What is meant by arc quenching in circuit breakers?

Short Answer:

Arc quenching in circuit breakers refers to the process of extinguishing the electric arc that forms between the breaker contacts when they open during fault conditions. When a circuit breaker interrupts high current flow, an arc is generated due to the sudden separation of current-carrying contacts.

The purpose of arc quenching is to quickly and safely break the current flow, prevent equipment damage, and ensure the breaker can be reset and reused. Different types of circuit breakers use different arc quenching methods, such as air, oil, vacuum, or SF₆ gas.

Detailed Explanation:

Arc quenching in circuit breakers

In an electrical system, when a fault such as a short circuit occurs, the current rises sharply. To protect the equipment, the circuit breaker operates to disconnect the faulty section by opening its contacts. However, during the separation of contacts, the current does not stop immediately. Instead, it jumps across the gap between the contacts in the form of an electric arc.

This arc has a very high temperature (up to several thousand degrees Celsius) and can damage the contacts and surrounding insulation. Therefore, the circuit breaker must be designed to quickly extinguish or quench this arc, which is the key function known as arc quenching.

Why arc quenching is important

  • Prevents contact damage: Reduces erosion and wear of contacts.
  • Ensures insulation safety: Avoids excessive heating of insulating material.
  • Minimizes re-ignition risk: Prevents the arc from re-forming after the initial break.
  • Allows quick circuit isolation: Enables fast fault clearing and system stability.
  • Improves breaker life: Helps maintain the durability and reuse of the circuit breaker.

Arc quenching methods in different circuit breakers

  1. Air Blast Arc Quenching:
    • High-pressure air is used to blow the arc away from the contacts.
    • The air cools and elongates the arc, causing it to break.
    • Used in high-voltage applications.
  2. Oil Arc Quenching:
    • The arc is formed in insulating oil, which decomposes and forms gas bubbles.
    • These gases help to cool the arc and interrupt current flow.
    • Common in older medium-voltage breakers.
  3. Vacuum Arc Quenching:
    • The arc is created and extinguished in a vacuum chamber.
    • Vacuum has no medium to sustain the arc, so it breaks very quickly.
    • Used in medium-voltage vacuum circuit breakers (VCBs).
  4. SF₆ Gas Arc Quenching:
    • Sulfur hexafluoride (SF₆) gas has excellent arc extinguishing properties.
    • The arc is cooled and diffused in the gas, leading to fast arc extinction.
    • Used in high-voltage and extra-high-voltage circuit breakers.
  5. Arc Chutes (in MCBs):
    • In miniature circuit breakers, metal plates divide the arc into smaller arcs.
    • This increases resistance and helps cool and quench the arc quickly.

Key steps in arc quenching

  1. Arc elongation: Stretching the arc increases its resistance.
  2. Arc cooling: Lowering the arc temperature reduces its energy.
  3. De-ionization of contact gap: Removing ionized particles from the arc path to stop conduction.
  4. Increasing dielectric strength: Ensuring the gap becomes non-conductive before re-ignition.
Conclusion:

Arc quenching in circuit breakers is the essential process of safely extinguishing the high-temperature arc formed when contacts open during a fault. Effective arc quenching protects equipment, maintains system safety, and ensures the reliable operation of the breaker. Various technologies like air, oil, vacuum, and SF₆ gas are used depending on the voltage level and application needs.