Ring Main Unit (RMU)

Product Description

Ring Main Unit (RMU)

The RMU is intended to be used in medium voltage (11KV to 33KV) Ring distribution system in any power supply/distribution utilities. RMU offers greater flexibility in supply system in maintaining continuity and offers practically maintenance free operation. The RMUs are very compact in size compared to MV switchgear panels. RMU cubicles form a system of small size modular equipment for Medium Voltage (11KV & 22KV for India ), each module having a specific function. Every function has its own metal enclosure, which contains a stainless steel/Epoxy tank filled with SF6 gas, in which the switching devices and busbar are kept. Full SF6 makes equipment pollution resistant and able to withstand even the flooding of MV sub-station. SF6 gas being inert, electro-negative and non-inflamable gas has many physical & chemical advantages.

The RMU gas tank is a permanently sealed system where the integrity of the seal is effective for 30 years in conformance with IEC-298. The tank charged with SF6 gas to the pressure of 0.03 Mpa which allows switch to break loads of upto 2600 amps at 12KV or 1300 Amps at 24KV (which under normal atmospheric conditions 630 amps at 12KV, that is, without SF6 gas). The RMU switching system consists of 3 phase, 3 position load breaking & fault making, the three positions being "close", "open", and "earth". The actuating is linked with the external earth switch as well as the panel door so as to provide safety interlocks which ensures that the operator may only access the panel when the system is isolated and earthed. The most common versions of RMU are Extensible & Non-extensible types - 3 Function & 4 Functions. A typical 3 function RMU shall comprise of 2 x 630 Amps Load Break Switch + 1 x 200/400/630 Amps Circuit Breaker (SF-6 or Vacuum type). Similarly, a typical 4 Function RMU will comprise of 2 x 630 Amps Load Break switch + 2 x 200/400/630 Amps Circuit-Breaker. The busbar is capable of carrying a load current of 630 Amps and the short circuit fault current of 20 KA for 3 secs. The system of mounting the bus bars conform to IEC 298.

The RMU generally conforms to the following standards

• Load Break Switch - IEC 265/BS 3804
• Load Break Sw. + Fuses - IEC 420/BS 16926
• Circuit Breaker - IEC 56/BS 5311/IS 13118
• Earthing Switch - IEC 129/BS 1985
• Monitoring & Control - IEC 801
• Degree of Protection - IEC 529
• Metal enclosed switchgear - IEC 298/BS 5311/BS 5227

General Characteristics:

• SF6 insulated compact switchgear
• Equipped with SF6 switch disconnector
• Equipped with vacuum circuit breaker
• Developed according to International Standard IEC 62271-200
• Insensitive to environmental conditions
• Modular or compact version
• Internal arc resistance
• Increased mechanical and electrical life (class E3 M2)
• Stainless steel tank

Technical Data: