Six Key Differences Between Ring Main Units and Switchgear Explained
Differences Between Ring Main Units (RMUs) and Switchgear
In power systems, both ring main units (RMUs) and switchgear are common distribution equipment, but they differ significantly in function and structure. RMUs are primarily used in ring-fed networks, responsible for power distribution and line protection, with the key feature being multi-source interconnection through a closed-loop ring network. Switchgear, as a more general-purpose distribution device, handles power reception, distribution, control, and protection, and is applicable to various voltage levels and grid configurations. The differences between them can be summarized into six aspects:
1. Application Scenarios
RMUs are typically deployed in distribution networks at 10kV and below, suitable for urban grids and industrial facilities requiring ring-fed power supply. A typical application is a dual-power supply system in commercial centers, where RMUs form a closed loop, enabling rapid power path switching during line faults. Switchgear has a broader application range, covering 6kV to 35kV voltage levels. It can be used on the high-voltage side of substations or in low-voltage distribution rooms. For example, high-voltage switchgear is required in outgoing feeder bays from the main transformer in a thermal power plant.
2. Structural Composition
RMUs commonly use gas insulation technology, with SF6 gas as the insulating medium. Typical components include three-position disconnectors, load-break switches, and fuse combinations. Their modular design reduces volume by over 40% compared to traditional switchgear; for example, the XGN15-12 RMU has a width of only 600mm. Switchgear typically uses air insulation, with standard cabinet widths of 800–1000mm. Internal components include circuit breakers, current transformers, and relay protection devices. The KYN28A-12 metal-enclosed switchgear, for instance, features a withdrawable circuit breaker trolley.
3. Protection Functions
RMUs usually rely on current-limiting fuses for short-circuit protection, with rated breaking currents up to 20kA, but lack precise relay protection systems. Switchgear is equipped with microprocessor-based protection relays, offering functions such as three-stage overcurrent protection, zero-sequence protection, and differential protection. For example, a certain switchgear model achieves overcurrent protection operation in as little as 0.02 seconds, enabling selective tripping with vacuum circuit breakers.
4. Expandability
RMUs use standardized interfaces, allowing expansion to up to six incoming/outgoing circuits. They can be quickly connected via busbar couplers—some models can be expanded in under 30 minutes. Due to high functional integration, switchgear expansion often requires replacing entire cabinets or adding new compartments, with typical retrofit times exceeding 8 hours.
5. Operating Mechanisms
RMUs typically use spring-operated load-break switches with operating torques under 50 N·m and visible break points. For example, the operating handle of one RMU model is limited to a 120° rotation to prevent misoperation. Switchgear circuit breakers are equipped with electric operating mechanisms; for instance, a spring mechanism can be charged in under 15 seconds and includes mechanical interlocks to ensure correct operation sequences.
6. Maintenance Costs
The annual maintenance cost of an RMU is about 2% of its equipment value, mainly involving SF6 gas pressure checks and mechanical lubrication. Switchgear maintenance costs reach 5% of equipment value, including circuit breaker mechanical testing and relay calibration. A project case shows that annual preventive testing for switchgear requires 8 man-hours per unit.
Typical Engineering Configuration
An industrial park’s 10kV distribution system uses eight RMUs to form a dual-ring network, each equipped with a DTU (Distribution Terminal Unit) for automatic fault section isolation. In contrast, a concurrently built 110kV substation uses 12 switchgear units in its 10kV outgoing bays, each fitted with microprocessor protection. The total investment shows the RMU-based system costs about 60% of the switchgear system.
Equipment Selection
Selection must consider reliability requirements. When continuity of supply needs to reach 99.99%, a dual-ring network using RMUs can meet the N-1 security criterion. For critical loads such as hospital operating rooms, switchgear with automatic dual-power transfer systems is required to ensure power interruption time remains under 0.2 seconds.
Technology Trends
New eco-friendly RMUs are replacing SF6 with dry air, achieving equivalent insulation performance with zero global warming potential. Intelligent switchgear integrates online monitoring systems; one model can monitor over 20 parameters (e.g., contact temperature, mechanical characteristics) in real time with a sampling frequency of up to 1000 Hz.
Answer and Analysis
Application Scenarios: RMUs (closed-loop distribution networks) – 15%, Switchgear (multi-voltage systems) – 15%
Structural Features: Gas-insulated, modular (RMUs) – 20%, Air-insulated, integrated (Switchgear) – 20%
Protection Systems: Fuse-based protection (RMUs) – 10%, Relay protection (Switchgear) – 10%
Expandability: Quick connection (RMUs) – 5%, Full cabinet replacement (Switchgear) – 5%
Operating Mechanisms: Manual spring charging (RMUs) – 5%, Electric control (Switchgear) – 5%
Maintenance Costs: Low maintenance (RMUs) – 5%, High maintenance (Switchgear) – 5%
Analysis: The scoring emphasizes structural features and application scenarios, as they directly determine equipment selection. The 20% weight for structural features reflects the impact of insulation differences on equipment size and space requirements—gas insulation reduces RMU volume by over 35%, a decisive factor in space-constrained urban distribution corridors. The 15% weight for application scenarios highlights the irreplaceability of each device in systems with different reliability needs; for example, data centers require RMUs to build redundant dual-power networks.
