Vacuum Switchgear Explained
Vacuum switchgear is medium voltage switchgear that uses a vacuum circuit breaker to switch and protect the circuit. The breaker interrupts the current inside a vacuum interrupter, a sealed bottle where the contacts open in a vacuum. It is the standard breaker technology across medium voltage distribution, found in substation incomers, plant feeders, motor circuits, and capacitor banks.
Is It Really "Vacuum Insulated"?
This is a common point of confusion. The vacuum does the arc interruption; it does not insulate the whole panel. Inside the sealed interrupter, the vacuum gives both excellent arc quenching and high dielectric strength across the open contacts. The rest of the switchgear, the busbar, the connections, and the cable termination, is insulated by air or by solid epoxy, the same as any air-insulated panel. So vacuum insulated switchgear really means switchgear whose breaker interrupts in vacuum, sitting in an air-insulated enclosure.
How a Vacuum Interrupter Works
The vacuum interrupter is a ceramic or glass cylinder evacuated to a very high vacuum, holding one fixed contact and one moving contact joined by a metal bellows. When the breaker trips, the moving contact pulls away and an arc forms in metal vapour boiled off the contact surface. Because there is almost nothing to ionise, the arc cannot sustain itself, and it goes out at the first natural current zero, within a few milliseconds. The instant the arc clears, the metal vapour condenses back onto the contacts and the vacuum recovers its full dielectric strength, so the gap withstands the returning voltage. Special contact shapes spin the arc with a magnetic field so it does not stay in one spot and erode the contacts.
Vacuum vs SF6 Circuit Breakers
Vacuum and SF6 are the two modern interrupting technologies, and they split by voltage. In medium voltage, vacuum has become the default: it needs no gas, so there is nothing to leak, monitor, or refill, maintenance is low, and the breaker lasts for tens of thousands of operations. SF6 breaks the arc in pressurised gas and holds the field at higher transmission voltages, but SF6 is a strong greenhouse gas under tightening regulation. For a medium voltage circuit that needs to avoid SF6, vacuum is the straightforward answer. The two are not rivals on every job: SF6 is still used as the insulating medium in compact gas insulated switchgear, and many of those panels actually use a vacuum interrupter for the breaking, with SF6 only for insulation.
Long Life and Low Maintenance
The sealed vacuum interrupter is maintenance-free inside, with no oil to change and no gas to top up. A standard vacuum circuit breaker is rated for around 10,000 to 30,000 mechanical operations and a high number of full-fault interruptions, which the standard classifies as mechanical endurance class M2 and electrical endurance class E2. For very high switching rates, such as arc furnaces, mining hoists, and soft starters, magnetic-actuator breakers reach far higher operation counts. That endurance is the main reason vacuum is specified for circuits that switch many times a day.
VS1, ZN63A, and ZN85
The common indoor vacuum breakers follow the ZN designation. The VS1, also called ZN63A-12, is the workhorse for 12 kV, rated 630 A to 4000 A and breaking up to 40 kA, with the breaker pole, mechanism, and interrupter built into one compact body. The ZN63A-24 is the 24 kV version with enhanced insulation for capacitor and furnace switching. The ZN85-40.5 covers the 40.5 kV class, with the mechanism arranged above and below the poles to keep the breaker shallow. All come in fixed or withdrawable form to suit the panel.
Where Vacuum Switchgear Is Used
Vacuum breakers sit inside metal-clad switchgear for substation incomers and feeders across utilities, industry, and renewables. Their long life makes them the clear choice for circuits with frequent operations: medium voltage motor starting, capacitor bank switching, and electric arc furnaces all switch far more often than a normal feeder, and they would wear out an oil or gas breaker. Data centres and hospitals favour vacuum for its clean, low-maintenance operation and the absence of any gas to manage on site.
Ratings and Standards
Our vacuum breakers cover the 12 kV, 24 kV, and 40.5 kV classes, with rated currents to 4000 A and breaking current to 40 kA, depending on the model. Rated insulation levels follow the voltage class, the same as any medium voltage panel: 42 kV and 75 kV at 12 kV, 65 kV and 125 kV at 24 kV, and 95 kV and 185 kV at 40.5 kV. Breakers are type-tested to IEC 62271-100, equivalent to GB 1984-2014, and the panels they sit in to IEC 62271-200. For the full switchgear range, see the medium voltage switchgear overview.
