26/10/2025
Medium-Voltage Switchgear for International Projects
We are currently engaged in numerous international projects. A common misconception is that switchgear can be applied directly so long as the system voltage matches. However, this overlooks critical technical distinctions.
Voltage specifications encompass both rated voltage and rated insulation voltage. For instance, in countries such as Australia and the Philippines, the rated voltage may also be 12 kV. According to IEC standards for solidly grounded systems, the power-frequency withstand voltage is 28 kV—considerably lower than the 42 kV specified in the GB standard. However, the lightning impulse withstand voltage is 95 kV, which is 20 kV higher than the GB standard’s 75 kV. Consequently, conventional 12 kV switchgear designed for the domestic market cannot satisfy these requirements.
For 17.5 kV switchgear, the power-frequency withstand voltage is 38 kV and the lightning impulse withstand voltage is 95 kV. As switchgear at this voltage level is not commonly manufactured in China, 24 kV switchgear is often used as a substitute, resulting in larger dimensions and higher costs.
Beyond voltage and insulation levels, the following differences must be considered early in the project to prevent issues during the Factory Acceptance Test (FAT):
- Rated Frequency: Frequency significantly affects switchgear performance, particularly circuit breakers. Strictly speaking, 50 Hz switchgear is not suitable for 60 Hz systems, and 60 Hz equipment cannot be fully utilized in 50 Hz systems. This is not simply a matter of derating; the interrupting capabilities of 50 Hz and 60 Hz circuit breakers are not interchangeable.
- Control Voltage: Some countries historically adopted the NEMA standard, which defines wider control voltage ranges. It is necessary to verify closing and opening characteristics at the lower voltage limits to ensure compliance. For example, with a nominal control voltage of 24 V, the minimum may be as low as 14 V, requiring larger conductor cross-sections to minimize voltage drop.
- Operating Sequences: IEC standards define two operating sequences: O–0.3 s–CO–3 min–CO and O–0.3 s–CO–15 s–CO. Some European and other international specifications require the second interval to be 15 s, which demands that the motor’s spring-charging time be less than 15 s and that switching operations remain reliable.
- Type Testing Requirements: Test requirements differ for grounded and ungrounded neutral systems and must be performed according to the standards recognized by the testing laboratory.
- Environmental Conditions: Ambient conditions vary by region. For example, the Middle East may require equipment rated for a maximum ambient temperature of 50°C, a 24-hour average of 45°C, an annual average of 30°C, and a minimum of –10°C. A maximum relative humidity of 100% may also be specified, which differs from standard conditions and requires appropriate validation and testing.
