From the series of articles "Lightning protection for beginners".
"Are you afraid of lightning" - I hate such a question. It essentially involves fatality and lack of initiative. It is better to discuss the danger of lightning. This word supposes specific impacts that we should be aware of and from which to protect ourselves. Knowledge is always active. It assumes response actions. The main purpose of this article is to eliminate the mythical fears and encourage the reader to effective protection against lightning.
First thing to be scared of is lightning voltage on its channel relative to the ground. One hundred million volts sounds very impressive, especially for a person who inadvertently put his fingers into a socket with a voltage of only 220 V. Do not try to compare these two voltages to evaluate the effect of lightning. This comparison won't give an honest result. Direct contact with the electrical network of 220 V make you fully feel these 220. And at a lightning strike into a human, the voltage will be distributed in direct proportion to the resistances of the lightning channel and the human body. Safety engineers orient to the human resistance of 1000 Ohms. How strong lightning is? While a lightning channel grows to the ground, its conductivity is not too high. Channel resistance may well be 100 ohms per meter. With the length of 5000 m (this figure is quite average for lightning) it is 500 times more than of a human. That means, a human will be hit by not 100 000 000 V but by just 200 000 V.
I understand, I did not bring you any joy, Still 200 kilovolts is quite an earthly figure. In a today's high-voltage laboratory there are sources with voltages ten times higher. Did laboratory staff had a chance to "get acquainted" with them? Unfortunately, the answer is yes. Consequences? Most sorrowful, though not necessarily fatal. My teacher, an electrical engineer in the third generation, E.Y. Ryabkova in front of the correspondent of "Pravda" (in those years newspaper number 1) inadvertently approached the powerful capacitor bank charged to almost 100 kV. She survived, but the correspondent fainted. To no longer return to the effects of a direct strike, I will say that after a contact with the sources of ultra-high voltage in the laboratory, nobody became a paranormalist, nobody acquired the ability to see through the clothes and did not receive any other miraculous properties about which our television likes to broadcast. In addition, a direct strike into a human is an extremely rare phenomenon.
Example with Ohm's law was necessary for me to draw your attention to the effects of a large lightning channel length. It often and remarkably weakens the hazardous effects. During the lightning flash the charge of up to 200 Kl flows along it channel. If you multiply this amount by the voltage of 100 mV (It is more convenient to record lightning voltage in megavolt) we will get 200 x 100 = 20,000 MJ - this is the value of energy dissipated by lightning during its flight. This is quite a weighty variable. It is enough to give power to an electric engine of 1 kW for almost 8 months. And yet no one dares to solve energy problems using lightning. All its energy is almost evenly distributed over a length of 3 - 5, or even 10 kilometers, where it warms the air without any use. Heat is generated at the point of contact with the metal surface. Fortunately, not so much - not more than 2000 J. This will be enough to boil about half a glass of water. However, this is enough to melt a roof of metal sheets or metal tile. Lightning cannot cope with a thick steel sheet (thicker than 4 mm). When a flagopole was removed from the Ostankino Tower that had been there for a few years, there could be seen many traces of penetration in the points of lightning strike on its steel tip. The surface looked like the skin after smallpox. The largest trace barely reached 1 cm in diameter and 1 mm in depth.
The tip of the flagpole from the Ostankino television tower. It edge was destroyed by about 200 lightning that hit it
Is it dangerous? For a construction of a tower crane it is definitely not. It is impossible to see a lightning scratch even on massive farms. Not so long ago I had to respond to a subpoena. It was necessary to estimate the fact of a lightning strike by a visual inspection of metal structures for the Arbitration Court. I could only make a helpless gesture. On the other hand, a burn-through of the metal house roof is a direct route to fire and it can disturb the homeowner. Burning of a thin metal tube of the sensor or a small-sized antenna is even more dangerous. It can be fatal for an aircraft.
Aviators respect lightning, although it almost never burns the duralumin skin of the aircraft. This skin is not thick, it is far from the 4 mm, which even a powerful discharge cannot burn through. The reason is that the air stream blows the point of hot channel contact with the skin. The action of the lightning current is distributed over a sufficiently noticeable length and there is no time to melt the metal.
In addition to the damage of sensors and antennas, the pilots will necessarily recollect about engine surge. As a teenager, I had to face with a lightning very closely. It hit in front of the window ten meters from our three-storey house. A thunderclap stunned me for a few minutes. Lightning creates a very strong shock wave. Penetrating into the air intake of a jet engine, a powerful air flow completely blows all the products of combustion away. The engine stalls. This is surge. A restart of the engine during the flight requires great courage and luck from the pilot. Let me note, that there can be no surge for modern aircraft with engines on pylons of wings. Lightning cannot reach there.
A shock wave in dense environments, such as water, is even stronger. There is the whole industry, which uses electro-hydraulic effect for crushing hard materials by electric spark discharges in liquid. Have you ever seen a teared off bark strip from the tree tops almost to the butt-end? It is the job of the electrohydraulic effect. After hitting a tree the lightning current was distributed on the most conductive part of the trunk, between the wood and the bark. Almost instantaneous evaporation increased the pressure there, having chipped off the bark. Lightning contact with modern composite materials is dangerous approximately for the same reason, when their components generate gas during heating. Broken metal tile in the place of the lightning strike is apporximately the same story. The experts are anxiously watching the massive introduction of composites in the construction of aircrafts. Not everything is clear here. Additional researches are required.
Everything listed above is directly connected with the direct contact of the object with the lightning channel. Therefore, the choice of protection means doesn't pose a special problem here. It is enough to, for example, put a lightning rod on the house roof to save its coating. It is also possible to protect the antenna, including "plates" desperately advertised nowadays, promising a hundred television channels. As for the antenna and aircraft sensors, an intelligent designer will be able to place them out of reach of lightning. The situation with the electromagnetic field of lightning is completely different. It was mentioned in the previous article. According to Faraday's law, this field induces EMF of magnetic induction in any contour, whether it is power line wires that go to your home, TV antenna or computer leads. Modern microprocessor technology is very sensitive to magnetic pickups. Not much is required to damage an electronic unit with the operating voltage of 3 - 5 V! It is ok if it happens to home appliances. Of course, it is expensive, but in case of damage only money will be lost. In the industry, the situation is incomparably more severe. Imagine a failure of an automatic production management system. The losses will cost millions. There was a case, when a single lightning strike disabled 8 completely healthy and innocent 110-kV lines due to a damaged relay protection. An area with the million population was left without electricity.
One of the stands with the SPD to protect control circuits and automation of a modern compressor station
It is impossible to protect from electromagnetic fields by lightning rods - it is able to affect at a distance. That is why it is necessary to equip critical circuits with special devices for protection against electromagnetic overvoltage (SPD). The number of such absolutely not cheap SPDs at a modern technical object is counted in hundreds, sometimes thousands. Their operaton is far from absolute reliability, and therefore it becomes necessary to equip another system, which should control the installed protective means. It turns out to be very difficult, and therefore not very reliable. The problem is waiting for its radical solution.
As you can see, lightning danger should not be exaggerated, but it's much worse to neglect it.
E. M. Bazelyan, DEA, professor
Energy Institute named after G.M. Krzyzanowski, Moscow
We hope that in the future this site will perform the role of an elementary textbook for self-defense against lightning. We plan to continually post articles about the real dangers of lightning electricity and modern means of lightning protection here. They are designed to help to sort out the existing problems and to estimate the ways to solve it.
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