What is RMU (Ring Main Unit) and Its Use in Electrical Power Distribution System?

Ring Main Unit (RMU) is a 11 KV or 33 KV HT panel having 3 nos. of switches (Circuit Breakers or Isolators or LBS) that are 2 for incoming one for outgoing. It enables consumer use 2 sources of HT power in at the same metering point. It is a totally sealed, gas-insulated compact switchgear unit.

RMU is to be used two incoming with mechanical or electrical interlock and one out going  to the load generally but some times one incoming and two outgoings medium voltage supplies. In engineering distribution this is called flexible power supply. Now a day SF6 (Sulfer Hexa-Floride) circuit breakers going to be used. These are maintenance free breakers. 

Figure 1 – Outlook of a typical three-feeder 24 kV RMU unit

Figure 2- 11 KV 2 Incoming and 3 Outgoing RMU at Bangladesh Film Archive.

Ring Main Unit is used in a secondary distribution system. It is basically used for an uninterrupted power supply. Alongside, it also protects your secondary side transformer from the occasional transient currents.