Oil Circuit Breaker: Working, Types, Advantages, Disadvantages and Maintenance

Search Description: Learn oil circuit breakers, their working principle, types, advantages, disadvantages, arc control methods and maintenance in simple beginner-friendly language.

Oil circuit breakers were widely used in electrical power systems for switching and protection. Their main job is to interrupt fault current safely and protect transformers, generators, busbars, feeders and other power system equipment. In this article, we will understand oil circuit breakers from basic level to advanced level in simple language.

Table of Contents

• What is an Oil Circuit Breaker?
• Why Oil is Used in Circuit Breakers
• Working Principle of Oil Circuit Breaker
• Advantages and Disadvantages
• Types of Oil Circuit Breakers
• Plain Break Oil Circuit Breaker
• Arc Control Oil Circuit Breaker
• Low Oil Circuit Breaker
• Maintenance of Oil Circuit Breaker
• Modern Relevance and Applications
• FAQs

What is an Oil Circuit Breaker?

An Oil Circuit Breaker is a type of circuit breaker in which insulating oil is used as the arc quenching medium. When the contacts of the circuit breaker open during a fault, an electric arc is formed between the contacts. The oil helps to cool, deionize and extinguish this arc.

In simple words, the oil circuit breaker protects an electrical circuit by opening the circuit during abnormal conditions such as short circuit or overload. The oil helps in breaking the current safely by reducing the effect of the arc.

Why is Oil Used in Circuit Breakers?

Oil is used because it performs two important functions:

• Arc extinction: Oil helps to extinguish the arc produced between the contacts.
• Insulation: Oil provides insulation between live parts and the earthed tank.

When an arc is produced inside the oil, the high temperature of the arc decomposes the oil and produces hydrogen gas. This hydrogen gas has good cooling properties and helps in removing ionized particles from the arc path.

Oil circuit breaker used for arc quenching and circuit protection.

Working Principle of Oil Circuit Breaker

Under normal operating conditions, the contacts of the oil circuit breaker remain closed and current flows through the circuit. When a fault occurs, the protective relay sends a trip signal and the moving contacts separate from the fixed contacts.

As the contacts separate, an arc is produced between them. The heat of the arc evaporates and decomposes the surrounding oil into gases, mainly hydrogen. This gas cools the arc and creates turbulence in the oil. Due to this cooling and turbulence, ionized particles are removed from the arc path and the arc is extinguished.

Step-by-Step Working

1. Fault occurs in the power system.
2. Relay detects the fault and sends a trip command.
3. Moving contacts separate from fixed contacts.
4. An arc is formed between the contacts.
5. Oil decomposes and produces hydrogen gas.
6. Hydrogen gas cools and deionizes the arc path.
7. The arc is extinguished and current is interrupted.

Advantages of Oil Circuit Breaker

• Oil absorbs arc energy and helps in cooling the arc.
• Hydrogen gas produced during arcing has excellent cooling properties.
• Oil provides insulation between live conductors and earthed parts.
• It allows smaller clearance between live parts.
• The surrounding oil provides a cooling surface near the arc.

Disadvantages of Oil Circuit Breaker

• Oil is inflammable, so there is a risk of fire.
• Oil may form an explosive mixture with air under some conditions.
• Carbon particles and arcing products contaminate the oil.
• Regular oil testing and replacement are required.
• Maintenance is higher compared with modern vacuum and SF6 circuit breakers.

Types of Oil Circuit Breakers

Oil circuit breakers are mainly classified into two types:

• Bulk Oil Circuit Breakers
• Low Oil Circuit Breakers

1. Bulk Oil Circuit Breaker

Bulk oil circuit breakers use a large quantity of oil. In this type, oil performs two functions: it extinguishes the arc and also insulates the current-carrying parts from each other and from the earthed tank.

Bulk oil circuit breakers are further classified into:

• Plain break oil circuit breakers
• Arc control oil circuit breakers

2. Low Oil Circuit Breaker

Low oil circuit breakers use a minimum amount of oil. In this type, oil is used only for arc extinction. The insulation between live parts is provided by air, porcelain or other insulating materials.

Plain Break Oil Circuit Breaker

A plain break oil circuit breaker is the simplest and oldest type of oil circuit breaker. In this breaker, contacts are separated under oil without any special arc control device. Arc extinction depends mainly on the increase in arc length and the cooling effect of oil.

Plain break oil circuit breaker construction.

Arc Extinction in Plain Break Oil Circuit Breaker

• Hydrogen gas bubble cools the arc column.
• Oil turbulence removes arcing products.
• Increasing contact gap increases dielectric strength.

Limitations of Plain Break Oil Circuit Breaker

• No special control over the arc.
• Long arc length is required.
• Arcing time is long and inconsistent.
• Not suitable for high-speed interruption.

Because of these limitations, plain break oil circuit breakers are mostly suitable for low-voltage and low-capacity applications.

Arc Control Oil Circuit Breaker

In arc control oil circuit breakers, special arc control devices are used to improve arc extinction. The main purpose is to extinguish the arc while the contact gap is still short. This reduces arcing time and improves the breaking performance of the circuit breaker.

Types of Arc Control Oil Circuit Breakers

• Self-blast oil circuit breakers
• Forced-blast oil circuit breakers

Self-Blast Oil Circuit Breaker

In a self-blast oil circuit breaker, the arc itself produces the pressure required for arc extinction. The gases produced during arcing are confined inside a pressure chamber or explosion pot. Due to restricted space, high pressure is developed and oil is forced across the arc path.

