RMU vs Metal-Clad Switchgear: Specify the Right Distribution Duty

1. Separate network function from construction

Ring main unit describes an arrangement commonly associated with ring feeders and transformer connections in secondary distribution. Metal-clad refers to an assembly construction concept; the applicable standard and tested design determine its actual compartment and access characteristics. The names therefore compare different aspects of equipment.

Do not reduce the choice to ‘RMU means switches, metal-clad means breakers’. Compact RMU platforms can include circuit-breaker functions. ABB’s secondary-distribution portfolio also includes both air-insulated and gas-insulated approaches. The requested feeder functions and the offered design matter more than a blanket label.

Secondary-distribution equipment context

2. Compare seven project requirements

Put both options against the same requirements and require written deviations. A generic quotation heading such as ‘12 kV panel’ is not enough to compare switching functions, future expansion or maintenance access.

3. Write the protection concept before choosing the cabinet

List what each circuit must switch and what must clear a fault. An earth switch, a load-break function and a fault-interrupting device have different duties. The presence of one does not establish the rating or capability of the others.

For a transformer connection, ask which protection scheme is proposed and which ratings and operating conditions it covers. For a relay-protected feeder, include the protection functions, instrument-transformer requirements, trip supply and communication interfaces. The designer must coordinate the complete scheme with the rest of the network.

4. Specify maintenance access and continuity separately

A withdrawable device does not automatically make every compartment safe to access or every maintenance task outage-free. Ask which parts remain in service during the specific activity, what isolation boundaries apply, and which classification and instructions support that arrangement.

IEC 62271-200 addresses metal-enclosed assemblies and includes service-continuity and internal-arc concepts. Treat loss-of-service-continuity category, internal-arc classification, enclosure protection and actual access conditions as separate specification fields. Do not infer one from another.

Give the supplier the room layout, cable route and permitted maintenance access. Include space for terminations, installation handling and any exhaust or pressure-relief arrangement required by the offered design. A cabinet-width comparison alone does not represent installed footprint.

Assembly standard and classification scope

5. Keep insulation medium separate from switching medium

A vacuum interrupter describes where interruption occurs. It does not, by itself, prove that every part of the assembly is free of SF6 or other fluorinated gases. Ask separately about the insulating medium, the interrupting medium and the full functional-unit arrangement.

Likewise, air insulation does not by itself establish an internal-arc classification, a particular footprint or a maintenance interval. Request the tested configuration and operating documentation. Evaluate the proposed arrangement on its evidence, not on a broad claim that one insulation category is always superior.

6. Check destination requirements before ordering

For EU projects, Article 13(9)(a) of Regulation (EU) 2024/573 prohibits, from 1 January 2026, putting into operation MV primary- and secondary-distribution switchgear up to and including 24 kV that uses, or relies on, fluorinated greenhouse gases in insulating or breaking media. The article also specifies conditions and exceptions; the responsible project reviewer must check whether they apply.

This is not a worldwide ban on every existing SF6 installation, and the relevant date is not simply the article’s publication date. Ask the operator and responsible project reviewer to confirm the destination requirements, timing and any applicable exception with supporting records. Do not assume that an offered ELCZEEN catalog model is SF6-free; obtain the configuration-specific declaration.

ABB’s 31 August 2026 industry article also emphasizes operating familiarity and maintenance when changing technologies. Translate that context into concrete tender questions: which interfaces change, what training is needed, and which spare parts and documents accompany the proposed design?

Regulatory scope and exceptions · Dated industry development, 31 August 2026

7. Send a function-based RFQ

Attach an SLD showing the ring, radial or sectionalized arrangement and identify each required function. Include voltage, current, fault duties, protection, cable interfaces, destination and installation conditions. State the maintenance tasks and outages that the owner needs to accommodate.

For a useful commercial comparison, require a completed deviation list and a document schedule with each offer. Separate included functions, optional functions and exclusions. Ask for a general arrangement, protection schematic and model-specific report list before treating two prices as equivalent.

• Ring ways, transformer feeders, bus sections and future spare functions
• System and assembly ratings with duty and duration
• Protection, CT/VT, interlocking and control requirements
• Cable type, termination arrangement and entry direction
• Room layout, installation environment and maintenance constraints
• Insulating and interrupting media, evidence and destination documents

Frequently asked questions