What Is a Voltage Class?
1. Definition of Voltage Class
Voltage classes (voltage levels) are a set of rated voltages adopted in power systems and electrical equipment. A rated voltage is the nominal voltage at which equipment is designed to operate under normal conditions. In essence, a voltage class indicates the working voltage range for which a system or device is intended.
2. Analogy to Consumer Appliances
Just as household appliances (e.g., refrigerators, TVs) have a nominal voltage—commonly 220 V—and may malfunction or incur damage if supplied with a significantly higher or lower voltage, power‑system equipment must also operate within its designated voltage class to ensure reliable performance.
Importance of Voltage‑Level Classification
1. Standardization
Unified voltage classes provide a clear specification basis for equipment design, manufacturing, and maintenance. Devices from different manufacturers that adhere to the same voltage class can interoperate seamlessly, enhancing interchangeability and simplifying maintenance and system management.
2. Efficiency
By selecting appropriate voltage levels for transmission, line losses are minimized. At a given power transfer, higher voltage reduces current and thus the I²R losses in conductors, improving overall transmission efficiency from generation to the end user.
3. Safety and Reliability
Clear delineation of voltage classes ensures that insulation requirements and protective measures are matched to the voltage stress. This prevents overloads and short circuits arising from mismatched voltages, safeguarding both personnel and equipment."
4. Facilitating Technological Evolution
As power demand grows and new technologies emerge (e.g., distributed generation, smart grids), well‑defined voltage tiers allow the grid to adapt, supporting the connection of renewables, energy storage, and advanced control systems, while maintaining system stability and resilience
Common Voltage Classes
1. Safety Voltage (≤36 V)
Used in applications with stringent safety needs (e.g., handheld power tools at 24 V, mine lighting at 12 V, medical endoscopes ≤6 V). Special environments (swimming pools) often use 12 V, and children’s toys ≤6 V. Equipment must comply with GB/T 3805‑2008, employ safety isolating transformers or batteries, double‑insulation, and ensure emergency lighting can run for 72 hours.
2. Low Voltage (220 V/380 V)
Constitutes the three‑phase four‑wire LV distribution network (220 V phase‑to‑neutral, 380 V phase‑to‑phase), with ±7% tolerance per GB/T 12325. Europe now uses 230/400 V; Japan uses a 100/200 V system. Protection includes RCDs (30 mA), overload/short‑circuit breakers (breaking capacity ≥6 kA), TN‑S earthing (PE conductor insulated throughout, earth resistance ≤4 Ω)
3. Medium Voltage (10 kV–35 kV)
10 kV is common for urban distribution (cable current capacity ~300 A/km), 35 kV for suburban/industrial feeders. IEEE 1547 limits distributed generation interconnection to ≤35 kV, requiring ±10% voltage regulation for PV plants.
4. High Voltage (110 kV–220 kV)
Economical for bulk transfer of 50–100 MW over 110 kV (e.g., LGJ‑240 conductors) and 200–500 MW over 220 kV. Typical substation transformer ratings are 180 MVA at 220 kV, with short‑circuit impedance of 12%–14%
5. Extra‑High & Ultra‑High Voltage (≥330 kV)
500 kV AC lines can carry ~1000 MW naturally; ±800 kV DC lines up to 8000 MW (conductors 6×720 mm²). 1000 kV AC UHV lines incur losses <0.8 ‰ per km
Basis for Voltage‑Level Determination
1. National Standards
China’s voltage tiers derive from GB/T 156‑2017 (“Standard Voltages”) and GB/T 156‑2007, which align with IEC 60038 yet reflect local grid needs for 50 Hz AC systems.
2. Transmission Distance
Higher voltages suit longer distances: 0.4 kV for short (<0.6 km) local distribution, up to 1000 kV AC for ultra‑long (800–1500 km) bulk transfer.
3. Transmission Distance
Higher voltages suit longer distances: 0.4 kV for short (<0.6 km) local distribution, up to 1000 kV AC for ultra‑long (800–1500 km) bulk transfer.
4. Technical Feasibility
Advancing to higher voltage levels entails challenges in insulation, cooling, and materials. UHV equipment uses specialized insulators and thermal management to maintain safe, stable operation, with ongoing R&D driving voltage‑class expansion.