What is a Circuit Breaker?
A circuit breaker is defined as a switching device that can be operated manually or automatically for controlling and protecting an electrical power system. It consists of two main contacts: a fixed contact and a moving contact. The contacts are normally closed and allow current to flow through the circuit. When a fault occurs, such as a short circuit or an overload, the contacts are separated by a mechanism that releases stored potential energy. This mechanism can be spring-operated, pneumatic, hydraulic, or magnetic.
The separation of contacts creates an arc between them, which is a highly ionized and conductive plasma. The arc can damage the contacts and the surrounding equipment if not extinguished quickly.
Therefore, circuit breakers use various methods to quench the arc, such as cooling, compressing, or replacing the ionized medium with fresh gas. The choice of arc quenching medium depends on the voltage level and current rating of the circuit breaker.
Circuit breakers protect electrical circuits from damage caused by overcurrent, short circuits, or overload. They interrupt the flow of current when a fault occurs and restores it when the fault is cleared. Unlike a fuse, which needs to be replaced after one operation, a circuit breaker can be reset and reused multiple times.
Note that while the current is shut off extremely quickly, engineers still need to design electrical systems that can withstand the maximum available fault current that occurs during a fault scenario.
Circuit breakers are essential for the safety and reliability of electrical systems. If a fault occurs, circuit breakers can protect people from electrical shocks.
Circuit breakers also help prevent equipment damage, fire hazards, and power outages caused by faults. They also allow for selective isolation and switching of different parts of the power system for maintenance and control purposes.
How A Circuit Breakers Works (Working Principle)
At a high level, a circuit breaker operates as follows:
- Detection of Fault: The circuit breaker detects a fault condition, such as an overload or a short circuit in the electrical system.
- Activation of Mechanism: Once a fault is detected, the circuit breaker activates a mechanism that triggers the release of stored potential energy.
- Separation of Contacts: This energy then separates the two main contacts of the circuit breaker: a fixed contact and a moving contact.
- Arc Formation: The separation of contacts creates a highly conductive and ionized plasma known as an arc.
- Arc Quenching: The circuit breaker uses various methods to extinguish or quench the arc, such as cooling, compressing, or replacing the ionized medium with fresh gas.
- Current Interruption: This process interrupts the flow of current, stopping any damage from the fault condition.
- Resetting: Once the fault is cleared, the circuit breaker can be manually or automatically reset, allowing the current to flow again.
- Re-establishment of Contacts: Resetting the breaker brings the fixed and moving contacts back together, re-establishing the flow of current through the circuit.
Elaborating on this, the working principle of a circuit breaker is based on the interruption of the electric current in an electrical circuit. A circuit breaker mainly consists of fixed contacts and moving contacts.
These two contacts are physically connected to each other in the normal “ON” condition of the circuit breaker due to applied mechanical pressure on the moving contacts.
The potential energy stored in the operating mechanism of the circuit breaker is released if the switching signal is given to the breaker. This potential energy can be stored in different ways, such as by deforming metal springs, by compressed air, or by hydraulic pressure.
Regardless of the source of potential energy, it must be released during operation. The release of this energy allows the moving contact to slide rapidly.
After a cycle of operation, the total stored energy is released, and the potential energy is again stored in the operating mechanism using a spring charging motor, air compressor, or other means.
However, the working principle of a circuit breaker isn’t merely mechanical; there are also electrical characteristics to consider.
The circuit breaker has to carry large rated or fault power. Due to this, there is always dangerously high arcing between moving contacts and fixed contact during operation.
This arcing can be safely quenched if the dielectric strength between the current-carrying contacts increases rapidly during every current zero crossing of the alternating current.
The dielectric strength can be increased in a number of ways, such as by compressing the ionized arcing medium (which accelerates the deionization process), cooling the arcing medium (which increases the resistance of the arcing path), or replacing the ionized arcing medium with fresh gases. Hence, arc quenching is an integral part of the operation of a circuit breaker.
Circuit Breakers in Substations
Substations are critical parts of the electrical system that transform voltage from high to low, or the reverse, and perform several other important functions. Circuit breakers play a crucial role in substations to ensure safe and efficient operation. They are responsible for isolating equipment for maintenance, clearing faults, and protecting equipment from overcurrents.
