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

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Minimum Oil Circuit Breaker (MOCB)
The minimum oil circuit breaker (MOCB) also known as the low oil circuit breaker is a major type of oil circuit breaker that uses oil only for extinguishing the arc. On the contrary, BOCB uses it for insulating the live part from earth parts as well. The main advantage of using less oil is to reduce fire hazards. It does not act as insulation between the live and earthed parts.

The MOCB has two chambers; the arc chamber and the supporting chamber. The arc chamber includes fixed and moving contact filled with oil. This chamber is used for extinguishing the arc. It is made from porcelain with bakelised paper encasing. It also includes vents for gas release at the top.
The supporting chamber provides support to the arc chamber. It is made of porcelain filled with oil for insulation. It insulates the arc chamber that is mounted on top of it. It includes an arm operated by a mechanism to move the moving contact upward and downward using a rod. The moving contact is also attached to a piston that develops the pressure necessary for arc quenching. Therefore, MOCB is a type of Forced Blast oil circuit breaker where the pressure is developed by external means independent of the arc current.
The working of MOCB is simple. Under fault conditions, the arm pulls the moving contact downward separating the contact. An arc is generated between the contacts. the arc energy vaporizes the oil generating necessary pressure. The pressure is confined to the arc chamber as it is blocked by the moving contact. As soon as the moving contacts unblock the hole, cool oil rushes in and the gases flow out through the vent.
The gas vents of the arcing chamber can be designed in one of the two following ways, therefore, MOCB is divided into two types.
Axial Venting MOCB
The following figure shows axial venting MOCB where the arc chamber vents are designed is such a way that when the contacts separate. The cool oil enters through the path unblocked by the moving contact. The arc is swept in an axial direction through the upper vent.

The axial venting generates very high pressure thus having very high de-ionization capabilities and consequently high dielectric strength of the oil. It is used for low arc current at high voltages.