Buying ring main units and compact substations from China: IEC 62271-202, enclosure classes and SF6-free options

This guide covers ring main units (RMUs) and prefabricated substations up to 52 kV, often sold as compact or box-type substations, bought from Chinese manufacturers for distribution networks, industrial sites, buildings and energy projects. Sinospect checked the standards and rules below at their sources on 26 September 2026.

Ring main units and prefabricated substations

A ring main unit is compact medium-voltage switchgear for secondary distribution. A common arrangement has two cable switches for the ring and one functional unit that protects a transformer, with a switch-fuse combination or a circuit-breaker.

IEC 62271-202 defines a prefabricated substation as a type-tested assembly: an enclosure holding at least a power transformer or high-voltage switchgear, and in general the low-voltage switchgear and the interconnections between them. It is operated from inside (walk-in) or from outside (non-walk-in), and it stands at ground level or partly or fully below ground. Since the 2022 edition the standard also covers substations that hold only high-voltage switchgear, and the components of a transformer substation can come as separate parts or as one compact equipment assembly (CEADS) to IEC 62271-212.

The standard for each part

IEC 62271-213:2021 replaced IEC 61243-5:1997 and IEC 62271-206:2011 for voltage detecting systems. The third edition of IEC 62271-201, published on 22 July 2026, aligned solid-insulation switchgear with IEC 62271-200:2021 and its 2024 amendment. For the transformer, see the guide to distribution transformers from China.

RMU catalogues use letter codes for the functional units, and the letters differ between makers. In one catalogue C is a cable switch, F a switch-fuse unit and V a vacuum circuit-breaker, so CCF means two cable switches and a switch-fuse unit; in another, L is the load-break switch and C the circuit-breaker. The specification describes each functional unit in words and names its standard.

In China: GB/T 17467 and the two box-substation types

China’s national standard for prefabricated substations is GB/T 17467-2020, in force since 1 October 2020. It is a modified adoption of IEC 62271-202:2014, the previous edition. A revision that adopts the 2022 edition was out for public comment in September 2026, under plan 20257012-T-604.

Chinese makers sell two types under the name box-type substation (箱式变电站). The European type has separate compartments for the medium-voltage switchgear, the transformer and the low-voltage board. The American type is a pad-mounted transformer, with the load-break switch and the protection fuses in the transformer’s oil-filled tank. China has a standard for this construction, JB/T 10217-2025, in force since 1 November 2025, for oil-immersed units from 6 to 35 kV and 30 to 2,500 kVA at 50 Hz.

Catalogue codes vary from maker to maker. One maker lists a YB unit as European-type and a YBW(ZGS) unit as American-type; another lists its YB series as American-type. The offer describes the construction in words and names the standard its type tests follow.

What IEC 62271-202 tests on the whole substation

Each part inside keeps its own standard and type-test reports. IEC 62271-202 adds type tests on the assembly, listed in its clause 7; Table 2 of the standard says which apply to a design.

The class of enclosure

The temperature-rise test measures the transformer’s temperature rise in open air (Δt1) and inside the enclosure (Δt2). The difference, in kelvin, is the class of enclosure. In a worked example from Schneider Electric’s installation guide, a transformer whose oil rises 60 K in open air rises 70 K in a class 10 enclosure.

The class and the site’s ambient temperature set the load the transformer can carry over its expected life. Annex D of IEC 62271-202 works this out with the IEC loading guides: IEC 60076-7 for mineral-oil-immersed transformers and IEC 60076-12 for dry-type transformers. In the same Schneider Electric guide, a site with a 30 °C summer average and a class 10 enclosure runs a transformer with 50 K oil and 55 K winding temperature-rise limits at a load factor of 0.9. For sunny sites, the 2022 edition adds an informative annex (Annex G) with a test method to evaluate the effect of solar radiation on temperatures inside the enclosure.

