What is a Lightning Strike and the Phenomenon of lightning
Lightning has been a source of curiosity for humans for thousands of years. lightning is a discharge of electrical charge between cloud and ground, the height of the cloud above ground level may vary from 500 to 30000 feet. The energy in the Lightning Strike may be an order of 250 kilowatt hours.
Mechanism of lightning strike
Lightning is the phenomenon of electrical discharge from the Cloud to the ground. during lighting stroke the cloud and ground act as two plates of a gigantic capacitor and air act as a dielectric medium between the ground and cloud. Since the lower part of the cloud is negatively charged Earth becomes positively charged by induction.
The Lightning discharge is required to puncture air between the cloud and the earth, At standard temperature and pressure breakdown voltage of air is 30kv/cm but clouds contain a large amount of moisture, and also because of high altitude and low pressure breakdown voltage of air dropped drastically it is only 10kv/cm
When a voltage gradient of about 10kv/cm is formed in the cloud the air surrounding gets ionized at this time streamer initial from the cloud to the ground this is not visible to the naked eye.
The current in a streamer is about an order of a hundred amperes and the speed of the streamer is 2.5ft/usec this starting streamer also called as a pilot streamer which causes lightning phenomena.
Depending upon the degree of ionization of air surrounding by the streamer it provides several paths and is known as a stepped leader as shown in figure b.
Stepped leader is about 50 meters long in length and lasts for few microseconds the charge brought from charge cloud true ionized path to pause, the air surrounding to pause is again ionized and the leader reaches the earth.
Once the stepped leader makes contact with the earth, the power return stroke moves very fast toward the cloud through the ionized path by the stepped leader.
This streamer is very strong and the current varies is between 1 KA to 200KA. At this instant where the negative charge of the cloud is neutralized by positively charged on the earth and gives rise to a lightning flash which we can see by eye.
There is another charge in the cloud near a neutralized charge. This charge tries to neutralize through an ionized path and this streamer is known as a dart leader. Shown in Fig C
Effect of dart leader is more than return stroke, the discharge current is more in return streamer but it last only for few seconds and due to this reason energy in streamer is less, Hence return streamer is also known as Cold lighting strike. And Dart leader called Hot lighting strike.
In Dart Leader current is small but it lasts for some milliseconds and therefore energy is greater in Dart Leader.
Types of lightning strikes
In a direct stroke, the lightning discharge is directly from the cloud to the subject equipment. From the line, the current path may be over the insulator down the pole to the ground.
Indirect stroke results from the electrostatically induced charges on the conductors due to the presence of charge clouds.