In an SF₆ (Sulphur Hexafluoride) circuit breaker, the fixed contact is:

Explanation:

SF6 (Sulphur Hexafluoride) Circuit Breaker

Definition: An SF6 circuit breaker is a type of circuit breaker that uses sulphur hexafluoride (SF6) gas as the insulating and arc-quenching medium. SF6 gas possesses excellent dielectric properties, making it highly effective in interrupting high-voltage electric currents. These breakers are widely used in high-voltage applications due to their high reliability, compact design, and ability to handle large fault currents.

Working Principle: In an SF6 circuit breaker, the arc generated during the interruption of current is extinguished by the SF6 gas. When the contacts of the circuit breaker separate, an arc is formed between them. The arc heats up the SF6 gas, causing it to decompose into ions and other reactive components. These components absorb energy from the arc, cooling and extinguishing it rapidly. The gas then recombines into its original form, ready for subsequent operations.

Components of SF6 Circuit Breaker:

• Fixed Contact: The stationary part of the breaker where the moving contact makes and breaks the circuit.
• Moving Contact: The movable part of the breaker that moves away from the fixed contact to interrupt the circuit.
• Arc Quenching Chamber: The chamber where the arc is extinguished using SF6 gas.
• SF6 Gas Reservoir: The storage for SF6 gas under high pressure.
• Operating Mechanism: The mechanism that controls the movement of the contacts.

Correct Option Analysis:

The correct option is:

Option 4: A hollow cylinder contact with an arc horn.

The fixed contact in an SF6 circuit breaker is typically a hollow cylindrical contact equipped with an arc horn. This design is crucial for the effective operation of the circuit breaker. The hollow cylindrical shape allows the SF6 gas to flow uniformly around the contact area, ensuring efficient arc quenching. The arc horn serves to guide the arc and facilitate its extinction by ensuring that the arc is stretched and cooled effectively. This design ensures that the arc is extinguished quickly, minimizing damage to the contacts and ensuring reliable operation of the breaker.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: A spherical contact with an insulating shield.

This option is incorrect. While spherical contacts may be used in some types of electrical devices, they are not typically used as fixed contacts in SF6 circuit breakers. The spherical shape does not facilitate the effective flow of SF6 gas around the contact area, which is critical for efficient arc quenching. Moreover, an insulating shield is not a standard feature of the fixed contact in SF6 circuit breakers.

Option 2: A solid cylindrical contact with no additional fittings.

This option is also incorrect. While a solid cylindrical contact may provide a basic connection, it lacks the arc horn, which is essential for guiding and stretching the arc for efficient extinction. The absence of additional fittings like an arc horn would compromise the performance of the circuit breaker, making it less effective in handling high fault currents.

Option 3: A flat plate contact with thermal insulation.

This option is incorrect as well. A flat plate contact is not suitable for SF6 circuit breakers because it does not allow for the optimal flow of SF6 gas around the contact area. Additionally, thermal insulation is not a standard feature of fixed contacts in SF6 circuit breakers. The design of the fixed contact must prioritize efficient arc quenching, which is not achievable with a flat plate contact.

Conclusion:

In conclusion, the fixed contact in an SF6 circuit breaker is a hollow cylindrical contact equipped with an arc horn. This design ensures efficient arc quenching by allowing the SF6 gas to flow uniformly around the contact area and effectively cool the arc. The arc horn plays a crucial role in guiding and stretching the arc, facilitating its rapid extinction. The other options provided do not align with the design and operational requirements of SF6 circuit breakers, making option 4 the correct choice.