Metal-Clad vs. RMU Switchgear: Which One Should You Choose?

Medium-voltage switchgear selection is one of the most commonly misjudged decisions in a substation project — it usually gets reduced to "which one is cheaper," when the real question should be "what growth and maintenance profile does this facility have." Metal-clad and RMU (Ring Main Unit) switchgear serve the same function — medium-voltage switching, protection, metering — through fundamentally different architectures, and that architectural difference directly determines decades of operating cost at an organized industrial zone (OIZ) site. A project started with the wrong switchgear type either falls short within a few years or ends up as unnecessary overinvestment. This article covers the technical differences between the two systems, which facility profile suits each, and the concrete footprint/cost tradeoffs involved.

What Is a Medium-Voltage Switchgear System?

Medium-voltage (MV) switchgear is a set of modular switching, metering and protection units that safely route power from the distribution grid to the facility transformer. A typical system lines up an incoming unit, a metering unit, a circuit-breaker/protection unit and a transformer feeder unit; each performs its own function while together forming the facility's MV backbone. Switchgear selection should be based not just on today's number of feeders but on the facility's 10-15 year growth plan, because once a switchgear system is installed, changing it later is costly and requires an outage. In Bursa's organized industrial zones, this decision is made alongside plot size, building type, and the distribution utility's connection requirements.

What Is Metal-Clad Switchgear?

Metal-clad switchgear is a modular switching system in which every functional section — busbar, circuit breaker, cable termination, control — is physically separated by metal partitions. This compartmentalized construction allows one compartment to be serviced or repaired while the rest of the system remains energized — meaning a single feeder can be isolated and worked on without shutting down the entire installation. The circuit breaker is typically withdrawable, moving to a test/service position where it is fully disconnected from both busbar and cable. Metal-clad switchgear is manufactured with internal-arc classification per IEC 62271-200, which guarantees that personnel outside the enclosure are protected in the event of an internal arc fault.

What Is an RMU (Ring Main Unit)?

The RMU gets its name from the ring-main feed topology it's designed for: it's typically a compact, factory-assembled sealed unit consisting of two incoming/outgoing units and one transformer-protection unit. Compartment separation is less granular than in metal-clad — in an RMU, the switching elements usually sit closer together within a single gas or vacuum chamber. This makes the unit smaller and simpler to install, but it increases the need to take the entire unit out of service during maintenance. RMUs are especially favored in facilities with a small number of feeders — typically single-transformer sites — fed from a grid section with a ring topology.

The Compartmentalization Difference

This is the single most decisive technical distinction between the two systems. In metal-clad switchgear, the busbar compartment, breaker compartment, cable compartment and low-voltage control compartment are separated by independent metal walls; each compartment can be isolated and earthed on its own. This means work can proceed on one feeder while neighboring feeders stay energized — in a large, multi-feeder substation, this is what allows scheduled maintenance to happen without stopping production. In an RMU, compartment separation is more limited; in most designs, the entire switching function sits inside a single sealed gas or vacuum block. This keeps the RMU cheaper and smaller, but a fault or maintenance event can mean the entire unit — and therefore multiple feeders — goes out of service at once.

Insulation Medium: SF6 or Vacuum?

In both switchgear types, the switching element (breaker, load-break switch) extinguishes the arc in a vacuum chamber, while busbar and connection-point insulation can be provided by SF6 gas, solid insulation, or air. SF6-insulated RMUs are common because of their compact design, but since SF6 is a potent greenhouse gas, there's a growing shift in Europe — and increasingly in Turkey — toward alternative insulation technologies such as solid-insulated, "gas-free" designs. Metal-clad switchgear can also use SF6, but its larger volume makes air-insulated or solid-insulated alternatives easier to implement. When choosing an insulation medium, factors to weigh include gas-leak monitoring requirements, maintenance staff SF6 certification, and long-term environmental regulatory trends.

Footprint and Site Area

The RMU's most tangible advantage is its compact footprint. Its single-body, narrow-base design saves significant space on OIZ plots with limited land or in indoor substation rooms. Metal-clad switchgear, by contrast, requires considerably more floor area and usually higher ceiling clearance because of its compartmentalized structure and larger breaker/busbar volumes. For a small manufacturing facility needing, say, 2-3 feeders, choosing an RMU produces measurable savings both in initial investment and substation room construction. Conversely, when a large OIZ factory tries to meet a 6-8 feeder requirement by lining up multiple RMU modules side by side, metal-clad's single, organized busbar system becomes the more sensible choice, both spatially and operationally.

Extensibility and Future Planning

Thanks to its modular structure, metal-clad switchgear is far better suited to adding a new feeder in the future — expansion is done by adding a new switchgear module to the busbar system, and most manufacturers offer an extensible busbar option for exactly this purpose. Extension is more limited on RMUs; while some models allow additional modules, most RMUs ship with a fixed number of bays (typically a 2+1 or 3+1 configuration), and once capacity is exceeded, a facility may need a second parallel RMU or a full system replacement. For facilities with uncertain or high growth expectations, choosing metal-clad despite the higher initial cost can reduce future expansion cost and outage risk.

