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Oil Circuit Breaker

Oil circuit breaker is such type of circuit breaker which used oil as a dielectric

or insulating medium for arc extinction. In oil circuit breaker the contacts of

the breaker are made to separate within an insulating oil. When the fault occurs

in the system the contacts of the circuit breaker are open under the insulating oil,

and an arc is developed between them and the heat of the arc is evaporated in

the surrounding oil. The oil circuit breaker is divided into two categories

  • Bulk Oil Circuit Breaker
  • Low Oil Circuit Breaker

Construction of Oil Circuit Breaker

Oil circuit breaker is very easy in construction. It consists of current carrying

contacts enclosed in a strong, weather-tight earth metal tank and the tank is

filled with transformer oil. The oil is both acts as an arc extinguishing medium

and as an insulator between the live part and earth.

At the top of the oil, air is filled in the tank which acts as a cushion to control

the displaced oil on the formation of gas around the arc and also to absorb

the mechanical shock of the upward movement of oil. The breaker tank is

securely bolted for carrying out the vibration caused on interrupting

very high current. Oil circuit breaker consists gas outlet which is fitted in the

tank cover for the removal of the gases.


Working Principle of Oil Circuit Breaker

During the normal operating conditions, the contact of the oil circuit breaker

is closed and carry the current. When the fault occurs in the system, the contacts

of the breaker are moving apart, and an arc is struck between the contacts.

Due to this arc, a large amount of heat is liberated, and a very high temperature

is reached which vaporises the surrounding oil into gas. The gas, thus liberated

surrounds the arc and its explosive growth around it displace the oil violently.

The arc is extinguished when the distance between the fixed and moving contact

reaches a certain critical value, depends on the arc current and recovery voltage.


The oil circuit breaker is very reliable in operation, and it is very cheap. The most

important feature of oil circuit breaker is that no special devices are used for controlling

the arc caused by moving contact. The oil as an arc quenching medium has certain

advantages and disadvantages

Advantages of Oil as an Arc Quenching

  1. The oil has a high dielectric strength and provides insulation between the contact after the arc has been extinguished.
  2. The oil used in circuit breaker provides a small clearance between the conductors and the earth components.
  3. The hydrogen gas is formed in the tank which has a high diffusion rate and good cooling properties.

Disadvantages of Oil as an Arc Quenching

  1. The oil used in oil circuit breaker is inflammable and hence, cause a fire hazard.
  2. There is a risk of formation of explosive mixture with air.
  3. Due to decomposition of oil in the arc, the carbon particles is generated which polluted the oil and hence the dielectric strength of the oil decreases.

Maintenance of oil circuit breaker

After a circuit breaker has interrupted by short-circuit current, sometimes their

contacts may get burnt due to arcing. Also, the dielectric oil gets carbonized in

the area of the contacts, thereby losing its dielectric strength. This results in

the reduced breaking capacity of the breaker. Therefore, the maintenance of

oil circuit breaker is essential for checking and replacement of oil and contacts.


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