DSJ Electrical Medium Voltage Ring Main Unit (RMU)

A Reliable Solution for North American Distribution Systems

In North American medium-voltage distribution systems, reliability, fault isolation capability, and compliance with IEEE standards are critical selection criteria.

With over 40 years of experience in power distribution equipment manufacturing, DSJ Electrical provides medium voltage Ring Main Units (RMUs) engineered for 5kV–38kV systems. Our RMUs are designed for utility, industrial, commercial, and renewable energy applications where service continuity and safety are essential.

Designed for Service Continuity

RMUs are primarily used in loop (ring) distribution networks. Their function is straightforward:

• Provide dual power paths
• Isolate faulted sections quickly
• Minimize outage areas
• Maintain service to unaffected loads
  
  

In urban grids, campuses, and industrial parks, loop configurations significantly improve system availability compared to radial systems.

Construction and Configuration

A typical DSJ Electrical RMU includes:

• Two load break switches for loop feeders
• One transformer feeder (fused switch or vacuum circuit breaker)
• Grounding switch
• Cable terminations
• Optional protection relays and CTs/VTs
  
  

Standard arrangements include 2L1T and 3L1T configurations, with modular expansion available as required.

Units can be supplied for indoor installation or in outdoor enclosures, and can be integrated into pad-mounted substations where space is limited.

Compliance with North American Standards

DSJ Electrical RMUs are engineered in accordance with:

• IEEE C37.20.3
• IEEE C37.74
• ANSI standards framework
  
  

Typical ratings include:

• Voltage class: 15kV / 25kV / 35kV
• Continuous current: 600A / 1200A
• Short-circuit withstand: 16kA–25kA
• BIL: 95kV–150kV
• Enclosure type: NEMA 1 or NEMA 3R
  
  

All ratings are selected based on available fault current, system grounding method, and utility requirements.

Insulation Options

Depending on project requirements, DSJ Electrical offers:

• SF6 Gas-Insulated RMU: Compact, sealed-for-life design with proven dielectric performance.
• Solid-Insulated RMU: SF6-free construction to support environmental compliance initiatives.
• Air-Insulated RMU: Conventional design with accessible components, suitable for specific industrial environments.
  
  

Insulation selection is typically driven by site conditions, maintenance philosophy, and environmental policy.

Typical Applications

DSJ Electrical RMUs are commonly used in:

• Utility distribution loops
• Wind and solar collector systems
• Industrial medium-voltage networks
• Healthcare and educational campuses
• Commercial developments
  
  

In renewable energy projects, RMUs are often integrated with transformers and low-voltage switchboards to form a complete medium-to-low voltage distribution package.

Engineering Considerations for Specification

For proper RMU selection, the following parameters should be defined:

• System nominal voltage
• Available short-circuit current
• Required BIL level
• Number of incoming and outgoing feeders
• Installation environment (indoor or outdoor)
• Utility interconnection requirements
  
  

Accurate system data ensures correct rating selection and long-term operational reliability.

Conclusion

A Ring Main Unit is more than a switching device—it is a reliability component within a modern distribution network.

DSJ Electrical delivers RMUs engineered for IEEE compliance, appropriate fault ratings, and practical integration into North American distribution systems. Our focus is straightforward: safe operation, dependable performance, and alignment with project specifications.

For 15kV or 25kV applications, our engineering team can provide configuration support based on your system parameters.