MV AC SwitchgearVacuum Switchgear

Vacuum circuit breaker contact shapes

Arc should be kept in a spiraling motion or in a diffuse.
the spiraling motion can be achieved by making incisions (or slits) in the arcing contacts. This is known as radial magnetic field(RMF) arc control. Electrodes are spiral shaped, a RMF is induced in which the arc rotates, and the arc is extinguished once the metal vapor recondenses on the electrodes and walls of the vacuum breaker chamber.
Axial magnetic field(AMF) arc control aims to keep the arc in the diffuse mode, holding the arc stationary, but using the whole inter-contact gap (diffuse mode). AMF can also be achieved by applying a horseshoe-shaped iron package located behind the contacts.
Butt contacts used in the initial VI
Butt contact design concentrated arc column that resulted in overheating of the contact surface and increased contact erosion.
RMF & AMF field contacts control the arc so that the arc is distributed and overheating is reduced increasing performance and life.
Butt-type contacts without arc control are usually applied for low current (switches, contactors), while the RMF contacts are a better choice for higher current applications and AMF contacts for applications where low erosion is a requirement as shown in Fig

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