There are two methods of extinguishing the arc in circuit breakers :
i. High resistance method.
ii. Low resistance or current zero method
1. High resistance method.
In this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc. Consequently, the current is interrupted or the arc is extinguished.
In this method enormous energy is dissipated in the arc. Therefore, it is employed only in d.c. circuit breakers and low-capacity a.c. circuit breakers. The resistance of the arc may be increased by :
• Lengthening the arc. 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.
• Cooling the arc. Cooling helps in the de-ionization of the medium between the contacts. This increases the arc resistance. Efficient cooling may be obtained by a gas blast directed
along the arc.
• Reducing X-section of the arc. If the area of X-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:
i. letting the arc pass through a narrow opening
ii. having smaller area of contacts.
• Splitting the arc.
The resistance of the arc can be increased by splitting the arc into a number of smaller
arcs in 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.
2. Low resistance or Current zero method.
This method is employed for arc extinction in a.c. circuits only. In this method, arc resistance is kept low until current is zero where the arc extinguishes naturally and is prevented from restriking inspite of the rising voltage across the contacts. All modern high power a.c. circuit breakers employ this method for arc extinction.
In an a.c. system, current drops to zero after every half-cycle. At every current zero, the arc extinguishes for a brief moment. However, the medium between the contacts contains ions and electrons with small dielectric strength that can be easily broken down by the rising contact voltage known as restriking voltage. If such a breakdown does occur, the arc will persist for another half cycle. On the contrary immediately after current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and the current will be interrupted.
The rapid increase of dielectric strength of the medium near current zero can be achieved by :
i. causing the ionised particles in the space between contacts to recombine into neutral molecules.
ii. sweeping the ionised particles away and replacing them by un-ionised particles
Therefore, the real problem in a.c. arc interruption is to rapidly de-ionize the medium between contacts as soon as the current becomes zero so that the rising contact voltage or restriking voltage cannot breakdown the space between contacts. The de-ionisation of the medium can be achieved by:
• Lengthening of the gap: The dielectric strength of the medium is proportional to the length of the gap between contacts. Therefore, by opening the contacts rapidly, higher dielectric strength of the medium can be achieved.
• High pressure: If the pressure in the vicinity of the arc is increased, the density of the particles constituting the discharge also increases. The increased density of particles causes higher rate of de-ionisation and consequently the dielectric strength of the medium between contacts is increased.
• Cooling: Natural combination of ionised particles takes place more rapidly if they are allowed to cool. Therefore, dielectric strength of the medium between the contacts can be increased by cooling the arc.
• Blast effect : If the ionised particles between the contacts are swept away and replaced by unionized particles, the dielectric strength of the medium can be increased considerably. This may be achieved by a gas blast directed along the discharge or by forcing oil into the contact.