What is an Oil Circuit Breaker?

What is an Oil Circuit Breaker? . An oil circuit breaker (OCB) is a crucial device in electrical systems designed to interrupt and isolate electrical faults. It utilizes oil as an arc-quenching medium, making it highly effective in high-voltage applications. Commonly used in power grids, industrial setups, and utility networks, OCBs play an essential role in ensuring electrical safety, stability, and efficiency.

An oil circuit breaker is a traditional type of circuit breaker. It has a separate contact. This contact’s primary function is to separate the insulating oil. When a fault or problem occurs, this includes good comparable properties to air, which opens in the lower part of the breaker contact oil.

When the arc strikes between the two breaking contacts, the heat from the arc dissolves the oil around it, and the high pressure separates some gaseous hydrogen. The unique feature of this circuit breaker is its low cost, reliable operation, and ease of use.

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.

Working Principle of Oil Circuit Breaker

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

Components of an Oil Circuit Breaker

Arc Quenching Chamber

This chamber contains the moving and fixed contacts where the arc forms and is extinguished.

Oil Reservoir

Houses the insulating oil, ensuring a stable medium for arc quenching.

Contacts and Insulators

The contacts initiate and interrupt the circuit, while insulators prevent current leakage.

Types of Oil Breakers

The oil circuit breaker can be classified into the following two main types

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

Bulk Oil Circuit Breaker- Such type of oil circuit breaker uses a large quantity of insulating oil. The oil is used for arc quenching as well as insulating the live parts from the earth parts of the breaker.

Minimum Oil Circuit Breaker- Such type of oil circuit breaker uses a small amount of oil only for arc quenching.

Bulk Oil Circuit Breaker (BOCB)

Bulk oil circuit breaker uses a large quantity of oil. The oil used serves two purposes. It extinguishes the arc when the contacts separate and it also insulates the live current-carrying parts from the earthed parts of the circuit breaker.

BOCB can be classified into the following two types

Plain Break Oil Circuit Breaker
Arc Control Oil Circuit Breaker

Plain Break Oil Circuit Breaker- Such BOCB does not have any control over the arc except for increasing the arc length by separating the contacts. they are fairly simple and easier to design

Arc Control Oil Circuit Breaker- Such BOCB has a special system for arc control that efficiently quenches the arc.

Plain Break Oil Circuit Breaker

Such BOCB is the simplest and oldest form of oil circuit breakers. There is no arc control system. Its operation is very simple. The current-carrying contacts are separated inside oil where the length of the arc increases with separation between the contacts. The arc quenches at the first zero current crossing.

Construction- Its construction is quite simple. It has a tank made from strong metal that contains the insulating transformer oil. The current-carrying contacts are submerged in the oil. The body of the tank is earthed. The oil acts as both an arc quenching medium as well as an insulator between the live contacts and earthed body. The tank is not completely filled with oil but some space is left for air that acts as a cushion for the displaced oil during oil vaporization. It absorbs the pressure released from the gas. The tank also includes vents for the gas outlet.

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Under fault conditions, the moving contact moves downward to separate from fixed contacts. During the contact separation, the separation gap increases. due to the high voltage difference, the oil starts to ionize and an arc is struck between the contacts.

The heat of the arc vaporizes the oil between the contacts and converts it into mostly hydrogen gas. The gas forms a bubble surrounding the contacts which increase in volume very rapidly. The gas bubble displaces the oil which is absorbed by the air cushion. The surrounding oil compresses the gas bubble that de-ionizes the medium. The dielectric strength of the medium between the contacts increases. The contact separation also increases the arc length increasing the medium dielectric strength. The arc is quenched at the next current zero-crossing.

The main disadvantage of a plain break oil circuit breaker is that it does not have arc control and it relies on contact separation that increases the arc length. It is inconsistent and takes longer arcing times. The arc is quenched when the contacts are fully separated. Due to these limitations, a plain break oil circuit breaker is used for low voltage 11kV systems.

Arc Control Oil Circuit Breaker

Such BOCB includes a special system designed to control the arc to efficiently quench it. These designs enable the CB to extinguish the arc even if the contact gap is small. Unlike plain-break OCB, these CBs can be used in high voltage circuits.

