SF6 as an Insulating Gas — Why It Is Used in Medium Voltage Switchgear
Sulfur hexafluoride (SF6) is a colorless, odorless, non-toxic, non-flammable gas that has been used as a high-voltage dielectric since the 1960s. The properties that make it valuable for switchgear are its dielectric strength (roughly two to three times that of air at the same pressure), its arc-quenching ability (a hot SF6 arc cools and recombines very quickly when the current crosses zero), and its chemical stability — SF6 does not break down, corrode metal parts, or react with insulating materials inside a sealed enclosure. These properties allow SF6 switchgear to be built much more compactly than air-insulated equivalents while still meeting the same voltage withstand and short-circuit ratings.
Modern SF6 gas insulated switchgear for medium voltage distribution uses a sealed-for-life design philosophy: the gas is filled at factory, the tank is welded shut, and the equipment operates for its full service life without the operator ever handling the gas. This is fundamentally different from earlier-generation SF6 equipment that required field filling and periodic top-up. The sealed-for-life approach addresses both reliability concerns (no field contamination, no human error in gas handling) and environmental concerns (no leakage path under normal operation).
SF6 GIS vs Ring Main Unit — Insulation vs Topology
These two terms are often used together but describe orthogonal characteristics of medium voltage switchgear.
SF6 GIS describes how the switchgear is insulated — the live parts sit inside a tank filled with SF6 gas instead of in open air. SF6 GIS is a category that includes everything from compact medium voltage distribution switchgear up to multi-hundred-kV transmission GIS in utility substations.
Ring Main Unit describes how the switchgear is connected into the distribution network. An RMU is purpose-built for ring main distribution — a closed loop of medium voltage cable feeding a series of distribution substations, with each substation containing one RMU that can switch its local transformer in and out of the ring. RMUs can be air-insulated, SF6-insulated, or solid-insulated; the topology is independent of the insulation choice.
EverWins HXGN switchgear is both: SF6 insulated and built as a ring main unit. The combination is by far the most common configuration for medium voltage secondary distribution worldwide because it pairs the space efficiency of SF6 insulation with the network resilience of ring main topology.
Inside a Ring Main Unit — Functional Unit Types
An RMU line-up is built from a mix of standardized functional units. Each unit handles a specific job and has a different cubicle width. The most common functional units in the HXGN range are:
• Cable feeder unit (V or L unit) — a load break switch and earthing switch for one cable feeder. Used as the in-feed and out-feed connections to the ring main cable. Narrowest cubicle in the line-up, typically 370 mm wide.
• Transformer protection unit (F unit) — a load break switch combined with three HRC fuses (one per phase) per IEC 62271-105. Used to feed and protect a single distribution transformer. Wider than the cable unit to house the fuses, typically 500 mm wide. The fuses provide short-circuit protection; the load break switch handles normal switching and earthing.
• Circuit breaker unit (C unit) — a vacuum circuit breaker inside the SF6 tank, used where the protection requirement exceeds what fuses can provide (large transformer feeders, motor feeders, capacitor bank protection). Equipped with overcurrent and earth-fault relaying. Typically 800 mm wide.
• Metering unit (M unit) — voltage and current transformers feeding revenue or check metering. Used at utility connection points and on the secondary side of distribution transformers.
• Bus coupler / bus section unit — used in larger RMU line-ups with two main busbars to split or couple the buses, supporting maintenance switching without losing service to either side.
A small substation might use just three functional units in a V-F-V arrangement: two cable feeders flanking a transformer protection unit. A larger substation can run twelve or more units.
Load Break Switch + Fuse vs Vacuum Circuit Breaker — When to Use Each
RMU protection comes in two configurations and the choice depends on the load profile and fault current.
Switch-fuse combination uses a load break switch to handle normal switching duty (closing and opening rated load current) and HRC fuses to clear short-circuit faults. HRC fuses are very fast — current-limiting fuses operate in under 10 milliseconds on a high fault current — and they are inherently coordinated with the downstream low-voltage protection. The trade-off is that fuses are single-shot devices: after a fault clears, all three fuses must be replaced (even if only one operated) to avoid the next fault landing on an unbalanced phase set. Switch-fuse combinations are economical and the standard choice for transformers up to about 1,250 kVA at 12 kV.
