Recommendation points
- Tasks and principle of operation
- Basic elements of lightning protection
- External lightning protection device
- Basic earthing
- Internal lightning protection
In order to protect residential infrastructure from atmospheric electricity discharges, a number of effective technical solutions have been developed. Today we will tell you in what cases a lightning protection device for a private house is required, what elements the system consists of and how to properly carry out the installation.
Tasks and principle of operation
Every year, more than ten million thunderstorms occur on Earth, during which up to thirty billion electric lightning occurs in the atmosphere, a third of which are discharged directly into the planet’s surface. The bulk of thunderstorm events occur in large water areas, however, in the continental area, thunderstorm activity is also quite high and at the same time territorially heterogeneous. Thunderstorm duration maps and the probable number of lightning strikes on an object determine the need for the construction of lightning protection devices and their type. According to the standards, lightning protection of category III is used for detached buildings in the countryside..
It is believed that the lightning discharge flows along the path of least resistance, and the only function of the lightning rod is to create this very path bypassing the vulnerable parts of the building structure. This is not entirely correct: the air around sharply rising conductors is ionized, due to which the strength of the electric field decreases, that is, by the very presence of spiers with good conductivity, it reduces the likelihood of a discharge. When the electric field is strained beyond the limit, the lightning protection system redirects the lightning current into the ground, preventing it from passing through the structural elements of the building, causing their destruction, threatening the health and life of people inside.
Lightning rod device: 1 – lightning rod; 2 – protected area; 3 – down conductor; 4 – ground loop
A lightning protection structure, as a rule, is represented by an elevated metal object and has a limited area of effect. The area of space in which the absorption of a lightning discharge is likely to occur has a complex geometrical configuration for which an unambiguous calculation method has not been defined. In practice, the tabular values set forth in the RD 34.21.122–87 standard and refined by the standard of the International Electrotechnical Commission IEC 62305 are used. In the first approximation, the area protected by a lightning rod is a cone, the top of which coincides with the upper point of the lightning rod, and the base radius is determined by the ratios indicated in the tables.
Protection of the house and electrical equipment from thunderstorms: 1 – remote lightning strike in power lines; 2 – electrical consumers; 3 – direct blow to the lightning rod; 4 – ground loop
Basic elements of lightning protection
Despite the seeming primitiveness, the building’s lightning protection system consists of a number of components, each of which performs a specific function. The main element is a lightning rod, which rises to a height sufficient to provide the required protection zone.
Contrary to popular belief, a mast or other supporting structure of an air terminal is not the main conductive link through which the lightning current flows to the ground. A steel cable or metal wire with a diameter of 6 mm or more is attached to the lightning rod with bolt clamps or welding. The conductor must be laid to the ground loop along the roof or wall of the building.
1 – rod lightning rod; 2 – lightning rod fastening; 3 – lightning rod mast; 4 – down conductor
The third element of lightning protection is a ground loop, which is combined with the main grounding conductors, provided that the protective conductors are separated from the working zero throughout. In contrast to the main circuit, the requirements for the conduction of lightning protection ground electrodes are less stringent. Contours on civil objects do not require mandatory control measurements.
External lightning protection device
Lightning rods are of three types: rod, cable and mesh. The first type, as a rule, is represented by a pipe with a diameter of 40 mm, divided into segments of 3-4 m for the convenience of construction. The links are connected either on flanges or socket couplings. Practice shows that the rod has sufficient stability at a height of up to 8-9 meters, a higher mast should be strengthened with guy wires. The majority of lightning protection systems for detached buildings with a non-metallic roof are based on lightning rods..
The second type of lightning rods is a metal wire string or cable with a thickness of 8 mm or more, stretched along the ridge of the roof on wooden posts about half a meter high. The permissible distance between the posts is 8 meters, with a greater length of the cable, it should be stretched from the posts to the ground, or additional supporting elements must be installed. With a cable length of up to 10 meters, it can be connected to grounding only on one side, longer lightning rods – necessarily on two sides. Catenary wire lightning protection is used on roofs that do not have non-metallic elements protruding above the ridge..
Lightning protection mesh is used on non-metallic roofs with a slope of less than 8: 1. The mesh consists of steel wire with a mesh size of at least 12×12 m, all intersections are connected by welding. The metal covering of the roof can also function as a lightning protection mesh. This type of lightning protection requires the mandatory integration of all metal elements of the roof into a single network with connection to grounding conductors at least every 100 m along the perimeter of the building..
All carrying and supporting devices of lightning rods must have sufficient mechanical strength and resistance to wind loads. Vertical masts of rod lightning rods necessarily require a reinforced concrete foundation, as well as fastening to the nearest walls at at least two points. Racks of cable receivers must be equipped with oblique supports located on the side of the tension forces. Fastening of supporting structures should be done not to the covering or lathing, but to the embedded elements of the roof.
Basic earthing
The rules for designing lightning protection first of all provide for the use of building foundations as grounding. For this, two conditions must be met: electrical connection between all elements of the reinforcement and the output of the contact rod for connection, as well as the absence of external epoxy or polymer waterproofing. Also, foundations cannot serve as natural grounding conductors with natural soil moisture at a depth of less than 3%.
In the absence of natural grounding conductors, a circuit is made with a normalized conductivity of no more than 20 ohms. As a rule, two electrodes with a cross section of at least 100 mm are sufficient2 and a length of 3 meters, driven vertically into the ground. The same electrodes can be placed horizontally by the beam method, but there should be at least three of them. The circuit electrodes must be driven in at a distance of no closer than 5 meters from each other and no closer than 5 meters to any underground communications, pedestrian paths, places of constant presence of people and animals. In this case, the location of the circuit should be chosen so that the length of the conductor between it and the lightning rod is minimal.
Internal lightning protection
The phenomenon, called high potential skidding, can occur not only with the participation of building structures, for the protection of which external lightning protection devices are intended. Gas pipes, any metal communications and wires of overhead power lines are also dangerous. Therefore, all inputs of metal structures into the house must be grounded..
SPD (surge protection device) is designed to protect circuits from lightning and switching surge voltages
The protection of the air entry line into the house is protected by special electrical devices called lightning arresters. At the moment, they are mass-produced in a compact version and can be installed in modular shields in combination with other protective devices. Basic protection against lightning strikes is provided by class 1 arresters, class 2 is provided to protect the wiring, and to protect household appliances, the installation of class 3 devices is required.
