Vacuum Circuit Breakers (VCBs) are widely used for medium-voltage applications where arc quenching takes place in a vacuum environment. The vacuum interrupter is the main part of the VCB, where opening and closing operations and arc extinction happen.
The interrupter consists of a steel chamber enclosed in ceramic insulators, maintaining a vacuum pressure of about 10−6 bar. Copper-chromium contacts are typically used for their excellent arc-handling and conductivity properties.
Advantages of Vacuum Circuit Breakers (VCBs)
- Longer service life compared to other circuit breakers.
- No risk of fire hazards unlike oil circuit breakers.
- Environment-friendly alternative to SF6 circuit breakers.
- User-friendly design and compact construction.
- Vacuum interrupters are easy to replace and maintain.
Operation of Vacuum Circuit Breaker
The core purpose of a circuit breaker is to interrupt fault current and isolate the faulty section by extinguishing the arc between contacts. In a Vacuum Circuit Breaker, this arc quenching occurs in a vacuum interrupter.
When the contacts separate under vacuum conditions, a vacuum arc forms due to the emission of neutral atoms, ions, and electrons from the contact surfaces. Depending on the current level:
- Low currents result in a single, highly mobile cathode spot.
- High currents generate multiple cathode spots, which form the primary vapor source for arc formation.
The arc is drawn due to:
- High electric field between separating contacts
- Resistive heating at micro-projections on contact surfaces
These micro-projections evaporate explosively, supplying vapor for arc formation. This process is highly localized and does not affect the entire surface. Hence, the vacuum arc is often referred to as a cold cathode arc.
Types of Electron Emission Mechanisms in Vacuum Arc
- Field emission
- Thermionic emission
- Combined field and thermionic emission
- Secondary emission due to positive ion bombardment
- Secondary emission by photons
- Pinch effect
Current Chopping Phenomenon
Unlike oil or air circuit breakers where arc instability causes current chopping, in VCBs it depends on vapor pressure and electron emission of the contact material. Selection of materials with suitable vapor pressure and conductivity helps minimize unwanted chopping:
- High vapor pressure, low conductivity metals limit current chopping.
- Low vapor pressure materials enhance dielectric recovery but may worsen chopping.
Hence, composite materials like Copper-Bismuth, Silver-Bismuth, Silver-Lead, Copper-Lead are used to balance performance.
Arc Recovery in Vacuum
After arc interruption, the space between the contacts is filled with metal vapor and plasma. The ability of the breaker to withstand voltage post-interruption depends on the decay of this residue — called the arc recovery phenomenon.
- At current zero, the arc extinguishes within 10–8 seconds.
- Dielectric strength rebuilds quickly due to lack of gas molecules.
- Recovery strength in vacuum = 1 kV/μs vs. air = 50 V/μs.
Working Principle of Vacuum Circuit Breaker
The core function of a circuit breaker is to quench the arc during fault conditions by restoring dielectric strength between contacts. Vacuum offers:
- Dielectric strength 8× greater than air and 4× greater than SF6 gas.
- Arc quenching in a small contact gap due to high vacuum strength.
During operation, when contacts separate, vacuum arc forms due to the emission of neutral atoms, ions, and electrons. Depending on the current, cathode spots are generated which become the primary source of metal vapor in the arc.
Arc formation occurs due to:
- High electric field between contacts
- Resistive heating at micro projections on contact surface
Types of Electron Emission in Cold Cathode Vacuum Arc
- Field emission
- Thermionic emission
- Field and Thermionic emission
- Secondary emission by positive ion bombardment
- Secondary emission by photons
- Pinch effect
Contact Material Considerations
Current chopping in vacuum breakers depends on vapour pressure and emission properties of contact materials, unlike oil or air CBs. Proper contact material selection helps balance chopping level and dielectric strength. Composite alloys like:
- Copper-Bismuth
- Silver-Bismuth
- Silver-Lead
- Copper-Lead
are used to combine conductivity with thermal resistance.
Arc Recovery Phenomenon
After arc interruption, the space between contacts fills with metal vapor and plasma. Recovery involves:
- Rapid cooling and condensation of metal vapor
- Quick restoration of dielectric strength (1 kV/μs for 100 A arc)
Vacuum's lack of gas molecules enhances the rate of dielectric recovery, making VCBs reliable and efficient in fault isolation.
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FAQs on Vacuum Circuit Breakers
What is a Vacuum Circuit Breaker (VCB)?
A Vacuum Circuit Breaker is a type of circuit breaker where arc extinction occurs in a vacuum environment. It is commonly used for medium-voltage applications due to its high reliability and fast recovery strength.
Why is vacuum used in circuit breakers?
Vacuum has a very high dielectric strength, allowing for quick arc extinction and dielectric recovery, making it ideal for switching and protecting circuits during faults.
What materials are used in VCB contacts?
VCB contacts are often made of composite materials like Copper-Chromium, Silver-Bismuth, or Copper-Lead to balance conductivity and vapor pressure for effective arc handling.
Is VCB better than SF6 circuit breaker?
VCBs are more environment-friendly and easier to maintain than SF6 breakers, which use greenhouse gas. However, SF6 breakers are preferred for higher voltage ratings.