- Vacuum Switchgear Definition: Vacuum switchgear is defined as a type of electrical switchgear that uses a vacuum as the arc quenching medium, providing high reliability and low maintenance.
- Dielectric Strength: Vacuum switchgear offers high dielectric strength, allowing for smaller contact gaps and effective arc quenching.
- Low Arc Energy: The energy dissipated during arcing in vacuum switchgear is much lower than in other types, resulting in minimal contact erosion.
- Simple Driving Mechanism: A vacuum switchgear’s driving mechanism is simpler due to the absence of a medium and the small contact gap, requiring less driving energy.
- Rapid Arc Quenching: Metal vapor produced during arcing re-condenses quickly in vacuum switchgear, ensuring fast recovery of dielectric strength.
Vacuum switchgear interrupts in a sealed vacuum bottle rather than in gas or oil. In medium voltage switchgear the usual voltage window is 3 to 36 kV. On that band vacuum now outsells air, SF6 and oil for new indoor boards, because a vacuum circuit breaker copes with frequent fault and load switching with little servicing.
Vacuum as an Interruption Medium
A circuit breaker tracks the medium used for arc quenching. Vacuum bottles need almost no gas handling, and the contact stroke is only millimetres, so the driving mechanism can be a modest spring.
Dielectric Strength of Vacuum
For a given gap, vacuum is often quoted as about eight times the dielectric strength of air and four times that of SF6 gas at one bar. Those ratios are handbook figures, not a universal law, yet they are why the gap can stay a few millimetres and arc quenching still works in that short gap. After arc interruption the gap returns to high strength at current zero. That fast recovery is why vacuum is widely used for capacitor switching.
Low Arc Energy in Vacuum
Energy in a vacuum arc is often put at about one-tenth of oil and one-fourth of SF6 gas. The short arcing time and short arc length both follow from the small gap. Contact wear is therefore low, so gas top-up and oil changes are not part of the job. breaking energy at the mechanism is also lower on a vacuum circuit breaker than on an air circuit breaker or an oil circuit breaker.
Simple Driving Mechanism
In SF6 oil and air breakers the contacts have to push through a compressed arc quenching filling. A vacuum bottle has no gas or liquid to squeeze, and the stroke is short, so this circuit breaker needs little driving energy. A spring-spring mechanism is therefore usual; hydraulic or pneumatic drives are not required on most indoor vacuum panels. The lighter drive also raises mechanical endurance. Magnetic actuators appear on some newer units.
Rapid Arc Quenching
When current-carrying contacts part, metal vapour fills the gap and electric current keeps flowing until the next current zero. That discharge is the vacuum arc. The arc dies near current zero. The vapour re-condenses on the contact faces in microseconds. Published figures often put about 1% of the vapour on the shield wall and 99% back on the face that boiled it.
Dielectric strength therefore returns quickly, and net metal loss from the contacts stays small.
Up to about 10 kA the arc stays diffuse and covers the face as a vapour discharge. Above that the self magnetic field pinches the column to the centre and local heating rises. Spiral, cup or axial-field contacts are shaped so the root runs around the face, spreading wear.
