What Is a Vacuum Circuit Breaker?
Vacuum circuit breakers (VCBs) are devices used to interrupt circuits during malfunctions in equipment or power systems.
Capable of interrupting all current types, including short-circuit, load, and overcurrent, VCBs excel in performance among circuit breakers. In the event of current surges reaching several thousand to tens of thousands of amperes, VCBs can extinguish the arc discharge and break the circuit. They are commonly used in large-scale, high-voltage power receiving and transforming facilities.
Uses of Vacuum Circuit Breakers
VCBs protect circuits from accidental currents and break circuits during abnormal currents. Preferred in high current scenarios, they are a cost-effective choice for high-voltage substations’ switchboards. Typical installations include substations, large factories, and commercial facilities.
As VCBs alone cannot detect abnormal currents, they are typically paired with an overcurrent relay for signal reception and automatic current interruption.
Principle of Vacuum Circuit Breakers
VCBs contain a vacuum valve with high vacuum levels, ranging from 10-3Pa to 10-5Pa. The valve includes two fixed and two movable electrodes forming a contact point, separating upon detection of abnormalities.
Interrupting the current requires separating the circuit contacts, generating an arc flash. To prevent potential fires or damage from arc discharge at high temperatures, VCBs use a vacuum environment to diffuse and extinguish arcs. When the electrodes in the vacuum valve separate, the arc dissipates spontaneously.
Regular maintenance of the vacuum valve mechanism in VCBs is essential to ensure reliability during emergencies. Unlike fuses that require replacement after each use, VCBs are reusable and notably quiet during operation.
Types of Vacuum Circuit Breakers
VCBs come in two types: fixed and pull-out, with variations including manual and electric spring types.
Differences in Mounting Method
Fixed VCBs are mounted directly onto the panel with N, R, and P installation directions. Compact yet requiring primary and secondary power shutdown for maintenance or breakdowns, they differ from pull-out VCBs, which are mounted on wheels for easy main circuit connection and disconnection. Pull-out VCBs simplify maintenance by only needing secondary side disconnection and are generally more expensive.
Differences Between Spring Types
VCBs use springs to physically break contacts. Manual spring VCBs involves energizing a mainspring by turning a handle, while the electric spring type employs a motor for spring loading. The electric variant is typically more costly. In both, the contacts remain open post-activation, requiring spring re-loading for restoration.
Other Variants
There are also tank VCBs, which are partially vacuumed gas circuit breakers used in larger substations.
How to Select Vacuum Circuit Breakers
When selecting VCBs, consider the rated interrupting current and capacity, ensuring they match the equipment’s requirements without exceeding the circuit’s short-circuit interrupting current. Overestimating this current can lead to accidents. This current can be provided by the power company or calculated considering wiring impedance.
Also, ensure the selected model fits the distribution panel’s shape.