Ring Main Units (RMUs): Configurations, Applications and Selection Considerations

A ring main unit (RMU) is a compact, factory-assembled set of medium-voltage switching functions for the points in a secondary distribution network where a cable ring is looped in and out and a transformer or customer supply is taken off. The same few functions are repeated at many sites across a network, so RMU selection is rarely about unusual ratings. It is about choosing the right functional units, insulation and operating features, and describing them clearly enough that offers can be compared.

This article answers the question: What configurations do ring main units come in, where are they used, and what should a specification state when selecting one?

What a ring main unit is

In an open-ring network, each secondary substation is connected into a cable ring by two switches: one towards each neighbouring substation. One switch somewhere in the ring is left open as the normally open point, so the ring is operated radially but can be re-supplied from the other end after a fault. The network-level reasoning is covered in MV Distribution Networks: Radial, Ring and Meshed. This article looks at the unit installed at each of those points.

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The RMU groups the two ring switches and the tee-off protection into a single compact assembly with a common busbar. In many designs the live parts of all functions share one sealed, gas-filled tank; in others each function has its own module or the insulation is air or solid.

Ring main unit vs switchgear

A common question is whether an RMU is “switchgear”. It is: an RMU is AC metal-enclosed switchgear within the scope of IEC 62271-200. The practical difference is between an RMU and a conventional MV switchboard made up of individual panels.

Neither is better in general. The RMU is the economical answer where the duty is repetitive ring switching with a small tee-off; a switchboard is the answer where many protected feeders, frequent fault interruption or withdrawable maintenance are needed. The ratings that apply to both are explained in Specifying MV Switchgear: Key Ratings Explained.

Functional units and typical configurations

Ring switches

Each ring cable is connected through a load-break switch, normally combined with an earthing switch. The load-break switch can make and break load current and is rated to close onto a short circuit, but it is not intended to interrupt fault current; that is the job of the circuit-breaker at the source substation. Switches for this duty are covered by IEC 62271-103, which defines general-purpose switches and electrical and mechanical endurance classes. The earthing switch, covered by IEC 62271-102, allows the cable to be earthed for work; its short-circuit making capability matters because it may be closed onto a cable that is unexpectedly live. Classes E0, E1, E2: none, two or five short-circuit making operations.

Transformer protection by switch-fuse combination

The most common tee-off for a single distribution transformer is a load-break switch combined with current-limiting fuses. Under IEC 62271-105, operation of any fuse causes its striker to trip the switch so that all three poles open, which avoids leaving the transformer on two phases. Designs usually provide earthing on both sides of the fuses so that fuse-links can be replaced safely. Fuse-links themselves are covered by IEC 60282-1, and the combination is tested with specified fuse types. A fuse-link from another source may not be compatible with the tested combination.

Transformer or feeder protection by circuit-breaker

Where the transformer is larger, where protection settings must be coordinated precisely with upstream and downstream devices, or where the tee-off feeds a cable rather than a transformer, a circuit-breaker (normally vacuum, to IEC 62271-100) with a protection relay is used. The relay may be self-powered from its current transformers or need an auxiliary supply. That choice affects how the relay behaves on energisation and whether a battery is needed.

Other functional units

Depending on the application, RMU ranges may also include metering units with voltage and current transformers, bus sectionalisers, direct cable connection without a switch, and cable riser units. These are usually available in extendible designs.

Describing a configuration

Manufacturers describe these combinations with their own letter codes, and the same letters do not always mean the same thing. A specification should list the functions in words, in order, with their ratings.

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Where RMUs are used

• Utility secondary substations, indoors in a building or room, or inside a compact / package substation alongside the transformer and LV board (see Compact / Package Substations: The Four Main Components).
• Commercial, industrial and institutional sites where an MV supply is looped through a site ring between several buildings.
• Infrastructure and renewable plants, where RMUs connect transformer stations along a collector cable circuit.
• Network switching points without a transformer, where only ring or branch switching is needed.

Insulation and switching technology

Many RMUs use a sealed, gas-filled tank. IEC 62271-1 describes this kind of enclosure as a sealed pressure system: it is not refilled during its expected operating life, and gas pressure or density is shown by an indicator. A sealed tank makes the live parts largely insensitive to dust, humidity and pollution, which is why gas-insulated RMUs are widely used in kiosks and harsh environments.

Sulphur hexafluoride (SF6) has historically been the usual insulating gas. It is a very potent greenhouse gas, and some jurisdictions now restrict its use in new MV switchgear on a phased timetable; the European Union’s F-gas rules are one example. Alternatives include dry or synthetic air and other gas mixtures, solid insulation, and air insulation, generally with vacuum interrupters for the switching duty. These alternatives can differ in size, weight, rated values and cold-climate behaviour, so compare complete offers rather than assuming equivalence.

For specification purposes, state which insulation media are acceptable in the destination, ask for the gas type and mass to be declared, and state any end-of-life or gas-handling expectations.

Ratings that matter for an RMU

The ratings follow the IEC 62271 series and are explained in more depth in the MV switchgear ratings article. For an RMU the following are the ones most often stated incompletely:

Under IEC 62271-1 the rated peak withstand current is normally 2.5 times the rated short-time withstand current at 50 Hz and 2.6 times at 60 Hz. This is one reason why the rated frequency must be stated rather than assumed.

