During the switching-off process, as the contacts separate, the contact area diminishes, leading to an increase in current density and contact resistance; this results in a rise in temperature, triggering thermionic emission. Simultaneously, the extremely narrow gap between the contacts generates an exceptionally high electric field strength, inducing field emission. The free electrons generated by these thermionic and field emissions accelerate under the influence of the electric field, and through ionizing collisions, they establish a conductive channel. Once the electric arc has formed, thermal ionization-driven by the high temperatures-becomes the primary factor sustaining the arc's combustion.
Common arc-extinguishing methods employed in switching devices include mechanical force extinction, dielectric arc-blowing, cooling extinction, short-arc extinction, multi-break extinction, parallel-resistor extinction, and the utilization of novel dielectric media. Mechanical force extinction-such as the "rapid-draw" method-extinguishes the arc by rapidly separating the contacts to elongate the arc column. Dielectric arc-blowing utilizes pressurized gas or oil directed into the arc gap; this technique encompasses both axial-blowing and cross-blowing configurations. Cooling extinction attenuates thermal ionization by lowering the temperature of the electric arc. Short-arc extinction employs a metal arc-splitting grid to segment a single long arc into a series of shorter arcs, thereby utilizing the near-cathode effect to limit current flow and extinguish the arc.
Multi-break extinction reduces the recovery voltage across each individual contact gap by arranging multiple gaps in series. Parallel-resistor extinction involves connecting a resistor in parallel across the main contact gap to improve arc-extinguishing conditions and limit switching overvoltages. Finally, the adoption of novel media involves utilizing superior arc-extinguishing substances, such as SF6 gas or a vacuum.
The extinction of an AC arc is determined by the interplay between the recovery process of the dielectric strength within the arc gap and the recovery process of the voltage across the gap; when the former exceeds the latter, the electric arc ceases to reignite.
