A126 IEC Standards Applicable to RMU Switchgear: A Comprehensive Guide (Part-1)

Introduction

Ring Main Units (RMUs) are critical components in medium-voltage (MV) power distribution networks, typically operating at voltages, ranging from 11 kV to 36 kV. Common voltage ratings include 12 kV, 15.2 kV, 17.5 kV, 24 kV, and 36 kV. These factory assembled, metal-enclosed compact switchgear assemblies enable efficient switching, protection, and isolation of electrical circuits in MV distribution networks. Ensuring safety, reliability, and interoperability across manufacturers requires adherence to international standards like IEC and IEEE. This article explores the key IEC standards governing RMU switchgear, outlining their scope and significance. In Part 1, we focus on the following standards: IEC 62271‑1, IEC 62271‑200 and IEC 62271‑100.

Overview of IEC Standards for RMU Switchgear

IEC standards provide a framework for the design, development, type testing, manufacturing and routine testing, and operation of electrical equipment. For RMUs, these standards ensure compliance with performance, safety, reliability, and normal and special service conditions to meet specific requirements for various applications. Below are the primary IEC standards applicable to RMU switchgear:

IEC 62271‑1: High-voltage Switchgear and Controlgear – Part 1: Common Specifications for Alternating Current Switchgear and Controlgear 

This foundational standard applies to all high‑voltage switchgear and controlgear rated above 1 kV up to and including 52 kV, defining common requirements for design principles, normal and special service conditions, ratings, design and construction, electrical, mechanical and thermal duties, operating functions, auxiliary and control circuits, and type and routine tests such as dielectric, short-time withstand current and peak withstand current, RIV, verification of protection, tightness test mechanical, EMC test and x-radiation test for vacuum interrupters. It serves as the reference document for the entire 62271 series.

IEC 62271-200: AC metal-enclosed Switchgear and Controlgear for Rated Voltages above 1 kV and up to and including 52 kV

This standard specifies requirements for metal‑enclosed assemblies including ring main units (RMUs) for alternating current of rated voltages above 1 kV and up to and including 52 kV for indoor and outdoor installation, for service frequencies up to and including 60 Hz. Enclosures may include fixed and removable components and may be filled with fluid (liquid or gas) to provide insulation. This publication is to be read in conjunction with IEC 62271-1. It covers:

·         Normal and special service conditions

·         Ratings

·         Design and construction

·         Type tests and routine tests

·         Guide to the selection of switchgear and controlgear

·         Information to be given with enquiries, tenders and orders

·         Transport, storage, installation, operation and maintenance

·         Safety and influence of the product on the environment

Key type‑test requirements for switchgear and controlgear are as follows:

Dielectric tests: Ensure insulation integrity under both power‑frequency and lightning‑impulse voltages.

Temperature‑rise tests: Validate thermal performance under continuous load.

Short‑circuit withstand tests: Verify mechanical and thermal resilience under specified fault currents.

Making‑and‑breaking current tests: Confirm that switching devices can reliably close onto and interrupt rated currents.

Mechanical operation tests: Assess the endurance of interlocks, shutters and operating mechanisms through repeated cycling.

Internal arc classification (IAC): Evaluate containment of arc faults to protect personnel (e.g., IAC AFLR class).

Contact‑resistance measurements: Measure the resistance of current‑carrying circuits to ensure low‑loss connections.

Mechanical interlocking & enclosure‑integrity tests: Verify interlock function, enclosure strength and protection against unauthorized access.

Radio‑interference voltage (RIV) tests: Assess electromagnetic noise generated during switching operations.

Tightness tests: Confirm that the enclosure is watertight and/or gas‑tight as required.

Electromagnetic compatibility (EMC) tests: Ensure the assembly neither emits nor is unduly affected by electromagnetic disturbances.

Additional tests on auxiliary & control‑circuits: Switchgear and controlgear include all auxiliary equipment and electrical circuits (electronic controls, supervision, monitoring and communication).A functional test of all low-voltage circuits shall be made to verify the proper functioning of auxiliary and control circuits in conjunction with the other parts of the switchgear and controlgear.

X‑radiation tests for vacuum interrupters: Verify that any X‑ray emissions remain within safe limits under specified operating conditions.

Verification of the IP coding: Subclause 7.7.1 of IEC 62271-1 is applicable

Verification of the IK coding: Subclause 7.7.2 of IEC 62271-1 is applicable

Each procedure is defined with precise test levels, durations and acceptance criteria so results are consistent and reproducible.

Switching Devices

RMUs integrate several elementary switching devices, each standardized by its own IEC 62271‑series part:

IEC 62271‑100: Alternating‑Current Circuit‑Breakers This part of IEC 62271 is applicable to AC circuit-breakers designed for indoor or outdoor installation and for operation at frequencies of 50 Hz and 60 Hz on systems having voltages above 1 kV. This document includes only direct testing methods for making-breaking tests. For synthetic test methods IEC 62271-101 should be referred.

This standard addresses the design, testing, performance requirements and ratings for medium and high-voltage circuit breakers. Various testing procedures, including dielectric, short-circuit, mechanical endurance, and temperature rise tests are described. The circuit breakers are critical components of power transmission and distribution systems, responsible for protecting electrical equipment from damage due to overloads or short circuits. The standard ensures that circuit breakers meet strict safety and reliability requirements.

This third edition cancels and replaces the second edition published in 2008, Amendment 1:2012 and Amendment 2:2017. This edition constitutes a technical revision. The main changes with respect to the previous edition are following:

– the document has been updated to IEC 62271-1:2017;

– Amendments 1 and 2 have been included;

– the definitions have been updated, terms not used have been removed;

– Subclauses 7.102 through 7.108 have been restructured.

Note: Annex B (normative): Tolerances on test quantities during type tests and Table B.1, and Annex C (normative): Records and reports of type tests, are mandatory parts of the standard and should be complied with during the performance and documentation of type tests.

Conclusion

Understanding and applying the requirements of IEC 62271‑1, IEC 62271‑200, and IEC 62271‑100 is fundamental to the successful deployment and operation of RMU switchgear. Compliance with IEC 62271‑1 ensures that basic design principles; such as insulation levels, clearances, and mechanical endurance are uniformly met. IEC 62271‑200 provides the framework for evaluating the enclosure, environmental resilience, and operational safety of metal‑enclosed assemblies, while IEC 62271‑100 governs the performance and testing of the circuit‑breaking elements essential for reliable fault interruption. Together, these standards form an integrated compliance pathway that enhances system safety, streamlines procurement, and reduces lifecycle costs. As distribution systems evolve, Engineers who stay current with revisions and best practices in these standards will be best positioned to deliver RMU solutions that combine technical excellence with long‑term operational integrity.

References

[1] IEC 62271-1:2017 - High-voltage switchgear and controlgear - Part 1: Common specifications for alternating current switchgear and controlgear.

[2] IEC 62271-200:2021 - High-voltage switchgear and controlgear - Part 200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV. 

[3] IEC 62271-100:2021 - High-voltage switchgear and controlgear - Part 100: Alternating-current circuit-breakers. 

[4] https://law.resource.org/pub/in/bis/S05/is.iec.62271.1.2007.pdf 

[5] https://law.resource.org/pub/in/bis/S05/is.iec.62271.200.2003.pd