A127 IEC Standards Applicable to RMU Switchgear: A Comprehensive Guide (Part-2)

Introduction

In medium voltage distribution systems, compact secondary substations, indoor and outdoor RMUs and smart RMU switchgears play a pivotal role in ensuring reliability, safety and flexibility. In Part 1 of this series, we reviewed the foundational standards IEC 62271-1, IEC 62271-200 and IEC 62271-100, which establish general definitions, common requirements, normal and special service conditions, design and construction, ratings, type tests and routines tests, performance requirements and specifications for RMU switchgear.

This Part 2 guide turns to three additional, function-specific standards in the IEC 62271 series, IEC 62271-102, IEC62271-103 and IEC 62271-105, which together define the design and construction, performance, testing, selection, safety and operation and maintenance requirements for the core switching functions within RMUs. By harmonizing terminology, classification rules and comprehensive type- and routine-testing protocols, these three standards form a coherent framework that guides Engineers, manufacturers and utilities through every phase of RMU switchgear development from component selection and mechanical design to factory acceptance testing and final field installation.

Unless stated otherwise, high-voltage switchgear and controlgear, including the operating devices and the auxiliary equipment which form an integral part of them, are intended to be used in accordance with their rated characteristics and the normal service conditions. For elevated humidity and temperature requirements typical of tropical countries, the relevant clause should apply.

IEC 62271‑102: High-voltage switchgear and controlgear - Part 102: Alternating current disconnectors and earthing switches

This part of IEC 62271 applies to alternating current disconnectors and earthing switches, designed for indoor and outdoor enclosed and open terminal installations for voltages above 1 kV and for service frequencies up to and including 60 Hz. It governs disconnectors and earthing switches integrated into RMUs .It also applies to the operating devices of these disconnectors and earthing switches and their auxiliary equipment. It covers manual or motor‑operated load and earthing switches, including contact spacing, mechanical endurance (M‑class), and interlocking with adjacent devices.  

Disconnectors are used to break the electrical circuit to isolate equipment for maintenance or repair, while earthing switches provide a path for grounding the equipment to prevent electrical hazards. The standard specifies various tests and verification procedures to ensure the reliability and safety of these devices, including dielectric tests, short-time withstand current and peak withstand current tests, resistance measurements, verification of protection, RIV and EMC tests etc.

Table 3a provide Mechanical Endurance for different classes of disconnectors. Number of operating cycles for class M1 is 2,000 operating cycles for class M2 is 10,000 operating cycles. The dielectric and dynamic current withstand ensures reliable isolation and earthing.

Historical Development: The First Edition was published in 2001, establishing basic definitions and tests for AC disconnectors and earthing switches. Amendment 1 (2011) and Amendment 2 (2013) incorporated clarifications on testing procedures and updated definitions. The Second Edition was published on 15 May 2018 (Edition 2.0). This second edition cancels and replaces the first edition published in 2001, Amendment 1:2011 and Amendment 2:2013. This edition constitutes a technical revision. In addition to the significant technical changes with respect to the previous edition, a new Annex B has been added with title: "Current-switching capability required of disconnectors and earthing switches"; a new Annex C has been added with title: "Tolerances on test quantities for type tests"; and a new Annex E has been added with title: "Extension of validity of type tests". The Amendment 1 was issued on 5 September 2022.

IEC 62271‑103:2021 High-voltage switchgear and controlgear - Part 103: Alternating current switches for rated voltages above 1 kV up to and including 52 kV

This part of IEC 62271 is applicable to three-phase, alternating current switches and switch-disconnectors for their switching function, having making and breaking current ratings, for indoor and outdoor installations, for rated voltages above 1 kV up to and including 52 kV and for rated frequencies from 16 2/3 Hz up to and including 60 Hz. This standard is also applicable to single-pole switches used on three phase systems. It applies to load-break switches and switch-disconnectors in RMUs as well. The normal and special service conditions, ratings, design and construction, type tests and routine tests are discussed in clauses 4, 5, 6, 7 and 8 respectively. The key requirements are: breaking capacity, which verifies reliable switching under both normal and overload conditions; and mechanical and environmental durability, which ensures long service life across diverse operating environments.

This standard is also applicable to the operating devices of these switches and to their auxiliary equipment. Switch-disconnectors are also covered by IEC 62271-102 for their disconnecting function.

Devices that require a dependent manual operation are not covered by this standard. General principles and provisions of this standard may also be applicable to single pole switches intended for application in single-phase systems. The requirements for dielectric tests and making and breaking tests should be in accordance with the requirements of the specific application. 

This standard establishes requirements for general, limited and special purpose switches used in distribution systems. It is assumed that opening and closing operations are performed according to the manufacturer's instructions. A making operation may immediately follow a breaking operation but a breaking operation should not immediately follow a making operation since the current to be broken may then exceed the rated breaking current of the switch.  

IEC 62271‑105:2021 High-voltage switchgear and controlgear - Part 105: Alternating current switch-fuse combinations for rated voltages above 1 kV up to and including 52 kV

This part of IEC 62271 is relevant for RMUs incorporating switch-fuse combinations. It applies to three-pole units for public and industrial distribution systems which are functional assemblies of switches including switch-disconnectors and current limiting fuses designed so as to be capable of

– Breaking, at the rated recovery voltage, any current up to and including the rated short circuit breaking current;

– Making, at the rated voltage, circuits to which the rated short-circuit breaking current applies.

This third edition supersedes the second edition (published in 2012) as a technical revision and incorporates the following significant changes:

- The document has been updated to be in alignment with the second edition of IEC 62271-1:2017.

- Rated TRV has been removed (TRV is only a test parameter), as in the latest revision of IEC 62271-100.

- Differentiation has been introduced between requirements expressed for fulfilling the function expected from a switch-fuse combination, from requirements only relevant when the function is performed by a stand-alone device. When a function is implemented within an assembly, the aim is to avoid duplicating requirements or conflicting with the applicable assembly standard.

In practice, different types of fuses may be combined with one type of switch, which give several combinations with different characteristics, in particular concerning the rated currents.

The key requirements are: fuse coordination, which ensures adequate protection during fault conditions; and switching tests, which validate performance under both fused and unfused operations.

Conclusion

In conclusion, this second part of comprehensive guide has mapped out the key requirements and applications of IEC 62271-102, IEC 62271-103, and IEC 62271-105 for RMU switchgear. By examining the service conditions, ratings, design and construction criteria, type tests and routine tests, transport and storage requirements, installation procedures, operation and maintenance protocols and safety considerations of each part, we can achieve desired results.

Integrating these standards into procurement, design, testing, installation and maintenance workflows will help us eliminate ambiguity, avoid redundant testing, and ensure interoperability across manufacturers. It also positions installations to meet reliability, safety and sustainability objectives.Bottom of Form

 References

[1] IEC 62271‑102: High-voltage switchgear and controlgear - Part 102: Alternating current disconnectors and earthing switches.

[2] IEC 62271‑103:2021 High-voltage switchgear and controlgear - Part 103: Alternating current switches for rated voltages above 1 kV up to and including 52 kV

[3] IEC 62271‑105:2021 IEC 62271‑105:2021 High-voltage switchgear and controlgear - Part 105: Alternating current switch-fuse combinations for rated voltages above 1 kV up to and including 52 kV

[4] https://law.resource.org/pub/in/bis/S05/is.iec.62271.102.2003.pdf