Oil Circuit Breaker Types and How to Maintain Them

Oil circuit breakers are a protective device that uses oil as an insulation medium for arc extinction. It is one of the oldest breakers that has its contacts inside the oil.

The switchgear has fixed and moving contact, which helps in circuit protection, both in substations and in transformers

Each time there is a faulty current or overload, the contact opens through the help of the insulating oil, and an arc stuck between them.

Types of oil circuit breakers.

There are two major types of oil circuit breakers, they include:

  • Bulk Oil Circuit Breaker (BOCB).
  • Minimum Oil Circuit Breaker (MOCB)

Bulk Oil Circuit Breaker (BOCB).

For bulk oil circuit breakers, the oil serves two purposes. It serves as an arc quenching medium, and an insulator to the current-carrying contacts and earth parts of the breaker.

Minimum Oil Circuit Breaker (MOCB)

These types of breakers also use oil as a quenching medium. But unlike bulk oil, the minimum oil has its insulation oil only in the interrupting chamber.

This feature distinguishes it from the bulk oil and helps reduce oil usage.

Parts of an oil circuit breaker

The oil circuit breaker comprises

  • An insulating cylinder
  • Arc chamber
  • Fixed and moving contacts
  • Gas expansion chamber
  • Oil level indicator
  • Oil sump
  • Terminals
  • Oil draining
  • Oil filling plugs.

Construction of oil circuit breakers

The construction of an oil circuit breaker is simple. It involves sinking the fixed and movable contacts of the breaker into a metal tank containing the transformer oil.

The oil serves both as an insulator and as a medium for arc quenching.

An air cushion between the tank roof and the oil surface is certain to help contain the displaced oil when gas forms around the arc.

That is why oil tanks are half-filled to a certain level where the air is tight in the tank.

The cover of the tank should be tight enough to prevent gas spillage during interruption of faulty current.

Working principle of an oil circuit breaker

A good oil circuit breaker carries currents along the circuit when closed (ON). But if a fault occurs, the movable contacts separate from the fixed, and an arc struck in-between them.

The arc causes a release of much energy that increases the temperature of the oil, turning them into gas.

The high-pressure gas surrounds the arc and its volatile bubbles displace the oil, pushing apart the contacts.

Advantages of oil circuit breakers

  • Hydrogen gas formed during arc separation helps in cooling the system.
  • It has a high dielectric strength that provides insulation between the arc after separation.
  • It acts as an insulator and also allows smaller clearance between the conductors and the earth components.


  • The oil is inflammable and may cause a fire outbreak.
  • It may form an explosive mixture with air.
  • The products of arcing such as carbon particles may pollute the oil and reduce its dielectric strength. This results in regular checking and replacement of oil.

Maintenance of oil circuit breakers

Maintenance of an oil circuit breaker is important and should be regular.

This includes the detection and maintenance of any conditions that may lower performance. The most common contaminants are carbon, moisture, and sludge.

Clean often, recondition the oil, and also remove carbon deposits to make the breaker last.

You can also follow the manual instructions provided by the manufacturer for greasing and lubricating.

Frequently Asked Questions

My circuit breaker is leaking oil, how can I repair it?

To repair an oil leaking circuit breaker, find the leaking point, which may be the bushing or the drain valves.

Repair it using the drill and injection method, custom design enclosures, or the clamp injection method with a rubber-based, non-adhering sealant to repair the breaker.

You can also contact us for the repair.

Frequently Asked Questions

Which oil is used in the oil circuit breaker?

We use mineral oil in oil circuit breakers because it is a better insulator.

Where are oil circuit breakers used?

Oil circuit breakers are used on grids, high voltage applications, substations, transmission and in distribution

Bulk oil circuit breakers are usually dead tanks, while minimum oils are live tanks.

What is the function of circuit breaker

The primary function of a circuit breaker is to protect an electrical circuit from power surge or short circuit. Any time it detects the overcurrent, it trips the current flow to prevent hazards.