Types of Circuit Breakers

1. Vacuum Circuit Breaker (VCB):

In VCBs, the fixed and moving contacts, along with the arc shield, are housed in an arc-interrupting chamber with a high vacuum. The vacuum's excellent dielectric strength allows for very short contact separation and rapid recovery of insulation strength after each interruption.

Merits:

• Compact size and quick operation due to minimal contact travel.
• Low maintenance and long operational life.
• No need for periodic refilling (unlike oil or gas CBs).
• Rapid recovery of dielectric strength.

Demerits:

• System failure is possible if there is a minor vacuum leakage.
• High cost of manufacturing.

2. SF₆ (Sulfur Hexafluoride) Gas Circuit Breaker:

These breakers utilize high-pressure SF₆ gas as an arc-extinguishing medium. The gas absorbs free electrons from the arc path, forming ions that increase the medium's dielectric strength. The gas is later recycled back to a high-pressure reservoir for reuse.

Merits:

• Excellent arc-extinguishing and insulating properties.
• Non-flammable and chemically stable.
• Does not produce toxic fumes or explosive decomposition products.
• Noiseless operation and requires minimal maintenance.

Demerits:

• SF₆ gas is a potent greenhouse gas and harmful if leaked.
• Requires a dry atmosphere; moisture can lead to operational failures.
• Suffocating gas that settles at the bottom due to its weight.

3. Oil Circuit Breaker:

When an arc forms, the surrounding oil evaporates and dissociates, producing hydrogen gas. The hydrogen displaces the oil around the arc, cools it, and provides a cooling effect to extinguish the arc.

Merits:

• The oil absorbs arc energy and provides effective cooling.

Demerits:

• Risk of fire and explosion due to the combustible nature of oil.
• Oil quality deteriorates over time, requiring periodic renewal.

4. Air Blast Circuit Breaker:

High-pressure air is introduced into the arc chamber through a nozzle when a fault occurs. The air cools the arc and sweeps away ionized particles, increasing the dielectric strength of the medium

Merits:

• Faster arc quenching and breaking speed.
• No risk of fire.
• Requires minimal maintenance.

Demerits:

• High maintenance requirements for the air compressor system.
• Possibility of air leakage from the system.

Arc Phenomenon:

When the contacts of a circuit breaker begin to separate under fault conditions, the contact area reduces rapidly. This reduction, combined with high fault current, increases the current density and causes a rise in temperature. The heat ionizes the surrounding medium, creating a conductive path for the current, which results in the formation of an arc between the breaker contacts. This arc persists as long as the ionized medium provides a low-resistance path, keeping the circuit energized.

Arc Mitigation Techniques:

• Increase the separation between the contacts to ensure that the potential difference across them is insufficient to sustain the arc.
• Use mediums like high-pressure SF₆ gas, vacuum, or air blasts to de-ionize the medium and extinguish the arc.
• Employ materials with high dielectric strength to recover insulation between contacts rapidly.