Circuit Breaker

What is a Circuit Breaker?

A circuit breaker is defined as, switching device which interrupts the faulty current and performs the function of a switch thus protecting the electrical system from damage.

Working Principle of Circuit Breaker

There are two essential contacts in a circuit breaker, and they are:

• Fixed contacts
• Moving contacts

When the circuit is closed, which is a normal condition, the contacts touch each other and carry the current under this condition. Under a closed circuit breaker, current-carrying contacts are known as electrodes that engage each other due to the pressure of a spring.

The switching and maintenance of the system are taken care of by either opening or closing the arms of the circuit breaker. The circuit breaker is opened by applying pressure to the trigger. When there is a faulty current flowing through any part of the system, the trip coil of the breaker gets energized thereby moving away from each other, thus opening the circuit.

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Types of Circuit Breakers

Following are the four types of circuit breakers:

• Air circuit breakers
• SF6 circuit breakers
• Vacuum circuit breakers
• Oil circuit breakers

Air Circuit Breakers

The air circuit breaker is also known as an air blast circuit breaker as they are air-based circuit breakers. In this circuit breaker type, the arc extinguishing medium used is a high-pressure air blast. In case of a faulty current flow, the blast valve opens the contact. The restriking of the arc is prevented by sweeping the ionized particles into the atmosphere, which increases the strength of the dielectric in the medium.

There are three types of air circuit breakers:

• Axial blast type
• Cross blast type
• Radial blast type

Advantages of Air Circuit Breaker

• There is an elimination of the risk of fire.
• The arcing products can be removed completely.
• The contact gap is less as the dielectric strength increases faster.
• It is suitable for frequent operations as the arcing time and arc energy are small.
• It is independent of the interrupting current.

Disadvantages of Air Circuit Breaker

• The compressor needs to be maintained in the air circuit breaker.
• It is sensitive to the variation in voltages.
• The properties of arc extinguishers are inferior.

SF6 Circuit Breakers

SF6 is a sulfur hexafluoride circuit breaker in which sulfur hexafluoride is used as the arc extinguishing medium. The sulfur hexafluoride gas attracts free electrons. As the circuit contacts are opened, the gas flows through the chamber striking the arc. The free electrons are then absorbed by the SF6 resulting in immobile negative ions. For the arc to extinguish completely, the Insulating strength of the medium should be increased. Both the contacts, fixed and moving contacts are placed in the arc chamber along with the gas.

Advantages of SF6 Circuit Breaker

• The arc extinguishing property is excellent.
• SF6 gas is three times greater than air, making the interruption of the current large.
• It’s environment-friendly as there is no exhaust released.
• Low maintenance.
• Suitable for harsh conditions such as in coal mines.

Disadvantages of SF6 Circuit Breaker

• SF6 gas is expensive.
• SF6 needs to be reconditioned after every use.

Vacuum Circuit Breakers

In vacuum circuit breakers, the vacuum is the arc extinguishing medium. The insulating strength of this circuit breaker type is excellent compared to other types. There is a formation of an arc when the vacuum is opened as there is the ionization of the metal vapours. The moving and fixed contact along with the arc shield is mounted in the vacuum chamber.

Advantages of Vacuum Circuit Breaker

• The arc energy released is low.
• It is capable enough to withstand the lightning strikes.
• Very little or no maintenance is required.
• The equipment is compact and has a long life.

Oil Circuit Breakers

In oil circuit breakers, oil is used as the arc extinguishing medium. The arc strikes when the contacts are opened in the oil thus, evaporating the surrounding oil as hydrogen gas. The arc region is surrounded by the hydrogen gas bubble. The hydrogen gas has a high thermal conductivity which cools the arc and deionizes the medium. There is turbulence caused in the surrounding oil pushing away the arcing products.

There are two types of oil circuit breakers:

• Bulk oil circuit breakers
• Low oil circuit breakers

Advantages of Oil Circuit Breaker

• This type of circuit breaker acts as an insulator between the live wires and earth.
• The cooling property is excellent.

Disadvantages of Oil Circuit Breaker

• There are high chances of fire hazard.
• There is no outlet of the arcing products to escape.

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