Hello learners, welcome to this topic on, Ring Main Units RMU. At the end of this topic, you will be able to describe ring main unit, explain the function of ring main unit. List the key specifications and tests offering main unit. Let us learn about ring main unit. A Ring Main Unit, RMU is a totally sealed gas insulated compact switchgear unit. These are utilized for medium voltage power distribution. It is basically used for an uninterrupted power supply, alongside it also protects your secondary side transformer, from the occasional transient currents. RMU is composed of one incomer feeder and more than one outgoing feeder. The following are the functions of a ring main unit. Normally an RMU is utilized to enable a feed supply from two directions and tapping of the transformer. One of these supplies might have several ring main units, in series creating a ring. When a section of the feeder requires being isolated because of work or failure, a ring main unit on every end of this area, could be opened without obstructing any supply. Circuit control, it controls the circuit to switch functions. Isolation, it isolates the faulty equipment from the rest of the circuit. Protection, it protects the circuit from ground fault current, short circuits, fault current and overload. Here is the typical drawing of a four way ring main unit. It depicts two incomers isolator one and two, and two outgoing transformers. The key specifications of RMU are rated voltage, bus bar rating, rated frequency, rated nominal voltage for ring switch, rated nominal current fort-off feeder. Internal arc classification IP class, that is gas tank or indoor or outdoor, rated lightning impulse withstand voltage. Rated power frequency withstand voltage, power frequency voltage withstand test on main circuit wet and dry. The following are the type test for RMU, power frequency voltage withstand test on main circuit, wet and dry. Lightning impulse voltage withstand test, dielectric test on auxiliary and control circuits, measurement of resistance of main circuit. Temperature rise tests, short time withstand current and peak withstand current tests on main and earthing circuits. Verification of rated making and breaking capacities for breaker isolator and earthing switches, mechanical operation tests for breaker isolator and Earthing switches. Verification of IP coding, IP41, IP54 and IP67, pressure withstand test for gas filled compartments. Internal arcing test that is for main tank AFLR and cable chamber. The acceptance test for RMU are as follows, visual inspection verification of BOM, dimension check as per approved GTP. Mechanical operation, control circuit checking, high voltage test 28kV RMS at one minute on power circuit, 5kV megger after and before HV test. Contact resistance for panel, high voltage 2kV RMS at control circuit for one minute, verification of closing and tripping time of vacuum circuit breaker for selected samples. Paint thickness checking by Elko meter that is, 50 microns minimum required. In this session we had described ring main units, explain the function of ring main unit. Listed the key specifications and tests of ring main unit.