Quiz On Circuit Breaker

The arcing contacts of circuit-breakers are generally made of copper tungsten alloy because this material has excellent electrical conductivity and high resistance to heat and arcing. Copper provides good conductivity, while tungsten increases the resistance to arcing, preventing damage to the contacts. This combination ensures efficient current interruption and prolongs the lifespan of the circuit-breaker.

SF6 gas is commonly known as sulphur hexafluoride.

The property of gas that should be considered while selecting a gas for a circuit breaker is high dielectric strength, non-inflammability, and non-toxicity. High dielectric strength ensures that the gas can withstand high voltages without breaking down. Non-inflammability is important to prevent fires or explosions in case of a fault. Non-toxicity ensures the safety of personnel working with the circuit breaker. Therefore, all of these properties are crucial in determining the suitability of a gas for a circuit breaker.

After the final current zero, when the circuit breaker pole opens, there is a sudden change in the voltage across it. This change in voltage is known as the recovery voltage. It occurs due to the interruption of the current flow and the subsequent re-establishment of the dielectric strength in the circuit. Therefore, the statement that the voltage across the circuit breaker pole after the final current zero is the recovery voltage is true.

SF6 oil circuit breakers are preferred for EHT (Extra High Tension) applications because they have excellent arc quenching properties and high dielectric strength. SF6 gas is used as the insulating medium in these circuit breakers, which allows for compact design and high reliability. Additionally, SF6 gas has a high thermal conductivity, which helps in dissipating the heat generated during the arc interruption process. This makes SF6 oil circuit breakers suitable for high voltage applications where high breaking capacity and reliable operation are required.

The interrupting medium in a contactor refers to the substance that is used to extinguish the arc that is formed when the contacts of the contactor open. It needs to be able to quickly and effectively extinguish the arc to prevent damage to the contactor and ensure proper functioning. Air, oil, and SF6 gas are all commonly used interrupting mediums in contactors. Therefore, the correct answer is (D) any of the above, as any of these substances can be used as the interrupting medium depending on the specific requirements and application of the contactor.

The medium employed for extinction of arc in an air circuit breaker is air. Air circuit breakers use the surrounding air as the medium to extinguish the arc. When the circuit breaker is opened, the contacts separate, creating a gap. The arc is formed in this gap due to the ionization of air. The air surrounding the arc cools and deionizes, which helps in extinguishing the arc and breaking the circuit. Air is a commonly used medium for arc extinction in low voltage circuit breakers.

In air blast circuit breakers, the pressure of air is of the order of 20 to 30 kg/cm2. This high pressure of air is necessary to quench the arc formed during the circuit breaking process. When the circuit breaker opens, an arc is formed between the contacts. The high pressure air is used to blow out the arc and rapidly cool the contacts, extinguishing the arc. This high pressure is required to ensure efficient arc extinction and prevent re-ignition of the arc.

SF6 gas has a density that is 5 times greater than that of air. This means that SF6 gas is much heavier than air.

A sudden short-circuit in an AC system causes a rise in current in the short-circuited phase. When a short-circuit occurs, the normal flow of current is disrupted, and a large amount of current is diverted through the short-circuited path. This leads to an increase in current in the short-circuited phase, as it becomes the path of least resistance for the electrical current.

Air circuit breakers are not typically used for voltages above 12 kV. Instead, they are commonly used for lower voltage applications, typically ranging from 400 volts to 10 kV. For higher voltage levels, other types of circuit breakers such as vacuum or SF6 circuit breakers are preferred due to their better insulation and interrupting capabilities. Therefore, the statement that air circuit breakers are used for voltages above 12 kV is incorrect.

The pressure of SF6 gas in circuit breakers is typically maintained at a range of 3 to 5 kg/cm2. This pressure is necessary to ensure effective insulation and arc quenching properties of the gas. It allows the circuit breaker to interrupt the flow of current and extinguish any resulting arcs safely and efficiently. Higher pressures may cause excessive stress on the equipment, while lower pressures may compromise the effectiveness of the SF6 gas as an insulating and quenching medium. Therefore, maintaining the pressure within the specified range is crucial for the proper functioning of circuit breakers.

After the final current zero, when the circuit breaker opens, a voltage is still present across its poles. This voltage is known as the "recovery" voltage. It occurs because the interruption of the current flow creates a transient voltage that needs to be suppressed or absorbed by the circuit breaker. The recovery voltage is important to consider because it can affect the performance and lifespan of the circuit breaker.

The rated short circuit current is the maximum current that a circuit breaker can safely interrupt without causing damage. For high voltage, AC circuit breakers, the rated short circuit current is passed for 30 seconds. This means that the circuit breaker can safely interrupt the maximum current for this duration without any issues.

The insulation requirements of switchgear are determined by transient voltage. Transient voltage refers to sudden and temporary changes in voltage levels that can occur in electrical systems. These voltage fluctuations can pose a risk to the insulation of switchgear, as they can cause insulation breakdown or damage. Therefore, the insulation requirements of switchgear need to be designed to withstand these transient voltage spikes and ensure the safe and reliable operation of the equipment.