Oil Circuit Breaker (ocb) – Types, Construction, Working and Applications ocb – Oil Circuit Breaker – Construction and Working Principle

Types of Oil Circuit Breaker
The oil circuit breaker can be classified into the following two main types

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  Bulk Oil Circuit Breaker (BOCB)
  
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  Minimum Oil Circuit Breaker (MOCB)
  

Bulk Oil Circuit Breaker- Such type of oil circuit breaker uses a large quantity of insulating oil. The oil is used for arc quenching as well as insulating the live parts from the earth parts of the breaker.
Minimum Oil Circuit Breaker- Such type of oil circuit breaker uses a small amount of oil only for arc quenching.
Bulk Oil Circuit Breaker (BOCB)
Bulk oil circuit breaker uses a large quantity of oil. The oil used serves two purposes. It extinguishes the arc when the contacts separate and it also insulates the live current-carrying parts from the earthed parts of the circuit breaker.
BOCB can be classified into the following two types

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  Plain Break Oil Circuit Breaker
  
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  Arc Control Oil Circuit Breaker
  

Plain Break Oil Circuit Breaker- Such BOCB does not have any control over the arc except for increasing the arc length by separating the contacts. they are fairly simple and easier to design
Arc Control Oil Circuit Breaker- Such BOCB has a special system for arc control that efficiently quenches the arc.
Plain Break Oil Circuit Breaker
Such BOCB is the simplest and oldest form of oil circuit breakers. There is no arc control system. Its operation is very simple. The current-carrying contacts are separated inside oil where the length of the arc increases with separation between the contacts. The arc quenches at the first zero current crossing.
Construction- Its construction is quite simple. It has a tank made from strong metal that contains the insulating transformer oil. The current-carrying contacts are submerged in the oil. The body of the tank is earthed. The oil acts as both an arc quenching medium as well as an insulator between the live contacts and earthed body. The tank is not completely filled with oil but some space is left for air that acts as a cushion for the displaced oil during oil vaporization. It absorbs the pressure released from the gas. The tank also includes vents for the gas outlet.

Under fault conditions, the moving contact moves downward to separate from fixed contacts. During the contact separation, the separation gap increases. due to the high voltage difference, the oil starts to ionize and an arc is struck between the contacts.
The heat of the arc vaporizes the oil between the contacts and converts it into mostly hydrogen gas. The gas forms a bubble surrounding the contacts which increase in volume very rapidly. The gas bubble displaces the oil which is absorbed by the air cushion. The surrounding oil compresses the gas bubble that de-ionizes the medium. The dielectric strength of the medium between the contacts increases. The contact separation also increases the arc length increasing the medium dielectric strength. The arc is quenched at the next current zero-crossing.
The main disadvantage of a plain break oil circuit breaker is that it does not have arc control and it relies on contact separation that increases the arc length. It is inconsistent and takes longer arcing times. The arc is quenched when the contacts are fully separated. Due to these limitations, a plain break oil circuit breaker is used for low voltage 11kV systems.