Effect of Arcing Time with Capacitive Making Current on Contact Welding in Vacuum

When using vacuum interrupters to switch capacitive current, the degree of insulation between the contacts tends to degrade as the broken welds are formed on the contact by making and opening operation. These broken welds locally enhance the electric field between contact by roughening the contact surface. Previous studies have found that the amplitude of the current has a major impact on breakdown electric field, while the impacts of arcing time and total arc energy remain ill-understood. This article reports an investigation of the impact of arcing time on the insulation between contacts. The arcing time affects the electric field in two distinct regions. Breakdown electric field decreases as arcing time increases for arcing times less than 1ms. On the other hand, breakdown electric field does not necessary degrade with further long arcing time. This trend is explained by vacuum arc diffusion. The equivalent current density of the vacuum arc decreases over time during each discharge process. The temperature of the contacts was calculated from the diffusion radius of vacuum arc observed using high speed cameras. These calculations reveal that the melted volume of the contacts increases for arcing times 1 ms, and subsequently declines with increasing arcing time. The resulting increased melted volume decreases the electric field. These results show that the relationship between electric field and total arc energy is complicated, as high arc energies do not necessary result in poor insulation performance.