In a substation, circuit breakers are typically organized in a way that allows for selective tripping. This means that if a fault occurs, only the circuit breakers connected to the faulty section of the network will open, leaving the rest of the network undisturbed.
High-voltage circuit breakers, like SF6 circuit breakers, are often used in substations due to their reliability, low maintenance requirements, and excellent arc extinguishing properties.
Circuit Breakers in Power Systems
Circuit breakers are an essential component of power systems, providing critical protective functions. They protect the power system from faults by interrupting the flow of abnormal currents and prevent damage to transformers, transmission lines, solar cells, PLCs, and other electrical equipment.
The role of circuit breakers in power systems extends to various applications, including power generation plants, transmission and distribution networks, and consumer end utility areas.
In power generation plants, circuit breakers protect generators and transformers from faults. In transmission and distribution networks, they isolate faulty sections to prevent the fault from spreading through the system.
The selection of circuit breakers in power systems depends on the voltage level, current rating, type and frequency of faults expected, environmental conditions, and maintenance
Types of Circuit Breakers
There are many types of circuit breakers based on different criteria, such as arc quenching medium, voltage level, installation location, and operating mechanism.
According to their installation location:
- Outdoor circuit breaker.
- Indoor breaker.
According to the operating mechanism:
- Spring-operated circuit breaker.
- Pneumatic circuit breaker.
- Hydraulic circuit breaker.
According to the voltage level of installation:
- High voltage circuit breaker.
- Medium voltage circuit breaker.
- Low voltage circuit breaker.
Most commonly, circuit breakers are classified by their arc quenching medium, which includes:
We’ll discuss each of these types of circuit breakers in a bit more detail.
Oil Circuit Breaker
An oil circuit breaker uses mineral oil as the arc quenching medium. The oil has high dielectric strength and good cooling properties. The contacts are immersed in an oil-filled chamber, which also acts as an insulator.
When the contacts open, the arc is drawn in the oil and decomposes it into hydrogen gas and other products. The gas bubbles surround the arc and reduce its temperature and pressure. The oil also acts as a barrier to prevent restriking of the arc.
Oil circuit breakers are suitable for high-voltage applications, such as transmission lines and substations. They have low maintenance costs and long service life.
However, oil circuit breakers have some risks that need to be managed, including:
- Oil leakage and fire risk
- Oil contamination and deterioration
- Large size and weight
- Environmental concerns
Air Circuit Breaker
An air circuit breaker uses compressed air as the arc quenching medium. The air has low dielectric strength but high thermal conductivity. The contacts are enclosed in an air-tight chamber, which is connected to an air compressor.
When the contacts open, the air blast is directed toward the arc and blows it away from the contacts. The air also cools down the contacts and removes the ionized particles from the gap.
Air circuit breakers are suitable for low-voltage applications, such as distribution panels and industrial circuits. They have a high speed of operation and low arc energy. However, they also have some disadvantages, such as:
- High noise level
- High operating pressure
- Sensitivity to voltage fluctuations
- High cost of installation and maintenance
SF6 Circuit Breaker
An SF6 circuit breaker uses sulfur hexafluoride gas as the arc quenching medium. The SF6 gas has high dielectric strength and excellent arc extinguishing properties. The contacts are housed in a sealed chamber filled with SF6 gas at low pressure.
When the contacts open, the gas flows through a nozzle and strikes the arc. The gas absorbs the free electrons in the arc and forms negative ions. This reduces the conductivity of the arc and increases its resistance. The gas also cools down the contacts and prevents restriking.
SF6 circuit breakers are suitable for medium and high-voltage applications, such as power plants and switchyards. They have low maintenance requirements and long service life. However, they also have some disadvantages, such as:
- High cost of gas
- Environmental impact of gas leakage
- Difficulty in the disposal of gas
- Special handling and safety precautions
Vacuum Circuit Breaker
A vacuum circuit breaker uses a vacuum as the arc quenching medium. The vacuum has very high dielectric strength and very low arc resistance. The contacts are enclosed in a ceramic or metal chamber that is evacuated to very low pressure.
