What is circuit Breaker ?

What is Circuit breaker ?A circuit breaker is an electrical switches operate automatically which is used to protect an electrical circuit from damage due to  excessive current flowing through it when the circuit is overload or short circuit.Basic function  of circuit breaker is to interrupt current flow  when the fault is detected

An electrical CB is an electrical switches which can be operated automatically or  manually for controlling and protecting the electrical circuits.

CB can be reset manually or automatically to resume the normal operation but a fuse  cannot reset and must be replace.

circuit breaker

Working Principle of Circuit Breaker

Circuit  breaker  consist  of  fixed and  moving  contacts Under normal condition these contacts touch each other and carry current with the rated value for which it is design.When the circuit breaker closed the current carrying contacts which is called electrodes engage each other under the pressure of a spring

Whenever a fault occurs at any part of the circuit, the current flow much in the tripping coil due to which the tripping  coil of the breaker gets energized and  as a result moving contacts are getting apart from fixed contacts  by some mechanism, thus opening the circuit.

Types of Circuit Breaker

Circuit breaker are mainly classified on the basis of  rated voltage .Circuit breaker below rated voltage of 1000v are known as low voltage circuit breaker and above 1000v are known as high voltage circuit breaker.

On the basis of  Arc extinction  circuit breaker are classified in to the following types .

 Oil circuit breaker

In this type of circuit breaker oil is used as a arc quenching medium as well as insulating medium, but mineral oil is preferable. Oil has better insulating property than air.

The moving contact and fixed contact are immerged inside the insulating oil with a seal packed chamber. When the separation of the contacts take place, then the arc in circuit breaker is initialized at the moment of separation of contacts, and due to this  arc in the oil  vaporization occur  and decomposed in hydrogen gas and finally creates a hydrogen bubble around the arc.

Due to this highly compressed gas bubble around and arc prevents re-striking of the arc after current reaches zero crossing of the cycle. It is the old type of circuit breakers.

Oil Circuit Breaker

Oil circuit breaker are futher classified in to two types

  • Bulk oil circuit breaker
  • Minimum oil circuit breaker

Bulk Oil Circuit Breaker (BOCB)

In the BOCB, oil is used as a arc the quenching medium and it is also used as insulating medium in between earth parts of circuit breaker and current carrying contacts. It is same as  transformer insulating oil is used.

The working principle of the BOCB says that when a current carrying contacts of CB which is immerse  in the oil are separated, then an arc is generated between the separated contacts.

Due to this arc which  will produce rapid growing gas bubble around the arc. The moving contacts move apart from the fixed contact of arc and as as result the resistance of the arc gets increased. Due to this  increasing  resistance the temperature become lower. Hence the reduced formations of gasses surround the arc.

When the current passes through zero crossing the arc quenching in the BOCB takes places. In the totally air tight If the vessel is totally air tight then  the gas bubble remain enclosed inside the oil. The oil surround with high pressure on the bubble, due to which highly compressed gas around the arc.

If the pressure of oil is increased the de- ionization of the gas also increases, which results in arc quenching. The hydrogen gas is used as cooling medium in arc quenching in the oil circuit breaker.

  • Its has good cooling property due to decomposition
  • The oil absorb arc energy while decomposing
  • Oil has high dielectric strength
  • It acts like an insulator between earth and live parts.
  • It will not permit high speed of interruption
  • It takes long arcing time.

Minimum Oil Circuit Breaker

In this type oc CB  oil is used as the interrupting medium. The minimum oil circuit breaker will place the interrupting unit in an insulating chamber at the live potential. But insulating material is available in interrupting chamber. It requires less amount of oil therefore it is called minimum oil circuit breaker.


  • It is suitable for both automatic operation and manual.
  • It requires less maintenance.
  • It requires smaller space


  • Oil deteriorates because of carbonization.
  • The cost for breaking capacity in MVA is also less.
  • There is a possibility of explosion and fire
  • As it has a smaller quantity of oil, so carbonization increases.
  • It is very difficult to remove gases from the space between the contacts.


Air Circuit Breaker

This circuit breaker will operate in the air means the arc  quenching medium is atmospheric pressure. Because of better insulating properties of oil in many countries the air circuit breaker is replaced by oil circuit breaker.

 The importance of ACB is still preferable choice to use an Air circuit breaker up to 15KV. Because the oil circuit breaker may catch fire when used at 15V.

Air Blast CB

Plain Air Circuit Breaker

Plain air circuit breaker which is also called Cross-Blast Circuit Breaker. In this typ , the circuit breaker is fitted along  a chamber which is  surrounded the contacts. This chamber is known  as arc chute.

The arc chute is made of  refractory material. The walls of  arc chute  inside are shaped in such a way that arc is not forced into close proximity. It will drive into the winding channel which is projected on an arc chute wall.

The arc chute will have a lot of small compartments and has many divisions which are metallic separated plates. Each of small compartments behaves as a mini arc chute and metallic separation plate behave like a arc splitter.

 Because of all the arc voltages will be higher than the system voltage therefore the arc will split into a series of arcs. It is only preferable for low voltage application.

