Characteristics of DC vacuum arc in the transverse axially symmetric magnetic field

The characteristics of DC vacuum arc in axially symmetric magnetic field under contact opening are experimentally investigated. The magnetic field is formed with the help of the constant magnet placed on the side of the fixed contact. The magnet position is chosen so that near the working area of contacts at radius approximate to 12 mm. there is mainly a radial component of the magnetic field of about 100-150 mT. Studies revealed two burning phases of the switching-off vacuum arc. At the initial phase the arc bums steadily, voltage slowly increases as contacts are opened, and discharge channels are concentrated inside the intercontact gap. Duration of this phase is of some milliseconds. Then the arc goes over into an unstable phase, which is characterized by a significant voltage noise. The unstable phase of arc burning is terminated by switching-off a current. During this phase the arc burns beyond the intercontact gap. Evolution of these phases is investigated depending on the arc current in the interval of 50-300 A.