What is an Oil Circuit Breaker : Working & Its Types

In the power system, an oil circuit breaker is an essential device because it will make the system strong. These are classified into different types based on the medium used to reduce the arc. An oil circuit breaker is one kind of circuit breaker where the insulating oil can be used as an arc quenching medium. Once the fault occurs within the system, then circuit breaker contacts will be separated and the arc will be struck among the contacts. This article discusses an overview of the oil circuit breaker and its working.

What is an Oil Circuit Breaker?

Definition: The oldest type of circuit breaker is an oil circuit breaker. It includes separate contacts and the main function of these contacts is to separate an insulating oil. It has good insulating properties compare with air. When the fault occurs then the contacts of the breaker will open beneath the oil. Once the arc is struck among the two contacts of the breaker then the heat of the arc will dissolve the surrounding oil & separates into a significant volume of gaseous hydrogen at high pressure. The main features of this circuit breaker are low cost, reliability, and simplicity.

Circuit Breaker
Circuit Breaker

Different Types

The types of oil circuit breakers include the following.

Bulk Oil Circuit Breakers

This kind of circuit breaker uses a large amount of oil for the destruction of arc. It is also called a dead tank circuit breaker. As the name suggests, the tank of this breaker is held at ground potential. The amount of oil used in this circuit breaker mainly depends on the system voltage. For instance, if the system voltage is 110 kV, it uses 8 kg to 10,000 kg of oil. Similarly, for 220 kV, it uses 50,000 Kg of oil. In this type of circuit, oil works like an arc extinguishing medium & insulates the existing parts from the earth.

Minimum Oil Circuit Breaker

This type of circuit breaker uses less oil. The oil tank is placed on a porcelain insulator for insulating it from the ground. It includes an arc chamber that is enclosed within a bakelite paper. This circuit breaker includes two portions; the upper portion is porcelain that is enclosed with contacts whereas the lower portion is supported through the porcelain

It uses less space as compared with bulk oil type. It is not used where the repeated operation is necessary. The main benefits of this circuit breaker are, it uses less oil, less space, less weight, tank size is small, less maintenance, etc.

Construction of Oil Circuit Breaker

The construction of this kind of circuit breaker is simple. It includes current-carrying contacts that are surrounded in a strong and metal tank. Here the tank is loaded with transformer oil. The transformer oil works as an insulator & arc extinguishing medium among the existing element & earth.

Construction of oil Circuit Breaker
Construction of oil Circuit Breaker

At the peak of the transformer oil, the air can be filled within the tank that acts as a pad to manage the transferred oil on the creation of gas in the region of the arc. It absorbs the mechanical shock of the rising movement of oil. The oil tank in this breaker will be bolted to carry out the vibration that is caused while interrupting the high flow of current. This includes a gas outlet which is fixed in the oil tank cover for the elimination of the gases.

Working Principle

In normal operation of this circuit breaker, the contact in the breaker will be closed as well as carries the current. Once the fault happens within the system, then the contacts will move apart & an arc will be struck among the contacts.

Because of this arc, a huge amount of heat will be released & high temperature can be achieved to vaporize the nearby oil to gas. So this gas will be surrounded by the arc & its unstable increase around it will move the oil violently.

The arc will be turned off once the space between the contacts like fixed & moving arrives at a certain critical value. It mainly depends on the recovery voltage & arc current. The operation of this circuit breaker is extremely reliable & cheap. The main feature of this circuit breaker is, there is no particular devices are used while controlling the arc which is caused by moving contact.

Maintenance of Oil Circuit Breaker

For every type of circuit breaker, maintenance is required. Similarly in oil type also, it is essential to verify and change the oil as well as contacts. Once the short circuit occurs, then a circuit breaker is interrupted. Sometimes, the contacts may get damaged because of arcing. So, the dielectric oil may get carbonized within the region of the contacts then its dielectric strength can be reduced and breaking capacity can be decreased. As a result, the maintenance of the breaker is necessary for verifying, replacing the oil & contacts.

The following points must be check before inspecting the circuit breaker

  • Observe the internal parts & arcing contacts. Once it gets a short circuit then the contacts must be changed.
  • Observe the coil’s dielectric strength
  • The surface of the breaker must be cleaned & eliminate carbon deposits through a dry fabric & strong.
  • Verify the level of oil.
  • Tripping as well as closing mechanism must be checked


The advantages of the oil circuit breaker include the following.

  • It uses less amount of oil
  • Dielectric strength of oil is high
  • Oil in the breaker will absorb the energy of arc when decomposing.
  • Less space
  • Risk of fire can be reduced
  • Maintenance also reduced


The disadvantages of the oil circuit breaker include the following.

  • It uses less quantity of oil so that carbonization amount will be increased
  • Removing of the gases within the contact space is difficult
  • The dielectric strength will declines the oil quickly because of the high quantity of carbonization.
  • Arcing time is high
  • Do not allow interruption with high-speed
  • Controlling of arc interruption can be done based on the length of arc.
  • It can form any volatile mixture through the air.

Thus, this is all about an overview of an oil circuit breaker, working principle, construction, types, advantages, disadvantages, and applications. Here is a question for you, what are the components used in oil circuit breaker?