When the contacts open, the arc is initiated in the vacuum and quickly extinguished. The vacuum has very low arc resistance and high dielectric strength, which makes the interruption process very fast and efficient.
Vacuum circuit breakers are suitable for low and medium-voltage applications, such as industrial and commercial circuits. They have low arc energy and noise level, and high reliability and durability. However, they also have some disadvantages, such as:
- High cost of manufacturing and testing
- The limited current rating and breaking capacity
- Difficulty in detecting vacuum loss
- Special care and handling of ceramic or metal chambers
How to Choose a Circuit Breaker
The selection of a circuit breaker depends on various factors, such as:
- The voltage level and current rating of the circuit
- The type and frequency of faults expected in the circuit
- The environmental conditions and installation location of the circuit breaker
- The maintenance requirements and service life of the circuit breaker
- The cost and availability of the circuit breaker
Some general guidelines for choosing a circuit breaker are:
- For high voltage applications (> 72.5 kV), oil or SF6 circuit breakers are preferred, as they have high breaking capacity and insulation strength.
- For medium voltage applications (1 kV to 72.5 kV), vacuum or SF6 circuit breakers are preferred, as they have low arc energy and noise level, and high reliability and durability.
- For low voltage applications (< 1 kV), air or miniature circuit breakers are preferred, as they have a high speed of operation and low cost of installation and maintenance.
- For outdoor applications, oil or SF6 circuit breakers are preferred, as they are weather-proof and resistant to corrosion and pollution.
- For indoor applications, vacuum or air circuit breakers are preferred, as they are compact and safe to operate.
- For frequent switching operations, air or vacuum circuit breakers are preferred, as they have low wear and tear of contacts.
- For generator protection, vacuum or SF6 circuit breakers are preferred, as they can handle high transient currents and asymmetrical faults.
Advantages and Disadvantages of Circuit Breaker
Circuit breakers have many advantages over fuses, as circuit breakers can:
- Be reset and reused multiple times without replacement
- Be operated manually or automatically for switching purposes
- Provide selective protection and isolation of different parts of the power system
- Interrupt high fault currents without producing harmful gases or flames
- Reduce the risk of fire hazards and power outages caused by faults
However, circuit breakers also have some disadvantages over fuses, including:
- They are more expensive and complex than fuses
- They require regular maintenance and testing to ensure the proper functioning
- They may fail to operate due to mechanical or electrical faults
- They may cause voltage surges or transients during switching operations
Applications of Circuit Breaker
Circuit breakers are widely used in various fields of electrical engineering, such as:
- Power generation: Circuit breakers are used to protect generators from short circuits, overloads, reverse power flow, etc. They also allow for the synchronization and parallel operation of generators.
- Power transmission: Circuit breakers are used to protect various types of transmission lines from faults, lightning strikes, switching surges, etc. They also allow for sectionalizing and load shedding of power system networks.
- Power distribution: Circuit breakers are used to protect distribution transformers, feeders, induction motors, etc., from faults, overloads, under-voltage, etc. They also allow for load management and demand response of power system loads.
- Industrial: Circuit breakers are used to protect industrial equipment, such as pumps, compressors, fans, etc., from faults, overloads, phase imbalance, etc. They also allow for speed control and soft starting of motors.
- Commercial: Circuit breakers are used to protect commercial buildings, such as offices, malls, hotels, hospitals, etc., from faults, overloads, power surges, etc. They also allow for energy management and automation of building systems.
- Rail: Circuit breakers are used to protect rail vehicles, such as trains, trams, metros, etc., from faults, overloads, short circuits, etc. They also allow for traction control and regenerative braking of motors.
Conclusion
Circuit breakers are vital devices for the protection and control of electrical power systems. They can interrupt high fault currents and restore the normal operation of the circuit.
They can also isolate and switch different parts of the power system for maintenance and control purposes. They use various arc quenching media, such as oil, air, SF6, or vacuum, depending on the voltage level and current rating of the circuit.
They have many advantages over fuses, such as resetability, selectivity, reliability, and durability.
However, they also have some disadvantages, such as complexity, cost, maintenance, and failure. They are widely used in various fields of electrical engineering, such as power generation, transmission, distribution, industrial, commercial, and rail.
