When a short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionise the air or vapourise and ionise the oil. The ionised air or vapour acts as conductor and an arc is struck between the contacts. The p.d. between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists.
During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller the current that flows between the contacts.
The arc resistance depends upon the following factors :
- Degree of ionization— the arc resistance increases with the decrease in the number of ionized particles between the contacts.
- Length of the arc— the arc resistance increases with the length of the arc i.e., separation of contacts.
- Cross-section of arc— the arc resistance increases with the decrease in area of X-section of the arc.
Factors responsible for maintaining ARC.
P.D.. between the contacts
When the contacts have a small separation, the p.d. between them is sufficient to maintain the arc. One way to extinguish the arc is to separate the contacts to such a distance that p.d. becomes inadequate to maintain the arc. However, this method is impracticable in high voltage system where a separation of many meters may be required.
Ionized particles between contacts
The ionized particles between the contacts tend to maintain the arc. If the arc path is deionized, the arc extinction will be facilitated. This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts.
Methods of ARC extinction.
Basically 2 methods:
1) High Resistance method.
2) Low resistance method or zero current method.
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. It is employed only in d.c. circuit breakers and low-capacity a.c. circuit breakers. Why?? Of course cause heat dissipation is very large that’s why.
Methods of increasing arc resistance.
- Lengthening of arc.
- Cooling of arc.
- Reducing cross section area of arc.
- Splitting the arc.
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 deionisation 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 letting the arc pass through a narrow opening or by 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.
Low Resistance or Current Zero Method.
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. Now the medium between the contacts contains ions and electrons so that it has small dielectric strength and 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 halfcycle. If 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 :
- Causing the ionized particles in the space between contacts to recombine into neutral molecules.
- Sweeping the ionised particles away and replacing them by un-ionised particles
The Deionization of the medium can be achieved by following methods:
- 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-ionization 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 space.