Vacuum interrupter is an electric vacuum device that uses high vacuum working insulating arc extinguishing medium and realizes the on-off function of power circuit by a pair of contacts sealed in vacuum. When it disconnects a certain amount of current, at the moment of separation of the dynamic and static contacts, the current shrinks to the point where the contacts just separate, resulting in a sharp increase in resistance between electrodes and a rapid increase in temperature, until the evaporation of electrode metal occurs, and at the same time, a very high electric field intensity is formed, resulting in extremely strong emission and gap breakdown, resulting in vacuum arc. When the power frequency voltage is close to zero, and at the same time, due to the increase of contact opening distance, The plasma of the vacuum arc quickly diffuses around. After the arc current passes zero, the medium in the contact gap quickly changes from a conductor to an insulator, so the current is cut off. Due to the special structure of the contact, the contact gap will produce a longitudinal magnetic field during arcing. This magnetic field can make the arc evenly distributed on the contact surface, maintain a low arc voltage, and make the vacuum arc extinguishing chamber have a high recovery speed of post arc dielectric strength, resulting in small arc energy and small corrosion rate. In this way, the interrupting current capacity and service life of the vacuum interrupter are improved.
Under certain circumstances, the vacuum circuit breaker can force the current in the circuit to zero before the natural zero (and reversal of current) in the alternating-current circuit. If interrupter operation timing is unfavorable with respect to the AC-voltage waveform (when the arc is extinguished but the contacts are still moving and ionization has not yet dissipated in the interrupter), the voltage may exceed the gap's withstand voltage. This can re-ignite the arc, causing abrupt transient currents. In either case, oscillation is introduced into the system that may result in significant overvoltage.
Nowadays, with very low current chopping, vacuum circuit breakers will not induce an overvoltage that could reduce insulation from surrounding equipment.