An electrical power system serves to distribute electricity to the different points for different applications. Electricity supply to industries, offices, schools, homes, or any other place forms a large circuit when it is coming from the power distribution grid. The electrical lines connected to the power plant forms the hot wire at one end with the other end being the line connecting to the ground. Electricity develops a potential between these two lines as it flows through the power grid. As such, it has to go through a circuit breaker box from where it is split into numerous circuits. The reason for this is because in the world of electrical and electronics, chances are high that a mishap can happen whenever the wires heat up from high voltage flow through the circuit. In case a mishap occurs, it can have catastrophic outcomes including damage to buildings, industries, offices, schools, and homes not counting the potential hazard it poses to humans. It is, therefore, not wise to trust voltage and current. In this regard, safety measures are implemented including the installation of circuit breakers. Circuit breakers or fuses serve to control the sudden upsurge of current and voltage or short circuiting in the electrical power connection thereby preventing incidences of accidents from electrical mishaps. As such, a circuit breaker controls and protects the power system by acting as a switching device whenever too much charge flows through the circuit. It can be operated either manually or automatically. Without a circuit breaker, the risk of electrical shocks, fires, and electrocutions is high.
Circuit breakers are available in several different types depending on different criteria, primarily the rating of an electrical power system in terms of voltage, as well as other criteria such as installation location, interrupting or mechanism, placement, and external design. As such, circuit breakers used in industries differ from those used at home. The design of circuit breakers in modern power systems has changed dramatically depending on the current and voltage as well as in preventing from arc while operating. Since the modern electrical power system is characterized with a rise in the demand for power for the different applications, they deal with huge power network as well as a massive number of associated electrical equipment. Special attention needs to be given in the design of CBs in order to make sure that it effectively serves to safely interrupt the arc produced when a circuit breaker is closed.
Based on the installation voltage level, CBs can be classified into high voltage CBs, medium voltage CBs, and low voltage CBs. Low voltage CBs are primarily used in small-scale industries as they are rated for for use up to 2kV. On the other hand, medium voltage CBs are rated for heavier use at electrical voltages between 2 kV and 72 kV. High voltage class are commonly rated for use at voltages above 123 kV. Both medium and high voltage circuit breakers are commonly referred to as transmission class breakers.
Based on location of installation, CBs can be categorized into indoor and outdoor CBs. Indoor CBs are devised to be employed inside buildings or in enclosures that are weather-resistant. The typical operation of indoor circuit breakers is through a metal clad switchgear enclosure at a medium voltage. On the other hand, outdoor circuit breakers are designed to be used outdoors as they feature a strong external enclosure arrangement. As such, they are capable of withstanding wear and tear and do not, therefore, need any roofing.
Based on external design, CBs can be categorized into live tank and dead tank circuit breakers. Dead tank CBs are those whose tank containing all the quenching and insulating medium encloses at ground potential. In other words, these circuit breakers have their tank at dead potential as it is shorted to ground. On the other hand, live tank circuit breakers are those whose tank encloses at a potential above ground-level. The tank, with the interrupter and insulating medium, features an insulation medium in between. According to the mechanism of operation, CBs can be categorized into hydraulic, pneumatic, and spring-operated CBs.
Based on the arc quenching medium that is used in their design, circuit breakers can be categorized into oil CBs, air CBs, vacuum CBs, and SF6 CBs among others.
Oil Circuit Breaker
Oil CBs employs oil, preferably mineral oil, to arc the quenching (interrupting) medium as well as an insulating medium between the carrying (moving) contacts and fixed contacts (earth sections) of the circuit breaker. Both the carrying and fixed contacts get submerged in oil. The arc gets initialized once the fixed and moving contacts are separated. Since oil has a better insulation property as compared to air, this type of circuit breaker cools the high voltage charges quicker as the oil gets vaporized and it then decomposes in hydrogen gas. Oil provides the right container to halt sparks. Sparks are referred to as automatic blackouts whenever there is oil damage. A highly compressed hydrogen bubble is created around the arc which thwarts it from re-striking once the power crosses zero point of the cycle. Based on the amount and pressure of oil used, these CBs are further divided into bulk oil and minimum oil CBs. Unlike the bulk oil CB, the minimum oil CB requires less amount of oil as it is primarily used as the interrupting (quenching) medium.
Air Circuit Breaker
The arcing contacts of this type of CB operates when exposed to the air. As such, the arc of the interrupting medium is formed at atmospheric pressure. Air circuit breakers commonly serve as substitutes for oil circuit breakers, especially for applications in circuits up to 15 kV. CB-Air can be further divided into plain air CBs and air blast CB.
