Minimum Oil Circuit Breaker (MOCB) - Construction, Working & Comparison

The minimum oil circuit breaker is also called a small or poor oil circuit breaker. It required a small percentage of oil (about 10% of oil) used in the case of the bulk oil circuit breaker. The oil is mainly used for the arc extinction process only. But in the case of bulk oil circuit breakers, the oil serves two purposes i.e., as a quenching medium and also used as an insulating medium between the parts.


Construction of Minimum Oil Circuit Breaker :

The construction of a minimum oil circuit breaker (MOCB) is simple which consists of two oil-filled chambers separated from each other. The two chambers are named supporting and circuit breaking chambers.


The circuit breaking chamber is a porcelain enclosure filled with oil which is responsible for quenching the arc at the time of circuit breaking. It consists of an arc control device, upper and lower fixed contacts. The lower fixed contact is ring-shaped, which consists of moving contact such that it can slide through the ring-shaped lower contact. There exists a continuous electrical contact between moving contact and lower fixed contact. The terminals of the circuit breaker are taken out through upper and lower fixed contacts.


The arc extinction process is carried in the arc control device which is fixed to the upper fixed contact. The arc control device consists of axial (for interrupting low currents) and radial (for interrupting high currents) vents. Hence, the circuit breaking chamber is also called an arc extinction chamber. The below shows the constructional diagram of a single-phase minimum oil circuit breaker.

Minimum Oil Circuit Breaker (MOCB)

The supporting chamber is also a porcelain enclosure filled with oil. There is no physical contact between the oils present in the two chambers (supporting and circuit breaking chambers). Also, the oil in the supporting chamber does not involve in the arc extinction process. This oil is used only for providing insulation. Hence the supporting chamber is also known as the dielectric supporting chamber.


The moving contact is made to move through the supporting chamber with an operating rod fixed to it. The operating rod provides vertical movement to the moving contact for making and breaking the circuit. Both supporting and circuit breaking chambers are separately enclosed chambers with cylindrical-shaped synthetic resin bonded papers within the porcelain insulators.

Minimum Oil Circuit Breaker (MOCB)

Working of Minimum Oil Circuit Breaker :

Under normal operating conditions the moving contact remains in physical (or electrical) contact with the upper fixed contact. Under abnormal conditions i.e., when there is a fault, the moving contact is pulled down by the operating rod which is controlled by the operating mechanism. When the moving contact starts separating an arc is drawn between the two contacts.


As the contacts are immersed in the oil, the arc energy increases the temperature of the oil. The oil surrounding the arc attains high temperatures and gets vaporized. Thus gases are produced and will expand inside the chamber so that pressure inside the chamber increases. The produced gases move upward by decreasing the arc energy (splitting the arc) and cooling the contacts.


When the gases move upwards, the oil surrounding the arc is filled with fresh oil. Again due to arc energy this oil will decomposes by realizing gases. This again increases the pressure inside the chamber and the gases move upward, by replacing it with fresh oil again.


This process of replacing the fresh oil between the contacts will continue until the arc is extinguished completely which occurs at current zero instant. But in order to avoid restriking of the arc after current zero, the dielectric strength of the medium (oil) should be maintained high. This can be achieved by following methods,

  • The oil pressure inside the chamber should be maintained high such that the fresh oil gets pushed between the contacts and gases move upwards.
  • Forcing fresh oil onto the arc with the help of piston action produced by the operating rod fixed to the moving contact.

Comparison Between Bulk Oil and Minimum Oil Circuit Breakers :

Bulk Oil Circuit Breaker (BOCB)Minimum Oil Circuit Breaker (MOCB)
The quantity of oil required is more.Only 10% of oil quantity is required.
If requires more space area.It requires less space area.
Because of its large size, it is difficult for transportation.Easy for transportation.
As the system voltage increases the maintenance cost also increases.Low maintenance cost.
Production of carbonization is less.The degree of carbonization produced is more severe.
More suitable for frequent operation.Less suitable for frequent operation.
It is easy to remove the gases from the contact space in time.Difficult to remove the gases from the contact space in time.
This type of circuit breaker is used up to 33kV.These can be used up to 132kV.

Advantages of Minimum Oil Circuit Breaker :

A minimum oil circuit breaker is an improvement over the bulk oil circuit breaker due to the following reasons,
  • Minimum Oil Circuit Breaker (MOCB) requires a smaller quantity of oil when compared with that required for Bulk Oil Circuit Breaker (BOCB).
  • The fire and explosion hazards are comparatively lesser in MOCB.
  • The weight of MOCB is appreciably lesser than BOCB due to the reduced amount of steel required for the tank.
  • The size of MOCB is quite smaller.
  • Access to the contacts is easier in MOCB.
  • MOCBs are suitable for both indoor and outdoor installations while BOCBs are suitable only for outdoor installations.
  • The problems associated with the storage of a large amount of high-grade mineral oil (or transformer oil) in the generating stations and substations are reduced with MOCBs.
  • For the same rating of circuit breakers, MOCB costs less.

Disadvantages of Minimum Oil Circuit Breaker :

A minimum oil circuit breaker has the following disadvantages as compared to the bulk oil circuit breaker,
  • The degree of carbonization is increased.
  • There is difficulty in removing the gases from the contact space in time.
  • The dielectric strength of the oil deteriorates rapidly due to a high degree of carbonization.


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