Before discussing the actual comparison, let’s look at the different mediums used in circuit breakers. These are air, oil, and SF6 gas. Nowadays, some other gaseous media like compressed air, C4-FN mixtures (C4-FN/CO2 or C4-FN/N2), C5-FK mixtures (C4-FK/air or C4-FK/CO2) and pure N2 or N2/CO2 mixtures are also being developed as insulating mediums. Although, some of these can contribute to arc quenching, but they are not yet widely adopted or standardized for high-voltage arc interruption. They are still only used as insulating mediums.
As we know, the medium in a circuit breaker must perform two main functions:
- Arc quenching — extinguishing the electrical arc when contacts open
- Providing dielectric strength between the contacts after they separate
Let’s first compare the dielectric strength of vacuum with other mediums.
- Normal dry air at atmospheric pressure has a dielectric strength of around 3 kV/mm
- Insulating oil has about 10 kV/mm
- SF6 gas at 1 bar provides around 9 kV/mm, and at 6 bar, it can reach 50 to 60 kV
- Vacuum offers around 30 kV/mm
Although vacuum has slightly less dielectric strength than high-pressure SF6, it is still excellent and far better than air or oil.
Air Circuit Breakers (ACBs) are not used for high-voltage applications. They are currently used only for low-voltage systems up to 1 kV. Interestingly, around 40 years ago, air blast circuit breakers were used in high-voltage systems, even up to 765 kV. These breakers used a sudden blast of compressed air to quench the arc. However, after the invention of SF6 circuit breakers, air circuit breakers became obsolete for high voltages.
Today, only standard ACBs for low voltage are available in the market.
SF6 circuit breakers are a comparatively newer technology and are suitable for all high-voltage levels, even up to 800 kV. This is due to the extremely high dielectric strength of pressurized SF6 gas.
Oil circuit breakers (OCBs) were widely used for low to medium voltages (3.3 kV to 36 kV) in distribution systems and high voltages (36 kV to 245 kV) in transmission systems. Advanced designs, particularly minimum oil circuit breakers (MOCBs), reached up to 420 kV, but with rare applications. They are bulky and contain inflammable oil, which creates a risk of fire hazards. Additionally, arcing is prolonged, causing faster erosion of contacts, and the oil requires frequent replacement. Oil leakage is a common issue, and regular maintenance of both contacts and the mechanism is needed.
Thanks to the excellent arc-quenching ability and high dielectric strength, Vacuum Circuit Breakers (VCBs) are compact and smaller in size. They are currently the most popular circuit breakers for low and medium-voltage levels, especially from 11 kV to 33 kV.
However, VCBs have a limitation: they are generally not suitable above 36 kV. This is because the contact gap in vacuum interrupters must remain short to allow very fast arc quenching. At higher voltages, the required contact gap becomes larger, and achieving the necessary high-speed contact movement becomes difficult and expensive.
In such cases, using SF6 gas as the arc-quenching medium is more cost-effective than building a complex high-speed mechanism for vacuum systems.
To summarize:
- Vacuum CBs are best for 11–33 kV, nearly maintenance-free, have fast arc interruption, and pose no environmental hazards.
- SF6 CBs support up to 800 kV, offer excellent performance, but leakage is a concern, and SF6 is a potent greenhouse gas, with 24,000 times the impact of CO₂.
- Oil CBs are becoming outdated due to bulkiness, fire risk, and frequent maintenance.
- Air CBs are presently used only in low-voltage applications, and air blast CBs are obsolete
Operating Noise Comparison:
- Air blast circuit breakers produce high noise due to compressed air discharge
- Oil circuit breakers are noisy, but less so than air blast CBs
- SF6 breakers produce moderate sound levels
- Vacuum breakers are the quietest among the four.