Lots of city power-grid builds, solar PV sites, and small-to-mid power jobs overseas use RMUs, short for Ring Main Units. These are common medium-voltage switchgear assemblies. A lot of engineers and project leads who are new to buying power gear mix up RMU swithregular high-voltage switchgear. They don't know what benefits RMUs bring, and they can't decide whether to pick air-insulated or gas-insulated models. We follow IEC global electrical standards and have many years of hands-on experience with power projects across the Middle East. This article goes over RMU internal parts, how they work, and where they fit best. We also compareRMUswith regular switchgear, to help you choose proper equipment for your work site.
What Is a Ring Main Unit (RMU)? Core Functions and Internal Components
RMU is short for Ring Main Unit. It is a compact medium-voltage switchgear built for ring-style power supply lines. Most units sold on the market work at 12kV.
Folks use standard switchgear like KYN28 mostly for large power incoming lines inside substations. RMUs, however, are more often installed at line ends and for branch power supply. These two pieces of equipment serve totally different working situations.
This gear handles two very important tasks.
1. It creates ring-shaped power loops. Power can travel in two directions. If one single line goes wrong, it will not knock out power for the whole area.
2. Its inner switches and protection parts can cut off fault current quickly. This keeps cables and other connected electrical gear from getting damaged.
A completeRMUincludes these key parts: load switches, earthing switches, current-limiting fuses, main busbars and cable compartments.
Our factory turns out two popular models in bulk.
1. The XGN15-12 air-insulated RMU. Regular air inside the cabinet acts as insulation.
2. The LMRM6-12 gas-insulated RMU. All internal parts are locked inside a sealed metal chamber filled with SF₆ gas. We have covered this kind of GIS insulation design in our previous article.
Ring Main Unit (RMU) Operating Logic: Principles and Mainstream Classifications
The operating logic of aring main unit is easy to understand: multiple power cables are connected end-to-end to form a ring circuit, and each line node is equipped with an RMU. During normal operation, the two branches of the ring circuit simultaneously transmit power. If a section of cable experiences a short circuit or damage, the load switch inside the cabinet will automatically isolate the faulty section, while the remaining intact lines can maintain normal power supply, effectively reducing the scope of power outages. This is a key reason why ring main units are widely used in urban power distribution projects.
Currently, the main streamring main unitson the market are divided into two main categories, corresponding to the AIS and GIS insulation structures introduced earlier:
1. Air-Insulated Ring Main Unit (AIS-RMU)
XGN15 and HXGN15 are representative models of this category. The live components inside the cabinet are directly exposed, using air as the insulation medium. This type of equipment has a lower procurement cost, allows for direct inspection of internal components by opening the cabinet, and simplifies the modification process for adding new outgoing circuits. It is more suitable for inland dry factory areas and township substations with ample space. Its drawback lies in its larger footprint. Long-term placement in coastal salt spray and dusty mining environments can easily lead to insulator accumulation and aging, requiring regular annual cleaning and maintenance.
2. Gas-Insulated Ring Main Unit (GIS-RMU)
The LMRM6-12 is a representative product. All live electrical parts sit inside a sealed metal gas chamber. The SF₆ as inside provides insulation and puts out electric arcs. This unit only takes up one-third the space of an air-insulated RMU. Its fully sealed housing keeps dust and coastal dampness out. It works well for underground power rooms, coastal solar projects and jobs along the Middle East coast. You hardly need to do much daily maintenance.
However, it costs more at the time of purchase. If you want to modify or add new output circuits later, you must release pressure from the gas chamber and run new pressure tests. All these steps make the whole construction work a lot more troublesome.
Common Application Scenarios of Ring Main Units in Power Distribution
Whether in domestic municipal engineering projects or overseas export projects, the application scenarios for ring main unitsare relatively concentrated. Below are four of the most common application needs encountered in engineering projects:
1. Power Distribution in Residential Communities and Commercial Complexes
Underground distribution rooms in residential communities generally have limited space. Gas-filled ring main units can significantly reduce the civil engineering construction area. The ring power supply mode can ensure continuous power supply for shopping malls and residents, and is also standard equipment in real estate power distribution projects.
