Ring Main Unit

Overview

A ring main unit (RMU) is the compact medium-voltage switchgear that sits in a distribution substation, a kiosk, a basement, a pad-mount, where a ring main cable feeds a distribution transformer. The ring lets each substation be supplied from either direction, so a fault on one section can be isolated while the rest stay energised. A typical RMU has two load-break switches for the incoming and outgoing ring cables and a protected tee-off to the local transformer, all packed into a single sealed tank. This unit is the SF6-insulated, sealed-for-life type, requiring no gas maintenance over its service life.

How it works

Everything live sits inside the Gas Tank, a welded stainless vessel filled with insulating gas through its SF6 Gas Fill, which lets the whole assembly be far smaller than an air-insulated panel. Cables enter through cast-epoxy Cable Bushing feed-throughs, and a Burst Disc vents the rare internal-arc fault safely downward.

The two ring cables are controlled by Load-Break Switch units. Each is a three-position switch, closed, open, or earthed, whose copper blades are thrown by a Spring Mechanism; the spring, charged through the handle or a small motor reusing a Stator Assembly and Rotor Assembly, snaps the contacts at a fixed speed so the operator's pace never affects the switching arc. The local transformer hangs off the Transformer Tee-Off, which protects the feeder with a Vacuum Interrupter backed by HRC fuses that fire a striker to trip all three phases on a fault.

Inside, the Busbar & Terminations joins the switches and lands the external cables on screened separable connectors. A self-powered Protection Relay relay senses the tee-off current and trips the interrupter, showing status on its LCD Panel. The Operating Handle and the cabinet's interlocks enforce the safe switching sequence, with the Earthing Switch grounding the cables only once the line switch is open. The mimic and controls are carried on the Front Enclosure front.