The basic function of a circuit breaker is to break/make the continuity of the circuit. It is the consideration of the effect on the circuit of doing this, that principally dictates the choice of the breaker. Therefore, circuit breakers are mechanical switching devices able to make, continuously carry, and interrupt currents under normal circuit conditions and also for a limited time under abnormal conditions, such as overloads and short circuits. There are various types of circuit breakers which are from small devices up to large switchgear that is used to protect low current circuits until high voltage circuits.
Types of electrical circuit breakers
Circuit breakers are categorized based on their voltage level: High voltage and low voltage circuit breakers.
High voltage circuit breakers
The main types of high voltage circuit breakers include Bulk oil, minimum oil, air, air blast, SF6, and vacuum circuit breakers.
Bulk oil circuit breakers
The bulk oil circuit breaker ranges from the lowest voltage with one break per phase to 330 kV with six breaks per phase. It has the characteristics of simplicity, robustness, and quiet operation.
The operation principally depends on the formation of gases in the oil by the arc, which creates high pressure in the restricted space. At high currents with very high pressure built up, the interruption takes place in hydrogen.
At lower currents, there is a lower pressure but a high cooling effect, which extinguishes the arc. Although the bulk oil circuit breaker is falling out of use at higher voltages due to its cost and weight, it has the great inherent advantage of internally mounted current transformers.
Minimum oil circuit breakers
Minimum oil circuit breakers start from the lowest voltage with one break per phase and have been designed for the highest voltages with 20 breaks per phase at 735 kV. The characteristics of this type are simplicity, quiet operation, and cheapness.
The operating principles are as that of the bulk oil but in a smaller enclosed volume to utilize the arcing energy to extinguish itself instead of the bulk oil volume pressure. It is becoming more and more acceptable at all voltages because of its characteristics and the large amount of development testing that has gone into the small oil circuit breaker design. It does, however, require more frequent maintenance than other types of circuit breakers.
Air circuit breakers
Air circuit breakers are generally used up to about 22 kV. For operation, they depend on lengthening and cooling the arc chutes. The features and characteristics are high ratings and low maintenance for a large number of operations. This type of circuit breaker is inherently expensive and is usually only used where a large number of operations are required, such as in auxiliary systems for power stations or substations.
Air blast circuit breakers
Air blast circuit breaker is used over a wide voltage range but, due to cost considerations, tends to be restricted to the high-voltage range (132 kV and above) where very high current ratings are required. They are traditionally used for arc furnace switching at 33 kV. The arc is drawn in high-pressure air, and extinction is obtained by deionization and cooling.
The performance and characteristics of this type are the highest ratings, low maintenance, and noise level is low when ﬁtted with silencers; it can be ﬁtted with the closing resistor to improve voltage recovery. The aforementioned factors make this CB most suitable for transmission systems of the highest capacity.
SF6 (Sulfur Hexaﬂuoride) circuit breakers
SF6 (Sulfur Hexaﬂuoride) circuit breaker is the most commonly used at present and its applications are from 11 kV upward. Its commercial use started in the early 1980s and it was then thought to be the principal type of circuit breaker at such voltages. Some problems have lately been discovered mainly with steep front voltage switching waveforms, which could produce Ferro resonance overvoltages in voltage transformers. Newer designs of the gas type are being developed, one of which is Freon gas, but further development was stopped as a result of the recent regulations.
The interruption mechanism depends on the electronegative gas SF6, which has a strong afﬁnity for free electrons. When an arc is drawn in this gas, the conducting electrons are captured to form relatively immobile negative ions.
This causes the arc to become unstable and easily extinguishable when the electric strength of the gap recovers very quickly.
The performance and characteristics of this type are the highest ratings, the noise level is low due to the closed-circuit gas system, and very suitable for metal-clad installations.
Vacuum circuit breakers
Vacuum circuit breakers are based on the use of vacuum interrupters that are now being produced up to 33 kV and current ratings above 1600 A. The breakers are virtually maintenance-free and where the number of interrupters in series is kept down, they are very simple to maintain. They are used largely in distribution and railway electriﬁcation systems.
Low voltage circuit breakers
The main types of low voltage circuit breakers include miniature, molded case, low voltage air, and earth leakage circuit breakers.
Miniature circuit breakers
Miniature circuit-breakers are only used at low voltage, mainly in domestic or light-industrial, or commercial applications. In general, they are used in the same applications as semi-enclosed or cartridge fuses and offer an alternative for protecting radial or ring circuits. They are single and multi-phase devices and have a typical rated load current range of up to 100 A with a maximum short-circuit rating of 16 kA at 240 V.
Miniature circuit-breakers usually employ a series overload coil for rapid short-circuit tripping and a bimetallic element for tripping on overloads.
Molded case circuit breakers
Molded-case circuit-breakers are also only used for low voltage applications. They are basically an upgraded version of the miniature circuit-breaker and are invariably three-phase devices. They have typical current ratings ranging from 100 to 2500 A and may have rated short-circuit ratings up to 50 kA at 415 V. Some designs also exhibit cut-off similar to the current-limiting fuse and all are provided with inherent short-circuit and thermal overload protection devices. They may also be provided with earth-leakage protection. They are commercially referred to as molded-case circuit breakers because they are constructed from some kind of molded material.
MCCBs can be found in all low-voltage applications: in the residential electrical distribution panel, in the industrial power distribution center, and in the main power feed panels used in large buildings such as offices, hospitals, and shopping centers.
Low voltage air circuit breakers
Air circuit-breakers can have very high rated currents of typically up to 6300 A and very high short-circuit interrupting capabilities of typically up to 120 kA. Their main application at low voltage is where an onerous performance is required in terms of load, the number of operations, and fault level. Mainly due to economic considerations, molded-case circuit-breakers have replaced many low voltage applications where previously air circuit-breakers were used but where high performance, maintainability, and long-term reliability are essential requirements air circuit-breakers are still used.
Earth leakage circuit breakers
Earth leakage circuit breaker (ELCB) is a combination of a fuse-free breaker and a leakage current detector for preventing fire or electric shock due to deterioration of electric insulation. It will provide perfect protection of a circuit against overload, short-circuit, or earth leakage.
The earth leakage circuit breakers (ELCB) consist of a circuit breaker unit composed of a switching mechanism, arc extinguishers, an overcurrent tripping device, etc., a zero-phase sequence current transformer, amplifier, and others for detecting earth leakage current. These components are accommodated in a heat-proof and arc-proof molded case and cover.