The first circuit breakers were produced by Thomas Alva Edison in 1879, but were only used in homes. Back then fuses were being used in commercial electrical systems. They were reliable to handle faulty conditions and current disruptions. Today, circuit breakers are used by both small and large scale to accommodate the growing demand for electricity.
A circuit breaker is an automatically or manually operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow.
Construction and working principle of circuit breaker :
The circuit breaker mainly consists of fixed contacts and moving contacts. In normal “on” condition of circuit breaker, these two contacts are physically connected to each other due to applied mechanical pressure on the moving contacts. There is an arrangement stored potential energy in the operating mechanism of circuit breaker which is realized if switching signal given to the breaker. The potential energy can be stored in the circuit breaker by different ways like by deforming metal spring, by compressed air, or by hydrolic pressure. But whatever the source of potential energy, it must be released during operation. Release of potential energy makes sliding of the moving contact at extremely fast manner. All circuit breaker have operating coils (tripping coils and close coil), whenever these coils are energized by switching pulse, the plunger inside them displaced. This operating coil plunger is typically attached to the operating mechanism of circuit breaker, as a result the mechanically stored potential energy in the breaker mechanism is released in forms of kinetic energy, which makes the moving contact to move as these moving contacts mechanically attached through a gear lever arrangement with the operating mechanism. After a cycle of operation of circuit breaker the total stored energy is released and hence the potential energy again stored in the operating mechanism of circuit breaker by means of spring charging motor or air compressor or by any other means.
Types of circuit breaker:
¤ Low-voltage circuit breakers
Low-voltage (less than 1,000 VAC) types are common in domestic, commercial and industrial application, and include:
MCB (Miniature Circuit Breaker):
rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.
There are three main types of MCBs: 1. Type B – trips between 3 and 5 times full load current; 2. Type C – trips between 5 and 10 times full load current; 3. Type D – trips between 10 and 20 times full load current. In the UK all MCBs must be selected in accordance with BS 7671.
MCCB (Molded Case Circuit Breaker):
rated current up to 2,500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.
¤ Magnetic circuit breakers
Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid’s pull releases the latch, which lets the contacts open by spring action. Some magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. A spring restrains the core until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker.
¤ Thermal magnetic circuit breakers
Thermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions. The thermal portion of the circuit breaker provides an “inverse time” response feature, which trips the circuit breaker sooner for larger overcurrents but allows smaller overloads to persist for a longer time.
¤ Medium-voltage circuit breakers
Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 40.5 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protectiverelays operated through current transformers. The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors andprotective relays, instead of relying on built-in thermal or magnetic overcurrent sensors.
Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:
Vacuum circuit breakers—With rated current up to 6,300 A, and higher for generator circuit breakers. These breakers interrupt the current by creating and extinguishing the arc in a vacuum container – aka “bottle”. Long life bellows are designed to travel the 6-10 mm the contacts must part. These are generally applied for voltages up to about 40,500 V, which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.
Air circuit breakers—Rated current up to 6,300 A and higher for generator circuit breakers. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models aremicroprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.
SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.
Medium-voltage circuit breakers may be connected into the circuit by bolted connections to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers in switchgear line-ups are often built with draw-out construction, allowing breaker removal without disturbing power circuit connections, using a motor-operated or hand-cranked mechanism to separate the breaker from its enclosure. Some important manufacturer of VCB from 3.3 kV to 38 kV are ABB, Eaton, Siemens, HHI(Hyundai Heavy Industry), S&C Electric Company, Jyoti and BHEL.
¤ High-voltage circuit breakers
Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. In substations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault.
High-voltage breakers are broadly classified by the medium used to extinguish the arc.
Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas to quench the arc.
Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line potential, ordead tank with the enclosure at earth potential. High-voltage AC circuit breakers are routinely available with ratings up to 765 kV. 1,200 kV breakers were launched by Siemens in November 2011, followed by ABB in April the following year.
High-voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability.
Such breakers would be useful to interconnect HVDC transmission systems.
Other circuit breaker :
The following types are described in separate.
¤ Breakers for protections against earth faults too small to trip an over-current device:
Residual-current device (RCD, formerly known as a residual current circuit breaker) — detects current imbalance, but does not provide over-current protection.
Residual current breaker with over-current protection (RCBO) — combines the functions of an RCD and an MCB in one package. In the United States and Canada, panel-mounted devices that combine ground (earth) fault detection and over-current protection are called Ground Fault Interrupter (GFI) breakers; a wall mounted outlet device or separately enclosed plug-in device providing ground fault detection and interruption only (no overload protection) is called a Ground Fault Circuit Interrupter(GFCI).
Earth leakage circuit breaker (ELCB)—This detects earth current directly rather than detecting imbalance. They are no longer seen in new installations for various reasons.
Recloser—A type of circuit breaker that closes automatically after a delay. These are used on overhead electric power distributionsystems, to prevent short duration faults from causing sustained outages.
Polyswitch (polyfuse)—A small device commonly described as an automatically resetting fuse rather than a circuit breaker.
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