Selecting the Right Circuit Breaker and Its Type

1. Selecting the Right Circuit Breaker and Its Type A circuit breaker is a power system protection device that can make or break a circuit A circuit breaker operates under fault conditions and isolates the faulty part of the circuit from the rest of it by breaking the circuit. This operation is performed automatically by employing a relay along with the circuit breaker. It should be noted that circuit breakers may also be manually operated and can also be operated under normal conditions. Hence circuit breakers are also useful switching devices that are used to make or break a circuit in normal conditions. Working Mechanism: In a general sense, a circuit breaker consists of two electrodes or contacts that under normal conditions remain in contact with each other allowing current to flow. But in case of a fault, the contacts open or become disconnected hence breaking the circuit and preventing the fault current from passing through. The opening of contacts is achieved by energizing the tripping coil of the circuit breaker which causes the contacts to move. It is also important to know that the tripping coil is energized by the relay, so basically it is the relay that signals to the circuit breaker to operate. These contacts can also be opened manually, for example during maintenance or switching. Arc Phenomena: Whenever a short circuit fault occurs, an extremely high current passes through the contacts of the circuit breakers. When these contacts start to open, the area of contact decreases and the current intensity increases rapidly. This causes the surrounding material to heat up rapidly and ionize. This ionized medium thus acts as a path for current to flow delaying the breaking of the circuit path. This can result in damage to the system and the heat produced can damage the circuit breaker itself. The potential difference between the contacts is quite small but enough to maintain the arc. Arc Extinction Methods: This arc needs to be eliminated for the successful isolation and breaking of the circuit. Therefore, it is a major factor in determining the type and size of the circuit breaker to be used in different applications. For this purpose, we have two methods for arc extinction. 1. High Resistance Method: In this method, the resistance of the arc is increased with time and is increased until the current value drops insufficient to maintain the arc. The disadvantage is the huge loss of energy and heat dissipated in the arc. 2. Low Resistance or Current Zero Method:

2. This method is used for A.C systems and is most widely used. All sinusoidal current and voltages pass through the zero crossings at every half cycle. The resistance is kept low until the zero crossings where the arc extinguishes naturally, after the zero crossings; the arc is prevented from reoccurring by the quenching medium. The fastest possible circuit breaker to date can extinguish the arc in 2 cycles while the most common mediums used for quenching the arc are air, oil, Sulphur Hexafluoride SF6, and Vacuum. Categories of Circuit Breakers: Circuit breakers can be categorized according to the corresponding voltage level of the system. Therefore, they can be divided into LV, MV and HV breakers. LV Circuit Breakers: These breakers are used for voltages up to 600 V and are further classified into 3 types. 1. Molded Case (MCCBS): They are used for currents of around 20 Amps to 2500 Amps and are often used to switch ON or OFF a circuit. They are placed in a sealed case hence they are not maintainable and are usually applied in switchboards and panel boards. It should be noted that MCCBS should be tested in compliance with UL489 and NEMA AB-1 standards. 2. Power CB: Power CBs have rated currents of 800 Amps to 6000 Amps. They are used for generator protection and motor protection. Power CBs are mounted on metal enclosures for LV switchgear and should be tested under ANSI C37.13 and UL1066 standards. 3. Insulated Case (ICCBS): ICCBS are essentially the same as Molded case circuit breakers. However, they include a 2-step stored energy closing mechanism. A charging handle or motor charges a spring, which is then released by a pushbutton or solenoid to finally close the breaker. They usually have frame sizes of 800 Amps to 4000 Amps. They usually have frame sizes of 800 Amps to 4000 Amps and are used in MCCs or as a main breaker in a switch board. ICCBs are also tested in compliance with UL489 and NEMA AB-1 standards MV and HV Circuit Breakers: MV breakers are used for 600 V to 69 KV systems, while HV breakers are applied to systems having voltages greater than 69KV. The type of medium that exists inside these circuit breakers is used to classify them. They are as follows: 1. Oil Circuit Breakers:

3. The Main contacts are immersed in oil which acts as the ionizing medium. Oil has a high dielectric strength to withstand the voltage across the contacts. The arc decomposes the oil into gases which have excellent cooling properties to extinguish the arc. However, Oil, as well as hydrogen gas, is flammable hence there is a risk of fire. These breakers also require timely inspection and replacement of oil. OCBs are used voltages up to 11KV. 2. Air Circuit Breakers: These Circuit breakers use a high-pressure air blast as the arc quenching medium. The air blast cools down the arc and pushes away the arc products into the atmosphere consequently extinguishing the arc. Air circuit breakers have now mostly replaced oil circuit breakers because there no fire hazards involved. They are also compact and have lesser arcing time. Most of the high voltage systems above 110 KV employ Air circuit breakers. 3. SF6 Circuit Breakers: Sulphur-hexafluoride (SF6) is an inert insulating gas which is used as the arc quenching medium. It has superior arc quenching properties because SF6 tends to absorb free electrons, hence an arc is quickly insulated due to the loss of conducting electrons.SF6 circuit breakers for voltages up to 115 KV and 230 KV with interrupting time of fewer than 3 cycles also exist. However, these circuit breakers are extremely costly. In these breakers, the vacuum is used as the arc quenching medium. It offers the strongest insulation properties than any other medium. Therefore, as soon as an arc is produced in this circuit breaker, it is immediately extinguished. These are used in 22KV to 66 KV systems. Visit https://aic.in/ for Switchgears and Circuit Breakers India, SPD Protective Device, Switchgear Suppliers, Residential Current Devices India, Air Circuit Breaker Suppliers India, ACB Switchgear Suppliers in India, Switch Gear India, MCCB Switchgear Suppliers India, ABB Switch gear, MCCB Switchgear, Soft Starters India, ABB Soft Starters, Mini and step power supply, Electronic products suppliers in india.