When a very high voltage is applied across two conductors of the overhead transmission line whose spacing is large as compared to their diameters. When this applied voltage exceeds a certain value (called critical disruptive voltage), then the conductors are surrounded by a faint violet glow which is known as the corona effect.
In practice, the corona effect is accompanied by a hissing sound, the production of ozone, power loss, and radio interference. The higher the transmission voltage, the larger and higher the luminous envelope becomes and the higher the power loss and the radio interference.
Air is not a perfect insulator, and even under normal conditions, air contains many free electrons and ions. When an electric field intensity establishes between the conductors, these ions and free electrons experience force upon them. Due to this effect, the ions and free electrons get accelerated and moved in the opposite direction.
The charged particles during their motion collide with one another and also with the very slow-moving uncharged molecules. Thus, the number of charged particles goes on increasing rapidly. This increases the conduction of air between the conductors and a breakdown occurs. Thus, the arc establishes between the conductors.