What is Arc Phenomenon in Circuit Breaker?
The Arc Phenomenon in Circuit Breaker refers to the electrical discharge that occurs when the contacts of a circuit breaker begin to open, typically during a fault or short circuit condition. As the contacts separate, the current density between them increases, causing a rise in temperature and the emission of electrons from the cathode. The ionized air surrounding the contacts acts as a conductor, allowing an arc to be struck between the two contacts, providing a low-resistance path for current to flow through.
During normal operation, a circuit breaker's contacts are closed, allowing current to flow through the circuit. However, if a fault or short circuit occurs, the fault sensing unit signals the circuit breaker to open its contacts, interrupting the current flow. As the contacts begin to separate, the current density between them increases, which can cause a rise in temperature. This increase in temperature results in the emission of electrons from the cathode, which can ionize the air surrounding the contacts.
Ionized air is a conductor, which means that it can carry an electrical current. When the ionized air is present between the contacts, it creates a path for the current to flow through, in the form of an arc. The arc is maintained by a small potential difference between the contacts, which is sufficient to keep the arc going. The resistance of the arc is relatively low, which allows the current to continue flowing through the circuit, even after the contacts have separated.
It should be noted that the arc can be very hot, and it can cause damage to the contacts and surrounding materials. Additionally, the arc can produce harmful gases, such as ozone and carbon monoxide. To minimize the damage caused by the arc, circuit breakers are designed to extinguish the arc as quickly as possible, typically by using techniques such as arc quenching, interrupting the current and cooling the contacts.
Principle of Arc Extinction:
The principle of arc extinction refers to the methods used to extinguish or interrupt the arc that forms between the contacts of a circuit breaker during a fault or short circuit condition. The arc is maintained by two factors: the potential difference between the contacts and the presence of ionized particles in the medium between the contacts. To extinguish the arc, one or both of these factors must be eliminated.
One method of extinguishing the arc is by decreasing the potential difference between the contacts. This can be done by increasing the distance between the contacts, which reduces the potential difference required to maintain the arc. However, this method is only applicable for low operating system voltages, as the separation distance required for higher voltages can be many meters, making it impractical.
Another method of extinguishing the arc is by deionizing the medium between the contacts. The ionized particles present in the medium between the contacts are what allow the arc to persist. By removing or neutralizing these particles, the arc can be extinguished. This can be achieved by cooling the arc, which reduces the temperature and ionization of the medium, or by removing the ionized particles between the two contacts. This can be done by passing a highly electronegative gas, such as sulfur hexafluoride (SF6) or Nitrogen, between the contacts. These gases have a strong tendency to attract electrons, which neutralizes the ionized particles present in the medium, effectively extinguishing the arc.
It should be noted that, different circuit breaker designs use different arc extinction techniques depending on the voltage level, current level and the application. For example, high-voltage circuit breakers generally use SF6 gas to extinguish the arc, while low-voltage circuit breakers may use air as the medium and cooling the arc by blowing air across the contacts.
Methods of arc Extinction:
There are two methods are used for arc extinction in circuit breakers:
1. High Resistance Method of arc extinction
2. Low Resistance Method or Current Zero Method
1. High Resistance Method of arc extinction:
The High Resistance Method of arc extinction is based on the principle that the current flowing in the circuit during an arc event is directly proportional to the resistance of the arc. By increasing the resistance of the arc, the current can be decreased to a very low value, which extinguishes the arc. This method of arc extinction can be implemented in several ways, including:
- Increasing the length of the arc: The resistance of an arc is directly proportional to the length of the arc and inversely proportional to the area of the cross-section of the arc (R ∝ ρl/A). By increasing the length of the arc, the resistance is increased, and the current is decreased, which extinguishes the arc.
- Reducing the area of cross-section of the arc: Similarly, by reducing the area of the cross-section of the arc, the resistance is increased, and the current is decreased, which extinguishes the arc.
- Cooling the Arc: Cooling the arc can also increase the resistance of the arc by reducing the ionized particles present between the contacts. This can be achieved by passing a coolant, such as air or water, across the arc. The reduction of ionized particles increases the resistance between the contacts and hence decreases the current, thus extinguishing the arc.
- Splitting the Arc: In this method, the arc is divided into a number of sections by using arc dividing shields. Each section is cooled, and the length of the arc is increased, which increases the resistance of the arc, thus decreasing the current and extinguishing the arc.
It should be noted that, High resistance method is suitable for high-voltage circuit breaker and not suitable for low-voltage circuit breaker because they require high resistance to extinguish the arc.
2. Low Resistance Method or Current Zero Method:
The Low Resistance Method or Current Zero Method of arc extinction is applicable only to AC circuits, because of the property of AC current to become zero twice in a cycle. When the current becomes zero, the arc extinguishes for a longer duration of time. However, during the next half-cycle, the arc can reappear again if the dielectric strength of the air gap is less than the voltage gradient between the contacts.
To prevent the arc from reappearing, it is necessary to improve the dielectric strength of the medium surrounding the ionized particles. This can be done by cooling the medium, sweeping out the ionized particles, or deionizing the medium. The different methods for deionizing the medium for extinguishing arc are:
- Increasing the Length of the Gap: The dielectric strength of the medium can be increased by increasing the length of the arc. As the dielectric strength ∈ is directly proportional to the length l i.e., ∈ ∝ l. Once the dielectric strength of the medium is increased, the arc fails to restrike again and the flow of current is interrupted, thus extinguishing the arc.
- Cooling: Cooling the medium reduces the temperature and ionization of the medium, which deionizes the ionized particles into neutral molecules. This increases the dielectric strength of the medium and extinguishes the arc.
- High Pressure: If the pressure near the contacts is maintained at a high value, it helps in the rapid deionization of the ionized particles to neutral molecules, thereby increasing the dielectric strength of the medium.
- Blast Effect: In this type of effect, either high-pressure oil is forced to pass through the arc gap or gas is used to sweep away the ionized particles and improve the dielectric strength of the medium. This method is also known as interruption by blast, as it uses a high-pressure blast of air or gas to interrupt the current and extinguish the arc.
It should be noted that, the High resistance method and the AC current dependent method are the two main methods to extinguish the arc in AC circuit breaker. The selection of the method depends on the voltage level and the current level of the circuit.
Important Terms:
In the context of circuit breaker, it's important to understand several key terms related to the arc phenomenon and arc extinction.
- Arc Voltage: Arc voltage is the voltage across the contacts of the circuit breaker during the arcing period. This is the voltage that maintains the arc and is responsible for the current flow through the arc. The arc voltage is usually higher than the normal system voltage and is dependent on the current flowing through the arc.
- Restriking Voltage: When the circuit breaker interrupts at the time of current zero, there will be a rapid increase in the voltage across the circuit breaker contacts. This transient voltage spike is known as restriking voltage. It is due to the rapid increase in voltage across the contacts after the current has been interrupted. This voltage spike is usually high in magnitude and can cause damage to the circuit breaker if not properly handled.
- Recovery Voltage: The voltage that appears across the contacts after the final arc extinction and after all the transients disappears is called Recovery Voltage. The recovery voltage is of normal frequency (50Hz) and is approximately equal to the system voltage. This voltage is the voltage that the circuit breaker is designed to handle, and it is the normal operating voltage of the system.
It's important to note that these terms are used to analyze the performance of the circuit breaker, and to ensure that the circuit breaker can extinguish the arc without getting damaged. By understanding these terms and the voltage levels involved during the arc event, it's possible to design and optimize the circuit breaker to operate reliably and safely.