The internal arc classification

IEC 62271-202 rates the substation’s internal arc classification (IAC) with three access types: A, where access is restricted to authorised personnel; B, where access is unrestricted, the general public included; and AB for both. The rating comes with the arc fault current and duration (IA and tA) and the phase-to-earth values (IAe and tAe). The internal arc test covers the high-voltage switchgear and interconnections inside the enclosure, and subclause 7.102.7 covers transferring test results between designs, so the offer says whether the report was made on the design quoted. The RMU’s own IAC under IEC 62271-200 is a rating of the switchgear alone; the offer states the IAC of the substation as well.

SF6 and SF6-free ring main units

For sites in the EU, Regulation (EU) 2024/573, Article 13(9), prohibits putting into operation medium-voltage switchgear that uses fluorinated gases such as SF6 as insulating or breaking medium: up to and including 24 kV from 1 January 2026, and above 24 kV up to 52 kV from 1 January 2030. The guide to medium-voltage switchgear from China sets out the exceptions. Chinese makers list SF6-free ring main units with dry-air or solid insulation and vacuum interrupters. Solid-insulation switchgear has its own standard, IEC 62271-201, for indoor installation in areas limited to authorised personnel. Outside the EU, the utility or project specification sets the insulating medium.

Routine tests and the factory acceptance test

IEC 62271-202 lists the routine tests made on each substation at the factory: a dielectric test on the main circuit, tests on the auxiliary and control circuits, measurement of the main-circuit resistance, a tightness test, and design and visual checks. Where the substation is assembled at its destination, the standard adds tests after assembly on site, which form a separate site-test phase. The factory acceptance test lets the buyer witness the factory routine tests before shipment: see switchgear FAT witnessing and transformer FAT witnessing.

What moves the price and the lead time

• the rated power, the voltages and the number of transformers;
• the RMU configuration and the transformer protection: switch-fuse, or circuit-breaker with its relay;
• the insulating medium: SF6, dry air or solid insulation;
• the class of enclosure and the IAC access type, arc current and duration;
• the enclosure: metal or concrete, walk-in or non-walk-in, above or below ground;
• the low-voltage board, with any metering, remote control and monitoring;
• the test regime: type tests at an independent laboratory, witnessed routine tests;
• transport: dimensions, weight, lifting and packing, and the delivery term.

Clause 10 of IEC 62271-202 lists the information an enquiry gives and a tender returns. A BOQ that follows it, substation by substation, gets quotations that can be compared line by line: see preparing a BOQ or specification.

Buyer checklist

Before award

• IEC 62271-202:2022 named for the substation, with the standard and edition of each part: RMU, transformer, low-voltage board.
• Rated power, class of enclosure and IAC (access type, arc current and duration), with the site’s ambient temperature and altitude.
• Type-test reports for the substation design quoted and for each part: laboratory, accreditation, date, edition.
• The construction in words, prefabricated substation or pad-mounted transformer, with the standard its tests follow.
• Each RMU functional unit described in words, with its standard.
• For EU sites: the insulating medium, checked against the 2026 and 2030 dates.

Before shipment

• Routine test records for the substation and for each part.
• The factory acceptance test witnessed where the contract provides for it.
• Lifting, transport and site assembly instructions, drawings and manuals in the destination language.

Frequently asked questions

Sources

Checked at the sources on 26 September 2026.

• IEC: IEC 62271-202:2022 and Corrigendum 1:2023, IEC 62271-202:2022 preview (contents, foreword, scope), IEC 62271-200:2021 Amendment 1:2024, IEC 62271-201:2026, IEC 62271-103:2021, IEC 62271-105:2021, IEC 62271-100:2021 Amendment 1:2024, IEC 62271-102:2018 Amendment 1:2022, IEC 60282-1:2020, IEC 62271-213:2021, IEC 62271-212:2022, IEC 61439-1:2020, IEC 61439-2:2020
• China: GB/T 17467-2020 and revision plan 20257012-T-604 (China’s national standards portal), JB/T 10217-2025 (National Digital Standards Library)
• European Union: Regulation (EU) 2024/573, Article 13
• Schneider Electric: Electrical installation guide 2015, chapter B, accessibility types under IEC 62271-200
• Makers’ catalogues: ABB SafeRing 36, LS Electric Susol RMU, Ruizhi Electric box substations, Yuanguang Electric YB series