Functions and Manufacturing Technology of Each Compartment in a Ring Main Unit (RMU)

✅ 1. Functional Compartments/Enclosures of a Ring Main Unit (RMU)

Compartment / Enclosure	Primary Function
Switchgear Compartment	Houses load-break switches, circuit breakers and earthing switches for making or breaking current, short-circuit interruption and earthing.
Fuse Compartment	Accommodates high-voltage current-limiting fuses that, in combination with the load-break switch, protect transformers against overload and short-circuit.
Operating-Mechanism Compartment	Contains manual or motor-operated spring-charged mechanisms for closing, opening and earthing operations.
Cable Compartment (Base Frame)	Provides space for cable entry/exit, fixing, earthing, and installation of current/voltage transformers and zero-sequence CTs.
Metering & Control Compartment (optional)	Installs secondary devices such as protection relays and DTU (Distribution Terminal Unit) to enable “remote measurement, remote signalling and remote control”.
Gas Tank / SF₆ Enclosure	A sealed tank that encloses the main primary components (three-position switch, busbars), filled with SF₆ or an eco-friendly gas for insulation and arc extinction.

✅ 2. Manufacturing Processes of Ring Main Units (SF₆-insulated type as example)

Stage	Description
Enclosure Fabrication	3 mm stainless-steel plates are laser-cut and welded, ensuring a leakage rate ≤ 0.01 % per year.
Gas-Tank Assembly	Load-break switches, busbars and three-position switches are modularised and installed inside the sealed tank, which is then filled with SF₆ or eco-friendly gas.
Cable-Compartment Assembly	Integrates plug-in type CT/VTs, cable clamps, PT elbows and zero-sequence CTs for reliable connections and easy maintenance.
Secondary Wiring	Secondary devices (protection relays, DTU) are mounted on the door or in the control compartment, followed by wiring and functional tests.
Testing & QA	Includes power-frequency withstand tests, partial-discharge tests, gas-tightness checks and lightning-impulse tests.
Automated Production Line	MES system and automatic helium-leak detection equipment realise full-process automation, traceability and quality control.

✅ 3. Typical Structural Types Comparison – American vs European

Type	Features
American Common-Tank	All circuits in one enclosure, compact footprint, maintenance-free, suitable for outdoor use.
European Common-Tank	Separate compartments for each functional unit (switchgear, busbar, etc.), modular design, easy extension and maintenance.
Individual Gas Tank (Air-insulated or Gas-insulated)	Each functional unit has its own sealed tank with SF₆ or eco-gas insulation, ideal for urban networks demanding high reliability.

Manufacturing Process of a Ring Main Unit (RMU)

The entire production cycle can be summarized in five main phases: enclosure fabrication → primary assembly → secondary wiring → testing → shipment. Each step is governed by strict workmanship and quality-control checkpoints.

1. Enclosure Fabrication

• Material: 2.5–3 mm non-magnetic stainless steel (SUS 304).
• Cutting & Bending: Laser cutting → CNC bending → robotic welding; welds are 100 % helium-leak tested.
• Surface Treatment: Seven-stage pre-treatment (degreasing → rinsing → phosphating …) → thermal-spray zinc–aluminium alloy → polyurethane powder coating, total thickness ≥ 80 µm for outdoor salt-fog resistance.
• Enclosure Features
  – Double-door design: front door for operation & metering, rear door for cable compartment, both with three-point locking.
  – Pressure-relief channel: bottom or top vent flaps to direct arc gases safely outward during internal arc faults.

2. Primary Assembly

• Gas-Tank Pre-assembly
  – Robotically welded “sealed for life” tank, leakage rate ≤ 0.1 % per year.
  – Vacuum evacuated, then back-filled with SF₆ or dry air slightly above atmospheric pressure; 30-year maintenance-free.
• Main-circuit Module Installation
  – Three-position switch (close–open–earth) + busbars + vacuum circuit breaker / load-break switch pushed into the tank as a complete module.
• Cable Compartment Fit-up
  – Integrated CTs/VTs, zero-sequence CT, cable clamps, separable PT elbows positioned for zero cable crossing and easy maintenance.
• Operating Mechanism Mounting
  – Spring-operated mechanism coupled to the three-position switch; manual and motor charging, mechanical position indication and interlocking adjusted simultaneously.

3. Secondary Wiring & Protection System

• Device Installation
  – Relays, DTU, auxiliary relays, temperature-humidity controller, compartment lighting mounted on door or dedicated metering cubicle.
• Wiring Practice
  – Harness routed in cable ducts or corrugated conduits, dual-end ferrules; CT secondary circuits earthed at one point.
• Terminals & Labelling
  – Screwless spring-type terminals, segregated incoming/outgoing sides; each wire carries both QR code and text labels for full traceability.

4. Testing & Quality Control

Test Item	Content / Standard
Power-frequency withstand	1.2 Ur for 1 min, no flashover
Partial discharge	≤ 5 pC @ 1.1 Ur
Gas tightness (helium)	≤ 0.1 % annual leakage
Mechanical endurance	3-pos switch ≥ 5 000 operations
Secondary injection	Verify pickup & timing of overcurrent, earth-fault, differential relays
Primary injection	100 % In through primary circuit, verify CT ratio & phase
Routine Factory Test (FAT)	Witnessed by customer or third party; test certificates & reports issued

5. Final Inspection, Packing & Shipment

• Final Check
  – Appearance, labels, keys, spare-parts list; door opening angle ≥ 120°, locks smooth.
• Packing
  – PE film wrap + fumigated plywood crate + desiccant; shipping marks “moisture-sensitive”, “keep upright”, “fragile”.
• Documentation
  – Factory test report, certificate of conformity, wiring diagrams (AutoCAD PDF), O&M manual, warranty card.

6. Typical Production-Line Layout (minutes per station)

1 Enclosure loading → 2 Gas-tank welding/leak test → 3 Primary module insertion → 4 Secondary wiring → 5 Complete unit testing → 6 Packing → 7 Warehousing/shipping
MES integration records welding parameters, SF₆ fill data and test curves in real time for full traceability.

The above process complies with IEC 62271-200, IEC 62271-103 and GB 3906 requirements and can be fine-tuned for solid-insulated, eco-gas or primary-secondary integrated RMUs.