Correct installation turns a well-built ring main unit into a safe, reliable part of your network. Most RMU problems in the first years trace back to site preparation, cabling or earthing mistakes. The equipment itself is rarely the cause. This guide walks through the essentials of RMU installation.
1. Ring Main Unit Site Preparation
The RMU needs a level, rigid foundation. This foundation must support its weight. It must also leave space for cable entries below. It must also allow safe access around the unit. Check the site’s ambient conditions first. Temperature range, humidity, altitude and pollution level should stay within the unit’s design envelope. Indoor rooms need adequate ventilation and, where specified, climate control.
2. Enclosure and Indoor/Outdoor Requirements
Choose an indoor or outdoor unit that matches the installation point. Enclosure protection ratings matter. Indoor units are typically IP3X or IP4X. Outdoor units need higher ingress protection against rain and dust. If the site is coastal or industrial, sealed units reduce contamination risk. In addition, gas-insulated or solid-insulated designs work best there. Compact units such as the GSRM6 Gas Insulated Ring Main Unit fit easily into small substation rooms.
3. Cabling and Termination
MV cables must be terminated according to the manufacturer’s instructions. Use the correct stripping lengths, stress control and torque values. Leave enough slack for re-termination. Keep phase identification clear. For example, seal the cable entries after termination. This prevents moisture ingress.
4. Earthing
Earthing is critical. Connect the RMU frame, cable screens and the earthing switch to the station earth grid. Size the conductors for the fault current. In practice, a poor earth connection defeats the purpose of the earthing switch. It also endangers personnel.
5. Pre-Commissioning Checks
- Verify all connections are torqued and phasing is correct
- Check the operating mechanisms, position indicators and interlocks
- For SF6 units, confirm gas pressure is within range
- Test the earthing switch operation and earthing circuit continuity
- Review the protection settings if relays or VCBs are fitted
- Complete an insulation resistance check before energizing, following local safety rules
For fixed-type air-insulated units, pre-commissioning is simple and fast. The GSM6-12~24 Air-insulated RMU requires no gas handling at all.
6. Common Installation Mistakes
- Under-sized earthing conductors
- Water ingress through unsealed cable entries
- Operating interlocks out of sequence during testing
- Storing the unit outdoors before installation without protection
Storing the unit correctly before installation is part of good practice. Keep the ring main unit in its original packaging. Store it in a dry, ventilated area. Protect it from condensation and vibration. Inspect it on arrival for shipping damage. If the unit stays in storage for more than six months, complete a full inspection. For example, for SF6 models, include gas pressure checks before commissioning. These simple steps prevent most early failures.
Site records are equally important. Keep the as-installed drawings, cable schedules and test results with the substation documentation. Future maintenance teams will then know exactly what was installed and how.
In practice, allocate enough time for the pre-commissioning checks. Rushing this stage causes most first-year failures. Also record the test results and keep them with the substation file. If the RMU includes SF6 compartments, confirm the gas pressure before energizing. A small leak at this stage is easy to fix. Finally, train the local operators before the unit goes live. Simple training on switching sequences prevents most operational errors.
7. Conclusion
Installation quality determines long-term reliability. Prepare the site properly. Follow the cable and earthing instructions. Run a disciplined pre-commissioning checklist. For outdoor RMU applications, take a look at the Outdoor ring main unit.
Electric-GS is a professional power and electrical equipment manufacturer in Yueqing, Zhejiang, China. The company produces air-insulated, SF6 and solid-insulated ring main units. OEM and ODM support is available. For installation documentation or technical support, contact info@electricgs.com or WhatsApp +86 135 8897 8050.
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GSM6-12~24 Air-insulated RMU (Fixed Type)
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