Opening a HV power line. Current continues to flow through the arcs on the three-phase line because the air becomes an ionised plasma, providing a conductive path (just like when arc welding). Under normal circumstances, air is an insulator (doesn't conduct electricity). However if voltage is high enough, it does breakdown and become conductive as evidenced by the phenomenon of lightning. Its dielectric strength is 3 kV/mm, meaning that between rounded electrodes, it takes 3000 V for air to breakdown and become conductive when conductors are spaced 1 mm apart. However conduction continues when the conductors are separated to beyond 1 mm, the plasma sustaining conduction. Eventually when the spacing is great enough, an arc between them will extinguish. Circuit breakers in sub-stations are filled with oil with a high dielectric strength so that when contacts open, the oil quickly extinguishes the arc.