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The characteristics of a DC vacuum arc in an 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 ≈12 mm, there is mainly a transverse 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 burns steadily, voltage slowly increases as contacts are opened, and discharge channels are concentrated inside the intercontact gap. The duration of this phase is several milliseconds. Then, the arc goes into an unstable phase characterized by a significant voltage noise. The unstable phase of arc burning is terminated by switching off the current. During this phase, the arc burns beyond the intercontact gap. Evolution of these phases is investigated as a function of the arc current in the interval of 50-300 A.