Plain Air Circuit Breaker:
Also known as Cross-Blast CB, this breaker is designed with a chamber, known as arc chute, which encircles the contacts. As such, it is built between the connections of air that collide in the air atmosphere. The chamber helps cool the air circuit breaker and its internal walls are shaped to prevent the arc forming in close proximity. As such, the arc moves along into the projected winding channel. There are several small compartments in the arc chute each divided by metallic plates. The small compartments act as mini-arc chutes with the metallic plates separating them behaving like arc splitters. Consequently, the arc voltages are typically higher as compared to the system voltages resulting in the arc splitting into a series of arcs. The plain air CB is commonly used for low voltage application with normal 3000 currents. Large electric motors are a feature in heavy industries and, as such, CB-plain air typically replace oil contamination and damage. It is typical to see these circuit breakers being used in electric stoves.
Air Blast CBs:
In this type of circuit breakers, sparks are extinguished by placing direct pressure on the air, either longitudinally or perpendicularly. The ionized hot air in the sparks zone is then quickly replaced by fresh and dry air. Consequently, the duration of the spark continues to increase steadily. CB-Air Blast are commonly used for system-voltages of between 245 kV and 420 kV. They can be further subdivided into 2 categories namely axial blast breaker and axial blast circuit breaker with sliding current-carring contact.
a. Axial Blast Breaker
The current-carrying contact in axial blast CBs are in contact. The contact of the CB is fixed with a nozzle orifice at a normal closed state. When an electrical mishap occurs characterized by high pressure initiated within the chamber, air flows through the orifice to sustain the voltage.
Air Circuit Breakers have a variety of applications including for protecting plants, generators, transformers, electrical machinery, and capacitors. Besides, they are also used in GND and electrical power sharing system as well as in low-high voltage and current applications.
Vacuum Circuit Breaker
In vacuum circuit breakers (VCB), vacuum acts as the quenching medium for the arc because of its high dielectric recovery property. This makes it an excellent medium of interrupting high frequency current that originates from the instability of arc when superimposed on the frequency of the current flowing through the electrical line. The CB-vacuum chamber has a pressure of 10-4 Pa. Therefore, it has a strong dielectric properties with high insulation capacity for other media, such as oil and compressed gas. In vacuum breakers, a contact, in this case an electrode, is only 1 cm apart. It takes little power owing to its tight design to open and close the circuit. The two electrodes remain closed under normal operation state between 10-7 and 10-5 Torr. Whenever an electrical fault takes place in any section of the electrical system, the trip coil of the CB is energized and the contacts finally separate. An arc is produced the moment the electrodes are opened in vacuum and they ionize, a process that quickly extinguishes the arc. The reason for this is because of the ionization of the metal vapors of electrodes which then condenses on the surface of the electrodes. Consequently, the system quickly recovers its dielectric strength.
SF6 Circuit Breaker
The contacts through which current flows in the SF6 CBs are surrounded by sulphur hexafluoride gas. This gas has a high electro-negativity and an excellent insulating property. Because of the high affinity of contacts absorbing free electrons, the SF6 gas molecules form a negative ion whenever they collide with the free electrons. Consequently, the negative ion gets absorbed by the gas molecule. The electrons attach themselves on the SF6 gas molecules in two distinctive ways. Since the negative ions formed during this process are much heavier than free electrons, their movement is slower. As such, the overall mobility (kinesis) of charged gas particles in sulphur hexafluoride is much less when compared with the mobility of other common gases. Consequently, the quenching medium acquires a very high-dielectric strength because of the heavier, less mobile charged particles of sulphur hexafluoride gas accountable for conducting electricity through a gas. Because of the low viscosity of the SF6 gas, it is 100 times more effecting as an interrupting medium as compared to air circuit breakers. SF6 CBs are generally used for high to medium voltage electrical power connections of between 33 kV to 800 kV.
Solid-State Circuit Breaker
Solid State CBs use solid materials such as triacs, thyristors, or transistor power as the interrupting and insulation medium. These breakers can clean electrical mishaps with just two cycles primarily because this circuit does not weaken the electromechanical contact. If the CB gets closed, there will be an error in the circuit, and the CB will still damage the arc with only 1.5 cycles
Miniature Circuit Breaker (MCB) switches off the circuit automatically whenever there is an overload or short circuiting incidence. A miniature CB is typically employed in low voltage electrical connections, instead of a fuse, as its current ratings are less than 100A. As such, handling an MCB is electrically safer. However, it is built with only one over-load protection and it trip settings cannot be adjusted.
Moulded Case Circuit Breakers (MCCB) use either fixed or interchangeable electromechanical trip units, especially in the traditional types. This CB provides support for the electrical system by merging a current-sensitive electromagnetic device and a temperature sensitive device, both of which operate automatically whenever the circuit mechanism trips. The current ratings for these CBs are over 1000A and they feature earth fault protection alongside current protection. However, unlike the miniature circuit breaker, moulded case circuit breakers have trip settings that can be easily adjusted.
Because of the large power rating and fault power that circuit breakers carry, there is a high risk of dangerous arcing between the fixed and carrying contacts during the working of a CB. As such, modern circuit breakers have included features to help in the quenching processes.
