What are the pros and cons of VCBs?

Pros (Advantages):

• High Reliability and Long Life: VCBs have a very long mechanical and electrical lifespan, often capable of tens of thousands of operations. The vacuum interrupter is a sealed, robust unit.
• Excellent Safety: They are inherently safer than alternatives. There is no flammable oil to catch fire or high-pressure gas that could leak.
• Environmentally Friendly: The interrupting medium is a pure vacuum, which has no environmental impact. This is a major advantage over SF₆ gas.
• Low Maintenance: The vacuum interrupter is sealed for life and requires no servicing. This significantly reduces lifetime maintenance costs compared to oil breakers, which require regular oil filtering and testing.
• Compact and Lightweight: VCBs are generally smaller and lighter than their oil-filled counterparts.
• Fast Operation: They have a very high dielectric strength recovery, meaning they can interrupt faults very quickly and efficiently.

Cons (Disadvantages):

• Loss of Vacuum: The primary failure mode is the loss of the vacuum inside the interrupter. While rare, if the seal is compromised, the entire interrupter must be replaced as it cannot be repaired on-site. This requires a specialized vacuum integrity test (Hipot test) during maintenance.
• Unsuitability for Very High Voltages: While dominant in the medium-voltage range, VCBs are generally not used for Extra High Voltage (EHV) transmission lines (e.g., 220kV, 400kV and above), where SF₆ breakers still have superior performance.
• Current Chopping: An older issue, now largely mitigated, where the breaker could force the current to zero before its natural zero-crossing. This could induce high voltage transients (spikes) that could stress equipment insulation. Modern VCBs use advanced contact materials to minimize this effect.