Oil Circuit Breaker


Oil Circuit Breaker

Mineral oil has better insulating property than air. In this breaker oil is use for quenching the arc. The circuit breaker which uses more oil or which is bulky is called bulk oil circuit breaker. The construction is simple and it consists of fixed and moving contacts enclosed in a strong weather and tight earthed tank containing oil up to a certain level and an air cushion above the oil level.


The fixed and moving contacts are closed. On the occurrence of fault.  when the moving contacts open the arc is struck between the contacts. due to high current rating.  Oil between the contacts gets decomposed and hydrogen gas bubble is formed around the contacts. The hydrogen gas cools the arc. The De-ionization of medium between contacts takes place and at some critical length of gap between the contacts ,the arc is extinguished.


  • oil has high dielectric strength.
  • Absorb the arc energy to decompose the oil into gases which have excellent cooling properties.
  • It acts as an insulator between the live parts and earth


  • hydrogen is inflammable and there is a risk of a fire.
  • it is necessary periodic checking and replacement of oil.

Types of Oil Circuit Breakers

  • Bulk oil circuit breakers

which use a large quantity of oil. The oil is used for quenching the arc during opening of contacts and  isolate the current conducting parts from one another and from the earthed tank.

plain-break oil circuit breaker

A plain-break oil circuit breaker is the simple process of separating the contacts under the whole of the oil in the tank. There is no special system for arc control other than the increase in length caused by the separation of contacts. The arc extinction occurs when a certain critical gap between the contacts is reached.

The air provides sufficient space to allow for the production of the arc gases without the generation of unusual pressure in the circuit breaker. It also absorbs the mechanical shock of the upward oil movement.

Bulk oil circuit breaker


  • Oil absorbs arc energy while decomposing.
  • Good cooling property of the gas formed due to decomposition.


  • Long arcing time.
  • Arc interruption control can be obtained only by increasing the length of arc.
  • Do not permit high speed of interruption.


  • Minimum oil circuit breaker

In this breaker, the supporting current interruption and top chamber are made of porcelain. Hence, clearance between live parts is small and requires less quantity of oil, hence the breaker is called Minimum Oil Circuit Breaker.

The chambers are completely filled with oil. The oil from upper chamber does not come to lower. The fixed contact is enclosed in the quenching chamber. Moving contact makes sliding contact with the lower fixed contact.

The operating rod is operated by operating mechanism, the three poles operate simultaneously. The voltage ratings are from 3.6kV to 420 KV.

minimum oil circuit breakerWorking

Under normal conditions, the moving contact is connected with the upper fixed contact. When a fault occurs, the contacts are separated in oil by the tripping springs and arc is formed. The arc energy vaporize the oil and produces gases under high pressure. This action constrains the oil to pass through a central hole in the moving contact and results in force of oil through the respective passages of the tabulator. The process of tabulation is orderly one, in which the sections of the arc are successively quenched by the effect of separate streams of oil moving across each section in turn and cooling its gases.

The features of designing Minimum Oil Circuit Breaker is to reduce requirement of oil, and hence these breaker are called minimum oil circuit breaker. These designs are available in voltages ranging from 1 kV to 765 kV using the multi-break technique.These types of circuit breaker are available up to 8000 MVA at 245 KV.


  • Require less quantity of oil compare to Bulk oil circuit breaker.
  • Cost per breaking capacity in MVA is less.
  • Requires smaller space.
  • Maintenance is less.
  • Suitable for both manual and automatic operation.


  • Possibility of fire and explosion.
  • Smaller quantity of oil, so carbonization increases.
  • Difficult to remove gases from the space between contacts.
  • Oil deteriorates rapidly due to carbonization.


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