Oil Circuit Breaker

An oil circuit breaker uses oil as an insulating medium to extinguish electrical arcs. The breaker’s contacts are isolated within an insulating oil. The circuit breaker’s contacts are opened under the insulating oil during a fault, creating an arc that generates heat and is dissipated by the surrounding oil.

Construction of Oil Circuit Breaker

The breaker has current-carrying contacts enclosed in a sturdy and weather-tight tank made of earth metal. The tank is filled with transformer oil, which acts as an arc-extinguishing medium and an insulator between the living part and the earth.


The oil tank has air filled at the top to cushion and control the displaced oil when gas is formed around the arc. It also helps absorb the mechanical shock caused by the upward movement of oil. The breaker tank is securely bolted to withstand the vibrations that occur when high currents are interrupted. The oil circuit breaker (OCB) includes a gas outlet fitted in the tank cover for removing gases.

Working Principle of Oil Circuit Breaker

When an electrical system has a fault condition, the contacts carrying the current are separated while submerged in insulating oil. This separation causes a high voltage difference between the contacts, which ionizes the insulating oil, creating an electrical arc. The arc produces an immense amount of heat, vaporizing the insulating oil that surrounds the contacts. The vaporized oil decomposes into several gases, mostly hydrogen gas and small quantities of methane, ethylene, and acetylene. These decomposed gases form a gas bubble surrounding the contacts.


When hydrogen gas is broken down into atomic form, it releases heat that heats the arc. This, in turn, increases the vaporization of oil. The volume of gas produced is much greater than the oil decomposed, with the gas bubble rapidly expanding due to high temperatures. Because the gas bubble is enclosed by oil, this puts a lot of pressure on it. As pressure increases, the medium surrounding the contacts starts to de-ionize and rapidly increases the dielectric strength of the medium. The arc is ultimately quenched at the following current zero-crossing.

Besides the cooling effect of the oil, gas bubbles also help in arc quenching.

Types of Oil Circuit Breaker

There are two types of Oil circuit breakers.

Bulk Oil Circuit Breaker

Bulk oil circuit breakers (BOCBs) are circuit breakers that use oil as both an arc quenching and insulating medium. This oil is placed between the current-carrying contacts and the earthed parts of the circuit breaker. The oil utilized in BOCBs is the same type used to insulate transformers.

Minimum Oil Circuit Breaker

Oil is used as a medium to break the electrical current in circuit breakers. In minimum oil circuit breakers (MOCBs), the interrupting unit is placed in an insulating chamber at the live potential. These breakers are designed to use less oil and are called “minimum oil circuit breakers.”

There is a classification for BOCB, which can be divided into two types.

  • Plain Break Oil Circuit Breaker
  • Arc Control Oil Circuit Breaker

Plain Break Oil Circuit Breaker- BOCB has no control over the arc except to increase its length by separating the contacts. They are simple and easy to design.

Arc Control Oil Circuit Breaker- BOCB features a unique arc control system that effectively extinguishes arcs.

Advantages of Oil as an Arc Quenching

  1. The oil maintains high dielectric strength, providing insulation between contacts even after the arc is extinguished.
  2. The oil serves as a thin layer of insulation between the conductors and the earth components.
  3. The oil has excellent cooling properties and a high diffusion rate of hydrogen gas that forms inside the tank.

Disadvantages of Oil as an Arc Quenching

  1. The oil that is utilized in the circuit breakers has a tendency to catch fire easily, which can create a hazardous situation.
  2. There is a potential danger of creating an explosive mixture when combined with air.
  3. When oil decomposes in the arc, it generates carbon particles that pollute the oil, reducing its dielectric strength.

Maintenance of Oil Circuit Breaker

If a short-circuit current occurs, the circuit breaker may experience burnt contacts due to arcing. Furthermore, the dielectric oil in the contact area may become carbonized, causing a loss of its dielectric strength. This reduction in strength decreases the breaker’s breaking capacity. Therefore, it is essential to maintain oil circuit breakers by frequently inspecting and replacing the oil and contacts.


Oil circuit breakers are utilized in high-voltage applications such as power grids, substations, power lines, transmission, and distribution systems with voltage ratings up to 220 kV.

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