# Theory of Circuit Interruption

## Theory of Circuit Interruption:

Arc in Circuit Breaker : Discharge in a.c. circuit breakers, generally in the form of an arc, occurs in two ways. When the contacts are being separated arcing is possible  even when the circuit emf is considerably below the minimum cold electrode breakdown voltage, because of the large local increase in voltage due to the circuit self-inductance. This way of drawing an arc is common to both d.c. and a.c. circuit breakers. The second method occurs only in a.c. circuit breakers. In this case the Arc in Circuit Breaker is extinguished every time the current passes through zero and can re-strike only if the transient recovery voltage across the electrodes already separated and continuing to separate reaches a sufficiently high value known as breakdown voltage. It is known that the arc phenomenon depends upon: The nature and pressure P of the medium. The external ionizing and deionizing agents present. The voltage V across the electrodes …

Arc Interruption Methods in Circuit Breakers : The two modes of Arc Interruption Methods in Circuit Breakers are: high resistance interruption, and low resistance or current zero interruption. High Resistance Interruption: In this case the arc is controlled in such a way that its effective resistance increases with time so that the current is reduced to a value insufficient to maintain it. The arc resistance may be increased with lengthening, cooling and splitting the arc. The main drawback with this type of Arc Interruption Methods in Circuit Breakers is that the energy dissipated is high and so it is only used in low and medium power a.c. circuit breakers and in d.c. circuit breakers. Low Resistance or Current Zero Interruption: This method is employed in a.c. Arc Interruption Methods in Circuit Breakers. A 50 Hz alternating current passes through zero 100 times per second. At every current zero the arc extincts for a brief moment and again …

Arc Properties : The manner in which the are medium is made to conduct has already been studied. A plot of instantaneous values of voltage eB between the electrodes of a burning arc against the corresponding values of current gives the Arc Properties. With the increase of arc current the temperature rises and the process of ionization becomes more active,: thereby increasing the conductivity of the medium and thus the arc voltage decreases. The increase of current in the higher range does increase the conductivity but has much less marked effect upon the voltage. The initial breakdown of the gap between the electrodes requires a high ignition voltage eZ at zero current (Fig. (13.3)). It is also seen that by changing the rate of change of current the characteristics shift. If the current changes rapidly with time the characteristics are known as dynamic and if the rate of change of current is small …