Circuit Breaker

A circuit-breaker is an automatic device capable of making and breaking an electric circuit under normal and abnormal conditions in the event of overloading or short circuits. A circuit-breaker makes and breaks the electrical circuits as and when required. During opening the circuit, separation of the circuit-breaker contacts is required. Separation of the contact must happen in an insulating fluid, i.e. air, oil or gas. Insulating fluid is used to:

• Extinguish the arc produced due to the fault current interruption.
• Provide insulation between the contacts and from each contact to the earth.

Insulating Fluids:

Fluids suitable for an arc extinction of a circuit-breaker depend on the size and rating of the circuit-breaker. Commonly used insulating fluids in the circuit-breakers are:

• Normal air at atmospheric pressure
• Compressed air
• Hydrogen producing oil
• Ultra-high vacuum
• Sulphur hexafluoride gas (SF₆)
• Gases are most preferred insulating fluids for fast-operating circuit-breakers.

Properties of Insulating and Arc Quenching Materials:

• Should have high dielectric strength.
• Should be thermally and chemically stable.
• Should be non-inflammable.
• Should have high thermal conductivity to keep properly the contacts, conductors cool and to help quick arc quenching.
• Should have low dissociation temperature.
• Should not produce carbon during arcing or any conducting products.
• Should be commercially available.

Air is preferred for most of the applications because it can be compressed to extremely high pressure at room temperature. Under compression, its dielectric strength becomes better than the electronegative gases. Sulphur hexafluoride gas (SF₆) has better insulating and arc quenching properties over air at equal pressure. But the circuit-breakers using SF₆ at 14 kg/cm² or more pressure are required to have heaters installed in high-pressure reservoir. Heaters are required to heat up the SF₆ above the pressure 14 kg/cm² in order to avoid to be liquefied. The dielectric strength of SF₆ at normal pressure and temperature is about three times that of air. SF₆ also has dielectric strength comparable to the transformer oil at twice of atmospheric pressure.

Arc Interruption:

The arc between the circuit-breaker contacts consists of ionized gas particles. An arc interruption is possible, if the contact gap is deionized. Deionization process of the contact gap can be achieved by the following ways:

• High pressure
• Forced convention and turbulence
• Arc splitting

Questionnaire:

• What is circuit breaker? Explain.