What is an Arc ? | Arc in Circuit Breaker

An electrical arc is a conductive plasma channel that can form between separating contacts. Through arc quenching or arc extinction, a circuit breaker controls this channel so that current stops and the contact gap withstands the recovery voltage. This page explains what is arc in that switching context.

What is an Arc?

When contacts carrying current separate, the electric field and heat can ionise the material in the widening gap. The resulting hot plasma conducts current after the metal contacts are no longer touching. This luminous conductive discharge is an electrical arc.

Arc in Circuit Breaker

An arc in circuit breaker can form when the breaker opens a current, although its behaviour depends on the current, circuit and interruption medium.

While the arc remains conductive, current continues across the open contacts. Successful arc quenching requires the arc to extinguish and the gap to regain dielectric strength fast enough to withstand the transient recovery voltage. Breaker design therefore controls arc energy and post-arc recovery for its rated duties. The physical processes described here support the later discussion of arc quenching techniques and the behaviour of an arc in circuit breaker.

Thermal Ionization of Gas

Ordinary gas contains a small background population of charged particles, but it remains a poor conductor until enough charge carriers are produced. Near 300^oK, molecules have random thermal motion with typical speeds of several hundred metres per second. Molecular collisions occur extremely often on microscopic timescales and can reach an order near 10¹⁰ per second under representative conditions; the exact values depend on the gas, pressure and temperature.

Heating increases particle energy. At sufficiently high temperatures, collisions can dissociate molecules and remove electrons from atoms. The mixture then contains electrons, positive ions and neutral particles. This electrically conductive state is plasma, and its production by heating is called thermal ionization of gas.

Ionization due to Electron Collision

A strong electric field accelerates available electrons between collisions. If an electron gains enough energy, its collision with a neutral particle can release another electron. This impact ionisation increases the number of charge carriers.

The newly released electrons also accelerate in the electric field and may cause further impact ionisation. This multiplication process can turn an insulating gap into a conductive discharge. It is called ionization of gas by electron collision.

Deionization of Gas

When energy input falls, ionization of gas decreases. Electrons and positive ions recombine, while charged particles can also diffuse to cooler boundaries and be neutralised. These processes reduce conductivity and are collectively described as deionisation.

Role of Arc in Circuit Breaker

The arc carries current while the contacts move apart, so current does not stop merely because the contacts separate. The circuit and breaker then determine the switching voltage. For an ideal inductive element with inductance L, the relation is V = L.(di/dt), where di/dt is the rate of current change. In an AC breaker, the arc current passes through a natural current zero. Interruption succeeds only if the gap recovers faster than the applied recovery voltage can cause breakdown and re-establish the arc in circuit breaker.

An arc is not a protective feature by itself. Its voltage and duration affect the rate of current change, energy dissipation and transient recovery voltage. The breaker must manage these effects and complete the transition from carrying current to the current breaking state within its tested ratings.

Arc Interruption or Arc Quenching or Arc Extinction Theory

Arc Column Characteristics

Charged particles in a hot gas have random thermal motion. An applied electric field adds a directed drift velocity. Mobility depends on particle species and on collisions with other particles, so gas composition, pressure and temperature affect arc conductivity. A useful interruption model must also account for energy transfer, gas flow, arc geometry and the changing contact gap.

The ionization process therefore depends on the medium, pressure, temperature and electric field. A gas-interruption chamber controls flow and cooling so that conductivity falls around current zero and dielectric strength recovers afterwards. Other circuit breaker technologies, such as vacuum interrupters, use different physical processes and design methods.

Heat loss from an Arc

An arc in a circuit breaker loses energy through conduction, convection and radiation. Their relative contribution depends on the medium and chamber design. In an air blast circuit breaker, forced gas flow cools and stretches the arc. For AC interruption, the chamber must reduce post-arc conductivity and restore dielectric strength after current zero.
The following sections describe two gas-based deionisation effects. They do not cover every interruption medium.

Deionization of Gas due to Increasing Pressure

For a given gas and temperature, higher pressure increases particle density and shortens the mean free path. More frequent collisions can reduce charged-particle mobility along the electric field and can increase recombination. Practical interruption performance is not set by pressure alone; gas properties, flow, temperature and chamber geometry act together.

Deionization of Gas due to Decreasing Temperature

Cooling reduces thermal ionisation and promotes recombination, which lowers plasma conductivity. As the ionization process weakens, the arc’s effective resistance rises and the gap can recover dielectric strength.
Different types of circuit breakers cool or replace the interruption medium in different ways. Their performance must be assessed as complete circuit breakers, not from one gas property alone.