What is Ring Main Unit RMU

What is Ring Main Unit (RMU)?

Ring main unit (RMU) is a set of high voltage switchgear installed in the steel plate metal cabinet or made into an assembly interval ring network power supply unit. The core part adopts load break switch and fuse, which has the advantages of simple structure, small volume, low price, improving power supply performance as well as power supply safety. The transformer will be protected by HV current-limiting fuse in combination with the load break switch. The load break switch is equipped with a stored spring energy mechanism which can be tripped by the fuse striker pin.

The load break switch is simple in structure, low in cost and small in volume. Most of it can be installed by the wall. Generally, it only has fuse protection and no relay protection. The load flow of the main bus of the load break switch is generally less than or equal to 630A, and its rated breaking current of the load switch is generally less than or equal to 630A (a few reach 1700A). Rated current of transformer cabinet (outlet cabinet) is generally no more than 125A.
Ring main unit is a kind of switch equipment that can realize the power supply of ring network. It is often used in ring type power supply, medium voltage divider connection, medium voltage terminal transformer room power supply. It is also used for opening and closing load current, breaking short-circuit current and transformer no-load current, and for controlling and protecting charging current of overhead lines and cable lines at a certain distance. Ring main unit is designed for application at city power or large industry area as ring network key switchgear!

A Ring Main unit is at least comprised of 3 independent unit switchgear cabinet, Its noramlly equiped with 2 ways Incoming unit  + 1 way outgoing unit.

Usually, the incoming unit is function as a load break switch, and the outgoing unit function as a fuse combination (Fuse + switch disconnector) or VCB + isolator (Disconnector switch), some special condition will specify the VCB for incomer unit.

How many different types of ring main unit?

Base on the present power technology requirement, worldside firms is developing out many type different RMU. There are half gas insulated RMU, dry air insualted RMU, solid insulated RMU, and Full gas insulated RMU. Also it can be devided by indoor type and outdoor with IP43 or IP65 class protection enclosure.

What is the typical type of ring main unit(RMU) in the world?

Nowadays, we call a ring main unit is basically mean a RMU which is a totally sealed with SF6 gas or other insulation material, Rockwill's RMU is a gas-insulated compact switchgear unit.

This major using RMU worldwide, it is the most relieable, stable one. It is metal enclosed set of switchgear used at the load connection points of a ring-type distribution network.

Rockwill's  RMU is combinated with minimum three ways together, The primary switching devices can be either switch disconnectors or fused switch disconnectors or circuit breakers (VCB or GCB).
Normally, a primary circuit breaker is the switching device, which can be vacuum circuit breaker or gas circuit breaker ,etc. it is always equipped with protective relay,  it is a very basic self-powered type or a more advanced one with communication capabilities.
It includes two switches in one unit that can connect the load to either or both main conductors, and a fusible switch or circuit breaker and switch that feed a distribution transformer.

The rated voltage and current ranges for RMUs typically reach up to 40.5 kV and 630 A respectively.

Rockwill's ring main unit

See further information of Rockwill's RMU pls check below link:

AIS RMU---https://www.cnrockwill.com/air-insulated-switchgear-secondary

GIS RMU---https://www.cnrockwill.com/gas-insulated-switchgear-secondary

SIS RMU---https://www.cnrockwill.com/solid-ring-main-unit

Outdoor RMU---https://www.cnrockwill.com/outdoor-ring-main-unit

Rockwill Youtube---https://youtu.be/cqHdDDGyqfM

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