International and Indian Standards for Ring main Unit

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Ring Main Unit (RMU) in High Voltage Systems: Components, Functions & Standards

Learn about Ring Main Units (RMU) in high-voltage systems – their components, functions, and applicable Indian and international standards. Essential for HT customers and transformer protection.

🔹 What is a Ring Main Unit (RMU)?

A Ring Main Unit (RMU) is an integral part of high-voltage (HV) and high-tension (HT) power distribution systems. It is widely used in HT networks where multiple consumers are connected to the same feeder line. The RMU ensures uninterrupted supply, easy switching, and effective protection of transformers.



RMUs serve two major purposes:

  1. Supplying power to distribution transformers
  2. Providing protection for transformers and switchgear

🔹 Components of a Ring Main Unit

An RMU typically consists of:

  • Load Break Switch (LBS) / Circuit Breaker (Vacuum type or SF6 gas type)
  • Earth Switch
  • Isolators

Important Note on SF6 Gas:

SF6 gas-insulated RMUs are mostly maintenance-free. The only critical check is the SF6 gas pressure indicator.

  • If the pointer shows green, the breaker can operate safely.
  • If it shows red, the breaker must not be operated, as doing so could cause an explosion.

🔹 Why Use RMUs in HT Systems?

  • Reliability: Enables uninterrupted power even if one section of the network is under maintenance.
  • Safety: Provides protection to transformers against overloads and faults.
  • Compact Design: Saves space compared to traditional switchgear.
  • Maintenance-Free: Especially in SF6 gas-insulated types.

🔹 Applicable Standards, Rules, and Codes for RMUs

The design, manufacturing, testing, and operation of Ring Main Units are governed by both Indian Standards (IS) and International Electrotechnical Commission (IEC) standards.

Standard / Rule

Title

Indian Electricity Rules

With latest Amendments

Indian Electricity Act, 2003

Legal framework for electricity supply

IS 3427

A.C. Metal enclosed switchgear & control gear above 1 kV

IS 9920 (Part 1, 3 & 4)

High voltage switches above 1 kV

IS 13118

General requirements of circuit breakers above 1 kV

IS 3231

Electric relays for power system protection

IEC 60265 (Part 1)

High-voltage switches

IEC 60056

High-voltage AC circuit breakers

IEC 60059

Preferred current ratings of HV switchgear

IEC 60185

Current transformers

IEC 60694

Specifications for HV switchgear

IEC 60298

A.C. Metal enclosed switchgear

IEC 60129

A.C. Disconnector and Earth Switches

IEC 60529

Degree of protection (IP ratings) for enclosures

IS/IEC 62271-102 (2003)

HV Switchgear and Controlgear – Disconnectors & Earthing Switches

IEC 60255

Electrical relays

✅ These standards ensure safe design, reliable operation, and proper protection schemes in HT switchgear applications.

🔹 Conclusion

The Ring Main Unit (RMU) plays a vital role in modern high-voltage distribution networks. It improves system reliability, ensures safety, and provides transformer protection with minimal maintenance. Adhering to the above-mentioned IS and IEC standards is crucial for manufacturers, utility companies, and engineers to guarantee safe and efficient HT operations.

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