The arc control Oil circuit breaker can be classified into the following two types.

Self-Blast Oil Circuit Breaker
Forced Blast Oil Circuit Breaker

Self-Blast Oil Circuit Breaker- In such OCB, the arc is controlled by internal means such as the energy of the arc is used for its own extinction.

Forced Blast Oil Circuit Breaker- In such OCB, the arc control is provided by external means.

Self-Blast Oil Circuit Breaker

In self-blast OCB, the contacts are surrounded by a chamber called pressure chamber or explosion pot. It is made from insulating material. The gas produced by the arc is accumulated inside the pressure chamber. It has a small volume thus having high pressure. The amount of pressure exerted is directly proportional to the arc current. It reduces the contact gap required for successful arc extinction, as well as reduces the arcing time.

Self-blast oil circuit breaker’s explosion pot can be designed in the following designs each providing its own benefits.

Plain Explosion Pot
Cross Jet Explosion Pot
Self-Compensated Explosion Pot

Plain Explosion Pot

Such a type of explosion Pot is very simple. It is a cylindrical-shaped pot made from insulating material. It includes fixed and moving contact. The moving contact pass through a narrow hole called the throat.

Under fault conditions, the contact separate and arc is struck. The arc vaporizes the oil, producing gas with high pressure. The high pressure compresses the oil to deionize the medium and quench the arc at zero crossings. When the moving contact leaves the pot, cool oil and gas rush through the throat that also helps in quenching the arc.

Plain explosion pot cannot be used for very low or very high currents. as the pressure is directly proportional to the arc current, low current produces very low pressure which may not be enough to quench the arc in time thus increasing the arcing time. on the other hand, a very high arc current may produce very high pressure that can explode the pot. Therefore, such an explosion pot is used for medium current.

Cross Jet Explosion Pot

Such type of explosion pot is a modified form of plain explosion pot. It includes channels on one side of the pot that also acts as arc splitters. The arc splitter splits the arc increasing its length that helps in quenching the arc. The side channels are used for allowing a path for cool oil to enter the chamber and cool off the arc.

During fault conditions, the contacts start to separate and an arc is struck at the top portion of the pot. The arc heat vaporizes the oil generating pressure that exerts a force on the oil through the passage. When the moving contact moves further down, it unblocks the channel arc splitters. It splits the arc increasing its length that helps in quenching the arc at zero crossing.

Cross jet explosion pot is used for a very high current with a very low arcing time. it is not suitable for low current because low current cannot generate enough pressure to quickly quench the arc.

Self-Compensated Explosion Pot

This type of explosion pot is made from a combination of plain and cross jet explosion pot. It has two chambers; the upper chamber is a cross-jet explosion pot while the lower chamber is a plain explosion pot as shown in the figure.

If the fault current is very high, the pressure generated will be high. As soon as the moving contacts unblock the arc splitter ducts, the oil rushes out and the arc is split and extinguish.

If the fault current is low, the pressure builds slowly. As the moving contacts take time to pass through the throat, the pressure build-up inside the chamber. There is a little leakage through the arc splitter ducts. The arc successfully extinguishes in the lower chamber when the moving contact completely passes through the throat.

Forced Blast Oil Circuit Breaker

In a force blast oil circuit breaker, the necessary pressure generated for arc quenching does not depend on the internal means (arc current) but it is generated by external means. A mechanical piston that is coupled with the movement of the moving contact presses against the oil to increase its pressure. it safely quenches the arc irrespective of the arc current.

Therefore, force-blast OCB has no limitation like self-blast OCB because it does not depend on the arc current. It can safely extinguish low, medium and high arc currents.

Based on the breaking of the current-carrying contacts, BOCB is divided into two types

Single Break Bulk Oil Circuit Breaker
Double Break Bulk Oil Circuit Breaker

Single Break Bulk Oil Circuit Breaker

As the name suggests, such BOCB has only one break between the current-carrying contacts. There is only one fixed contact and one moving contact as shown in the figure. The arc control OCB has a single break OCB design. Upon fault condition, such breakers generate a single arc. Since arc control OCB depends on the pressure generated by the heat of the arc, a single arc generates much more heat and high pressure to deionize the medium.

