Distribution system planning

(Ring Main Unit (RMU

Ring Main Unit (RMU) The panel known as RMU is used to connect substations to each other within one ring or to connect the distribution transformer of a particular building to the medium voltage network in which the building is located. Usually, the RMU panel is used to connect transformers with a capacity less than 5MVA .

The following figure is an RMU board with three 3-cells inside:

One of these cells has an entry cable coming from a station, and the second cell has an exit cable heading to a station equipped with a Load Break Switch (LBS) at 24KV, the rated current of the LBS is 1250A.

As for the third cell, it contains a protection for the transformer feeding this RMU, containing a fuse of 100A to protect the transformer capacity of 1000KVA.

The role of the Load Break Switch is limited to separating the entry and exit cables for maintenance operations and has no role in protecting or preventing the transformer from malfunctions.

The RMU panel contains Earthing switches, and it is used to ensure the leakage of any shipments after disconnecting the panel from service and before performing maintenance inside it, meaning that it represents a safety element during the work of the maintenance team.

The board is also provided with an Interlock that connects each LBS to the Earthing Switch to ensure that the two are not in "Close" mode at the same time so that there is no shorting.

A fuse is used in series with LBS to protect against very high short-circuit currents.