2. Centralized Photovoltaic and Wind Power Stations
Outdoor photovoltaic sites are mostly built in suburban or coastal areas. Gas-filled ring main units have excellent moisture-proof and salt spray resistance, which can significantly reduce the daily operation and maintenance workload of outdoor sites.
3. Power Distribution in Municipal Roads, Tunnels, and Rail Transit
The internal environment of tunnels is humid and dark, and the installation space is small. Sealed gas-filled ring main units are not affected by water accumulation and moisture, resulting in stronger power supply stability.
4. Overseas Small and Medium-Sized Power Projects
For solar projects along Middle East coasts like Dubai and Qatar, gas-filled RMUs work best to stop damage caused by salty sea wind. If you have lots of free space on site, such as dry industrial zones in inland Saudi Arabia or rural power grids in Africa, go with the XGN15 air-insulated RMU. This helps you keep your total purchase costs under good control.
What Are the Differences Between Ring Main Units (RMUs) and Conventional Switchgear? Selection Reference Suggestions
Many customers inquire whetherring main units(RMUs) belong to switchgear. Strictly speaking, RMU sare a subcategory of switchgear, but their product positioning is significantly different.
Conventional medium-voltage switchgear like the KYN28 truck-mounted switchgear has a larger rated current and is mostly used in substation main incoming lines and large-capacity main distribution rooms in factories. It offers high flexibility for future expansion and adding circuits, and its cabinet size is also larger.
Ring main units (RMUs) have a compact overall structure and a smaller load capacity. Their core function is line segmentation and building a ring network, making them only suitable for branch lines and end-point power distribution scenarios.
Simple and clear selection criteria:
1. If your project requires main power feed for substations or heavy-load industrial equipment, go with draw able switchgear like the KYN28.
2. UseRMUsfor housing areas, solar power plants and local power branches that need ring-loop power supply.
3. Go for GIS-type RMU swhen installation space is limited, or you are working at salt-heavy coastal sites and underground power rooms. If you have plenty of open dry ground and your project budget is tight, the XGN15 air-insulated RMU gives you better value for money.
Frequently Asked Questions
Q1: Is gas-insulated switchgear the same type of equipment as ring main units?
They are not equivalent.Ring main unitsinclude bothair-insulated and gas-insulated products; agas-insulated RMU is just one sub-model within thering main unit category.
Q2: Which type of RMU should be selected for an overseas 11kV rural power grid project?
For inland Africa and arid industrial areas of Saudi Arabia with ample space, the XGN15 air-insulatedring main unit(RMU) is a suitable option. However, in coastal areas of the UAE and Qatar, where sea wind corrosion is severe, it is recommended to directly configuregas-insulated GIS RMUs.
Q3: Can outgoing circuits be added to the RMU later?
It's pretty easy to adjust and add new output circuits for the XGN15 air-insulated RMU. But working on the gas chamber of agas-insulated RMU is tricky, and this work will take a lot more time. So you'd better leave enough spare output ports when you first plan out your project.
Summary
In general, the RMU is an indispensable small switchgear for building a ring distribution network. It comes in two versions:air-insulated and gas-insulated, each suitable for different sites and climates. Understanding its applicability to large-capacity main switchgear allows for quick matching of equipment models to meet project requirements.
Are you working on housing estates, solar plants or overseas power-distribution jobs and cannot figure out which gear to use: regular switchgear or RMU? If you also can't tellair-insulated gear fromgas-insulated types, just get in touch with us. Tell us about your project's working voltage, site conditions, power load, and whether you plan to add more equipment down the line. Our LVMA electrical experts will make a custom-fit solution for you, and give you a full detailed price quote.
About us
Zhejiang Lvma Electric Co., Ltd. (founded 2018) draws on 17 years of specialized electrical equipment expertise under ISO 9001:2015 certification. Our core products are intelligent switchgear systems and oil-immersed and dry-type distribution transformers, exported to Europe, the Middle East, South America, Southeast Asia, and Africa.
Our R&D team, supported by over 40 patents, is driving the company's evolution into a smart energy solution provider. By embracing digital manufacturing and real-time monitoring, we consistently supply electrical systems that are safe, efficient, and built for longevity.