Engineer's Notes: ring main units and HV switchgear
What an RMU does
A ring main unit ties a substation into an HV ring. Two load break switches handle the ring cables, so the utility can move the normal open point and restore supply after a cable fault. One or two transformer feeders, with fuses or a vacuum breaker, feed the transformers. Everything sits in a sealed tank that needs very little maintenance.
Choosing ratings
Match the rated voltage to the system (12 kV for 11 kV, 17.5 kV for 13.8 kV, 24 kV for 22 kV, 36 kV for 33 kV), the short-time current to the network fault level, and the busbar current to the ring load. Ask the utility for its fault level and approved types, because most RMUs in building substations belong to the network.
Interlocks keep you safe, up to a point
Mechanical interlocks stop you closing the earthing switch while the main switch is closed, and stop you opening the cable door until the unit is earthed. They cannot see the far end of the cable. That is why every switching program says: check the VPIS, prove dead, then earth.
SF6 or SF6-free?
SF6 made switchgear compact and reliable, but it is the strongest greenhouse gas. Dry air, clean air and solid-insulated RMUs with vacuum interrupters now cover the same ratings. The EU has started to phase SF6 out of new MV switchgear, and many networks elsewhere are following.
Frequently asked questions
What is a ring main unit (RMU)?
A ring main unit is compact, factory-built medium-voltage switchgear that connects a substation into an HV ring. A typical RMU has two load break switches for the incoming and outgoing ring cables and one or two transformer feeders, protected by fuse-switch combinations or vacuum circuit breakers.
When should I use a fuse-switch instead of a circuit breaker for a transformer?
A fuse-switch combination is simple and cheap and suits transformers up to about 1250 to 1600 kVA at 11 kV. Use a vacuum circuit breaker with a protection relay for bigger transformers, for customer HV feeders, where selectivity with upstream protection matters, or where the utility requires it.
What do LSC2 and IAC AFL mean?
LSC2 (loss of service continuity category 2) means you can open one compartment, such as a cable compartment, while the busbar and other units stay in service. IAC AFL is the internal arc classification: the switchgear has been arc tested with authorised persons at the front (F) and lateral (L) sides. AFLR adds the rear.
Why must I check the VPIS before earthing a cable?
The mechanical interlocks only stop you closing the earthing switch while the local switch is closed. They cannot know whether the far end of the cable is still live. The voltage presence indicator (VPIS) shows whether the cable is energised from the other end. Earthing a live cable causes a full short circuit.
Is SF6 switchgear being phased out?
In the EU, the F-gas Regulation 2024/573 bans most new medium-voltage switchgear using SF6 up to 24 kV from 2026 and up to 52 kV from 2030. Other regions still use SF6 widely, but many utilities in Australia, New Zealand and Asia are trialling or adopting SF6-free dry air, clean air or solid-insulated alternatives.
Topics: ring main unit · RMU configurations CCF, CCV · IEC 62271-200 · LSC2 · IAC AFL / AFLR · fuse-switch combination · HV fuse selection · transfer current · VCB relay settings · 3-position switch · interlocks · VPIS · switching program · SF6-free switchgear · RMU installation
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