Phases of the arc current in a vacuum circuit breaker

The vacuum arc can be divided into 3 regions:

1. cathode spot region
2. interelectrode region
3. space charge sheath in front of the anode

This 3 phase show as photo:

1. Before current zero, ions are launched from the cathode towards the anode. At current zero, the ions that have just been produced continue to move towards the anode as a result of their inertia. Electrons match their velocity with the ion velocity to compensate for the ion current, so the total electric current is zero.
2. Immediately after current zero, the electrons reduce their velocity, and the flux of positive charge arrives at the post-arc cathode. This process continues until the electrons reverse their direction; until that moment, the net charge inside the gap is zero. With no charge, the voltage across the gap remains zero. As soon as the electrons reverse their direction, the post-arc current enters its second phase, in which the electrons move away from the cathode, leaving an ionic space charge sheath behind.
3. The sheath continues to expand until it reaches the new anode. At that moment, the post-arc current starts its third phase. The electric current drops, since all electrons have been removed from the gap.

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