Formation of an arc is due to Corona Effect.
Theory of corona formation:
- Some ionization is always present in the air due to cosmic rays, ultraviolet radiations, and radioactivity.
- Therefore, under normal conditions, the air around the conductors contains some ionized particles (i.e., free electrons and +ve ions) and neutral molecules.
- When p.d. is applied between the conductors, a potential gradient is set up in the air which will have a maximum value at the conductor surfaces.
- Under the influence of potential gradient, the existing free electrons acquire greater velocities.
- The greater the applied voltage, the greater the potential gradient and more is the velocity of free electrons.
- When the potential gradient at the conductor surface reaches about 30 kV per cm (max. value), the velocity acquired by the free electrons is sufficient to strike a neutral molecule with enough force to dislodge one or more electrons from it.
- This produces another ion and one or more free electrons, which in turn are accelerated until they collide with other neutral molecules, thus producing other ions.
- Thus, the process of ionization is cumulative. The result of this ionization is that either corona is formed or spark takes place between the conductors.