The cost of a selection mistake far exceeds the price of the equipment.

On project sites for urban power distribution, renewable energy power stations, and industrial substations, medium-voltage switchgear (1 kV–52 kV) is unquestionably the backbone of the power system. Yet a common misconception is to simplify selection into “whatever the system voltage is, choose that.”

In fact, the voltage class is only the starting point. If equipment is matched solely on nominal voltage, insulation coordination failures may occur during the commissioning stage, and years into service, spare parts may even be discontinued — hidden risks whose losses far exceed the equipment purchase cost.

Truly professional selection is a systematic engineering exercise that takes the system's highest voltage as its baseline and comprehensively considers the lightning impulse withstand level (BIL), clearances, creepage distances, and short-circuit withstand capability. Today, taking the three most widely applied classes — 12 kV, 24 kV and 40.5 kV — as examples, we uncover the technical logic behind selection and see how NATUS Electric (NATUS) delivers the optimal answer with robust products.

Air-insulated switchgear of the same type at 12 kV and 24 kV looks almost identical from the front of the cubicle, but the real differences lie in the insulation parameters:

12 kV

Corresponds to 10 kV systems; the mainstay of urban distribution and small commercial applications, and the most mature application. Typical BIL is 75 kVp, with a minimum phase-to-phase / phase-to-earth clearance of about 100 mm.

24 kV

Corresponds to 20 kV systems (in accordance with IEC 60038); a growth segment for dense urban distribution and renewable energy collection, gaining rapid adoption in Asia and Europe. BIL rises to 95–125 kVp, and clearance increases to about 180 mm.

40.5 kV

Corresponds to 35 kV systems, serving large-scale PV and wind farm collection as well as captive power plants. BIL reaches 150–185 kVp, with clearance of about 290 mm.

💡 Engineering note: Above 1,000 m altitude, air-insulated external insulation must be corrected in accordance with the standard; the rule of thumb of “adding 1% clearance per 100 m” cannot simply be applied. All parameters should ultimately be based on the product type test report.

Selection depends not only on the voltage class but also on matching the installation environment and insulation technology. Today's mainstream options fall into three categories:

AIS air-insulated switchgear

Low initial cost and simple maintenance, but sensitive to dust, moisture, and salt, and with a relatively large footprint.

GIS/C-GIS gas-insulated switchgear

Live parts are sealed within a metal enclosure, reducing the footprint by about 90% compared with AIS, with exceptionally strong environmental immunity. It is the preferred choice for urban substations, underground distribution, and data centers.

SF₆-free eco-gas equipment

SF₆ is a potent greenhouse gas and is strictly regulated. IEC 63360:2025 has been published to set quality standards for alternative gases, making eco-friendly equipment an inevitable trend.

In this field, NATUS is among the first to launch the NRTem-24kV SF₆-free gas-insulated ring main unit. Featuring a fully sealed stainless steel gas tank, an IP67 protection rating, IAC (Internal Arc Classification) 20 kA/1 s protection, and a rated current of 630 A, it provides a proven SF₆-free solution for projects with clear environmental objectives.

As the Asia-Pacific R&D and manufacturing base of the German NATUS Group, NATUS's product line covers 12 kV to 40.5 kV, encompassing GIS/C-GIS, ring main units, and eco-gas insulated equipment. Two representative products deserve particular attention:

NRTem-24kV SF₆-free gas-insulated ring main unit

→ Fully sealed stainless steel gas tank with IP67 protection, suited to harsh environments such as dusty and humid conditions;

→ IAC 20 kA/1 s internal arc protection, safeguarding personnel;

→ SF₆-free design, aligned with environmental trends and suitable for renewable energy collection and urban distribution networks.

NXTs-40.5 C-GIS gas-insulated switchgear

→ Three-phase common enclosure design, with rated current up to 3,150 A and a rated short-circuit breaking current of 31.5 kA;

→ Supports top and side extension, flexibly accommodating the expansion needs of substations and industrial scenarios;

→ Compact design that greatly saves floor space, particularly suited to space-constrained 40.5 kV systems.

To avoid selection pitfalls, we recommend following these five steps:

1. Establish the system parameter baseline

Use the system's highest voltage as the baseline, not the nominal voltage.

2. Short-circuit verification

The short-circuit rating of the switchgear must exceed the prospective fault current at the installation point.

3. Insulation coordination and BIL verification

Determine the withstand voltage and specific creepage distance in combination with altitude and pollution level.

4. Installation environment assessment

Consider temperature, humidity, dust, salt spray, and available area when choosing between AIS and GIS.

5. Safety and expansion

Confirm the internal arc classification (IAC) and reserve electrical margin for future expansion.

Common pitfalls to watch for:

❌ Selecting solely on rated voltage → may result in a BIL or short-circuit capacity mismatch;

❌ Treating 40.5 kV equipment as a “higher-rated” version of 24 kV equipment → the two have completely different insulation designs and test requirements;

❌ Focusing only on purchase price → life-cycle costs such as installation, O&M, downtime, and spare parts often only emerge years later.

Choosing among 12 kV, 24 kV and 40.5 kV switchgear is essentially a systems engineering decision. Inheriting the technological heritage of the German NATUS Group, NATUS covers everything from eco-gas insulation to compact C-GIS, with a full range from 12 kV to 40.5 kV, consistently answering the stringent demands of global customers with its principle of “technical quality given equal weight.”

Whether it is an urban distribution upgrade, the construction of a renewable energy power station, or an overseas grid project, NATUS can provide system-level empowerment from products to capabilities. For technical specifications, configuration advice, or project selection support, please contact NATUS Electric — let our expertise safeguard your power system.

Note: IEC and national standards are updated and revised over time; engineering applications should adopt the version in force at the time of project execution.

· IEC 62271-1:2017 — High-voltage switchgear and controlgear – Part 1: Common specifications

· IEC 62271-100 — High-voltage alternating-current circuit-breakers

· IEC 62271-200:2011 — AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV (a 2020 edition is available)

· IEC 60038 — IEC standard voltages

· IEC 63360:2025 — Specification of SF₆ alternative gases for power equipment

· GB/T 11022 — Common specifications for high-voltage switchgear and controlgear standards

· GB/T 3906 — AC metal-enclosed switchgear and controlgear for rated voltages above 3.6 kV and up to and including 40.5 kV

· DL/T 404 — Technical conditions for ordering AC metal-enclosed switchgear and controlgear for rated voltages above 3.6 kV and up to and including 40.5 kV

· DL/T 593 — Common specifications for high-voltage switchgear and controlgear standards