Vacuum versus SF6 circuit breakers — arc-quenching principle, and why SF6 is being phased out

The MCCB-versus-automaat guide covers low-voltage circuit breaker technology. This article covers a question that mainly arises at medium-voltage level, where two fundamentally different interrupting media dominate the market: vacuum and SF6 (sulphur hexafluoride) — and why the second of these is now being actively phased out.

How a vacuum interrupter extinguishes the arc

Inside a vacuum interrupter, the contacts open inside a sealed bottle evacuated to a very low pressure (typically below 10⁻⁴ Pa). Separating the contacts under load initially still draws an arc — sustained by metal vapour boiled off the contact surfaces themselves, not by any gas — but because there is essentially no gas present to keep ionising and sustaining that arc column, the metal vapour condenses back onto the contacts and the surrounding shield extremely quickly once the current passes through a natural zero crossing. Special contact materials (typically copper-chromium, CuCr) are used specifically because they give a good balance between low contact erosion and rapid, clean de-ionisation at current zero. Vacuum interrupters are the dominant technology for medium-voltage switchgear up to around 38-40.5 kV and are increasingly used higher.

How an SF6 breaker extinguishes the arc

SF6 is a heavy, chemically stable, electrically electronegative gas: its molecules readily capture free electrons from the arc plasma to form larger, much less mobile negative ions, which sharply reduces the arc's conductivity as current approaches zero. Combined with a high dielectric strength (several times that of air at the same pressure), this gives SF6 breakers excellent interrupting performance and compact dimensions across a very wide range, from medium voltage up to the highest transmission voltages (in some designs beyond 800 kV), which is why SF6 became the dominant technology at higher voltage levels where vacuum interrupters are less commonly applied.

Why SF6 is being phased out despite its technical performance

SF6's technical qualities are not in question — the issue is environmental. SF6 has a global warming potential (GWP) of approximately 23,500 times that of CO₂ over a 100-year horizon, and an atmospheric lifetime measured in thousands of years, making it one of the most potent greenhouse gases regulated under EU law. The revised EU F-gas Regulation sets out a phased ban on new SF6-insulated switchgear, moving progressively through medium-voltage equipment before reaching higher voltage classes later in the phase-down schedule. For anyone specifying or replacing switchgear, this means the choice of interrupting/insulating medium for new equipment is no longer purely a technical or cost decision — it is also a regulatory compliance question tied to installation date.

The practical alternatives

Vacuum interruption remains unaffected by the F-gas phase-down (it uses no fluorinated gas at all) and is already the default choice for new medium-voltage switchgear at most distribution voltage levels.
Low/zero-GWP gas mixtures are the emerging alternative where a gas-insulated design is still preferred (for compactness or for voltage levels where vacuum is less established): mixtures such as AirPlus (dry air with a small admixture of a fluoronitrile) and g³ (a fluoroketone/CO₂/O₂ mixture) are marketed with a GWP below 1, a small fraction of SF6's, while retaining acceptable dielectric and interrupting performance for the switchgear designs built around them.
Pure air or CO₂-based designs, without any fluorinated component at all, are also emerging for some applications, in direct response to the regulation's eventual move toward banning fluorine-containing gases altogether at MV level.

Practical relevance

When specifying replacement medium-voltage switchgear, or assessing an existing SF6-insulated installation for future maintenance and end-of-life planning, it matters which technology is being installed or retained: existing SF6 equipment is not immediately banned from continued use, but new installations are increasingly restricted by date and voltage class under the F-gas Regulation, and SF6 handling during maintenance, leak-testing, and end-of-life gas recovery already carries specific regulatory obligations regardless of the phase-down timeline.

Common mistakes

Assuming vacuum and SF6 breakers are interchangeable technologies with only a cost difference — the arc-quenching principle, typical voltage range, and now the regulatory status differ substantially.
Overlooking the F-gas Regulation's phase-down schedule when planning a new medium-voltage switchgear installation, resulting in a design choice that is compliant today but not for the equipment's full service life.
Treating a low-GWP gas mixture (AirPlus, g³) as functionally identical to SF6 without checking the specific switchgear design it is qualified for — these mixtures are not simply a drop-in replacement gas for existing SF6 equipment.
Neglecting SF6 handling procedures (leak checking, recovery equipment, records) on existing installations on the assumption that the phase-down only concerns new equipment.