Dielectric Recovery and Interruption Variability of Air Arcs at Atmospheric Pressure

The dc dielectric recovery method has been used extensively to study processes in post-current-zero arc columns in circuit breakers. This paper describes recovery experiments in air at atmospheric pressure following arcs drawn between butt and cone-shaped electrodes of different materials and surface profiles for contact gap spacings of 2 mm and 1 cm. The experiments were carried out with 400 pusec duration current pulses between 10 A and 1000 A for reignition voltage between 300 V and 30 kV. A statistical recovery experiment was performed using the Normal Approximation to the Binomial and Probit analysis methods to determine the standard deviation of the recovery data. The experiments show that recovery is faster: (i) with Ag-CdO contacts than with any other material tested, (ii) with flat contacts than with those having a high-point at the contact center and (iii) with cone-shaped rather than butt contacts. These results are then related to ac interruption, and are explained in terms of physical processes related to: (i) the electrodes, such as thermal properties of electrode materials or contact surface properties; (ii) the gas, such as density reduction and ionizing and electron-attaching processes; and (iii) magnetic pumping and looping effects.