Arc Formation:

1. When the fixed and moving contacts are separated under load ( Current flowing ) .
2. High temperature at contact surface causes metal evaporation.
3. A metal vapor cloud forms between contacts.
4. Current continues through this metal vapor particles.
5. So the arc is generated . 
6. So we can call this as metal vapor arc but not air arc.



Arc Quenching:

1. As this is vacuum there are no gases inside to sustain ionization.
2. And also the vacuum has extremely high di-electric strength , it condenses the metal vapor instantly and brings the currents to zero immediately.
3. And the arc will be completely extinguished.

Time taking to Quench:

10 or less milliseconds.

Thankyou.