The preceding paragraphs explain what is required, and how to achieve it, but a heavy current arc is an intractable thing and one of its main tendencies is to keep itself as short as possible since that helps to maintain its ionisation level, and hence its current-carrying capability. Thus, while it is not difficult to design convoluted walls to arc chutes, it is also not easy to persuade the arc to enter them! The usual arrangement of an air circuit breaker is to arrange the arc chute above the contacts, and enable the contacts and the connections leading to them to form a tight loop so that a magnetic field is produced within the contact gap acting upon the arc in such a manner as to drive it up into the chute.

12a shows the closed position of interrupter. 12b shows contacts in partially open position with arcing between fixed and moving contact. 12c shows contacts in full open position with arc extinguished. The interrupting capability of air blast breaker is usually increased by increasing the normal pressure range. Normally the pressure level is around 30 to 35 bars. In order to maintain the insulation level and reliability of operation, it is also necessary for the condition of the air to be very dry.

It is thus evident that the arc plays an important role in the process of current interruption and therefore must not be regarded as an undesirable phenomenon. It must also be realised that, in the absence of the arc, the current flow would be interrupted instantaneously, and due to the rate of collapse of the associated magnetic field, very high voltage would be induced which would severely stress the insulation of the system. On the other hand, the arc provides a gradual, but quick, transition from the current-carrying to the current-breaking states of the contacts.