Explanation of Thermal Ionization process of producing an Arc
When a fault occurs, the current carrying contacts of the circuit breaker are opened and the medium between the contacts get highly ionised.
Under faulty conditions, a heavy current flows through the circuit breaker before they are opened by the protective system. As soon as the contacts starts separating, the area of contact decreases which will increase the current density. The increase in current density causes temperature rise and is known as thermionic emission. The medium between the contacts in a circuit breaker is usually air or oil. This medium will be ionised due to the high temperatue and the ionised medium acts as a conductor. As the temperature increases, more electrons will be emitted from cathode. Thus an arc strucks between the contacts. The potential difference between the two contacts will be sufficient to maintain the arc. It is essential that the arc should be extinguished as early as possible. There are different methods for arc interruption.
Energy balance theory
This theory suggests that the re-establishment or the interruption of an arc is an energy balance process. This theory states that, if the rate of heat dissipation between the contacts is greater than the rate of heat generation, arc will be extinguished .Otherwise it will restrike.
At current zero, the hot arc between the contacts of circuit breaker needs to be cooled down to such a low temperature that it no longer conducts.
The following assumptions are made in this theory.
a) The temperature remains constant.
b) Arc consists of a cylindrical column having uniform temperature at its cross section. So the energy distributed in the column is uniform
c) The cross section of the arc adjusts itself to accommodate the arc current.
d) Power dissipation is proportional to cross sectional area of arc column.