Arc Phenomenon in Circuit Breaker :
We know that a circuit breaker is a mechanical switching device that operates automatically. It can make, carry, and interrupt the current during some abnormal conditions like short circuits during the on-load condition.
When there is a fault or short circuit on the system, it causes to flow heavy current of high magnitude in the system and also through the contacts of the circuit breaker. As soon as the fault occurs the fault sensing unit signals the circuit breaker to open its contacts.
When the circuit breaker contacts begin to open, the contact area between the two contacts decreases. Due to heavy inrush current, during separation, the current density increases consequently and liberating heat at the contacts causing a temperature rise. The increase in temperature results in the emission of electrons from the cathode.
The air surrounding the breaker contacts already consist of electrons, positive ions, and neutral molecules. During a fault condition, the emitted electrons from the cathode are sufficient to ionize the air. This ionized air acts as a conductor and due to this, an arc is struck between the two contacts. A small potential difference between the contacts is sufficient and can be responsible for maintaining the arc.
The arc struck between the contacts provides a path for current flow due to its low resistance and thus the current in the circuit remains uninterrupted as long as the arc persists. Thus it can be seen that the current flow in the circuit depends upon the resistance of the arc.
Principle of Arc Extinction :
The factors that are responsible for maintaining the arc between the contacts of the circuit breaker are,
- The potential difference between the contacts
- Ionized particles present in the medium (air or oil) between contacts.
If the gap between the contacts is small, the potential difference between them is sufficient to maintain the arc. So, firstly the potential difference needs to be decreased.
Suppose if the contacts are separated by a certain distance such that the potential difference becomes small to maintain the arc and thus the arc can be extinguished. This method is applicable for low operating system voltages and not suitable for higher operating voltages because the separation distance required is many meters.
The presence of ionized particles between the contacts of the circuit breaker will cause for persistence of the arc. If the arc path is deionized then it is possible to extinguish the arc. This can be achieved by cooling the arc or by removing the ionized particles between the two contacts which can be done by passing highly electronegative gas between contacts.
Methods of Arc Extinction in Circuit Breaker :
The arc extinction in circuit breakers is done by two methods, namely, the high resistance method and low resistance method (current zero method).
High Resistance Method :
From the phenomenon of arc, it is clear that the current flowing in the circuit entirely depends upon the arc resistance. In this method, resistance is increased to such a value that the current is decreased to a very low value. Once the current is decreased the arc is extinguished. The resistance of arc can be increased in following ways,
- Increasing the Length of the Arc - We know that, resistance R ∝ ρl/A. If the length of the arc is increased then the resistance is increased, once the resistance is increased, the current decreases.
- By Reducing the Area of Cross-section of the Arc - If the area of cross-section of the arc is reduced, the resistance increases as R ∝ ρl/A. When the resistance is increased, the current is automatically interrupted thereby, the arc is extinguished.
- Cooling the Arc - By providing any cooling agent, the ionized particles present between the contacts can be reduced. The reduction of ionized particles increases the resistance between the contacts and hence decreases the current.
- Splitting the Arc - The arc is made to split into a number of sections. In each section, resistance is increased due to the effect of cooling and lengthening of arc.
Low Resistance Method or Current Zero Method :
It is applicable only to ac circuits. We know that the current becomes zero twice a cycle in case of ac. When it becomes zero, the arc extinguishes for a longer duration of time.
During the next half-cycle, it reappears again if the dielectric strength of the air gap is less than the voltage gradient between the contacts. Using this property of ac circuits, the arc can be extinguished easily when the current becomes zero.
For such conditions to prevail, the dielectric strength of the medium surrounding the ionized particle has to be improved. It is possible by either cooling or sweeping out the ionized particles and deionizing the medium.
Moreover, if the dielectric strength of the medium increases more rapidly than the potential difference between the contacts, then the current can be interrupted and arc can be extinguished. The different methods for deionization of the medium for extinguishing arc are,
- Increasing the Length of the Gap - The dielectric strength of the medium can be increased by increasing the length of the arc as the dielectric strength ∈ is directly proportional to the length l i.e., ∈ ∝ l. Once the dielectric strength of the medium is increased, the arc fails to restrike again and the flow of current is interrupted, thus extinguishing the arc.
- Cooling - During cooling the ionized particles are deionized into neutral molecules. By means of cooling the dielectric strength of the medium is increased and the arc is extinguished.
- High Pressure - If the pressure near the contacts is maintained at a high value, it helps in the rapid deionization of the ionized particles to neutral molecules thereby increasing the dielectric strength of the medium.
- Blast Effect - In this type of effect either high-pressure oil is forced to pass through the arc gap or gas is used in order to sweep away the ionized particles and improve the dielectric strength of the medium.
Important Terms :
The arc formed between the contacts is nothing but an electric discharge. In the circuit breaker, the arc is formed when the contacts are opened. The circuit breaker used in the system from the protection of fault currents should have the capability of extinguishing the arc without getting damaged. For this purpose, some terms are used for the analysis of the circuit breaker.
Arc Voltage :
Arc voltage is the voltage across the contacts of the circuit breaker during the arcing period.
Restriking Voltage :
When the circuit breaker interrupts at the time of current zero, there will be a rapid increase in the voltage across the circuit breaker contacts. The voltage that appears will be transient in nature and it is known as Restriking Voltage which is shown below.
Recovery Voltage :
The voltage that appears across the contacts after the final arc extinction and after all the transients disappears is called Recovery Voltage. The recovery voltage is of normal frequency (50Hz) and is approximately equal to the system voltage as shown above.