Air Blast Circuit Breaker

This type of CB is used for high voltage  245 KV, 420 KV and also even more.Air blast circuit breakers are of two types:

  • Axial blast breaker
  • Axial blast with sliding moving contact.

Axial Blast Breaker

In this type of CB the moving contact of the axial blast breaker will be in contact. The nozzle orifice is a fixed to the contact of a breaker at a normal closed condition. when high pressure is introduced into the chamber, a fault occur.. To sustain high-pressure air votage is sufficient when flowed through nozzle orifice.

Advantages of ACB
  • Because of lesser arc energy it is used where frequent operation is required.
  • It is risk free from fire as compare to OCB
  • It is Small in size.
  • ABCB requires less maintenance.
  • Much faster arc quenching
  • The CB has high speed .
  • For all value of current the time duration of arc is same .
Disadvantages of ACB
  • It requires additional maintenance which increase the cost.
  • Arc extinguishing properties of air is low
  • It has high capacity air compressor.
  • There may be a chance of air pressure leakage from the pipe junctions.
  • There is the chance of a high rate rise of re-striking current and voltage chopping.

Application and Uses of ACB

  • It  is used for plant protection,transformer capacitors , electrical machines and generators.
  • It is used in Low as well as High Currents and voltage applications.
  • It is  also used in the Electricity sharing system and GND about 15Kv

SF6 Circuit Breaker

In this type of CB the current carrying contacts operate inside  sulphur hexafluoride gas is known as an SF6 circuit breaker. SF6 gas has excellent insulating property and high electro-negativity. It has high affinity of absorbing free electron. when a free electron collides with the SF6 gas molecule , negative ion is formed ; it is absorbed by that gas molecule.

SF6 + e = SF6
SF6 + e = SF5- + F

The negative ions which are formed will be much heavier than a free electron. Therefore, overall mobility of the charged particle in the SF6 gas is much less then the other gases.. The conducting current through a gas is mainly due to mobility of charge particles. Therefore for heavier and less mobile charged particles in SF6 gas, required very high dielectric strength. SF6 has very good heat transfer property due to low gaseous viscosity. SF6 is 100 times more effective as a arc quenching medium than air circuit breaker. SF6 gas is used for both medium and high voltage electrical power system from 33KV to 800KV.

Types of SF6 Circuit Breaker

  • Single interrupter SF6 CB which is applied up to 220
  • Two interrupter SF6 CB which is  applied up to 400
  • Four interrupter SF6 CB which is applied up to 715V

Vacuum Circuit Breaker

In vacuum CB  vacuum is used to extinct the arc. Vacuum has dielectric recovery characteristic, excellent interruption and can interrupt the high frequency current which results from arc instability, superimposed on it. The principle of operation of VCB is that the two contacts called electrodes will remain closed inside the vacuum chamber under normal operating conditions. Suppose when a fault occurs in any section of circuit, then the trip coil of the CB gets energized and due to which contact gets separated.

The contacts of the CB are opened in vacuum within  10-7 to 10-5 Torr an arc is produced between the contacts by the ionization of metal vapors of contacts. The arc extinguished quickly due to the metallic vapors, electrons and ions which is produced during the arc, which is condense quickly on the surface of the CB contacts, resulting in quick recovery of dielectric strength.

Vacuum CB


  • Vacuum circuit breake are  more reliable, compact and long life
  • VCB can interrupt any fault current.
  • In VCB there will be no fire hazards.
  • There is no noise during operation
  • Vacuum has higher dielectric strength.
  • VCB has  required less power for control operation.

What is MCB,MCCB ,ELCB and RCCB?


MCB (Miniature Circuit Breaker)

The current rating of this circuit breaker is less then 100A and has only one over current protection built within it. In this type the tripping characteristic are normally not adjustable It has thermal or magnetic operation for contact opening and closing  

Minature CB

MCCB (Molded case Circuit Breaker)

The current rating of this type of breaker is  up to 1000A.The triping current may be adjustable to the desired current rating .It has thermal or  thermal-magnetic operation for contact opening and closing .MCCB have earth fault protection along with current protection.

Molded case CB

RCD or RCCB ( Residual current Device or Residual current circuit breaker )

RCCB Trip the circuit whenthere is earth fault current .Phase and neutral both wire are connected through RCCB .The amount of current through phase wire should return through neutral wire .

The most widely used are 30 mA and 100 mA devices’ 300/500 mA may be used where fire protection is required in electric circuit where the risk of electric is small.

A specially modified RCCBs is available which will operate on normal AC and pulsating DC.RCCB don’t offer protection against current overload..RCD detect an imbalance  in the live and neutral current.

residual Current circuit breaker

ELCB ( Earth leakage circuit Breaker )

In this type the phase ,neutral and earth wire is connected to the ELCB.Its work on the base of earth leakage current.

Earth leakage circuit breaker

The safest limit of current which human body can withstand is 30 mA sec.Suppose human body resistance is 500 ohms and voltage to ground is 230 volt the the body current will be 500/230 =460 mA,Hence ELCB must be operated in  30mA/460mA=0.65 mA.

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