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You never know when lightning will strike...
With the spread of advanced information technology throughout society in recent years, lightning damage is having an increasing effect on the electronic equipment, computer systems, and networks that play an important role and are indispensible for comfortable living.
Conventional lightning rods guide lightning through lightning conduction rods, such as a building's steel framework, so that a large current flows to the ground. As a result, electromagnetic induction occurs, and a large surge current flows in the ground, which causes damage to electronic equipment by penetration from power supplies or communication cables, etc.
The diagram on the right is the “Number of days of lighting and rain per year” for all parts of Japan over the 10 year period from 1954 to 1963, and shows the average number of days (Source: Japan Meteorological Agency). As can be seen from this data, the North Kanto, Hokuriku, Kinki, and Kyushu regions have particularly high numbers of lightning and rain days per year.
Number of days of lightning and rain per year
In most cases the mechanism for occurrence of thunder clouds is rising air currents due to the air being heated near the ground by strong sunlight. The air in this rising air current is cooled, and the charge is separated when hail is generated, so the thunder clouds grow large due to the effect of electricity being generated.
Normally, a positive charge accumulates in the top of a thunder cloud and a negative charge accumulates in the bottom, so positively charged static electricity is induced near the ground surface. In this way, a strong electric field is generated between the thunder cloud and the earth, and when this exceeds the insulation capacity of the air, lightning is generated.
Lightning generation mechanism
When lightning strikes a lightning rod, a large lightning current flows into the ground through the lightning rod conduction wires and the steel framework of the building.
A magnetic field is generated by this excessive electric current, and a surge voltage is induced in cables and electronic equipment by electromagnetic induction. Also, an induced voltage is generated in equipment near the grounding cables and in buried electric lines by the surge current that flows into the earth.
Lightning current is large in the range 1 to 200 kA, and the rise time is large (9 to 65 kA/µs), so a large surge voltage is generated that is capable of causing damage to nearby electronic equipment such as computer systems and telephone exchanges (PBX).
When the lightning strikes the ground, the potential of the ground is raised and because of the neutralization phenomenon, the induced charge of the thunder cloud that charged the ground surface and structures moves towards the point of the lightning strike, which generates a surge current due to the large ground current.
Also, at the same time the electric field near the ground surface generates an electrostatic induction surge due to the sudden change.
Induced lightning surges that penetrate from power supply lines, etc.
As a result of the discharge in the lightning and thunder clouds, etc., the balance in the previously balanced electrostatic charge in overhead electric lines becomes unbalanced, so surge current flows into equipment from power supply lines and telephone lines, etc., which have surge current, so damage can result.
Note: A lightning surge is an unbound charge flowing into the ground.