With new improvements in vacuum breaker design, vacuum circuit breakers are becoming more common at the lower voltage ranges of transmission networks. Today they can handle voltages up to 252 kV but are still very expensive. Vacuum circuit breakers are more commonly used at system voltage levels up to 72 kV. Because there is no gas to ionize to form the arc, the insulating gap is smaller than in other circuit breakers. An arc does form from the vaporized contact material. The insulation distance in a vacuum circuit breaker is about 11 – 17 mm between plates. Normally there is one break per phase but there can be two or more interrupters in series for higher voltages.
The contact plates are formed to conduct the current in a way that creates a magnetic field that causes the arc to rotate and extinguish. A benefit with a rotating arc is uniform heat distribution resulting in more even contact erosion. Other advantages with vacuum breakers are their relatively long operational life time and their relatively limited impact on the environment since they are designed without greenhouse gases and with relatively few components. Vacuum circuit breakers also suffer less wear on the main contact than air and oil circuit breakers.
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