Types of Oil Circuit Breakers (OCBs)

Oil circuit breakers (OCBs) are used for performing the following two important functions -

• Switching of electric supply
• Protection of electrical system

They are called oil circuit breakers because in this type of circuit breakers, insulating/dielectric oil is used for quenching the arc.

In this article, we will learn about the basics of oil circuit breakers and their different types.

What is an Oil Circuit Breaker?

An oil circuit breaker is an electrical switch in which an insulating or dielectric oil is used for arc quenching. As we know, when a fault occurs in the electrical system, we have to open the circuit breaker to interrupt the power supply.

When the circuit breaker is opened or the contacts of the breaker are separated, an electric arc strikes between them. To quench this arc and dissociate the heat generated due to arcing, an insulating oil or dielectric oil is used.

The oil used in the circuit breaker performs the following three functions -

• It quenches the arc created between the separating contacts.
• It provides insulation between conducting parts of the circuit breaker.
• It helps in dissipating heat produced due to arcing.

The oil circuit breakers are high power and high current switching device. Therefore, these are generally used in power transmission and distribution systems.

Types of Oil Circuit Breakers

Depending on the amount of dielectric oil, the oil circuit breakers can be classified into the following two types -

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

Let us discuss each of these two types of oil circuit breakers in detail.

Minimum Oil Circuit Breaker

A type of oil circuit breaker in which minimum amount of insulating oil is used for quenching the arc is called a minimum oil circuit breaker or MOCB. It is used in applications where space limitation is a major concern.

In a minimum oil circuit breaker, solid insulating materials like porcelain are used for insulation and a small amount of dielectric oil is used for arc quenching.

The following are some major characteristics of the minimum oil circuit breaker -

• Minimum oil circuit breakers have smaller size and compact design. Thus, they can be easily installed in a limited space.
• In minimum oil circuit breakers, a small amount of dielectric oil is required that reduces the overall cost of the breaker.
• The design of the minimum oil circuit breaker is highly optimized for arc quenching purpose. This results in increased efficiency of the circuit breaker.
• Due to smaller size and less weight, the maintenance of minimum oil circuit breakers is easier.
• Since a less amount of oil is used in minimum oil circuit breakers, therefore, these circuit breakers have a reduced impact on the environment.

Advantages of MOCB

Minimum oil circuit breakers (MOCBs) have numerous advantages. Some of the main benefits of minimum oil circuit breakers are listed below -

• MOCBs require a small space for installation.
• MOCBs have a small amount of dielectric oil. This results in reduced size and weight of the breaker.
• MOCBs are relatively environment friendly due to less volume of oil used.
• The reduced amount of oil decreases the oil-related issues and make the MOCBs safer.
• MOCBs have better arc quenching capabilities that enhance their performance and reliability.
• Due to less weight and smaller size, MOCBs require less efforts and cost in maintenance.

Disadvantages of MOCB

However, the minimum oil circuit breakers have several advantages as given above. But they also have certain disadvantages. The following are some common disadvantages of the minimum oil circuit breakers -

• Minimum oil circuit breakers have a complex design due to presence of specialized components.
• MOCBs are relatively more expensive to manufacture. Thus, their initial cost is high.
• Minimum oil circuit breakers are susceptible to damages due to heavy arcing in certain fault conditions.
• MOCBs have certain limitations in terms of operating voltage, current, and trip capabilities.

Bulk Oil Circuit Breaker

A type of oil circuit breaker in which a larger amount of dielectric oil is used for quenching the arc produced between the contacts is known as a bulk oil circuit breaker or BOCB.

In the case of bulk oil circuit breakers, the volume of insulating oil used depends on value of the system voltage. For example, 8000 to 10000 kg of oil is used in the breaker if it is used in a system of 110 kV.

The insulating oil used in a bulk oil circuit breaker performs the following two functions -

• Quench the arc, and
• Provides insulation between live parts of the breaker.

Although, there is a large volume of insulating oil used in the bulk oil circuit breakers. Hence, these breakers can effectively quench the arc and there is no risk of damages due to heavy arcing.

Advantages of BOCB

The following are some main advantages of the bulk oil circuit breakers -

• Bulk oil circuit breakers have high interruption capacity. Therefore, these breakers can be used in high voltage electrical systems.
• The design and construction of bulk oil circuit breakers is simpler.
• Bulk oil circuit breakers have robust construction. Hence, they are more durable.
• Due to simple design, BOCBs are easy to repair and maintain.
• Bulk oil circuit breakers have a longer service life.
• Bulk oil circuit breakers have a large volume of insulating oil that reduces the chances of arcing damages. Hence, these breakers are more reliable.

Disadvantages of BOCB

The major disadvantages of bulk oil circuit breakers are listed below -

• Bulk oil circuit breakers have a large physical size and they are heavy. Hence, they pose several difficulties in installation and transportation.
• In bulk oil circuit breakers, a large volume of insulating oil is used which is very difficult to store and handle.
• The maintenance of bulk oil circuit breakers is relatively more time consuming which increases the labor cost and down time.
• The large volume of insulating oil used in bulk oil circuit breakers have more impacts on the environment.

Difference between Bulk Oil and Minimum Oil Circuit

The following table highlights all the major differences between bulk oil circuit breaker and minimum oil circuit breaker -

Conclusion

In this article, we explain in detail about two main types of oil circuit breakers namely, minimum oil circuit breakers (MOCBs) and bulk oil circuit breakers (BOCBs).

MOCB is one in which the insulating oil is used just for quenching the arc, while in a BOCB, the insulating oil is used for arc quenching as well as insulating purpose. However, the function and working of both MOCB and BOCB is similar.

FAQs Related to Oil Circuit Breakers

This section is meant for answering some commonly asked questions related to oil circuit breakers.

1. What is the oil used in circuit breaker?

In an oil circuit breaker, a highly refined mineral oil is used for arc quenching and insulation. It is also known as dielectric oil or insulating oil.

2. What are two functions of oil in a circuit breaker?

In an oil circuit breaker, the oil performs the following two functions - Arc quenching and Insulation between live parts.

3. What is a minimum oil circuit breaker used for?

Minimum oil circuit breaker is used in medium voltage applications for switching and protection of electrical circuits.

4. What is the full form of OCB breaker?

OCB stands for Oil Circuit Breaker.