A Ring Main Unit (RMU) is one of the most common pieces of medium voltage distribution equipment worldwide. Installing one correctly is critical — a poorly installed RMU creates safety hazards, reliability problems, and premature equipment failure. This guide covers the complete installation and commissioning process from site preparation to first energization.
Pre-Installation: Site Preparation
Before the RMU arrives on site, these preparations must be completed:
Foundation: A level concrete plinth, typically 100-200mm above finished ground level, sized to the RMU footprint plus 100-200mm overhang on all sides. The foundation must include pre-cast cable entry holes (typically 300x300mm or larger) aligned with the RMU's cable compartment positions. Foundation bolts (M12 or M16, stainless steel) should be cast in place.
Cable trench: Underground cable trenches from the RMU position to the incoming and outgoing cable routes. Trench depth typically 600-1000mm, width 300-600mm. Cables should enter the RMU from below through the foundation.
Earthing grid: A grounding grid connected to the site's main earth electrode system. The earth resistance at the RMU location should be below 10 ohms (below 1 ohm for substations). Earth bars (copper, minimum 25x3mm flat) should be laid along the cable trench and connected to the foundation bolts.
Access: Verify that the delivery vehicle (typically a flatbed truck with crane) can reach the installation site. RMUs weigh 200-1500 kg and require a crane or forklift for positioning. Minimum clearance of 1000mm on all sides for installation and future maintenance.
Step 1: Receiving and Inspection
When the RMU arrives on site:
- Inspect the packaging for transport damage. Check all corners and edges of the enclosure.
- Verify the nameplate data matches the purchase order: rated voltage, rated current, short-circuit rating, number of ways, switch/fuse/breaker configuration.
- Check that all accessories are included: operating handles, fuse links, cable lugs, instruction manual, test certificates.
- For SF6 insulated RMUs: check the gas pressure gauge — pressure should match the factory certificate. Low pressure indicates a leak during transport.
- For vacuum or solid-insulated RMUs: no gas check needed — one of the advantages of SF6-free technology.
Step 2: Positioning and Anchoring
Lift the RMU onto the prepared foundation using the designated lifting points (marked on the enclosure or in the manual). Do not lift by the operating handles, cable compartment doors, or any protruding components.
- Align the cable entry holes in the RMU base with the foundation openings.
- Level the unit using a spirit level on the top surface. Shim if necessary — maximum allowable tilt is typically 1 degree.
- Bolt the RMU to the foundation through the base mounting holes. Torque foundation bolts to the manufacturer's specification (typically 40-60 Nm for M12).
- Seal the gap between the RMU base and foundation with silicone sealant or a compressible gasket to prevent water and vermin entry.
Step 3: Cable Termination
Cable termination is the most skill-intensive phase of RMU installation. Incorrect termination causes partial discharge, insulation failure, and potentially catastrophic cable faults.
- Cable preparation: Strip the cable outer sheath, screen, and insulation to the dimensions specified by the cable termination kit manufacturer (heat-shrink or cold-shrink kits). Follow the termination kit instructions exactly — dimensions are critical.
- Termination type: Most modern RMUs use screened separable connectors (type A, B, or C per IEC 62271-209) for plug-in cable connection. Older designs may use bare conductor termination with heat-shrink boots.
- Torque: Tighten cable lug bolts to the specified torque (typically 25-50 Nm for M10 bolts). Under-torque causes high-resistance connections; over-torque damages the bushing.
- Phasing: Verify cable phases match the RMU phase markings (L1, L2, L3 or R, S, T). Phase sequence errors cause motor reversal and protection relay maloperation.
Step 4: Earthing Connections
Earthing is non-negotiable for safety. All of the following connections must be made:
- RMU enclosure frame to the site earthing grid via minimum 70mm² copper earth conductor.
- Cable screens/sheaths terminated and connected to the RMU earth bar.
- Neutral connection (if applicable) from transformer to the earth bar.
- Foundation reinforcement (if steel) bonded to the earthing grid.
Test earth continuity from the RMU frame to the main earth electrode. Resistance must be less than 0.5 ohms.
Step 5: Secondary Wiring
Connect all secondary (low voltage) circuits:
- Current transformer (CT) secondary leads to the protection relay or metering panel.
- Motor operating mechanism power supply (if motorized switches).
- SCADA/RTU communication cables (RS485, Ethernet, or fiber optic).
- Auxiliary contacts wiring for remote status indication.
- Heating circuit (anti-condensation heater) power supply.
Verify CT polarity using a polarity test set before connecting to protection relays. Reversed CT polarity causes directional protection elements to operate incorrectly.
Step 6: Testing and Commissioning
Before first energization, complete the following test sequence:
1. Insulation resistance (megger test): Apply 5 kV DC between each phase and earth, and between phases. Reading must exceed 100 megohms. Low readings indicate moisture ingress, contamination, or insulation damage.
2. Contact resistance: Measure the resistance across each closed switch or breaker contact using a micro-ohmmeter. Maximum allowable resistance: 200 micro-ohms for new equipment. High readings indicate insufficient contact pressure or surface contamination.
3. Mechanical operation test: Operate each switch, circuit breaker, and earthing switch through full open-close-open cycles. Verify smooth operation, correct mechanical interlocks (e.g., cannot close earthing switch while main switch is closed), and operation counter increment.
4. Protection relay verification: Inject test currents into the CT secondary circuits to verify relay pick-up values, time delays, and trip output contacts. This confirms the protection settings match the coordination study.
5. SCADA signal verification: Verify all remote indication signals (switch position, alarm contacts, metering values) are correctly received at the control center.
Step 7: Energization
Only after all tests pass:
- Remove all temporary earths from the incoming cables.
- Close the RMU door and secure all access panels.
- Verify all personnel are clear of the installation.
- Close the incoming switch or breaker to energize the RMU bus.
- Verify bus voltage on all three phases using the built-in voltage indicators.
- Close outgoing switches/feeders one at a time, verifying correct voltage and current on each.
The RMU is now in service. Record all commissioning test results, take photographs, and hand over the documentation package to the asset owner.
NAIJI Electric RMU Products
NAIJI Electric manufactures a complete range of ring main units for 12-24 kV distribution networks, including compact SF6-free designs and extended configurations with integrated vacuum circuit breakers. Our RMUs are delivered factory-tested and ready for site installation. Contact us for product specifications and installation support.