and bismuth oxide metal oxides, eliminating spark gaps and improving the reliability of protection. Its characteristic is no discharge delay, no power frequency current after atmospheric overvoltage, can withstand various lightning strikes, low residual voltage, large current capacity, long service life, and is widely used for lightning protection and overvoltage protection of 0.25-550kV electrical systems and equipment, as well as low-voltage side overvoltage protection.
The down lead of the lightning protection device should meet the requirements of mechanical strength, corrosion resistance, and thermal stability.
(1) The down lead is generally made of round or flat steel, and its size and anti-corrosion requirements are the same as those of the lightning protection network and lightning strip. Use steel strands as down conductors, with a cross-sectional area not less than 25mm2. When using non-ferrous metal wires as down conductors, copper wires with a cross-sectional area of no less than 16mm2 should be used.
(2) The down lead should be laid along the exterior wall of the building and should avoid bending, and should be grounded through the shortest possible path.
(3) When using multiple down conductors, in order to facilitate grounding resistance and check the connection of down conductors and grounding wires, it is advisable to set up a disconnect card at a height of approximately 1.8m above the ground for each down conductor.
(4) When multiple down conductors are used, at least two down conductors should be installed in the first and second class lightning protection buildings, with distances between them not exceeding 12m and 18m respectively; When the perimeter of the third type lightning protection building exceeds 25m or the height exceeds 40m, there should also be two down conductors, with a distance between them not exceeding 25m.
(5) In areas prone to mechanical damage, a section of down lead from 0.3m below the ground to 1.7m above the ground should be protected with bamboo pipes, angle steel, or steel pipes. When using angle steel or steel pipe protection, it should be connected to the down lead to reduce the reactance when passing through lightning current.
(6) If the section of the down lead is corroded by more than 30%, it should be replaced.
Lightning protection and grounding device
Grounding devices are an important component of lightning protection devices. The grounding device releases lightning current to the ground, limiting the lightning protection device's voltage to the ground from being too high. In addition to the independent lightning rod, the lightning protection grounding device can be shared with other grounding devices on the premise that the grounding resistance meets the requirements.
(1) Materials for lightning protection and grounding devices. The materials used for lightning protection grounding devices should be greater than those for general grounding devices. The lightning protection grounding device should undergo thermal stability verification.
(2) Lightning protection grounding resistance generally refers to impulse grounding resistance, and the grounding resistance value depends on the type of lightning protection and the type of building. The impulse grounding resistance of independent lightning rod shall not be greater than 10 Ω generally; For less important third class buildings, it can be relaxed to 30 Ω. The power frequency grounding resistance of the induction lightning protection device should not exceed 10 Ω. The grounding resistance of lightning protection electric intrusion waves and impulse grounding resistance should not exceed 5-30 Ω, among which the grounding resistance of valve type lightning arrester should not exceed 5-10 Ω.
(2) Suppression of step voltage. In order to prevent step voltage from injuring people, the distance between the anti direct lightning grounding device and the entrances and exits of buildings and structures, as well as pedestrian crossings, should not be less than 3m.
When it is less than 3m, one of the following measures should be taken:
① The horizontal grounding body is partially buried at a depth of more than 1m;
② The horizontal grounding body is partially covered with insulation (such as a 50 to 8Ocm thick asphalt layer);
③ Pave an asphalt pavement with a width exceeding 2m of the grounding body and a thickness of 50-8Ocm;
④ Buried brim style or other types of uniform pressure strips.