Double Break Bulk Oil Circuit Breaker

Double break OCB has two breaks between its current-carrying contacts when it opens. There are two fixed contacts and one moving contact. When it separates, two arcs at both ends of the contacts are generated. It lengthens the arc but decreases the heat produced by the arc at one spot.

Plain-Break OCB has a double-break design because it quenches the arc by lengthening the arc and double-break design offers that by breaking the arc into two.

Minimum Oil Circuit Breaker (MOCB)

The minimum oil circuit breaker (MOCB) also known as the low oil circuit breaker is a major type of oil circuit breaker that uses oil only for extinguishing the arc. On the contrary, BOCB uses it for insulating the live part from earth parts as well. The main advantage of using less oil is to reduce fire hazards. It does not act as insulation between the live and earthed parts.

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The MOCB has two chambers; the arc chamber and the supporting chamber. The arc chamber includes fixed and moving contact filled with oil. This chamber is used for extinguishing the arc. It is made from porcelain with bakelised paper encasing. It also includes vents for gas release at the top.

The supporting chamber provides support to the arc chamber. It is made of porcelain filled with oil for insulation. It insulates the arc chamber that is mounted on top of it. It includes an arm operated by a mechanism to move the moving contact upward and downward using a rod. The moving contact is also attached to a piston that develops the pressure necessary for arc quenching. Therefore, MOCB is a type of Forced Blast oil circuit breaker where the pressure is developed by external means independent of the arc current.

The working of MOCB is simple. Under fault conditions, the arm pulls the moving contact downward separating the contact. An arc is generated between the contacts. the arc energy vaporizes the oil generating necessary pressure. The pressure is confined to the arc chamber as it is blocked by the moving contact. As soon as the moving contacts unblock the hole, cool oil rushes in and the gases flow out through the vent.

The gas vents of the arcing chamber can be designed in one of the two following ways, therefore, MOCB is divided into two types.

Axial Venting MOCB

The following figure shows axial venting MOCB where the arc chamber vents are designed is such a way that when the contacts separate. The cool oil enters through the path unblocked by the moving contact. The arc is swept in an axial direction through the upper vent.

The axial venting generates very high pressure thus having very high de-ionization capabilities and consequently high dielectric strength of the oil. It is used for low arc current at high voltages.

Radial Venting MOCB

The radial venting MOCB has multiple vents designed radially along with the arc chamber as suggested by its name. When the arc is stretched between the contacts, the arc is swept radially through the vents. Such design developed low pressure therefore, it is used for high arc current.

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Advantages of Oil as an Arc Quenching

The oil has a high dielectric strength and provides insulation between the contact after the arc has been extinguished.
The oil used in circuit breaker provides a small clearance between the conductors and the earth components.
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

The oil used in oil circuit breaker is inflammable and hence, cause a fire hazard.
There is a risk of formation of explosive mixture with air.
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.

Applications of Oil Breakers

High-Voltage Power Systems

OCBs are extensively used in power grids to handle faults and maintain reliability.

Industrial Electrical Distribution

Industries rely on OCBs for stable and efficient power distribution in high-voltage networks.

Comparing Oil Circuit Breakers with Other Types

Oil vs. Air Circuit Breakers

Oil Breakers: Excellent arc control and insulation but require maintenance.
Air Circuit Breakers: Easier to maintain but less effective in high-voltage applications.

Oil vs. SF6 Circuit Breakers

Oil Breakers: Cost-effective but environmentally challenging.
SF6 Circuit Breakers: Superior performance but expensive and dependent on gas availability.

Safety Features of Oil Breakers

Overload Protection: Prevents damage during electrical surges.
Fire Safety Considerations: Equipped with fire-resistant designs and protective measures.
Operational Safety: Advanced locking mechanisms to avoid accidental trips.

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12. Frequently Asked Questions about Oil Circuit Breakers

Why is oil used in circuit breakers?
Oil serves as both an insulating medium and a coolant to manage high-energy arcs effectively.

How often should oil be replaced in an OCB?
The frequency depends on usage and environmental conditions, but regular inspections are recommended.

What are the safety precautions when working with OCBs?
Always isolate the breaker from the power source, wear protective gear, and ensure proper ventilation.