Since ancient times, mankind has been fearing various natural phenomena. One of them is the thunderstorm and its spectacular manifestation - the lightning. Lightning presents the greatest danger for residential buildings, industrial enterprises and so-called critical infrastructure, the failure of which can cause considerable damage both for large cities and some regions.
According to the World Meteorological Organization, about 50,000 thunderstorms happens in the world every day. But the thunderstorm is not just the thunder and rain. The most dangerous phenomenon which it carries with itself is lightning.
History remembers an incident that happened to Russian scientists M. Lomonosov and G. Richmann during their experiments to capture atmospheric electricity in 1753 in St. Petersburg. These famous natural scientists of the 18th century had developed "lightning diverters" for their premises and tested them during a severe thunderstorm over the city. But the experiment was interrupted by the sudden appearance of a fireball, so the experiment failed.
Researchers are also trying to find a way to use lightning in the energy sector. A great inventor and scientist Nikola Tesla was examining the opportunities to use (store) the power of lightning in the first half of the 20th century in the United States. However, the US authorities classified the results of his studies and experiments and destroyed the original experimental facilities. This article is a plain-language explanation of important applied aspects associated with this natural phenomenon, namely, the protection of facilities, infrastructure and people from the devastating effects of storms and lightning.
Basic concepts and definitions
The thunderstorm is a natural phenomenon, which manifests itself in the form of atmospheric electricity discharges, which we are used to identifying as the lightning. Thunderstorms are also accompanied by strong acoustic effects (thunder), sometimes by strong gusty winds (flurry), and rain or hail. When an electrical potential difference of several million volts is reached between the various parts of a cloud, as well as between the cloud and the ground, then a natural phenomenon of electric discharge happens, known as the lightning. Lightning may have a length of 2 to 50 km, and their current can reach up to 100,000-200,000 Amps. The temperature in the lightning channel rises 10,000 and even 50,000 degrees centigrade. When the thunderstorm is caused by lightning aimed directly at an object, it presents threats considered primary ones. And if the storm is manifested as induced voltage from lightning strikes, such threats are considered secondary ones.
By their direction of discharge lightnings are divided into:
- downward-moving. We see a discharge in the downward direction that strikes into a building or another object;
- upward-moving. In this case, we can see a ground-to-cloud lightning strike. Most strikes into high-rise facilities (over 200 m) are upward-moving. Both downward- and upward moving lightning strikes can be both negative and positive, ie, positive or negative polarity current can flow through the lightning channel;
- in- and intercloud. These are lightning strikes between opposite charges inside a cloud or between nearby clouds. They do not present danger to any objects located on the ground surface.
Lightning protection is a set of measures to ensure the security of facilities, equipment, people, wildlife and public infrastructure from the destructive effects of lightning. Selecting the design of lightning protection systems is an important part of a construction project in terms of ensuring a safe human environment, the protection of buildings, structures, critical infrastructure and industrial communications from the effects of atmospheric electricity. It should be noted that there are Russian standards that categorize the security levels of facilities and the effectiveness of lightning protection measures. This is reflected in the "Instructions for lightning protection of buildings and structures. RD 34.21.122-87”, Instructions for the lightning protection of buildings, structures and industrial communications SО 153 – 34.21.122 – 2003», as well as in other regulations.
Lightning protection operating principles
In the most general terms, this device is a combination of components such as lightning rods, a down conductor and a ground electrode system. Three components generally form a lightning diverter that intercepts the lightning itself and leads its current into the soil. In such a way the facility is protected from the consequences of this natural phenomenon. Complex measures taken in order to protect humans, object, houses and other structures from direct lightning strikes with the help of special devices are called external lightning protection system.
In addition to the commonly used devices mentioned above, an alternative lightning protection system called early streamer emission is becoming popular in recent years. Such systems are much more expensive than the common ones, and their "increased efficiency" has not been proven by technical experts in practical operation.
Specialists install free-standing lightning diverters, but it is also possible to install a lightning diverter on a facility itself.
What is the function of a free-standing lightning diverter? This device can prevent the effect of current distribution throughout the facility by diverting current to the ground.
A simple formula shows the radius of such a diverter at the ground level: R = 1,5 * h, (the formula is given for B RD 34 zone), where h is the height of lightning rod measured from ground level. Now let's consider the case when a lightning diverter is installed by a technician directly on the facility.
