Ring Main Unit (RMU)
A compact factory-assembled HV switchgear set for ring-type or radial distribution networks.
An RMU is a compact MV switchgear set at distribution points where distribution transformers connect to the MV network.
Combines: load-break switches, fuse-switch combinations or circuit breakers for transformer protection, busbar connections for ring topology. Enables flexible reconfiguration and supply continuity.
Configurations: 3-way, 4-way, 6-way. Modern types use SF6 or vacuum technology. See grounding guide for RMU grounding.
QDTB manufactures RMUs up to 24 kV / 630A, used in urban distribution and public facility projects.
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Related Terms
Switchgear
Equipment for switching, controlling, and protecting electrical circuits including breakers and relays.
Distribution Transformer
A transformer stepping down voltage from the distribution network (11-33 kV) to utilization voltage (400/230V) for end consumers.
Prefabricated Substation
A factory-assembled complete substation integrating transformer, switchgear, and protection for rapid installation.
More Terms Starting with “R”
Reactor
An inductive device in power systems to limit current, absorb reactive power, or filter harmonics.
Rectifier Transformer
A transformer designed to supply rectifier/inverter loads with enhanced harmonic and DC magnetization handling.
Voltage Regulation
The change in secondary voltage from no-load to full-load as a percentage, indicating voltage stability.
Rated Capacity
The maximum continuous power output a transformer can deliver at specified conditions without exceeding temperature limits.
More in “Components & Accessories”
Bushing
An insulated device allowing a conductor to pass through a grounded barrier while maintaining insulation integrity.
Busbar
A metallic strip conducting electricity within switchgear, transformers, and distribution equipment.
HV/LV Bushing
High-voltage and low-voltage insulated conduits for transformer winding connections through the grounded tank.
Frequently Asked Questions
What is a ring main unit used for?
An RMU is the switching point where distribution transformers tap into a medium-voltage ring. It lets operators isolate one section of the ring for maintenance or a fault while power continues to flow the other way, which is why it is standard equipment in urban cable networks, industrial parks and renewable collection systems.
What is the difference between a load break switch and a circuit breaker in an RMU?
A load break switch makes and breaks normal load current and provides visible isolation, but cannot clear a fault on its own. A circuit breaker interrupts fault current and can be tripped by protection relays, so it is used where automatic fault isolation or reclosing is required.
How many ways can an RMU have, and which voltage rating do I need?
QDTB RMUs are built from 2 to 6 ways with load break switch, fuse-switch or vacuum circuit-breaker compartments, so the ring can be extended as the network grows. Ratings are 12 kV or 24 kV, 630 A main busbar, with 16–20 kA short-time withstand, selected to match the system fault level.
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