Lightning is the heavy discharge of accumulated charge from clouds into the earth. Its voltage may be as high as 50,000 volts or more. There exists a potential difference between the sky and earth depending on the climatic conditions. In normal sunny days, this potential difference is around 100 volts with negligible current because the air acts as a good insulator. When clouds start to accumulate in the sky, this atmospheric charge increases to 1000 volts or more. If this charge increases further to thousands of volts, sudden discharge of charge takes place which is the lightning strike.The explosion during lightning is the result of Air column breaking.
The major hazardous effect of lightning is the Arcing in plug sockets. During lighting, heavy transients and spikes develop in power lines. The heavy transients occurs both in the phase and neutral lines.
In the wall sockets, the metal contacts for phase and neutral are closely located so that heavy Arc flash develops between the contacts creating a bright blue flash and a mild explosion. This is known as Arc blast. The Arc blast will short circuit the instrument and will be destroyed. Since the switch is connected to phase line only, it cannot prevent the damage from Arc blast. So to prevent Arc blast damage, it is necessary to unplug all electrical and electronic equipments and telephone if there is heavy lightning.