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Experimental studies have been carried out on characteristics of the direct current vacuum arc during contact separation in axially symmetrical magnetic field that has predominantly a radial component. The study results on the dynamics of the plasma flow and voltage at the vacuum arc are presented for the stable stage of the arc at currents in the range of 50-300 A. Experimental relationships between the stable stage duration and the values of the current and the induction of the magnetic field are obtained. The performed studies enabled to determine and interpret typical peculiarities of the development of the stable stage of vacuum arc during the contact separation. It is shown that during the contact separation the voltage of the arc increases slowly from 15 to 25-30 V up to the transition into unstable stage of the arc. In this period, one discharge channel burns in the intercontact gap. The duration of the stable stage has a statistical character and its average value increases with current and decreases with the radial component of magnetic field induction. The observed phenomena are explained in terms of a 1-D stationary model of plasma flow in the transverse magnetic field. The instability is caused by a crisis in the plasma flow resulting from the decrease of velocity of fast cathode ions to the ion-sound velocity. The calculations of the stable stage duration of arc burning made within this model are in fair agreement with the obtained experimental results.