Vacuum circuit breaker units use a fully-rated breaker capable of switching both normal load current and short-circuit fault current, with proper relay protection coordinating the trip. The breaker can be re-closed and tested after a fault without replacing parts. CBs are required for transformers above ~1,600 kVA, for motor feeders where frequent switching is needed, and for any feeder where relay coordination with upstream protection is critical. The trade-off is higher initial cost and a wider cubicle.
Most substations end up with a mix — switch-fuse units for the standard distribution transformers and a vacuum circuit breaker unit for the incoming feeder or the largest transformer.
HXGN Type Designation
Chinese GB 3906 uses a type code system that encodes the cubicle's construction class, mounting style, installation environment, and voltage rating. The HXGN prefix used on EverWins SF6 RMUs breaks down as:
• H — Hézhuāng (盒装), meaning box-type or compartment-type construction. The cubicle is built around a sealed compartment rather than open framework.
• X — Xiāng (箱), reinforcing the box / cabinet construction.
• G — Guī (柜), cabinet.
• N — Nèibù (内部), indoor service.
The number after the prefix (12, 24, 40.5) is the rated voltage class. So HXGN-12 is an indoor box-type cabinet switchgear rated 12 kV, and HXGN-40.5 is the same family at 40.5 kV. The HXGN designation maps directly onto the IEC 62271-200 description: LSC2 service-continuity category with a sealed gas-tight compartment, the standard construction class for medium voltage secondary distribution worldwide. (The XGN designation without the leading H usually refers to air-insulated fixed switchgear; EverWins SF6 product uses HXGN, where the H signals the sealed compartment construction.)
How to Specify an SF6 Ring Main Unit
At the quotation stage, confirm the following. Anything you cannot specify, our engineering team can derive from your single-line diagram and protection schedule.
1. Rated voltage class. 12 kV for 10/11 kV systems, 24 kV for 20/22 kV systems, 40.5 kV for 33/35 kV systems.
2. Number and function of cubicles. List the line-up by functional unit type: cable feeder, switch-fuse transformer protection, circuit breaker, metering, bus coupler. The single-line diagram defines this directly.
3. Rated main bus current. Set by the largest continuous current flowing through the bus. 630 A is most common; 1250 A for higher-load substations.
4. Rated short-time withstand current. From your fault-level study; 20 kA for 4 seconds covers the great majority of medium voltage distribution. Higher values available on request.
5. Transformer fuse rating (for switch-fuse units). Sized to the transformer rated current with current-limiting fuse curves coordinated with the downstream LV protection.
6. Cable entry direction. Bottom entry is standard; top entry available where the substation layout requires it.
7. Relay scheme (for circuit breaker units). Overcurrent, earth fault, and where applicable transformer differential or sensitive earth fault.
8. Auxiliary supplies. Control voltage (commonly 110 V DC, 220 V DC, or 220 V AC) and source. Some HXGN units operate fully mechanically without auxiliary power.
Maintenance and Service Life
SF6 gas insulated switchgear is among the lowest-maintenance medium voltage equipment available. The sealed stainless steel tank keeps the switching contacts, busbars, and earthing switch in a contamination-free atmosphere, so there is no dust ingress, no moisture absorption, and no oxidation of the contact surfaces. Routine maintenance reduces to:
• Annual visual inspection — check the SF6 pressure indicator, confirm the position indicator matches the operating state, inspect bushings and cable terminations for damage or pollution.
• Annual mechanical operation test — operate the load break switch and earthing switch through their full sequence to confirm the mechanism remains free.
• Three- to five-yearly interlock function test — verify the five-prevention mechanical interlocks and the low-pressure lock-out.
• Periodic relay testing — for circuit breaker units, secondary injection of the protective relays at the interval specified by the protection coordination study.
• Infrared scan of external bus connections under load — catches loose terminations before they fail.There is no scheduled SF6 gas handling under normal operation. If the pressure indicator shows a slow loss over years of service (an indication of gasket leakage), the unit should be returned to the factory for repair and re-sealing rather than topped up in the field. With this routine, a properly specified HXGN unit reliably delivers the 25+ year design life, often longer in protected indoor environments.