When such a design solution is used, the process of current spreading along strictly controlled paths to divert current takes place. The probability of fire or explosion is reduced to a negligible value because the thermal action on the facility is excluded.
Depending on the type of air termination system, lightning rods, catenary wires and mesh cages are most commonly used. The idea and design of a lightning rod were first proposed and patented in 1752 in the United States by a prominent American political figure and a talented scientist Benjamin Franklin. Since then, this type of lightning diverter is called Franklin's rod.
Catenary wire lightning protection, as its name implies, is a steel wire drawn between two masts, which intercepts a lightning strike into a facility under the wire. More than one catenary wire can be used to protect a building.
Another type of protective device is called lightning protection mesh cage . To install such a protection, technicians lay a mesh cage directly on a non-metallic roof of the building; the roof must be horizontal. Any inclination of the roof could mean the risk of being struck by lightning.
Another type of air terminal is called
The popularity of a particular air terminal type varies by regions and facilities, so it is almost impossible to distinguish a "leader". If a very tall building is located near your facility, then the design of the lightning protection system must take into account the distance between such structures, as well as a number of economic factors and other features of the protected structures. The down conductor is built into the wall of the building; it is laid down to the soil and should be fairly short to pass the charge into the ground very quickly.
Internal lightning protection is a set of preventive measures to prevent the secondary threats of the lightning strike. Such threats are usually presented in the form of a surge and an inbound high potential. Surge protection devices (SPD) are usually used against such threats. Sometimes it is also called lightning surge protection. Surge can be caused by both by a direct and indirect lightning strike. If a lightning strikes near the structure or facility, then we are talking about the secondary threat of the lightning, i.e. surge.
Experts subdivide surge protection devices into 3 classes. Class I SPDs are installed at all facilities protected by an external system, as well as at facilities near tall structures such as masts and any other structures, with which they have a common grounding device. This class of SPDs is designed for reliability, which enables them to sustain a full lightning current, limit it to the desired value and remain intact.
Class II SPDs should be able to limit the surges passing through Class I SPDs, as well as induced and switching surges. They are Installed downstream of Class I SPDs and intended to limit surges to a level that most devices can sustain. Class II SPDs can be used as the first protection tier when the object is not equipped with an external lightning protection system, and there is no risk of a direct lightning strike into a facility and networks and communications entering it (for example, when power is supplied by a cable). Class III SPDs are installed downstream of the Class II SPDs and are designed to protect sensitive electronic devices, as when a power supply cable has a big length, which leads to the induced surge. When installing lightning protection systems, either the principle of security or fail-safety is applied. If safety is considered first, then it the surge protection device destruction is unacceptable and the lightning protection can not be turned off. But in this case, users disconnection from the power supply is allowed. SPD protective fuse is inserted into a phase conductor (serial phase connection).
Lightning protection and grounding application
Let's refer to regulations mentioned above in this article. If you are planning to protect a house having a height of up to 60 m from the effects of thunderstorms, please bear in mind that there are 3 categories of such protection. Class III protection applies to all conventional residential premises with a typical layout, as well as public buildings. But for multi-storey high-rise building lightning protection becomes even more important, as in such cases all kinds of low-voltage electrical installations may be damaged. This is especially true if modern control systems for "smart homes" are used. Therefore, experts equip such buildings with special surge protection devices. Such systems are installed in special locations defined at the overall design stage of the lightning protection system. But there are no mandatory requirements for the lightning protection of houses in Russia's rural areas. Such houses have Class III lightning protection category.
Some facilities (sometimes called critical)are buildings and the objects of infrastructure that can cause heavy material damage or threats to human health or even life in case of their destruction. Such objects require the installation of lightning protection systems having a category established by special requirements at the project documentation drawing stage. These include power generation and telecommunication facilities, pipelines, railways, highways, airports, maritime and river terminals, gas and oil fields, information and communication infrastructure, as well as military and defense facilities. As information technology and the Internet become more and more widespread in manufacturing and other industries, the equipment used in these fields should have grounding protection.
To sum up, lightning protection as an integral safety component of modern industrial and public infrastructure that ensures cost savings and the protection of people, installations and buildings from the destructive effects of atmospheric electricity.
- Grounding. What is it and how to make it.
- Lightning protection and grounding
- Professor's article: "The lightning protection of residential and public buildings"
- Off-the-shelf solutions for the surge protection of various systems