Maintenance Access and Fault Response

The withdrawable breaker design in metal-clad switchgear allows the breaker to be moved to a test position without disturbing its connection to the busbar and cable — this enables periodic maintenance to be performed without affecting the rest of the system. Each compartment also has its own earthing switch, which simplifies establishing a safe working zone for maintenance staff. In RMUs, maintenance is typically performed per the manufacturer's service procedure by de-energizing the relevant section of the unit; some modern RMUs integrate load-break and earthing switches for safe isolation, but they generally lack the same granular compartment isolation found in metal-clad. This difference is an important decision criterion for facilities expecting frequent fault response or requiring near-continuous production.

Cost and Investment Comparison

In terms of initial investment, RMUs are generally significantly more economical than metal-clad switchgear — less material, a smaller enclosure and standardized manufacturing drive this difference. But when calculating total cost of ownership, it's not just the initial investment that matters — also weigh: substation room construction cost (RMU needs a smaller enclosure), future expansion cost (lower with metal-clad), production-loss risk during scheduled maintenance (lower with metal-clad), and spare-part/service availability. For small and mid-sized facilities, the RMU's lower upfront cost usually wins out; for large facilities with critical production processes or a high feeder count, metal-clad's operational advantages offset the initial cost difference over the long run.

Which Facility Profile Should Choose Which?

The decision becomes clear based on the following profiles:

• RMU-suited facilities: single-transformer small-to-mid-sized manufacturing sites, operations needing 2-4 feeders, OIZ plots with limited land area, sites requiring an indoor substation room, areas fed by a ring-topology grid section.
• Metal-clad-suited facilities: large industrial sites with many feeders (5 or more), factories requiring near-continuous production (automotive, food, chemical), facilities with confirmed future capacity expansion plans, critical infrastructure requiring frequent maintenance/test access, main distribution centers feeding multiple transformers.

This assessment should account not only for today's needs but for the facility's 10-year production plan — switchgear selection is, to a large extent, an irreversible decision.

Installation and Commissioning Process

Regardless of which switchgear type is chosen, the installation process follows similar stages: project approval and coordination with the distribution utility, equipment procurement, mechanical assembly and cable termination, insulation resistance and continuity testing, protection relay setting and function tests, earthing measurement, and finally preparation of the provisional acceptance file. We cover this process end-to-end in our substation installation guide. Switchgear selection should be finalized at the very start of this process — during the design phase — because room dimensions, foundation details and cable routing all differ depending on the switchgear type.

Common Mistakes

• Choosing an RMU based on initial cost alone: In a facility with a growth plan, this can force a parallel RMU installation or a full system change within a few years.
• Oversizing metal-clad unnecessarily: Choosing metal-clad for a small facility that only needs two feeders wastes both space and capital.
• Ignoring SF6 maintenance requirements: In gas-insulated systems, neglecting periodic gas-pressure checks degrades insulation reliability over time.
• Not accounting for expansion margin at the design stage: Adding a switchgear bay later usually means shutting down the existing system and incurring unplanned cost.

FAQ

Is RMU switchgear less safe than metal-clad? No. Both systems are equally safe when manufactured to the relevant IEC standards (IEC 62271-200 and related product standards). The difference lies in maintenance granularity and extensibility, not in safety.

Can a facility start with RMU and later switch to metal-clad? Theoretically yes, but in practice it requires substantial reconfiguration and an outage. That's why basing the initial choice on the growth plan is far more economical than changing it later.

Up to how many feeders is an RMU practical? The general rule of thumb is that RMUs are practical and economical for 2-4 feeder requirements. Beyond that, a multi-RMU arrangement or a switch to metal-clad should be evaluated.

Is maintenance more expensive for metal-clad switchgear? While unit maintenance cost is comparable, metal-clad's compartment-based isolation reduces the risk of production loss during maintenance — which indirectly lowers total cost.

Would you recommend a solid-insulated alternative over SF6-based RMUs? This depends on environmental regulatory trends and the facility's long-term plans. Solid-insulated or "gas-free" designs are a growing trend; we recommend evaluating this option for any new investment.

Who should make the switchgear selection? Switchgear selection should be evaluated by an EMO-registered electrical engineer alongside the short-circuit calculation, protection coordination, and the facility's growth plan — picking from an equipment catalog alone does not guarantee overall system compatibility.

Which type is more common in Bursa's OIZs? RMUs are common in small and mid-sized manufacturing facilities; metal-clad is preferred in large factories and main distribution points. The choice varies with plot size and the distribution utility's connection requirements.

Is a hybrid solution possible between the two? Yes, some facilities use a hybrid approach — RMU on the incoming/metering side, metal-clad for critical protection and transformer feeders. This requires project-specific engineering evaluation.

Conclusion

The choice between metal-clad and RMU isn't "which one is better" but "which one better fits this facility's growth, maintenance, and outage-tolerance profile." The right decision should be made during the design phase through an engineering evaluation that considers not just today's feeder count but the future expansion plan, maintenance strategy, and site constraints. SOREAS provides design, installation, and commissioning services for both switchgear types across Bursa's 17 organized industrial zones; we can determine the right switchgear system for your needs together under our MV switchgear and transformer systems service.