Causes of lightning
In the cumulonimbus cloud generated by lightning and when the positive and negative charges are accumulated in different parts of the same cloud or in different clouds, the amount of charge accumulated in different parts of the same thunderstorm or between different clouds is different, or The polarity is different, and the atmospheric electric field is generated. When the intensity of the atmospheric electric field reaches the level that can penetrate the air, a strong discharge phenomenon occurs, and lightning occurs. The lightning at this time is called cloud flash or cloud lightning.
When the charged cloud layer moves close to the ground or buildings, due to static induction, the surface of the ground or the building will generate an opposite charge. As the amount of charge increases or the distance between the clouds/buildings decreases, the charged cloud layer and the ground or building When the electric field strength can penetrate the air insulation strength, it starts to discharge instantaneously, and lightning (cloud flash) occurs.
Type of lightning
There are four main types of lightning discharge: lightning in the clouds, lightning in the clouds, lightning in the clouds, and lightning in the clouds. Among them, cloud lightning is the most dangerous to humans.
From the perspective of lightning hazard, there are several kinds of lightning phenomena such as direct lightning (cloud lightning), inductive lightning, lightning intrusion wave and spherical lightning.
Direct lightning refers to the phenomenon of discharge between the charged cloud and the earth in the sky. It accounts for about one-fifth to one-sixth of the world’s annual lightning, discharge current can reach more than 200kA, and has a high voltage of more than 1Mv, which is the most harmful. Inductive lightning is also called lightning induction, and it is divided into static induction and electromagnetic induction. Static induction is caused by a large amount of charge induced on the top of an overhead line or other bulge when the thundercloud is close to the ground; electromagnetic induction is caused by a large magnetic field that rapidly changes in the surrounding space after a lightning strike. This strong magnetic field induces a very high voltage on the metal conductors in its vicinity.
A lightning intruding wave is a lightning wave that generates a surge voltage on an overhead line or an airborne pipeline due to a lightning strike and propagates in two directions along the line or the pipeline at an extremely fast speed.
A spherical lightning is a kind of red or white light fireball formed during lightning. In the thunderstorm season, spherical mines may sometimes invade the room from doors, windows, chimneys and the like.
The harm of lightning includes three aspects:
- Direct lightning strikes (direct lightning strikes) which is what we usually call lightning. Direct lightning strike has three major effects: thermal effect, electrical effect and mechanical effect, and the lightning energy is huge, which can instantly cause physical damage and electric shock damage such as damage and collapse of the object.
- Inductive lightning during the formation of thunderclouds, due to the accumulation of electric charge in the thundercloud and the sharp decrease of the charge in the thundercloud when lightning occurs, a wide range of electrostatic induction and electromagnetic induction phenomena will be formed, resulting in lightning damage (lightning) The metal conductors within a radius of 2 km occur high potential (strong voltage) and instantaneous inrush current (surge). The main hazard may be caused by the electric potential spark caused by the potential difference, the breakdown of the power supply and the signal line caused by the surge, the short circuit of the line, and the intrusion of the electrical equipment to cause equipment damage. In particular, it is more harmful to low-voltage electrical systems and electronic information systems.
- Conducted lightning: When a lightning strikes a ground object, especially a building, the metal current, power and signal lines that enter and exit the building are conducted outward during the lightning current discharge process (about 50% of the total lightning current), thus Lines and facilities in other buildings cause damage.
Lightning protection measures
For the harm of lightning, lightning protection measures are divided into external lightning protection measures and internal lightning protection measures.
External lightning protection measures mainly include installation of lightning receptors (such as lightning rods, lightning protection belts, lightning protection nets, etc.), down conductors and grounding devices. The lightning receptor is used to intercept lightning and avoid direct lightning strike by the lightning protection; the grounding device is used for the leakage of lightning current to the earth and has the requirement of grounding resistance; the down conductor is used to connect the lightning receptor and the grounding device.
Internal lightning protection measures include: shielding, reasonable wiring, installation of lightning arresters (SPD), equipotential bonding, and grounding. Shielding and reasonable wiring can reduce the influence of static induction and electromagnetic induction on lines and equipment; the installation of lightning arrester can limit the surge voltage on the line and guide the leakage of lightning current; equipotential bonding can avoid counterattack between adjacent metal objects and lines. Grounding is an important guarantee for shielding and arresters to function.
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