SOLUTION

An electric arc is a visible plasma discharge between two electrodes that is caused by electrical current ionizing gasses in the air. Electric arcs occur in nature in the form of lightning. With proper control, electric arcs can be harnessed and used industrially for welding, plasma cutting, and even certain types of lighting such as fluorescent lighting where a high voltage ionizes the inert gas within a glass tube; the flow of current across the ionized gas liberates visible light.

The electric arc is a self-sustained electrical discharge that exhibits a low voltage drop, is capable of sustaining large currents, and behaves like a non-linear resistor. Though the most Commonly observed arc discharge occurs across the air at atmospheric conditions, the arc discharge is also observed at high and low pressures, in a vacuum environment, and in a variety of gases and metal vapors.

As the volt-ampere characteristics of an arc voltage are negative i.e arc voltage is high when the arc current is low and vice-versa. In order to maintain the ionization of the gas, when the arc is effectively cooled, the magnitude of the arc voltage must increase. What this means is simply that as the arc cools, the cooling efficiency increases the deionization of the arc space, which in turn increases the arc resistance. As a consequence of the increase in resistance, the short circuit current and the phase angle are reduced and thus the likelihood of a successful interruption is significantly enhanced.

In an air circuit breaker, increasing the resistance of the arc in effect ins creases the arc voltage. Thus, to effectively increase the arc voltage any of We following means can be used:

1. Increasing the Length: The resistance of the arc is directly proportional to its length. The length of the arc can be increased by increasing the gap between contacts thereby increasing the resistance and therefore the arc voltage. In general, arc voltage increases almost linearly with an increase in arc length (within reasonable limits)
   and the same is attributed to increasing in resistance to the flow of current due to the reduction in charged particle density in arc zones with an increase in arc length. 
2. Cooling of Arc:– Cooling helps in the deionization of the medium between the contacts. This increases the arc Resistance. Efficient cooling Slav, be obtained by a gas blast directed along the arc.
3. Reducing the cross-section area of an arc:- If the area of the cross-section of the arc is reduced, the voltage necessary to maintain the arc is increased. In other words, the resistance of the arc path is increased. The cross-section of the arc can be reduced by letting the arc pass through a narrow opening or by having a smaller area of contact.
4. Splitting of arc:- The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in the series. Each one of these arcs experiences the effect of lengthening and cooling. The arc may be split by introducing some conducting plates between the contacts.