In Oil Circuit Breakers, mineral oil is used for arc extinction. Mineral oil has good insulating properties. These are the oldest type of circuit breakers. Oil circuit breakers employ between 25 MVA at 2.5 KV and 5000 MVA at 250 KVA. In such circuit breakers, the contacts (fixed contact and moving contact) are made to separate within insulating oil. The oil has better insulating properties than air.

Oil Circuit Breakers
Oil Circuit Breaker

During Operation, when contacts begin to separate, an arc is struck between them. This heats up the oil. The heat of the oil decomposes the oil, which boils at 658ko K. At this instant, the hydrogen, acetylene, methane and ethylene gases are liberated at high pressure. The hydrogen gas occupies a volume about one thousand times that of the oil decomposed. The heat conductivity of hydrogen gas is very high.

The arc extinction is facilitated by two processes. Firstly, due to the high conductivity of hydrogen gas, it cools the arc, thus aiding the de-ionization of the medium between the contacts. Secondly, the gas sets up turbulence in the oil and forces it into the space between contacts, thus arcing products from the arc path are eliminated.

Types of Oil Circuit Breaker

These circuit breakers are extensively used in the power system, due to reliability, simplicity and low cost. These are classified into the following types:

Bulk Oil Circuit Breakers

These circuit breakers use large quantity of insulating (mineral) oil for the purpose of arc extinction. An insulating oil, insulates the current conducting parts of the circuit breakers from one another and from the earthed tank.

Minimum Oil Circuit Breakers

Minimum oil circuit breaker use minimum oil for their operation. This circuit breaker also known as Low oil circuit breaker.


  • It has high dielectric strength.
  • It absorbs arc energy while decomposing.
  • It acts as an insulator between live parts and earth.


  • It is easily inflammable.
  • It can cause an explosion by mixing with air.
  • It requires maintenance.
  • It requires periodic maintenance.