It is a device designed to close/make or open/break the electrical circuit under
abnormal and normal conditions automatically and also manually.
Duties of a circuit breaker are:
It carries the full load current continuously without overheating and damage.
It opens and closes the circuit on load.
It makes and breaks the normal operating current of magnitude up to which it
is designed for (automatically).
It makes and breaks the circuit the normal operating current (manually for
How does a circuit breaker operate automatically?
Insulating Fluids commonly used for circuit breakers are:
Air at atmospheric pressure
Oil producing Hydrogen for Arc Extinction
Ultra high Vacuum
Sulphur Hexa Floride
Arc Resistance depends on factors like:
Degree of Ionization
Length of the arc
Cross section of arc
Methods of Arc Quenching:
1) High Resistance Method:
Arc resistance is made to increase with time so that current is
reduced to a value insufficient to maintain the arc.
2) Low Resistance Method:
Dielectric Medium between the contacts is built up more rapidly
so that the arc fails to restrike and the current will be
High Resistance Method:
Lengthening of arc
Reducing the cross section of the arc
Splitting the arc
Low resistance Method:
Lengthening the gap
High pressure in the vicinity of the arc
Cooling the dielectric medium
Significant Terms associated with Circuit Breakers:
Short Time Capacity
Pros of Oil Circuit Breakers:
It absorbs the arc energy to decompose the oil into gasses which
have excellent cooling properties.
It acts as an insulator and permits smaller clearance between live
conductors and earthed components.
Cons of Oil Circuit Breakers:
Oil is inflammable there is a risk of fire.
Oil may form an explosive mixture when it comes in contact with
Arcing products like carbon remain in the oil and its quality
deteriorates with successive operations. This requires periodic
checking and replacement of oil.
Rating: Bulk Oil CB= Up to 12 KV
Minimum Oil CB= 13.6 KV to 146 KV
Pros of Air Blast Circuit Breakers:
Risk of fire is totally eliminated.
The size of the circuit breaker is reduced.
High speed operation and suitable for frequent operation.
Cons of Air Blast Circuit Breakers:
Air insulation is lesser than other medium like as oil etc.
Considerable Maintenance required for the compressor plant
which supplies air blast.
Rating: 110 KV & above
Pros of SF6 Circuit Breakers:
Very Sort Arcing Time
Can Interrupt very large currents due to high dielectric strength.
Noiseless Operation due to its closed/sealed gas circuit.
No problem of moisture.
No risk of fire as SF6 is inflammable.
Low Maintenance Cost.
Cons of SF6 Circuit Breakers:
SF6 Circuit Breakers are costly due to high cost of SF6.
Since SF6 gas has to be reconditioned after every operation of the
breaker additional equipment is needed for this purpose.
Rating: 3.6 KV to 760 KV
Advantages of VCB:
Compact & reliable.
Less Maintenance and quite in
Withstand Lightning Surges.
Small Power for control
Perfect breaking of any heavy
No risk of Fire.
Routine tests conducted on VCB:
Voltage Test with different
High Voltage Test
Therefore by using appropriate circuit breaker for appropriate
operating voltages we can provide protection for busbar and
electrical equipments like Power transformers, generators, C.T.’s ,
P.T.’s, wave traps etc. from different faults that occur in power system.