Our dependency on electrical and electronic equipment continues to increase, in both our professional or private lives. Data networks in companies or emergency facilities such as hospitals and fire stations are lifelines for an essential real time information exchange. Sensitive databases, e.g. in banks or media publishers, need reliable transmission paths.
It is not only lightning strikes that pose a latent threat to these systems. More and more frequently, today›s electronic aids are damaged by surge voltages caused by remote lightning discharges or switching operations in large electrical systems.
During thunderstorms too, high volumes of energy are instantaneously released.
These voltage peaks can penetrate a building though all manner of conductive connections and cause enormous damage.
Some 90% of all lightning discharges between a cloud and the ground are negative cloud-earth strikes. The lightning begins in a negatively charged area of the cloud
and spreads to the positively charged surface of the earth.
Additional discharges are divided into:
• Negative earth-cloud strikes
• Positive cloud-earth strikes
• Positive earth-cloud strikes.
The most common discharges actually occur within a
cloud or between different clouds.
Creation of lightning discharges
When warm, damp air masses rise, the air humidity condenses and ice crystals are formed at great heights. Storm fronts can occur when the clouds expand to heights of up to 15,000 m. The strong upwind of up to 100 kilometeres per hour causes the light ice crystals to enter the higher area and the sleet particles enter the lower area. Knocks and friction cause electrical discharge.
Negative and positive charges
Studies have proved that the sleet falling down (area warmer than −15 °C) has a negative charge and the ice crystals being thrown upwards (area colder than −15 °C) has a positive charge. The light ice crystals are carried into the upper areas of
the cloud by the upwind and the sleet falls to the central areas of the cloud. This divided the clouds into the three areas:
• Top: Positively charged zone
• Centre: Weakly negative charged zone
• Bottom: Weakly positive charged zone
This separation of charges forms a
voltage in the cloud.
Typical load distribution:
• Positive at the top, negative in the centre and weakly positive at the bottom.
• Positive charges can also be found in the area near the ground.
• The field strength required to trigger lightning is dependent on the insulating ability of the air and is between 0.5 and 10 kV/cm.