What is the working principle of VCB

Vacuum circuit breaker or VCB and vacuum interrupter

A Vacuum circuit breaker is one such type of circuit breaker where the arc quenching takes place in a vacuum. The technology is particularly suitable for medium voltage applications. High-voltage vacuum technology was developed, but not commercially feasible. The process of opening and closing current-carrying contacts and the associated arc interruption takes place in a vacuum chamber in the aforementioned interrupter Vacuum breaker. The vacuum switch consists of an arched steel chamber in the middle of symmetrically arranged ceramic insulators. The vacuum pressure in a vacuum interrupter is normally maintained at 10- 6 Bar.

The material used for the live contacts plays an important role in the performance of the Vacuum circuit breaker. Cu / Cr is the ideal material for making VCB contacts. Vacuum interrupter technology was first introduced in 1960. Still, it is an evolving technology. Over time, the size of the vacuum interrupter is reduced due to the various technical developments in this field of technology from the early 1960s. The contact geometry also improves over time. From the impact contact of earlier days, it gradually changes into a spiral shape, cup shape and axial magnetic field contact. The vacuum circuit-breaker is now recognized as the most reliable current interruption technology for medium-voltage switchgear. Minimal maintenance is required compared to other circuit breaker technologies.

Advantages of the vacuum circuit-breaker or the VCB

Lifespan of Vacuum circuit breaker is much longer than other types of circuit breakers. There is no fire hazard as with an oil circuit breaker. It's much more environmentally friendly than SF6 Circuit breaker. Other than that, VCB's contraction is easy to use. Replacing the vacuum interrupter (VI) is very practical.

Operation of the vacuum circuit-breaker

The main goal of a circuit breaker is to extinguish the arc during the current zero crossing by creating a high withstand voltage between the contacts, so that the restoration of the arc after the current zero point becomes impossible.

The dielectric strength of the vacuum is eight times greater than that of air and four times greater than that of SF6 Gas. This high dielectric strength makes it possible to extinguish a vacuum arc within a very small contact gap. With a short contact distance, low contact mass and no compression of the medium, the drive energy required in the vacuum circuit-breaker is minimal. If two face-to-face contact areas are in the process of being separated, they will not be separated immediately, the contact area on the contact area will be reduced and eventually come to a point, and then eventually they will be touched. While it does so in a fraction of a microsecond, it is a fact. At this point of touching contacts in vacuum, the current through the contacts concentrates at that last point of contact on the contact surface and forms a hot spot.

Since it is a vacuum, the metal on the contact surface is easily vaporized through this hotspot, creating a conductive medium for the arc path. Then the arc is initiated and continued until the next current zero.

At zero current, this vacuum arc is extinguished and the conductive metal vapor is recondensed on the contact surface. At this point the contacts are already separated, so that renewed evaporation of the contact surface is not an option for the next current cycle. This means that the arch cannot be restored. In this way Vacuum circuit breaker prevents the re-establishment of the arc by generating a high dielectric strength in the contact gap after the current zero point

There are two types of arch shapes. To interrupt the current up to 10 kA, the arc remains diffuse and vapor-dissipating and covers the entire contact surface. Above 10 kA, the diffuse arc is considerably restricted by its own magnetic field and contracts. The phenomenon leads to overheating of the contact in its center. To prevent this from happening, the contacts should be designed so that the arc does not remain stationary, but moves through its own magnetic field. Specially developed contact form from Vacuum circuit breaker causes the narrowed stationary arc to move along the surface of the contacts, causing minimal and even contact erosion.