Air Blast Circuit Breakers Advantages Disadvantages

In air blast circuit breakers, compressed air at a pressure of 20-30 kg/cm² is employed as an arc quenching medium. Air blast circuit breakers are suitable for operating voltage of 132kV and above. Switching resistors and equalizing capacitors are generally connected across the interrupters. The switching resistors reduce the transient over voltages and helps arc interruption. Capacitors are employed to equalize the voltage across the break. The number of breaks depends on the system voltage (eg: for 132kV 2 to 4 breaks and for 220kV 2 to 6 breaks)

 Some of the advantages and disadvantages of air blast circuit breakers are discussed below

Advantages of Air Blast Circuit Breaker:

• Cheapness and free availability of the interrupting medium, chemically stable and inertness of air
• Air blast circuit breakers have advantage of high speed operation
• In air blast circuit breaker fire hazard is eliminated unlike in oil circuit breakers
• Short and consistent arcing time and therefore, less burning of contacts
• Air blast circuit breakers require less maintenance
• They are suitability for frequent operation
• Facility for high speed re-closure

Disadvantages of Air Blast Circuit Breaker:

• An air compressor plant has to be installed and maintained
• In air blast circuit breaker upon arc interruption, the air blast circuit breaker produces a high level of noise when air is discharged to the open atmosphere
• In air blast circuit breaker current chopping problem exists
• There is a problem of re-striking voltage

Circuit Breakers Interview Questions Answers

What is meant by Current Chopping?

Answer: Current Chopping is a phenomenon of current interruption before the natural current zero is reached.

Why Isolation switches are provided along with the circuit breakers?

Answer: Isolation switches are provided for separating the disconnected element from the live portion of the system for the purpose of safety. 

Why asymmetrical breaking current is higher than the symmetrical breaking current in a circuit breaker?

Answer: Asymmetrical breaking current is the rms value of the combined sum of the dc and ac components of the short circuit current at the instant of separation of the breaker contacts while symmetrical breaking current is the rms value of the ac component alone. Thus asymmetrical breaking current is higher than the symmetrical breaking current. 

What is recovery voltage in circuit breaker?

Answer: Recovery voltage in circuit breakers is defined as the power frequency rms voltage which appears across the breaker contacts after the transient oscillations die out and the final extinction of the arc has resulted in all the poles of the circuit breaker.

It is the voltage developed after the arc extinction is competed 

When does an arc extincts?

Answer: If the rate of rise of re-striking voltage (RRRV) is less than rate of rise of dielectric strength between the breaker contacts  arc will not sustain and extincts

What is re-striking voltage?

Answer: The Re-striking voltage may be defined as the resultant transient voltage which appears across the breaker contacts at the instant of arc extinction

For an Resistive Load and Inductive load considered for switching operation of circuit breaker, Switching of which load is difficult and Why?

Answer: In case of switching resistive load, current and voltage will be in phase i.e,when during current zero voltage also will be zero in case of resistive load. Hence the voltage at the breaker terminals during arc extinction is zero or minimal. On the other hand, for switching inductive load during current zero voltage across the breaker will be maximum. This results in re-striking of arc once again. Therefore switching inductive load will be difficult