Types of Explosion Pots

• Plain explosion pot: Suitable for moderate short-circuit currents.
• Cross jet explosion pot: Efficient for heavy fault currents.
• Self-compensated explosion pot: Works effectively for both low and heavy fault currents.

Forced-Blast Oil Circuit Breaker

In forced-blast oil circuit breakers, oil pressure is produced by external mechanical means, such as a piston-cylinder arrangement. The pressure does not depend on the fault current. This gives better performance even at lower fault currents.

Advantages of Forced-Blast Oil Circuit Breaker

• Performance is more consistent at low currents.
• Oil pressure is independent of fault current.
• Quantity of oil required is reduced.

Low Oil Circuit Breaker

In a low oil circuit breaker, only a small quantity of oil is used for arc extinction. Solid insulating materials are used for insulation. This reduces the size, weight, oil quantity and fire risk compared with bulk oil circuit breakers.

Main Parts of Low Oil Circuit Breaker

• Supporting chamber: Provides insulation and mechanical support.
• Circuit-breaking chamber: Contains fixed contacts, moving contact and turbulator.
• Top chamber: Provides expansion space for oil and gases.

Low oil circuit breaker construction.

Advantages of Low Oil Circuit Breaker

• Requires less oil.
• Needs smaller space.
• Reduced fire risk.
• Maintenance problems are reduced.

Disadvantages of Low Oil Circuit Breaker

• Oil carbonization is higher due to less oil quantity.
• Gas removal from contact space can be difficult.
• Oil dielectric strength deteriorates quickly.

Maintenance of Oil Circuit Breakers

Maintenance of oil circuit breakers is very important because repeated operation can burn the contacts and reduce the dielectric strength of the oil. If maintenance is ignored, the rupturing capacity of the breaker may reduce and the breaker may fail during a fault.

Important Maintenance Checks

• Check current-carrying parts and arcing contacts.
• Replace badly burnt contacts.
• Test the dielectric strength of oil.
• Change or recondition badly discolored oil.
• Clean insulation surfaces and remove carbon deposits.
• Check oil level.
• Check closing and tripping mechanisms.

As a common maintenance practice, oil circuit breakers are inspected at regular intervals such as every 3 to 6 months, depending on operating conditions and number of fault interruptions.

Modern Relevance of Oil Circuit Breakers

Today, many new installations use vacuum circuit breakers and SF6 circuit breakers because they require less maintenance and offer faster interruption. However, oil circuit breakers are still important for students and electrical engineers because they explain the basic concept of arc extinction, insulation and fault interruption very clearly.

In older substations and industrial installations, oil circuit breakers may still be found. Therefore, understanding their operation and maintenance is useful for electrical maintenance engineers, power system students and substation technicians.

Comparison: Bulk Oil vs Low Oil Circuit Breaker

Feature	Bulk Oil Circuit Breaker	Low Oil Circuit Breaker
Oil Quantity	Large quantity of oil	Small quantity of oil
Function of Oil	Arc extinction and insulation	Mainly arc extinction
Size	Larger	Smaller
Fire Risk	Higher	Lower
Maintenance	More maintenance	Less maintenance than bulk oil type

Student Notes

• Oil circuit breakers use transformer oil for arc extinction.
• Hydrogen gas helps in cooling and deionizing the arc path.
• Bulk oil circuit breakers use oil for both insulation and arc extinction.
• Low oil circuit breakers use oil mainly for arc extinction.
• Maintenance mainly includes oil testing, contact inspection and mechanism checking.

Interview Questions and Answers

1. What is an oil circuit breaker?

An oil circuit breaker is a circuit breaker that uses insulating oil as the arc quenching medium.

2. Why is oil used in circuit breakers?

Oil is used because it helps in arc extinction and also provides insulation between live parts.

3. What gas is mainly produced during arcing in oil?

Hydrogen gas is mainly produced during arcing in oil.

4. What is the difference between bulk oil and low oil circuit breaker?

Bulk oil circuit breakers use a large quantity of oil for both insulation and arc extinction, while low oil circuit breakers use a small quantity of oil mainly for arc extinction.

5. Why is maintenance necessary in oil circuit breakers?

Maintenance is necessary because oil gets contaminated by carbon particles and contacts may burn due to repeated arcing.

Frequently Asked Questions

Are oil circuit breakers still used?

Oil circuit breakers are less common in modern new installations, but they are still found in older substations and industrial power systems.

What is the main disadvantage of oil circuit breakers?

The main disadvantage is fire risk because oil is inflammable. Regular maintenance is also required.

Which is better: oil circuit breaker or vacuum circuit breaker?

For modern medium-voltage applications, vacuum circuit breakers are generally preferred because they are compact, safer and require less maintenance.

What is arc control in oil circuit breakers?

Arc control means using special devices such as explosion pots or turbulators to extinguish the arc quickly and efficiently.

Conclusion

Oil circuit breakers played an important role in the development of power system protection. They use oil to extinguish the arc and protect electrical equipment from fault currents. Although modern systems now prefer vacuum and SF6 circuit breakers, oil circuit breakers remain an important topic for understanding power system protection, substation equipment and arc interruption methods.