A circuit breaker is an automatically-operated electrical circuit designed to protect an electrical circuit from damage caused by overload or short circuit. A circuit breaker is a switch device which is used for making and breaking and electrical circuit, once or repeatedly, several times, during normal as well as abnormal operating conditions.
FUNCTIONS
1.Circuit breakers are used to protect an electrical circuit from overloading or short-circuiting. This prevents damage to equipment and wiring, as well as reducing the risk of fire.
2.Closing, opening and carrying full load rated current of the circuit for a prolonged period without any excessive temperature rise.
3.To carry the full load current at all times.
4.Breaking the heavy currents in case of short circuits.
5.Carrying current of short circuit magnitude until the fault is cleared by a circuit breaker nearest to the fault if it is used as back up protection.
6.Withstanding the effect of arcing at its contact terminal and thermal and electromagnetic stresses due to flow of heavy current during fault.
7.To open and close the circuit on no loads.
BASIC PRINCIPAL OF CIRCUIT BREAKER
All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker.The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.
Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.
The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by the erosion due to interrupting the arc. Miniature and molded case circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.
When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms. Different techniques are used to extinguish the arc including:
•Lengthening of the arc
•Intensive cooling
•Division into partial arcs
•Zero point quenching
•Connecting capacitors in parallel with contacts in DC circuits
Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit
ARC PHENOMENON IN CIRCUIT BREAKER
When a loose connection (a gap) is made in the faulted circuit so loose that the current flow is non-continuous, it is called an arcing or arc fault. An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. The arc consists of a column of ionized gas having molecules which have lost one more electrons. The electrons being negatively charged are attracted towards the positive contact with high velocity and on the way they detach more electrons by impact. The positive ions are attracted towards the negative contact but they comprise almost the entire weight of the atom, they move relatively slow.
Initiation of arc
To initiate the arc, when the fault current occurs and the electrodes separate, the electron from the cathode must be emitted.
The emission of the arc takes place due to the following reasons.
1. When the contacts are separated than the area and pressure between the contacts is decreased and the resistances will increases but the fault current is same. So due to the high current the high potential gradient is formed which can dislodge the electrons from the contacts.
2. When the contact separates the contact area is also decreases. Due to the high current flowing, the current density will increase. Due to the high current densities the temperature will increase resulting in the thermionic emission.
Maintenance of arc When the electron are emitted from the cathode, they collide with other neutral electron and other particles with great velocities and make them also the negatively charge and swifts them along with itself toward the cathode. The process is known as ionizations. The ionized ions further ionize other particle and create the avalanche of the electron to reach to the anode. The process of ionization is furthered achieved by the following reasons:
•The high temperature caused by the high current densities when the contact separates.
•The high voltage gradient formed when the contact separates.
•Due to increase in the mean free path when the contact separates.
Arc extinction-The phenomenon of the arc formation is inevitable when the contacts separated during the faults.
The main reasons to form the arc are
1. The high voltage gradient between the contacts
The arc formed by the high gradient can be reduces by making the high separation between the contacts but this is not possible for the high voltage system because the separation need very large.
2. The ionization of neutral particles.
Also the formation of ions is proportional to number of electron per cubic centimeters, so to avoid this we need to have the high diameter which is again not practical.
Following methods are use for the arc extinguishing.
A. High resistance method
The high resistance method uses the process to increase the effective resistance of the arc with the time so that the current is reduced to such a value that heat produce by it is not sufficient to produce the arc. During this the heat is dissipated inside the circuit breaker the circuit breaker should be designed to withstand such large quantities of energy.
The resistance of the arc can be increased by the following ways.
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Cooling of arc The arc resistance can be increased by adding the neutral or cold air between the contacts.
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Increasing the length of arc The resistance of the arc can be increased by increasing the mean length between the contacts. This decreases the voltage gradient of the contact and the arc phenomenon can be reduces. But this process is not practical because this increases the length of the contacts for the high voltage system.
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Reducing the cross section area The cross section of the arc can be reduced by having the small contact surface area or letting the arc pass through the small hole to reduce the arc. This process can help to reduce the voltage necessary to maintain the arc.
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Splitting the arc This is the best method of increasing the resistance of the arc. The arcs so formed are spitted into the small channels to reduce the effect of it. The provision of splitter is designed in the circuit breaker and the formed arc is passed through it to form the series of arc into the splitter. This increases the mean length of the arc and the cooling is better. B. Low current or the current zero interruption. This method is applicable for only the ac supply because in this we get have the zero current in each half cycle. In this process when the current reaches the zero value it has the minimum effect and the fresh air is supplied to turn down the arc. This method is widely used in the modern circuit breaker. This phenomenon is explained by the given theories.
• Energy balance or Cassie theory: This theory states that if the rate of heat dissipation between the contacts is greater than the rate at which heat is generated, the arc will be extinguish, otherwise it will restrike. During the faults the high heat is produced due to the higher voltage gradient or the high current densities between the contacts. Thus if the heat generated could be removed by cooling, lengthening or by arc splitter at a higher rate than the generation of the arc, then the arc will be extinguish.
• Recovery rate or Slepian’s theory: This theory states that is the rate at which the ions and the electrons combine to form or are replaced by the neutral molecules i.e. the rate at which the gap recovers its dielectrics strength is faster then the rate at which voltage stress rises, the arc will be extinguished: if otherwise the arc may be interrupted for a brief period but it again restrike. So in this process when the current is at zero value, the fresh air is entered to neutral the electrons. For this the following process are applied:
a) Lengthening the gap The dielectrics strength and the length between the contacts are proportional to each other. Lengthening the contact gap can be the obvious process. The permissible arc length is limited; however, by other considerations e.g. arc energy and possibility of transient voltages due to the current chopping.
b) Increasing the pressure in the vicinity of the arc By increasing the pressure the density of the particle constituting the discharge also increases. The increased density of particle causes higher rate of deionization and thus the dielectric strength of the medium between the contacts is increased.
c) Cooling If the particle is allowed to cool the natural combination of ionized particles will take place more rapidly resulting increase in dielectric strength of the medium. Cooling by conduction to adjacent parts e.g. baffles or by the use of gas such as hydrogen that has as high diffusion and great absorption rate is, therefore, effective. d) Blast effect By blowing a stream of air through the arc ionized particles between the contacts are swept away and replaced by unionized particles. These unionized particles increased the dielectrics strength of the, medium considerably.
RESTRIKING VOLTAGE IN CIRCUIT BREAKER
•In ac circuit breakers, the current interruption takes place invariably at the natural zeroes of current wave.
•At current zero, a high frequency transient voltage appears across the breaker contacts and is caused by the rapid distribution of energy between the magnetic and electric fields associated with the plant and transmission line of the power system.
•This transient voltage is known as the restriking voltage. This voltage appearing across the breaker contacts at the moment of final current zero has a profound influence on the arc extinction process.
•Under the influence of this voltage the arc tries to restrike and hence it is named as restriking voltage. After current zero, the arc gets extinguished if the rate of rise of restriking voltage between the contacts is less than the rate at which dielectric strength of the medium between the contacts recovered.
•Thus, restriking voltage may be defined as the resultant transient voltage which appears across the breaker contacts at the instant of arc extinction.