Internal arc classification and service continuity

IEC 62271-200 defines internal arc classification (IAC). An IAC designation states accessibility type A (restricted access), B (public access) or C (out of reach), the accessible sides (F front, L lateral, R rear), and the arc fault test current and duration, for example “IAC AFLR 20 kA 1 s”. An RMU that will stand against a wall in a locked substation room needs a different classification from one in a kiosk with public access around it. When the RMU sits inside a prefabricated substation, IEC 62271-202 addresses the internal arc behaviour of the complete substation, so the RMU and housing classifications should be considered together.

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Specify also where the hot gases from an internal arc are vented: upward into the room, downward into a cable trench, or through a duct. This affects room height and trench design.

The loss of service continuity category (LSC) describes which parts remain in service when an accessible compartment is opened. Sealed gas tanks are not accessible compartments, so for an RMU the LSC category mainly concerns the cable and fuse compartments. The partition class (PM for metallic, PI for insulating partitions) should be declared.

Earthing, interlocking and operation

• Interlocks. A switch and its earthing switch cannot be closed together. The cable compartment cover is accessible only when the cable is earthed. Fuse compartments are interlocked with earthing on both sides of the fuses.
• Voltage indication. A voltage detecting and indicating system shows whether each cable is live before earthing. IEC 62271-213 now covers such systems; older designs often refer to IEC 62271-206.
• Cable testing. State whether cable testing is to be done through the earthing switch or by other means, and whether test accessories are required.
• Padlocking and operating handles. State padlocking needs for each operating position and how many handles are to be supplied.

Cable connections

RMUs normally use plug-in or bolted bushings with separable cable connectors. The bushing interface type and current rating determine which connectors can be used; in European practice these interfaces are standardised in EN 50181. Specify the cable type, conductor material and size, and the number of cables per phase for each function. Also state whether surge arresters must fit in the cable compartment, since this affects compartment depth and connector choice. If cable connectors are not in the supply scope, say so; they are often assumed to be.

Automation and network readiness

Remote control may be added at supply or later. Typical elements are motor operating mechanisms on selected switches, fault passage indicators, current and voltage sensors, an auxiliary supply with battery, and a remote terminal unit. Communication protocols such as IEC 60870-5-104 or IEC 61850 must match the operator’s control system.

“Automation-ready” means different things to different suppliers. If automation is a future intention, specify exactly what must be fitted now: motor-ready mechanisms, auxiliary contacts, wiring, space for the remote terminal unit, and sensor provisions.

Site conditions and enclosure

RMUs are often installed in small, unheated rooms or kiosks where condensation, dust and temperature swings are significant. State ambient temperature range, humidity, pollution, altitude and whether the installation is indoor, in a kiosk or outdoors; Defining Site and Service Conditions explains how to document these. Gas-insulated designs also need minimum operating temperatures to be confirmed for cold climates.

IEC and ANSI/IEEE practice

In IEC markets RMUs are specified and type-tested to the IEC 62271 series. In North American practice, the same duties are usually met by pad-mounted or vault-type load-interrupter switchgear specified to IEEE C37.74, with different voltage classes, insulation levels and nomenclature. The two rating systems do not map one-to-one, so a specification should use one system consistently and state the rated frequency explicitly.

Specification and procurement considerations

An RMU enquiry is quoted fastest and compared most fairly when it contains:

• network data: nominal and highest voltage, frequency, system earthing, fault level and its duration;
• the configuration, listed function by function in words, with extendible or non-extendible design;
• ratings for each function: rated normal current, short-time withstand, making capacity and endurance classes;
• transformer data for each tee-off: rating, voltage, impedance and inrush expectations, so fuses or relay settings can be selected;
• for circuit-breaker tee-offs: protection functions required, relay supply (self-powered or auxiliary), CT requirements;
• IAC classification required (accessibility, sides, current and duration) and gas-venting direction;
• acceptable insulation media and any gas-related restrictions in the destination;
• interlocking, voltage indication, padlocking and cable test provisions;
• cable interfaces, cable sizes and whether connectors and surge arresters are included;
• automation scope: motorised functions, sensors, fault passage indicators, RTU, protocol;
• site and service conditions, and the required type test evidence and routine tests.

For a structured way to present this, see Preparing an RFQ for Electrical Power Equipment.

Common specification mistakes

• Using a manufacturer’s configuration code as the specification. Codes differ between ranges; describe functions in words.
• Leaving fuse selection “by supplier” without transformer data. The fuse rating depends on transformer rating, inrush and coordination with the LV protection.
• Specifying a circuit-breaker tee-off without the protection details. Without functions, CT requirements and relay supply, offers will not be comparable.
• Stating an IAC value without accessibility and sides. The classification is meaningful only for the installation arrangement it was tested for.
• Ignoring rated frequency. Ratings declared at 50 Hz are not automatically valid at 60 Hz.
• Assuming cable connectors are included. They are a frequent source of later variation.
• Assuming the insulating gas does not matter. Local restrictions on SF6 or end-of-life handling can affect acceptance.
• Treating “automation-ready” as a defined term. List the fitted provisions instead.

Conclusion

An RMU is a small piece of switchgear with a large influence on how safely and reliably a distribution network can be operated. Define the configuration function by function, match the ratings to the network, and be explicit about arc classification, insulation medium, interlocking, cable interfaces and automation. Offers can then be compared on equal terms.

Related AdepaTrade solutions

AdepaTrade sources ring main units, MV switchgear and protection equipment up to 36 kV, as well as compact / package substations, matched to the network configuration and specification. On request, AdepaTrade can arrange on-site engineering supervision and commissioning support under a separate scope. Medium Voltage Distribution & Protection · Compact / Package Substations

Relevant standards

IEC standards are the primary reference for ring main units. Confirm the applicable edition for each project; network operators may add their own requirements.