12KV Outdoor Vcb Breaker High Voltage Three Phase 1000 Mechanical Life

During contact separation of circuit breaker arc is formed due to ionization of particles in the medium between the contacts. Idea behind the vacuum circuit breaker is to eliminate the medium between the contacts i.e., vacuum. Every medium that has a pressure below atmospheric, which is 760 mm of Hg, is known as vacuum. Low pressures are measured in torr, where 1 torr = 1 mm of Hg. The pressures below about 10–5 torr are considered to be high vacuum.
The breakdown voltage of certain contact gap varies with the absolute pressure in the vacuum interrupters. When vacuum is used to serve the purpose of insulation, it implies that pressure of gas at which breakdown voltage becomes independent of pressure. Such a condition is believed to be realized at values of pressures from 10–4 to 10–6 torr.
Vacuum Arc:
If an arc is to be formed in vacuum it is essential that the pressure should be low because only then it approaches the ideal arc. The ideal arc should have a low pressure, when it is initiated during its burning period and after its extinction.
Arc formed in vacuum is of different type from those formed in other mediums as it is formed by the neutral atoms, ions and electrons emitted from the electrodes themselves. The cathode surface normally, is not perfectly smooth but have many micro-projections. During the separation of contacts the current will be concentrated in these micro-projections as they are last points of contact. Due to their small area of cross section the projections will suffer explosive evaporation by resistive heating and supply sufficient quantity of vapour for the arc formation.
In vacuum arc the electron emission occurs only at the cathode spots and not from the entire surface of the cathode. For this reason the vacuum arc is also known as the cold cathode arc.