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How RMU Works in Medium Voltage System

Classification:

Industry News

Release time:

2026-06-21

How RMU Works in Medium Voltage System

Ring topology · Switching logic · Fault isolation · Five-prevention interlock — explained with the HXGN series

Luoyang Aisite Transformers Co., Ltd. • ~7 min read • Updated June 2025

A Ring Main Unit (RMU) is the switching node that makes a medium voltage ring network work. Whenever a city block, industrial park, or solar farm is fed by a looped cable rather than a single end line, an RMU sits at each connection point, deciding which way power flows and which section gets isolated if a fault occurs. This article walks through exactly how an RMU works step by step — from the basic ring topology, through normal switching operation, to what happens during a cable fault — using the HXGN series ring main unit (10 kV–35 kV indoor AC metal-enclosed, IEC 62271-200 / IEEE C37.20.3 compliant) as the reference design.

2+1 Ring switches + transformer feeder per unit

10–35kV rated d voltage range (HXGN series)

630–1600A Rated current options

5 Mechanical/electrical interlock protections

≥10,000 Rated mechanical operations

01 · Network Topology

Most conventional distribution lines are radial: power flows in one direction from the substation to the end user, with no alternative path. If a cable fails anywhere along that radial line, every downstream customer loses power until the fault is repaired.

A medium voltage ring network solves this by connecting the far end of the feeder back to another source point, forming a closed loop. Power can now reach any point on the ring from either direction. The Ring Main Unit is the equipment installed at every junction of this loop — it is the device that actually performs the switching that makes the ring topology functional rather than just a piece of looped cable.

Source A — RMU1 — RMU2 — RMU3 — Source B

Each RMU has two ring switches connecting neighbors, plus one transformer feeder switch for local load.

Each RMU in the chain typically integrates three switching positions in one sealed cabinet: two ring switches (incoming and outgoing, connecting it to the cable ring) and one transformer feeder switch (tapping power off to a local distribution transformer). This is the structural reason a ring is the structural reason a ring main unit looks different from an ordinary single-feed switchgear cabinet — it is built specifically to sit inside a loop.

02 · Normal Operation

Under normal conditions, the ring is typically operated as an open ring — one switch somewhere in the loop is deliberately left open to prevent circulating current and to simplify fault location, even though the cable itself forms a closed physical loop. Here is the step-by-step sequence of how power actually moves through the system:

1	Power Enters the Ring Switch Medium-voltage power arrives at the RMU's incoming ring switch. The load switch inside the ring main unit is closed, allowing current to pass through.
2	Busbar Distributes Current Internally Internal busbar connects three switch positions, power supplies next RMU and local transformer simultaneously.
3	Transformer Feeder Switch Taps Local Load Feeder switch with fuse or VCB steps down medium voltage for factory, building or PV station.
4	Outgoing Ring Switch Continues the Loop Remaining power flows to next RMU until open ring point or second substation source.

03 · Fault Isolation

This is the scenario that justifies the entire ring topology investment. When a fault — typically a cable short circuit or insulation breakdown — occurs on the section of cable between two ring main units, the system must isolate only that section while keeping every other customer on the ring supplied. Here is how the sequence unfolds:

⚡ Fault between RMU2 & RMU3

→

🔌 Relay detects fault current

→

🔒 Open switches on both sides of fault

→

✅ Restore power from opposite source

In practice: protection relays detect the abnormal current and trip the nearest breaker or fuse. Field crews or the SCADA system open ring switches beside the fault cable to isolate the damaged segment, and close the ring-open point to supply loads from the reverse direction. The net result: only customers on the fault cable lose power temporarily.

ℹ️ This is the core functional advantage of a ring main unit over a radial cabinet: it creates a self-healing power network without a full blackout.

04 · Internal Components

The switching behavior described above depends on specific components working together inside the sealed cabinet. Understanding these is essential to understanding how the RMU works at the equipment level.

🔀 Load Break Switch Core ring switching element, makes and break normal load current for ring reconfiguration.	⚡ Fuse or Vacuum Circuit Breaker Protect transformer feeder, VCB handles heavy short-circuit current with reset function.
🧊 Insulation Medium Air / SF6 / N2 mixed gas for dielectric insulation inside compact cabinet.	📡 Protection Relay & Monitoring Detect abnormal current, auto trip and send running data to remote SCADA platform.

05 · Safety Interlock (Five-Prevention)

Every HXGN ring main unit equips five-prevention mechanical & electrical interlock to avoid dangerous misoperation.

1 Prevent Accidental Operation

Block switch handle without standard operating sequence.

2 Prevent Closing Onto Live Fault

Forbid closing switch when circuit still energized or faulty.

3 Prevent Grounding Live Conductor

Ground switch cannot close under charged circuit.

4 Prevent Energizing While Grounded

Ring switch cannot close if ground switch is engaged.

5 Prevent Entry Into Live Compartment

Cabinet door locked while inner compartment energized.

💡 Five-prevention interlock allows engineers isolate fault safely without full ring power cut, realizing practical safety for daily maintenance.

06 · HXGN Series Reference Specifications

Parameter	Specification
Rated Voltage	10 kV / 24 kV / 35 kV
Rated Current	630 A / 1250 A / 1600 A
Short-Circuit Withstand	20 kA/4s standard; 25 kA/4s optional
Insulation Type	Air / SF₆ gas / Eco-friendly N₂ mixed gas
Switching Configuration	2 ring switches + 1 transformer feeder switch (load switch + fuse, or VCB)
Mechanical Life	≥ 10,000 operations
Applicable Standards	IEC 62271-200 / IEEE C37.20.3

Aisite's HXGN series Ring Main Unit applies to the scenarios:

🏘️ Residential distribution 🏢 Industry and commerce 🔋 Energy storage ☀️ Photovoltaic power generation 🏭 Metal melting ⚡ State Grid feeders

07 · FAQ

Q What does an RMU actually do in a medium voltage network?

An RMU sits at a node on a medium voltage ring network and performs three jobs: it receives power from two directions of the ring, it switches power to a local transformer feeder, and it isolates a faulted cable section so the rest of the ring keeps supplying power.

Q How does an RMU isolate a fault without cutting off the whole ring?

Because each RMU sits between two ring switches, a fault between two units can be isolated by opening the switches on either side of the faulted section. Power is then restored to downstream units by closing a switch elsewhere in the ring, supplying them from the opposite direction.

Q What is the difference between a ring main unit and a normal switchgear cabinet?

A normal distribution switchgear cabinet is typically built for a single radial feed. A ring main unit is purpose-built with two ring switches plus a transformer feeder switch in one compact, sealed enclosure, allowing it to sit inside a closed ring topology rather than a simple radial line.

Q Why is the five-prevention interlock important to how an RMU works?

The five-prevention interlock is what makes safe switching possible inside the RMU. It mechanically blocks dangerous sequences such as closing a switch onto a live fault or opening a cabinet door while the compartment is energized, so operators can safely reconfigure the ring during normal operation and fault isolation.

08 · Summary

A Ring Main Unit works by combining two ring switches and a transformer feeder switch in one safety-interlocked enclosure, allowing medium voltage power to flow around a closed loop, supply local transformers, and — most importantly — isolate a single faulted cable section without interrupting the rest of the network. That combination of ring topology and five-prevention interlock safety is what makes the HXGN series RMU the standard choice for resilient medium voltage distribution worldwide.

Need technical support or a custom HXGN RMU configuration for your ring network project?
Aisite's engineering team provides one-stop ring main unit selection and supply.

Contact the Technical Team View RMU Product Page